Volume 08 - Mechanical Testing and Evaluation Part 13 pdf

Fatigue Testing of Brittle Solids J.A Salem, Glenn Research Center at Lewis Field; M.G Jenkins, University of Washington Summary Fatigue testing of ceramics and glasses is performed by either indirect or direct methods Indirect or strength methods employ smooth tensile or flexure specimens and infer the fatigue parameters from strength measurements without crack length measurements Direct methods employ either long cracks or short cracks, and the crack length is measured by observation of the crack or by inference from devices such as strain gages and electrical resistance grids Long crack test specimens include fracture mechanics specimens such as the DT, DCB, CT, and SEPB Short crack methods employ surface cracks formed by indentation, or surface cracks that develop naturally on the surface of a smooth test specimen Structural ceramic and glass components that are designed to have long lives will fail from small cracks developed over long periods of time The cracks will develop from either inherent processing flaws or from damage generated in component machining and handling (e.g., machining cracks) Cyclic loading, though not required to induce growth in glasses and many ceramics, tends to accelerate fatigue crack growth Thus the measurement of fatigue parameters should be done with tests employing realistic crack sizes, environments, and the applicable load histories As a result, the development of standardized static and cyclic fatigue test methods has revolved around the use of small, inherent flaws Fatigue Testing of Brittle Solids J.A Salem, Glenn Research Center at Lewis Field; M.G Jenkins, University of Washington References C Gurney and S Pearson, Fatigue of Mineral Glass under Static and Cyclic Loading, Proc R Soc A., Vol 192, 1948, p 537–543 E.B Shand, Experimental Study of Fracture of Glass, Part I: The Fracture Process, J Ceram Soc., Vol 37, 1954, p 52–60 S Pearson, Delayed Fracture of Sintered Alumina, Proc Physicl Soc., Vol 69, Part 12, Section B, 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Exercise,” NPL Report DMM (D) 72, National Physical Laboratory, Teddington, Middelsex, U.K., Crown Copyright, Dec 1990 69 “Standard Test Method for Tensile Strength of Monolithic Advanced Ceramics at Ambient Temperatures,” C 1273, Annual Book of ASTM Standards, Vol 15.01, ASTM, 1999, p 671–678 70 Cyclic Fatigue Testing of Silicon Nitride, Ceramics (newsletter), Ceramics Division of IIT Research Institute, Chicago, IL, 42, March 1978; Ceram Eng and Sci Proc of the 18th Annual Conference on Composites and Advanced Ceram Mater.-A, Part of 2, 9–14 Jan 1994, Vol 15 (No 4), July–Aug 1994 (Cocoa Beach, FL), Am Ceram Soc., Westerville, OH, 1994, p 32–39 71 H.N Ko, Cyclic Fatigue Behavior of Ceramics under Rotary Bending, Materials Research Society International Meeting on Advanced Materials, Materials Research Society, Vol 5, 1989, p 43–48 72 H.N Ko, Cyclic Fatigue Behavior of Sintered Al2O3 under Rotary Bending, J Mater Sci Lett., Vol 6, 1987, p 801–805 73 R Kossowsky, “Cyclic Fatigue of Hot Pressed Silicon Nitride,” J Am Ceram Soc., Vol 56, 1973, p 10, 531–535 74 Y Matsuo, Y Hattori, Y Katayama, and I Fukuura, Cyclic Fatigue Behavior of Ceramics, Progress in Nitrogen Ceramics, F.L Riley, Ed., Martinus Nijhoff, 1983, p 515–522 75 K Ohya, K Ogur, and M Takatsu, Effect of Loading Waveform on Cyclic Fatigue Behavior of PSZ, J Soc Mater Sci., Jpn., Vol 38 (No 425), 1989, p 144–148 76 K Ohya, K Ogura, and M Takatsu, Cyclic Fatigue Testing Device for Fine Ceramics by Using PiezoElectric Bimorph Actuator, J Soc Mater Sci., Jpn., Vol 38 (No 424), 1989, p 44–48 77 J.A Salem, L.J Ghosn, and M.G Jenkins, A Strain Gage Technique to Measure Stable Crack Extension in Ceramics, Post Conference Proceedings of the 1997 SEM Spring Conference on Experimental Mechanics, Society for Experimental Mechanics, Bethel, CT, 1997, p 1–8 78 J.A Salem, L.J Ghosn, and M.G Jenkins, Back-Face Strain as a Method for Monitoring Stable Crack Extension in Ceramics, Ceram Eng Sci Proc., Vol 19 (No 3), 1998, p 587–594 79 J.O Outwater and D.J Gerry, “On the Fracture Energy of Glass,” NRL Interim Contract Report, Contract NONR 3219(01)(x), AD 640848, University of Vermont, Burlington, VT, Aug 1966 80 J.O Outwater and D.J Gerry, “On the Fracture Energy, Rehealing Velocity and Fracture Energy of Cast Epoxy Resin,” Paper 13-D, presented at the 22nd Society of Plastic Industry Conference, 1967; also, J Adhes., Vol 1, 1969, p 290–298 81 G.W Weidmann and D.G Holloway, Slow Crack Propagation in Glass, Phys Chem Glasses, Vol 15 (No 5), Oct 1974, p 116–122 82 C.D Beacham, J.A Kies, and B.F Brown, A Constant K Specimen for Stress Corrosion Cracking Testing, Mater Res Standards, Vol 11, 1970, p 30 83 A.G Evans, Method For Evaluating the Time-Dependent Failure Characteristics of Brittle Material and Its Application to Polycrystalline Alumina, J Mater Sci., Vol (No 10), 1972, p 1137–1146 84 A.G Evans and S.M Wiederhorn, Crack Propagation and Failure Prediction in Silicon Nitride at Elevated Temperatures, J Mater Sci., Vol 9, 1974, p 270–278 85 A.G Evans, L.R Russel, and D.W Richerson, Slow Crack Growth in Ceramic Materials at Elevated Temperatures, Metall Trans A, Vol 6A (No 14), 1975, p 707–716 86 K.R McKinney and H.L Smith, Method of Studying Subcritical Cracking of Opaque Materials, J Am Ceram Soc., Vol 56 (No 1), 1973, p 30–32 87 P.N Thornby, Experimental Errors in Estimating Times to Failures, J Am Ceram Soc., Vol 59 (No 11–12), 1976, p 514–517 88 E.R Fuller, Jr., an Evaluation of Double Torsion Testing: Analysis, Fracture Mechanics Applied to Brittle Materials, STP 678, S.W Frieman, Ed., ASTM, 1979, p 3–18 89 B.J Pletka, E.R Fuller, Jr., and B.G Koepke, An Evaluation of Double Torsion Testing: Experimental, Fracture Mechanics Applied to Brittle Materials, STP 678, S.W Frieman, Ed., ASTM, 1979, p 19–37 90 C.G Annis and J.S Cargill, Modified Double Torsion Method for Measuring Crack Velocity in NC-132 Si3N4, Fracture Mechanics of Ceramics, Vol 4, R.C Bradt et al., Ed., Plenum Press, 1978, p 737–744 91 D.P Williams and A.G Evans, A Simple Method for Studying Slow Crack Growth, J Test Eval., Vol (No 4), 1973, p 264–270 92 K.R Linger and D.G Holloway, Fracture Energy of Glass, Philos Mag., Vol 18 (No 156), 1968, p 1269–1280 93 J.A Salem, M.G Jenkins, M.K Ferber, and J.L Shannon, Jr., Effects of Pre-Cracking Method on Fracture Properties of Alumina, Proceedings of Society of Experimental Mechanics Conference on Experimental Mechanics, 10–13 June 1991 (Milwaukee, WI), Society for Experimental Mechanics, Bethel, CT, 1991, p 762–769 94 S.W Freiman, D.R Mulville, and P.W Mast, J Mater Sci., Vol (No 11), 1973, p 1527–1533 95 S Mostovoy, P.B Crosley, and E.J Ripling, Use of Crack-Line-Loaded Specimens for Measuring Plane-Strain Fracture Toughness, J Mat., Vol (No 3), Sept 1967, p 661–681 96 A.G Evans and H Johnson, The Fracture Stress and Its Dependence on Slow Crack Growth, J Mater Sci., Vol 10, 1975, p 214–222 97 T Sadahiro, Transverses Rupture Strength and Fracture Toughness of WC-Co Alloys, J Jpn Inst Met., Vol 45, 1981, p 291–295 98 D.J Martin, K.W