JOURNAL OF MATERIALS SCIENCE 31 (1996) 1081-1085 Fracture of brittle multiphase materials by high energy water jets A. MOMBER* WOMA Apparatebau, Germany R. KOVACEVIC University of Kentucky, Center for Robotics and Manufacturing Systems, Lexington, KY, USA High energy water jets are established in processing brittle, inhomogeneous materials like rocks and concrete. Despite their wide field of application, the failure mechanisms of these materials, especially the influence of inclusions, are not well known. This work examines the influence of grain inclusions on the fracture behaviour of a multiphase brittle material exposed to high energy water jet processing. The behaviour of the specimens is detected by mass removal measurements, microscopical observations and the mercury penetration technique. It is found that the failure is based on microcrack growth due to hydrostatic pressure. The fracture mechanical behaviour of the reference material changes considerably with the addition of aggregates. The addition of grains leads to a reduction of the threshold tool energy for the start of mass removal. On the other hand, the presence of inclusions permits a more reduced and controlled removal progress. The interfaces between matrix and grains are the preferred locations for crack growth and also for crack branching. The inclusions act as crack arresters and crack branchers. In the case of cracks growing through the grains, a higher amount of fracture energy is absorbed and the fracture performance is weakened. 1. Introduction For many years, high energy water jet units have been competing regarding machining performances. They are the state-of-the-art for machining plastics and deburring metals, and they are widely used for removing concrete and cutting and drilling rocks [i]. The jets, as tools for material removal, are generated in nozzles. Inside these nozzles potential energy of high pressurized flows is transformed into kinetic energy of fast moving jets. The energy density of these jets is comparable with that of laser beams. On the surface of the machined material a stagnation pressure profile is created after the jet has hit it. This profile enables the water to penetrate into cracks, flaws and pores. Inside these instabilities the water flow generates forces on walls and flanks which results in stresses. If these stresses exceed critical values the instabilities start to grow and the material fails. General contributions to these topics were made in previous studies [2-4]. However, all these investigations did not consider the influence of inclusions and interfaces which are found in multiphase and composite materials. The importance of inclusions in conventional failure and several erosion modes was shown, among others, in earlier work [5-8]. Certainly, the interfaces between inclusion grains and the matrix where they are embedded - the grain boundaries - have a significant influence also on the material removal process in the case of water jet machining. The objective of this work is to compare the behaviour of a plain homogeneous matrix material with respect to a multiphase material which contains the matrix as one of its phases, during their cutting with pressurized water. Also of interest is the influence of the interfaces between the matrix and the embedded inclusion grains on the erosion characteristics of the brittle multiphase materials. 2. Materials and experimental methods 2.1. Materials Two different material groups are designed and investigated. In this work they are called "matrix" and "multiphase material". The matrix is a hardened mixture of water (w) and binding agent (b) in a ratio of w/b = 0.55. As the latter, a portland cement (PZ 35 F, DIN 1164)) was used. After mixing, this composition was cured and hardened for 28 days. The same procedure was followed for the multiphase material. In this case the mixture consisted of water (w), binding agent (b) and limestone grains (g). Fig. 1 shows the size distribution of the used inclusions. The relation between water and binder was changed depending on *Feodor-Lynen Fellowship holder of the Alexandervon Humboldt Foundation, Germany. 0022 2461 9 1996 Chapman & Hall 1081