| In this thesis, different crack detection and monitoring techniques are studied including surface replication, marker bands, and electrical potential drop (EPD). The EPD method is the most successful short crack detection technique detecting cracks as small as 42 mum in radius in the 2024 aluminum alloy.; From post-fracture analysis, it is concluded that almost all the fatigue failures originate at constituent particles, ranging from approximately 50 mum 2 to 550 mum2 in size, with many cracks nucleating along the notch wall. This leads to the hypothesis that cracks nucleate early during fatigue testing but then become arrested. This process repeats until one crack breaks through the arresting barrier and propagates to failure. The time required for the crack to break through barrier is the reason why the majority of the fatigue life is spent in the nucleation and short crack regimes.; A chemical analysis of the constituent particles reveals a compositional difference between the large particles at the nucleation sites and the smaller particles that have no role in fatigue crack nucleation. The larger particles contain significant iron and manganese and increased copper compared to the smaller particles. Therefore, to improve the fatigue life of 2024, the iron content must be lowered. |
