Experimental and Numerical Nonlinear Dynamics and Stress Field Analysis of Post-Buckled Composite Plates

Abstract

Thermally-buckled composite panels in aircraft may experience dynamic snap-through due to aerodynamic loading, which can accelerate damage growth (or delamination) in these structures. Therefore, characterizing post-buckled dynamic response and corresponding stress fields can be an important step to help assure the structural integrity of composite structures. To address this issue, this work experimentally and numerically investigates nonlinear dynamics and snap-through of post-buckled laminated composite plates under harmonic loading. In addition, stress fields induced by these phenomena are simulated to explore their potential impact on the fatigue failure of composite structures. The experimental investigation made in this work is intended to lead to a better fundamental understanding of the aforementioned phenomena and to provide benchmark data for robust model validation in the nonlinear regime. The dynamic response of a post-buckled composite specimen under various harmonic scenarios is captured using a full-field digital image correlation system. The spatio-temporal complexity and parameter sensitivity of the dynamic response are explored and the snap-through boundaries of the specimen are characterized. Several numerical models are built using in-house finite element codes written in MATLAB and are calibrated using the static full-field measurement of the buckled shape of the specimen. The primary objectives of the modeling done in this work are to develop reliable simulation tools for numerical analysis of nonlinear dynamics, and to investigate the impact of nonlinear dynamics and snap-through on stress fields. A model based on the classical laminated plate theory and nonconforming (semi-C1 continuity) cubic Hermite elements (free of shear locking) is shown to achieve excellent agreement with the experimental observations including the snap-through boundaries. As a ‘truth’ model in the thin plate limit, this model provides a new tool for developing benchmark data (displacement fields) for validation of computationally-demanding models which involve high computational costs or potential locking issues. For accurate and direct computation of stress fields, another model is generated based on the first-order shear deformation theory and bi-linear elements (C0 continuity). The shear and membrane locking issues of this model are exposed through linear and nonlinear analyses of its displacement and stress fields. To address these locking issues, two types of assumed strain methods are employed: the mixed interpolation of tensorial components (MITC4) and the enhanced assumed strain (EAS) method. The geometrically-nonlinear analysis presented herein shows that the EAS method effectively controls both shear and membrane locking, whereas the MITC4 method alleviates only shear locking with strong oscillations of membrane stresses which imply the occurrence of membrane locking. The simulation results obtained using the EAS model reveal that post-buckled response generates larger-amplitude stresses than the pre-buckled cases and that snap-through can induce a significant increase in stress amplitudes, thereby potentially affecting fatigue life. By developing reliable simulation tools for analysis of nonlinear dynamics and consequent stress fields, this work makes an important step forward in investigating the impact of these phenomena on the structural integrity of composite structures for supersonic- or hypersonic-speed flight.