Journal of Composite Materials

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.

Three-dimensional fiber-reinforced foam cores may have improved mechanical properties under specific strain rates and fiber volumes. But its performance as a core in a composite sandwich structure has not been fully investigated. This study explored different manufacturing techniques for the three-dimensional fiber-reinforced foam core using existing literature as a guideline to provide a proof of concept for a low-cost and easily repeatable method comprised of readily available materials. The mechanical properties of the fiber-reinforced foam were determined using a three-point bend test and compared to unreinforced polyurethane foam. The foam was then used in a sandwich panel and subjected to dynamic loading by means of a gas gun (10³ s⁻¹). High-strain impact tests validated previously published studies by showing, qualitatively and quantitatively, an 18–20% reduction in the maximum force experienced by the fiber-reinforced core and its ability to dissipate the impact force in the foam core sandwich panel. The results show potential for this cost-effective manufacturing method to produce an improved composite foam core sandwich panel for applications where high-velocity impacts are probable. This has the potential to reduce manufacturing and operating costs while improving performance.

The need for accurate material models to simulate the deformation, damage, and failure of polymer matrix composites under impact conditions is becoming critical as these materials are gaining increased use in the aerospace and automotive communities. To attempt to improve the predictive capability of composite impact simulations, a next generation material model is being developed for incorporation within the commercial transient dynamic finite element code LS-DYNA. The material model, which incorporates plasticity, damage, and failure, utilizes experimentally based tabulated input to define the evolution of plasticity and damage and the initiation of failure as opposed to specifying discrete input parameters such as modulus and strength. The plasticity portion of the composite constitutive model is based on an extension of the Tsai-Wu composite failure model into a generalized yield function. For the damage model, a strain equivalent formulation is used to allow for the uncoupling of the deformation and damage analyses. For the failure model, a tabulated approach is utilized in which a stress- or strain-based invariant is defined as a function of the location of the current stress state in stress space to define the initiation of failure. Failure surfaces can be defined with any arbitrary shape, unlike traditional failure models where the mathematical functions used to define the failure surface impose a specific shape on the failure surface. In the current paper, the complete development of the failure model is described and the generation of a tabulated failure surface for a representative composite material is discussed.

In this paper, experiments have been performed and finite element models have been developed for studying the influence of low-velocity impact damage on the four-probe electrical resistance of carbon fiber-reinforced polymer matrix laminates. Sixteen-ply and 32-ply AS4/3501-6 laminates with quasi-isotropic layup were analyzed. Electrical resistance was evaluated using a four-step procedure. First, finite element models were created in Abaqus Finite Element Analysis (FEA) for simulating low-velocity impact using a quasi-static loading approach. Second, matrix rupture in the inside plies was evaluated, and delamination analysis was performed at the corresponding interfaces to determine delamination patterns. Third, four-probe electrical finite element models were developed in Abaqus FEA for specimens before and after impact using the concept of effective conducting thickness and the delamination patterns obtained from the delamination analysis. Effects of the low-velocity impact delamination on four-probe top and oblique electrical resistance were studied. Electrical resistance predictions were compared to the experimental data. Both top and oblique resistance planes were sensitive to presence of delamination with the oblique resistance measurement being more sensitive as compared to the top resistance measurement. In addition, the resistance of the 16-ply specimens was more greatly affected by the delamination compared to the 32-ply specimens. The proposed analysis can be utilized for design of carbon fiber-reinforced polymer matrix composites with optimized damage sensing capabilities.

The increasing popularity of composite materials in aerospace applications is creating the need for a new class of predictive methods and tools for the simulation of progressive damage in laminated fiber-reinforced composite structures. The unique challenges associated with modeling damage in these structures may be addressed by means of thin-shell formulations which are naturally developed in the context of Isogeometric Analysis. In this paper, we further validate our recently developed Isogeometric Analysis-based multi-layer shell model for progressive damage simulations using experimental data for low-velocity impact on a 24-ply flat panel. The validation includes a careful comparison of delamination and matrix damage patterns predicted by the Isogeometric Analysis-based simulation and those obtained from post-impact non-destructive evaluation of the damaged coupon. The Isogeometric Analysis-based formulation is then deployed on two additional examples: a stiffened panel and a full-scale UAV wing, to demonstrate its suitability for, and ease of application to, typical aerospace composite structures.

