PSI - Issue 33

Emanuele Sgambitterra et al. / Procedia Structural Integrity 33 (2021) 1073–1081 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1075

3

under long term operations with complex/extreme service loading conditions (Niccoli et al. 2017, Ionatjis et al. 1995). Within this context, the combination of SMAs with conventional polymer matrix composites would offer unique features, as they combine the exceptional active features of SMAs with very high structural properties of composite materials. In last years, several studies were carried out with the aim of developing new design and manufacturing strategies for the realization of SMA-based polymer composites made by SMA wires embedded in polymeric matrices (Wei et al. 1998, Hebda et al. 1995, Tsoi et al. 2002, Jang et at. 2005). In particular, Thairi et al. (2004) and Winzek et al. (2003) studied the difference between the phase transition temperature of the SMA and the glass transition temperature of the polymer. Barrett and Gross (1996) developed a low stiffness active composite where SMA filaments are embedded in a silicone matrix to be used for biomedical, surgical and prosthetic applications. However, some technical issues related to SMA-polymer composites are still unsolved, mainly arising from the high strain, stresses and temperatures generated by SMA thermal activation, that become even more complex during cyclic loading due to the unique fatigue (Di Cocco et al. 2014, Maletta et at. 2017, Sgambitterra et al. 2018, Sgambitterra et al. 2019) and fracture response of SMAs (Maletta and Young 2011, Sgambitterra et al. 2014, Sgambitterra et al. 2015). In particular, the limited metal-polymer interface strength, under the serious thermomechanical conditions caused by SME, represents the main technical difficulty for the development of SMA based composites. In fact, during the SMA thermal activation, the combination of the high temperature together with the high recovery strain/stress could lead to premature debonding mechanisms of the composite structure (Murasawa et al. 2004, Payandeh 2012). The choice of a suitable matrix, with proper strength (Jang and Kishi 2005), deformation to failure, curing and glass transition temperature and the chemical composition of the shape memory alloy are of great interest for designing and manufacture smart composite materials. In fact, their mechanical properties and response strongly depends on the efficiency of stress and strain transfer at the wire-matrix interface. The aim of this work is to analyze the basic mechanical interaction of SMA/polymer interfaces resulting from both mechanical stresses and thermal activation of SMAs. To this purpose, results of preliminary studies to assess the interfacial strength of SMA/polymer samples, subjected to pull out tests and thermal activation cycles, are reported in this paper. This work is carried within the R&D industrial project ARIA (Active Responsive Intelligent Aerodynamics) whose aim is to develop SMA-based composite systems with shape morphing capabilities, to improve the aerodynamic performance of motorized vehicles. Three different experiments were carried out: i) pull-out test on samples as manufactured, ii) pull-out tests after cyclic activation of the sample and iii) pull-out test during the activation of the SMA wire. The aim is to catch the influence of the mechanical/thermal stress and/or their combination on the interfacial strength of SMA/polymer samples. Results revealed the main technical limitations for SMA-based composites that are related to the weakening of the mechanical interaction between the resin and the SMA wire when this latter is thermally activated. 2. Materials and experiments 2.1. Material selection Both SMA and polymer materials to be used for SMA composites were preliminary selected based on their thermomechanical properties, that is according to the both manufacturing and operative constraints as schematically shown in Fig. 1 and described below. • Condition #1: manufacturing constrain Curing temperature of the polymer (T C ) must be lower than the activation temperature of the SMA (Austenite start TT, A s ) to avoid premature SMA activation during polymer setting; • Condition #2: operative constrain Glass transition temperature of the polymer (T G ) must be higher than the activation temperature of the SMA (Austenite finish TT, A f ). This is to avoid material damage during SMA thermal activation; • Condition #3: operative constrain