Analysis of fatigue crack growth retardation due to overloading by using AFGROW

Abstract

The effect of mode I and mode II overload on subsequent mode I crack propagation is studied on single edge notch specimen of Aluminum alloy. The application of overload spike during constant amplitude high cycle fatigue introduces a large plastic zone which enhances the magnitude and size of compressive residual stress field in the vicinity of the crack tip. This enhanced compressive residual stress field reduces the available crack tip driving force, thus causing a reduction in fatigue crack growth rate. It has been observed that the number of cycles to failure increases with increase in the overload application angle. The material used in the present investigation was aluminum alloy (Zn- 4.6, Mg- 1.4, Mn-0.5, Cr-0.1, Zr-0.1, Ti-0.03) having yield strength of 250 MPa. Single edge notched specimens of dimensions 52 mm*170 mm*6.5 mm were prepared in the LT- direction. The notches were of flat type cut with jewellary saw up to a length of 15mm. Before the fatigue test, the notched specimens were precracked up to a length of 16mm. The fatigue tests were carried out in tension-tension constant stress amplitude mode using sinusoidal loading conditions in an Instron-4553 electromagnetic resonance (EMR) machine. The tests were performed at a stress ratio R=0.1 and loading range of ∆P=7000 Newton’s. For the values obtained from the experiments we get the graphs. To calculate the plastic zone size for the applied overload we have to write the maximum fitting curve for the values obtained from the experimental data. The experimental work is carried out for the specimen on INSTRON machine and the experimental results can be noted down. In the present work the fatigue crack growth retardation is obtained by AFGROW software using different boundary conditions and plastic zone sizes are obtained for mode I overloads. Plastic zone sizes are also calculated from experimental results and comparison of the experimental results with the simulation results are carried out.