Fluid jet erosion as a non-linear fracture process: a discussion

Abstract

Fluid jets are increasingly used to process and machine strain-softening materials. Typical applications are drilling, cutting, fragmentation, hydrodemolition and 3D-machining. Strain-softening materials, such as cement-based composites, ceramics, most rocks, ice and solidified impurities, are quasi-brittle behaving materials characterized by a so-called fracture process zone. Therefore, linear elastic fracture mechanics do not suitably cover these materials. Based on a literature review, it is also found that conventional strength parameters, such as compressive strength, cannot describe the resistance of this group of materials against fluid jet erosion, and that the erosion process is strongly determined by the size of the aggregate (inclusions) in the materials. Also, structure and behavior of non-linear behaving materials are characterized. An SEM-study performed on specimens of concrete, mortar, and hardened cement paste eroded by waterjets with velocities up to 470m/s clearly illuminates features of quasi-brittle behavior, including grain bridging, microcracking and crack branching. It is concluded that a non-linear fracture resistance parameter may be suitable to estimate the erosion resistance of the materials. It is found that the characteristic length of a material is a very suitable resistance parameter. An already existing approximate empirical formula for estimating this length is used, and a very good relationship between the estimated values and the volumetric erosion rate taken from published results for 20 different types of concrete could be obtained. Finally, the application of the proposed resistance parameter for fracturing and drilling rocks and refractory ceramics by plain and abrasive-laden waterjets is checked by discussing results found in the literature.