An investigation of the mechanical performance of Z-pin reinforced composites

Abstract

Fibrous composites, having excellent mechanical properties in the direction of the fibres, have lower mechanical properties in the through thickness direction, controlled by resin. Z-pinning improves the delamination toughness (up to 500%) with a relatively modest reduction to the in-plane mechanical properties (typically 5–15%). This experimental study investigates the mechanical performance of Z-Pins bridging an existing delamination in fibre reinforced resin composites under pull-out (Mode I), shear-out (Mode II) and mixed mode loading conditions using a specially designed testing rig. In Mode II the opening displacement was restricted and measured by springs of three different stiffnesses. A new technique of needle assisted Z-Pin insertion was developed, in which prepreg panels were perforated with a steel needle in order to insert Z-Pins. This technique ensured the desired orientation of Z-Pins, improved pinning quality and removed the necessity of costly preforms used in the traditional UAZ method.Test specimens were blocks (15 mm x 15 mm x 6mm thick) of carbon-epoxy IM7/8552 composite in unidirectional (UD) and quasi-isotropic (QI) stacking sequences, with PTFE delamination film in the mid-plane recreating an existing crack, bridged with a single T300/9310 Z-Pin or a group of four pins of either 0.28 mm or 0.51 mm diameter. Three phases of pull-out were identified: Linear Phase (linear force-displacement curve), Crack Formation (unstable crack propagation phase) and Frictional Sliding (friction-controlled pull-out). Two phases of shear-out were identified: Linear Phase (with no energy loss) and Breaking Phase (where the fibrous structure of the Z-Pins is fractured, ending with Z-Pin breakage). In mixed mode specimens behaved similarly to pull-out for the pin angles up to 45°. For higher angles the behaviour was more similar to pure shear-out. The influence of the Z-Pin diameter, z-pinning depth, distance between adjacent Z-Pins, composite stacking sequence and pull-out speed on the Z-Pins behaviour were investigated. The results will be useful in the formulation of improved Z-Pin bridging laws for use in finite element models.