Fatigue Tests of Un-Notched and Notched Specimens and Life Prediction Using a Variable Critical Distance Method

Fatigue is one of the most common failure mechanism in engineering structures. The statistical nature of fatigue life and the stress gradient are the two challenges among many while designing any component or structure for fatigue. Fatigue lives of the identical components exhibit the considerable variation under the same loading and operating conditions due to the difference in the material micro-structures and other uncontrolled parameters. Stress concentration at the notch causes stress gradient and therefore, applying the plane specimen results for actual engineering components with notches does not give quantitatively reliable results if the stress gradient effects are not considered. The objective of the work presented here was to carry out the fatigue tests of un-notched, U and V-notch specimens which were die cast using aluminum alloy (A380) and to obtain fatigue life using a variable critical distance method which considers the stress gradient due to the notch geometry. Specimens were prepared in the foundry shop in a way to minimize the microstructural variations and a radiography study was carried out to ensure that cast specimens have minimum or no porosity. Geometric variations of the specimens were studied in the metrology lab to ensure that these variations are minimum. Laboratory tests were conducted in the fatigue test lab using un-notched, U and V-notch specimens at several loads and load vs life curves were obtained. A simulation method was proposed for the fatigue life prediction, which considers the effect of different stress gradients due to difference in the notch geometry. A good correlation was obtained between the test and simulation results. The method offers a better way for quantitative prediction of fatigue life and may be very helpful to design robust automotive components.