Sheet sensor using SrAl2O4:Eu mechanoluminescent material for visualizing inner crack of high-pressure hydrogen vessel

A new non-destructive evaluation technique to detect cracks emanating from the inner surface (inner cracks) of a high-pressure hydrogen storage cylinder was developed by means of mechanoluminescence (ML) sensor consisting of SrAl2O4:Eu ML material and epoxy resin. To visualize the inner crack, a sheet ML sensor was attached onto the outer surface of the storage cylinder subjected to hydraulic pressure cycling with the maximum pressure of 45 MPa. The ML pattern was changed with an increase in the cycle number and the ML sensor could visualize the inner crack. The stress analysis by the finite element method clarified that the ML sensor provided unique equivalent strain distribution associated with stress concentration at the crack tip, i.e. the distance between two points having high equivalent strains was inversely proportional to the crack depth; consequently, the growth behavior of the inner crack was non-destructively quantified with the ML sensor attached on the outer surface.