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7. Apparatus
7.1 The readings from the individual strain sensors shall be repeatable at the rated force within 10 % of the permitted bending strain, during five successive force applications made after the first force application without reducing the applied force to less than 5 % of the rated force.
7.2 When multiple strain sensors are used as in 6.1.1 and 6.1.2, specimen size limitations may dictate the use of electrical resistance strain gages rather than extensometers employing mechanical linkages. Strain sensors, such as mechanical, optical, or electrical extensometers, as well as wire resistance or foil strain gages, can provide useful displacement data. The sensitivity of displacement measurement required by an applicable standard or specification depends on the amount of bending permitted.
7.3 For verification by Method 2, a single extensometer of the nonaveraging type may be used by rotating it to various positions around the perimeter during successive force applications and repeating the measurements as described in 10.5. In general, repeated force applications are not permitted in Type T tests (see 5.3) because they may affect the subsequently measured results.
NOTE 6—Repositioning the extensometer around the specimen does not usually give highly precise and reproducible results, but nevertheless is a technique which is useful for detecting large amounts of bending.
7.4 For determining maximum bending strain during Type T Tests (see 5.3), the use of three or four separate extensometers or an extensometer with multiple strain sensors which reads strain at three or more positions about the perimeter is recommended.
7.5 In most cases, the strain sensors will reference displacements between points on the specimen surfaces. However, it is also possible to reference displacements of surfaces attached to the specimen. Such an arrangement might consist of two plates firmly fixed to each end of the gage length of a specimen which is free of initial bending. Displacement measurements are made between corresponding pairs of points on these plates.
Each pair of points is in a plane containing the specimen axis and is equally distant from this axis. For specimens of circular cross section, it is recommended that three or four pairs of points be used. A suitable extensometer may then be used to measure the displacement of the pairs of points as force is applied to the specimen. The strain at the specimen surface in the plane containing the pairs of points may, for small displacements, be taken equal to the strain computed at the measurement points multiplied by the ratio of the distance between the specimen applied force axis and the specimen surface to the distance from this axis to the measurement points.
An apparatus that measures displacements at points external to the specimen surfaces should be qualified by showing that the bending strains calculated from these measurements agree with those calculated from strains measured directly on the specimen surface using the same application of force.
NOTE 7—When multiple extensometers are used, the strain may be determined by arithmetically averaging outputs. Electrical outputs are thought to be more accurate and reproducible than mechanical outputs.
8. Test Specimen
8.1 This practice refers to cylindrical specimens, thickrectangular specimens, and thin rectangular specimens.
8.2 This practice is valid for metallic and nonmetallic test specimens.
8.3 Quality of machining of test specimens is critical, for example, straightness, concentricity, flatness, and surface finish.
NOTE 8—Geometry and dimensions of test specimens taken from different product forms are described in the Test Specimen section of Test Methods E 8.
9. Calibration and Standardization
9.1 When three or more strain measurements are made at one or more longitudinal positions, the bending strains are determined from ratios of strain measurements.
Consequently, the absolute accuracies of the extensometers are not significant. The sensitivities and reproducibilities of the instruments used are significant. All sensors should be calibrated by the same means (see Method E 83) and correction factors should be applied, if necessary, to bring their readings into agreement.
10. Procedure
10.1 Temperature variations during the verification test should be within the limits specified in the methods or practices which require the alignment verification.
10.2 The zero-force reference value of the strain sensors should be measured at a force no greater than approximately 1 % of that force at which the alignment verification is to be made.
10.3 To verify the alignment of the testing apparatus, repeated force applications are necessary. The amount of bending introduced by the load-train depends on the relative position of the various components which transmit force to the specimen and also on the care with which these parts are machined and assembled. Aspects of the test specimen, such as straightness and concentricity, are critical.
10.4 Repeated loadings should include assembly and disassembly of the components of the load-train, including the test specimen. Rotation in 90° increments (0°, 90°, 180°, 270°, repeat 0°) are recommended for a systematic study of the effects of rotational position of components of the load-train.
Calculate the bending value for each combination of the components of the load-train. The maximum value should not exceed the specified values in the standard practices, testing methods, or material specifications.
