In situ measurement of three-dimensional deformation and stress fields with applications to soft material fracture

软聚合物和凝胶由于其高可变形性、生物相容性和对各种外部刺激的反应性而成为一类令人兴奋的材料。它们广泛应用于可拉伸电子，压敏粘合剂，生物医学植入物和具有大形状变化的致动器等领域。这些应用的瓶颈是软材料抵抗由小缺陷生长引起的断裂的能力。由于复杂的非线性力学和缺乏实验工具来检测缺陷附近的机械应力，人们对软材料如何断裂的理解有限。提出的研究计划旨在推进对软材料断裂的基本理解，并确定具有增强抗断裂能力的坚韧聚合物和凝胶的工程指导方针。这将极大地扩展其应用范围，例如用于人造软骨的坚韧凝胶和用于软机器人的耐用弹性体。为了实现这一目标，我们将开发一种新的实验方法，可以测量软材料的三维应变和应力场。该方法将与理论和计算分析相结合，绘制裂纹或空洞等缺陷附近应力的详细分布，并了解应力如何导致缺陷生长并最终导致宏观断裂。重点还将放在理解非线性弹性和材料微观结构对缺陷生长的影响。除了理解软材料断裂外，本提案中开发的实验方法可能成为表征软材料力学的变革性工具。通过拟开展的研究活动，将培养两名博士生。

Soft polymers and gels have emerged as an exciting class of materials due to their high deformability, bio-compatibility and responsiveness to various external stimuli. They are widely applied in areas including stretchable electronics, pressure sensitive adhesives, biomedical implants, and actuators with large shape change. A bottleneck for these applications is the ability of soft materials to resist fracture caused by the growth of small defects. There is a limited understanding on how soft materials break, largely due to the complex nonlinear mechanics and the lack of experimental tools to examine mechanical stresses near defects. The proposed research program aims to advance fundamental understandings on soft material fracture, and to identify guidelines for the engineering of tough polymers and gels with enhanced ability to resist fracture. This will greatly expand the scope of their applications, examples including tough gels for artificial cartilage and durable elastomers for soft robotics. To achieve this goal, we will develop a novel experimental method that can measure three-dimensional strain and stress fields in soft materials. This method will be combined with theoretical and computational analyses to map the detailed distribution of stresses near defects such as cracks or cavities and understand how the stresses cause defect growth and eventually lead to macroscopic fracture. Emphasis will also be placed on understanding the effect of nonlinear elasticity and material microstructure on defect growth. In addition to understanding soft material fracture, the experimental method to be developed in this proposal can potentially become a transformative tool for characterizing mechanics of soft materials. Two PhD students will be trained through the proposed research activities.