The effect of residual stress on the fatigue crack growth behavior of cast Al-Si-Mg alloys – Mechanisms and mathematical models

Introduction and background

Fatigue crack growth behavior of various types of alloys is significantly affected by the presence of residual stress induced by manufacturing and post-manufacturing process. There is a quantitative understanding of the effects of residual stress on fatigue behavior, but the effects are not comprehensively quantified and/or accounted for. The difficulty in quantifying these effects is mostly generated by the complexity of residual stress measurements (especially, considering that parts produced in similar conditions have different residual stress levels) and the lack of mathematical models able to convert experimental data with residual stress into residual stress free data.

Residual stress are self-equilibrating internal or locked-in stress remaining in a material that is free of applied (external) forces, external constraints, or temperature gradient. In most cases, residual stresses are an undesired result of material processing and they persist in material unless eliminated through subsequent stress relieving techniques. They are commonly found in weldments, complex forged and extruded parts, castings, especially when heat treated. In some cases, compressive near surface residual stresses are purposively introduced (e.g. shot peening) to improve fatigue life. The major difference, however, consists in the type of residual stress (comprehensive-retards fatigue crack growth or tensile-accelerates fatigue crack growth) and the knowledge of residual stress level. In the first case, the residual stress level is difficult to predict, while in the second case, it is known and quantified.

Knowledge of residual stress level in the component is very important, particularly when techniques to account for it are developed. There are several methods (non-destructive, partly-destructive, and destructive) to measure initial residual stress fields in the material. All stress measurements are based on the evaluation of actual strain or changes in strain, and they can be either qualitative or quantitative. Most commonly used stress measuring techniques are: photostress coating, ultrasonic (acoustic), photoelasticity, X-ray diffraction, neutron diffraction, hole drilling, positron annihilation, spectroscopy, chemical etching, sectioning strain gauged samples, and indentation and microhardness mapping. X-ray diffraction is recognized to be the only one truly non-destructive technique that is reliable.

Determining the presence, magnitude, and distribution of residual stress is vital for the correct interpretation of fatigue crack growth experimental data and implicitly the real service life prediction.