Improvement in fracture toughness of an epoxy/copper composite through the use of various end shaped fibers

Improvement in composite fracture toughness can be achieved by using short ductile fibers with shaped ends which utilizes the plastic work potential of the fiber volume through anchoring of the fiber end into the matrix. Single fiber pullout tests performed on copper wire demonstrates that a 2D flat-end-impacted fiber geometry that is easier to produce improves the fracture toughness increment at least as well as a 3D "axisymmetric" end-impacted fiber. Calculations based on pull out tests and a model for predicting the fiber contribution to the fracture toughness increment "DeltaG" show a 46% higher DeltaG for the flat-end-impacted fiber compared to a straight fiber at 0degrees orientation. Results further indicate that for a given fiber geometry there is an optimum end volume; above or below this volume results in a lower DeltaG. Due to the small fiber end volume, the packing density of the fibers will not be significantly affected by the end shaping of the fiber. Annealing and subsequent oxide removal of the end-impacted fiber is not necessary because there is no significant improvement in the DeltaG. However, a moderate 3.5-15% decrease in the debonding force is seen in the annealed fibers, depending on the embedment depth. This indicates that annealing weakens the fiber-matrix bond. Furthermore, results indicate that a fiber that has multiple mechanical interlocks in the matrix is not as effective in fracture toughening as single anchoring of a shaped fiber end. However, the load displacement curve for the light rippled fiber shows a unique "wavy" behavior related to the geometry of the fiber end which may be useful in other applications. Round fiber ends produced with an acetylene torch had a 70% lower DeltaG compared to a straight fiber because of the large fiber end volume and the poor microstructure resulting from the high torch temperature. (C) 2003 Elsevier B.V. All rights reserved.