The design of many critical structural components needs a careful description of the propagation characteristics of fatigue cracks. The propagation of long and short cracks has been extensively studied, and the role of crack closure has been widely confirmed.

Crack front curvature is commonly obtained in most fatigue experiments. Nevertheless, classical linear elastic fracture mechanics generally considers bi-dimensional crack geometries corresponding to straight crack fronts, and only few studies consider 3D effects [1].

The objective of this study is then to provide a 3D numerical predictive tool of the evolution of the crack front curvature during the propagation of a fatigue crack taking into account plasticity-induced crack closure. Tests carried out in CT-50 specimen of 304L austenitic stainless steel have given the stabilized crack front shape for a long crack with different load ratios R and different stress intensity factor (SIF) ranges DK. In order to avoid any influence of the loading history, constant ∆K have been applied. A 3D numerical tool, using the ABAQUS® code and the programming language PYTHON has been developed. A frictionless contact placed on the crack plane allows determining the opening load. Two parallel calculations are done (elastic and plastic), in order to measure, on each node of the current crack front, the value of the local effective SIF range. The latter has been assumed to be the driving force for the whole propagation. Several methods, in order to calculate the local stress intensity factor along the crack front, have been implemented, where a previous study [2] has demonstrated that the Shih and Asaro method used by ABAQUS® cannot consider accurately free edge effects in plane stress conditions. It appears that the use of the stress field in the crack vicinity leads to more accurate results of the SIF along the crack front. This technique also has an additional advantage of considering no hypothesis for the evolution of the stress state throughout the specimen thickness.

The following crack front is then formed using the resulting advances at each point while considering a semi elliptical crack front shape. A re-meshing procedure is then developed, and numerous steps are done, reconstructing each time the plastic wake.

Two different stabilization conditions are considered, allowing a comparison of the experimental and predicted stabilized crack front shapes. This shows globally a good agreement and a remarkable improvement compared to the previous predictions.

References:

[1] Vor K, Gardin C, Sarrazin-Baudoux C, Petit J. Wake length and loading history effects on crack closure of through-thickness long and short cracks in 304L: Part II- 3D numerical simulation. Eng. Fract. Mech. 2013; 99:306-323.

[2] C. Gardin, S. Fiordalisi, C. Sarrazin-Baudoux, M. Gueguen, and J. Petit, “Numerical prediction of crack front shape during fatigue propagation considering plasticity-induced crack closure,” Int. J. Fatigue, vol. 88, pp. 68–77, Jul. 2016.