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

Evolution of dislocation structure during low cycle fatigue in 3003 aluminu m alloy was investigated for the purpose of finding a possible parameter to indicate fatigue damage prior to crack initiation. In low cycle fatigue, i t is observed that by increasing the number of cycles, the dislocation stru cture develops to random cells, then changes to clear cells and finally to sub-grains. In other words, the cell-wall thickness decreases with increasi ng number of cycles. It is also found that measuring the cell-wall thicknes s is a valuable method to evaluate fatigue damage when the cell structure i s formed. The fatigue damage index (the number of cycles/cycles up to fract ure) is found to be inversely proportional to the average cell-wall thickne ss, regardless of the applied stress level. This phenomenon can be explaine d by dynamic recovery of dislocation during fatigue, i.e. mutual annihilati on of dislocations, which takes place during each half cycle. (C) 2001 Else vier Science B.V. All rights reserved.