A theoretical framework for the compressive properties of aligned fibre composites

Abstract

New experimental results have made necessary the reformulation of the theory for compressive strength. The new theory is based on the precept that a number of different mechanisms can cause composite ifailure. The active one in a particular situation is that which gives the lowest failure stress. Thus composite strength can be dominated by fibre strength when the fibres are ductile, or controlled by matrix yielding when the matrix is soft. Lack of linearity in the fibres has a very important effect also, as does the adhesion between the matrix and the fibres. Modulus is affected as well as strength. Governing equations are developed for six different mechanisms and the agreement with experiment is very good. It is concluded that to make composites with good compressive properties the fibres should be hard, as straight as possible and well bonded to the matrix. The matrix should have a high yield stress, tensile strength and compressive strength. Hybridization is useful tO improve the compressive strength of Kevlar composites, but should be avoided with brittle fibre systems due to unfavourable "hybrid effects".

Compressive Failure of Fiber Reinforced Composites

“…An interesting observation is that the same type of glass composite changes it's failure mode when tested under different loading ratios. This observation was also made by Piggott (1981) who found that the failure mode changed when the composites were tested under pure compression with partially cured matrix. The reason for the change in failure mechanism can be attributed to the magnitude of the shear modulus of the matrix and hence the composite.…”

“…The splitting failure mode in glass composites has been reported by Piggott (1981); Piggott and Harris (1980), Lee and Waas (1999) and . Splitting is a distinctly different failure mechanism from kinking.…”

“…Compressive splitting occurs when a preexisting flaw inside the composite specimen starts to grow when the specimen is under compression load. Splitting failure has been observed before by a number of researchers who examined the compressive strength of aligned glass fiber composites ( Fleck (1997);Piggott (1981); ). Yet, it is noticeably absent in carbon fiber composites.…”

“…Hence, when the torsional load is sufficiently high, beyond the magnitude required to cause shear yielding of the composite and the matrix, glass composites tend to fail by kinking instead of splitting. Similarly, when a resin is partially cured (as was the case with the experiments conducted by Piggott (1981)), it results in a matrix with a lower shear yield stress, inducing the glass fiber reinforced composites to fail by kinking instead of splitting, since, splitting would have required higher compressive stresses to cause failure.…”

“…Splitting failure mode has been observed in glass composites by Piggott (1981), Lee and Waas (1999) and . The micrographs of the no failed specimens as seen in Figure 4.11 and Figure 4.12 reveal that the fiber/matrix interfaces split during failure and the fibers are broken.…”