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GE Bolt Mike III Operating Manual

1. Page 26 74 Material Variations Many materials exhibit very uniform material constants However material constants in samples of some materi als will vary widely A material s elastic modulus has a direct effect on that material s acoustic velocity Vo and stress factor K Hardening or heat treatment of the material or relaxation of the hardening will affect the accuracy of the standard values of these constants In fact the constants in some materials can vary dramatically as a result of work hard ening of the material Therefore it is strongly suggested that a sample of the bolts be tested to confirm the accu racy of the material properties you ve chosen under ac tual loading conditions Guide to Ultrasonic Inspection of Fasteners Chapter 8 BoltMike Formulas The BoltMike uses the following collection of formulas as a basis for all calculations and derived values If us ing the formulas manually be certain to convert all val ues to the units listed below and to adhere to accepted rounding practices and number of significant digits Fi nally keep in mind that all BoltMike calculations are per formed in metric units When English units are displayed the conversion from metric to English takes place after values are calculated Units Temperature Thermal Coefficient Cp Time of Flight TOF Acoustic Velocity Vo AII values of length Modulus of Elasticity Eo Stress Factor K Degrees C 1 D
2. ase 2650 sem 234 o 2750 578 O 278 eso 6r 3 sow 684 28 THREAD SERIES 28UN Tensile Stress Area Secondary Basic Major Diameter i i Sq in 12 216 i N E 17 16 11 2 Page 36 Guide to Ultrasonic Inspection of Fasteners Appendix Tabular Data 32 THREAD SERIES 32UN Sizes Primary Secondary Basic Major Diameter Tensile Stress Area Sq in in In In woj NO o waj o IE Des esso IE ene eno com __ DECO esse com _ es ess sos _ SEC o NT m RN O o e esso om es ess e s eee em ose ee ese _ RETI IO N o e se oe _ REC O o IE se sens ese _ AAA RS Guide to Ultrasonic Inspection of Fasteners Page 37 Appendix Tabular Data THIS PAGE WAS INTENTIONALLY LEFT BLANK Page 38 Guide to Ultrasonic Inspection of Fasteners
3. ROTATE TO FIND MAXIMUM A SCAN AMPLITUDE MARK POSITION ON FASTENER HEAD CONTINUE TENSIONING CONFIRM THAT MARKED POSITION CONTINUES TO RETURN THE MAXIMUM AMPLITUDE A SCAN FIGURE 2 3 A consistent approach to transducer placement ensures accurate results Guide to Ultrasonic Inspection of Fasteners Page 13 Chapter 2 Fastener Preparation maximum amplitude waveform may change If the maximum response location has changed adjust the position of the transducer to the new location on the bolt head This assures the optimum sound path is being used both before and after tightening Step 5 Position the transducer in the marked location or at the newly identified maximum amplitude location to continue recording tensioned readings 2 2 2 Fixtures for Non Magnetic Fasteners When fasteners are made of non magnetic materials fixtures are sometimes used to hold the transducer in place Note that the fit between the transducer and the head of the bolt is extremely critical and some provision must be made in the fixture to allow the transducer to float while finding the position where contact is at ts best NOTE Ultrasonic inspection techniques evaluate the change in length of a fastener Fastener elongation occurs when a significant portion of the fastener known as the effective length is exposed to tensile loading However ultrasonic techniques are not ef fective when only a small percentage of the f
4. BoltMike calculations while others have a significant effect It is important to understand how each geometric property affects the BoltMike s out put 6 1 Approximate Length In the BoltMike the approximate length is the total length of the fastener In terms of ultrasonics this is the dis tance from the ultrasonic transducer to the opposite re flecting end of the fastener The approximate length is used to determine the distance at which the BoltMike s receiver is enabled While the accuracy of the quantity entered for total fas tener length does not directly affect the accuracy of the BoltMike readings entering a significantly incorrect value for total length may result in unstable or no readings at all If the value entered for approximate length is too large the first echo that returns from the bolt will be ig nored If the value entered for approximate length is too short the BoltMike will not detect the correct returning echo These two cases are shown in Figure 6 1 INCORRECT POSITION APPROX LENGTH TOO SMALL 6 2 Determining Effective Length When a fastening system is tensioned the length of the fastener to which the tensile load is applied is known as its effective length When considering a constant applied load the amount of fastener elongation is directly pro portional to a fastener s effective length In other words if two fastening systems are identical in all ways includ ing the tensile load on
5. EFFECTIVE LENGTH FIGURE 6 2 This is a typical stud configuration The effective length of a stud with nuts on each end is found by adding the stud diameter to the clamp length 1 2 BOLT DIAMETER 1 CLAMP LENGTH gt 1 3 BOLT DIAMETER gt EFFECTIVE LENGTH _ gt FIGURE 6 3 This is a typical through bolt configuration The effective length of a bolt with a single nut is found by adding half the diameter to one third the diameter 5 6 of the diameter total to the clamp length Page 22 Guide to Ultrasonic Inspection of Fasteners Chapter 6 Fastener Geomet 1 2 BOLT X CLAMP LENGTH gt DIAMETER Ta BOLT DIAMETER gt EFFECTIVE LENGTH FIGURE 6 4 This is typical of a configuration with a bolt screw turned into a threaded hole When a headed fastener is threaded into a metal block such as an automotive head bolt calculate the effective length by adding half the diameter to one third the diameter 5 6 of the diameter total then adding this amount to the clamp length 1 2 STUD CLAMP LENGTH 1 2 STUD DIAMETER EFFECTIVE LENGTH DIAMETER FIGURE 6 5 This is typical of a configuration with a stud turned into a threaded hole When a stud is threaded into A blind hole and a nut is placed on the opposite end find the effective length by adding the stud diameter to the clamp length Guide to Ultrasonic Inspection of Fasteners Page 23 Chapter 6 Faste
6. Figure 1 2A a sound pulse is sent through the fastener The BoltMike s triggering gate is positioned based on the user inputted value of the fastener s approximate length to detect this sound pulse s first returning echo The BoltMike measures the time duration between transmit ting and receiving the sound pulse and uses this value as the basis for its calculations In M E measurement mode a sound pulse is again trans mitted into the fastener This time however the BoltMike utilizes two triggering gates These gates are positioned so that the first returning echo triggers the first gate and the second returning echo triggers the second gate The gates are again positioned based on the user in PAS Triggering point putted value of the fastener s approximate length In this mode the BoltMike measures the time duration between triggering of the two gates by two consecutive echoes It is critical however that similar features on the two con secutive packets be used to trigger the gates An advantage of operating in M E mode is that all mea surements are taken between the first and second re turning echoes This means that variations in transducer to fastener coupling caused for instance by varying couplant thickness and instrument zeroing are factored out of the BoltMike s measurement This is shown in Figure 1 2B 1 1 4 Time of Flight and Ultrasonic Length The elapsed time between transmitting and receiving the
7. Figure 2 1 this parallelism al lows for identical sound path distance regardless of the transducer s position The degree to which these two surfaces are machined parallel determines the upper limit of an ultrasonic inspection system s accuracy MACHINE FASTENER ENDS TO BE PARALLEL TO EACH OTHER AND PERPENDICULAR TO CENTER LINE POOR END PREPARATION PRODUCES INCONSISTENT SOUND PATHS FIGURE 2 1 Fastener ends must be uniform parallel and perpendicular to the fastener s centerline to ensure acceptable ultrasound transmission Guide to Ultrasonic Inspection of Fasteners Page 11 Chapter 2 Fastener Preparation While the surface finish of the reflective surface is not as critical very rough or uneven finish can produce errors Use care when machining fastener ends A common problem occurs when a small peak is left in the center of a fastener end after facing on a lathe This small bump prevents the transducer from achieving proper contact and greatly reduces the signal amplitude NOTE The use of Multi Echo measurement mode re duces some types of variation and measurement in accuracies especially those that are due to couplant thickness and instrument probe zeroing However errors introduced by inconsistent transducer place ment or surface preparation techniques are not elimi nated with the use of M E mode 2 2 Methods Of Transducer Placement Unless fastener ends and transducer surfaces are per fectly