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.

In this study, X-ray micro-computed tomography is employed to characterize the impact damage mechanisms in foam core sandwiched composites, paying particular attention to the influence of extreme low temperature effects. Investigation on impact response reveals that more energy absorption with lower impact damage force occurs at lower temperature. Results evidently show that test temperature has a significant influence on the impact damage behavior. Post-mortem inspection portrays clear relationships between damages in both foam core and carbon fiber reinforced polymer facesheets, as well as exposed test temperature. Specimens impacted at extreme low temperature (−70℃) exhibit less strength, and higher susceptibility to damage, verified by severer penetration of the impactor. Micro-computed tomography is exploited to examine cross-sectional views of the impacted specimens, showing detailed damage mechanisms of the carbon fiber-reinforced polymer facesheets and the foam core, thereby evidently revealing multiple complex impact damage modes such as fiber breakage, delamination, core shearing and crushing, facesheet-core debonding, which are all strongly influenced by arctic low temperature. The findings of this work will lead to improved design for advanced composite structures with enhanced impact resistance and damage tolerance in extreme cold environment particularly in the arctic region.

X-ray computed tomography has recently become an increasingly popular non-destructive imaging method in composites research. However, due to the complexity of 3D computed tomography data sets, it can be difficult to accurately and quantitatively assess the damage state of a composite structure without additional post-processing. A new segmentation procedure has been developed that takes a 3D computed tomography data set of an impacted composite laminate and separates internal damage into information about intraply and interlaminar damage within each ply and at each interface. Impacted flat T800/3900-2 unidirectional carbon/epoxy composite panels were scanned and then segmented to create comprehensible maps of internal damage states. Based on the types of data extracted by the developed computed tomography segmentation, techniques to input these datasets into numerical modeling have been developed. Additionally, various damage visualization and interpretation techniques made possible by the computed tomography segmentation have been explored.

A single-ply unidirectional IM7/8552 carbon fiber reinforced plastic composite with artificially introduced circular defects is subjected to dynamic tensile loading using a modified Kolsky tension bar. A high-speed X-ray phase contrast imaging method is integrated with the Kolsky bar setup to record the crack initiation from the defects and subsequent propagation in the material in real time during the tensile loading. The tensile loading was applied either in longitudinal (0° to fibers) or transverse (90° to fibers) direction of the specimens. Shear failure of the matrix and axial splitting along the loading/fiber direction were observed in longitudinal specimens to initiate from the edge of the artificial circular defects. Debonding of fiber and matrix was observed in transverse specimens, which initiated from the top and bottom edge of the hole. The dynamic tensile loading history during the crack propagation was recorded using a piezoelectric load cell and synchronized with the observed damage and failure processes.

Ballistic impact experiments were performed on triaxially braided polymer matrix composites to study the heat generated in the material due to projectile velocity and penetration damage. Triaxially braided (0/+60/−60) composite panels were manufactured with T700S standard modulus carbon fiber and two epoxy resins. The PR520 (toughened) and 3502 (untoughened) resin systems were used to make different panels to study the effects of resin properties on temperature rise. The ballistic impact tests were conducted using a single stage gas gun, and different projectile velocities were applied to study the effect on the temperature results. Temperature contours were obtained from the back surface of the panel during the test through a high speed, infrared thermal imaging system. The contours show that high temperatures were locally generated and more pronounced along the axial tows for the T700S/PR520 composite panels; whereas, tests performed on T700S/3502 composite panels, using similar impact velocities, demonstrated a widespread area of lower temperature rises. Nondestructive, ultrasonic C-scan analyses were performed to observe the failure patterns in the impacted composite panels and correlate the C-scan results with the temperature contours. Overall, the impact experimentation showed temperatures exceeding 252℃ (485°F) in both composites which is well above the respective glass transition temperatures for the polymer constituents. This expresses the need for further high strain rate testing with measurement of the temperature and deformation fields in order to fully understand the complex behavior and failure of the material and to improve the confidence in designing aerospace components with these materials.