Theoretical modelling of residual and transformational stresses in SMA composites

Abstract

SMA composites are a class of smart materials in which shape memory alloy (SMA) actuators are embedded in a polymer matrix composite. The difference in thermal expansion between the SMA and the host material leads to residual stresses during processing. Similarly, the SMA transformations from martensite to austenite, or the reverse, also generate stresses. These stresses acting in combination can lead to SMA/epoxy interfacial debonding or microcracking of the composite phase. In this study the residual and transformational stresses are investigated for a nitinol wire embedded in a graphite/epoxy composite. A three-phase micromechanical model is developed. The nitinol wire is assumed to behave as a thermoelastic material. Nitinol austenitic and martensitic transformations are modelled using linear piecewise interpolation of experimental data. The interphase is modelled as a thermoelastic polymer. A transversely isotropic thermoelastic composite is used for the outer phase. Stress-free conditions are assumed immediately before cool down from the cure temperature. The effect of nitinol, coating and composite properties on residual and transformational stresses are evaluated. Fiber architectures favoring the axial direction decrease the magnitude of all residual stresses. A decrease in stresses at the composite/coating interface is also predicted through the use of thick, compliant coatings. Reducing the recovery strain and moving the transformation to higher temperatures were found to be most effective in reducing residual stresses.

Modeling porous shape memory alloys using micromechanical averaging techniques

“…In a later work the same authors (Boyd and Lagoudas, 1996b) have reported that by proper selection of the manufacturing process, namely, by proper pre-straining of the SMA fibers in the martensitic phase, a two-way composite SME is possible even though the fibers exhibit only one-way SME. Berman and White (1996) have studied the effect of the phase transformation of SMA wires embedded in a polymer matrix on the development of residual stresses. It has been found that by properly designing the SMA composite system, the residual stresses can be controlled or totally eliminated.…”

Stress and Cure Sensing of Single-Shape Memory Alloy (SMA) Fiber/Epoxy Composites using Electro-micromechanical Technique

“…The lattice-invariant shear mechanism accommodates the different shape of the new martensite within the transformed austenite. The uniqueness of shape memory lies in the fact that the latticeinvariant shear step of this transformation occurs almost solely by twinning [14,15]. Twinning forces the BCC shape into a monoclinic phase with no volumetric change and no broken bonds.…”

Self-powered sensory nerve system for civil structures using hybrid forisome actuators

“…Stimuli-sensitive materials cover a broad spectrum of materials that respond to external fields such as pH, temperature, and electric/magnetic field [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] . These materials show promise in applications ranging from pumps and valves, to shock tube instrumentation.…”