Mechanical behavior and damage kinetics in nodular cast iron: part I. Damage mechanisms

Numerous authors consider that in nodular cast iron, the interface between the graphitic nodule and the alpha -matrix is so weak that nucleation appea rs suddenly at the beginning of plastic strain.([1,2]) Nevertheless, the pr esent experimental study indicates that the nucleation of voids is a contin uous process as a function of the imposed mechanical parameters (Sigma (m), epsilon (peq)). This result is explained in terms of interaction effects b etween closely spaced particles, A critical parameter for interaction is de termined. Growth kinetics are also investigated and comparison with the Gur son-Tvergaard law allows identification of the q(1) and q(2) parameters of the potential. A critical void volume fraction f(c) is finally defined and is shown to depend on the stress triaxiality level. At last, the role of th e various microstructural heterogeneity levels on the damage kinetics is em phasized. Then, it is clearly demonstrated that all these heterogeneity lev els must be considered to estimate a critical interfacial stress.