| In the present research, fatigue performance, fatigue failure mode and fatigue failure mechanism of aluminum resistance spot welds have been experimentally investigated and spot weld fatigue life prediction model been developed.Quasi-static tensile tests have been performed on AA5754 and AA6111 similar spot welds and AA5754-AA6111 dissimilar spot welds in various specimen types and stack-up conditions. Test results indicate that, static failure modes of aluminum spot welded tensile shear and coach peel specimen are interfacial fracture and nugget pull-out, respectively. Ultimate tensile loads of AA5754 and AA6111 spot welds agree well with each other, which is primarily due to consistent failure mode and micro-hardness profile in weld nugget.Fatigue tests have been conducted to explore effects of base material, cyclic load ratio, stack-up condition and fatigue failure mode on fatigue life of aluminum spot weld.Fatigue test results indicate that effect of cyclic load ratio on fatigue life of spot weld is existent while not significant. Increasing cyclic load ratiowill, to some extent, decrease spot weld fatigue life. Fatigue data and fatigue failure modes of AA5754 and AA6111 spot welds are consistent, indicating that effect of base material is negligible in fatigue model establishment. Fatigue life of spot weld with unequal thickness sheets is dominated by the failure process of thinner sheet. Increasing thickness of thinner sheet will significantly improve fatigue performance of the spot weld.SEM analysis and micro-structure analysis have been carried out to investigate fatigue damage process and fatigue crack development of specimens failed in different failure modes. Observed fatigue failure modes of spot welded tensile shear(TS) specimen are eyebrow failure, interfacial fracture and nugget pull-out. Due to the common feature of those three failure modes, a three-stage crack growth mode could be used to describe fatigue damage process of TS specimen: Stage I, fatigue crack initiation and early growth; Stage II, crack propagation in sheet thickness until visible through thickness crack and Stage III, fatigue crack propagation in specimen width. Stage II is the primary stage in fatigue damage process, of whichthe duration could account up to 80% of total fatigue life. As for CP specimen, crack initiates at nugget circumference and advances along nugget interface until interfacial fracture. Welding defects(such as voids) might affectcrack growth path in fast fracture zone, but have little influence on fatigue life of CP specimen.Loads transferred through weld nugget have been analyzed based on a simplified beam model and transfer matrix method, andgeneral solutions of reactions at weld nugget been derived. Validated by ACM1 modelbased FEM analysis, the derived analytic solutions could be accurately and conveniently employed to perform load analysis for various spot welded specimens.Reactions at weld nugget have been employed to calculate stress intensities and a new spot weld fatigue damage parameter has been proposed accordingly to correlate fatigue data.The proposed local stress intensity factor based fatigue damage parameter is proven effective in capturing failure mode of TS and CP specimen and consolidating a large amount of fatigue data into a narrow band.The fatigue damage parameter has been further modified using base material Young’s modulus and a general fatigue life prediction equation is then generated. This fatigue life prediction equation was validated using fatigue data of aluminum alloy AA5182, AA6061 and magnesium alloy AZ31 spot welded joints. Validation results indicate that this equation could be employed to estimate fatigue life of spot weld of various materials as well as different specimen geometries. Compared withcurrent fatigue models, the proposed model in this thesis is more effective and is especially suitable for the comparativefatigue strength evaluation of components and specimens. |
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