Fracture of High-Performance Quasibrittle Materials: Effects of Loading Rate & Fatigue

准脆性材料（包括高性能材料，如高强度混凝土）的断裂力学在过去几年中已经出现，但一个关键方面仍然被忽视-超前率（超过动态范围），载荷循环和疲劳的影响。实验和理论，集中在混凝土和覆盖岩石将进行。缺口断裂试样的试验包括：1)各种恒定位移速率下的载荷-挠度曲线，2)峰后软化范围内的载荷松弛试验，3)疲劳试验，4)再缺口试验。部分测试将与芝加哥材料服务公司合作进行。这些理论公式，按照逼近性提高、简洁性降低的顺序，将包括：1)R曲线方法的速率泛化；2)试件体具有粘塑性，桥接应力与开口位移关系具有粘塑性的内聚线裂纹模型，采用基于裂纹涂抹-尖端叠加的奇异积分-微分方程求解；3)断裂过程区具有裂纹损伤、蠕变和速率相关非线性三轴本构的非局部有限元模型；4)推广模拟混凝土微观结构的随机颗粒模型。这个问题的解决将为开发更好的材料铺平道路；更有效、更安全地利用它们；为超越现有经验范围的特殊或超大型混凝土结构提供更好和更安全的设计；更可靠地预测混凝土结构的长期开裂；对岩石边坡、开挖、地下洞口和钻孔的破坏进行更现实的预测。

Fracture mechanics of quasibrittle materials, which include high performance materials such as high strength concrete, has emerged during the last few years but one key aspect is still ignored- the effects of leading rate (beyond the dynamic range), load cycling and fatigue. Experiments and theory, focused on concretes and also covering rocks will be conducted. The tests of notched fracture specimens will include: 1) load-deflection curves at various constant displacement rates, 2) load relaxation tests in the post- peak softening range, 3) fatigue tests and 4) renotching tests. Part of the testing will be carried out in cooperation with industry, Material Service Corp., Chicago. The theoretical formulations, in the order of improving approximation but decreasing simplicity, will include: 1) Rate generalization of R- curve approach; 2) Cohesive line-crack model with viscoplasticity in the bulk of the specimen and viscoplasticity in the relation of bridging stress to opening displacement, solved by means of a singular integro-differential equation based on smeared-tip superposition of cracks: 3) nonlocal finite element model with cracking damage, creep and rate-dependent nonlinear triaxial constitutive model for the fracture process zone; and 4) generalization of the random particle model simulating the microstructure of concrete. Resolution of this problem will pave the way for development of better materials; for their more effective and safer utilization; for better and safer designs of special or very large concrete structures beyond the range of existing experience; for more reliable predictions of long-time cracking in concrete structures; and for more realistic predictions of the failure of rock slopes, excavations, underground openings and boreholes.