Wave Filtering Effects in Fractured Media

From a strength of materials point of view materials weakened by damage and/or fractures such as aged metallic and non-metallic materials, polymers and jointed rock mass, under certain conditions behaves like a material with reduced stiffness. When elastic waves (high frequency, seismic and blasting waves) propagate through such a material complex and important phenomena can be identified which depend on the characteristic parameters of the weakening discontinuities. In many applications these discontinuities are individual cracks or sparsely or densely distributed cracks which may even form an interlinked network. Parameters such as the type of cracks, their density, spacing, quality and type of filler material, surface topography and preferential orientation, and possibly other parameters become important. When the wave/pulse length of the dynamic disturbance is much larger than the characteristic dimension of the fractured medium, it is appropriate to average the effect of the physical discontinuities over a representative volume. This is the case when the discontinuities are closely spaced as compared with the length of the wave/pulse. If, however, the length of the wave/pulse becomes comparable to the fracture spacing or the delamination size on the fracture surface, the discontinuities must be explicitly modelled. In this case the ensemble of fractures acts as a filter on the frequency spectrum of the pulse and a strong interaction between the fracture and the wave/pulse is expected. This research project focuses on the properties of the induced elastic wavefield when discontinuities are modelled explicitly. The experimental program includes the application of dynamic photomechanics, particularly dynamic photoelasticity to the propagation and interaction of elastic stress waves with fracture type discontinuities. In the scaled laboratory models the fractures will be modelled as planes or wavy surfaces, completely broken or with variable degree of continuity, empty or filled with a filler material of specified quality, variable fracture spacing and orientation. In particular the transmission of curved wave fronts across planar weak interfaces will be studied experimentally and the results will be compared with data from various applications in mechanical engineering and geotechnics. A problem of particular interest is the transmission of waves across partially contacting interfaces and the ensuing filtering effect. In addition, analytical investigations and numerical simulations will be performed. The analytical studies will focus on the use of a displacement discontinuity model whereas the numerical simulation studies will be using conventional dynamic numerical codes including Finite Elements, Boundary Elements and Finite Differences. The results will focus on a better understanding of wave-fracture interaction and wave filtering effects of fractured (and dissimilar) media. They play an important role in the prediction of response behaviour of damaged and fractured structures. In particular, the integrity of structural components fabricated from materials weakened by fractures will be investigated and assessed. A particularly important application will be wave propagation through micro-crack damage material and the seismic stability of a jointed rock mass. When applied to geotechnology the results obtained will assist in the design of preventive measures for the reduction of damage and vibration control in seismically hazardous and blast disturbance effected urban regions.