10.5 When using a single, nonaveraging extensometer to evaluate apparatus (Type A), move the extensometer from one side of the specimen to the opposite at the rated force, then rotate 90° at the lower force limit (see 10.2), and repeat the process. Calculate a bending value from the four readings, that is, the readings from two applications of force and two removals of force. Remove the specimen from the grips, and repeat the loading force application sequence for systematic rotations of the components of the load-train as described in
10.3. The largest bending strain resulting from this procedure should not exceed the values permitted by the standard practices, testing methods or material specifications.
10.6 Location of Strain Sensors:
10.6.1 Cylindrical Specimens—To measure strain, place the strain sensors at equally spaced positions around the circumference of specimens of circular cross section.
10.6.2 Thick Rectangular Specimens—If the specimen is of sufficient thickness, to measure strain, place the four strain sensors at the center of each side of the specimens of the rectangular cross section (see Positions 1 through 4 in Fig. 3a).
10.6.3 Thin Rectangular Specimens—If the specimen thickness is not sufficient, then place the four strain sensors on opposite sides of the wide faces, near the edges, and equidistant from them (see Positions 5 through 8 in Fig. 3b).
10.6.4 If eight strain gages are used for determination of maximum bending strain, place the gages opposing each other across the specimen longitudinal axis, with two pairs near the upper end of the reduced portion and two pairs near the lower end. The errors in the bending strains are less than the difference between the highest and the lowest value of the four values of axial strain.
NOTE 9—For sheet specimens where the foregoing placement of strain sensors cannot be made, axial strain can be determined using two sets of back-to-back sensors which are equidistant from the longitudinal midpoint of the specimen. (For example, see Fig. 3b.)
NOTE 10—Mechanical hysteresis in the strain sensor may influence the strain measurement.
12. Report
12.1 Report the following information:
12.1.1 Values of bending strain or percent bending, and method used, including the location of the strain sensors. (See Section 6.)
12.1.2 Test temperature.
12.1.3 Rated maximum force used in verification.
12.1.4 Description of specimen (material and dimensions).
12.1.5 Description of strain measuring equipment, including precision and sensitivity and method of fastening strain sensors to specimens.
12.1.6 Description of load-train, including method of gripping dimensions of pull bars, types of couplings and joints, and length of load train.
12.1.7 Sample calculation.
12.1.8 Estimate of precision and bias, if strains were measured at four locations. (See Section 13.)
13. Precision and Bias
13.1 The precision of the measurement of specimen alignment under applied tensile forces varies with such test conditions as temperature, stress, configuration of load train, and material. At present, the available data are not of a type that permits meaningful analysis of the precision of the measurement.
It is the intention of Committee E-28 to obtain the necessary data from an interlaboratory test program based on this practice.
13.2 The bias of the measurement of specimen alignment under tensile loading varies with such test conditions as temperature, stress, quality of machining of test specimens, and load-train components and material. Since the bending strains used to measure alignment are determined from ratios of strain measurements from three or more strain sensors, the absolute accuracy of the strain sensor calibration is not important (see 9.1). No direct measure of bias is available, because the identical test conditions cannot be duplicated during a calibration run and an actual test.
APPENDIX
(Nonmandatory Information)
X1. SOURCES AND EFFECTS OF MISALIGNMENT UNDER TENSILE LOADING
X1.1 Source of Misalignment
X1.1.1 The usual procedure in a uniaxial tension test is to apply a tensile force to a specimen through grips attached to a load-train and then correlate the strain response of the specimen, as measured with an appropriate extensometer, with the applied stress. In the case of ideal alignment, the top and bottom grip centerlines are precisely in line with one another and with the centerlines of other components of the loading train. Moreover, they are precisely in line with the specimen centerline. Finally, the specimen is symmetric about its centerline.
Departures from the ideal situation are caused by poor alignment of the top and bottom grip centerline, poor conformance of specimen centerline to top and bottom grip centerlines, and asymmetric machining of the test specimen itself. A combination of these three sources of misalignment always operates in any test under tensile forces. The occurrence of misalignment is recognized in the ASTM standards referenced in Section 2.
