Graphite/epoxy laminate coupon testing programme

Abstract

This paper presents a summary of some of the work undertaken at the Australian Defence Science and Technology Organisation Aeronautical Research Laboratory in a programme to study graphite/epoxy materials and components. The paper covers work in the areas of fatigue behaviour of impact-damaged material, moisture absorption by graphite/epoxy material and details of damage inspection and damage growth behaviour in this material. Work has been undertaken to investigate the effects of spectrum modifications on the fatigue-life behaviour of impact-damaged coupons manufactured from XAS-914C as a 56 ply laminate. Impact damage of 8 J absorbed energy was introduced into coupons which were then subjected to compression-dominated spectrum fatigue loading using a number of modified FALSTAFF loading sequences. It was found that for coupons tested under ambient conditions, the removal of low-load excursions and the introduction of 10 s holds at high load levels in the spectrum had no significant effect on thefatigue life. The fatigue performance under hot/wet conditions was significantly degraded with the most severe effect being due to the incorporation of holds at high load levels. The paper also describes experimental work carried out to assess the moisture absorption characteristics of several graphite/epoxy laminates under a range of environmental conditions. The effects of thermal spiking on the moisture absorption characteristics was also investigated. Thermal spiking was found to be a significant factor in moisture absorption behaviour when the laminate had a moisture gradient and when the moisture level at a particular location in the laminate was above a minimum level which was determined to be approximately 1% moisture for AS4/3501-6 laminates. Another important part of the present investigation described in this paper deals with the assessment of damage in graphite/epoxy laminates. An NDE technique developed in the laboratory is described and was used, together with optical microscopy, to study the configuration of impact damage. Preliminary observations of impact damage and damage growth are presented.