When brittle hydrogels fail, several mechanisms conspire to alter the state of stress near the tip of a crack, and it is challenging to identify which mechanism is dominant. In the fracture of brittle solids, a sufficient far-field stress results in the complete loss of structural strength as the material ‘unzips’ at the tip of a crack, where stresses are concentrated. Direct studies of the so-called small-scale yielding zone, where deformation is large, are sparing. Using hydrogels as a model brittle solid, we probe the small-scale yielding region with a combination of microscopy methods that resolve the kinematics of the deformation. A zone over which most of the energy is dissipated through the loss of cohesion is identified in the immediate surroundings of the crack tip. With direct measurements, we determine the scale and structure of the process zone, and identify how the specific loss mechanisms in this hydrogel material might generalize for brittle material failure.

当脆性水凝胶失效时，几种机制共同改变了裂纹尖端附近的应力状态，并且很难确定哪种机制占主导地位。在脆性固体的断裂中，足够的远场应力会导致结构强度完全丧失，因为材料会在应力集中的裂纹尖端“解开”。对变形较大的所谓小规模屈服区的直接研究很少。使用水凝胶作为模型脆性固体，我们结合使用显微镜方法来探测小规模屈服区域，以解决变形的运动学问题。在紧邻裂纹尖端的区域中可以识别出大部分能量因内聚力损失而耗散的区域。通过直接测量，