The present study was started to examine the effect of ductile metallic fiber on the toughness and fracture behavior of engineering ceramics. Helical fibers were used for the purpose of better ensuring stress transfer from the matrix to the fiber than straight fibers.
A length of tantalum fiber 0.4 mm in diameter was formed into two types of helix. Helical or straight fibers were incorporated in a silicon carbide (SiC) matrix. The fibers were unidirectionally aligned to the longest axis of a specimen with uniform spacings. Specimen dimensions were 10 by 10 by 55 mm. Specimens were prepared by means of vacuum sintering. The volume fraction of the fiber was varied from 0.01 to 0.04. Three-point bending tests were carried out at room temperature in air. Measurement of the acoustic emission signal was made to investigate the fracture process of the composites. Results were compared with three kinds of specimens: helical fiber reinforced composites, straight fiber reinforced composites, and SiC matrix alone.
It was found that the fracture of the composites reinforced with tantalum fiber, whether straight or helical, was not so catastrophic as to be shattered to pieces like that of the SiC matrix alone. This arises because in fiber reinforced composites, fibers bridge the crack surfaces which have occurred in the matrix and carry the load which the matrix supported before fracture. But the fracture behavior of both types of composites differs in detail depending on their fiber geometry. The maximum load supported by the specimen after the matrix fracture was larger in the helical fiber composites than in the straight fiber composites. This seems to be due to the fact that the stress-transfer effect between the matrix and fiber was better for the helical fiber composites than for the straight fiber composites. Also, multiple fracture of the SiC matrix was observed in the helical fiber composites. Moreover, the total work done up to the complete fracture of a specimen, which was estimated from the load-displacement relation, was different by the fiber geometry and this was due to the differences of the energy absorption mechanism of the composites.