R, Srilakshmi
(2014)
Experimental and Numerical Investigation of Adhesively Bonded Composite Patch Repair of an Inclined Center Cracked Aluminium Panel under Static and Fatigue Load.
PhD thesis, Indian Institute of Technology Hyderabad.
Abstract
Composite patch repair is gaining importance in extending the fatigue life of an aging aircraft. An aircraft, during its service life, it is subjected to severe structural and aerodynamic loads which results from repeated landings and take off, fatigue, ground handling, bird strikes and environmental degradation such as stress corrosion. However due to limited budgets and escalated procurement costs in replacing the aircraft, aircrafts service life need to be extended beyond their design life. Hence, a reinforcement or repair of damaged aircraft is essential to improve its service life. Among various available repair techniques bonded composite repair is mostly preferred. There is lot of research carried out in the safety and life prediction of composite patch repair applied on straight center cracked panels under in plane tensile load. In field, always cracks that appear on structures are of mixed mode and therefore, it is necessary to study the behavior of composite patch repair applied to inclined center cracked panels under in plane tensile static and fatigue loads.In the present work, a three dimensional finite element analysis (FEA) is carried out to study and compare the performance of single and double sided patch bonded over an cracked aluminum panel (2014-T6) having an inclined crack at 45˚. Carbon fiber reinforced polymer (CFRP) is the patch material chosen as part of this work. From FEA based study, it has been found that in case of single sided repair the stress intensity factor (SIF) at the unpatched surface tends to be higher than that of the unrepaired panel SIF. This is due to additional bending load arising due to shift in neutral axis after repair. Further, there are different parameters such as patch lay-up, patch thickness and patch shape and dimensions which affect the performance of the repaired panel. Out of them, patch shape plays a major role on SIF reduction. A detailed finite element based study has been carried out to arrive at the effective patch shape. Later, a genetic algorithm (GA) based optimization technique is employed in conjunction with FEA to arrive at an optimum patch dimensions resulting in higher reduction in SIF near the crack-tip. Further, to predict the whole field strain over the patch surface and also the shear strain distribution over the thickness of adhesive layer, an experimental investigation has been carried out using digital image correlation (DIC) technique. Lastly, three dimensional fatigue analysis using FEA has been conducted to study the crack growth in repaired and unrepaired panel. DIC is also effectively used to monitor the crack growth during the fatigue loading. The obtained experimental results have been compared with FEA estimates for their accuracy and they are in good coherence. It is found that the static strength and fatigue life of double sided repaired vii panel is higher than single sided repaired one. The utility of DIC as an accurate experimental technique for whole field strain prediction in repair applications is shown and turned out to be accurate when compared with FEA prediction thereby recommended for repair studies.
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