Date of Award
Master of Science (MS)
Oliver J Myers
Composite laminates constructed in an asymmetric/unsymmetric cross-ply layup orientation exhibit two stable static equilibrium positions and may be actuated to snap from a primary stable cure shape to an inversely related secondary stable cure shape. This study aims to add functional descriptions of thick square bistable laminates, whose increased thickness risk the loss of bistability, through previously established analytical and finite element analysis (FEA) approaches as well as experimentation. Rayleigh-Ritz and Abaqus CAE 6.14 FEA software were both used to determine the cure shapes of carbon fiber reinforced polymer (CFRP) laminates composed of DA409 and TR50S-12k carbon fibers. These laminates were modeled to act as square thick bistable composites with sidelengths up to 0.914m. Visualizations of the out-of-plane displacements are shown for both methods with descriptions of each approach. Experiments using DA409/8552 and TR50S-12K/Newport 301 prepreg are used to further describe and develop the fundamental description for thick bistable laminates in terms of loss of bistability, deflection, curvature, actuation load, and shape.
The analytical model is an extension of Hyer's (2002) and Mattioni's (2009) work applied to thick bistable laminates where the primary assumption was the x-axis curvature equaled the negative y-axis curvature for the primary and secondary stable positions, respectively. This assumption leads to the already cemented conclusion that bistable laminates, once cured, take on one of two equal, yet inversely related, paraboloid shapes. Fourth order polynomials were used to describe the curvature along the principle laminate axes, differing from the previously used Menger curvatures, (three-point approximation). Bifurcation plots using peak deflections and average curvature generated from Rayleigh-Ritz, FEA simulations, and experiments clearly showed bistability existed to approximately 30 plies; however, FEA showed predictions upwards of 70 plies. On the other hand, energy landscapes generated from FEA indicated a significant degradation of bistability starting at 36 plies. Experimentation was performed on a test stand mimicking the same boundary conditions used in FEA while applying a centroidal out-of-plane transverse load. Experimental observations showed decreased peak displacements of stable cure shapes in addition to indications that the x-axis curvature had a significant difference in magnitude compared to the negative y-axis curvature. However, the existence of bistability agreed closely with Rayleigh-Ritz and FEM energy landscape plots, with clear "snaps" ending at thicknesses of 28-36 plies. Moreover, actuation force was found to be significantly similarly during experiments when compared to FEA simulations. Significant differences in curvature predictions and bistability loss from FEA was attributed to the combination of material characteristic differences for DA409/G35 and TR50S-12K, limitations of the experimental setup, and hand layup fabrication errors, curvature calculation methods, and the exclusion of defects in models. Lastly, although this paper raises more questions, it also shows viability of thicker bistable laminates to be used in macroscale applications where shape morphing or shape-retention attributes are a necessary constraint.
Knippenberg, Christopher Henry, "Functional Description for Thick Bistable Carbon Fiber Laminates with Rayleigh-Ritz, Abaqus, and Experiments" (2020). All Theses. 3407.