A macroscopic phenomenological framework is used for developing a closed-form solution for analyzing the pure
bending of shape memory alloy (SMA) beams. In order to study the effect of tension-compression asymmetry
on the bending response, two different transformation functions are considered; a J2-based solution with symmetric
tension-compression response, and a J2 - I1-based solution capable of modeling the tension-compression
asymmetry. The constitutive equations are reduced to an appropriate form for studying the pseudoelastic bending
response of SMAs, and closed-form expressions are obtained for the stress and martensitic volume fraction
distributions in the cross section. These expressions are used for calculating the bending moment-curvature
analytically. Both circular and rectangular cross sections are considered and several case studies are presented
for analyzing the accuracy of the presented method and also the effect of considering the tension-compression
asymmetry on the bending response of SMAs.
We have studied the corrosion of phosphorus-doped polySi when contacted to a gold metallization layer and exposed to various hydrofluoric acid (HF) based chemistries, including mixtures with HCl, C2H6O, H2O, NH4F, Triton-X-100, as well as vapor-based HF. Here, we utilize optical-, electron-, and atomic-force-microscopy, optical interferometry, as well as instrumented indentation ("nanoindentation") to characterize test and reference specimens exposed to the various HF solutions. These measurements provide information concerning the appearance, roughness, physical dimensions, hardness, elastic modulus, and reverse phase transformation activity of the various polysilicon specimens. In general, some of the chemistries produced time-dependent darkening or "staining" visibly seen on free surfaces, roughening and attack at grain boundaries, nano-scale pitting of the free surfaces, decrease in thickness, decrease in hardness and mechanical modulus, and diminished elbow and reverse excursion activity for those silicon specimens electrically connected to metal. Change in performance is attributed to the formation of a galvanic cell during the HF immersion, and the corresponding damage driven by an anodic current. The results here can be used to explain previous work, which focused on the change in performance of designated MEMS diagnostic structures.
The use of smart materials and multifunctional components has the potential to provide enhanced performance, improved economics, and reduced safety concerns for applications ranging from outer space to subterranean. Elastic Memory Composite (EMC) materials, based on shape memory polymers and used to produce multifunctional components and structures, are being developed and qualified for commercial use as deployable components and structures. EMC materials are similar to traditional fiber-reinforced composites except for the use of a thermoset shape memory resin that enables much higher packaging strains than traditional composites without damage to the fibers or the resin. This unique capability is being exploited in the development of very efficient EMC structural components for deployable spacecraft systems as well as capability enhancing components for use in other industries. The present paper is intended primarily to describe the transition of EMC materials as smart structure technologies into viable industrial and commercial products. Specifically, the paper discusses: 1) TEMBO EMC materials for deployable space/aerospace systems, 2) TEMBO EMC resins for terrestrial applications, 3) future generation EMC materials.
We examine creep of thin film Au on curved bimaterial Au/Si microcantilevers. Time-dependent inelastic strains in the Au film lead to gradual changes in the microcantilever curvature over time. Curvature-temperature-time experiments are used to examine the effects of hold temperature and maximum annealing temperature on the inelastic response of the Au films. Experiments reveal inelastic strains in the Au films due to creep, recovery, and microstructural coarsening. At moderate hold temperatures, 30 °C < T < 175 °C, inelasticity in the Au films is observed to be a competition between creep and recovery. Creep strains are driven by tensile stresses in the film and serve to decrease the microcantilever curvature towards the equilibrium curvature of the underlying Si beam. Strains due to recovery of the metastable Au cause contraction of the film and the development of intrinsic tensile film stresses. The recovery leads to "anomalous’ changes in microcantilever curvature since the curvature gradually increases or decreases away from the equilibrium curvature of the underlying Si. The inelastic behavior of the Au film is shown to depend on annealing temperature through changes in initial film stress after thermo-elastic cooling and degree of recovery.
The next generation of shape memory alloy constitutive models must explicitly or implicitly account for the physics of deformation. Such thermodynamic based micro-mechanical or internal state variable models yield a class of constitutive models with predictive capabilities extending past traditional phenomenological models. Unfortunately, a lack of quantitative understanding of pertinent deformation mechanisms in polycrystalline NiTi shape memory alloys has hampered the advancement of the aforementioned state-of-the- art constitutive modeling approaches. With a goal to improve our fundamental quantitative understandings of the deformation of polycrystalline NiTi shape memory alloys, the circumvent modeling roadblocks, the present talk outlines recent experimental results on single crystal NiTi. Particular attention is paid to macroscopic phenomenon which have recently been elucidated based on the mechanical testing of NiTi single crystals such as texture effects, tension-compression asymmetry, coupled detwinning and transformation stains, and cyclic loading effects. Ultimately, the aim of the talk is to present an overview of our recent findings while concurrently outlining the needs for state-of-the-art constitutive modeling efforts to address some longstanding issues. Throughout the paper, we will also outline future experiments on single crystal NiTi necessary to strengthen our fundamental understandings.
Conference Committee Involvement (3)
Reliability, Packaging, Testing, and Characterization of MEMS/MOEMS V
25 January 2006 | San Jose, California, United States
Reliability, Packaging, Testing, and Characterization of MEMS/MOEMS IV
25 January 2005 | San Jose, California, United States
Reliability, Testing, and Characterization of MEMS/MOEMS III
26 January 2004 | San Jose, California, United States