Finite Element Validation and Calibration of a Modified SENT Specimen to Study Fracture

Abstract

In materials testing, it is frequently an interest to measure the crack arrest toughness of a material which experiences a ductile to brittle toughness transition, characterized by the variation of property with the test temperature. Crack arrest toughness is most frequently quantified in terms of the value of K1 occurring at crack arrest (K1a). To determine higher K1a values SENT specimens are frequently employed. The data from the review shows that extremely long SENT specimens are needed to achieve maximum applied K1 values that significantly exceed the applied K1 at crack initiation, implying that the K1a measurement capacity of these specimens is limited by specimen length. Thus the variation of K1 values with a/W ratios for the specimen capable of achieving higher applied K1 values at fixed grip boundary conditions than can the conventional SENT specimen was developed. Using Finite element analysis a Single Edge Notched Tension SENT specimen, modified by introduction of a semi-circular and semi-elliptical stress concentrating cutouts in front of the crack, was developed and characterized. Path independence of the contour integral values is sufficient to indicate mesh convergence for stress, strain, or displacements. The modified specimens was found to produce a drastically increase in K1 values, than is possible with a conventional SENT specimen of the same size, assuming an initial crack depth of 0.1 W and fixed grip boundary conditions. There is very small variation in the K1 values of the two modified specimens up to the crack lengths calculated. This new specimen (modified SENT specimen) could be used to investigate upper transition crack arrest phenomena with smaller specimen and testing machine capacities than have been possible previously.