In civil, road-bridge and underground engineering, there are many bi-materials foundations under various structures. The growths of cracks in these bases will cause structure failure. To avoid these cases, the mechanism of this kind crack growth must be investigated. In order to study whether an original crack existing in a weak medium (such as in concrete) can propagate into a hard medium (such as in rock), under a loading condition of compression and shearing, bi-materials specimens have be designed. By the specimens, a series I-II mixed mode fracture experiments have been conducted. The experiments demonstrate that in some situations crack can grow from the weak medium concrete into the hard medium marble under the uniaxial compression loading condition. The stress intensity factors K1۱ KII and the first principle stress σ1 near the crack tips have been calculated by finite element method. Different numerical results between the single-material and bi-materials specimens have also been made. According to the mixed mode fracture theory e.g. the σ θ max criterion, the conditions which estimate when the mixed mode crack in the bi-materials specimens may propagate and whether a crack in a weak medium can pass through the interface and go into the hard medium have been constructed. Experimental results are agreement with the theoretical predictions.
Lots of researches have been done on mixed mode crack fracture in single-rock-material (Rao et al. 2003, Wong et al. 2004, Li et al.1994, and 2006), to meet the needs of rock engineering study. However, there is bi-materials crack fracture problem in large-scale construction projects. Building these kinds of structures always refers to the together use of a great deal of rocks and concretes. Whether the crack in one kind of material can grow into another one, especially, whether a crack in weak-material can grow into a strong one? What is the condition of smooth expansion? Under which conditions will the expansion be cumbered? Research on these issues has great importance in civil, road-bridge and underground engineering. At present, the researches on bi-materials cracks are mainly limited to researches on composite materials; and the researches on bi-materials of rock and concrete are still in the very preliminary stage. This paper studies the difference in crack propagation ability between the single-material and the bi-materials specimens and studies how a center inclined crack in weak medium concrete can grow into strong medium marble by the experimental observation, finite element method calculation and theoretical analysis.
In this paper as Figure 1(a) shows, the by-materials specimens (150mm×150mm) are designed by two marble blocks stuck by concrete, with a center inclined crack in concrete. The original length of crack in concrete is 2a=3cm, the crack inclination angle is α=15°, 30°,45°,60° and 75° respectively, The distance from the crack tip to the interface of each specimen is fixed to d=5mm. The thickness of each specimens is B=3cm. The mechanical parameters of marble and concrete are shown in table 1.
