Takao Okada

Fatigue crack propagation property of friction stir welded 2024-T3 aluminum alloy

Fatigue crack propagation tests on friction stir welded aluminum alloy sheets were carried out to investigate the effect of residual stress on the crack propagation rate. In these tests, the effect of the inclination angle of the weld line on the direction of crack propagation was also investigated. The test results show that crack propagation rate is accelerated by high tensile residual stress around the weld line. The angle of the weld line does not affect the crack propagation rate and direction. This behavior of crack propagation cannot be explained by the consideration of residual stress value alone. The estimation of effective stress range from the crack opening stress is required to satisfactorily evaluate the crack propagation rate of friction stir welded specimens.

Fatigue Crack Growth of Friction-Stir-Welded Aluminum Alloy

Crack propagation tests were conducted to clarify the effect of stress ratio on crack growth rate in friction-stir-welded 2024-T3 aluminum alloy. The effect of the distance between the weld line center and the center of the specimen was also evaluated. The result indicates that the peak of the acceleration is not at the area of maximum tensile residual stress. The modified stress ratio that considers the residual stress identifies that the location of the peak modified stress ratio corresponds to that of the peak maximum acceleration. The relationship between stress ratio and crack growth acceleration in friction-stir-welded panels is also evaluated by an analysis using the stress intensity factor range with and without residual stress and the crack growth rate for the master curve obtained by the base material with a different stress range. The maximum acceleration decreases with the increase of the distance between the weld line center and the center of the specimen and with the increase of the stress r...

Energy dissipation in a riveted lap joint of aircraft structure under in-plane tensile and shear loading

The energy dissipation per load cycle in a riveted lap joint of aircraft structure was investigated as structural damping characteristics. The specimens were riveted lap joints with 2024-T3 clad skins and NAS-1097 rivets in this research. Energy dissipation per cycle under in-plane shear loading (DS), and under in-plane tensile and shear loading (DTS) were measured on specimens using an extensometer, a tensile test machine, and rail shear jigs for riveted lap joint specimens. FE-simulations were conducted in order to estimate rivet transferring loads in specimen, and an area of slip surface (m) on interfacial surface of riveted lap joint. The experimental results under varying load condition on specimens showed that DS and DTS are proportional 3-powered the in-plane load ranges. The FEsimulations results showed that relationship between m and load applied to a riveted lap joint is linear, and the area of slip surface is associated with the amount of rivet transferring load, regardless of load direction. The discussions from these results showed that the energy dissipation in a riveted lap joint under in-plane tensile and shear loading can be estimated from the amount of in-plane tensile and shear loads applied to a riveted lap joint.

Evaluation of Fatigue Crack Growth Behavior in FSW Joint by Experiment, Analysis and Elasto-Plastic FEM

Elasto-plastic FEM is used to examine the crack opening stress of a 2024-T3 Aluminum alloy sheet and a friction stir welded (FSW) panel. To investigate the effect of plastic deformation around the crack on the crack opening stress, the effects of the following two parameters, the distance between the weld line and the center of the crack starter, and the magnitude of the tensile residual stress on the crack opening stress, are evaluated. The da/dN-ΔK curves and a-N curves for the FSW plate are obtained numerically using an experimental da/dN-ΔK curve for the base material and the calculated crack opening stress. In addition, the da/dN-ΔK curves where ΔK is evaluated by correction factor and its a-N curves are obtained analytically. Comparison of these numerical results with the results of empirical tests shows while that the FEM result conforms to experimental data, the calculations based on the correction factor show poorer correspondence. These results demonstrate that FEM can reasonably predict the da/dN-ΔK curves and a-N curves for an FSW panel.