Cyclic Viscoplastic-Viscodamage Analysis of Shape Memory Polymers Fibers With Application to Self-Healing Smart Materials

Abstract

The cold-drawn, programmed shape memory polymer (SMP) fibers show excellent stress recovery property, which promotes their application as mechanical actuators in smart material systems. A full understanding of the thermomechanical-damage responses of these fibers is crucial to minimize the trial-and-error manufacturing processes of these material systems. In this work, a multiscale viscoplastic-viscodamage theory is developed to predict the cyclic mechanical responses of SMP fibers. The proposed viscoplastic theory is based on the governing relations for each of the individual microconstituents and establishes the microscale state of the stress and strain in each of the subphases. These microscale fields are then averaged through the micromechanics framework to demonstrate the macroscale constitutive mechanical behavior. The cyclic loss in the functionality of the SMP fibers is interpreted as the damage process herein, and this cyclic loss of stress recovery property is calibrated to identify the state of the damage. The continuum damage mechanics (CDMj together with a thermodynamic consistent viscodamage theory is incorporated to simulate the damage process. The developed coupled viscoplastic-viscodamage theory provides an excellent correlation between the experimental and simulation results. The cyclic loading-damage analysis in this work relies on the underlying physical facts and accounts for the microstructural changes in each of the micro constituents. The established framework provides a well-structured method to capture the cyclic responses of the SMP fibers, which is of utmost importance for designing the SMP fiber-based smart material systems.

Thermomechanical constitutive modelling of shape memory polymer including continuum functional and mechanical damage effects

“…F D ij , the damage strain rate, i.e. D D ij , and the scalar MD parameter has been discussed in detail by Shojaei et al [22,23].…”

“…As depicted in figure 1, the holding time during the programming has a direct impact on the magnitude of the shape fixity ratio (SFR) obtained after the unloading process and also on the SRR measured through unconstrained heating process, i.e. free recovery [2,4,13,14,22,36,55]. The effect of the holding time (or structural relaxation time) on the SRR is captured through F t rel function, and an empirical constitutive relation is defined for this function as follows:…”

“…The accumulation of the failed chains may result in the loss of the mechanical properties of polymers such as a gradual reduction in the elastic tensile modulus [55,57]. In the case of ductile or low cycle fatigue, the MD damage parameter is constitutively developed by [22,23,54,58] …”

“…Thermomechanical modelling of SMPs has been extensively studied in the literature [4,5,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Most recently Qi and co-workers [25] reported the influence of programming conditions on shape fixity and free shape recovery and developed a unified model for shape memory behaviours.…”

“…In general, the damage mechanisms in SMPs can be categorized into two broad classes: (i) mechanical damage (MD) or physical damage: which is associated with the mechanical loads, e.g. impact or fatigue, during the service life of the SMP structures, represented by degradation in physical or mechanical properties such as modulus of elasticity [22,23,[29][30][31][32][33][34]; and (ii) functional damage (FD) or thermomechanical cyclic damage: besides the service loads, SMPs experience thermomechanical loads during their TM cycles and the associated damage in these cycles give rise to loss of their functionality [4,13,14,22,23,35,36]. Under cyclic thermomechanical loadings, SMP may exhibit continuous degradation in functionality, such as reduction in shape recovery ratio (SRR) or stress recovery ratio.…”