Fatigue-damage evaluation in aluminum heat-transfer tubes by measuring dislocation cell-wall thickness

Evolution of dislocation structure during low cycle fatigue in 3003 aluminum alloy was investigated for the purpose of finding a possible parameter to indicate fatigue damage prior to crack initiation. In low cycle fatigue, it is observed that by increasing the number of cycles, the dislocation structure develops to random cells, then changes to clear cells and finally to sub-grains. In other words, the cell-wall thickness decreases with increasing number of cycles. It is also found that measuring the cell-wall thickness is a valuable method to evaluate fatigue damage when the cell structure is formed. The fatigue damage index (the number of cycles/cycles up to fracture) is found to be inversely proportional to the average cell-wall thickness, regardless of the applied stress level. This phenomenon can be explained by dynamic recovery of dislocation during fatigue, i.e. mutual annihilation of dislocations, which takes place during each half cycle.