In this paper, a traction-separation-based cohesive modeling approach is proposed to predict the effect of z-pinning on laminated composites. A detailed experimental characterization of the z-pin pullout process using the flatwise tension test is presented. Utilizing these flatwise tension results, numerical simulation of the progressive damage due to delaminations in a double cantilever beam with z-pinning has been performed. Experimental details of the z-pinned double cantilever beams are presented for IM7/977-3 graphite/epoxy. The approach taken in this study utilizing the cohesive elements within the Abaqus® finite element software has proven that the models can predict the behavior of z-pinned composites close to experimental observations. It was found that the discretization of the fracture resistance curve along the z-pin field is essential to capture the dynamics of the delamination accurately.

An optical fiber-based full-field strain measurement technique was used to investigate delamination growth in laminated composites. An experimental setup to load the test samples under idealized modes of delamination was used to investigate the ability to capture the shape and location of the delamination front. It is envisioned that the demonstrated approach has significant field applications in controlled laboratory settings where delaminations have to be located accurately. Furthermore, the ability of this measurement system to provide full-field strain measurements at any given preimplanted location through the thickness overcomes the surface strain measurements obtained by digital image correlation. In order to demonstrate the technique, distributed fiber optic sensing is used to monitor the propagation of delaminations under pure mode I and II loading. Optical fibers were embedded one ply from the crack plane of both double cantilever beam and end notch flexure specimens. To establish a repeatable fabrication methodology, manufacturing techniques for embedding the optical fibers during the laminate layup process were established. Specimens with and without embedded fibers were tested to verify the fibers did not affect measured fracture toughness values. Crack lengths measured with the optical fibers compared well with true crack lengths, and measured strain distributions compared well with results from finite element analysis.

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This study investigates the feasibility of using embedded optical fibers in polymer matrix composite laminates to characterize delaminations caused by low-velocity impacts with energies between 30 J and 50 J. Impact damage can occur in composite structures during manufacture, in-service, storage and routine maintenance. Because of their small size and light weight, optical fibers can be embedded in composite structures during the manufacture of composite parts, allowing the structure to be monitored for impact-induced delaminations without being removed from service. In this study, optical fibers are embedded in a grid configuration at four selected locations (one-third from impact surface, midplane, two-thirds from impact surface, and farthest ply from impact) in thick autoclave-cured graphite/epoxy laminates. Low-velocity impact testing is performed at four energy levels. Manufacturing procedures for embedding the optical fibers within the composite laminates are investigated. The strain distribution from the optical fibers is correlated with ultrasonic C-scans of the laminates in which they are embedded. X-ray computed tomography scan images are also compared to those from ultrasonic C-scans. Results indicate that embedded optical fibers can provide post-impact strain responses and delamination area from each embedded site within the impacted laminates.

In this study, a new approach for predicting damage and specific failure modes in laminated fiber reinforced composites is presented. The new method is based on the peridynamic theory and models individual plies, and represents fiber and matrix materials in each ply explicitly. These features enable analysis of laminates with arbitrary fiber orientation in a convenient manner. Additionally, a new failure mode identification algorithm has been developed and implemented. Instead of the conventional peridynamic damage parameter, the new algorithm works with individual broken bonds, which makes identification of different failure modes including matrix cracking, fiber breakage, and delamination straight-forward and unambiguous. The new peridynamic approach is demonstrated by considering the low-velocity impact damage on composite laminates with and without translaminar reinforcements. The translaminar reinforcement technique considered in this study is z-pinning; two different geometric configurations of z-pins are explored. The impact testing and the post-impact nondestructive evaluations with ultrasonic c-scans are performed at the Air Force Research Laboratory to characterize the delaminations. The impact tests on different samples are simulated using the current peridynamic approach. The predicted impact damage failure modes are compared against the experimental measurements. The new approach is shown to capture low-velocity impact damage both quantitatively and qualitatively.