Davido, and W.D Scott, Slow Crack Growth Measurement Using an Electric Grid, Am Ceram Soc Bull., Vol 65 (No 7), 1986, p 1052–1156 99 P.K Liaw, H.R Hartmann, and W.A Lodgson, A New Transducer to Monitor Fatigue Crack Propagation, J Test Eval., Vol 11 (No 3), 1983, p 202–207 100 R.H Dauskardt, R.O Ritchie, J.K Takemoto, and A.M Brendzel, Cyclic Fatigue in Pyrolytic Carbon-Coated Graphite Mechanical Heart-Valve Prostheses: Role of Small Cracks in Life Prediction, J Biomed Mater Res., Vole 28, 1994, p 791–804 101 R.H Dauskardt, M.R James, J.R Porter, and R.O Ritchie, Cyclic Fatigue-Crack Growth in a SiC-Whisker-Reinforced Alumina Ceramic Composite: Long- and Small-Crack Behavior, J Am Ceram Soc., Vol 75 (No 4), 1992, p 759–771 102 “Standard Test Methods for Fracture Toughness of Advanced Ceramics,” C 1421, Annual Book of ASTM Standards, ASTM, Vol 15.01, 2000, p 631–662 103 L Ewart and S Suresh, Dynamic Fatigue Crack Growth in Polycrystalline Alumina under Cyclic Compressive Loads, J Mater Sci., Vol 5, 1986, p 774–778 104 L Ewart and S Suresh, Crack Propagation in Ceramics under Cyclic Loads, J Mater Sci., Vol 22, 1987, p 1173–1192 105 F Sudreau, C Olagnon, and G Fantozzi, Lifetime Prediction of Ceramics: Importance of Test Method, Ceram Int., Vol 10, 1994, p 125–135 106 E.M Rockar and B.J Pletka, Fracture Mechanics of Alumina in a Simulated Biological Environment, Fracture Mechanics of Ceramics, Vol 4, R.C Bradt et al., Ed., Plenum Press, 1978, p 725–735 107 Pletka and Wiederhorn, Subcritical Crack Growth in Glass-Ceramics, Fracture Mechanics of Ceramics, Vol 4, R.C Bradt et al., Ed., Plenum Press, 1978, p 745–759 Multiaxial Fatigue Testing Yukitaka Murakami, Kyushu University, Japan Introduction TESTS for combined-stress fatigue and multiaxial fatigue have been conducted since the early stages in the history of fatigue testing In particular, the combined-stress fatigue test for cylindrical specimens has been used by many researchers The main objective for classical studies of combined-stress fatigue was to obtain fatigue data for axles and to find the criterion for the fatigue limit under combined stress Although recent studies still use essentially the same testing method, the main objective is to elucidate the factors that control the fatigue mechanism and particularly the behavior of small fatigue cracks The influence of loading history and phases is also a topic of recent studies Cylindrical specimens or tubular specimens are mostly used for these studies Perhaps the most important recent topic in multiaxial fatigue studies is the behavior of cracks The threshold condition of macrocracks and crack propagation paths in large structures have been investigated by many researchers Although cracks mostly propagate by mode I (the opening tension mode), even under mixed mode loading, the propagation behavior is affected by mixed mode loadings due to various factors such as the size of the yield zone at the crack tip, crack closure, and friction between crack surfaces On the other hand, a crack seldom grows by pure mode II (sliding or shear mode) or mode III (tearing mode) in real structures Some examples of mode II fatigue are contact fatigue damage in rolls of steelmaking mills, contact fatigue of rails and bearings, and fretting fatigue In these cases, the criteria for the threshold condition for mode II cracks and the resistance to mode II crack growth are needed Crack growth by mode III is the form studied in the torsional fatigue test of circumferentially notched specimens Thus, the fatigue testing method, specimen geometries, and stress intensity factors are all important factors in the study of multiaxial fatigue Many factors of multiaxiality make the testing method more complicated than mode I fatigue testing, and, accordingly, many researchers, working independently, have developed their own original methods This article first explains stress states of combined stress and stress fields near crack tips and then describes various multiaxial fatigue testing methods Multiaxial Fatigue Testing Yukitaka Murakami, Kyushu University, Japan Stress States Most engineering designs and/or failure analyses involve three-dimensional combinations of stress and strain (multiaxiality) in the vicinity of surfaces and notches, which can be limiting in fatigue applications This section provides a brief review of these stress states Additional information is provided in the article “Multiaxial Fatigue Strength” in Fatigue and Fracture, Volume 19 of Asm Handbook Two dimensional stress states without cracking are defined in Fig 1, where the basic relations are: (Eq 1) where σ1, σ2 are the principal stresses σx = σ0 and σy = -σ0 in Fig 1(d), and this is equivalent to the case shown in Fig 1(b) if τxy = σ0 Fig Two-dimensional stress states without cracking The yield criterion or yield stress, σY, is: Tresca: σ1 - σ2 = σY Von Mises: − σx σy + (Eq 2) + = (Eq 3) Stress states at the tip of a crack in combined mode I and mode II are defined by stress-intensity factors KI is the mode I stress intensity factor, and KII is the mode II stress intensity factor Radial stress (σr), normal stress (σθ, and shear stress (τrθ) in polar coordinate (r, θ) in the vicinity of the crack tip are given as follows (Fig 2): (Eq 4) (Eq 5) (Eq 6) Fig Stress state near a crack in a polar coordinate The direction (θ0) where σθ has the maximum value is given by: KI sinθ0 + KII (3 cosθ0 - 1) = (Eq 7) (Eq 8) This equation gives θ0 = ± 70.5° for pure mode II (KI = 0) The stress intensity factor that prescribes σθ is defined by: (Eq 9) The maximum value (Kθmax) of Kθ for pure mode II is derived, substituting θ0 = ±70.5° into Eq Thus: Kθmax = 1.155KII (Eq 10) Stress State at the Tip of a Crack in Mode III If there is a semielliptical surface crack and the crack is subjected to pure shear (Fig 3), the condition at the deepest point of crack front, A, is pure mode III, and the condition at surface corner points, B and C, is pure mode II, which means the branching angle at B and C by mode I crack growth is 70.5° under reversed torsion (Ref 1) (There have been some discussions among researchers about the irregular singularity close to the corner point where a crack meets the free surface It is known that a mode I stress component in tension has a singularity different from - If KIII ≠ at the surface point, it means that there exists a shear stressb τyz on the free surface Therefore, KIII must be zero at points B and C.) Fig 50 Release angle signature Torque-angle plots and release-angle plots can be used to directly estimate bolt tension, or preload, which is the ultimate goal of the fastener-tightening process The release angle of approximately 95° in the example shown in Fig 50 confirms the tightening angle measured on the torque-angle diagram for the hand-torque audit Clearly, the release-angle method of audit provides a direct measure of the capability of a given tool to develop tension in the tightened fastener Frictional Analysis Audits To provide an example of how audit techniques can be used to the effect of differences in frictional characteristics, the fastener type used in the previous examples (Fig 