Strip yield model for multiple straight cracks with coalesced yield zones: A theoretical study

The paper presents a complicated case of coalescence of yield zones between two internal cracks out of four collinear straight cracks weakened an infinite isotropic plate. Two solutions are presented for the case of opening and closing of multiple cracks under general yielding conditions. Using these two solutions and the principle of superposition, we found the analytical expressions for load-bearing capacity of the plate using complex variable method. A numerical study has been carried out to investigate the behavior of yield zone length concerning remotely applied stresses at the boundary of the plate and the impact of two outer cracks on the propagation of inner cracks due to coalesced yield zones. Results obtained are reported graphically.

Strip-Yield Modeling of Load-Time-Temperature Effects on Crack Growth in Engine Materials

Plastic and creep deformations around a crack front and in the wake of a moving crack under cyclic loading are implemented into the life-prediction code, FASTRAN (a strip-yield model). Creep deformations are modeled by stress relaxation around the crack-tip location, since the crack-front material is under displacement control due to the surrounding elastic material. Sinusoidal and trapezoidal loading are considered. A modified linear superposition model was used to compute the cyclic- and time-dependent damage, which was based on the stress-intensity-factor concept for creep-brittle materials. Application of the modified strip-yield model was made on two sets of test data on Inconel-718 alloy. The environments were laboratory air or helium gas. From the literature, the “environment” had been shown to be a major contributor to damage magnitudes. Thus, the time-dependent crack-growth constants were selected to match the test data. In addition, the effects of a small overload on time-dependent damage, and the effects of stress relaxation and varying temperatures on crack-opening stresses and cyclic crack-tip-opening displacements, were studied.

Elastic-Plastic Analogy for Examination of Softening Response for Strip Yield Models

The manner in which the spread of inelastic deformation of a softening cohesive zone affects the load capacity is examined. The analysis makes use of an elastic-plastic analogy to the strip yield model applied to pure bending. The example of pure bending is one case where inelastic deformation contributes to enhancing the load capacity. The analytical solution to the elastic-plastic case is developed for zero hardening (baseline for strip yield case for which analytical solution is known) as well as for a range of linear softening rates. Evaluation of the results shows that the maximu m bending load capacity is always reached before the stress at the surface becomes zero.

Mode II Fracture of an Elastic-Plastic Sandwich Layer

The shear strength of a pre-cracked sandwich layer is predicted, assuming that the layer is linear elastic or elastic-plastic, with yielding characterized either by the J2 plasticity theory or by a strip-yield model. The substrates are elastic and of dissimilar modulus to that of the layer. Two geometries are analyzed: (i) a semi-infinite crack in a sandwich layer, subjected to a remote mode II K-field and (ii) a center-cracked sandwich plate of finite width under remote shear stress. For the semi-infinite crack, the near-tip stress field is determined as a function of elastic mismatch, and crack tip plasticity is either prevented (the elastic case) or duly accounted for (the elastic-plastic case). Analytical and numerical solutions are then obtained for the center-cracked sandwich plate of the finite width. First, a mode II K-calibration is obtained for a finite crack in the elastic sandwich layer. Second, the analysis is extended to account for crack tip plasticity via a mode II strip-yield model of finite strength and finite toughness. The analytical predictions are verified by finite element simulations, and a failure map is constructed in terms of specimen geometry and crack length.

Modeling of creep-fatigue interaction effects on crack growth at elevated temperatures

The well-known load frequency effect on creep-fatigue crack growth is explained by the interactions between fatigue and creep loading and is quantified using the concept of plasticity-induced crack closure. It is shown that the hold time during creep loading affects crack growth rates during subsequent fatigue cycles. Longer hold times lead to lower crack-tip opening stresses and faster crack growth rates during fatigue loading. To model the impact of hold time on crack opening stresses during fatigue loading, a strip-yield model was developed for creep-fatigue crack growth. The strip-yield model computes crack-tip opening stresses, which determine the effective stress intensity factor range and crack growth rate during the fatigue portion of each loading cycle. Maximum stress intensity factor is used to compute the crack growth rate during the creep portion of each cycle. The proposed strip-yield model is used to compute creep-fatigue crack growth rates for several structural materials, i.e., an Astroloy, aluminium alloy 2650 and 316 stainless steel. The model predictions of crack growth rates compare well with published experimental data for these alloys. This model achieves reliable predictions of crack growth rates and life prediction on components subjected to creep-fatigue loading at elevated temperatures by considering loading interaction effects.

Comparison of Strip Yield and Net Section Plasticity Models for a Bar in Bending With a Single Edge Crack

The strip yield model is widely used to describe crack tip plasticity in front of a crack. In the strip yield model the stress in the plastic zone is considered as known, and stress and deformation fields can be obtained from elastic solutions using the condition that the crack tip stress singularity vanishes. The strip yield model is generally regarded to be valid to describe small scale plasticity at a crack tip. The present paper examines the behavior of the strip yield model at the transition to large-scale plasticity and its relationship to net section plasticity descriptions. A bar in bending with a single edge crack is used as an illustrative example to derive solutions and compare with one-sided and two-sided plasticity solutions.

Strip Yield Model Applicability to Crack Growth Predictions for Various Types of Fatigue Loading

The capability of the strip yield (SY) model to predict crack growth in structural steel is investigated. To this end the SY model implementation developed by the present authors is applied to simulate crack growth observed in S355J2 steel specimens under constant amplitude and simple variable amplitude loading. A particular attention is given to the calibration of the model using the constraint factors and examining whether tuning the model, based on constant amplitude loading, allows the adequate predictions for variable amplitude loading.