An investigation into damage initiation and propagation in carbon fibre reinforced plastics
In this investigation into the initiation and propagation of damage in CFRP materials, a method was first established whereby damage could be induced, during flexure, at the centre of thin plate-like cantilever specimens. An acceptable clamping shape was found using 3-D Finite Element modelling. This technique was also useful in deducing the types of stress that were responsible for delamination, and in assessing the fatigue resistance of various layups. Results were confirmed by Thermography and Scanning Electron Microscope results. Experimental results indicated that damage initiation during flexural fatigue occurred earlier for higher surface strains, and that subsequent delamination affected damping, natural frequency and static stiffness of specimens. There was also shown to be the possibility of the existence of a fatigue strain limit for the material. Experimental and theoretical work was extended to examine the condition of in-plane stress with superimposed dynamic excitation. Here, difficulty was experienced in designing experiments complementary to F.E. models. However, results predicted the direction of damage propagation and that the presence of in-plane stresses could be detrimental to fatigue life. A more refined F.E. model in which delamination could be opened between desired layers, indicated that shear stresses were greater the further the delamination was sited from the neutral plane. Since failure during experiments always occurred between the same two layers, this suggested that high shear stresses had a stronger influence upon crack initiation than small defects which might have occurred anywhere throughout the thickness. Acoustic Emission work demonstrated the existence of `micromechanical' damage, which was suspected to be fibre breakage. At a constant amplitude of dynamic flexural loading, emissions occurred initially, but after a certain time ceased almost completely. A hypothesis was suggested to account for such mechanisms, and their possible influence upon delamination.