43, 44, 45, 46, 47, and 48) was stripped of all thread and underhead lubricants to create higher friction coefficients in the thread and underhead regions The torque-angle diagram for tightening to 81 N · m (60 lbf · ft), shown in Fig 51, indicates a tightening angle of only 25° projected from the elastic origin Compared with the lubricated fastener, where the tightening angle was 85°, the predicted preload of 9786 N (2200 lbf) was confirmed by the clampforce measurement The breakaway audit for the dry-tightened fastener (Fig 52), confirms that the installation torque was approximately 81 N · m (60 lbf · ft) and also reveals the expected very low angle of turn from the elastic origin Fig 51 Torque-angle diagram for a fastener stripped of all thread and underhead lubricants, tightened dry Fig 52 Breakaway torque-angle audit for the dry-tightened fastener Application of Torque-Angle Signature Analysis The torque-angle signature method of analysis applied to tightening and loosening curves is plain, simple, and straightforward It is a basic engineering analysis technique using fundamental stress, deflection, and material strength properties to model and measure the bolted-joint tightening process Torque angle signatures can be analyzed to determine installation torque, thread strip, underhead embedment, bolt yield, and most importantly, fastener tension While there are many factors that can alter the tightness of a given bolted joint, the torqueangle signature analysis method provides a practical method for direct verification of clamp force to assure a quality fastener assembly The technique can be applied to fasteners of all sizes and all grip lengths The release-angle signature, when compared to the installation-torque angle, can be used to evaluate the clamp load retained after a dynamic test Material creep and embedment phenomena, which lead to loss of preload, are readily analyzed and quantitatively evaluated through the use of release-angle analysis methods The results of release-angle audits, being directly related to the achieved tension, are significantly more meaningful than the torque magnitudes obtained from breakaway torque audits An improved version of the breakaway torque audit, which uses the torque-angle signature of the audit, can be used to directly estimate fastener tension This analysis process correlates precisely with the release-angle-signature method The only limitation is that the breakaway audit must be conducted in the elastic tightening region for the bolted joint where bolt yield or thread strip are not present Mechanical Testing of Threaded Fasteners and Bolted Joints Ralph S Shoberg, RS Technologies, Ltd Measurement Accuracy It is important that all standards for fasteners be reviewed in regard to the specification for measurement accuracy The specification of measurement accuracy for torque and clamp force should be in compliance with the ISO 25 methods used by the American Association for Laboratory Accreditation (A2LA) in the certification of laboratories Both torque and clamp-force measurement specifications should refer to accuracy at the point of measurement It is desirable to certify the accuracy and uncertainty of dynamic measurements at the real-time testing speeds (i.e., rpm) that are required by each test specification The following paragraphs of this section illustrate some examples of the accuracy specifications found in current standards that need to be reviewed and perhaps changed or revised It is hoped that future revised editions of existing standards and all new standards will give proper attention to understanding the science of measurements and the capability of current state-of-the-art measurement equipment From a practical point of view, it is also important to consider the true need of accuracy so that unnecessarily tight tolerances are not specified This is especially true where broader tolerances that are capable of properly meeting the desired end result can be demonstrated as sufficient to qualify the measurement with regard to its intended purpose Defining the Measurement All measurements are comparisons to standards, and practically no measurement is without a degree of uncertainty To be valid, a measurement must have an unbroken chain of traceability to well-defined, established primary standards Each step of the traceability chain introduces additional uncertainty, which has a cumulative effect on the accuracy and uncertainty of the final measurement It should be recognized that the accuracy of a measurement cannot be any better than the resolution of the measurement system For most practical purposes, the measurement system resolution is defined as the smallest increment displayed on the measurement dial, such as the scale on a torque wrench or the pressure read on a hydraulic load-indicating gage In digital electronic measurement systems, the smallest increment for the analog-to-digital (A/D) converter establishes the basic measurement resolution U.S Standards The American Society of Mechanical Engineers (ASME), the Society of Automotive Engineers (SAE), ASTM, the American National Standards Institute (ANSI), and the Industrial Fasteners Institute (IFI) all have established fastener standards in the United States However, to meet the needs of a global economy where products manufactured in the United States can be marketed worldwide, it is important to recognize that a large amount of effort may be necessary to align U.S testing standards with the international marketplace Significant progress has been made in the conversion of U.S manufacturing to the metric (Système International d'Unites) (SI) system Recognizing the United States' continued slow progress in this matter, the European commission has recently delayed the mandatory metric-only labeling requirement from 31 Dec 1999 to the year 2010 International Standards The European Committee for Standardization (CEN) centralizes the establishment of standards for the European Union (EU) member nations By coordinating U.S testing standards with those approved by CEN, it will then be possible to avoid or at least minimize any costly duplicate testing on products intended for sale to customers in EU member countries One area in which the United States needs to address serious deficiencies is in its method of definition of accuracy with regard to the test measurement procedures specified by U.S standards A number of U.S standards contain specifications that are obsolete, impractical, or not precise enough to be acceptable when subjected to the qualifying analysis applied to European testing methods For a number of years, many U.S companies have been striving to become world class manufacturers with certification to ISO 9002 Fastener testing that meets the requirements of ISO/International Electrotechnical Commission (IEC) 25, titled “General Requirements for the Technical Competence of Calibration and Testing Laboratories,” would qualify to meet the requirements of ISO 9002 Failure to specify accuracy and uncertainty of measurements in a manner consistent with ISO 25 requirements will very likely make testing unacceptable for product sales to the