Effects of an Open Hole on the Biaxial Strengths of Composite Laminates

Experimental biaxial data are essential to validate failure criteria for composite laminates. In this study an optimized design of cruciform specimen used for in-plane biaxial loading was proposed. The uniformity of strain field in the gauge region of the smooth specimen under biaxial loadings was confirmed via finite element analysis. Both smooth and open-hole specimens fabricated by π/4 quasi-isotropic carbon epoxy laminates were tested under three different biaxial stress schemes, where stresses {σx, σy} are expressed in normalized forms as {1, 0}, {1, 1}, and {1, -1}, respectively. The relations between far-field stresses and global biaxial loadings were established based on test data, and the strengths of both smooth and open-hole specimens under different biaxial loading schemes were acquired. Digital image correlation (DIC) technique was employed to measure strain field in the neighborhood of the central hole of the open-hole specimens during test, and the measurements were compared with strain distribution obtained from numerical simulations. Good agreement between DIC images and FEA results was achieved. First-ply-failure (FPF) envelopes based on micro-mechanics of failure (MMF) for both smooth and open-hole cases were drawn and compared with test data.