The compression strength of composites with kinked, misaligned and poorly adhering fibres

Abstract

Experiments carried out on pultruded fibre reinforced polyester resins show that, at moderate fibre volume fractions, the compressive strength of aligned fibre composites depends linearly on the volume fraction. The strength falls off when the fibre volume fraction, Vf = 0.4 with Kevlar and high strength carbon fibres; The effective fibre strength at Vf < 0.4 is much less than the tensile strength but it is close to the tensile strength with E-glass fibres and high modulus carbon fibres. Poor adhesion between fibres and matrix reduces the compressive strength, as does kinking the fibres when the fibre radius of curvature is reduced to below 5 mm. Misalignment of the fibres reduces the compressive strength when the average angle of misalignment exceeds about 10 ~ for glass and carbon fibres. However, with Kevlar no such reduction is observed because the compression strength of Kevlar reinforced resin is only a very little better than that of the unreinforced resin.

Compressive Failure of Fiber Reinforced Composites

“…Apart from the strain gage data, data was collected on the cross head displacement, cross head rotation, axial force and the torque acting on the specimen. As has been reported in the literature (Martinez et al (1981); Vogler and Kyriakides (1999)), the type of gripping influences the failure strength of the fiber reinforced polymer composite under compression. As can be seen in Figure 4.1 the rectangular block grips provide uniform continuous contact with the sides of the specimen.…”

“…(1.1). Several improvements and modifications have been suggested (Sadowsky et al (1967), Lanir and Fung (1972), Steif (1988), Waas et al (1990) and Lagoudas et al (1991)) to the elastic microbuckling model of Rosen (1965) but none of the models based on the elastic behavior of matrix could predict compressive stresses comparable to the experimental results, as reported, for example in Weaver and Williams (1975), Piggott and Harris (1980), Parry and Wronski (1981), Piggott and Harris (1981) and Martinez et al (1981). Argon (1972) came up with an important observation about the dependancy of the compressive strength of fiber reinforced composites on the misalignment of the reinforcing fibers.…”

“…The experimental work of Weaver and Williams (1975) revealed that the kink bands consisted of broken fibers and that the kinking compressive strength of carbon composites tested increased with increasing confining pressure. Piggott and Harris (1980), Piggott and Harris (1981) and Martinez et al (1981) in a series of compression experiments on composites reinforced with carbon, glass and kevlar fibers provided a new insight to the problem of compressive failure behavior of fiber reinforced composites. Piggott and Harris (1980), observed that the compressive strength of the fiber reinforced composites dropped with the reduction in the shear yield stress of the matrix.…”

“…They also observed that the failure mode in carbon fibers was kinking whereas the glass fiber reinforced composites failed by splitting. In another set of experiments reported by Martinez et al (1981), the effect of fiber matrix adhesion and misalignment was studied. They observed that the compressive strength is hnearly varying with fiber volume firaction, Vf for values of Vf less than 0.4 and beyond this fiber volume fraction the compressive strength was seen to drop below this linear relation.…”

Influence of Ply Waviness on the Stiffness and Strength Reduction on Composite Laminates

“…Fiber kinking failure is closely related to fiber waviness. Consequently, numerous investigations have been conducted on the kinking failure of initially misaligned fibers (Argon 1972;Budiansky 1983;Steif 1990aSteif , 1990bMartinez et al 1981). …”