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.
