Non-linear buckling analysis of delaminated hat-stringer panels using variational asymptotic method

This research proposes a computationally efficient methodology using a Constrained Variational Asymptotic Method (C-VAM) for non-linear buckling analysis on a hat-stringer panel with delamination defects. Starting with the geometrically non-linear kinematics, the VAM procedure reduces the three-dimensional (3-D) strain energy functional to an analogous 2-D plate model and evaluates the closed form warping solutions. Utilising the resulting warping solutions and recovery relations for the skin and the stringer, displacement continuity at the three-dimensional level is enforced between the stringer and the skin based on the pristine and delaminated interface regions. Consequently, the constrained matrices obtained from C-VAM is incorporated into an inhouse developed non-linear finite element framework. Using the developed formulation, a stiffened panel with delamination of 40 mm between the stringer and the skin is analysed under compression. The results have been validated locally and globally, employing experimental data and 3-D finite element analysis (FEA). Experiments are carried out on the co-cured panel by applying quasi-static loading with displacement-controlled conditions, and 3-D FEA is carried out in Abaqus. Load-response plots have been obtained to validate the results at the global level, and they are in excellent agreement with experiments and 3-D FEA. Subsequently, out-of-plane displacement contour plots are obtained; the number of half waves and wave intensity in 3-D FEA and C-VAM are comparable, although there are minor differences compared to the experimental findings. The proposed framework is shown to be computationally efficient by over 55% as compared to 3-D FEA for performing non-linear buckling analysis on the stiffened composite structure considered in the current work

Influence of Alkali Treatment on Mechanical Properties of Short Cocos nucifera Fiber Reinforced Epoxy Based Sustainable Green Composite

The present study deals with finding the optimum fiber content and effect of alkali treatment on the mechanical properties of the proposed Cocos-nucifera-reinforced polymer matrix composite. The present work is carried out in two different phases. First, the optimum fiber weight fractions for untreated Cocos nucifera composite are found out and later the effect of alkali treatment on the mechanical behavior of composite with optimum weight fraction of fibers selected in the first phase is determined. Proposed composites are mechanically characterized to determine the optimum fiber percentage to be used in composite and the effect of NaOH treatment on the mechanical properties of the composite with optimum fiber percentage is studied along with the determination of optimal NaOH concentration to be used. The results were analyzed by comparing the mechanical properties of the various composites and the effect of alkali treatment on the fibers of the composites was also studied. The obtained results showed that proposed composite with 40 wt% untreated fiber exhibits better mechanical properties compared to its counterparts and treating the fibers of the composite with 10% NaOH concentration further enhances the mechanical properties of the composite. The proposed composite can be a potential candidate for automobile interior application

Elucidating the effect of cohesive zone length in fracture simulations of particulate composites

The influence of the cohesive zone length on the crack driving force is quantified and analyzed in a representative system of particles dispersed in a matrix of a composite material. For heterogeneous material systems, e.g. particulate composites, it is known that as a crack approaches the particles, the crack driving force may increase (shielding) or decrease (anti-shielding) depending on the relative stiffness of the particles. These results have been established in numerous studies using the classical linear elastic fracture mechanics approach (LEFM). The cohesive zone method (CZM) introduces a length scale parameter, referred to as the cohesive zone (or fracture process zone) length scale, into the formulation of fracture mechanics. It is generally established that fracture mechanics predictions using the CZM are similar to those obtained using LEFM in the limit case where the process zone is very small relative to a suitable characteristic dimension of the problem. However, the influence of the length scale parameter has not been clearly demonstrated for crack propagation in a heterogeneous material system, especially when the cohesive zone length is not negligible. By considering a simple crack-particle-matrix system, it is shown that, in addition to the elastic properties, the process zone length scale parameter exhibits a critical influence on the crack driving force. For this study, the concept of configurational forces is utilized and the eXtended Finite Element Method (XFEM) is employed as a tool to simulate crack propagation. Through numerical simulations, it is shown that (i) the magnitude of the driving force vector directly depends on the length scale parameter and (ii) the direction of the driving force is largely influenced by the presence of a cohesive zone. This, in turn, alters the crack trajectory in the particulate system if the criterion for the direction of crack propagation depends on the orientation of the driving force vector. Towards this end, two different criteria for direction of crack propagation, namely maximum principal stress and maximum energy dissipation, are compared in the presence of a cohesive zone and the results are reported. The study reveals the crucial influence of the inherent length scale associated with the cohesive zone method when applied to crack propagation in particulate composite systems and elucidates important differences when comparing predictions from distinct theories of fracture mechanics.Aerospace Structures & Computational MechanicsNovel Aerospace Material

Dataset for the publication: "Coupon scale Z-pinned IM7/8552 delamination tests under dynamic loading"

Datasets for a paper published in Composites Part A: Applied Science and Manufacturinghttps://doi.org/10.1016/j.compositesa.2019.105565The quasi-static tests were done at velocity of around 0.01mm/sDynamic tests were conducted at norminal velocity of around 4m/s, some of the WDCB tests were done at 7m/s.EPSR