A NEW DESIGN OF UNIAXIAL CREEP TESTPIECE WITH SLIT EXTENSOMETER RIDGES FOR IMPROVED ACCURACY OF STRAIN-MEASUREMENT

A new design of the uniaxial creep testpiece with slit extensometer ridges is proposed, to improve the accuracy of strain measurement during creep testing, Numerical techniques were developed for finite element analysis of the creep-rupture behaviour and of the accuracy of the measured strain of the new testpiece. Physically based constitutive equations with the stress sensitivity described by both hyperbolic sine and power-law relationships were used in the numerical computation. The technique models the formation and evolution of damaged material and determines the accuracy of strain measurement during testing. It is shown that the distribution of axial stress in the gauge area for the new testpiece is more uniform than that for the standard uniaxial creep testpiece presently used with unslitted extensometer ridges. Variations in the accuracy of the measured strain with life fraction, gauge length and stress level for the testpiece with slit extensometer ridges are presented and compared with those computed by Lin et al. [2] for the standard testpiece. The maximum error of the measured strain obtained from the new testpiece is only half that found for the traditional one. It is concluded that reliable test results can be obtained with shorter testpiece gauge lengths when slit extensometer ridges are used; this may be an important advancement for low-cycle fatigue testing where the use of short gauge lengths is desirable to avoid buckling.