Micromechanical modeling of anisotropic behavior of oriented semicrystalline polymers

Some manufacturing processes of polymeric materials, such as injection molding or film blowing, cause the final product to be highly anisotropic. In this study, the mechanical behavior of drawn polyethylene (PE) tapes is investigated via micromechanical modeling. An elasto-viscoplastic micromechanical model, developed within the framework of the so-called composite inclusion model, is presented to capture the anisotropic behavior of oriented semicrystalline PE. Two different phases, namely amorphous and crystalline (both described by elasto-viscoplastic constitutive models), are considered at the microstructural level. The initial oriented crystallographic structure of the drawn tapes is taken into account. It was previously shown by Sedighiamiri et al. (Comp. Mater. Sci. 2014, 82, 415) that by only considering the oriented crystallographic structure, it is not possible to capture the macroscopic anisotropic behavior of drawn tapes. The main contribution of this study is the development of an anisotropic model for the amorphous phase within the micromechanical framework. An Eindhoven glassy polymer (EGP)-based model including different sources of anisotropy, namely anisotropic elasticity, internal stress in the elastic network and anisotropic viscoplasticity, is developed for the amorphous phase and incorporated into the micromechanical model. Comparisons against experimental results reveal remarkable improvements of the model predictions (compared to micromechanical model predictions including isotropic amorphous domains) and thus the significance of the amorphous phase anisotropy on the overall behavior of drawn PE tapes

Mechanics of amorphous solids-identification and constitutive modelling

Both polymers and metals can be in an organised crystalline or amorphous glassy state, where for polymers usually at least a part of the structure is amorphous and metals are in a glassy state only when processed under special conditions. At the 15th European Mechanics of Materials Conference in September 2016 in Brussels, Belgium, a session focussing on the mechanical properties of amorphous or partly amorphous solid materials was organised, attempting to bridge descriptions found for metallic glasses and polymers, which share some common features, such as a rate- and temperature-dependent response, being prone to strain localisation in the form of shear bands, the occurrence of damage by cavitation, etc

Multi-scale microstructure evolution of tungsten under neutron and plasma loads

Tungsten (W) owing to its excellent high temperature properties, is the candidate material for plasma facing components in fusion reactors such as ITER and DEMO. However, the lifetime of the tungsten based plasma facing component and thereby the lifetime of the reactor, is dictated by the extreme particle (neutron and ions) and heat loads, and is not very well understood. The fast neutrons result in the generation of point and clustered lattice defects, which further interact with the plasma based helium ions, leading to nucleation and growth of helium bubbles. Additionally, these interactions in combination with high temperatures influence the microstructural evolutionby grain growth and recrystallization process, ultimately affecting the mechanical and thermal properties. Thus, an in-depth understanding on the role of helium ions in conjunction with heat and neutron loads is crucial for predicting the microstructure evolution under fusion conditions accurately.In the present work, a multi-scale model describing the simultaneous effect of the defect generation by neutron irradiation and helium implantation from the plasma, considering irradiation time scales of hours and component length scales is developed. At atomic length scales, the generation of defects such as the vacancies, self-interstitial atoms, their clustering and the trapping of helium at defects and their clusters are modelledusing a kinetic rate theory approach. Additionally, these microstructurallevel interactions are linked to mesoscopic length scales by considering the diffusion of mobile defects along the tungsten monoblock depth (ITER specifications). The spatially varying defect concentrations from the model are also used to obtain a measure of the spatially varying lattice stored energy, thereby allowing to link the effect of helium with mechanisms such as recrystallization and grain growth. The influence of the helium resolution from existing bubbles and microstructural sinks on the helium diffusionlength scales is investigated. Furthermore, the effect of helium cluster mobility on the overall helium retention in tungsten is found to be less-pronounced

Micromechanics of oriented semi-crystalline polymers: from structure to properties

The microstructure of semi-crystalline polymers, in terms of for example the degree of crystallinity, crystal type, size and orientation, may vary drastically depending on subtle details of the manner in which the polymer is shaped into the final product. For this material, often an oriented microstructure is formed, leading to anisotropic yield and failure kinetics.\ua0To obtain a fundamental and quantitative understanding of how these anisotropic properties depend on the structure, a multiscale micromechanical model is developed. The modelling approach is based on a mean field framework, accounting for the crystalline phases, which are modelled by crystal plasticity and amorphous domains. The anisotropy of these amorphous regions is incorporated in the micromechanical model in the form of a pre-stretch of the amorphous network and anisotropic visco-plastic flow. Both aspects are found to be crucial for predicting the experimentally observed orientation dependence of the yield kinetics. With this combined experimental-modelling approach, new insight in the physical processes that govern the mechanics of semi-crystalline polymers are obtained