4. Specific detection of fatigue crack propagation

The appearance of acoustic multiplets, during loading cycles, at a specific stress threshold σ is illustrated in figure 5 , for a test on 99.95% pure aluminum (coarse-grained polycrystalline). A remarkable feature of this representation is the highlighting of alignments of points associated with the repeated triggering of signals at each cycle, at an almost identical stress threshold or evolving very slowly from one cycle to the next. These sequences can last for hundreds of cycles. Three of these groups are shown in figure 5 as typical examples for further analysis.