A method for the metallographical measurement of the CTOD at cracking initiation and the role of reverse plasticity on unloading

A method is proposed for accurately estimating the Crack Tip Opening Displa cement (CTOD) at physical cracking initiation in ductile materials. The met hod requires a few precracked specimens to be loaded in such a way as to ex hibit various crack extensions. The total opening of the blunted precrack, delta(1), the opening of the tearing crack, delta(2), and the ductile teari ng extension are measured for each specimens. The unloaded CTOD at cracking initiation is equal to the difference of delta(1) - delta(2) extrapolated for zero crack extension. Results obtained in two different applications, o ne on plane strain specimens of copper and the other on TRIP assisted multi phase steel plates, illustrate the potential of the method. A model based o n fracture mechanics arguments is proposed in order to account for the reve rse plasticity during unloading. This model is compared to results obtained using more exact large strain finite element computations. 2000 Elsevier S cience Ltd. All rights reserved.