5 research outputs found
Modeling of NiTiHf Using Finite Difference Method
NiTiHf is a high temperature and high strength shape memory alloy with transformation temperatures above 100oC. A constitutive model based on Gibbs free energy is developed to predict the behavior of this material. Two different irrecoverable strains including transformation induced plastic strain (TRIP) and viscoplastic strain (VP) are considered when using high temperature shape memory alloys (HTSMAs). The first one happens during transformation at high levels of stress and the second one is related to the creep which is rate-dependent. The developed model is implemented for NiTiHf under uniaxial loading. Finite difference method is utilized to solve the proposed equations. The material parameters in the equations are calibrated from experimental data. Simulation results are captured to investigate the superelastic behavior of NiTiHf. The extracted results are compared with experimental tests of isobaric heating and cooling at different levels of stress and also superelastic tests at different levels of temperature. More results are generated to investigate the capability of the proposed model in the prediction of the irrecoverable strain after full transformation in HTSMAs
A multiscale study of NiTi shape memory alloys
Shape memory alloys (SMAs) are widely used in a broad variety of applications in multiscale devices ranging from nano-actuators used in nano-electrical-mechanical systems (NEMS) to large energy absorbing elements in civil engineering applications. This research introduces a multiscale analysis for SMAs, particularly Nickel-Titanium alloys (NiTi). SMAs are studied in a variety of length scales ranging from macroscale to nanoscale. In macroscale, a phenomenological constitutive framework is adopted and developed by adding the effect of phase transformation latent heat. Analytical closed-form solutions are obtained for modeling the coupled thermomechanical behavior of various large polycrystalline SMA devices subjected to different loadings, including uniaxial loads, torsion, and bending. Thermomechanical responses of several SMA devices are analyzed using the introduced solutions and the results are validated by performing various experiments on some large SMA elements. In order to study some important properties of polycrystalline SMAs that the macroscopic phenomenological frameworks cannot capture, including the texture and intergranular effects in polycrystalline SMAs, a micromechanical framework with a realistic modeling of the grains based on Voronoi tessellations is used. The local form of the first law of thermodynamics is used and the energy balance relations for the polycrystalline SMAs are obtained. Generalized coupled thermomechanical governing equations considering the phase transformation latent heat are derived for polycrystalline SMAs. A three-dimensional finite element framework is used and different polycrystalline samples are modeled. By considering appropriate distributions of crystallographic orientations in the grains obtained from experimental texture measurements of NiTi samples the effects of texture and the tension-compression asymmetry on the thermomechanical response of polycrystalline SMAs are studied. The interaction between the stress state (tensile or compressive), number of grains, and the texture on the thermomechanical response of polycrystalline SMAs is also studied. For studying some aspects of the thermomechanical properties of SMAs that cannot be studied neither by the phenomenological constitutive models nor by the micromechanical models, molecular dynamics simulations are used to explore the martensitic phase transformation in NiTi alloys at the atomistic level. The martensite reorientation, austenite to martensite phase transformation, and twinning mechanisms in NiTi nanostructures are analyzed and the effect of various parameters including the temperature and size on the phase transformation at the atomistic level is studied. Results of this research provide insight into studying pseudoelasticity and shape memory response of NiTi alloys at different length scales and are useful for better understanding the solid-to-solid phase transformation at the atomistic level, and the effects of this transformation on the microstructure of polycrystal SMAs and the macroscopic response of these alloys.Ph.D
Defect-Tolerant Bioinspired Hierarchical Composites: Simulation and Experiment
Defect tolerance, the capacity of
a material to maintain strength
even under the presence of cracks or flaws, is one of the essential
demands in the design of composite materials, as manufacturing induced
defects, or those generated during operation, can lead to catastrophic
failure and dramatically reduce the mechanical performance. In this
paper, we combine computational modeling and advanced multimaterial
3D printing to examine the mechanics of several different classes
of defect-tolerant bioinspired hierarchical composites, built from
two base materials with contrasting mechanical properties (stiff and
soft). We find that in contrast to the brittle base constituents of
the composites, the existence of a hierarchical architecture leads
to superior defect-tolerant properties. We show that composites with
more hierarchical levels dramatically improve the defect tolerance
of the material. We also examine the effect of adding both self-similar
and dissimilar hierarchical levels to the materials architecture,
and show that the geometries with multiple hierarchical levels can
retain a significant portion of their fracture strength in the presence
of either large edge cracklike flaws or multiple small distributed
defects in the material. We compare the stress distributions in materials
with different numbers of hierarchies in both simulation and experiment
and find a more uniform stress distribution in the uncracked region
of materials with higher hierarchy levels. These results provide micromechanical
insights into the origin of the higher defect tolerance observed in
simulation and experiment