TWI Industrial Member Report Summary 153/1981
By A A Willoughby
The unloading compliance technique for deriving R-curves has been applied mainly to compact tension specimens, where the load line displacement is readily obtained. The difficulties experienced in extending the technique to bend specimens are outlined. Various solutions, including the use of a double clip gauge arrangement, are investigated. Finite element calculations show that the best estimate of crack length is likely to be obtained from the conventional clip gauge/load compliance, using a/W ~> 0.5. The results are compared with those from blunt-notched calibration testpieces and with fatigue cracked specimens, both with and without side-groves, of two pressure vessel steels. It is found that the initial crack length may be estimated accurately, but that the estimate becomes less precise as tearing occurs. Consequently, predictions of crack extension are rather inaccurate. However, there is considerable scope for improvement in the measurement technique employed, and for the use of correction factors.
The use of two clip gauges allows good estimates of load point displacement, COD and J to be made, particularly when plastic displacements become significant. It is suggested that the technique could prove valuable in routine measurements of COD and J on bend specimens.