Fracture of welded aluminum thin-walled structures

Abstract

A comprehensive methodology was developed in the thesis for damage prediction of welded aluminum thin-walled structures, which includes material modeling, calibration, numerical simulation and experimental verification. An extensive experimental program was conducted on large-scale welded panels used on Inter City Express (ICE) high-speed European passenger trains. These panels consist of geometrically complex extrusions, which are welded together to form the final structure. A wealth of data was generated to validate the proposed methodology. The current work has demonstrated the efficiency and robustness required for mainstream industrial applications. As the first step, a local fracture criterion was validated on two types of aluminum components without welds: (i) S-rails under quasi-static and dynamic axial loading; (ii) large-scale extruded aluminum panels under 4-point bending. With the fracture parameter calibrated from uniaxial tensile tests, numerical simulations gave excellent predictions of crack formation for test articles. A novel technique was developed to calibrate heterogeneous weldments for plasticity and fracture. This technique eliminates the need for machining and testing of miniature tensile specimens, cut from different zones within the weldment.

(cont.) The calibrated data was validated by comparing the numerical results with small and intermediate-scale tests. Excellent agreement was achieved. A wide range of aluminum weldments, including those developed as part of this study and relevant examples found in the literature, were examined from the point of view of microstructure, hardness distributions, stress-strain relations, etc. This study concludes that aluminum weldments exhibit very different mechanical characteristics than comparable steel weldments considering the above factors. The relative strength mismatch ratio between the weld zone and the Coarse Grain Heat Affected Zone (CGHAZ) MR, was identified as the most critical parameter for the global load/deformation response, and for fracture initiation of typical aluminum weld joints. Finally, a unique series of large-scale Mode I and III fracture tests was performed on full-scale welded ICE panels. The mechanism for crack initiation and growth under these two types of loadings was then investigated numerically and compared with the test results. Prediction of crack growth using the discrete element removal technique in combination with the proposed fracture locus, was shown to be accurate and robust.

(cont.) The most impressive result from the Mode I simulation was its ability to model a sudden jump of the crack from the weld zone to the HAZ, which was witnessed in the tests. Despite the differences in global loading from Mode I and Mode III cases, fracture in both loading modes was shown to be tension dominant. The new technique is now ready for industrial applications.