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Composites are finding lot of applications in aerospace, automobile and many other sectors due to their high strength to weight ratio and longer fatigue life. For assembly or electrical wiring purposes, often hole(s) are drilled into the laminate thereby reducing its strength. The strength prediction and damage mechanics study is of great importance in such structural applications. In this work, a three-dimensional finite element based progressive damage model (PDM) is presented for unidirectional carbon fiber reinforced polymer (CFRP) laminates having two holes in different configurations subjected to tensile loading. The developed model is suitable for predicting failure and post failure behavior of fiber reinforced composite materials. The material is assumed to behave as linear elastic until final failure. The three broad steps involved in this study are stress analysis, failure analysis and damage propagation which are implemented as a PDM involving finite element analysis. Hashin's failure criteria for unidirectional fiber composite is used for the damage prediction. It utilizes a set of appropriate degradation rules for modeling the damage involving material property degradation method. Digital image correlation (DIC) experiment is also carried out to perform whole field strain analysis of CFRP panel with different hole configurations. Whole field surface strain and displacement from finite element prediction are compared with DIC results for validation of the finite element model. Load-deflection behavior as well as path of damage progression is predicted by both PDM simulation and experiment. They are found to be in good agreement thereby confirming the accuracy of PDM implementation. Effect of spacing between the holes on stress concentration factor (SCF) is also further investigated.

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The damage evolution in composite material is a complex phenomenon, comprising several interacting failure modes like matrix cracking, fiber breakage, debonding and delamination. Damage initiation, its propagation and ultimate strength prediction of composite structure is of paramount importance for developing reliable and a safer design and utilizing them as primary load bearing one. During service life, these structures get damaged and are often repaired for extending their service life. In the present work, a 3D finite element-based progressive damage model is developed for predicting the failure and post-failure behaviour of notched and repaired panel under tensile load. Failure initiation load, ultimate strength and failure mechanisms are investigated through the developed progressive damage model. The accuracy of developed finite element model is assessed by comparing its prediction with the experimental results obtained from digital image correlation technique and they are found to be in good agreement. In this study, the panels made of carbon/epoxy composite laminates of pure unidirectional and quasi-isotropic stacking sequence are considered. The damaged panel is repaired with both single- and double-sided circular patch of same parent material. Stress-based 3D-Hashin’s failure criterion is used for predicting the damage mechanism. Maximum shear stress and strain criteria are considered to account for patch debonding. It is found that the damage in notched panel always initiates with matrix cracking around the hole. However, damage in repaired panel is influenced by localized patch debonding.

Composites are significantly used in aerospace, automotive and civil structures due to their high specific strength, high stiffness, corrosion resistant and longer fatigue life. During service life, composite structures are susceptible to damage, which reduces their structural integrity. For extending its service life, the damage needs to be repaired. In case of low velocity impact damage adhesively bonded patch repair is found to be effective in extending the service life of damaged parts. The repair performance is mainly influenced by patch stacking sequence, patch shape, patch thickness, overlap length and adhesive thickness and its shear strength. In the present work, both numerical and experimental works are carried out to study the mechanics of composite patch repair on damaged carbon fiber reinforced polymer panel of configuration [45/−45/0/90]s subjected to tensile load. The influence of patch stacking sequence, patch thickness, adhesive thickness and overlap length on repair performance is investigated through a mechanics-based design approach involving finite element analysis. Stress concentration factor and shear stress in adhesive layer are considered to analyze the repair performance. Later, a genetic algorithm-based approach in conjunction with finite element analysis is implemented for arriving at an optimized patch dimension and adhesive thickness. Experimental study is then carried out with optimized geometry using non-contact optical-based technique namely digital image correlation. The strain field from digital image correlation is compared against the finite element results.

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In the present work, critical strain field in thin adhesively layer of double-sided (symmetrical) patch-repaired carbon fiber-reinforced polymer composite panel under tensile load is investigated using digital image correlation technique. Longitudinal, peel, and shear-strain distribution in adhesive layer is analyzed thoroughly in repaired panel by performing global cum local strain field analysis involving digital image correlation. Effective load/shear transfer length in repair configuration is estimated based on global strain analysis, and further, it is compared with the one predicted from finite-element analysis. Localized strain analysis using magnified optics provides higher resolution, and it is found useful in revealing complex strain field in small but critical zones responsible for failure initiation. The global and local-strain analyses are found complementary to one another, and therefore both are essential to fully characterize the strain field in thin-adhesive layer. The critical failure mechanism is also investigated and correlated with the load–displacement behavior. DIC is found to be suitable and accurate for analyzing the global and local strain field over small but critical locations and helps in predicting the damage-initiation location based on strain anomalies. Finally, the experimental results are compared with the numerical predictions, and they are found to be in good correlation.