The use of digital image correlation to monitor delaminations in composite structures.

Abstract

The range of applications for composite materials is growing, but understanding of the effect of defects is limited, as is the ability to detect them. Versatile non-destructive testing (NDT) techniques, that can be deployed rapidly and reliably, to detect and monitor damage in composite components are vital to the continued growth of this sector. This research investigates the application of Digital Image Correlation (DIC) as an NDT technique for monitoring delamination defects in Fibre Reinforced Polymer (FRP) composites. The research has matched physical experiments with Finite Element (FE) modelling. Four types of glass-fibre reinforced epoxy matrix composite structural elements were designed and manufactured to assess this application of DIC. The first two types of structural elements were flat coupons containing fully embedded delaminations, artificially introduced using two different methods. It was found that by placing these specimens in three-point bending, near surface delaminations, one ply below the surface being monitored, would cause a plateau in the surface strains. This plateau in strains was used to measure the embedded defect sizes. The size of the delamination was consistently overestimated from the interpretation of the strain fields. This was improved with the assistance of FE modelling to identify the relationship between the feature and the delamination. Pulse thermography was found to be a better technique for measuring the size of these defects. The third type of specimen was a flat coupon containing a milled-slot, which was fatigued to grow a delamination at the foot of the milled slot, and the delamination measured visually. For this specimen, the DIC results showed good correlation with the visually determined delamination lengths with an empirical fit applied to the strain results. Both lock-in thermography and pulse thermography were used to measure the delamination size of the same specimens and showed reasonable correlation with the visually determined delamination lengths. Finally, tubular specimens containing embedded PTFE delamination-defects were fatigued at different ratios of tension and torsion. DIC of the specimens loaded at the fatigue load ratio at which the delaminations were grown could not be used to quantify the size of the delaminations. The work has shown that DIC can be used to monitor delaminations in some structural elements, however the type of loading needs to be considered to ensure sufficient influence on the surface strains to enable strain features that can be used to measure the size of delamination