8. along a fastener s length before and after a tensioning force is applied to the fastener The fastener material s acoustic velocity together with difference in the measured times allows the instrument to calculate the change in fastener length under the tensile load Provided the fastener s dimensional and material prop erties are known and the constants that represent the material properties are entered into the instrument the BoltMike will calculate the load and stress present when the fastener is in its tensioned state 1 1 Important Concepts To best understand exactly how ultrasonic sound waves are used to determine loads stress and elongation of threaded fasteners it is necessary that you understand the concepts described in this section Chapter 8 lists the actual formulas used by the BoltMike to calculate many of the quantities described below 2 WAVE REFLECTS HERE SOUND PATH BOLTMIKE MEASURES ITS DUARTION AND DIVIDES THAT BY TWO TO DETERMINE A TIME OF FLIGHT 1 1 1 Acoustic Velocity Applying a large electric pulse to a piezoelectric element in a transducer creates an ultrasonic shock wave This type of shock wave known as longitudinal wave travels through a fastener at a speed equal to the fastener material s acoustic velocity A material s acoustic veloc ity represents the speed with which sound moves through it All materials have a representative acoustic velocity but true velocity can vary from one
9. center of the fastener less of the actual stress is observed 1 1 12 Fastener Geometry Several geometrical characteristics of fasteners affect the ultrasonic measurement of load stress and elonga tion While these characteristics are described in great detail in Chapter 6 and the Appendix Figure 1 7 briefly illustrates them Guide to Ultrasonic Inspection of Fasteners Chapter 1 Ultrasonic Measurement of Fasteners VECTOR CORRELATION SLOPE STRESS RATIO TRUE RELATIONSHIP BETWEEN ACTUAL AND ACTUAL MEASURED LOAD TENSILE LOAD Y INTERCEPT STRESS OFFSET AS REGRESSION CORRELATION SLOPE STRESS RATIO IN THIS AREA VECTOR CAN BE MORE ACCURATE BOLTMIKE CALCULATED LOAD FIGURE 1 6 When the Calibration Group feature is used known and measured loads for a group of fasteners are entered into the BoltMike The correlation method chosen vector or regression determines if a stress ratio or a stress ratio and offset correction factor are then calculated As you ll learn in Chapter 6 the quantities inputted for fastener geometry have varying effects on the accuracy of the BoltMike s calculations In general Cross Sectional Area Affects the calculation of LOAD Effective Length Affects the calculation of ELON GATION LOAD amp STRESS Approximate Total Length Affects only the position of the triggering gates EFFECTIVE LENGTH AVERAGE CROSS SECTIONAL AREA A TOTAL LENGTH
10. gt 1 2 Principles of BoltMike Operation NOTE This section offers a brief description of fas tener elongation measurement using ultrasonics For more details on ultrasonic inspection techniques in general refer to ULTRASONIC TESTING OF MATE RIALS by Josef and Herbert Krautkramer 3rd Edition 1983 IBSN 0 318 21482 3 324 published by the American Society of Nondestructive Testing FIGURE 1 7 The geometrical characteristics of a fastener greatly affect the results obtained by ultrasonic inspection techniques Included in these important characteristics are total length effective length and average cross sectional area Guide to Ultrasonic Inspection of Fasteners Page 7 Chapter 1 Ultrasonic Measurement of Fasteners The BoltMike measures the time it takes for a sound wave to travel through a fastener The sound wave more spe cifically known as an ultrasonic shock wave or longitudi nal wave is created in the transducer The wave is gen erated when a large electric pulse is sent to the trans ducer from the instrument This pulse excites a piezo electric element in the transducer The wave s frequency varies with the thickness of the piezoelectric element Frequencies most useful for measuring fasteners range from 1 to 20 MHz This range of ultrasound will not travel in air Couplant which is a dense liquid substance usually glycerin or oil must be used to provide a pathway for the ultrasound to t
11. parallel as discussed in section 2 1 of this manual the reflected ultrasonic signal will vary with changes in the transducer s orientation with respect to the fastener This condition is illustrated in Figure 2 2 Optimal re peatability and accuracy are achieved by leaving the transducer attached to the fastener in exactly the same location and angular orientation throughout the tensioning process As this ideal approach is often not possible or practical the next best practice is to consis tently return the transducer to the same location and angular orientation with respect to the fastener This practice improves the chances that the path followed by the shock wave when the reference length was mea sured is identical or close to identical to the path fol lowed after the fastening system is tightened TRANSDUCER PLACEMENT AND ORIENTATION EFFECT MEASURED VALUES 2 2 1 Practical Methods Several practical methods are used to ensure consis tent transducer placement The most common method utilizes a magnetic transducer which is placed in the center of the bolt s head When inspecting bolts with di ameters above one inch refer to Figure 2 3 and follow these steps Step 1 First measure the reference non tensioned length by coupling the transducer to the fastener end and adjusting its orientation while observing the A scan display Position the transducer in the center of the fas tener end and identify the angular transduc
12. sample to another of the same material type and even throughout the material in a particular sample It is important to realize that the actual acoustic velocity is not truly a constant Instead it varies between fasteners of like material even when the fastener s material composition is tightly controlled 1 1 2 The Use of Ultrasound The ultrasonic wave is transmitted from a transducer into the end of a fastener When the ultrasonic wave encoun ters an abrupt change in density such as the end of the fastener most of the wave reflects This reflection trav els back the length of the fastener and back into the transducer When the shock wave re enters the piezo electric element a small electrical signal is produced This signal is represented on the BoltMike s display panel by the triggering of a measurement gate This signal is used by the BoltMike to indicate the returning wave Figure 1 1 ELEMENT mm CABLE TO BOLTMIKE TRANSDUCER S ELEMENT SENDS AND RECEIVES ULTRASONIC SHOCK WAVE FIGURE 1 1 The BoltMike determines the length of a fastener by measuring how long it takes for sound to travel its length Guide to Ultrasonic Inspection of Fasteners Page 1 Chapter 1 Ultrasonic Measurement of Fasteners 1 1 3 Initial Pulse and Multi Echo Measurement Modes The BoltMike III can be operated in one of two ultrasonic measurement modes initial pulse I P and multi echo M E In I P mode as illustrated in
13. tables contained in this appendix give the The BoltMike stores data in metric form If a number is cross sectional stressed area for many standard sizes entered in English units it is converted to metric for in of bolt The operator may choose to use these tables ternal use and then converted back to English to be to determine the area of a fastener IMPORTANT displayed The following table shows the displayed units These tables are provided for convenience only of the BoltMike in both English and metric StressTel does not assume liability for errors METRI O In this appendix you will find these tables e Material Constants Units of measurement Inches per microsecond imus Meters per Second m s English and Metric for each of the BoltMike s constant or measured values e Metric Standard Thread Eo elast mod j j K dV force Inches per Second per Pounds Metric Fine Thread apa Cp Temp coef x per degree x per degree Fahrenheit 9 Centigrade 9 Pounds per Square Inch psi MegaPascals MPa Meters per Second per Pascal m s Pa e Metric Standard Thread Waist Bolts Y Weld Pounds per Square Inch psi MegaPascals MPa e Metric Fine Thread Waist Bolts Extra Fine Thread Series UNEF and NEF inches in Millimeters mm Fine Thread Series UNF and NF LEfecive inches in Millimeters mm Coarse Thread Series UNC and NC Square Inches ir Square Milimeters mr 6 Thread Series UN inc
14. the fastener except that the ef fective length of the first fastener is twice the effective length of the second then the elongation of the first fas tener will be twice the elongation of the second The effective length must be entered into the BoltMike in order to make any measurement other than the refer ence length However the accuracy of the value en tered as the effective length has almost no influence on the accuracy of the elongation measurement And then the affect on elongation measurement is only noticeable at very high tensile loads approaching the material s yield strength Because the measurement of elongation is virtually independent of the effective length tension loading is specified in terms of elongation in applications where the ability to accurately determine effective length is questionable However the accuracy of the value entered for effective length has a direct influence on the accuracy of mea sured stress and load If the value entered for effective length is ten percent less than the actual value the er ror in load and stress measurements will be ten percent DESIRED GATE POSITION INCORRECT GATE POSITION APPROX LENGTH TOO LARGE FIGURE 6 1 The value of approximate total length is used only to set the position of the gate s on the A scan display screen Guide to Ultrasonic Inspection of Fasteners Page 21 Chapter 6 Fastener Geometry The effective length is calculated differently
