A Method to Identify the Crack Closure and Opening in Cyclic Tearing Tests on Fracture Mechanics Specimens

To measure crack propagation in compact tension specimen, many methods can be used. The electric drop potential measurement is one of them and allows the detection of crack initiation. In our case, CT specimens, which have been taken from a carbon steel pipe (Tu42C) used in the secondary circuit of French PWR, are employed for cyclic tearing test. The detection of crack closure and crack opening should provide information for energetic analysis. However, the electric signal is unusable due to the cyclic loading. Indeed, because of the clearance between the pin and the specimen, each direction loading change causes a discontinuity in the signal. The roughness of the lips surface or the crack closure during compression loading returns also an unusual signal. Moreover, local measurement is required and there is high strain level around the crack tips, so strain gages are not suitable. Thus, displacement field are measured with digital image correlation and a specific image acquisition is employed. These methods allow a direct measurement of strain fields on the surface of the specimen. Thereby an interpretation of the previous electric signal and the crack opening and closure detection is realizable. Then, F.E. simulation, with non-linear kinematic hardening and node release method, are performed. These simulations allow the check of crack opening and closure detection through the specimen thickness.