Experimental and theoretical evaluation of a high-accuracy uni-axial creep testpiece with slit extensometer ridges

The paper evaluates a previous uni-axial creep testpiece design which has slitted extensometer ridges to relieve the constraint to axial deformation provided by conventional, or unslitted, ridges. The results of experiments are reported on slitted and unslitted ridged testpieces; and they are used together with theoretical studies to evaluate the effectiveness of the slitted ridged testpiece design for different specimen gauge lengths. Firstly, the test results are used to derive true constitutive equations (i.e. those equations for testpieces without ridges), these are then used to predict the response of the majority of the testpieces, using the creep Continuum Damage Mechanics (CDM) Finite Element solver DAMAGE XX. The creep rupture mechanisms, lifetimes and accuracy of the measured creep strains have been determined theoretically for testpieces with unslitted and slitted extensometer ridges and gauge lengths of 50, 30 and 10 mm. Close agreement has been obtained between the experimental and theoretical results for the creep failure mode and lifetimes. It has been shown how the accuracy of the creep behaviour measured using short gauge length testpieces (10 mm), of the type used in tension-compression testing, may be greatly improved by using slitted extensometer ridges.