15. ture increases the amount of time it takes for sound to travel through the fastener also increases In other words when a fastener is subjected to increased temperature its acoustic velocity decreases and therefore its ultra sonic length increases In fact temperature s effect on ultrasonic length is even greater than its effect on physi cal length The thermal expansion of the fastener and the ultrasonic velocity change with changing tempera ture are two separate effects However in the BoltMike s logic they are compensated for with a single combined factor known as the Temperature Coefficient Cp The BoltMike relies on its temperature compensation system to normalize the time of flight of a fastener and thus correct for temperature caused changes in its physi cal and ultrasonic length The compensation system normalizes the TOF to the value expected at 22 22 de grees C 72 degrees F before attempting to calculate the change in the fastener s ultrasonic length This compensation greatly improves accuracy when the tem perature has changed during the time period between recording a reference length and a tensioned length 4 1 Measuring Fastener Temperature In some applications significant differences in tempera ture exist from one portion of the fastener to another Compensating for these temperature gradients is extremely difficult Instead the fastener s average tem perature is used for temperature compensation Whi
16. with surfaces are indicated by poor signal quality on the waveform display Guide to Ultrasonic Inspection of Fasteners 1 3 4 The Limitations of I P and M E Measurement Modes Because M E measurement mode determines the elapsed time between two consecutively returning ech oes it eliminates some inconsistencies introduced in I P mode such as variation of couplant thickness and probe instrument zeroing However because M E mode relies on the second re turning echo and the quality of ultrasonic signals dimin ishes substantially with each returning echo there are certain conditions under which the subsequent return ing echoes will be distorted beyond acceptable limits and M E mode will not be effective For instance ultrasonic interference resulting from echoes off of the fastener s sidewalls increases the level of distortion present when the second returning echo is received To some extent the sidewall distortion effect can be compensated for with the use of a larger diameter transducer Similarly the effects of frequency dispersion attenuation and sidewall distortion can also be compensated for by using a lower frequency transducer In general lower transducer fre quencies produce greater amplitude returning echoes Ultimately however some small diameter longer length fastener measurements must be conducted in I P mode Page 9 Chapter 1 Ultrasonic Measurement of Fasteners THIS PAGE WAS INTENTIONALLY LEFT BLA
17. 2500 1806 tas fo teo ise so 1856 Ll pp tes tes tie po 150 168 ll ims 1565 17 se 1seso te ime tes 20 1984 io ae 11306 18025_ 24 pte do ETC E pis tes 277 2 20 es k 2 212350 f ss ema sw am ass aso aa ete 2500 47 Ll Pass 2650___ 516 ess __ 2700 s s 2 so ee o lap 300 67 sw siso 737 sia so 79 s aso 86s en 350 f wwe s 3650 8 esa ao f toeg 378 aso onma O 4 __ 400 tete aso po m te aa eo ne O 4 aso mez ste sw o s Ll Pess seso tes 434 ro ws 4 so tes ls po 50 we swe sio er sw poo sesw f at1 ll 5 sao 22 k sn 55000 2332 __ 5 5650_ 243 ss 570 254 pss 58750__ es e __ 60m 27 12 THREAD SERIES 12UN Sizes Secondary Basic Major Diameter Tensile Stress Area Primary in in in Sq in ste oss ome se __ osso oz ie oes e 3 __ osso 0985 ae 040 ze __ osso 04 aser ose __ oo 06 _ ss ose ae teso oss oe ie os aia teso ora sie ass 9 ie 1 o 1816 O f om es 7 ae __ o is A es __ rr _ ine tes 206 _ ia so 210 A es 28 e 880 28 A ss 20 _ __ 200 28 f oee 200 328 D
18. 6 __ iti tos 0696 pte 1250 oo _ 136 mss 0802 pata __ teo 100 1806 tas ima 138 __ 1350 tes pre 1475 1360 ito 150 tee 1916 1566 16 1588 __ 1650 178 ame ter g ia 170 zos imss ies es BON DE EM a 20 Li Ps ar 28 peer 2220 o O o 2 21250 as zia so 356 238 eso ss ame 2500 44 256 2650 49 a 2550 54 27 28780 59 3 __ 30 est aw s 7os sua seso 7s 338 aso est ate 3580 ass 3650 ou 334 aro 08 s aso noe A A GO A A 1 sie caso gt 1259 ama eo 138 aso 49750 142 42 asomo a 458 4650 150 434 __ 4750 tes ES SEO BU A pos __ 5 w8e7 5w sismo ww Pom ao 538 sso 217 sie f 550 ez 5 seso ws 534 __ srmo _zs 5 ses HO e 600 zi Page 34 Guide to Ultrasonic Inspection of Fasteners Appendix Tabular Data 16 THREAD SERIES 16UN Sizes nali Basic Maj Diameter Tensile Stress Area Prima A gt Sq in fee A e ie __ osoo oxtst one 0565 os s __ 0650 0250 ne 0666 0308 ss ozo osa Ll pp ime 08125 044 78 oso osa e os7s 0604 lp too oes tute tos 0788 ate iso osso 1816 ties 0997 pt
19. Guide to Ultrasonic Inspection of Fasteners N STRESSTEL ULTRASONIC TESTING EQUIPMENT Part No 021 002 175 Rev B 2003 STRESSTEL 50 Industrial Park Road Lewistown PA 17044 Phone 866 243 2638 Fax 717 242 2606 www stresstel com Guide to Ultrasonic Inspection of Fasteners N STRESSTEL Copyright 2003 StressTel Important Notice Important Notice The following information must be read and understood by any user of a StressTel measurement instrument Failure to follow these instructions can lead to errors in stress measurements or other test results Decisions based on erroneous results can in turn lead to prop erty damage personal injury or death StressTel assumes no responsibility for the improper or incorrect use of this instrument General Warnings Proper use of ultrasonic test equipment requires three essential elements Selection of the correct test equipment e Knowledge of the specific test application require ments e Training on the part of the instrument operator This operating manual provides instruction in the basic set up and operation of the StressTel BoltMike III mea surement instrument There are however additional fac tors which affect the use of ultrasonic test equipment Specific information regarding these additional factors is beyond the scope of this manual The operator should refer to textbooks on the subject of ultrasonic testing for more detailed information Operat
20. In preparation for temperature calibration create a group containing enough fasteners to store one L REF for each of the fasteners you wish to sample Measurements made as described below will only be stored as L REFs Create a custom material type with the correct acoustic velocity then assign it along with a temperature coefficient of 0 zero to the group Page 18 Allow plenty of time for the bolt in the oven to reach the target temperature One way to tell when the internal temperature of the fastener has stabilized is to watch the L REF change on the BoltMike When the L REF has been stable for two minutes the temperature in the fastener is constant This occurs because the displayed L REF is temperature compensated Record the fastener s measured length and the probe s tempera ture reading Identify these as L1 and T1 Change the oven setting to the higher temperature monitor the bolt length until it again stabilizes and re peat the process described above Identify the second measured length and temperature as L2 and T2 You should now have recorded at least two ultrasonic length measurements at different temperatures Two measurement points will allow you to calculate a value of Cp These calculated values of Cp must be averaged over a temperature range to find the best value of Cp in the temperature range of your test In the following for mula L1 and T1 are the reference length and tempera ture for data point 1
21. NK Page 10 Guide to Ultrasonic Inspection of Fasteners Chapter 2 Fastener Preparation Prior to measuring a fastener it must be properly pre pared for ultrasonic inspection The fastener ends must be machined to be parallel and the end that willbe mated with a transducer must be machined to a controlled smooth surface finish Further to allow for proper cou pling of the transducer and fastener a suitable couplant must be applied Finally consistent placement of the transducer on the bolt head or stud end improves the instrument s accuracy and repeatability NOTE Most fastener materials are excellent conduc tors of ultrasound However certain cast irons and many plastics absorb ultrasound and cannot be mea sured with the BoltMike 2 1 Fastener End Surface Machining The ends of bolt heads and threaded sections bolts or studs must be prepared before the fastener is suitable for ultrasonic inspection The fastener end that will be mated with a transducer must be perpendicular to the fastener s centerline and machined to a very flat smooth surface to allow for proper coupling of the transducer The ideal finish for the transducer coupling point is be tween 32 to 63 min CLA 0 8 to 1 6 mm Ra Inadequate surface finishes are indicated by poor signal quality on the A scan display The reflective surface at the opposite end of the fas tener must be parallel to the surface that mates with the transducer As shown in
22. am __ 22500 36 238 29550 _ 40 zie __ 250 460 A 2660 506 e __ 2550 o em 2850 60 os es eoe sie 72 s __ o 78 ss eso e sie __ eo os 86 seo 986 sa 3760 dos se ess n 4000 deo 6 4280 es aa __ 4250 e 38 asso 446 ae 480 ss 8 40250 162 aaa asso id id ave 8580 sso 5 sowo sso _ si steso 200 _ sme 52500 oo s sas 20 _ sie _ ss0 a 556 seo 26 sa sso se _ s seo IE e cow 278 Guide to Ultrasonic Inspection of Fasteners Page 35 Appendix Tabular Data 20 THREAD SERIES 20UN Primary in in in Sq in ga __ 0250 0088 sie __ osws 00547 3g osso 00886 ne __ 0475 ong te osoo oo one __ 0528 0207 se oeso_ 021 __ une 0675 030 aa omo ose o ias osis 048 o ove oso 0586 ll see oss ose too om O ie 1065 oso7 ie n oso O o 1356 11878 tots tia 125800 ts o 1816 13128 tes 138 187580 aa o 16 1478 ist tie 180 185 is 15868 o 158 teo S O a es ii col 184 __ 180 27 Co 11356 tes aa gt ire 187580 as ll 1186 19878 aso pao 200 2 218 a ass zia 2250 ass ll 238 23780 425 zie 250 472
