The capability of a material to display two equilibrium states, called bistability, has been previously observed in carbon fiber reinforced polymers (CFRPs). For bistability to occur, the laminate must consist of an asymmetric layup about a mid-plane. This layup generates internal residual stress from thermal contraction allowing for the multiple equilibrium positions. Prior studies have observed bistability in CFRPs with small-scale rectangular geometries where all sides were less than ten inches. The aim of this paper is to demonstrate the existence of bistability in large-scale CFRPs with rectangular and non-rectangular geometries. Experimental and computational analyses (finite element analyses) were conducted to determine the viability of bistability in large-scale CFRPs where at least one length aspect of the specimen is greater than or equal to 12 inches.

 

Specimens, whose shapes included rectangles, rhombuses, triangles, and circles, were fabricated and tested to determine the presence of bistability and the associated curvature for each equilibrium state. Rectangular specimens had a length of 36 inches and widths that varied from 6 to 18 inches. For the rhombuses, triangles, and circles, one length aspect (i.e. the diagonal, height, or diameter, respectively) equaled 12 inches. The specimens could be considered thin laminates relative to their overall geometric dimensions as the maximum number of plies used during fabrication was eight plies. Finite element models of the same geometries and dimensions as the physical specimens were created to compare the equilibrium shape, specifically the curvature, and the existence of bistability. A multi-step loading procedure was developed for the large-scale rectangular plates to eliminate their tendency to buckle under a centroidal load. A parametric analysis on the aspect ratio of the rectangular laminates was conducted using finite elements to examine its effect on bistability and curvature.

 

Bistability was found to occur in the non-quadrilateral specimens examined in this work; the rectangular specimens with a side length of 36 inches displayed bistability for the experimentally tested widths. The remnants of the 12-inch rectangular laminates which the non-rectangular specimens were fabricated from also displayed bistability. The curvature of the fabricated specimens decreased an average of 40% over time due to hygroscopic effects. The finite element models produced results for a pristine specimen absent of fabrication imperfections and environmental effects such as humidity. The non-traditional shaped CFRPs followed bistability trends found for traditional, small-scale, rectangular laminates. An inverse relationship between the ply count and curvature was exhibited for the large-scale, rectangular laminates; curvature decreased as the number of plies in the laminate increased. The ability of a laminate to display bistability was found to be related to both the number of plies and aspect ratio of the rectangular laminates. Through morphing, the rectangular specimens’ equilibrium shapes mirrored the shapes demonstrated in small-scale rectangular laminates; however, the circular and triangular specimens displayed symmetric and asymmetric curving, respectively.


Non-Traditional Shape Variations on Bistable Carbon Fiber Reinforced Polymer Laminates

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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