Effect of gap on strengths of automated fiber placement manufactured laminates

Tensile and compressive tests of automated fiber placement (AFP) manufactured quasi-isotropic laminates with repeating embedded gaps or overlaps are conducted and reveal significantly lower tensile and compressive strengths compared to those of normal laminates in spite that the gaps and overlaps cut no fiber. The reduction in tensile strength is greater than that of the compressive strength. A two-dimensional mechanical model of the quasi-isotropic laminates with an arbitrary inclined gap is proposed and analytically solved using a super-position technique as the sum of a global uniform stress field that satisfies equilibrium and compatibility con-ditions by applying a virtual force at both ends of the gap anda local stress field caused by the virtual force. The local problem is approximately solved using a Green function technique. The stress and strain fields under a unidirectional load are obtained in a closed-form expression in which the effect of the tapered portions typi-cally observed at both sides of the gap is incorporated. The present analytical solution agrees with the finite element solution. The derived expression shows that significant stress concentration occurs at the end portion of the gap.