X1.1.2 The characteristic elastic strain gradients resulting from misalignment are such that the extreme elastic strains occur at the surface. These gradients can significantly influence the results of a tension test, especially results at strains less than 0.002 where significant plastic strain and accompanying strain hardening have not yet contributed to evening out the gradients. Therefore, it is important to recognize the effects of misalignment on the stresses and strains measured in studies of the fracture strength of materials in a brittle state, stress-rupture life, creep, notched-tensile specimens, fatigue, plastic microstrain, alloy strengthening, and surface-sensitive strength.
X1.1.3 The objective of any effort to improve alignment is to bring the centerlines of all load-train components into precise alignment. Logically, the first piece of hardware on which to focus attention is the testing machine itself. Testing machines as-received from manufacturers may have deviations between top and bottom grip centerline positions of 0.001 to 0.125 in. (0.03 to 3.18 mm). Moreover, further misalignment may develop as applied forces cause machine frame deflection or as nonaxial crosshead separation occurs. In the worst case, deviations in this range have been reported to lead to eccentricities resulting in a 50 to 100 % difference between extreme surface bending strains and average strain.
X1.1.4 After the testing machine comes a consideration of the tolerances specified for the machining of load-train components and test specimens. In ordinary machine shop practice, tolerances usually range from 60.002 to 60.010 in. (60.05 to 60.25 mm). These tolerances may cause poor alignment when the components of a loading train are assembled, for example, in the worst case, these tolerances have been reported to lead to eccentricities resulting in a 50 to 100 % difference between extreme surface bending strains and average strain.
X1.1.5 There are two further considerations for the development of good alignment. One deals with the type of couplings in the load-train, such as threaded-versusnonthreaded joints, spherical seats and universal joints with low friction, cross flexures, fluid couplings, and other couplings which tend not to transmit a bending stress. The other relates to specimen design, such as length and length-todiameter ratio. The approach to promoting good alignment has
been discussed in several papers (1-11).3
X1.2 Effects of Misalignment on Test Results
X1.2.1 Bending stresses associated with misalignment between the load-train and the specimen axes have been shown to affect the results of tension tests (12-16). In routine tension tests of most engineering materials, bending stresses will be insignificant if sufficient plastic flow occurs during the test to eliminate the bending stresses. However, when testing under conditions where plastic flow is limited by inherent brittleness of the test specimen material, or by need for measurements near the elastic limit, or when plasticity is confined to a small volume (specimens with stress concentration such as notches), small misalignment may give rise to variable bending stresses which have noticeable effects on the test results. For example, Morrison (8) noted that the yield stress of carefully machined mild steel specimens tested in torsion exhibited a 61 % variation from the mean, whereas the yield stresses of the same steel specimens tested in tension exhibited a 65 % variation.
Morrison concluded that the larger variation in tensile yield stresses resulted from misalignment rather than from microstructural variations, and he stated that “with the ordinary standard of accuracy in cutting the screwed ends of the specimens, the slackness in the thread was quite sufficient to allow the specimen to take up and retain under load an eccentricity in the shackles which would account for the variation in results.”
X1.2.2 Schmieder et al (9, 10) found that bending ranged from 5 to 27 % and depended on specimen coupling to the load-train, prior force application, and type of testing machine.
These authors concluded that “most of the nonaxiality of loading appears to be due to loose threads or machining imperfections in the couplings.” Jones and Brown (11) demonstrated that, at fixed stress, simply rotating a load-train component through 360° about the longitudinal axis changed the percentage of bending by a factor of more than 5, from 8 to 43 %. In an experiment with other equipment, Jones and Brown (11) found that bending could be varied between about 2 and 14 %, depending on the relative rotational positions of the specimen and of the top and bottom grips. Hence, a fourth item which influences bending might be added to the three cited by Schmieder et al, namely, the rotational registry of the components of the load-train.
X1.2.3 Robinson (12) reported a 40 to 60 % decrease in the uniaxial tension–tension fatigue life of steel bolts when the bending microstrain increased by a factor of two. Jones et al (13) demonstrated a continuous decrease (ranging from 80 to 90 %) of notch-rupture life of a chromium-molybdenumvanadium steel, at 60 ksi 1000°F (414 MPa 538°C), as eccentricity increased from a negligible value to 0.1 in. (2.5 mm). Christ (14) showed that results of plastic microstrain studies and other pre-yield studies are ambiguous unless effects of misalignment on the average microstrain are recognized.
