Post-impact compression behaviour of continuous fibre composite materials
Compression-after-impact testing is widely used to assist in the development and selection of materials for aircraft applications. Presently, there are no standard test methods in existence. The most widely used industrial tests require large specimens which are expensive to manufacture and test. The results of an experimental study of the compression-after-impact test are reported. A miniaturised testing arrangement was used to investigate the effects of specimen width, thickness and lay-up on the measured compression strength of undamaged and impact damaged specimens. A toughened carbon I epoxy was used for the above work. In addition three other materials were tested (a carbon / polyetheretherketone (APC), a glass I epoxy (GRP) and another carbon / epoxy). The in-plane extent of delamination damage after impact was measured using an ultrasonic C-scanning method. The carbon and glass reinforced epoxy materials had similar resistance to the initiation and propagation of impact damage. The APC was much more resistant to the formation of impact damage. The measured strength of undamaged specimens was dependent upon specimen geometry, decreasing with width increase and increasing with thickness increase. The strength of impact damaged specimens was independent of width. Increasing the thickness increased the incident impact energy required to initiate damage and, therefore, delayed the onset of residual strength reductions. The strength of undamaged quasi-isotropic and 0/90 laminates was very similar and higher than for ±45 laminates. After impact the 0/90 material was strongest. The residual strength of the quasi-isotropic and ±45 materials were very similar. The APC retained the highest proportion of its initial strength over a range of incident impact energies. This was attributed to its resistance to the formation of impact damage. The GRP was the most damage tolerant material.