A Finite Element and Experimental Investigation of Specimen Geometry and Constraint Effects On Mixed Mode Ductile Fracture Initiation

Abstract

In practical situations, the loading experienced at the tip of a crack in a structural component can be complex, involving combinations of modes I, II and III. The main objective of this thesis is to examine the applicability of a fracture criterion for mixedmode (modes I and II) ductile crack initiation, based on the Jintegral and a neartip plastic modemixity parameter Mp. Further, the effect of cracktip constraint and specimen geometry on ductilefracture processes (such as void growth and coalescence), as well as on the relationship between the critical value of the Jintegral at fracture initiation (Jc) and Mp, is examined. In the first part of the thesis, the validity and range of dominance of the planestrain elasticplastic cracktip fields under mixedmode loading is investigated. To this end, smallstrain elasticplastic finiteelement analyses of mixedmode fracture are performed using a twoterm boundarylayer (K-T) formulation. Analyses were carried out for different values of the remote elastic mixity parameter Me and the Tstress. Several commonly used mixedmode fracture specimens, such as the compact tension shear (CTS) specimen, the fourpoint bend specimen, and the modified compacttension specimen, were considered. The results suggest that a twoparameter characterization of cracktip fields by J and Mp is adequate for commonly used mixedmode fracture specimens, except for mixities close to pure mode I and under largescale yielding. In the second part of the thesis, constraint effects on the ductilefracture process of void growth near the crack tip under mixedmode loading are investigated using the modified boundarylayer approach. The numerical simulations were carried out employing a largedeformation elasticplastic finiteelement procedure. The interaction between a notch and a prenucleated hole is studied by examining plasticstrain localization and hydrostaticstress levels in the ligament connecting them, as well as the growth of the hole. In the third part, this issue is examined in greater detail using a continuumdamage model that accounts for microvoid nucleation, growth and coalescence. Finiteelement computations were carried out up to the stage where the ligament between the notch tip and hole undergoes complete failure by microvoid coalescence. These analyses predict that the constraint level at the crack tip can strongly influence the functional relationship between the fracture toughness Jc and the neartip plastic modemixity Mp. In the final part of the thesis, mixedmode fracture experiments were conducted on a ductile 2014O aluminium alloy. The results were used to examine the micromechanics of mixedmode ductile fracture initiation and to establish the dependence of Jc on Mp. Furthermore, the effect of the cracktowidth ratio on these aspects was investigated, allowing comparison with numerical simulations performed earlier.