Attention was directed to this point by McVetty (15) as early as
1928, but it has been frequently overlooked since then.
REFERENCES
(1) Christ, B.W., and Swanson, S. R.,“ Alignment Problems in the Tensile Test,” Journal of Testing and Evaluation, Vol 4, No. 6, November 1976, pp. 405–417.
(2) Wu, H. C., and Rummler, D. R., “Analysis of Misalignment in the
Tension Test,” Transactions: Series H, Journal of Engineering Materials and Technology, Vol 101, n.1, ASME January 1979, pp, 68-74.
(3) Holmes, A. M. C., “Continuous Servo-Controlled Alignment of Specimens in Materials Testing,” Experimental Mechanics, Vol 15,
No. 9, September 1975, pp. 358–364.
(4) Webb, J. N., “A System For the Axial Loading in Creep Specimens,”
Structures Dept., Royal Aircraft Establishment, Farnborough, England.
Her Majesty’s Stationary Office, London, 1977.
(5) Jones, M. H., Bubsey, R. T., Succop, G., and Brown, W. F., Jr., “Axial
Alignment Fixture For Tension Tests of Threaded Specimens,” Journal
of Testing and Evaluation, Vol 2, September 1974, p. 378.
(6) Jones, M. H., and Brown, W. F., Jr., “Note on Performance of Tapered
Grip Tensile Loading Devices,” Journal of Testing and Evaluation, Vol
3, No. 3, 1975, pp. 179–181.
(7) Penny, R. K., Ellison, E. G., andWebster, G. A., “Specimen Alignment and Strain Measurement in Axial Creep Tests,” Materials Research and Standards, Vol 6, No. 2, 1966, pp. 76–84.
(8) Morrison, J. L. M., “The Yield of Mild Steel with Particular Reference to the Effect of Size of Specimen,” Journal and Proceedings, The Institution of Mechanical Engineers, London, Vol. 140, No. 3, 1940, p.193–223.
(9) Schmieder, A. K., “Measuring the Apparatus Contribution to Bending in Tension Specimens,” Elevated Temperature Testing Problem Areas, ASTM STP 488, ASTM, 1971, p. 15.
(10) Schmieder, A. K., and Henry, A. T.,“ Axiality Measurements on Fifty Creep Machines,” Elevated Temperature Testing Problem Areas,
ASTM STP 488, ASTM, 1971, p. 43.
(11) Jones, M. H., and Brown,W. F., “An Axial Loading Creep Machine,” ASTM Bulletin, ASTM No. 211, January 1956, p. 53.
(12) Robinson, D., “Misalignment Detector for Axial Loading Fatigue
Machines,”Technical Note 480, National Bureau of Standards, Washington,DC, 1972.
(13) Jones, M. H., Shannon, J. L., Jr., and Brown, W. D., Jr., “Influence of Notch Preparation and Eccentricity of Loading on Notch Rupture Life,” Proceedings, ASTM, Vol 57, 1957, p. 833.
(14) Christ, B. W., “Effects of Misalignment on the Pre-Macro Yield
Region of the Uniaxial Stress–Strain Curve,” Metallurgical Transactions, AIME, Vol 4, No. 8, 1973, pp. 1961–1965.
(15) McVetty, P. G., “Testing of Materials at Elevated Temperatures,” Proceedings, ASTM, Vol 28, 1928, p. 60.
(16) “Private Communication to B. W. Christ from H. S. Starrett,”
Southern Research Institute, Birmingham, AL, April 1979.
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7. Apparatus
7.1 The readings from the individual strain sensors shall be repeatable at the rated force within 10 % of the permitted bending strain, during five successive force applications made after the first force application without reducing the applied force to less than 5 % of the rated force.
7.2 When multiple strain sensors are used as in 6.1.1 and 6.1.2, specimen size limitations may dictate the use of electrical resistance strain gages rather than extensometers employing mechanical linkages. Strain sensors, such as mechanical, optical, or electrical extensometers, as well as wire resistance or foil strain gages, can provide useful displacement data. The sensitivity of displacement measurement required by an applicable standard or specification depends on the amount of bending permitted.
