Estimation of the Onset of Crack Growth in Ductile Materials

In this paper, the ductile fracture mechanism is discussed. The results of numerical and experimental analyses were used to estimate the onset of crack front growth. It was assumed that the ductile fracture in front of the crack starts at the location along the crack front where the accumulated effective plastic strain reaches a critical value. According to numerous research articles, the critical effective plastic strain depends on the stress triaxiality and the Lode angle. The experimental program was performed using five different specimen geometries, three different materials, and three different temperatures of +20 °C, −20 °C, and −50 °C. Using the experimental data and results of the finite element computations, the critical effective plastic strains were determined for each material and temperature. However, before the critical effective plastic strain was determined, a careful calibration of the stress–strain curves was performed after modification of the Bai–Wierzbicki procedure. It was found that critical effective plastic strain was a function of triaxiality factor and Lode parameter, as expected, and that the fracture locus was useful to estimate the onset of ductile crack growth

Stress distribution in front of the crack – analytical solutions vs. numerical. Can the differences be minimized?

It is shown that it is possible to obtain such parameters as α and Q, which, when used in the analytical formulae proposed by O’Dowd and Shih, can lead to stress distributions similar to those obtained numerically. The numerical solution obtained after calibration of the stress-strain uniaxial curve and assuming large strains is expected to be close to the “real” stress distribution. Thus, the analytical solution after correction is also close to the “real” stress distribution. These new values of α and Q can now be used in fracture criteria proposed within the scope of classical nonlinear fracture mechanics

Application of modified Ritchie-Knott-Rice criterion to cellular automata

In this paper, the cellular automata model is applied to analyse cleavage and ductile fracture in front of a crack in three-point-bend specimens made of Hardox-400 steel. The research programme was composed of experiments followed by fractographic and numerical analyses. On the basis of microscopic observations, the sizes of cells used in the automata were determined. The algorithm enabled mapping of the two-dimensional crack surface as well as a simulation of temperature-dependent failure mechanisms by defining transition rules based on the modified Ritchie-Knott-Rice cleavage fracture criterion. The critical stress values were estimated and verified by the cellular automata model

Estimation of the Onset of Crack Growth in Ductile Materials

In this paper, the ductile fracture mechanism is discussed. The results of numerical and experimental analyses were used to estimate the onset of crack front growth. It was assumed that the ductile fracture in front of the crack starts at the location along the crack front where the accumulated effective plastic strain reaches a critical value. According to numerous research articles, the critical effective plastic strain depends on the stress triaxiality and the Lode angle. The experimental program was performed using five different specimen geometries, three different materials, and three different temperatures of +20 °C, −20 °C, and −50 °C. Using the experimental data and results of the finite element computations, the critical effective plastic strains were determined for each material and temperature. However, before the critical effective plastic strain was determined, a careful calibration of the stress–strain curves was performed after modification of the Bai–Wierzbicki procedure. It was found that critical effective plastic strain was a function of triaxiality factor and Lode parameter, as expected, and that the fracture locus was useful to estimate the onset of ductile crack growth