23. and L2 and T2 the reference length and temperature for data point 2 aa it If readings are taken across a temperature range for example at four temperatures you can calculate a Cp for T1 and T2 as well as a Cp for T3 and T4 Then average the two calculated values for Cp to produce an average Cp over the temperature range e Guide to Ultrasonic Inspection of Fasteners Chapter 5 Selecting Phase When recording a reference non tensioned fastener length the operator must first select a measurement phase This setting determines if the triggering gate is positioned above or below the A scan zero level and therefore if the gate detects positive or negative head ing portion of the signal Once the measurement phase is selected and an L Ref is recorded the phase may not be changed again for that fastener Therefore it is critical that the user first examine the A scan shape in non tensioned and tensioned loading conditions As shown in Figure 5 1 there are often low amplitude half cycle features visible on the A scan These echoes should not be used to trig ger the gate as they are not valid representations of a returning echo However the first valid echo available should be used to trigger the gate especially in Multi Echo mode as later echoes may be substantially af fected by sidewall distortion Sidewall distortion results from sound energy reflecting off of the fastener s sidewalls into the primary sound p
24. arge variation in temperature in the range of fifty degrees Centigrade or Page 17 Chapter 4 Temperature Compensation more the nonlinear thermal reaction becomes a factor and significant errors may occur When temperature variations are relatively large and increased accuracy is desired the temperature coefficient may be adjusted to the specific temperature range 4 3 Adjusting the Temperature Coefficient If measurements are to be made over a large tempera ture range 50 degrees C or greater the best results will be obtained by adjusting the temperature coefficient to the particular bolt and the specific temperature range Select at least two temperature levels that fall within the temperature range anticipated during the actual ultra sonic measurement For example the extremes of the temperature range may be 20 degrees C representa tive of the shop temperature when the fasteners refer ence length is recorded and 70 degrees C the tem perature of the structure to which the bolt will be con nected In this case you might wish to examine the fas tener at 20 40 50 and 70 degrees C Proper temperature calibration requires a means of con trolling the bolt temperature such as a temperature oven Place the bolt to be measured in the oven set to the lower of your two target temperatures with a transducer and temperature sensor attached It is not necessary to load the bolt to determine the temperature coefficient
25. astener s length experiences tensile loading such as a screw holding a piece of sheet metal or where load levels are below 10 of ultimate tensile stress Page 14 Guide to Ultrasonic Inspection of Fasteners Chapter 3 Transducer Selection A wide variety of ultrasonic transducers are available Suitability for a specific application is determined based on the transducer s center frequency diameter and damping However because there is often a broad range of applications for which transducers are suitable and these ranges often overlap it can be difficult to pick the best transducer for a specific job NOTE It is a generally accepted practice that the same style and model probe be used when taking non tensioned L Ref and tensioned fastener measure ments of a fastener group Further it is preferable that the same probe be used to make tensioned and non tensioned measurements of a fastener group 3 1 General Acceptability There is no single rule of thumb to follow when selecting a transducer for a specific application For some fasten ing systems many different types of transducers will measure with acceptable results In the case of a hard to inspect fastener transducer selection becomes more critical The best way to evaluate an application is to use the Bolt Mike s waveform display and an assortment of transducers Try making readings on a fastener that s similar or identical to the ones you ll be ins
26. ath and back towards the transducer ALWAYS TRIGGER ON EARLIEST VALID ECHO LOW AMPLITUDE LEADING ECHOES SHOULD NOT BE USED TO TRIGGER GATES GATE POSITION IS SET TO TRIGGER ON A NEGATIVE HEADING PORTION OF THE SIGNAL GATE REFERENCE NON TENSIONED A A LEADING HALF CYCLE ECHOES MAY GROW WITH LOAD BUT SHOULD STILL NOT BE USED TO TRIGGER GATES Jl TENSIONED READING B FIGURE 5 1 Select the PHASE to trigger off of the first valid echo available in both the non tensioned and tensioned condition Note that invalid echoes before the first valid echo and distortion affected later echoes should not be used to trigger gates Guide to Ultrasonic Inspection of Fasteners Page 19 Chapter 5 Selecting Phase THIS PAGE WAS INTENTIONALLY LEFT BLANK Page 20 Guide to Ultrasonic Inspection of Fasteners Chapter 6 Fastener Geometry As explained throughout Chapter 1 of this guide many of the calculations made by the BoltMike rely directly on user input fastener dimensions A fastener s material type nominal length average diameter and effective length also known as working or grip length must be input in order for the BoltMike to perform all calculations While material types and the constants that define their properties are described in Chapter 7 this chapter deals with the geometric properties that define a fastener s shape Some of a fastener s geometric properties have little effect on certain
27. cific segment Figure 6 6 Add all of the re sulting values and then divide the total by the sum of the lengths In the appendix of this manual you will find tables of average cross sectional areas for various types and sizes of common fastener For the best accuracy of load readings calibrate the BoltMike for the specific application This will cancel er rors due to cross sectional area uncertainty In this ap proach a calibration group is formed using fasteners that are the same or similar to the ones being tested The fasteners are inserted in a fixture that loads them at the same effective length with a known quantity of load es eee 6 A3 A2 A1 A1xL1 A2xL2 A3xL3 A4xL4 A3xL3 A2xL2 A1xL1 L1 L2 L3 L4 L3 L2 L1 FIGURE 6 6 Follow this procedure to determine the average cross sectional area over the effective length of an irregular fastener Page 24 Guide to Ultrasonic Inspection of Fasteners Chapter 7 Material Constants As described in Chapter 1 of this guide several con stants are used by the BoltMike to represent the mate rial properties of a specific fastener You have the option of using constants already stored in the BoltMike for standard material types or defining constants for a custom material type 7 1 Standard Material Constants While constants are stored in the BoltMike for twelve stan dard material types as shown in Table 7 1 any other material type and it s related constan
28. de to Ultrasonic Inspection of Fasteners Appendix Tabular Data COARSE THREAD SERIES UNC AND NC 4 THREAD SERIES 4UN Basic Major Threads per In Tensile Stress Area Diameter in pe E N in ae ae aa one com _ ques eee e ome ee RE NT SE I Ps a ee e oem ee Teo am e Guide to Ultrasonic Inspection of Fasteners Page 33 Appendix Tabular Data 6 THREAD SERIES 6UN Primary in in in Sq in 138 18750 11555 me deso ir Apri 1500 il 1408 o ten ases a ise __ tes0 16s ime tes 18es 134 __ 1580 198 o tishe tes 244 _ ire temo aso __ 1186 is es O 2 200 25 218 242580 309 zia 2250 3a 238 2350 ss _ zie 250 429 o 258 2620 476 234 2750 S o 278 eso 578 3 __ 300 6s 3198 341280 689 aw sso 338 33750 em aw sso ez ass 3650 os 334 3750 tom 378 38780 toss 4 po 400 tis7 O 418 41250 was sia sw _ aa ll 438 49750 139 ate 450 tazas O ll 458 4650 156 O 434 4750 tes ll 47 aso 175 sd _ sm 184 518 Sio 193 sia seso 23 __ 588 60 as sie 55o 24 __ 558 sswso 234 534 57580 45 ss sesso 256 6 __ 600 8 THREAD SERIES 8UN Primary in in in Sq in i __ tow 06
29. depending on the fastener application The directions for calculat ing the effective length in four different cases are out lined in Figures 6 2 through 6 5 Note that the resulting values for effective length are approximate and may vary due to certain other factors For example consider an application using a bolt in a blind hole Suppose the material strength of the bolt is greater than the threaded hole The weaker threads in the hole will flex more than the threads of the bolt and the effective length will be longer than if the materials were of the same material For the best accuracy of load or stress readings cali brate the BoltMike for the specific application This will cancel errors due to effective length uncertainty In this approach a calibration group is formed using fasteners 1 2 STUD gt o CLAMP LENGTH 3 that are the same or similar to the ones being tested The fasteners are inserted in a fixture that loads them at the same effective length with a known quantity of load Refer to Figures 6 2 through 6 5 to identify the fasten ing system closest to the one you are evaluating Then follow the instructions in the applicable figure to calcu late effective loading The figures show e Stud fastening system Figure 6 2 e Through bolt fastening system Figure 6 3 Bolt screw turned into a threaded hole Figure 6 4 Stud turned into a threaded hole Figure 6 5 k 1 2 STUD DIAMETER DIAMETER k
30. e BoltMike Type m s 1 deg C MPa m s Pa MPa ASTM A193 B7 0 00000009062990 655 8621 0 00000009468245 655 8621 0 00000011457682 627 9593 0 00000013999740 720 6897 0 00000011162954 882 9428 0 00000010720853 991 0345 0 00000008989307 1058 9310 0 00000009210355 209 9862 ASTM A193 B16 ISO 8 8 ISO 9 8 1 0 9 ISO 10 9 11 9 ISO 11 9 12 9 ISO 12 9 304SS 304 Stainless Steel 316SS 316 Stainless Steel 0 00010304 0 00000008841941 209 9862 0 00000008105113 295 1724 0 00000009210355 274 9821 0 00000008068271 896 5517 Guide to Ultrasonic Inspection of Fasteners 1020S 1020 Mild Steel MONEL MONEL A490 A490 Structural Steel Page 25 Chapter 7 Material Constants the CUSTOM material mode is activated you edit the material name and any material property to match those of your non standard material type Even if you are able to obtain published constants for a non standard material type it is best to perform some amount of testing to determine the accuracy of the re sulting measurements Another way to determine the bolt type is to measure a group of bolts and use the built in calibration function to determine which material type gives the minimum error In this approach a calibration group is formed using fas teners that are the same or similar to the ones being tested The fasteners are inserted in a fixture that loads them at the same effective length with a known quantity of load
