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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: Significance of damage in composite materials

Author: ARAMAH, Simon

Year: 2001

Supervisor(s): Paul Hogg

A quasi-isotropic glass/epoxy Non-Crimp-Fabric (NCF) based composite material was chosen to conduct this work. This novel fabric preform represents one of the current methods in improving composite damage tolerance. Three basic in-plane loading conditions were prescribed for this research, tension, compression and in-plane shear. These were considered to be adequate to describe material behaviour. Development of a test method to evaluate the effects of damage upon in-plane shear properties was also undertaken during this study.

Two damage conditions were applied, circular through thickness notches and impact damage. The circular notches represented the most extreme form of damage, whilst impact damage is the condition most harmful to composite materials. Environmental effects were also sought, whereby specimens were immersed in distilled water @ 90oC for six months, then tested in the above mentioned loading conditions, in both undamaged and damaged states.

A methodology was developed, based on the Whitney and Nuismer stress fracture criteria, which enabled predictions to be made upon the residual strength of an NCF containing damage under the prescribed loading conditions. A novel method, involving a basic mechanical test and composite laminate theory was developed to assess the stiffness properties of an impact damaged NCF.

The glass/epoxy NCF composite exhibits levels of damage tolerance due to its inherent structure, which inhibits complete damage propagation through the thickness. Notched NCF's reveal greater strength loss in tension than in either compression or shear, due to the significant stress magnification effects and reliance upon fibre properties. Impact damaged NCF's are susceptible to greater strength loss in compression than in either tension or shear. The NCF was observed to behave according to Fickian moisture absorption laws. Harsh environmental exposure nullified the effect of impact damage upon strength, whilst reducing the effect of notches for all loading conditions.