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  1. Bistable composite laminates have large-scale applications in morphing and energy-harvesting structures, but their fatigue performance remains largely unexplored. This study investigates the stiffness and damage progression and evaluates bistable performance to develop protocols for long-term applications. We analyze the effects of displacement-controlled fully reversible high cycle fatigue-loading on stiffness, damage, curvature, and snap-through load in the out-of-plane loading direction at eight different combinations of parameters with frequency from 1 to 10 Hz, two boundary conditions, and temperature from 22°C to 150°C up to 3 to 10 million cycles. Stiffness and damage evolution analysis demonstrate the first two stages in out-of-plane fatigue loading. The study proposes a damage definition in terms of load adapting with two fatigue damage models: (1) Shiri Model and (2) Wu Model, while both models exhibit reasonable accuracy in predicting damage for the first two stages despite deviating at the final cycle due to assuming this cycle as the final failure cycle. Of the two models, the Shiri model provided a smaller range of model parameter values, 0.22 and 0.43, for parameters p and q, respectively, which reflects adjustability to different test conditions by maintaining a moderate range. Specimens encountered no final failure by fiber breakage and did not lose bistability for any combination. Curvature and snap-through load measurements have not substantially changed due to fatigue loading. These findings confirm application protocols with a broad range of parameters for which the laminates can operate without significant fatigue damage and maintain their bistable performance for an infinite lifetime.

     
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  2. Abstract

    Bistable composite laminates have exhibited enormous potential in morphing and energy harvesting followed by a wide range of application in aerospace, power generation and automobile industries. This study presents the fatigue analysis of bistable laminates in terms of stiffness degradation and loss of bistability. Moisture saturation of the specimens are ensured by keeping them in a controlled laboratory environment for an extended period of time. Mass of the specimens have been measured to quantify the moisture saturation. Fatigue tests are performed at 1 Hz frequency, and R = −1 stress ratio which is the ratio of minimum stress to maximum stress. Specimens are tested for 3 million cycles in displacement control. Load-displacement plot from the test is divided into 5 stiffness regions. A piecewise study of each region has demonstrated good agreement with existing analytical model. Stiffness degradation in 4 regions corresponding to 2 stable configurations follows general trend for composites up to the second stage of damage in three stage damage progression model while the remaining region corresponding to unstable configuration is not considered in this analysis. Test results have been reproduced with minor discrepancy at the specified environmental and loading condition, ply configuration, and size of the laminate. Test protocols, results, and damage analysis presented in this study can be utilized to evaluate the fatigue performance of multistable CFRP structures subjected to higher amplitudes and frequencies.

     
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