Aspects of Testpiece Design Responsible for Errors in Cyclic Plasticity Experiments

A uniaxial cyclic plasticity copper testpiece has been simulated, using the Continuum Damage Mechanics based finite element programme Damage XX, under conditions of cyclic plasticity at temperatures of 20°C and 500°C. The results obtained have been compared with the results of experiments carried out under the same conditions. It has been shown that inhomogeneous fields of strain and stress are generated in the gauge length region of the testpiece, which result from standard cyclic plasticity testpiece features, namely, the blend radius linking testpiece shank and gauge length, and the extensometer ridges. The influence of these features on the inhomogeneity is exacerbated by the short gauge length which is necessary in cyclic plasticity testpieces to prevent buckling. At the temperatures of 20°C and 500°C, cracks were predicted to occur, and found experimentally, at the extensometer ridges. Testpiece failure at 20°C occurred outside the gauge length; and at 500°C failure took place from cracks growing within the gauge length region. The formation and growth of the latter gauge length crack was accurately predicted. The results obtained clearly demonstrate the dependence of cyclic plasticity data on testpiece design and suggest that the range of scatter observed experimentally in cyclic plasticity testing for nominally identical testing conditions may result from variations in testpiece design. This is of some concern, and the establishment of a methodology which overcomes these problems will be the subject of a future paper.