Fracture initiation under impact

Abstract

In the first part of this paper the influence of temperature T and loading rate K I upon the fracture toughness K IC of structural steels is considered. A review of experimental results is presented over a wide range of loading rate and temperature in the form of the cross-sections of the constitutive surface K INC = f(K I,T). The hypothesis is proposed that both yield stress σ y in uniaxial tension and fracture toughness K IC are controlled by the same process of thermally activated movements of dislocations. Consequently, an introduction of the characteristic time t c leads to the master plot K IC ( σ y ) in double logarithmic coordinates which is temperature and rate-independent. Such an approach provides a simple method for estimating the value of K IC under a given set of imposed conditions (T,K̇ I) 1 provided it is known for another set of imposed conditions (T,K̇ I) 2. In the next part of this paper an attempt is presented to model the effect of T and K̇ I on fracture toughness K IC [15]. A model is discussed which combines correlations between critical cleavage stress σ F , yield stress σ y and the concept of thermally activated plastic flow from side and the local fracture criteria from the other [15]. It has been demonstrated that this approach can be useful in the proper predictions of changes of K IC as a function of loading rate and temperature. For some steels, however, a minimum of fracture toughness is observed and typically occurs for K I ≊ 1×10 4 MPa/pv/m/s at room temperature. The last part of this study deals with this important phenomenon [34]. It is concluded that the behavior of the constitutive surface K IC = f(K I,T) is highly nonlinear for steels.