European community Definitions of Accuracy Accuracy can be defined only in terms of the entire measuring system and the environment, pertaining to both the instrumentation and the physical nature of the phenomena to be measured Measurement accuracy is dependent on the capability of the measuring system to dynamically track the signal (frequency response) and provide sufficient resolution to permit comparison to be made between readings Because a measurement reading is always an approximation of the true value, the uncertainty quantifies the limits of accuracy that can be expected In the example illustrated in Fig 53, the stated accuracy is 1% of the reading Fig 53 Range of accuracy Measurement Error There are two general components of measurement error, bias and random Bias is a constant value, such as a zero offset of a transducer or measuring amplifier, or a digital reference point The random component is a complex function of system noise or the least-bit resolution in digital systems (Fig 54) Fig 54 Nature-of-measurement error Frequency response effects where the phenomena measured may occur dynamically in a region beyond the flat response of the measurement system are not random errors Such errors are the result of poor measurement system performance resulting from improper understanding of the engineering physics of the process being measured Measurement Error Observations Measurement errors of a random nature typically have a normal distribution as shown in Fig 55 To evaluate a measurement system, it is necessary to input constant known values and determine the scatter in the measurements over a number of samples Statistics, such as sample standard deviation, can be used to quantify the probability function for the readings that define the uncertainty for the test conditions simulated Fig 55 Measurement error observations Allowable Uncertainty Versus Percent Full-Scale Reading A number of currently used standards specify the accuracy of readings in terms of “percent full scale.” As shown in Fig 56, the 2% full-scale accuracy specification results in a 4% error at a point of reading taken at 50% of the full scale of the measuring device range (in this case, a force transducer) Similarly, the error would extrapolate to 8% of the reading at 25% of measurement-transducer range In an attempt to limit the error in measurement, this standard stipulates that “transducers cannot be used below 50% of their full-scale range.” If this stipulation is followed closely for locknut testing, the tester would need to use one set of transducers to obtain prevailing on-torque and clamp-load values, then stop the test and use a higher capacity set of sensors to obtain the torque at clamp load This process would be followed by a return to the lower-range sensor to obtain the prevailing off-torque data to complete the test This situation is impractical at the least and impossible in most cases Fortunately, with properly calibrated and qualified modern testing systems and specifications based on accuracy at the point of measurement, the complicated process described is rendered totally unnecessary Fig 56 Allowable uncertainty It is far more practical and consistent with modern testing procedures according to ISO 25 that accuracy be specified at the point of measurement In this example, it would be proposed that the accuracy specification be changed to “within 2% of reading.” Taking a modern approach in revising this standard, the totally impractical and unnecessary limitation on the use of the transducer below 50% of its full-scale range would be eliminated There are a number of test procedures where accuracy specifications can be adjusted to different values at various points within the test, thus permitting a valid comparison of samples while using a single transducer over a broad measurement range One possible example would be testing of prevailing torque locknuts where the accuracy of torque measured for achieving clamp load could be 2% of the reading (50 N · m) and a fifth offtorque accuracy of 5% (1.1 N · m) In this case, testing could be done with a single transducer for all torque measurements Frequency Response Analysis All test specifications should call for accuracy of measurements at the speeds at which the test is run rather than contain statements that specify reading rates or dB points on filters, which can introduce errors as high as 30% A typical example is shown in Fig 57, where a 30° hard joint is used to evaluate fastening tools according to ISO 5393 With a 100 rpm tool and the filter specified in the standard, an error of about 30% would be expected because the filter will have reduced the measured signal at least 30% for equivalent waveforms above 500 Hz To properly capture the peak torque for a 1000 rpm tool, the system needs to be flat to 500 Hz, not down dB in response at 500 Hz Fig 57 Frequency response analysis In this case, the intent of the standard was to minimize errors due to electrical noise and perhaps due to a perception that higher-frequency components did not contribute to the tightening process The dB point specified at 500 Hz in the standard clearly places a limitation on the upper rpm for the testing of tools Measurement Resolution The measurement resolution and accuracy for a test should be verified at the specified testing speed The example shown in Fig 58 illustrates several important features verifying measurement capability In the following test, two torque transducers in series are driving an M12 prevailing nut at 100 rpm The initial position of the nut is such that the nylon patch is not engaged The transducers have full-scale ranges of 68 and N · m Fig 58 Measurement resolution using transducers with full-scale ranges of 68 and N · m The maximum applied torque is slightly less than N · m, or about 2%, of full scale on the larger capacity unit and 30% of full scale on the smaller-capacity unit It is clear from the recorded data plot that at 100 rpm, the transducers are reading precisely the same torque values This test verifies that the measurement system is capable of simultaneously capturing readings at the 100 rpm test speed It also illustrates the possibility of the use of the high-capacity torque sensor to make valid measurements in the range of 2% of full-scale capacity when used with this measuring system ISO 9000 Registration Versus Laboratory Accreditation There are significant differences between laboratory accreditation using ISO 25 and quality system registration The key difference can be summarized in that the essence of ISO 25 is to ensure the validity of test data, whereas technical credibility is not addressed in ISO 9000 ISO 9000 and QS 9000 registrations require that procedures are in place, well documented, and followed ISO 25 goes further in that it requires the science of the laboratory measurements be understood and that the tester is capable of demonstrating that valid measurements are the product of his or her testing procedures All testing procedures should be evaluated to account for certain basic considerations American testing standards for fasteners may not receive international recognition unless