Results from an effort to fabrication shape memory alloy hybrid composite (SMAHC) test specimens and characterize the material system are presented in this study. The SMAHC specimens are conventional composite structures with an embedded SMA constituent. The fabrication and characterization work was undertaken to better understand the mechanics of the material system, address fabrication issues cited in the literature, and provide specimens for experimentM validation of a recently developed thermomechanical model for SMAHC structures. Processes and hardware developed for fabrication of the SMAHC specimens are described. Fabrication of a SMAHC laminate with quasi-isotropic lamination and ribbon-type Nitinol actuators embedded in the 0 °layers is presented. Beam specimens are machined from the laminate and are the fiJcus of recent work, but the processes and hardware are readily extensible to more practical structures. Results of thermomechanical property testing on the composite matrix and Nitinol ribbon are presented. Test results from the Nitinol include stress-strain behavior, modulus versus temperature, and constrained recovery stress versus temperature and thermal cycle. Complex thermomechanical behaviors of the Nitinol and composite matrix are demonstrated, which have significant implications for modeling of SMAHC structures.

The temperature dependence of the plane-strain initiation fracture toughness (KJICi) is modeled micromechanically for a variety of advanced aluminum alloys that fail by microvoid processes. Materials include precipitation-hardened ingot metallurgy, spray formed, submicron-grain-size powder metallurgy, and metal-matrix composite alloys. A critical-plasticstrain-controlled model, employing tensile yield strength, elastic modulus, work hardening, and reduction of area measurements, successfully predicts KJICi versus temperature for eight alloys, providing a strong confirmation of this approach. Modeling shows that KJICi is controlled by the interplay between the temperature dependencies of the intrinsic failure locus εfp (σmσfl) and the crack-tip stress/strain fields governed by alloy flow properties. Uncertainties in εfp (σmσfl), as well as the critical distance (volume) for crack-tip damage evolution, hinder absolute predictions of KJICi. Critical distance (calculated from the model) correlates with the nearest-neighbor spacing of void-nucleating particles and with the extent of primary void growth determined from quantitative fractography. These correlations suggest a means to predict absolute plane-strain fracture toughness.

Shape memory alloys (SMAs) have enormous potential for a wide variety of applications.

Previous NASA work has included fabrication and modeling of hybrid composite (HC) specimens with embedded Nitinol ribbon actuators and thermomechanical testing of the constituents. The Nitinol tensile behavior depended significantly on the thermomechanical condition (TMC). A Nitinol microstructure/mechanical property characterization was conducted on four TMCs. Differential scanning calorimetry and x-ray diffraction were used to rationalize the microstructures present. Tensile tests determined the effect of TMC on the Nitinol tensile behavior and stress state of the microstructure. Three TMCs showed typical shape memory behavior. The TMC that simulated the HC autoclave process on the actuator resulted in an irreversible microstructure. The microstructural constituents and their stress states probably govern the Nitinol stress-strain behavior. The critical stress to achieve an initial stress plateau was dependent on the amount and stress state of R-phase present in the initial microstructure. Thus, prior TMC critically affects the Nitinol tensile behavior. Numerical model inputs must therefore account for these effects on the Nitinol actuator.

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