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A study was undertaken to evaluate the fracture behavior of sand and clay-filled polyester composites. A two-pronged effort was made (1) to characterize the fracture toughness of sand-polyester and sand-clay-polyester composites and (2) to identify the mechanisms which account for fracture resistance in these composites.
Separate laminates of sand-resin and sand-clay-resin were made using a polyester resin, sand (300-μm mesh), and clay (0.5- to 3-μm mesh) particulates. The volume fraction of the constituents were 30/70 and 23/7/70 in the laminates, respectively. Three-point bend notch tests were conducted to record the load-deflection response of the composites. Fracture energy was calculated based on linear elastic fracture mechanics. Results show that the fracture energy of the sand-polyester composite was about 25% greater than that of the sand-clay-polyester composite. Fractographic study was undertaken using a scanning electron microscope to determine the mechanisms which contribute to fracture resistance in the two composites. This evaluation was considered critical to evaluate the role of the sand and clay fillers on the overall fracture behavior.
In the sand-filled specimens, fracture steps were observed behind the particles, which are characteristic of the pinning mechanisms. These features arise from the crack front breaking away from the pinning position, that is, from the rigid sand particles. The toughening mechanism in this composite is essentially attributed to the crack-pining phenomenon and subsequent interaction of various crack fronts. In this case, the particles act as obstacles to the moving crack front, and additional energy is required to break away from the pinned positions, contributing to higher fracture energy. This increase in line energy of the crack front is analogous to the line-tension effect due to dislocation pinning in metals. However, in the sand-clay-polyester composite, the pinning mechanism appears to be absent. Rather, the fracture surface is rough and is reminiscent of quasicleavage-type damage. Fracture energy in this case may be related to the size and density of cavities. The toughening mechanism in this type of hybrid particulate-filled composite is not well understood and requires further investigation.
fracture energy, particulate composite, crack-pinning mechanism, fracture step, crack propagation
Principal research scientist, Battelle Memorial Institute, Columbus, OH