7.3 For verification by Method 2, a single extensometer of the nonaveraging type may be used by rotating it to various positions around the perimeter during successive force applications and repeating the measurements as described in 10.5. In general, repeated force applications are not permitted in Type T tests (see 5.3) because they may affect the subsequently measured results.
NOTE 6—Repositioning the extensometer around the specimen does not usually give highly precise and reproducible results, but nevertheless is a technique which is useful for detecting large amounts of bending.
7.4 For determining maximum bending strain during Type T Tests (see 5.3), the use of three or four separate extensometers or an extensometer with multiple strain sensors which reads strain at three or more positions about the perimeter is recommended.
7.5 In most cases, the strain sensors will reference displacements between points on the specimen surfaces. However, it is also possible to reference displacements of surfaces attached to the specimen. Such an arrangement might consist of two plates firmly fixed to each end of the gage length of a specimen which is free of initial bending. Displacement measurements are made between corresponding pairs of points on these plates.
Each pair of points is in a plane containing the specimen axis and is equally distant from this axis. For specimens of circular cross section, it is recommended that three or four pairs of points be used. A suitable extensometer may then be used to measure the displacement of the pairs of points as force is applied to the specimen. The strain at the specimen surface in the plane containing the pairs of points may, for small displacements, be taken equal to the strain computed at the measurement points multiplied by the ratio of the distance between the specimen applied force axis and the specimen surface to the distance from this axis to the measurement points.
An apparatus that measures displacements at points external to the specimen surfaces should be qualified by showing that the bending strains calculated from these measurements agree with those calculated from strains measured directly on the specimen surface using the same application of force.
NOTE 7—When multiple extensometers are used, the strain may be determined by arithmetically averaging outputs. Electrical outputs are thought to be more accurate and reproducible than mechanical outputs.
8. Test Specimen
8.1 This practice refers to cylindrical specimens, thickrectangular specimens, and thin rectangular specimens.
8.2 This practice is valid for metallic and nonmetallic test specimens.
8.3 Quality of machining of test specimens is critical, for example, straightness, concentricity, flatness, and surface finish.
NOTE 8—Geometry and dimensions of test specimens taken from different product forms are described in the Test Specimen section of Test Methods E 8.
9. Calibration and Standardization
9.1 When three or more strain measurements are made at one or more longitudinal positions, the bending strains are determined from ratios of strain measurements.
Consequently, the absolute accuracies of the extensometers are not significant. The sensitivities and reproducibilities of the instruments used are significant. All sensors should be calibrated by the same means (see Method E 83) and correction factors should be applied, if necessary, to bring their readings into agreement.
10. Procedure
10.1 Temperature variations during the verification test should be within the limits specified in the methods or practices which require the alignment verification.
10.2 The zero-force reference value of the strain sensors should be measured at a force no greater than approximately 1 % of that force at which the alignment verification is to be made.
10.3 To verify the alignment of the testing apparatus, repeated force applications are necessary. The amount of bending introduced by the load-train depends on the relative position of the various components which transmit force to the specimen and also on the care with which these parts are machined and assembled. Aspects of the test specimen, such as straightness and concentricity, are critical.
10.4 Repeated loadings should include assembly and disassembly of the components of the load-train, including the test specimen. Rotation in 90° increments (0°, 90°, 180°, 270°, repeat 0°) are recommended for a systematic study of the effects of rotational position of components of the load-train.
Calculate the bending value for each combination of the components of the load-train. The maximum value should not exceed the specified values in the standard practices, testing methods, or material specifications.
10.5 When using a single, nonaveraging extensometer to evaluate apparatus (Type A), move the extensometer from one side of the specimen to the opposite at the rated force, then rotate 90° at the lower force limit (see 10.2), and repeat the process. Calculate a bending value from the four readings, that is, the readings from two applications of force and two removals of force. Remove the specimen from the grips, and repeat the loading force application sequence for systematic rotations of the components of the load-train as described in
10.3. The largest bending strain resulting from this procedure should not exceed the values permitted by the standard practices, testing methods or material specifications.
10.6 Location of Strain Sensors:
10.6.1 Cylindrical Specimens—To measure strain, place the strain sensors at equally spaced positions around the circumference of specimens of circular cross section.