31. een tensile stress and elongation in a fastener The material s modulus of elasticity equals the slope of the straight portion of this curve this area is known as the material s elastic region The point at the top of the curve where it is no longer linear represents the material s yield strength Note that the graph actually plots stress verses strain Strain is simply the amount of elongation divided by the original length of the stressed section Page 4 Guide to Ultrasonic Inspection of Fasteners Chapter 1 Ultrasonic Measurement of Fasteners 1 1 9 Stress Factor K The velocity at which a longitudinal wave moves through an object is affected by stress When a fastener is stretched there are two influences on its ultrasonic length as determined by multiplying the sound wave s time of flight by the constant value of acoustic velocity First the length of material through which the sound must travel increases Also the fastener s actual acoustic velocity decreases as stress increases In other words even when the stretching effect on the fastener s physical length is ignored tensile stress leads to an increase in the fastener s ultrasonic length In the BoltMike a mate rial constant known as the Stress Factor K compen sates for the effect stress has on the fastener s actual acoustic velocity A great deal of confusion surrounds this effect Con sider the example shown in Figure 1 5 as you read the following descri
32. egrees C s Seconds m s Meters per sec m Meters Pa Pascal m s Pa meters per second per Pascal Yield Strength Y Pa Pascal Uncorrected Stress Pa Pascal Corrected Stress Pa Pascal Stress Offset Pa Pascal Cross Sectional Area m Square meters Load kN KiloNewton NOTE The units of measurement listed above are those units used in the following equations These are not in all cases the same units that are displayed by the instrument nor are they necessarily the same units as listed in tables throughout this guide Guide to Ultrasonic Inspection of Fasteners Measured Time of Flight TOF TOF measured Sound Path Duration 2 Reference Length LREF LREF TOF V measured Temperature Normalization TOF ormai 3 TOF reasured 1 Cp i Temp 22 22 measured Change in Ultrasonic Length Change in Ultrasonic Length V TOF V TOF PIRRO class Stress Calculation and Correction Stres uncorrected M Change in Ultrasonic Length K Change in Ultrasonic Length Effective Length StreSS ected z Stress corrected 1 Stress Ratio Stress Offset 100 Load Load Stress Cross Sectional Area corrected Elongation Elongation Stress Effective Length Eo corrected Page 27 Chapter 8 BoltMike Formulas THIS PAGE WAS INTENTIONALLY LEFT BLANK Page 28 Guide to Ultrasonic Inspection of Fasteners Appendix Tabular Data NOTE The
33. er position that returns the A scan waveform of greatest amplitude At this point consider the accuracy of the selected mea surement mode M E mode can increase repeatability and improve accuracy if the subsequent returning ech oes are free enough of distortion to be measured prop erly Step 2 Mark the transducer location and angular orien tation on the fastener end Step 3 Continue with the fastener tightening procedure If possible the transducer should remain connected to the fastener end in exactly the same position and orien tation If this is not possible proceed to step 4 Step 4 Before proceeding reconfirm that the position marked on the fastener end remains the location that returns the greatest amplitude waveform and the short est length and or lowest load or stress reading This step is important because in some cases as the fastener is tensioned a small amount of bending occurs When bending occurs the angular orientation that returns the PLACE THE TRANSDUCER TO OBTAIN THE STRONGEST A SCAN WAVEFORM AND THE SHORTEST LENGTH LOWEST LOAD MEASUREMENT FIGURE 2 2 Changing the transducer s position with respect to the fastener s end can change the shape and or amplitude of the returned waveform This effect is especially significant when inspecting long or large diameter fasteners Page 12 Guide to Ultrasonic Inspection of Fasteners Chapter 2 Fastener Preparation POSITION TRANSDUCER IN CENTER OF HEAD
34. ermined using the corrected stress and cross sectional area 1 3 Practical Limitations Of Ultrasonic Measurement Included in the list of fastening system types that are quite successfully inspected using ultrasonic techniques are those where equal distribution of load is critical such as pipe flanges and head bolts where gaskets must be compressed evenly for optimum performance Not all threaded fastening systems are suitable for mea surement by ultrasonic methods and some systems are better suited to either multi echo or initial pulse mea surements An understanding of ultrasonic inspection s practical limitations will reduce frustration and errone ous results 1 3 1 Material Compatible with Ultrasonic Inspection Most metals are excellent conductors of ultrasound How ever certain cast irons and many plastics absorb ultra sound and cannot be measured with the BoltMike 1 3 2 Significant Fastener Stretch Since ultrasonic techniques measure a fastener s change in length a significant amount of stretch is re quired to produce accurate measurements Accuracy is a significant problem in applications where the effective length of a fastener is very short such as a screw hold ing a piece of sheet metal These applications may be poorly suited to ultrasonic measurement because the tensile load and therefore tensile stress is applied over a very short effective length of the fastener Because Guide to Ultrasonic Inspectio
35. fastener s cross sectional area AVERAGE CROSS SECTIONAL AREA CORRECTED TENSILE STRESS x CROSS SECTIONAL AREA TENSILE LOAD CORRECTED TENSILE STRESS x EFFECTIVE LENGTH Eo ELONGATION FIGURE 1 3 As the threaded fastening system is tightened tensile loads are applied to the bolt or stud and elongation occurs Guide to Ultrasonic Inspection of Fasteners Page 3 Chapter 1 Ultrasonic Measurement of Fasteners 1 1 6 Stress Stress occurs when load is applied to a fastener When a tensile load like the one shown in Figure 1 3 is ap plied to a fastener the tensile stress is equal to the ten sile load divided by the fastener s average cross sec tional area see the Appendix for average cross sec tional areas The BoltMike calculates tensile stress in units of pounds per square inch psi or mega Pascal MPa This calculation is performed using the change in ultrasonic length the effective length acoustic veloc ity described in section 1 1 1 the material s stress fac tor a property that is described below and stress com pensation parameters known as Stress Ratio and Stress Offset These are instrument correction parameters that are described in section 1 1 11 1 1 7 Elongation As a tensile load is applied a fastener stretches in the same way a spring would The amount of stretch known as elongation is proportional to the tensile load as long as the load is within the fastener s working range which
36. he accuracy of the BoltMike s stress load and elon gation calculations depends on many factors Two ma jor influences on the accuracy of these calculations are the material property constants inputted and the fastener s geometric characteristics While the material property constants including elastic ity acoustic velocity and stress factor are considered to be standard values actual material properties vary widely This variation is even found among fasteners produced in the same manufacturer s lot The BoltMike s accuracy depends partly on the difference between the fastener s actual material properties and those proper ties represented by the standard material constants Similarly variations in fastening systems physical char acteristics affect the accuracy of load and elongation calculations Page 6 When BoltMike III users desire to calculate load elonga tion stress or TOF time of flight values with a higher degree of accuracy they generally choose to create calibration groups During the process of creating a cali bration group the BoltMike uses inputted values of ac tual tensile load as well as its own measured load data to calculate two correction factors Stress Ratio and Stress Offset These correction factors are used to con vert the BoltMike s raw stress value into a corrected stress as shown in Chapter 8 of this guide The BoltMike uses one of two methods to determine these correction factors T
37. he first method called a regression correlation uses a linear regression technique to deter mine the stress factor and offset Figure 1 6 The stress factor is actually the slope of a line that represents the relationship between actual and calculated load The stress offset represents the Y intercept of the actual verses calculated load line This value can be thought of as the level to which actual load can increase before the BoltMike can measure an observable load The second method used to determine correction fac tors is known as vector correlation With this approach the BoltMike calculates only a stress ratio The value of the stress offset is set to zero Figure 1 6 When creating a calibration group the user must de cide which correction method to use This decision should be based on the application If accuracy over a wide range of loads including low level loads is desirable the vector correction is usually preferred If the highest level of accuracy at a single target load is desired the regression method is best Why are two methods required Often the relationship between actual and measured stress is non linear especially at the low end of the curve as shown in Figure 1 6 This can be caused by a skin effect When a small amount of load is applied to a fastener most of the stress is in the surface layers not evenly distributed across the cross section Since the longitudinal wave travels predominantly down the
