Fatigue damage mechanisms in advanced polymer matrix composites

Recently, advances have been made in the design, manufacture and application of composite materials. A great deal of this progress has been made in the field of Fibre-Reinforced-Plastics (FRP). FRP often have greater strength to weight and stiffness to weight ratios than traditional materials such as metals, which makes them ideal for use in many application especially in aeronautical and aerospace sectors. For example Carbon-Fibre-Reinforced-Plastics (CFRP) are becoming more common in civil and military aircraft structures. However, there remain many unanswered questions regarding the behaviour of these materials especially under in-service conditions such as fatigue. There is an increasingly urgent need to gain an understanding of how FRPs behave. To fully understand the fatigue of a material it is necessary to gain an understanding of how damage initiates and accumulates and how the damage will affect the materials properties. It is also clear that to fully utilise FRPs it is necessary to be able to model the relationship between microstructural damage and the materials mechanical properties. This work has characterised the fatigue life of a quasi-isotropic carbon fibre reinforced composite, HS/919, at four R ratios. These are R=0.1 (tension-tension), R=+10 (compression-compression), R = -0.3 and R=-3.3 (both tension-compression). Those R ratios with a majority compression loading cycle experienced lower fatigue lives than those with a mainly tensile loading cycle. The work also highlighted the delaminations were a major damage mechanism. Post failure analysis of the fatigue specimens showed that the primary delamination, was occurring at different interfaces dependent on the loading cycle. With a mainly tensile loading cycle the delamination was occurring at the 0°/90° interface. While the mainly compressive loading cycle showed delaminations at the 0°/45° interface. This phenomenon was investigated using a modified mixed-mode bending technique developed by Reeder and Crews at Nasa. Static and fatigue tests were carried out on both the highlighted interfaces at three mixed-modes, MI/MII of 1/3, 1/1 and 3/1. Static tests showed that the 0°/45° interface was the weaker. In the fatigue tests two phenomena were observed, 1) that the strain energy release rate steadily decreased with crack length, 2) the strain energy release rate initially increased and then decreased. This is due to fibre bridging which was seen in both interface but was more apparent for the tests at the 0°/90° interface. There was a large amount of scatter in the fatigue data, especially at the 0°/45° interface. This made fitting a Paris type law to crack growth rates impossible for this interface. A Paris law was fitted to the 0°/90° data. It was hoped to transfer this knowledge to fatigue coupons with inserts.EThOS - Electronic Theses Online ServiceGBUnited Kingdo