Biaxial flexural failure of woven composite plates

Biaxial flexural stress states are frequently encountered in thin composite structures used in a wide variety of engineering applications. For reliable operation and efficient design of these thin engineering structures, it is essential to characterize and understand their failure behavior under biaxial flexure, among other stress states. Despite a large body of research into the biaxial strength of fiber reinforced composites, studies involving biaxial flexural failure of plates are comparatively less common. Existing literature shows that several testing standards may be found for isotropic brittle materials; however, no standard test for anisotropic composite materials has been reported. One of the most commonly used methods for biaxial flexure testing of isotropic materials involves supporting a disc or plate shaped specimen on a circular ring and applying an out of plane loading at or near the center. The out of plane loading is applied either through a ball (ball on ring, BOR), or by applying uniform pressure (pressure on ring, POR) or via a smaller circular ring (ring on ring, ROR). The ROR test, being the axisymmetric analog of the four-point bending test, is particularly popular and is in fact an ASTM standard (C1499) for ceramics and has also been applied to glass, dental ceramics and also concrete.

Plate bending tests are not usually common for anisotropic fiber reinforced polymer matrix composites. To advance further the knowledge in this field, researchers from the Stony Brook University: Daniel Deland, Zongyan Zhang and led by Professor Kedar Kirane performed an in-depth analysis of the biaxial flexural failure of woven fabric composite plates. Their work is currently published in the research journal, Thin–Walled Structures.

They selected the ROR bending test for this purpose. 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 researchers focused on applying this test method to analyze the biaxial flexural failure of woven fabric composite plates.

The authors reported that their work revealed interesting similarities and differences when compared to ceramics. To be specific, it was 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, was reported to be similar to ceramics, and consisted of several radial kink bands which initiated near the center and propagated outwards towards the specimen edges. Further, subsequent numerical stress analysis of the test revealed that the stress state in the region of the plate encompassed by the loading ring was almost exactly equibiaxial. This implied near axisymmetry of the stress state despite material orthotropy, which enabled a simple determination of the biaxial flexural strength using the in-situ measured principal strains. It also allowed approximate determination of the equibiaxial compressive strength for the woven composite, directly measuring which is usually a challenging task.

In summary, the study demonstrated the successful application of the ring on ring bending test to analyze the biaxial flexural failure of woven composite plates, which are orthotropic. Further conditions for the suitability of this test method were explored via further numerical modeling. In a statement to Advances in Engineering, Professor Kedar Kirane said their work is the first successful application of the ROR test to orthotropic materials, such as woven composites. The results show significant potential of the method for easy measurement of the equibiaxial flexural, and equibiaxial compressive strengths of woven composites.