Operational Manual - DCU

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SECTION 6 TYPICAL EXPERIMENTAL RESULTS The Bauschinger Effect When a metal bar is subjected to torsional overstrain and the load then removed, the load-free bar is full of residual stresses. These stresses are of two kinds: I. Body stresses which affect a relatively large volume of metal (i.e. macro stresses), and 2. Textural stresses which are really the residual stresses in between and within the crystals of the metal caused by deformation of each crystal (i.e. they are in the actual texture of the metal). Fortunately body stresses (which are beneficial) are more stable than textural stresses (which are harmful), the latter being removed by a low temperature heat treatment of 200°C. greater dIan dIe torque at G. This shift of dIe strength range in dIe direction of dIe plastic defonnation is sometimes called the 'Bauschinger Effect'. Upper and Lower Yield Strength Mild StEtel Normalised mild steel has the peculiar propert)' of having an upper and lower yield strength. That is, the initiation of yield occurs at a greater stress than the propagation of yield along the bar. This is demonstrated in Figure 6 where, after the ulitial yielding of the specimen, point A, the load immediately fallen to a lower value, point B. The strain reduces until the specimen is again in the elastic range, point C, but when reloaded it yields at the lower yield strength, lJOint D, showing that with mild steel the yield propagates at the lower yield strength stress value. Figure 5 Reverse torsion tests Relationship between Torque and Surlace Stress Reverse torsion tests are possible on the Torsion Testing Machine, allowing residual stress phenomena to be readily demonstrated, as shown in Figure 5. In the initial load cycle the specimen yields at A, is plastically defoTDled to B, then unloaded and plastically deformed in the reverse direction to point C. It is then loaded in the positive direction to point 0, unloaded and given a low temperature heat treatment, and then reloaded. It now yields at F rather than O. Thus the harmful effects of the textural stresses, which were removed by the heat treatment, were equal to OF. The vertical distance of point F represents the beneficial effect of the body stresses. If the material is now strained to point E and then strained in the reverse direction to point K (i.e. sight negative plastic strain). On reloading it arrives back at the strain represented by point E at a lower torque value G. Thus, during the strain cycle the strength range has moved in the negative direction that is the torque at F is During both the elastic and plastic range of torsional strain, the relationship between applied torque, 7: and the maximum shear stress, which occurs at the surface 'tmax, is proportional to 1/ d', the actual relationship depending upon the stress strain characteristics of the material being tested. In the elastic range the precise relationship is: 16T 't= -r Jtd Normally this relationship for stress is used throu~~out the test, but in the plastic region. t is a nominal !;tress and not the real stress. The real stress is less thw:l the nominal stress.
APPENDIX A TORSION TEST SPECIMEN ~ A standard range of metric specimens can be supplied For specimens of gauge 6", see catalogue order no. by TQ. Each specimen is stamped with a code reference TRIO6O to TRIO85. and has dimensions as shown in Figure 9. .I Figure A 1 Standard torsion specimen r
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Operational Manual - DCU

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