they can be shown to meet the basic requirements of ISO 25 The most important questions to answer when reviewing standards are the following: • • • • Will the procedure produce accurate results? How have the procedures been validated to ensure accuracy? Does the tester understand the science behind the test procedures? Are the limitations of the procedures known? The science of measurement engineering is a critical area of technology needed to properly qualify both the specifications and the measurement systems used to verify the quality of threaded fasteners The pure acquisition of data for its own sake is one of the greatest technical crimes of this era All measurements must reflect a technical knowledge of the testing process, as well as a full understanding of the capability and limitations of the measuring system Mechanical Testing of Threaded Fasteners and Bolted Joints Ralph S Shoberg, RS Technologies, Ltd Reference “Standard Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers, and Rivets (Metric),” F 606M, Annual Book of ASTM Standards, ASTM Mechanical Testing of Threaded Fasteners and Bolted Joints Ralph S Shoberg, RS Technologies, Ltd Selected References Fastener Design and Application Engineering • • • • • • • • • • • • • • • J.H Bickford, An Introduction to the Design and Behavior of Bolted Joints, 3rd ed., Marcel Dekker, Inc., New York, 1995 A Blake, Design of Mechanical Joints, Marcel Dekker, Inc., New York, 1985 A Blake, What Every Engineer Should Know about Threaded Fasteners: Materials and Design, Marcel Dekker, Inc., New York, 1986 V Moring Faires, Design of Machine Elements, 3rd ed., The Macmillan Co., 1955 P.G Forrest, Fatigue of Metals, Pergamon Press, Addison-Wesley Publishing Co., Inc., 1962 Fastener Standards, 5th ed., Industrial Fasteners Institute, Cleveland, OH, 1970 Metric Fastener Standards, 2nd ed., Industrial Fasteners Institute, Cleveland, OH, 1983 R.C Juvinall, Engineering Consideration of Stress, Strain, and Strength, McGraw-Hill Book Co., 1967 R.C Juvinall and K.M Marshek, Fundamentals of Machine Component Design, 3rd ed., John Wiley & Sons, 1999 G.L Kulak, J.W Fisher, and J.H.A Struik, Guide to Design Criteria for Bolted and Riveted Joints, 2nd ed., John Wiley & Sons, 1987 J.E Shigley, Mechanical Engineering Design, 5th ed., McGraw-Hill Book Co., 1989 R.S Shoberg, Torque-Angle Signature Analysis, Fastener Technology International, Vol 19 (No 1), 1996 R.S Shoberg, Engineering Fundamentals of Torque-Turn Tightening, Fastening Technology 1996, March 1996 (Cleveland, OH), Clemson University College of Engineering & Science, 1996 R.S Shoberg, Analyzing Torque-Angle Signatures For Reliable Bolted Joints, Assembly Technology Expo '96 (Chicago, IL), Sept 1996 R.S Shoberg, Coordination of Bolted Joint Design and Test Methods, Fastening Design & Application Engineering Conference (Novi, MI), Oct 1996 Fastener Testing • • • • • • • “Metric Screw Threads—M Profile,” B1.13M-1995, American Society of Mechanical Engineers “Standard Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers, and Rivets,” F 606, Annual Book of ASTM Standards, ASTM Determination of Coefficient of Friction of Bolt/Nut Assemblies under Specified Conditions, DIN 946, Deutsches Institut für Normung e.V., Berlin, Germany, 1990 “Steel Metric Threaded Fasteners Torque/Clamping Force Performance,” Ford WZ100, Ford Motor Co., 1993 “Torque Tension Test,” GM 9064P, General Motors Corp., 1998 IFI 101, 100/107, and 543 standards (drafts), Industrial Fastener Institute, Cleveland, OH “Torque-Tension Test Procedure for Steel Threaded Fasteners—Inch Series,” SAE J 174, Society of Automotive Engineers, 1996 • “Systematic Calculation of High Duty Bolted Joints, Part I,” VDI 2230, Translation of the German Edition 7/1986, C Junker and J Newnham, SPS Laboratories, Naas, Ireland, 1988 Measurement Accuracy • • • • • • • • • “Rotary Tools for Threaded Fasteners—Performance Test Method,” ISO 5393, 2nd ed., 1994-0501, Ref No ISO 5393:1994(E), International Organization for Standardization “Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results,” NIST Technical Note 1297, 1994 F.R Schraff, Defining Data Accuracy, Sensors, Vol 15 (No 6), 1998 R.S Shoberg, “Special Report: Measurement Resolution for Locknut Testing,” RS Technologies, Ltd., April 1994 P.K Stein, Measurement Engineering, Vol 1, Stein Engineering Services, Inc., 1962 J.L Taylor, Measurement Uncertainty: A Basis for Assessing and Improving the Quality of Measurement Processes, Sverdrup Technology, May 1998 P.S Unger, ISO/IEC Guide 25 Versus ISO 9000 for Laboratories, American Association for Laboratory Accreditation, 1995 “Test for Evaluating the Torque-Tension Relationship of Both Externally and Internally Metric Threaded Fasteners,” USCAR Fastener Strategic Standards Technical Committee, draft, May 1998 C.P Wright, Applied Measurement Engineering, Prentice Hall, 1995 Testing of Adhesive Joints K.L DeVries and Paul Borgmeier, University of Utah Introduction MOST ENGINEERING DESIGNS require the connecting or joining of component parts Means available to accomplish these goals can be broadly classified as mechanical connections, welding, and adhesives Mechanical connections such as bolts, pins, and rivets have the advantages of being easy to install, they can be inspected often, and they can be repaired or replaced On the other hand, such connections require making holes in the component members, which act as stress risers (with typical stress concentration factors of or greater) Structures that experience dynamic loading, such as airframes, often fail from fatigue cracks originating at their stress concentrations The heads of connections such as nuts and bolts can affect aesthetics, streamlining, and other parameters Another type of mechanical connection that is becoming increasingly popular is the “snap” connector These connections are wedge-like tabs and grooves that, when forced together, snap into place to hold containers or housings together Snap connectors greatly facilitate assembly but have limited strength, are easily damaged in assembly (or even more likely during disassembly), and may deteriorate with time and wear In welding, the joining segments of the component parts are locally melted, brought together, and fused (additional material may be supplied from a welding rod) Welded joints have the advantage of being relatively continuous, resulting in high strength They have the disadvantage of requiring a relatively expensive process The required heating can damage other parts of the structure or its contents Not all combinations of materials can be welded Most metals and plastics can be welded only to themselves or to a limited number of other similar materials Some materials not lend themselves to welding at all Adhesive joints involve joining parts by bonding component parts together with an adhesive In some plastic applications, one of the parts may act as the adhesive, and in some cases, a solvent may be used to dissolve/soften the materials, which are then bonded by diffusion of the polymer chains into the respective parts, forming essentially one material Examples of this type of bonding include