LOCAL DISPLACEMENTS AND LOAD-TRANSFER IN SHAPE-MEMORY ALLOY COMPOSITES

Although there has been a significant amount of research dedicated to characterizing and modeling the response of shape memory alloys (SMAs) alone, little experimental work has been done to understand the behav ior of SMAs embedded in a host material. The interaction between SMA w ires and a host polymer matrix was investigated by correlating local d isplacements and stress fields induced by the embedded wires with SMA/ polymer adhesion. Most SMA composite applications require transfer of strain from the wire to the matrix. In these applications, maximum int erfacial adhesion between the SMA wire and the polymer matrix is most desirable. The adhesion was varied by considering four different surfa ce treatments: untreated, acid etched, hand sanded and sandblasted. Th e average interfacial bond strength of the SMA wires embedded in an ep oxy matrix was measured by standard pull out tests. Sandblasting signi ficantly increased the bond strength, whereas hand sanding and acid cl eaning actually reduced interface strength. In situ displacements of e mbedded, surface-treated SMA wires were measured using heterodyne inte rferometry, whereas the resulting stresses induced in the polymer matr ix were investigated using photoelasticity. Increased wire adhesion re sulted in lower axial wire displacement and higher interfacial stresse s due to the restraining effect of the matrix on the actuated wire, A simplified theoretical analysis was carried out to estimate the shear stress induced in the matrix due to wire actuation. The maximum shear stress predicted for the case of a perfect interfacial bond was about 7 percent larger than the value measured experimentally for the sandbl asted wire.