10.6.2 Thick Rectangular Specimens—If the specimen is of sufficient thickness, to measure strain, place the four strain sensors at the center of each side of the specimens of the rectangular cross section (see Positions 1 through 4 in Fig. 3a).
10.6.3 Thin Rectangular Specimens—If the specimen thickness is not sufficient, then place the four strain sensors on opposite sides of the wide faces, near the edges, and equidistant from them (see Positions 5 through 8 in Fig. 3b).
10.6.4 If eight strain gages are used for determination of maximum bending strain, place the gages opposing each other across the specimen longitudinal axis, with two pairs near the upper end of the reduced portion and two pairs near the lower end. The errors in the bending strains are less than the difference between the highest and the lowest value of the four values of axial strain.
NOTE 9—For sheet specimens where the foregoing placement of strain sensors cannot be made, axial strain can be determined using two sets of back-to-back sensors which are equidistant from the longitudinal midpoint of the specimen. (For example, see Fig. 3b.)
NOTE 10—Mechanical hysteresis in the strain sensor may influence the strain measurement.
12. Report
12.1 Report the following information:
12.1.1 Values of bending strain or percent bending, and method used, including the location of the strain sensors. (See Section 6.)
12.1.2 Test temperature.
12.1.3 Rated maximum force used in verification.
12.1.4 Description of specimen (material and dimensions).
12.1.5 Description of strain measuring equipment, including precision and sensitivity and method of fastening strain sensors to specimens.
12.1.6 Description of load-train, including method of gripping dimensions of pull bars, types of couplings and joints, and length of load train.
12.1.7 Sample calculation.
12.1.8 Estimate of precision and bias, if strains were measured at four locations. (See Section 13.)
13. Precision and Bias
13.1 The precision of the measurement of specimen alignment under applied tensile forces varies with such test conditions as temperature, stress, configuration of load train, and material. At present, the available data are not of a type that permits meaningful analysis of the precision of the measurement.
It is the intention of Committee E-28 to obtain the necessary data from an interlaboratory test program based on this practice.
13.2 The bias of the measurement of specimen alignment under tensile loading varies with such test conditions as temperature, stress, quality of machining of test specimens, and load-train components and material. Since the bending strains used to measure alignment are determined from ratios of strain measurements from three or more strain sensors, the absolute accuracy of the strain sensor calibration is not important (see 9.1). No direct measure of bias is available, because the identical test conditions cannot be duplicated during a calibration run and an actual test.
APPENDIX
(Nonmandatory Information)
X1. SOURCES AND EFFECTS OF MISALIGNMENT UNDER TENSILE LOADING
X1.1 Source of Misalignment
X1.1.1 The usual procedure in a uniaxial tension test is to apply a tensile force to a specimen through grips attached to a load-train and then correlate the strain response of the specimen, as measured with an appropriate extensometer, with the applied stress. In the case of ideal alignment, the top and bottom grip centerlines are precisely in line with one another and with the centerlines of other components of the loading train. Moreover, they are precisely in line with the specimen centerline. Finally, the specimen is symmetric about its centerline.
Departures from the ideal situation are caused by poor alignment of the top and bottom grip centerline, poor conformance of specimen centerline to top and bottom grip centerlines, and asymmetric machining of the test specimen itself. A combination of these three sources of misalignment always operates in any test under tensile forces. The occurrence of misalignment is recognized in the ASTM standards referenced in Section 2.
X1.1.2 The characteristic elastic strain gradients resulting from misalignment are such that the extreme elastic strains occur at the surface. These gradients can significantly influence the results of a tension test, especially results at strains less than 0.002 where significant plastic strain and accompanying strain hardening have not yet contributed to evening out the gradients. Therefore, it is important to recognize the effects of misalignment on the stresses and strains measured in studies of the fracture strength of materials in a brittle state, stress-rupture life, creep, notched-tensile specimens, fatigue, plastic microstrain, alloy strengthening, and surface-sensitive strength.
