Impact damage tolerance of carbon fibre reinforced plastics

Thesis

Carbon fibre reinforced plastic (CFRP) is a composite composed of very high strength and stiffness carbon fibres within a polymer matrix. Low mass and cost make CFRP attractive for aerospace applications. At present application of CFRP is limited by the poor residual strength and fatigue properties of the material following impact damage, and because of the limited understanding of the problem.

Damage to CFRP results in a combination of fibre fracture and matrix cracking. The strength of CFRP perpendicular to the fibre direction is relatively poor and so the material is generally used as a laminate of unidirectional plies of different fibre orientations. Laminates are particularly prone to delamination, this is matrix cracking which separates plies.

In this thesis techniques for measurement of damage type and severity in CFRP are evaluated. These are used to quantify the distribution of damage resulting from hard body impacts at high, low and quasi-static incident velocities. A composite material damage model in a dynamic finite element analysis computer program (DYNA 3D) was used to predict the experimentally measured impact damage. However the prediction was found to be insufficiently accurate for reliable use by designers. Recommendations are made for improvements in the numerical modelling of impact damage in CFRP.

Mechanical tests that provide data on the effect of impact damage on the residual strength of CFRP specimens are reported. These show that providing impact damage is limited to delamination, with little fibre fracture ocurring, the compressive residual strength will be more severely reduced than the tensile property. Strain gauge measurements show that this effect is caused by local compressive buckling of the delaminated plies in the impact damaged area. Lquivalent hole sizes, causing the same residual strengths as in the impact damaged specimens, are given.

Cyclic load tests were conducted providing stress-life data for impact damaged specimens of CFRP. The data fit a stress-life diagram divided into three scatter band regions: static fracture; cycle dependent damage growth; and infinite life. Qualitative and quantitative observations are given of the progression of damage in a fatigue after impact damage test. Strain gauge and force/extension measurements show the presence of creep and energy absorption in undamaged and impact damaged CFRP. Much of the observed behaviour may be explained by a viscoelastic model. Because of the presence of viscoelastic creep it is suggested that the fatigue mechanism must depend on interacting cycle dependent and time dependent mechanisms.

Authors: Symons, D

• DOI: 10.5287/ora-wvb4n2xjx
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford