Microstructure-sensitive computational modeling of fatigue crack formation

A damage model for the simulation of delamination propagation under high-cycle fatigue loading is proposed. The basis for the formulation is a cohesive law that links fracture and damage mechanics to establish the evolution of the damage variable in terms of the crack growth rate dA/dN. The damage state is obtained as a function of the loading conditions as well as the experimentally-determined coefficients of the Paris law crack propagation rates for the material. It is shown that by using the constitutive fatigue damage model in a structural analysis, experimental results can be reproduced without the need of additional model-specific curve-fitting parameters.

Simulation of delamination in composites under high-cycle fatigue

The effects of grain size and crack length on the threshold condition of fatigue growth of small cracks are analysed both theoretically and experimentally. The theoretical model is based on the assumption that the threshold condition is determined by whether the crack-tip slip band blocked by the grain boundary propagates into an adjacent grain or not. By comparing the model analysis with the experiments of mild steel, it was found that the threshold stress estimated from the fatigue limit of the smooth specimen gave the effective components of the applied stress. When the threshold stress normalized by the fatigue limit of the smooth specimen was correlated to the crack length normalized by a certain crack length, the theoretical relation was found to agree well with the experimental data. From a further analysis of the published data of various materials, the theoretical normalized relation was found to give a lower bound of the threshold values of the stress and the stress intensity factor. A deviation from the theoretical relation seen in the cases of hard metals was explained through an extension of the model by regarding that hard metals contained initial, inherent flaws even in the smooth specimen.

Fatigue growth threshold of small cracks

The dislocation dipole accumulation model for fatigue crack initiation previously proposed by the authors is extended to an analysis of the fatigue strength reduction due to inclusions in high strength alloys. The initiation of a fatigue crack is determined by an energy criterion under the assumption that the crack initiation takes place when the self strain energy of dislocation dipoles accumulated at the damaged part in the material reaches a critical value. Explicit formulae for the crack initiation criterion in several cases are derived as functions of the applied stress, the inclusion size, the slip band shape, and the shear moduli of the inclusion and matrix. The following three types of fatigue crack initiation at inclusions are considered: the slip-band crack emanating from a debonded inclusion, the inclusion cracking due to impinging of slip bands, and the slip-band crack emanating from an uncracked inclusion. The first mechanism was reported to be operative in high strength steels, while the last two mechanisms were reported in high strength aluminum alloys. The present theoretical results are in good agreement with the experimental data published for each case of fatigue crack initiation at inclusions.

A theory of fatigue crack initiation at inclusions

A status report on delamination resistance testing of polymer-matrix composites

Effect of stress ratio on near-threshold propagation of delimination fatigue cracks in unidirectional CFRP

The propagation and non-propagation of small fatigue cracks at the notch root were studied in the first part of the present paper. A resistance-curve method was proposed for predicting the growth threshold of mechanically short cracks on the basis of the experimental results on the development of crack closure. The effects of material, notch geometry, and mean stress on the growth threshold of short cracks and the notch fatigue strength were successfully predicted by the resistance-curve method. In the second part, the propagation behaviour of small fatigue cracks in smooth specimens was examined. A micromechanical model of small crack growth was combined with statistical simulation of crack growth for life prediction. The fatigue failure diagram was constructed to judge the threshold condition for small crack growth.

/pdf/propagation-and-non-propagation-of-small-fatigue-cracks-21y7phoqfc.pdf

Propagation and non-propagation of small fatigue cracks

The present paper reviews some recent developments of the measurements of the macrostress and microstress by diffraction using X-rays, synchrotron and neutrons especially in Japan. These three methods are based on the same principle of the diffraction of crystals, and have different advantages. The conventional X-rays detect the stress very near the surface, while the neutron diffraction takes the stress in the interior of the materials. High-energy X-rays from synchrotron sources have the penetration depth in between and are suitable for the measurement of subsurface stresses. After describing the developments of the fundamentals of the methods, the paper covers the recent applications of the diffraction methods to the residual stress analysis in textured thin films, the nondestructive determination of the subsurface distribution of residual stress in shot-peened materials, local stress measurements near the crack tip, the stress measurements of single crystals, macrostress and microstress measurements in composites, and the determination of the internal distribution of the residual stress in welded joints.

https://www.jstage.jst.go.jp/article/jsmea/47/3/47_3_252/_pdf

Keisuke Tanaka

Papers

Effect of stress ratio on near-threshold propagation of delimination fatigue cracks in unidirectional CFRP

Propagation and non-propagation of small fatigue cracks

Diffraction Measurements of Residual Macrostress and Microstress Using X-Rays, Synchrotron and Neutrons

Local Residual Stress near Fatigue Crack Tip

X-Ray Measurement of Residual Stress Near Fatigue Fracture Surfaces of High Strength Steel