The second ASTM/ESIS Symposium on Constraint Effects in Fracture; an overview

Abstract

The Second ASTM/ESIS Symposium on Constraint Effects in Fracture attracted a total of 24 contributions. These papers addressed various models to characterize, quantify and predict constraint effects, as well as experimental/validation studies and application studies. Available constraint models include the mechanics-based approaches of two-parameter fracture mechanics (2PFM) (i.e. J-T, J-Q, J-A 2 , J- α g ), statistical techniques based on the Weibull model, and micro-mechanical approaches applicable to fracture by both cleavage and ductile mechanisms. Collectively, these strategies extend significantly the range of loading conditions to which a fracture mechanics methodology can be applied to assess the integrity of an operating structure. At this stage, the following general statements can be made: 1. (1) In the lower transition regime where cleavage fracture occurs before or just shortly after the onset of ductile tearing, all of the 2PEM constraint models can be applied to parameterize the variation of critical fracture toughness with constraint. Of the various models available, the J-Q approach of O'Dowd and Shih applies rigorously to the highest deformation levels and to the broadest range of materials. Experimental evidence is available which shows the validity of this approach. All of the 2PEM approaches, however, suffer from the disadvantage that they complicate considerably the task of characterizing material toughness because toughness becomes a function of constraint at every temperature rather than a single value. 2. (2) In the lower transition regime it is also possible to predict without resort to empirical argument this variation of toughness with constraint using the results of standard fracture toughness tests coupled with the micromechanics approach of Dodds and Anderson. At the second Symposium, the applicability of this model was extended into the upper transition regime where significant stable tearing may precede the onset of cleavage. Again, experimental evidence is available which shows the validity of this approach. Certain issues remain with respect to the proper treatment of 3-D effects; these are currently under investigation. 3. (3) A “master curve” approach to the analysis of fracture toughness data in the transition regime has been proposed in a draft ASTM standard on this topic. Combination of this approach with a statistical correction for thickness effects based on the Weibull model appears to provide a powerful tool for the predicting toughness of geometrically similar specimens from one another (e.g. thick C(T)s predicted from thin C(T)s) across a wide range of thicknesses. 4. (4) 2PFM models can be applied on the upper shelf to parameterize constraint effects on R-curve behavior. However, in this application the theoretical basis of these approaches is lost as a reference infinite body field solution that is self-similar to the field solution for growing cracks in finite bodies is not available. As a consequence, it can be expected that “size effects” would likely reveal themselves in such an application. On the upper shelf the way forward appears to be through application of some form of local approach wherein sub-continuum material variables are incorporated into the models to provide a capability to predict accurately structural behavior from test results.