X1.1.3 The objective of any effort to improve alignment is to bring the centerlines of all load-train components into precise alignment. Logically, the first piece of hardware on which to focus attention is the testing machine itself. Testing machines as-received from manufacturers may have deviations between top and bottom grip centerline positions of 0.001 to 0.125 in. (0.03 to 3.18 mm). Moreover, further misalignment may develop as applied forces cause machine frame deflection or as nonaxial crosshead separation occurs. In the worst case, deviations in this range have been reported to lead to eccentricities resulting in a 50 to 100 % difference between extreme surface bending strains and average strain.
X1.1.4 After the testing machine comes a consideration of the tolerances specified for the machining of load-train components and test specimens. In ordinary machine shop practice, tolerances usually range from 60.002 to 60.010 in. (60.05 to 60.25 mm). These tolerances may cause poor alignment when the components of a loading train are assembled, for example, in the worst case, these tolerances have been reported to lead to eccentricities resulting in a 50 to 100 % difference between extreme surface bending strains and average strain.
X1.1.5 There are two further considerations for the development of good alignment. One deals with the type of couplings in the load-train, such as threaded-versusnonthreaded joints, spherical seats and universal joints with low friction, cross flexures, fluid couplings, and other couplings which tend not to transmit a bending stress. The other relates to specimen design, such as length and length-todiameter ratio. The approach to promoting good alignment has
been discussed in several papers (1-11).3
X1.2 Effects of Misalignment on Test Results
X1.2.1 Bending stresses associated with misalignment between the load-train and the specimen axes have been shown to affect the results of tension tests (12-16). In routine tension tests of most engineering materials, bending stresses will be insignificant if sufficient plastic flow occurs during the test to eliminate the bending stresses. However, when testing under conditions where plastic flow is limited by inherent brittleness of the test specimen material, or by need for measurements near the elastic limit, or when plasticity is confined to a small volume (specimens with stress concentration such as notches), small misalignment may give rise to variable bending stresses which have noticeable effects on the test results. For example, Morrison (8) noted that the yield stress of carefully machined mild steel specimens tested in torsion exhibited a 61 % variation from the mean, whereas the yield stresses of the same steel specimens tested in tension exhibited a 65 % variation.
Morrison concluded that the larger variation in tensile yield stresses resulted from misalignment rather than from microstructural variations, and he stated that “with the ordinary standard of accuracy in cutting the screwed ends of the specimens, the slackness in the thread was quite sufficient to allow the specimen to take up and retain under load an eccentricity in the shackles which would account for the variation in results.”
X1.2.2 Schmieder et al (9, 10) found that bending ranged from 5 to 27 % and depended on specimen coupling to the load-train, prior force application, and type of testing machine.
These authors concluded that “most of the nonaxiality of loading appears to be due to loose threads or machining imperfections in the couplings.” Jones and Brown (11) demonstrated that, at fixed stress, simply rotating a load-train component through 360° about the longitudinal axis changed the percentage of bending by a factor of more than 5, from 8 to 43 %. In an experiment with other equipment, Jones and Brown (11) found that bending could be varied between about 2 and 14 %, depending on the relative rotational positions of the specimen and of the top and bottom grips. Hence, a fourth item which influences bending might be added to the three cited by Schmieder et al, namely, the rotational registry of the components of the load-train.
X1.2.3 Robinson (12) reported a 40 to 60 % decrease in the uniaxial tension–tension fatigue life of steel bolts when the bending microstrain increased by a factor of two. Jones et al (13) demonstrated a continuous decrease (ranging from 80 to 90 %) of notch-rupture life of a chromium-molybdenumvanadium steel, at 60 ksi 1000°F (414 MPa 538°C), as eccentricity increased from a negligible value to 0.1 in. (2.5 mm). Christ (14) showed that results of plastic microstrain studies and other pre-yield studies are ambiguous unless effects of misalignment on the average microstrain are recognized.
Attention was directed to this point by McVetty (15) as early as
1928, but it has been frequently overlooked since then.
REFERENCES
(1) Christ, B.W., and Swanson, S. R.,“ Alignment Problems in the Tensile Test,” Journal of Testing and Evaluation, Vol 4, No. 6, November 1976, pp. 405–417.
(2) Wu, H. C., and Rummler, D. R., “Analysis of Misalignment in the
Tension Test,” Transactions: Series H, Journal of Engineering Materials and Technology, Vol 101, n.1, ASME January 1979, pp, 68-74.
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