Research On The Fatigue Damage Of Spot Weld Of Aluminum Alloy

Resistance spot-welding is a widely used technology in modern automotive manufacture industry.Under working condition, there exists big stress concentration around the nugget of spot weld. Fatigueis one of the main failure forms for spot-weld. In order to improve the durability and reliability ofvehicles, it is necessary to look for a reasonable way to describe the fatigue damage evolution for thespot weld, which is helpful for the evaluation of fatigue life of it as well.In this thesis, the lap shear specimen (LP) is used for AA5754aluminum alloy spot weld in theresearch. An extensometer is used to measure the rigidity of LP specimen, and a special self-madevoltage gauge is used to measure the voltage of the nugget area, which can also be used to evaluatethe fatigue evolution of LP specimen. Two fatigue damage parameters are introduced, and the damageevolution equations are proposed based on the theory of damage mechanics. The test results show thatthe fatigue evolution of LP of AA5754aluminum alloy is non-linear, and it can be divided into fourstages, which are1) damage initiation,2) slow increases of damage,3) acceleration of damage, and4)fatigue failure.The strain distributions around the nugget are measured by moiré interferometry in the research.The maximum strains at different fatigue cycles are measured in the second stage. The test resultsindicate that the edge of the nugget is the area with high strain concentration. In the second stage, themaximum elastic strains around the nugget decrease while the fatigue cycles increase, which meansthat there exists cyclic hardening near the nugget of AA5754aluminum alloy spot weld.Finally, a unified equation for the fatigue life prediction of LP of AA5754aluminum alloy isproposed which is based on the test data and FEA analysis, in which, the fatigue life of LP specimenwith different plate thicknesses can be evaluated.