the solvent bonding of polyvinyl chloride (PVC) pipe and acrylic materials Very dissimilar materials can, in principle, be joined by adhesives, for example, wood to plastic, metal to ceramics, and aluminum to steel Likewise, it appears that the “load” could be distributed over a larger area compared with joining materials with, for example, bolts This aspect can be even further enhanced by appropriate design as, for example, in the familiar finger-scarf joint used to increase the length of lumber This type of joint design accomplishes several beneficial goals: it increases the bonded area, changes what for a butt joint would be largely tensile stresses to more shear stress, and produces a smooth, attractive (almost invisible) joint Along with their advantages, adhesive joints pose their own design problems Prediction of load-carrying capacity is often neither straightforward nor reliable Adhesive joints also have problems with inspection because the bonded surfaces are usually not visible after assembly Stresses in an adhesive joint are generally not uniformly distributed on the bonded surfaces, and in fact, elastic stress analysis often exhibits singularities analogous to the stress risers associated with the other methods of joining already mentioned Because adhesive joints usually involve very dissimilar materials in the adherends and adhesives, cure stresses, thermostresses, deformation mismatches, and so on may cause problems Despite these drawbacks, the advantages and positive features of adhesive joints are so attractive that the use of adhesives has enjoyed phenomenal growth over the last 50 years One compilation of adhesives lists more than 5000 different commercial adhesives available to the U.S designer (Ref 1) Adhesive selection, testing, use in design, and application is a very complex subject covered in a variety of large reference books (for example, Ref 2, 3, 4, 5, and Adhesives and Sealants, Volume of the Engineered Materials Handbook published by ASM International) and literally thousands of journal articles; dedicated journals include the Journal of Adhesion and the International Journal of Adhesion Science and Technology Articles on adhesives and adhesion also appear in journals or publications of the following organizations: American Society for Testing and Materials, Society for Experimental Mechanics, American Society of Mechanical Engineers, Materials Research Society, and many others The Society for Adhesion, the Gordon Research Conferences, and the American Chemical Society hold annual or periodic conferences dedicated to the subject of adhesives Despite this extensive research and study, there is still much to be done, and adhesive joint design is still as much art as it is science Brazing and soldering are methods of joining that have similarities to adhesives and welding These methods might be viewed as falling between welding and adhesive bonding Brazing is closer to welding in that, while it uses a different material for “bonding” the “adherends,” its application typically involves extensive heating and perhaps some interpenetration of the brazing and host materials The material used for brazing is melted in a puddle, in which there may be some alloying with the host material(s) on fusion In soldering, on the other hand, usually only the solder material melts, and if it wets the host material(s), it attaches by adhering to the joined pieces It might be viewed as a metallic form of hot-melt adhesive Adhesive science and technology remains an important area of active research New adhesives continue to be developed, and work continues on the understanding of stresses; methods of analyzing stresses, strains, displacement, and load-carrying capacity; improved experimental techniques; and so on References cited in this section Adhesives, Edition 6, D.A.T.A Digest International Plastics Selector, 1991 R.L Patrick, Ed., Treatise on Adhesion and Adhesives, Vol 1–6, Marcel Dekker, 1966–1988 G.P Anderson, S.J Bennett, and K.L DeVries, Analysis and Testing of Adhesive Bonds, Academic Press, 1977 A.J Kinlock, Adhesion and Adhesives, Chapman and Hall, 1987 A Pizzi and K.L Mittal, Ed., Handbook of Adhesive Technology, Marcel Dekker, 1994 K.L Mittal, Adhesive Joints, Plenum Press, 1984 Testing of Adhesive Joints K.L DeVries and Paul Borgmeier, University of Utah Purpose of Testing Adhesive Joints In no area of materials is testing more important than it is for adhesives In the United States, the largest organization devoted to standardizing test procedures is the American Society for Testing and Materials (ASTM) ASTM and its counterparts in other countries work through the International Organization of Standardization (ISO) in an effort to coordinate testing on an international scale All these activities are very important and time consuming, entailing countless hours of volunteer efforts in the United States alone Other organizations, besides ASTM, have established testing procedures that relate to the testing of adhesive joints Such organizations include the military (MIL specs), other professional societies, such as the Society of Automotive Engineers (SAE), the American Society of Mechanical Engineers (ASME), and industrial organizations Many examples of the wide variety of standardized tests for adhesives can be found in the Annual Book of ASTM Standards, Volume 15.06, which is updated and republished annually (Ref 7) Except where specifically noted, the standards and practices listed in this treatise can be found in this volume There are also many other less formalized tests used by various organizations and a few specialized tests for adhesives found in other ASTM volumes ASTM practices, specifications, and test methods cover a wide variety of topics ranging from measurements of shelf life and pot life through resistance to mold or insect attack to determination of viscosity and various aspects of strength This article concentrates on tests and methods used in the measurement of adhesive joint strength A logical question in the discussion relative to the selection of the specific adhesive might be “How strong is it compared with the other available adhesives?” The answer to such a query is neither as simple nor as straight-forward as might be assumed The reasons for this complexity are addressed in this article Testing of adhesive joint strength might be conducted for a number of reasons, including the following: • • • Quality control: To ensure that changes such as adhesive age, mix method, and surface preparation that will affect bond quality have not been introduced Comparative analyses: To determine which of a series of adhesives, primers, surface preparations, cure methods, and so on is best suited for a given bond application Generation of engineering design data: To predict the load-carrying capability of a given bonded joint The geometry of the test specimen selected is very important even for quality control tests In fact, an adhesive that exhibits “high strength” in a tensile test may have poor peel strength and vice versa In fact, many variables may affect bond strength Each variable may affect the bond strength in a