Among novel non-destructive evaluation techniques for structural health monitoring, the magnetostrictive (MS)-tagged fiber-reinforced composites stands out as especially suitable due to: (a) distributed sensory properties, (b) non-contact damage detection, and (c) straightforward manufacturing implementation. Experimental data and mathematical modeling of a MS-tagged fiber reinforced composite beam under bending (flexural) loading are presented. A brief review of the state of the art identifies previous work on axially loaded MS composites, but finds no previous work on bending. Description of bending beam design and fabrication is followed by theoretical analysis and by the description of the experimental set-up and equipment used. Several analysis models were used. Test data, with and without applying magnetic bias field between loading cycles, is presented and results are discussed. Numerical values for the stress and strain versus magnetic flux density coefficients are given for both annealed and non-annealed cases. Piezomagnetic coefficients for the MS composite are calculated. The correlation between the results developed in the present paper for bending and previously published results for axial loading is found to be within 10% after correction factors depending on the quantity of the MS material.

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.

No abstract

Nitinol shape memory wires were embedded in epoxy beams to study their thermomechanical behavior in the embedded state. Beams were made with ten wires placed below the geometric mid-plane. A room temperature cure epoxy was used to minimize residual stresses acting on the wires. The beams were placed in an oven and heated from room temperature to a temperature well above the austenitic finish temperature of the wires. The beams were photographed through a window in the oven door at periodic intervals during the experiment. The photographs were analyzed to measure beam deflections during the experiment resulting from thermal and shape memory transformational strains. A beam model was developed and predictions of beam curvature were compared to the experimental data. The results show that agreement between the model and the experimental data is dependent, and extremely sensitive to, an accurate knowledge of the thermomechanical properties of both the nitinol and epoxy phases. Large deflections of the beams were obtained and some evidence of a transformation front propagation from the edges to the center of the beam is shown.

No abstract