The Effects of Non-Proportional Biaxial Loading Paths on Ductile Fracture Initiation: A Void Growth Analysis

The effects of non-proportional biaxial loading paths on ductile fracture initiation toughness are studied in this paper. To this end, the growth of a cylindrical void (hole) located in front of a mode I plane strain crack has been studied using large deformation finite element analysis (FEA). A specific microstructural feature of a steel alloy was thoroughly studied by having a single void positioned at a fixed distance from the crack tip and void that was equal to 10 times the diameter of the void. In particular, the non-proportional biaxial loading path effects on the crack tip blunting, void-growth, ligament reduction and near-tip stress fields are investigated computationally. Under small-scale yielding conditions, one proportional loading and two non-proportional loading paths are applied to the modified boundary layer (MBL) model. It is observed that the non-proportional load paths have a marked effect on the void growth, crack tip blunting and their interaction. By applying the criteria for the coalescence of the crack tip and void, the ductile fracture initiation toughness is estimated. It is shown that the ductile fracture toughness is dependent on loading paths, and the T-stress ratios. Results from this study are of relevance to ductile fracture assessment of components or pressure vessels that operate under non-proportional biaxial loading conditions.

Load Normalization Method Accounting for Elastic and Elastic-Plastic Crack Growth

A single specimen technique to estimate crack length, standardized in ASTM E1820, is the so called load-normalization technique, also known as the Key-curve technique. The method is based on the separability between deformation and crack length. This means that if the load is normalized by a suitable function of crack length, the result will be a single crack length independent load-displacement curve. If this “Key”-curve is known, then based only on load and displacement information it is possible to estimate the corresponding crack length. The load normalizing method assumes a plastic response of the specimen during crack growth. If there is crack growth already in the elastic regime, non-linearity in the load-displacement record is not due to plasticity, but due to the crack growth. In this case the standard load-normalization method does not work since it assumes that the non-linearity is due to plasticity or crack tip blunting. Such materials require a modified approach. Here, a modified load normalization method accounting for possible elastic crack growth is presented. The method is shown to produce realistic crack growth estimates regardless of plasticity level of the specimen. The method applies an improved load normalization equation compared to the one presently used in ASTM E1820.

Sideways and stable crack propagation in a silicone elastomer

We have discovered a peculiar form of fracture that occurs in a highly stretchable silicone elastomer (Smooth-On Ecoflex 00–30). Under certain conditions, cracks propagate in a direction perpendicular to the initial precut and in the direction of the applied load. In other words, the crack deviates from the standard trajectory and instead propagates perpendicular to that trajectory. The crack arrests stably, and thus the material ahead of the crack front continues to sustain load, thereby enabling enormous stretchabilities. We call this phenomenon “sideways” and stable cracking. To explain this behavior, we first perform finite-element simulations that demonstrate a propensity for sideways cracking, even in an isotropic material. The simulations also highlight the importance of crack-tip blunting on the formation of sideways cracks. Next, we provide a hypothesis on the origin of sideways cracking that relates to microstructural anisotropy (in a nominally isotropic elastomer). To substantiate this hypothesis, we transversely prestretch samples to various extents before fracture testing, as to determine the influence of microstructural arrangement (chain alignment and strain-induced crystallization) on fracture energy. We also perform microstructural characterization that indicates that significant chain alignment and strain-induced crystallization indeed occur in this material upon stretching. We conclude by characterizing how a number of loading conditions, such as sample geometry and strain rate, affect this phenomenon. Overall, this paper provides fundamental mechanical insight into basic phenomena associated with fracture of elastomers.

Effects of Notch Acuity on Fracture Toughness Measurements

Effects of initial crack-tip acuity on toughness measurements has been evaluated through extensive small-scale toughness testing — single-edge notched tension and bend (SE(T) and SE(B)) — at room temperature using two X70 pipeline steels and girth welds. In order to investigate effects of different crack tip radii on toughness, some specimens were notched to the target initial crack size using electrical discharge machining (EDM), and no further fatigue precracking was made. On the other hand, other specimens were EDM-notched and then fatigue-precracked to the target crack size according to the current practice of ASTM E1820. The results show that effects of crack tip acuity on toughness measurement are significant especially at the crack-tip blunting stage. The EDM precrack increased toughness measurements by a factor of up to 1.6 and 2 for SE(B) and SE(T), respectively, at 0.2 mm of crack extension, compared to the fatigue precrack.

Path-dependent J-integral evaluations around an elliptical hole for large deformation theory

An exact expression is obtained for a path-dependent J-integral for finite strains of an elliptical hole subject to remote tensile tractions under the Tresca deformation theory for a thin plate composed of non-work hardening material. Possible applications include an analytical resistance curve for the initial stage of crack propagation due to crack tip blunting.