An investigation of fatigue crack propagation under mode I and mixed mode I/II loadings.

摘要

An investigation on fatigue crack propagation under mode I and mixed mode (I and II) loadings has been performed. Fatigue crack growth data in the case of plane strain mode I have been obtained by performing experiments on compact tension specimens of 4340 steel for increasing {dollar}Delta{dollar}K{dollar}sb{lcub}rm I{rcub}{dollar}, decreasing {dollar}Delta{dollar}K{dollar}sb{lcub}rm I{rcub}{dollar} and constant {dollar}Delta{dollar}K{dollar}sb{lcub}rm I{rcub}{dollar} loading conditions.; Under mixed mode plane stress condition, the fatigue crack growth trajectory has been determined by performing experiments on the center cracked thin disk specimen of aluminum 2024. With two mixed mode loading ratios, the subsequent propagation of the main crack has been performed by producing a series of kinks and forks. For practical applications such as the case of a flaw or a crack in aircraft or ship structure in which the loading axis (with respect to the crack) frequently changes, a zig-zag or complicated crack trajectory is very important in both experimental and theoretical studies. In order to apply any specific crack growth criterion for the analysis of the experimental results, a detailed analysis for the stresses near the crack front and the extent of yielding is necessary.; A full field solution, based on small deformation, three-dimensional elastic-plastic finite element analysis of the centrally cracked thin disk under mode I loading has been performed. The solution for the stresses under small-scale yielding and locally fully plastic state has been compared with the HRR plane stress solution. At the outside of the 3D zone, within a distance of {dollar}rsigmasb{lcub}it 0/{rcub}{dollar}/{dollar}J{dollar} = 18, HRR dominance is maintained in the presence of a significant amount of compressive stress along the crack flanks. Ahead of this region, the HRR field overestimate the stresses. These results demonstrate a completely reversed state of stress in the near crack front compared to that in the plane strain case. The combined effect of geometry and finite thickness of the specimen on elastic-plastic crack tip stress field has been explored.; Under mixed mode loading, the dominance of the elastic K-field outside the elastic-plastic region has been demonstrated at the load level at which the crack extension experiment has been performed in the thin disk.; The complex crack trajectory has been analyzed on the basis of the strain energy density criterion for the prediction of propagated crack path under subsequent changes in orientation of the crack to the loading axis or equivalently saying the changes in loading direction with respect to the crack. (Abstract shortened by UMI.)