Challenges in detecting damage in the presence of microstructural inhomogenities in a Friction Stir Welded aluminum alloy for reusable cryotanks

Continuous real time structural health monitoring will be a requirement for future space launch missions. Reusable metallic cryotanks manufactured using Friction Stir Welding (FSW) technology for multiple missions, demands weld and microstructural integrity. The FS weld contains multiple interfaces and a variety of microstructures. To develop NDE-based health monitoring capability which detects damage and monitors the progression of damage, in the presence of these microstructural inhomogeneities, is a challenging task. Most structural health monitoring techniques are based on acoustic wave propagation. To design and develop efficient and optimized health monitoring capability based on acoustics, it is necessary to incorporate local elastic property variations that arise due to differences in the weld microstructure. These local elastic property changes across FSW regions have been measured using a focused acoustic beam. Measurements across the weld line show variations with a maximum change of 1% in the sound velocities. Macroscopic measurements of velocity of surface acoustic waves propagating across and also parallel the weld line in a large plate show significant variation. Experimental results of local and macroscopic sound velocity measurements from the changing microstructure along with their impact on the design of structural health monitoring system are discussed.