Biaxial flexural failure of woven composite plates investigated by the ring on ring bending test

Abstract

Abstract The ring on ring (ROR) bending test is an ASTM standard (C1499) for characterizing the equibiaxial flexural strength of ceramics. This simple test protocol induces biaxial flexure by applying an out of plane uniaxial load via a small loading ring, to a plate or disc shaped specimen, simply supported on a larger ring. The main objective of this paper is to apply this test method to analyze the biaxial flexural failure of woven fabric composite plates. Ring on ring bending tests are conducted on square shaped plate specimens of epoxy/carbon plain woven composites, until failure. Experiments reveal interesting similarities and differences when compared to ceramics. First it is seen that while for ceramics the bottom surface fractures under biaxial tension, for woven composites the top surface fractures under biaxial compression. The failure pattern however, is similar to ceramics, and consists of several radial kink bands which initiate near the center and propagate outwards towards the specimen edges. Subsequent numerical stress analysis of the test reveals that the stress state in the region of the plate encompassed by the loading ring is almost exactly equibiaxial. This is primarily because the test method induces pure bending and because the two in-plane principal moduli are equal to each other, and much larger than all the other moduli. This implies near axisymmetry of the stress state despite material orthotropy. This aspect is seen to enable a simple determination of the biaxial flexural strength using the in-situ measured principal strains. It also implies that this biaxial flexural strength could be a good approximation of the equibiaxial compressive strength for the woven composite, directly measuring which is usually a challenging task. Conditions for the suitability of this test method are explored via further numerical modeling.