Brent L. Volk

Analysis of the finite deformation response of shape memory polymers: II. 1D calibration and numerical implementation of a finite deformation, thermoelastic model

This study presents the analysis of the finite deformation response of a shape memory polymer (SMP). This two-part paper addresses the thermomechanical characterization of SMPs, the derivation of material parameters for a finite deformation phenomenological model, the numerical implementation of such a model, and the predictions from the model with comparisons to experimental data. Part II of this work presents the calibration of a previously developed thermoelastic constitutive model which is capable of handling finite deformations. The model is proposed in a general three-dimensional framework; however, this work focuses on reducing the model to one dimension and subsequently calibrating the model using experimental data obtained in part I. The one-dimensional numerical implementation of the model is presented, including the handling of the system of nonlinear equations and the integral term resulting from the constitutive model. The model is then used to predict the uniaxial shape memory effect. Results indicate good agreement between the model predictions and the experimental results, but the predictions do not capture the irrecoverable deformation present at the end of recovery.

Analysis of the finite deformation response of shape memory polymers: I. Thermomechanical characterization

This study presents the analysis of the finite deformation response of a shape memory polymer (SMP). This two-part paper addresses the thermomechanical characterization of SMPs, the derivation of material parameters for a finite deformation phenomenological model, the numerical implementation of such a model, and the predictions from the model with comparisons to experimental data. Part I of this work presents the thermomechanical characterization of the material behavior of a shape memory polymer. In this experimental investigation, the vision image correlation system, a visual–photographic apparatus, was used to measure displacements in the gauge area. A series of tensile tests, which included nominal values of the extension of 10%, 25%, 50%, and 100%, were performed on SMP specimens. The effects on the free recovery behavior of increasing the value of the applied deformation and temperature rate were considered. The stress–extension relationship was observed to be nonlinear for increasing values of the extension, and the shape recovery was observed to occur at higher temperatures upon increasing the temperature rate. The experimental results, aided by the advanced experimental apparatus, present components of the material behavior which are critical for the development and calibration of models to describe the response of SMPs.

Characterizing and modeling the free recovery and constrained recovery behavior of a polyurethane shape memory polymer.

In this work, tensile tests and one-dimensional constitutive modeling are performed on a high recovery force polyurethane shape memory polymer that is being considered for biomedical applications. The tensile tests investigate the free recovery (zero load) response as well as the constrained displacement recovery (stress recovery) response at extension values up to 25%, and two consecutive cycles are performed during each test. The material is observed to recover 100% of the applied deformation when heated at zero load in the second thermomechanical cycle, and a stress recovery of 1.5 MPa to 4.2 MPa is observed for the constrained displacement recovery experiments.After performing the experiments, the Chen and Lagoudas model is used to simulate and predict the experimental results. The material properties used in the constitutive model - namely the coefficients of thermal expansion, shear moduli, and frozen volume fraction - are calibrated from a single 10% extension free recovery experiment. The model is then used to predict the material response for the remaining free recovery and constrained displacement recovery experiments. The model predictions match well with the experimental data.

Thermomechanical characterization of the nonlinear rate-dependent response of shape memory polymers

This study presents the testing techniques used to thermomechanically characterize the material behavior of a shape memory polymer as well as the resulting data. An innovative visual-photographic apparatus, known as a Vision Image Correlation system was used to measure the strain. A series of tensile tests were performed on specimens in which strain levels of 10%, 25%, 50%, and 100% were applied to the material while above its glass transition temperature. After deforming the material to a specified applied strain, the material was constrained and cooled to below the glass transition temperature. Finally, the specimen was heated again to above the transition temperature, and the resulting shape recovery profile was measured. The dependence of the recoverable strain on the heating and cooling rate was investigated in this work. Results showed that strain recovery occurred in a nonlinear fashion with respect to temperature. Results also indicated that the ratio of recoverable strain to the applied strain was a constant value, and was independent of the level of applied strain.

Thermomechanical Characterization and Modeling of Ni60Ti40 SMA for Actuated Chevrons

This work describes the thermomechanical characterization and preliminary FEA modeling of commercial jet engine chevrons incorporating active Shape Memory Alloy (SMA) beam components. The SMA beams, when activated, induce the necessary bending forces on the chevron structure to deflect it into the fan flow and reduce noise. The primary focus of this work is the characterization of the SMA material (Ni60Ti40 wt%) chosen to actuate these chevrons and the preliminary modeling of the active chevron system behavior. To fully understand the material and calibrate the model, various thermomechanical experiments are performed on both untrained and trained standard SMA tensile specimens. Material properties for the shape memory alloy components are derived from this tensile experimentation. By using this data, a 3-D FEA implementation of a phenomenological SMA model is calibrated and used to analyze the response of a system motivated by the active chevron. The problem modeled consists of a pre-curved SMA beam clamped firmly against a straight aluminum substrate. The model proves to be an accurate tool for predicting the mechanical response of such a system subject to defined thermal inputs.© 2006 ASME

Feasibility of Crosslinked Acrylic Shape Memory Polymer for a Thrombectomy Device

Purpose To evaluate the feasibility of utilizing a system of SMP acrylates for a thrombectomy device by determining an optimal crosslink density that provides both adequate recovery stress for blood clot removal and sufficient strain capacity to enable catheter delivery. Methods Four thermoset acrylic copolymers containing benzylmethacrylate (BzMA) and bisphenol A ethoxylate diacrylate (Mn~512, BPA) were designed with differing thermomechanical properties. Finite element analysis (FEA) was performed to ensure that the materials were able to undergo the strains imposed by crimping, and fabricated devices were subjected to force-monitored crimping, constrained recovery, and bench-top thrombectomy. Results Devices with 25 and 35 mole% BPA exhibited the highest recovery stress and the highest brittle response as they broke upon constrained recovery. On the contrary, the 15 mole % BPA devices endured all testing and their recovery stress (5 kPa) enabled successful bench-top thrombectomy in 2/3 times, compared to 0/3 for the devices with the lowest BPA content. Conclusion While the 15 mole% BPA devices provided the best trade-off between device integrity and performance, other SMP systems that offer recovery stresses above 5 kPa without increasing brittleness to the point of causing device failure would be more suitable for this application.

Characterizing and Modeling the Free Recovery and Constrained Recovery Behavior of a Polyurethane Shape Memory Polymer

In this work, tensile tests are performed on a polyurethane shape memory polymer for both free recovery (extension recovery at zero load) and constrained recovery (stress recovery at constant extension) conditions. The experimental characterization is conducted on an electromechanical screw driven test frame, and a laser extensometer is used in conjunction with the electromechanical frame to provide a non-contact technique for measuring the deformation of the material. The specimens are deformed, above the glass transition temperature, to 10% extension. The SMP is then cooled, at a constant value of extension, to below the glass transition temperature to ‘lock’ the temporary shape. The extension recovery at zero load as well as the stress recovery at a constant value of extension is measured during the first shape memory cycle as the SMP is heated to above its glass transition temperature. The material is observed to recover 93% of the applied deformation when heated at zero load. In addition, a stress recovery of 1.5 MPa is observed when heated while holding a constant value of deformation (10% extension). After performing the experiments, the Chen and Lagoudas model, implemented in 1-D by Volk, et al., is used to simulate and predict the experimental results. The material properties used in the model — namely the coefficients of thermal expansion, shear moduli, and frozen volume fraction — are calibrated from a single free recovery experiment. The calibrated model is then used to simulate the material response for the free recovery tests as well as predict the response for the constrained recovery condition. The model simulations agree well with the free recovery experimental data but predict a larger compressive stress than what is observed during the constrained recovery experiment.Copyright