Critical compressive stress for continuous fiber unidirectional composites

Abstract

Dow and Rosen’s work in 1965 formed an intellectual framework for compressive strength of unidirectional composites. Compressive strength was explained in terms of micro-buckling, in which filaments are beams on an elastic foundation. While groundbreaking, the discrepancy between model predictions and observed compressive strength is well known. This study builds on Dow and Rosen’s model specifically with respect to the dominant shear mode instability. A new method that accounts for matrix thermal residual strain, matrix compressive and shear stresses, with associated reductions in tangent modulus due to matrix non-linearity, is proposed. The method is validated using a specific test case, which is conducted to precisely account for microscopic fiber alignment and matrix non-linearity. Accordingly, a method of measuring and accounting for a continuum of fiber misalignment is developed. Predictions are compared to literature values, with variations in fiber modulus, matrix modulus, and volume fraction. Good agreement is shown, both in terms of trend and magnitude. The approach successfully preserves initial in-plane shear stiffness, while showing that in-plane shear stiffness decreases significantly as compressive stress increases.

Theoretical and experimental compressive strength of a glass fiber – vinyl ester pultruded composite

“…Thompson et al . 6 proposed the Combined Stress Model to estimate the compressive strength of unidirectional continuous fiber composites. Their approach incorporated a fiber misalignment cumulative distribution function (CDF), matrix nonlinearity, thermal prestress, fiber elastic properties, and fiber volume fraction.…”

“…Thompson et al . 6 also supplied variable sensitivity plots. Hypothetically, the model suggested that a glass-resin composite of identical constituents as the test case, but with perfect fiber alignment, would have a compressive strength of 1.3 GPa.…”

“…8 Accordingly, this study addresses this theoretical prediction via the following steps: Prototype a continuously protruded glass-resin rod using the same constituents as the test case presented by Thompson et al . 6 Measure the fiber misalignment CDF within the pultruded rod. Estimate the rod compressive strength with the Combined Stress Model from Thompson et al. 6 Develop a test methodology for measuring compressive strength of the rod. Measure compressive strength of the rod. Compare and discuss model predictions and experimental results. …”

“…6 Measure the fiber misalignment CDF within the pultruded rod. Estimate the rod compressive strength with the Combined Stress Model from Thompson et al. 6 Develop a test methodology for measuring compressive strength of the rod. Measure compressive strength of the rod. Compare and discuss model predictions and experimental results.…”

“…Prototype a continuously protruded glass-resin rod using the same constituents as the test case presented by Thompson et al . 6…”