Issue 35

Chahardehi et al, Frattura ed Integrità Strutturale, 35 (2016) 41-49; DOI: 10.3221/IGF-ESIS.35.05 46 Figure 2: Dependence of U on crack length. Reprinted with permission from [32]. Copyright (1985) The Society of Materials Science, Japan. During early stages of crack growth, the effective stress range ratio ( U ) decreases with an increase in crack length. This parameter then approaches a constant value. For more negative R-ratios, this decrease continues over a longer crack size. This is believed to be due to the larger size of the zone of residual stress due to compressive loading at more negative R- ratios [32]. Effect of loading mode Only uniaxial loading is considered in the present work under mode I, i.e. ‘opening mode’ loading. Work on biaxial stress fields has shown similar R-ratio dependence of fatigue crack growth [51]. Chung-Seong et al [52] looked at the effect of loading mode (pure bending vs. axial loading) in closure behaviour of surface and through-thickness cracks. The loading mode does not seem to affect the relative ratio of crack opening in through-thickness and surface cracks. However, effect of loading mode on residual stresses from compressive cycle is not directly obtained. Therefore one hypothesis may be that the shape and extent of the residual stress field developed due to compressive stress could be dependent on loading mode. Miscellaneous considerations Care should be exercised when dealing with closure in corrosive environment, as the mechanisms involved may not have been fully understood. Also, sensitivity of closure behaviour to waveform and frequency has not been experimentally investigated, however it may be assumed that mechanistically the behaviour should not be dependent on these two parameters, as is the case in general crack growth under constant amplitude loading. Compressive behaviour at variable amplitude has also not been investigated. C ONCLUSIONS he two important features recognized in fatigue of compressively loaded components are the beneficial effect of closure (for low R-ratio loads with a tensile part), and the finite albeit limited growth in fully compressive stress mode. The current papers highlights the observations, performed over the last five decades, among with some of the explanations given for certain features. Fatigue crack growth in compression is clearly confirmed to be a complicated phenomenon, where a simple, broad-brush method for its assessment has not been developed. Observations include the