Reliability Engineering Approach to Fatigue Crack Growth Rate by Constant <DELTA> K Tests

点击翻译按钮获取中文摘要

The purpose of this investigation is to clarify <DELTA> K dependence of variability of the fatigue crack growth rate da/dN by conducting statistical fatigue crack growth tests under the condition of constant <DELTA> K, and to estabilish the probabilistic formulation of da/dN, where <DELTA> K is the stress intensity range. The condition of constant <DELTA> K was attained using the contoured double cantilever beam specimen. The material used was the high strength steel HT60. 4 to 6 specimens were tested at each of six levels of <DELTA> K. In all 31 specimens were tested. da/dN was measured using DC electric potential method. In order to automize measurement and data processing, an automatic crack length measurement system using micro-computer was developed. Various efforts were made to attain high accuracy of measurement. For obtaining an effective method for analysis of the experimental data under constant <DELTA> K, the privious data under the condition of increasing <DELTA> K were reanalysed. The analysis showed that it was appropriate to regard variability of da/dN as composed of the two components : inter-specimen variability and intra-specimen variability. Next, the experimental data under constant <DELTA> K was analysed. First, variability of da/dN was decomposed into inter-specimen variabity and intraspecimen variabity using the method of analysis of variance. It was found that variability of da/dN under constant <DELTA> K was also composed of the above two components. Then, <DELTA> K dependence of these two components was examined. Neither inter-specimen variability nor intra-specimen variabity showed clear dependence on <DELTA> K, and thus, both the components could be regarded as consatnt independent of <DELTA> K. From the above result, it is concluded that an appropriate probabilitistic formulation of da/dN is to randomize C and to treat m as constant, when Paris's law da/dN = C( <DELTA> K)m is applied at each point in materials.