NIAR Research Publications

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    Failure Analysis of Composite C-Spars Under Inter-Laminar Tensile (ILT) Configurations: An Experimental & Numerical Evaluation
    (DEStech Publications, 2023-09) Shafie, Mohamed Z.; Seneviratne, Waruna P.; Tomblin, John S.; Rathnaweera, Ruchira; Nadason, Harishanker
    In designing composite structures, it is fundamental to understand critical areas of structural deficiencies that lead to failures. One such critical design element is the failure caused by out-of-plane loads propagating through the bending of a composite laminate. In such cases, load offsets and element heights are generally the primary contributors to moments and forces leading to the degree of bending. In curved beams, these load conditions result in inter-laminar tension (ILT) type failures between plies, resulting in delamination and eventual failure. This work investigates such an ILT failure mechanism on composite C-Spars. The C-Spar specimen was constructed entirely of a quasi-isotropic [45/0/-45/90]4S layup and consisted of two 90-degree radii along with straight web sections defining the height of the element. To examine the contribution of the bending moment, resultant force, and rotation on the unfolding radii, varying loading arm configurations were investigated with a pinned load applicator. High-fidelity finite element (FE) analysis using B-Spline Analysis Method (BSAM), developed under the Air Force Research Laboratories (AFRL), was also utilized to predict failure of the C-Spars. The FE models showed good agreement with that of the experimental data and predicted failure loads within 10% on multiple configurations while also mimicking the geometric nonlinearity under larger deformation. Having successfully modeled the rotation and failure of the C-Spar configuration, this enables further investigation into the critical inter-laminar stresses induced within the design of such structures.
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    A Progressive Fatigue Model on the Bearing Failure of Composite Bolted Joints
    (DEStech Publications, 2023-09) Cui, Xiaodong; Kariyawasam, Supun; Albrecht, Quinten; Lua, Jim Y.
    Bolted composite joints have distinctive failure mechanisms due to the bolt clamping force, which could induce a hydrostatic pressure in the bearing region. Compressive failure near a bolt hole under bearing, clamp-up pressure, and constraint from non-zero degree plies takes a variety of forms at the microscales including fiber micro-buckling, fiber kinking, fiber compressive failure, matrix cracking, delamination, and out-of-plane shear cracking. The hydrostatic pressure would cause the increase of compressive strength of the composite material and hence grant a large load-carrying capability to the bolted joint from damage initiation to ultimate failure. A physics-informed modeling approach is developed for the static bearing failure analysis by considering the residual stress along the longitudinal and transverse direction in the bearing region. However, the mechanisms become more complicated for the fatigue load when hole elongation is involved. The permanent hole elongation could be caused by the releasing of the formed debris in the bearing region during the unloading stage under fatigue load. To consider this permanent deformation, two permanent strain components in the longitudinal direction and transverse direction are introduced into the developed fatigue model and their evolution with the increase of fatigue cycles. With the evolution of permanent strain implemented, the fatigue model can be utilized in the failure prediction for the composite bolted joints of various configurations such as single shear bearing (SSB), double shear bearing (DSB), and bearing and bypass (BB) coupons. Validation studies are performed using test data from National Institute for Aviation Research (NIAR) and damage characterization results to correlate the damage progression and final failure mechanisms. The validated bearing damage and failure prediction module for CB2ATA will be used as a virtual testing tool to generate additional points on the bearing and bypass interaction curve without testing.
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    Rx-FEM Modeling of Fatigue Damage Growth in Composites with Local R-Ratio by Using Strength Tracking Method
    (DEStech Publications, 2023-09) Lu, Wei-Tsen; Curry, Grayson; Adluru, Hari K.; Seneviratne, Waruna P.; Iarve, Endel V.
    A computationally efficient fatigue methodology is proposed that uses automatic cycle jump step selection algorithm within implicit framework to predict fatigue damage accumulation in laminated composites. Stress state scan within a specified load profile such as one or more cycles is performed to determine local stress ratio (R-ratio) in each integration point. S-N ply level-based strength criterion was applied for matrix Mesh Independent Crack (MIC) insertion in each ply as well as within respective Cohesive Zone Model (CZM) formulation at the damage initiation stage. Paris Law based propagation was utilized for MIC and delamination opening within CZM. The local R-ratio is implemented within CZM for initiation and propagation stages. In addition to the matrix failure modes fatigue degradation in the fiber direction under compression loading was implemented. The concept is based on shear strength degradation in the misalignment frame and LaRC failure criterion. Finite element simulations of [45/0/-45/90]3S open hole laminate under tension-tension (T-T) and compression-compression (C-C) fatigue were performed. Fiber damage, delamination, and matrix cracks are seen to be the appropriate size and location compared with the experimental result.
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    Effects of Impact Damage on the Energy Absorption Capabilities of Composite Materials
    (DEStech Publications, 2023-09) Bhasin, Akhil; Keshavanarayana, Suresh R.; Maichan, Tanat; Gomez, Luis; Olivares, Gerardo
    In the current investigation, the effects of in-service impact damage on the energy absorption capabilities, and damage evolution of two different geometries, corrugated beams, and c-channel stanchions, have been presented. Both the energy absorber geometries were manufactured using two composite material systems, Hexcel IM7/8552 and Hexcel AS4 PW/8552. Two different stacking sequences evaluated were: [90o/0o]2S and [45o/90o/-45o/0o]s. The pristine specimens were secured in a nonstandard fixture and impacted at the center using a drop tower [1]. The specimens were impacted at different energy levels to introduce varying levels of damage. Post-damage introduction, compression tests were performed at 1 in/s using a high-speed servo-hydraulic test frame. All the tests were supported with high-speed Digital Image Correlation (DIC) to obtain damage evolution and surface strains. A comparison of the energy absorption capabilities and damage modes between the pristine specimens and specimens with impact damage has been reported.
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    Effects of thermal plasma on the microstructure and mechanical behavior of alumina-mullite based oxide/oxide ceramic matrix composites
    (Elsevier Ltd, 2023-10) James, Hannah; Singh, Abhendra K.; Hyde, Truell; Andrulonis, Rachael; Opliger, Matthew S.
    Oxide/oxide ceramic matrix composites have potential applications in thermal protection systems due to their ability to withstand very high temperatures while maintaining their load bearing capability and oxidation resistance. Under hypersonic flight conditions vehicle components may be exposed to temperatures above the melting points of alumina and mullite, as well as to chemically active ionized gases. Therefore, it is necessary to understand the response of these CMCs to not just thermal loads, but also the added challenges of plasma environments. In this study, alumina-mullite based oxide/oxide CMCs were exposed to an inductively generated plasma jet for 30 min at a heat flux of 24 W/cm$^2$ in near vacuum conditions. Resulting microstructures were characterized with XRD and SEM/EDS. Silicon rich, sub-micron globules were observed on the surface of the impinged region. Also, the first 5 μm below the surface were enriched with silicon. The back surface showed no silicon enrichment but did have a granulated surface texture in comparison to as manufactured samples. Short beam strength tests were performed as a measure of structural performance. Differences in the overall response and short beam strength were noted between the as manufactured and plasma exposed cases.