different manner, and the manner in which the bond strength is affected may be different for each test geometry The amount of adhesive used in a mixing batch and the delay time between mixing and casting will likely affect strength and may affect peel strength differently than it affects tensile strength As another example, bond thickness often affects lap shear strength in a different way than it affects tensile button strength If a series of lap shear joint test are completed for various adhesive thickness is optimum for a given joint unless the geometry, adhesive, and loading are nearly identical to those of the lap shear joints tested Thus, even for comparative analyses, a great deal of care must be used in applying laboratory test data to joints whose geometries differ in even subtle ways from that of the laboratory test specimens There are many standard adhesive tests available to the engineer for determining the strength of adhesive joints In the example just discussed, reference was made to tensile and peel specimens Indeed, most standard tests for adhesive joints fall within three general categories, namely, peel tests, lap shear tests, and tensile tests It is important to recognize, however, that the stresses at the tip of the debond region are not necessarily of the character implied by the name In fact, the stresses are usually of mixed character For example, as discussed in the section “Lap Shear Tests”, the stress in the critical region of the so-called lap shear specimens are apt to be more crack opening (tensile) in nature than shear One purpose for obtaining adhesive joint engineering data is in an effort to ensure that a bonded joint will withstand the loads for which it was designed This might be accomplished by “proof testing” every joint, that is, by loading every joint manufactured to its design load However, this approach has its problems First, such procedures are apt to be very costly Furthermore, the joint may withstand such loading but be damaged by the loading such that it subsequently fails when subjected to aging, creep, and fatigue loading or the environment In addition, when a joint is in place in a structure, it is often difficult to apply the actual or even a simulated load The load may be prohibitively large and/or in an awkward location, making testing difficult, very expensive, or even impossible There is also a danger that applying such proof loads might damage neighboring or adjoining parts of the structure For these reasons, an engineer is generally required to obtain data from laboratory size samples and use this data to infer the strength of a given “practical” joint For many of the standard adhesive joint tests, the results are reported as the load at failure divided by bonded area In one straightforward design approach, this stress is then compared with the average stress that exists in the joint being evaluated when its maximum load is applied However, if the joint geometry, loading time, and other conditions are not identical to the laboratory test conditions, the direct comparison can lead to unsafe joint designs This problem is discussed in subsequent sections (“Lap Shear Tests” and “Adhesive Fracture Mechanics Tests”) Reference cited in this section Adhesives, Annual Book of ASTM Standards, Vol 15.06, ASTM (updated annually) Testing of Adhesive Joints K.L DeVries and Paul Borgmeier, University of Utah Factors Influencing Mechanical Strength of Adhesive Joints Before proceeding with a description of specific tests, a few general comments might be helpful The strength of an adhesive joint should not be viewed as being essentially (or even largely) an inherent property of a given adhesive Typically, strength depends on many factors The nature of the adherends may have a dramatic effect on joint strength A given adhesive may not even “adhere” to some potential substrates Even where the inherent adhesion may be good, its quantitative value is typically highly dependent on proper cleaning, surface treatment, and details of curing The presence of coupling agents and surface roughness can affect strength Thickness of adhesive and adherends and other geometric factors also play strong roles in the strength of a joint In short, it is safe to say that the strength of an adhesive joint is a system property that depends on many factors beyond the chemical and physical nature of the adhesive per se Some ASTM and other standards and practices address these other factors For example, the following ASTM standards cover surface treatments for various materials: • • • • D 2093, “Standard Practice for Preparation of Surfaces of Plastics Prior to Adhesive Bonding” D 2651, “Standard Guide for Preparation of Metal Surfaces for Adhesive Bonding” D 2674, “Standard Test Method for Analysis of Sulfochromate Etch Solution Used in Surface Preparation of Aluminum” D 3933, “Standard Guide for Preparation of Aluminum Surfaces for Structural Adhesives Bonding (Phosphoric Acid Anodizing)” Coupling agents such as silanes, titanates, zirconates, and chrominates are sometimes applied to the surface before application of the adhesive The agents are also sometimes incorporated into the adhesive In at least some cases, they are thought to form covalent bonds between the adhesive and the adherend surfaces (Ref 8) As with most materials, aging can affect the properties of an adhesive and the strength of an adhesive joint It is often not convenient (or possible) to simply wait to see how time and exposure to the elements might cause deterioration Accelerated aging tests are an effort to infer from short-term tests, usually under very harsh conditions, how well materials might hold up under longer-term, more realistic service conditions A comparison of how different adhesive joints in wood exposed to boiling water for several hours or days might, for example, be used to estimate their relative weatherability (ASTM D 3434, D 5572) While the comparisons might not be perfect, they are likely better than nothing Several other standards that address accelerated testing of adhesives include ASTM D 1101, D 1183, D 2559, D 3632, and D 4502 Reference cited in this section E.P Plueddemann, Silane Coupling Agents, Plenum Press, 1982 Testing of Adhesive Joints K.L DeVries and Paul Borgmeier, University of Utah Qualitative Tests Most “adhesive strength tests” are quantitative in nature The test results are typically given as “average stress” at failure (tensile and lap joints) or force per unit width (peel tests) The preparation and testing of quantitative samples are expensive and time consuming It would often be advantageous to have a means of making a qualitative determination of adherence to ascertain if the cost of conducting the quantitative tests would likely be justified Such quick screening of candidate adhesive/adherend pairs might result in significant time and cost savings by eliminating unlikely candidates and assisting in selecting those worthy of further study The first effort along this line with which the authors are familiar is found in the patent literature (U.S Patent 4,025,159, Cellular Retroreflective Sheeting) In this patent, Dr J.M McGrath of 