Crack nucleation and growth during dynamic indentation of chemically-strengthened glass

Dynamic point impact loading is a primary cause of fracture of glass screens on mobile devices. An improved understanding of crack initiation and evolution under high-speed indentation could contribute to the development of materials with better performance, but experimental observations are challenging due to the short timescales and limited depth-of-field of optical microscopy. To address this need, we have observed fracture of a chemically-strengthened glass during dynamic indentation using in situ x-ray phase-contrast imaging (XPCI). Median cracks initiate below the surface of the glass, at a depth approximately corresponding to the depth at which the surface residual compressive stress diminishes to zero. These cracks initially propagate at ∼10ms−1, rapidly accelerating to >100ms−1. We also observe some evidence for rate-dependent behavior, in that indentation at the lowest rates studied here (below about ∼0.15ms−1) fails to initiate cracks regardless of the depth of the indentation (up to 18μm), while indentation at higher rates produces median cracks that either arrest or cause complete fracture, depending on the depth of indentation.