Characterization of the Nonlinear Rate Dependent Response of Shape Memory Polymers

Shape Memory Polymers (SMPs) are a class of polymers, which can undergo deformation in a flexible state at elevated temperatures, and when cooled below the glass transition temperature, while retaining their deformed shape, will enter and remain in a rigid state. Upon heating above the glass transition temperature, the shape memory polymer will return to its original, unaltered shape. SMPs have been reported to recover strains of over 400%. It is important to understand the stress and strain recovery behavior of SMPs to better develop constitutive models which predict material behavior. Initial modeling efforts did not account for large deformations beyond 25% strain. However, a model under current development is capable of describing large deformations of the material. This model considers the coexisting active (rubber) and frozen (glass) phases of the polymer, as well as the transitions between the material phases. The constitutive equations at the continuum level are established with internal state variables to describe the microstructural changes associated with the phase transitions. For small deformations, the model reduces to a linear model that agrees with those reported in the literature. Thermomechanical characterization is necessary for the development, calibration, and validation of a constitutive model. The experimental data reported in this paper will assist in model development by providing a better understanding of the stress and strain recovery behavior of the material. This paper presents the testing techniques used to characterize the thermomechanical material properties of a shape memory polymer (SMP) and also presents the resulting data. An innovative visual-photographic apparatus, known as a Vision Image Correlation (VIC) system was used to measure the strain. The details of this technique will also be presented in this paper. A series of tensile tests were performed on specimens such that strain levels of 10, 25, 50, and 100% were applied to the material while it was above its glass transition temperature. After deforming the material to a specified applied strain, the material was then cooled to below the glass transition temperature (Tg) while retaining the deformed shape. Finally, the specimen was heated again to above Address all correspondence to this author: [email protected] 1 the transition temperature, and the resulting shape recovery profile was measured. Results show that strain recovery occurs at a nonlinear rate with respect to time. Results also indicate that the ratio of recoverable strain/applied strain increases as the applied strain increases. INTRODUCTION Intelligent systems, or systems that can sense and react to their environment autonomously, represent a rapidly growing sector of technology. These systems often exploit the properties of active materials to accomplish the desired sensing or actuation response Shape Memory Alloys (SMAs), shape memory ceramics, and piezoelectrics are a few examples of active materials that have been heavily researched and have been developed and utilized in a wide range of applications, such as oil exploration, medical, and aerospace industries [1, 2]. Another such type of material is Shape Memory Polymers (SMPs). While both SMPs and the more widely researched SMAs have the ability to recover an apparently permanent deformation due to thermal manipulation, SMPs have the unique ability to recover strains up to 400% [3]. In addition, SMPs have a lower density and lower manufacturing and processing costs than SMAs. Recent research efforts exemplify the heightened interest in the use of SMP in modern applications. For instance, Lockheed Martin and Hypercomp/NextGen are developing and testing morphing wings using smart materials. These wings are expected to adjust the surface area based on the current flying conditions, with possible area increases of 300%. In this project, funded by the Defense Advanced Research Projects Agency (DARPA), SMPs are being considered as a possible choice for the skin of the wing [4]. Furthermore, in an effort to boost the naturally low stiffness and low recovery stress of SMPs, efforts are underway to create composites using a shape memory polymer matrix material [5, 6]. The material response allowing for strain recovery in SMPs is known as a shape memory effect. This phenomenon includes the transition between two material phases glass and rubber. Materials in the glass phase possess a higher elastic modulus and will not deform easily. Conversely, materials above the glass transition temperature, and thus in the rubber phase, are much softer and can be deformed to large values of applied strain. The thermomechanical cycle for recovering a seemingly irrecoverable deformation, discussed in detail in references [7, 8], is summarized in the following steps: 1. Heat the material to above the glass transition temperature, Tg while maintaining a zero-stress. 2. Deform the material at the elevated temperature to the desired strain level. 3. At a constant applied strain on the SMP, cool to below Tg. 4. Release the load on the specimen. 5. Heat the material a second time to above Tg to recover original shape. Using the knowledge of the shape memory mechanism employed by SMPs, predictive models are created to accurately depict the material response. Such models require thorough experimental characterization. Initial modeling efforts have represented SMPs as a discrete spring-dashpot system [7, 9]. Additional models have been created to capture the small deformation material response. At small levels of deformation, the strain recovery behavior can be approximated as linear behavior [8]. While advances have been made in SMP modeling, existing models still cannot adequately capture the unique large-deformation recovery. Chen and Lagoudas recently developed an SMP model which accounts for the nonlinear material response due to large deformations. [10,11] The purpose for the study presented in this paper is to provide shape recovery test data to support current SMP model development and calibration. Shape memory polymer specimens were stretched to specific levels of applied strain, and upon removing any residual load, the specimen returned to its original shape. The degree to which the polymer returned to its original shape is measured by observing the recovered strain. Specimen preparation techniques and details of the experimental set up will be presented in this paper, followed by a discussion of the fundamental experimental properties of the SMP and the methodology for testing such material. EXPERIMENTAL DETERMINATION OF THERMOMECHANICAL PROPERTIES To thoroughly quantify the shape memory polymer material response, a variety of thermomechanical experiments is necessary. The origin and preparation of the specimens are described in detail in this section. In addition, the components of the experimental setup as well as the experiments performed are explained. Specimen Preparation The material used for testing was received from Cornerstone Research Group, Inc. (CRG) in Dayton, Ohio. The styrene-based shape memory polymer was manufactured in 305 x 305 x 3.18 mm sheets. With the first experimental focus strictly on tension tests, the experimental specimens were prepared and tested according to the ASTM standard D638 Standard Test Method for the Tensile Properties of Plastics [12]. The resulting samples were a dog-bone shape with a 57-mm gage length and a 12.7-mm gage width. The portions of the sample where the grips attached were 25.4-mm x 25.4-mm. The complete length of the specimens was 114-mm. A water jet cutting procedure was used to cut the experimental specimens. The water jet technique resulted in fewer variations in dimensions from specimen to specimen, and significantly reduced the likelihood of the ma-