Compression properties of z-pinned sandwich composites

Abstract

Sandwich composites are ultra-light weight structural materials consisting of thin face skins covering a lowdensity core. The skins are made of metal alloy sheet or fibre-reinforced polymer laminate, and provide the sandwich composite with high in-plane mechanical properties. A variety of low-density materials are used in the core, with the most common being metallic honeycomb, polymer foam, syntactic foam, Nomex and balsa. Sandwich composites are used in a large number of structural applications because of their high in-plane properties and light-weight, including in aircraft, ships, rail carriages and buildings. A major limitation of sandwich composites is their low through-thickness compression properties, which are determined by the compliant and weak core material. The compression modulus and strength in the through-thickness direction is less than 10% of the in-plane properties for most types of sandwich composites, and this has limited their use in structures that must support high throughthickness compression loads. A method to improve the through-thickness compression properties is z-pinning, which involves inserting high modulus, high strength pins through the sandwich material. The mechanical performance of z-pinned sandwich composites have not been thoroughly characterised, and there are few micromechanical models to predict their structural properties. Previous research has revealed that z-pinned sandwich composites possess improved damage tolerance [1, 2], and Cartie and Fleck [3] recently showed that the throughthickness compression strength of foam core sandwich materials is increased with fibrous composite or metal pins. However, the effects of the volume content and diameter of the pins on the compression properties has not been determined. This paper presents a study to determine the effect of z-pinning on the through-thickness compression properties of sandwich composites. Improvements to the through-thickness compression modulus and strength with increasing volume content and size of z-pins is determined by experimental testing, and a model for predicting the compression properties of z-pinned sandwich composites is presented. A sandwich composite consisting of fibreglass skins and a poly(vinyl chloride) foam core was reinforced with fibrous composite pins. The skins were a woven glass/ epoxy laminate and the core was a closed cell PVC foam with a density of 90 kg/m. The sandwich material was reinforced with thin (0.28 mm diameter) T300 carbon/ bismaleimide pins to volume contents of 0.5%, 2% and 4%. The material was also pinned using thick (0.51 mm diameter) T650 carbon/bismaleimide pins to the volume content of 2%. Aztex Inc. (Waltham, MA) produced the pins using a pultrusion process that aligned the carbon fibres in the lengthwise direction. The mechanical properties of the pins are given in Table 1. The z-pins were inserted through the sandwich composite using a hand-held ultrasonically actuated horn in a process described by Partridge et al. [4]. The process used is the standard z-pinning technique for reinforcing composite laminates and sandwich materials. A problem with this process is the difficulty in accurately inserting the z-pins in the orthogonal orientation, and the pins are usually offset at a shallow angle. This is a problem often encountered with the z-pinning process, and is difficult to avoid when the pins are inserted manually. Figure 1 presents a histogram showing the percentage of pins offset at different angles from the orthogonal direction in the sandwich composite. The pins are A. P. Mouritz (&) School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Vic 3001, Australia e-mail: adrian.mouritz@rmit.edu.au J Mater Sci (2006) 41:5771–5774 DOI 10.1007/s10853-006-0114-8