The Effect Of Staking Sequence On Low Velocity Impact Damage Tolerance And The Post Impact Compression Properties Of Carbon Epoxy Laminated Composites

Abstract

In this study, the effect of impact damage tolerance and the ply orientation stacking sequence was studied to evaluate laminated carbon epoxy composites; one was cross-ply, and the other was quasi-isotropic. This research will be focusing on the low velocity impact characteristics. The low velocity impact studies are very significant to the aviation industry since there may be significant damage through-the-thickness before any surface damage is present. The entire structure will eventually be deform during contact as the low velocity impact especially with a large mass will result in global response in the structure. This may result in internal damage that may cause the structural integrity to occur and may suddenly cause the structure to fail. The in-plane compression properties of the specimens of the composite panels were measured after impact testing. Chapter 1 in this thesis outlines the aim of this research. This research aims to investigates the effect of stacking sequence on low-velocity impact damage tolerance of composite laminates by comparing cross-ply and quasi-isotropic orientations. The effect to stacking sequence to the absorption of the impact damage to these multidirectional composite were studied. This research also investigates whether the key parameters such as the impact energy and laminate ply stacking pattern are affecting the impact damage resistance and the post-impact compression strength and modulus properties of carbon/epoxy laminate composite. A comprehensive and critical review of the scientific literature review regarding the mechanical properties and failure mechanisms of the composite laminates in through thickness is presented in this thesis. The various properties of composites are reviewed, including their material, ply-stacking sequence, low velocity impact tolerance, compression after-impact properties and the mode of failure. The literature review which is presented in vii chapter 2 reveals that there were published information regarding the study on improvement to increase the in-through-thickness strength of composites. The chapter 2 also presents the literature review on the barely visible impact damage (BVID) conditions to the composite in the aviation environments due to low velocity impact damage. The methodology and the process flow of the research were presented in chapter 3. And, the result of this study was presented in chapter 4. Both of the composites were impact damaged with energies 0J, 8.5J and 25.5J with low velocity impact. The compression test was also done following the impact test. This is because the objective of this study are to presents an experimental study of the effect of manipulation of orientation in plies stacking towards the low velocity impact damage tolerance of the quasi-isotropic and cross-ply laminates carbon/epoxy composite materials and the post-impact compression properties. Based on the conclusion that the research derive in chapter 5, the cross ply is better in term of the low velocity impact damage tolerance based on the tests result on the chapter 4. This suggests that for containment of damage, laminates with abrupt changes in fibre direction should be avoided. The research also reveal two distinct energy regions in the low velocity impact where increasing the ply orientation provide no improvement to the damage resistance in the low energy region and a slight improvement in the high energy region. This behavior thereby provides no significant improvement to the low velocity impact tolerance against barely visible impact damage (BVID) condition. The compressive properties were also prove to be inter-related with the stacking sequence. This research also recommend that the in-through-thickness strength should be improve to increase the low velocity impact tolerance as Z-pin reinforcement is the most potential reinforcement to be used in improving this situation.