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 $\sim 10\mathrm{m}{\mathrm{s}}^{-1}$, rapidly accelerating to $>100\mathrm{m}{\mathrm{s}}^{-1}$. We also observe some evidence for rate-dependent behavior, in that indentation at the lowest rates studied here (below about $\sim 0.15\mathrm{m}{\mathrm{s}}^{-1}$) fails to initiate cracks regardless of the depth of the indentation (up to $18\mu \mathrm{m}$), while indentation at higher rates produces median cracks that either arrest or cause complete fracture, depending on the depth of indentation.

动态点冲击载荷是移动设备上玻璃屏幕破裂的主要原因。更好地理解高速压痕下的裂纹萌生和扩展可能有助于开发性能更好的材料，但是由于时间尺度短且光学显微镜的景深有限，因此实验观察具有挑战性。为了满足这一需求，我们使用原位X射线相衬成像（XPCI）在动态压痕过程中观察到化学强化玻璃的破裂。在玻璃表面下方以大约对应于表面残余压缩应力减小至零的深度的深度开始产生中部裂纹。这些裂纹最初在$〜10\mathrm{米}{\mathrm{s}}^{-\mathrm{1个}}$，迅速加速到 $>100\mathrm{米}{\mathrm{s}}^{-\mathrm{1个}}$。我们还观察到一些与速率相关的行为的证据，即此处研究的最低速率下的缩进（如下$〜0。15\mathrm{米}{\mathrm{s}}^{-\mathrm{1个}}$）不论压痕的深度如何（直至 $18\mu \mathrm{米}$），但以较高的速度压入会产生中值的裂纹，该裂纹会根据压入的深度而止住或引起完全断裂。