38. hes in Millimeters mm 8 Thread Series 8UN inches in Mlimeters mm e 28 Thread Series 28UN 32 Thread Series 32UN NOTE The following tables give the cross sectional stressed area for many standard sizes of bolt Use these tables to determine the area to enter into the bolt group IMPORTANT These tables are provided for convenience only StressTel cannot assume li ability for errors Guide to Ultrasonic Inspection of Fasteners Page 29 Appendix Tabular Data METRIC STANDARD THREAD Tensile Stress Area Sq mm METRIC FINE THREAD Tensile Stress Area Sq mm 39 2 1110 1028 Page 30 Guide to Ultrasonic Inspection of Fasteners Appendix Tabular Data METRIC FINE THREAD WAIST BOLTS Gili mm Sq mm BER CE ne cli ei mo to w as Deere e METRIC STANDARD THREAD WAIST BOLTS Waist ee Tensile Stress Area e mm HEREJE Guide to Ultrasonic Inspection of Fasteners Page 31 Appendix Tabular Data EXTRA FINE THREAD SERIES UNEF AND NEF n Basic Major Threads per in Tensile Stress Area Diameter in pe Sq in 12 121216 0 2160 SA E ER 0 0270 DICO IRE ee Ds se e IEC Cioe am ee FINE THREAD SERIES UNF AND NF Sizes Basic Major Thread nerin Tensile Stress Area in Diameter in perin Sq in oa oom 2 oo O Lao ona ew _ fue e mm sa oso a 78 osso 1 1 000 1 1250 0 856 ICA REC REN 0 7500 Page 32 Gui
39. ick glove Alternatively you may carefully remove the temperature sensor by pulling on and handling only its cable 4 2 Limits of Accurate Temperature Measurement Errors in temperature compensation can have several causes including e Manual input of air rather than fasten tempera ture e Contact between the operator s hand and the temperature sensor e Variation of the material s temperature coefficient e Materials non linear response to changes in temperature The last two of these sources of error should be further explained If asample of physically identical bolts is tested for temperature coefficient some bolt to bolt variation will be found The amount of variation will depend on the type of material and the uniformity with which the fas teners were manufactured One way to compensate for this variation is to determine the range of actual tem perature coefficients in the sample then decide of the difference between the actual and average values is too significant Alternatively a temperature calibration can be preformed for each fastener A materials actual response to changes in temperature as represented in the BoltMike by the temperature co efficient is not necessarily linear over a large range of temperatures Although the thermal expansion of a fas tener when plotted against change in temperature is very nearly linear non linearity is present in all materi als When trying to compensate for a l
40. ing rep resents only the condition of test piece material that is exposed to the sound beam Operators must exercise great caution in making inferences about the test mate rial not directly exposed to the instrument s sound beam When a less then complete inspection is to be per formed the operator must be shown the specific areas to inspect Inferences about the condition of areas not inspected based on data from evaluated areas should only be attempted by personnel fully trained in appli cable techniques of statistical analysis Sound beams reflect from the first interior surface en countered Operators must take steps to ensure that the entire thickness of the test material is being examined Calibrating the instrument transducer combination is particularly important when the test piece is being ultra sonically tested for the first time or in any case where the history of the test piece is unknown Transducer Selection The transducer used in testing must be in good condi tion without noticeable wear of its contact surface Badly worn transducers will have a reduced effective measur ing range The temperature of the material to be tested must be within the transducer s temperature range If the transducer shows any signs of wear it should be re placed Page iv Guide to Ultrasonic Inspection of Fasteners Contents Chapter 1 Ultrasonic Measurement of Fastnet SS ii io iii 1 1 1 Important Concepts 1 1 1 1 Acoustic Ve
41. le the BoltMike allows manual input of temperature it is preferable to input fastener temperature using the tem perature probe The BoltMike s temperature sensor provides a conve nient way to input fastener temperature Because it magnetically couples to the metal of the fastener joint it provides a very accurate temperature reading Typically the temperature sensor is attached to the superstructure or frame that is being fastened not each individual bolt The probe is then left in place while the lengths of all fasteners in the area are ultrasonically measured NOTE In most cases air temperature has very little effect on fastener temperature and should not be entered as the temperature of the fastener For opti mum accuracy use the temperature sensor and au tomatic temperature compensation Guide to Ultrasonic Inspection of Fasteners NOTE The range of the BoltMike temperature sen sor is 55 degrees to 150 degrees C 67 to 302 de grees F Use of the sensor outside of these ranges will damage the sensor NOTE Large accuracy problems can occur from han dling the temperature sensor Body heat conducted into the housing of the sensor will greatly increase the temperature reading After holding the sensor in a bare hand allow approximately ten to fifteen min utes for the temperature probe to stabilize If while fastener measurement is underway a temperature sensor must be moved handle it only while wearing a th
42. locity n e 1 1 1 2 The Use of Ultrasound 1 1 1 3 Initial Pulse and Multi Echo Measurement Modes 2 1 1 4 Time of Flight and Ultrasonic Length 2 1 1 5 Tensile Load neee 3 11 6 SESS ui dedi nett 4 11 7 EIONQatiON vices aaa eri 4 1 1 8 Modulus of Elasticity Eo 4 1 1 9 Stress Factor K 5 1 1 10 Temperature Coefficient Cp 6 1 1 11 Calibration Group Correction Factors Stress Ratio and Offset 6 1 1 12 Fastener Geometry 6 1 2 Principles of BoltMike Operation 7 1 3 Practical Limitations Of Ultrasonic Measurement i 8 1 3 1 Material Compatible with Ultrasonic INSPECCION ciccia 8 1 3 2 Significant Fastener Stretch 8 1 3 3 Fastener End Surface Configuration 9 1 3 4 The Limitations of I P and M E Measurement Modes 9 Chapter 2 Fastener Preparation 11 2 1 Fastener End Surface Machining 11 2 2 Methods Of Transducer Placement 12 2 2 1 Practical Methods 12 2 2 2 Fixtures for Non Magnetic Fasteners 14 Chapter 3 Transducer Selection 15 3 1 General Acceptability eee 15 3 2 Transducer Frequency 15 3 3 Transducer Diameter 15 Purpose of Instrument and Tra
43. means at loads that are less than the fastener s yield YIELD STRENGTH LOAD AREA N y Lu cc kE 0 Lu wm Z Lu kE SLOPE Eo MODULES OF ELASTICITY strength a term we ll describe shortly Using the effec tive length the materials modulus of elasticity and the calculated value for corrected stress the BoltMike calcu lates elongation Figure 1 3 1 1 8 Modulus of Elasticity Eo When a fastener is loaded with a tensile force its length increases As long as the loading does not approach the fastener s yield strength defined as the loading point beyond which any change in material shape is not com pletely reversible the relationship between the tensile stress and elongation is linear By this we mean that if the stress level increases by a factor of two the amount of elongation also increases by a factor of two For load levels in the fastener s elastic region meaning that the loads are less than the yield strength of the fastener the relationship between stress and elongation is de scribed by a material constant known as the modulus of elasticity The variable Eo in the BoltMike represents the modulus of elasticity The concepts of tensile stress elon gation modulus of elasticity and yield strength are illus trated in Figure 1 4 ELASTIC LIMIT _ ELASTIC REGION STRAIN CHANGE IN LENGTH LENGTH SUBJECTED TO TENSILE LOAI FIGURE 1 4 This graph shows the relationship betw
44. n of Fasteners Chapter 1 Ultrasonic Measurement of Fasteners the stressed length is so small little or no measurable elongation of the fastener occurs In the same way it is difficult to measure the effects of very low loads Negligible elongation occurs when ten sile stress levels are less than about 10 of the material s ultimate tensile stress The small errors in measurement introduced by removing and replacing the transducer as described in section 2 2 become very significant when trying to measure such a small amount of elongation 1 3 3 Fastener End Surface Configuration The ends of bolt heads and threaded sections bolts or studs must be prepared before the fastener is fit for ultrasonic inspection The fastener end that will be mated with a transducer must be machined to a very flat smooth surface to allow for proper coupling of the transducer The ideal finish for the transducer coupling point is be tween 32 to 63 micro inch CLA 0 8 to 1 6 micro meter Ra Refer to section 2 1 to learn more about the re quirements of fastener end surface preparation Similarly the surface at the opposite end of the fastener known as the reflective surface must be parallel to the surface that supports the transducer This parallelism allows the reflective surface to reflect the ultrasound back to the transducer While the finish of the reflective sur face is not as critical very rough or uneven finish can produce errors Problems
