Effect of compression precracking on near threshold region for AlSI 4340 steels considering compliance measurements


Determining long crack threshold as a design criterion is not a new research topic; however, some anomalies because of different test methods in literature keep it voguish. Inaccurate estimations may result in considerable variation on corresponding endurance stress for components with small crack size. Accepted approach compared to traditional methods is to perform compression-compression precracking before crack growth tests by eliminating history effects and accurately estimating long crack threshold value. Load history effects may result from specimen configuration and test procedure applied. The scale of yielding during precracking cannot be underestimated and these residual stresses have an effect on crack growth rates, even if limited by stress relief in the plastic region. Naturally formed precrack will propagate until diminishing size of the corresponding tensile zone responsible for crack growth. Compression-compression precracking under constant amplitude loading is performed to form non-propagation crack. The advantage of compression precracking is to provide a fully open crack that eliminates possible closure effects. In the scope of this paper, different stress ratios will be tested and the effect of compression precracking on near threshold will be investigated. To present reliable and accurate crack measurement that affects long threshold directly, compliance curves will be also introduced with this article.