Short fatigue crack growth behavior including mixed-mode loading and notch effects.

摘要

Short fatigue crack growth behavior under mixed-mode loading and from notches has become more important with the developed application of fracture mechanics in many engineering fields. Mixed-mode loading represents the true loading condition in many practical situations and notches cannot be avoided in many structures and machines. In addition, most of the fatigue life of many components is often spent in the short crack growth stage. The study of short crack growth behavior under mixed-mode loading and from notches has, therefore, much practical significance.; Previous studies mainly focused on some specific aspects of short cracks, such as short crack closure effects or short crack growth in some special material. This work investigated the overall short crack growth behavior under mixed-mode loading and from notches for a commonly used steel and with a broad scope. The effects of load mixity, notch constraint, crack closure and load ratio on short crack growth behavior were investigated.; In this study experiments and analyses were carried out using four-point bending specimens made of commonly used SAE 1045 steel. Several different initial KII/KI mixed-mode ratios covering a wide range (0 to 13.08) were used to study short fatigue crack growth under mixed-mode loading. Key-hole notched specimens were subjected to mode I loading to study short crack growth from a notch. Two different load ratios, R, of nearly zero and 0.5 were used in both mixed-mode short crack and short crack from notch studies. All the finite element analysis in this study were carried out using fracture mechanics software package FRANC2D (Fracture Analysis Code 2D). Before it was applied in this study, applicability and accuracy of the FRANC2D program was verified with some well-known analytical methods and other numerical solutions from the literature.; From the study of short crack growth under mixed-mode loading, cracks were observed to grow along the paths with very small K II/KI ratios (i.e. mode I), similar to that of long crack under mixed-mode loading from a previous study of the material used. The criteria used to predict long crack growth direction under mixed-mode loading including the maximum tangential stress (MTS) and the minimum strain energy density criteria were applied to predict short crack growth directions. The MTS criterion option of FRANC2D program was also used to predict the crack growth paths. The predicted paths were found to be fairly close to the experimental paths, except when cracks grew long. (Abstract shortened by UMI.)