Effects of misalignment on the pre-macroyield region of the uniaxial stress-strain curve
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THE
use of high resolution strain gages has been fruitful for precision measurement of the transition from elastic to microplastic strain, l-s as well as for measurement of slow strain rate phenomena, s An informative review of m i c r o s t r a i n techniques was p r e sented by Brown 4a in a recent volume 4b entirely devoted to the subject of microplasticity. The influence of misaUgnment, bending s t r e s s e s and nonuniform local strains on the premacroyield region of the uniaxial s t r e s s - s t r a i n curve was dealt with only briefly by Brown. A recent review s demonstrates that small misalignments inherent in gripping devices and other loading-system components can significantly influence the results of a tension test, Particularly at small strains. The purpose of this paper is to discuss the influence of misalignment on p r e - m a c r o y i e l d strains. Measurements made in this laboratory with a three-point (120 deg apart) parallel-plate capacitance strain gage fastened to cylindrical tensile samples demonstrated that: 1) r e g a r d l e s s of the nonuniformity of local elastic strains, the average applied s t r e s s divided by the average elastic strain always gives a unique number, Young's modulus, and 2) the average microplastic strain measured in a m i c r o s t r a i n study is not uniquely related to the average applied s t r e s s , but rather, depends upon test system misaltgnment. The validity of these two observations is evident only if strain is measured at three or four noncoltnear points on the sample perimeter (thus facilitating analysis of bending p a r a m eters)S. '1~ Although the arguments used to explain these two observations are presented in t e r m s of uniaxial tension tests, the same arguments apply to uniaxial compression tests.
I. EXPERIMENTAL ASPECTS A. Special Testing Equipment 1) CAPACITANCE STRAIN GAGE AND LINEARIZED CIRCUIT The strain gage used throughout this study, pictured earlier by Ceil and Feinberg, 8 is composed of t h r e e parallel-plate capacitors (hereafter r e f e r r e d to as probes) a r r a y e d at 120 deg intervals on a 1.5 in. diam gage circle concentric with the cylindrical tensile sample. The set of high voltage plates and the set of ground potential plates were embedded in two separate (split) holders, which clamped to the samples. Each of the three high-voltage plates was fastened to the stem of a m i c r o m e t e r which facilitated calibration. Guard rings on the ground-potential plates and appropriate shielding on all other electrical components minimized pick up of s t r a y signals. Several features of this strain gage, especially its bridge circuit which was designed by R. Cutkosky, v are noteworthy. For example, the strain gage was used in an inductively-coupled capacitance bridge c i r cuit, Fig. l(a), which gave a linear output with displacement, e r a t h e r than the nonlinear output c h a r a c teristic of the usual parallel-plate capacitance bridge, Fig. l(b). The linear output was achieved by balancing the bridge circuit with a
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