Enhancing the deformability of elastic memory sandwich composites with elastic memory/conventional epoxy hybrid facesheets
Date
2016
Authors
Antonio, Allyson Melia, author
Radford, Donald W., advisor
James, Susan P., committee member
Belfiore, Laurence A., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Shape memory polymer (SMP) composites have the ability to return repeatedly, and with great accuracy, to their cured geometry when heated. By taking advantage of the inherent ability of polymers to exist in a rubbery state at higher temperatures and in a glassy state at lower temperatures, shape memory composites (SMC), which incorporate continuous fiber reinforcement, can accommodate strains up to 5%, compared to the ultimate strain of 1% - 1.5% typical of carbon fibers in aerospace composites. This is possible because the rubbery modulus of an SMP is at least two orders of magnitude lower than its corresponding glassy modulus. The limited lateral stability provided by the matrix in the rubbery state allows the fibers to buckle elastically in sinusoidal waves, called microbuckles, under compressive load. Elastic memory composites (EMC) are a class of SMC that are able to use elastically stored strain energy to exert force, and thereby perform mechanical work. The elastic energy stored by the EMC is not large enough to drive the reverse deformation once cooled to the glassy state, but can be released with applied heat. Because the EMC polymer can processed into low density foam, the combination of EMC polymer foam with continuous fiber EMC to create sandwich panels, is an attractive one. Although microbuckling is the enabling mechanism for strain accommodation, excessive microbuckling results in either matrix failure (delamination) or fiber breakage. Therefore, the conformability EMCs is limited by the amount of flexure that can be achieved without incurring permanent damage. Bending an EMC in the rubbery state induces compressive stress that is amplified by the difference in tensile and compressive moduli. Whereas tension is resisted by the fibers, compression is only supported by the matrix. This causes a shift in the location of the neutral stress plane, from the center of the thickness, far toward the maximum tension surface. Additional bending section thickness from adding foam to the EMC to create a sandwich panel, exponentially raises the maximum stress on the compression surface. This leads to microbuckling failure of EMC sandwich panels at much lower deflections than typical solid laminate EMCs. It is hypothesized that by incorporating permanently bonded conventional carbon epoxy composite plies on the compression surface of the EMC sandwich panel, compression microbuckling failure can be delayed, increasing bending capability. Three base laminate configurations were selected to assess relative deformability in 3-point bending: shape memory only, shape memory with one conventional ply, and shape memory with two conventional plies. To simulate local heating in some specimens, phenolic blocks were bonded to the ends of the foam core and incorporated into the laminate with conventional epoxy, to prevent facesheet shear. The following variables were used to further investigate the viscoelastic behavior of the specimens: bending temperature, deflection rate, and hold time at maximum deflection. Results are presented in the form of force-deflection, rather than stress-strain, as the use of shear end constraint significantly affected the shape of the bent specimens. Global compression facesheet buckling into the foam core caused mid span thinning that was more prominent at higher temperatures. Loading cycle hysteresis due to stress relaxation was recorded at temperatures above and below Tg, being greatest below Tg. It was found, regardless of variables, that the specimens with hybrid elastic memory/conventional epoxy matrix facesheets achieved more than double the deflection of pure EMC.