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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Factors effecting the failure mechanisms in compression after impact of continuous fibre composites

Author: KAY, Marcel

Year: 1995

Supervisor(s): Paul Hogg

Damage tolerance is of particular interest to those manufacturing and designing with composite materials as it provides a means of categorising materials according to their resistance to damage propagation. However, the mechanisms of failure of a material when damage is initiated and then propagates are complex and therefore a study of damage tolerance must cover a wide range of issues. Various factors can influence the failure mechanisms and these can be as diverse as different environments and different fibre systems. The following study attempts to answer some of the questions surrounding damage tolerance. Research is focused on the compression after impact test as this is widely accepted by the aerospace industry to measure the damage tolerance of composite materials.
Progress has been made into determining the exact mechanisms of damage development in compression after impact. This included whether compressive failure results from the propagation of delaminations which then cause ultimate failure by buckling (slow delamination growth with final sudden buckling) or the alternative of sudden failure due to stress concentrations at the damage zone. In one case delamination growth is the cause of ultimate failure while in the other it is a consequence of ultimate failure. Different fibre forms were assessed and these included woven non-crimped fabrics and continuous unidirectional fibres. Changes in the damage state after impact and subsequent compression were noted for the non-crimped fabrics as compared to laminates made from unidirectional fibre prepregs.

Importance is also placed on how the mechanisms of failure may be altered by environmental exposure.