3M Corporation explores means of increasing the bond strength between the cover sheet and the base sheet (polymer binder with embedded reflective glass spheres) in retroreflective sheeting This sheeting is used in stop signs and other reflective signs To accomplish this goal, McGrath proposes curing (cross linking or chain extension) the sheeting after the thermoforming operation He points out that the proposed process does not work for all potential pairs of cover sheet and base materials As an aid in selecting materials for further study, he suggests casting a small amount of candidate base materials on potential cover sheets and curing them in place After curing, McGrath proposes a single-edged razor blade be used to lift, scrape, or otherwise separate the base material from the cover sheet The relative effort required to facilitate this separation is accessed and used as guide in the selection of candidates for further study More recently, Committee D-14 of ASTM formalized and adopted ASTM D 3808 “Standard Test Method for Qualitative Determination of Adhesion of Adhesives to Substrates by Spot Adhesion.” The stated purpose of the document is to provide “a simple qualitative procedure for quickly screening whether an adhesive will, under recommended application conditions, bond to a given substrate without actually making bonded assemblies.” In this test method, spots of adhesive are placed onto a substrate using the application procedure and curing conditions acceptable to the user and supplier of the adhesive To test adhesion, the document recommends the use of “a thin stainless steel spatula or similar probe” as a prying lever It states “If the results are acceptable, then standard quantitative adhesive test procedures can be used to obtain quantitative measurement of the adhesive's performance.” The authors believe such methods of preliminary screening are certainly worthy of consideration in many instances They can often save time and testing costs Testing of Adhesive Joints K.L DeVries and Paul Borgmeier, University of Utah Peel Tests Peel tests are easy to visualize albeit their analysis and the interpretation of peel test results is neither so easy nor straightforward When a strip of adhesive tape is placed partially on paper or on another substrate and then removed by pulling the free end (the portion of the tape that is not attached), in essence, a peel test is being conducted, as shown schematically in Fig Several quantitative observations might readily be made from such a test (it is important to note that the tape is a very flexible material) The force required to propagate the peel failure is a strong function of the angle at which the peel force is applied If the angle is small (i.e., close to the direction of the substrate surface), large peel surfaces are required As the angle increases to the point where the peel angle is 90°, perpendicular to the surface, the force is reduced significantly As the angle is further increased, it reaches a limit at 180° (ASTM D 903 describes a 180° peel test for a “flexible” peel specimen) The force per unit width required to facilitate peel is called the peel force or the stripping strength It can also be observed that the forces required to sustain peeling depend on the rate of peel For tapes bonded with pressuresensitive adhesives, very slow peel rates require relatively small peel forces, while at extremely high rates, the peel forces are much more substantial It can also be noted in this simple peel test that the force required to initiate the peel may differ from that required to sustain the peel once it has started It may also be observed that the force required to peel the tape from the substrate depends on the nature of the surface to which the tape is attached For example, it may adhere very tightly to a clean glass surface but hardly at all to moist or oily glass Fig Schematic of the peel test ASTM has formalized a variety of different peel tests in its Annual Book of ASTM Standards, Volume 15.06 Several of the standard peel test geometries (described in detail in ASTM D 903, D 3167, and D 1876) are illustrated in Fig (Ref 7) ASTM D 1781 describes the climbing drum peel test, which is used to determine the peel resistance of adhesive bonds between a relatively flexible adherend and a rigid adherend Fig Typical peel test specimens (a) Stripping strength specimen (ASTM D 903) (b) Roller drum peel test specimen (ASTM D 3167) (c) T-peel test specimen (ASTM D 1876) Source: Ref Several other peel tests are in common usage that have not been standardized by organizations such as ASTM For example, the authors have found a test they have adopted from researchers at 3M Corporation to be very useful for measuring peel strength This method makes use of a test jig incorporating a platform that uses rollers that allow the platform to move horizontally with extremely low friction It is used to test the adhesion between two thin flexible sheets For testing, one of the sheets is bonded to a thin aluminum sheet with a strong adhesive A region of debond along the adhesive between the thin flexible sheets is initiated with a razor blade or by other means, leaving a loose tab The aluminum sheet with the attached bilayer sheeting is inserted into slots on the jig platform, or otherwise held fixed to it The jig is then attached to the lower crosshead of a universal-testing machine such that the platform is horizontal The loose tab is then attached by a grip to the upper crosshead During testing, motion between the two crossheads then peels the upper sheet from the lower The peel angle is maintained at very nearly 90° as the platform moves horizontally (facilitated by the low friction rollers), keeping the peel region directly below the upper grip In some cases where the adherence between the sheetings is high and the sheeting cohesive strength is relatively low, it has been helpful to reinforce the upper sheet by applying strapping tape that is gripped and peeled along with the upper sheet It is important to note, however, that even though the peel failure may follow the same path, it would not be anticipated that the peel force would be the same as for the nonreinforced sheet Adhesive joint strength is a “system property,” and the addition of another layer can modify the energy absorption during the peel process, thereby altering the associated peel forces It is, therefore, important to compare the results from tests on reinforced materials with those of other materials that are similarly reinforced and vice versa ... 59 0 .081 565 82 352 51 600 87 0 .086 724 105 296 43 538 78 0.096 634 0.69 1.76 -0 .92 1.78 -0 .85 0.48 -0 .67 1.75 -0 .80 0.46 -0 .67 92 5454 5456 6061 0.092 0 .137 0.116 0. 108 0.103 -0 .11 0.079 -0 .75... 0.106 0 .081 -0 .11 0.124 -0 .11 1.8 -0 .69 0.85 -0 .86 0.21 0.22 -0 .52 -0 .59 1.33 - 5086 F temper 296 43 600 87 0.11 572 83 5182 O temper 296 43 469 68 0.075 841 122 5454 O temper 234 34 400 58 0 .084 ... Ferber, H.T Lin, and J Keiser, Oxidation Behavior of Non-Oxide Ceramics in a High-Pressure, High-Temperature Steam Environment, Mechanical, Thermal and Environmental Testing and Performance of