Fatigue crack propagation after overloading and underloading at negative stress ratios

This paper intends to evaluate the influence of the intrinsic properties of the materials, namely plastic and cyclic plastic properties, on the overloading/underloading effect on crack propagation rate, at baseline negative stress ratios, under plane strain conditions. The importance of the negative loading part of the fatigue cycle on crack propagation rate has been shown by previous works of this same author. In those works has also been shown that under baseline negative stress ratios there exists negative open loads and crack propagation rate does not correlate properly with the crack closure concept. These features were shown to be strongly related to plastic properties and cyclic plastic properties of the materials. It has been concluded that the Bauschinger effect may be the explanation for the different sensitivity to negative loads. Thus, some materials may be very sensitive to negative loads and some others may not be so sensitive. Tensile overload and compression underload tests, at positive and negative baseline stress ratios were made in different materials, with different plastic properties, in order to predict their influence on crack propagation rate. The main emphasis in this paper is the importance of the compressive part of the loading cycle under negative baseline R ratios on overloads/underloads effect on crack propagation rate. Results will show that the effect of overloads and underloads on crack propagation rate, at baseline negative stress ratios, are not fully accounted for by crack propagation models and that the generalized accepted behaviour of OL/UL may not be the same at baseline negative stress ratios. It will be shown that Overloads may produce acceleration instead of the accepted retardation effect. A physical understanding on the effects of OL/UL is also provided in the paper.