Invisible crack visualization and depth analysis by mechanoluminescence film

We experimentally demonstrated the depth analysis of artificial invisible notches on the backside of aluminum alloy (A6061-T6) plate using a high-sensitive mechanoluminescence (ML) film. The ML film was developed by a screen-printing method utilizing a mixture of Eu-doped SrAl2O4 powder and optical epoxy resin. From the analysis of luminescence distribution images (ML patterns) obtained from the ML film attached to the aluminum alloy specimen, the ML film visualized the notches of 37.5–100% depth levels from the opposite side of the notched surface of the specimen under dynamic tensile testing. Moreover, it was clarified that the discovered two parameters from the ML patterns are worthwhile to estimate the depth level of the invisible notch. The first parameter is the ratio of the time derivative of the ML intensity curves located on the stress concentration area and nominal stress area. The other is the distance between two points with maximum ML intensity located around the back-side of the invisible notch. These parameters were found to vary almost linearly with the depth level of the back-side notch, although their detection ranges are different. Also, it should be noted that the second parameter is hardly affected by the strain magnitude and its rate. This implies that the high-sensitive ML film is a promising candidate as non-destructive testing to predict a residual life of a fatigue crack in a structure under dynamic behavior. [Display omitted] •The developed mechanoluminescence (ML) film could visualize the invisible cracks in an aluminum alloy (A6061-T6).•The observed ML pattern indicated not only a position but also the depth level of the invisible crack.•We newly proposed the two methods for estimating the crack depth using ML parameters due to the stress concentration.•The ML pattern derived from the invisible crack is independent of both strain and strain rates in structural material.