45. nal stress have no effect on the acoustic velocity when measured along the fastener s length 1 1 10 Temperature Coefficient Cp The temperature of a fastener affects its physical length As the temperature of a fastener increases its physical length increases In addition as a fastener s tempera ture increases the amount of time it takes for sound to travel through the fastener also increases In other words when a fastener is subjected to increased temperature its acoustic velocity decreases and therefore its ultra sonic length increases In fact temperature s affect on ultrasonic length is even greater than its affect on physi cal length The thermal expansion of the fastener and the ultrasonic velocity change with temperature are two separate effects However for the purpose of the BoltMike they are compensated for with a single com bined factor known as the Temperature Coefficient Cp The Bolt Mike relies on a temperature compensation system to normalize the measured time of flight TOF and thus correct for temperature caused changes in its physical and ultrasonic length The compensation sys tem normalizes the TOF to the value expected at 72 degrees Fahrenheit 22 degrees C before attempting to calculate the fastener s stress load and elongation This compensation greatly improves accuracy when the temperature has changed during tightening 1 1 11 Calibration Group Correction Factors Stress Ratio and Offset T
46. ner Geometry 6 3 Fastener Cross Sectional Area The cross sectional area is the average area of that portion of a fastener that is subjected to tensile loading In other words it s an average cross sectional area taken over only the fastener s effective length The cross sectional area in threaded portions of the fastener should be calculated based on the thread s minor diameter The accuracy with which cross sectional area is entered only affects the BoltMike load calculation It has no effect on the stress or elongation measurement The accuracy of the value entered for cross sectional area has a direct influence on the accuracy of measured load If the value entered for cross sectional area is ten percent less than the actual value then the measured value of load will be ten percent lower than the actual value If a fastener s geometry is more complex with varying values of cross sectional area along its effective length the various areas over the effective length may be aver it Veto aes L4 A1 A2 A3 A4 AVERAGE AREA aged to arrive at an overall average cross sectional area In the case of a hollow fastener the area of the hole must be subtracted from the overall average cross sec tional area to determine the actual cross sectional area To calculate the average cross sectional area of a fas tener multiply the length of each segment along the ef fective length of the fastener by the cross sectional area of each spe
47. nsducer LIM adas 15 Guide to Ultrasonic Inspection of Fasteners Chapter 4 Temperature Compensation 17 4 1 Measuring Fastener Temperature 17 4 2 Limits of Accurate Temperature Measurement cceeceeeeeeeeeeeeeeeeeeeeettteeeees 17 4 3 Adjusting the Temperature Coefficient 18 Chapter 5 Selecting Phase 19 Chapter 6 Fastener Geometry 21 6 1 Approximate Length eeeeeeeeeeeeees 21 6 2 Determining Effective Length 21 6 3 Fastener Cross Sectional Area 24 Chapter 7 Material Constants 25 7 1 Standard Material Constants 25 7 2 Custom Material Constants 25 7 3 Selecting a Material Constant 25 7 4 Material Variations eenen 26 Chapter 8 BoltMike Formulas 27 Appendix Tabular Data 29 Page v Chapter 1 Ultrasonic Measurement of Fasteners When threaded fastening systems comprised of a bolt or stud and a nut are tightened the threaded fastener is said to be tensioned The tensioning force in the fas tener identified in the BoltMike as its load is equal to the fastening system s clamping force The BoltMike determines the load on a fastener by mea suring the amount of time it takes for a sound wave to travel
48. or Training Read the information in this manual prior to use of a StressTel instrument Failure to read and understand the following information could cause errors to occur during use of the instrument Failure to follow these instruc tions can lead to error in stress measurement or other test results Decisions based on erroneous results can in turn lead to property damage personal injury or death Operators must receive adequate training before using ultrasonic test equipment Operators must be trained in general ultrasonic testing procedures and in the set up required before conducting a particular test Operators must understand Soundwave propagation theory e Effects of the velocity at which sound moves through the test material e Behavior of the sound wave e Which areas are covered by the sound beam Guide to Ultrasonic Inspection of Fasteners More specific information about operator training quali fication certification and test specifications is available from various technical societies industry groups and government agencies Testing Limitations Information collected as a result of ultrasonic testing rep resents only the condition of test piece material that is exposed to the sound beam Operators must exercise great caution in making inferences about the test mate rial not directly exposed to the instrument s sound beam When a less then complete inspection is to be per formed the operator must be sh
49. own the specific areas to inspect Inferences about the condition of areas not inspected based on data from evaluated areas should only be attempted by personnel fully trained in appli cable techniques of statistical analysis Sound beams reflect from the first interior surface en countered Operators must take steps to ensure that the entire thickness of the test material is being examined Calibrating the instrument transducer combination is particularly important when the test piece is being ultra sonically tested for the first time or in any case where the history of the test piece is unknown Transducer Selection The transducer used in testing must be in good condi tion without noticeable wear of its contact surface Badly worn transducers will have a reduced effective measur ing range The temperature of the material to be tested must be within the transducer s temperature range If the transducer shows any signs of wear it should be re placed Soundwave propagation theory e Effects of the velocity at which sound moves through the test material e Behavior of the sound wave e Which areas are covered by the sound beam More specific information about operator training quali fication certification and test specifications is available from various technical societies industry groups and government agencies Page iii Important Notice Testing Limitations Information collected as a result of ultrasonic test
50. pecting Use several different transducers and observe the waveform display and the stability of the reading produced with each transducer While you re using a transducer ob serve the effects of removing and replacing it Select the transducer that provides a large amplitude signal and stable repeatable readings 3 2 Transducer Frequency A transducer s frequency rating refers to the resonant frequency of the piezoelectric crystal This is determined by the thickness of the crystal material A thin crystal has a higher resonant frequency than a thick crystal The BoltMike will work with transducers in the 1 to 15 MHz megahertz range The frequency of the transducer affects the transmis sion of ultrasound in two different ways beamwidth and absorption The beamwidth also referred to as directiv ity identifies how dispersed the shock wave becomes as it travels over a specific distance Beamwidth de creases that is the wave becomes more tightly focused as transducer frequency increases This means that a 10 MHz transducer has a tighter beam with a lower beamwidth than a 5 MHz version of the same transducer A tightly focused beam is desirable since it allows more Guide to Ultrasonic Inspection of Fasteners energy to reach the end of the fastener making the noise that reflects off the thread and shank areas less of an issue However as frequency increases the absorption of the ultrasound by the material also increase
51. ption In Figure 1 5A no load is applied to the fastener when the reference ultrasonic length UL1 is recorded In Figure 1 5B a load is applied and a new ultrasonic length UL2 is recorded Note that Figure 1 5A and B also identify the physical length when unloaded Physical Length 1 and loaded Physical Length 2 The actual physical elongation of the fastener equals Physical Length 1 Physical Length 2 The dif ference between the ultrasonic lengths UL1 and UL2 is about three times the actual physical elongation of the fastener ULTRASONIC LENGTH 1 UL1 NO LOAD A PHYSICAL LENGTH ULTRASONIC LENGTH 2 gt TTT AND ELONGATION TENSILE LOAD B UL2 UL1 CHANGE IN ULTRASONIC LENGTH 3 x CHANGE IN PHYSICAL LENGTH FIGURE 1 5 Applied tensile stress affects the ultrasonic measured length of a fastener in two ways First it stretches the fastener thus increasing the actual length Second tensile stress reduces the fastener s acoustic velocity further increasing its ultrasonic length In the BoltMike the material constant K stress factor is used to compensate for the effect of tensile stress on acoustic velocity Guide to Ultrasonic Inspection of Fasteners Page 5 Chapter 1 Ultrasonic Measurement of Fasteners It is important to note that in order to change the acous tic velocity stress must be applied in the same direction traveled by the ultrasonic shock wave Thus shear and torsio
