Failure Analysis of a Cast A380 Aluminum Alloy Casting Using a Microstructurally Based Fatigue Model

When evaluating the fatigue life of a die cast A380 aluminum alloy, the combination of microstructures, inclusions, and stress concentrations is particularly important. This paper presents a failure analysis of an A380 die cast aluminum pivot arm used in a clothing press. This study includes finite element analyses, optical microscopy of the fracture surface, fatigue testing of the A380 aluminum alloy, and the application of a microstructurally-based fatigue model. Finite element analyses reveal stress concentrations exceeding the yield strength in the failed area while optical microscopy exposed large pores in the castings. Fatigue testing of specimens cut from a pivot arm validated the use of some input parameters for the fatigue model. Fatigue life predictions for the pivot arm were below the component design life but in the range of the actual performance data. This analysis shows that a redesign to reduce stress concentrations and/or a reduction in porosity levels would significantly improve the pivot arm’s life. This methodology also exemplifies that in general a cast component with complex microstructure, inclusion state, and stress state can be analyzed with the microstructurally-based fatigue model. INTRODUCTION The objective of this paper is to analyze the failure of a pivot arm in a commercial clothing press designed for routine, repetitive pressing operations. The pivot arm is a die cast A380 aluminum alloy. The pivot arm’s service history showed that it was not subjected to any abnormal working conditions. A visual examination of the pivot arm shows minimal deformation and suggests failure initiated at the drilled hole shown in Figure 1. There were no signs of corrosion, indicating the aluminum was not exposed to a chemical attack. Because of operating conditions requiring cyclic loadings, fatigue failure is an obvious culprit. Fig. 1. The pivot arm fracture shows a drilled hole as a possible failure initiation site. The pivot arm’s interaction with other components was considered to determine the constraints and forces acting on the part. Figure 2 shows a drawing of the clothing press assembly. The compressive spring of the assembly transmits a force to the pivot arm via the spring bracket. As the press is closed, an iron attached to the end of the pivot arm contacts the iron matting. A handle fixed to the spring bracket is used to rotate the spring bracket up, clockwise as shown in Figure 2, clamping the iron into place. The motion of the pivot arm is stopped when either the matting force counter balances the spring’s moment or the pivot arm reaches the stop screw located on the iron base. If the pivot arm never reaches the stop screw then the pivot arm must support the full load produced by the spring. Fig. 2. The schematic of the press mechanism and pivot arm depicts the interaction of the pivot arm with other parts. The pivot arm failure analysis consists of stress, fracture surface, and fatigue analyses. The first section investigates stresses in the pivot arm using finite element simulations. Analytical hand calculations of stress at several points in the pivot arm are determined and compared to the finite element results. A second section presents an analysis of the fracture surface using optical microscopy to evaluate the porosity level and pore diameters in the casting. The third section uses results from the stress and fracture surface analyses, fatigue testing of specimens machined from the casting, and a microstructurally-based fatigue model (McDowell et al., 2003) to estimate the fatigue life of the pivot arm. The paper concludes with suggestions for increasing the fatigue life of the pivot arm. ANALYTICAL AND FINITE ELEMENT STRESS CALCULATIONS To obtain a better understanding of the stresses experienced by the pivot arm, analytical stress calculations were performed. Figure 3 shows the free body diagram of the pivot arm with the applied force, F, from a compression spring and the resulting reaction force, R, located at the iron attachment point. The spring force applied to the assembly varies as the press moves from an open to a closed position. The maximum spring force occurred at an angle of 55 degrees and was found to be 1321 N (297 lb) from experiments. The reaction force was calculated by summing moments about point L. Fig. 3. The free body diagram of the pivot arm illustrates the reaction force (R), spring force (F), and component forces (Fx and Fy). Analytical stress estimations are made at four locations along the length of the pivot arm labeled A, B, C, and D in Figure 4. The bending moment at each point was calculated using the reaction force and the distances shown in Figure 4. The area moments of inertia and distances from the neutral axis to the points of interest for each cross section were determined using CATIA. The analytical stress results are given in Table 1 and are used to validate the finite element analysis. Fig. 4. A top view of the pivot arm illustrates the geometry and four points of interest A, B, C, and D. Table 1. Analytical and Finite Element Stress Analysis Comparison Analytical Results FEA Results Cross Section (MPa) (MPa) A 85.0 100.6 B 50.2 44.8 C 49.0 58.6 D 78.2 79.3 A linear elastic stress analysis was initially conducted using CATIA to determine the magnitude and location of the maximum stresses present in the pivot arm. A finite element mesh was generated with CATIA using 10-node tetrahedron elements and consisted of 36,718 nodes and 20,979 elements. This three dimensional linear elastic finite element calculation was performed using the material properties in Table 2 for a cast A380 aluminum alloy. Figure 5 shows the part constraints and boundary conditions applied in the finite element simulation. The constraints consisted of fixing the through-pin holes, labeled as attachment point in Figure 2, in the yand z-directions while the reaction holes were fixed in the y-direction. The components of the 1321 N force were applied to the holes as shown in Figure 3. The CATIA finite element results revealed a stress concentration where stresses exceeded the yield strength. Since CATIA can only be used for linear elastic stress analyses, the mesh was exported and reformatted for a stress analysis using ABAQUS. This elastic-plastic finite element analysis provided the plastic strains and stresses within the pivot arm. Table 2. Mechanical Properties of Cast A380 Aluminum Alloy (Makhlouf, et al., 1998) Ultimate Tensile Strength 318 MPa Yield Strength 159 MPa Elongation to Failure 4% Modulus of Elasticity 71 GPa Density 2.7 g/cc Fig. 5. This solid model shows the loads and constraints applied to the pivot arm finite element mesh. Stress values given by the ABAQUS finite element simulation at locations A, B, C, and D are shown in Table 1 and are compared with the analytical stress values. The finite element stresses agreeing with the analytical stresses validate the finite element results. Stresses were found to be below the yield strength throughout the pivot arm except for a stress concentration at a drilled hole. This stress concentration is shown in Fig. 6 where the local von Mises equivalent stress reaches a maximum value of 177.9 MPa on both sides of the hole and a local maximum strain of 0.6%. The pivot arm does fail at this drilled hole, but since the stresses only exceed the yield strength at two small areas, this suggests the part did not fail due to an overload. However, the cyclic loading conditions of the pivot arm would likely initiate a crack in this higher stressed region.