Noninvasive three-dimensional visualization of defects and crack propagation in layered foam structures by phase-contrast microimaging

Layered polymer foam structures used to insulate the fuel tank on the Space Shuttle were investigated by combining phase-contrast X-ray three-dimensional imaging and loading. It is shown that the interlayers between layers were distinctly different from the layers, resulting in incoherent interfaces where defects developed due to severe mismatch of cell structure between the mechanically weaker layers and stronger interlayers. These images reveal noninvasively that cracks nucleated readily at the interfaces and grew preferably along the direction of foam rise.