52. ravel from the transducer into the fastener When the ultrasonic wave encounters an abrupt change in material density such as at the end of the fastener most of the wave reflects This reflection travels back the length of the fastener through the layer of couplant and back into the transducer When the shock wave enters the piezoelectric element a small electrical signal is produced The BoltMike detects this signal In I P mode Initial Pulse mode is described in section 1 1 3 the BoltMike measures the elapsed time between the sound entering the material and the returned signal This elapsed time is known as the wave s time of flight Of course the time of flight actually represents the time taken by the wave to travel the length of the fastener two times The TOF reported by the BoltMike equals half of this value In M E mode Multi Echo mode is described in section 1 1 3 the BoltMike measures the elapsed time between two consecutive returning signals This elapsed time is equal to the wave s time of flight As in I P mode this time of flight actually represents the time taken by the wave to travel the length of the fastener two times The TOF reported by the BoltMike equals half of this value The BoltMike then determines the ultrasonic length by first using the temperature coefficient Cp to correct the TOF for any changes in temperature The BoltMike then multiplies the corrected TOF by the fastener s acoustic velocity Aco
53. rger than the piezoelectric crystal size For example a 1 4 inch 5 MHz non magnetic transducer has a case with a 3 8 inch outside diameter However when a transducer with the same 1 4 inch crystal is mounted in a magnetic housing the transducer s outside diameter is 3 4 inch Purpose of Instrument and Transducer Zeroing The BoltMike s zeroing procedure occurs whenever the user presses the Inst Zero key and follows the steps as prompted The procedure compensates for the actual delay that occurs while the transmitted pulse travels through the instrument s circuitry the probe cable and the probe s head and contact surface Variations in dif ferent probes and cables as well as changes in the trans Page 15 Chapter 3 Transducer Selection ducer cable length affect the necessary amount of time delay compensation Repeat the transducer calibration whenever changing transducers or cables As the probe s contact surface wears with use the instrument should be periodically re zeroed to compensate for any change in time delay NOTE When operating in multi echo measurement mode the transducer and instrument zero do not af fect the instrument s accuracy Page 16 Guide to Ultrasonic Inspection of Fasteners Chapter 4 Temperature Compensation The temperature of a fastener affects its physical length As the temperature of a fastener increases its physical length increases In addition as a fastener s tempera
54. s Absorption refers to the material s ability to absorb rather than re flect ultrasonic sound energy It interferes with the shockwave reducing the received signal s resolution Lower frequency ultrasound travels around small flaws or air bubbles in the fastener s without significant inter ference to the shock wave Absorption is an especially significant problem when inspecting more granular ma terial such as is found in castings In conclusion lower transducer frequencies are better suited as fastener lengths increase 3 3 Transducer Diameter A transducer s rated diameter actually refers to the di ameter of its crystal A transducer s diameter affects the efficiently with which it transmits sound as well as the beamwidth of the transmitted ultrasound Remember beamwidth identifies how dispersed the shock wave be comes as it travels over a specific distance Beamwidth decreases that is the wave becomes more tightly fo cused and transmitting efficiency increases as the di ameter of the transducer s crystal increases Again a tightly focused beam is desirable since it allows more energy to reach the end of the fastener making the noise that reflects off the thread and shank areas less of an issue It s generally preferable to select the largest diameter transducer available that will still fit on the fastener to be measured Note that external diameter of a transducer equipped with a built in magnet is much la
55. shock wave is known as the sound path duration Of course as shown in Figure 1 1 the sound path dura tion actually represents the elapsed time taken by the E TTS2 FASTENER 1 STAGE 1 9 2613 n MEASUREMENT BASED ON TIME FROM INITIAL PULSE TO ZERO CROSSING Baseline Zero crossing STABILITY METER MAA I P COMMNT PHASE I P OPE PEAK 123 GAIN 200M 57 dB DOMO BOLT BOLT RATING MODE 22 06 17 Y O Elio L REF 10 077 in JAN EFFECT OF INSTRUMENT ZERO AND TRANSDUCER COUPLING IGNORED STABILITY METER MEAN M E COMMNT PHASE FASTENER 1 Zero crossing Zero 1st packet crossing i ida 2nd packet PEAK gt 127 Z GAIN 1 GAIN 2 200M 21 dB 26 dB DOMO BOLT BOLT M E OPERATING MODE B FIGURE 1 2 In Initial Pulse I P mode the BoltMike measures the time to the first gate triggering In Multi Echo mode the time between two consecutive gate crossings is measured Page 2 Guide to Ultrasonic Inspection of Fasteners Chapter 1 Ultrasonic Measurement of Fasteners wave to travel the length of the fastener two times This duration is divided by two to find the time of flight TOF which represents the time it takes for the shock wave to travel once down the length of the fastener The BoltMike then determines the ultrasonic length by first correcting the measured TOF for any changes in temperature and then multiplying by the fastener s acoustic velocity Aco
56. ts may be entered using the CUSTOM material type feature Material constants used by the BoltMike include Vo Acoustic Velocity described in section 1 1 of this guide Eo Modulus of Elasticity described in section 1 1 8 of this guide Cp Thermal Coefficient described in sections 1 1 10 and 4 3 of this guide K Stress Factor described in section 1 1 9 of this guide Y Yield Strength described in section 1 1 8 of this guide The material constants listed in Table 7 1 are stored in the BoltMike for the twelve standard material types listed 7 2 Custom Material Constants StressTel offers laboratory material calibration at a nomi nal cost This service is highly recommended for users of exotic material or in applications where highest accu racy is required 7 3 Selecting a Material Constant There are several ways to select a bolt material con stant The best way is to compare the published specifi cations for the material you wish to evaluate against those of the standard material types listed in Table 7 1 First identify the standard material type that s closest in prop erties to the non standard material type you wish to test Next while creating a Group in the BoltMike first select the standard material type that most closely resembles the properties of your non standard material and then press D to enter the CUSTOM material mode When Table 7 1 Standard Material Types and Constants Stored in th
57. us tic velocity is represented in the BoltMike with the vari able V and is determined by the fastener s material type Further corrections as described below are then made to this ultrasonic length to determine a measured physi cal length Because the actual acoustic velocity is not truly a con stant the uncorrected ultrasonic length is not exactly the same as the physically measured length Even if two identical fasteners physical lengths are very tightly con trolled the measured time of flight through each fas tener may vary by as much as one percent Because of CLAMPING FORCE ON FLANGES gt this variability the change in measured time of flight re corded before and after each fastener is tensioned must be used to accurately determine the tensile stress in a fastener As you will learn shortly acoustic velocity also varies with factors other than material type including stress sections 1 1 9 and temperature section 1 1 10 For this reason the BoltMike incorporates logic to com pensate for these effects on ultrasonic length 1 1 5 Tensile Load As you may be aware when the nut in a threaded fas tening system is tightened the clamping force the fas tening system nut and bolt or stud places on the joint is equal to the tensile load placed on the fastener This effect is shown in Figure 1 3 The BoltMike calculates Load L by first determining tensile stress as described below then multiplying by the
58. ustic velocity is represented in the BoltMike with the variable V and is determined by the fastener s material type The stress constant K and effective length are then used by the BoltMike logic to determine an un corrected stress As explained in Chapter 8 when the calibration group feature is used the stress ratio and offset are applied to this stress value to find a corrected stress Since the actual acoustic velocity is not truly a constant and can vary significantly between fasteners of like ma terial composition the change in measured time of flight recorded before and after each fastener is tensioned Page 8 must be used to accurately measure a fastener s stress load and elongation To determine the change in time of flight the BoltMike first records a reference length by determining a nor malized time of flight for a non tensioned fastener A normalized time of flight measurement of the same fas tener this time while tensioned is then recorded The two normalized TOF s which have already been cor rected for the effects of temperature are then used with the effective length stress factor K and acoustic ve locity V to determine the uncorrected stress e The uncorrected stress is then corrected using the stress offset and stress ratio these values are produced using a Cal group e Elongation is calculated using the corrected stress effective length and the modulus of elasticity e Load is also det

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