Thermomechanical Characterization of a Shape Memory Polymer Based Syntactic Foam

• Main Author: Nettles, Damon
• Other Authors: Li, Guoqiang
• Format: Others
• Language: en
• Published: LSU 2009
• Subjects: Mechanical Engineering
• Online Access: http://etd.lsu.edu/docs/available/etd-11102009-173141/

Shape memory polymers (SMPs) are a type of smart material capable of remembering multiple shapes and transitioning between them in response to an external stimulus. They offer the potential to self-seal macro-length scale damage in a nearly autonomous fashion. Syntactic foams are lightweight structural materials currently used in the marine and aerospace industries. This study seeks to bring syntactic foams and SMPs together, retaining characteristics of both components, to create a low-density smart composite. The SMP based syntactic foam is used as the core of a grid stiffened sandwich structure capable of healing impact damage multiple times. In order to better understand the sealing efficiency and the effect different programming and recovery conditions have on it, the current research is concerned with characterizing the SMP based syntactic foam using T_g determination by DMA, isothermal uniaxial compressive behavior at three temperatures and three strain levels, quasi strain-controlled programming followed by free recovery, volume change, and through thermomechanical cycles using stress-controlled programming (at two different stresses) followed by free, strain-controlled, and stress-controlled recovery. Compression above the T_g, at 79°C, revealed that the stiffness and strength were significantly lowered, with the foam being less affected. At 121°C, all compositions behaved like a rubber. Creep effects were observed after the initial loading during stress-controlled programming. Additional deformation occurred during cooling due to thermal contractions, viscoelasticity around the T_g, and viscoplasticity below the T_g. Stress-controlled programming offers the best results and allows for the most control. However, quasi strain-controlled compressive programming can be used to achieve reasonable shape fixities. Shape fixity values were close to 100% for all compositions using stress-controlled programming. Free shape recovery was near 100% for non-foam and around 86% for the foam. The majority of the recovery occurred in the T_g region. Strain-controlled recovery can be used to recover all of the programming stress, during which time there was an initial build up of thermal stress followed by a decrease and then a plateau. Confined recovery is effective at sealing damage by either controlling the strain or stress and allowing the shape memory effect to recover into the internal free volume.