The constitutive modelling of semi-crystalline polymers (SCP) has to consider several aspects as rate- and temperature-dependence, self-heating, and in particular, the double yield (DY) phenomenon. A full characterization of all these complex features involves prominent efforts in terms of material testing and parameter identification (PI). The contribution of the crystalline phase plays an important role in the evolution of the plastic yield in the SCPs. In this work, a constitutive model, named Unified SCP (USCP), is proposed by modifying the physically-based Boyce-Parks-Argon (BPA) glassy model. The contribution of the crystalline phase is introduced in the strain softening/hardening evolution law of the strength, providing an alternative interpretation of the underlying morphological changes caused by the crystalline phase embedded in the amorphous phase. The proposed formulation extends the BPA model with a new contribution to capture the crystalline phase. A full thermo-mechanical coupled numerical framework is developed for the USCP model validation. The DY phenomenon at different strain rates with self-heating and thermal softening effects is investigated and predicted. The proposed model extension needs three material constants with clear physical meaning. To identify them, a fast in-house optimization process based on Nelder-Mead is used, in which only a single element test is required. The model accurately predicts the experimental results for both thermosets and thermoplastics such as epoxy, nylon 101, PA6 and LDPE under monotonic loadings reported by different authors.

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The double yield (DY) phenomenon observed in a wide variety of semi-crystalline polymers (SCP) adds difficulties in the material characterization. In this paper, a constitutive model, termed as explicit semi-crystalline polymer (ESCP) model, is proposed to study DY phenomenon as well as the rate- and temperature-dependent thermomechanical response below the glass transition temperature. The underlying yield kinetics due to the morphological changes of the spherulite micro-structure is represented by a rheological analogue described by a physically-based amorphous intermolecular resistance and a rate-independent crystalline interlamellar resistance. Independently-identified viscoelastic response and network resistance are also implemented to complete the model description. The activation and disclosure of the crystalline component depend on the saturated state of amorphous phase. The proposed model is validated against ex- perimental data obtained from different authors for three commonly used SCPs: nylon 101, LDPE and PA6. A straightforward parameter identification procedure, requiring a minimum number of calibration tests, is presented to illustrate the model usage. The thermomechanical-coupled analyses provide satisfactory predictions using simulated models of a cylinder compression and dogbone tensile tests at different rates, where the self-heating and thermal softening effects are naturally captured by the model.@en