Multi-field modelling and simulation of large deformation ductile fracture

In the present contribution we focus on a phase-field approach to ductile fracture applied to large deformation contact problems. Phase-field approaches to fracture allow for an efficient numerical investigation of complex three-dimensional fracture problems, as they arise in contact and impact situations. To account for large deformations the underlying formulation is based on a multiplicative decomposition of the deformation gradient into an elastic and plastic part. Moreover, we make use of a fourth-order crack regularization combined with gradient plasticity. Eventually, a demonstrative example shows the capability of the proposed framework

Optimisation and characterisation of graphene-based microporous layers for polymer electrolyte membrane fuel cells

The viability of graphene-based microporous layers (MPLs) for polymer electrolyte membrane fuel cells is critically assessed through detailed characterisation of the morphology, microstructure, transport properties and electrochemical characterisation. Microporous layer composition was optimised by the fabrication of several hybrid MPLs produced from various ratios of graphene to Vulcan carbon black. Single cell tests were performed at various relative humidities between 25% and 100% at 80 °C, in order to provide a detailed understanding of the effect of the graphene-based MPL composition on the fuel cell performance. The inclusion of graphene in the MPL alters the pores size distribution of the layer and results in presence of higher amount of mesopores. Polarisation curves indicate that a small addition of graphene (i.e. 30 wt %) in the microporous layer improves the fuel cell performance under low humidity conditions (e.g. 25% relative humidity). On the other hand, under high humidity conditions (≥50% relative humidity), adding higher amounts of graphene (≥50 wt %) improves the fuel cell performance as it creates a good amount of mesopores required to drive excess water away from the cathode electrode, particularly when operating with high current densities

Gas permeability, wettability and morphology of gas diffusion layers before and after performing a realistic ex-situ compression test

The through-plane gas permeability, wettability, thickness and morphology have been investigated before and after a compression test, which is important to the GDL design. The compression tests were designed to simulate the initial assembling compression and the cycles of loading and unloading arising as a result of hydration/dehydration of the membrane. Owing to the presence of the microporous layer (MPL), the results show that the coated gas diffusion layers (GDLs) are slightly more resistive to deformation than the uncoated GDLs. Amongst all the tested carbon substrates (i.e. the uncoated GDLs), Toray carbon substrate was found to show the least reduction in thickness and gas permeability after compression, and this was attributed to its relatively high density and low porosity. As for the coated GDLs, the level of MPL penetration for one of the tested GDLs (i.e. SGL 35BC) was significantly higher than that of the other GDL (i.e. SGL 34BC), resulting in substantially less reduction in thickness and gas permeability of the former GDL after compression. Finally, the contact angles of all the tested GDL materials were found to decrease after compression due to the decreased surface roughness

Phase field modeling of ductile fracture at finite strains: A variational gradient-extended plasticity-damage theory

This work outlines a rigorous variational-based framework for the phase field modeling of ductile fracture in elastic-plastic solids undergoing large strains. The phase field approach regularizes sharp crack surfaces within a pure continuum setting by a specific gradient damage modeling with geometric features rooted in fracture mechanics. It has proven immensely successful with regard to the analysis of complex crack topologies without the need for fracture-specific computational structure such as finite element design of crack discontinuities or intricate crack-tracking algorithms. Following the recent work Miehe et al. (2015), the phase field model of fracture is linked to a formulation of gradient plasticity at finite strains. The formulation includes two independent length scales which regularize both the plastic response as well as the crack discontinuities. This ensures that the damage zones of ductile fracture are inside of plastic zones, and guarantees on the computational side a mesh objectivity in post-critical ranges. The novel aspect of this work is a precise representation of this framework in a canonical format governed by variational principles. The coupling of gradient plasticity to gradient damage is realized by a constitutive work density function that includes the stored elastic energy and the dissipated work due to plasticity and fracture. The latter represents a coupled resistance to plasticity and damage, depending on the gradient-extended internal variables which enter plastic yield functions and fracture threshold functions. With this viewpoint on the generalized internal variables at hand, the thermodynamic formulation is outlined for gradient-extended dissipative solids with generalized internal variables which are passive in nature. It is specified for a conceptual model of von Mises-type elasto-plasticity at finite strains coupled with fracture. The canonical theory proposed is shown to be governed by a rate-type minimization principle, which fully determines the coupled multi-field evolution problem. This is exploited on the numerical side by a fully symmetric monolithic finite element implementation. The performance of the formulation is demonstrated by means of some representative examples

Multi-field modelling and simulation of large deformation ductile fracture

In the present contribution we focus on a phase-field approach to ductile fracture applied to large deformation contact problems. Phase-field approaches to fracture allow for an efficient numerical investigation of complex three-dimensional fracture problems, as they arise in contact and impact situations. To account for large deformations the underlying formulation is based on a multiplicative decomposition of the deformation gradient into an elastic and plastic part. Moreover, we make use of a fourth-order crack regularization combined with gradient plasticity. Eventually, a demonstrative example shows the capability of the proposed framework

Curvilinear virtual elements for contact mechanics

The virtual element method (VEM) for curved edges with applications to contact mechanics is outlined within this work. VEM allows the use of non-matching meshes at interfaces with the advantage that these can be mapped to a simple node-to-node contact formulation. To account for exact approximation of complex geometries at interfaces, we developed a VEM technology for contact that considers curved edges. A number of numerical examples illustrate the robustness and accuracy of this discretization technique. The results are very promising and underline the advantages of the new VEM formulation for contact between two elastic bodies in the presence of curved interfaces

Magnetic and structural characterization of Nb 3+ -substituted CoFe 2 O 4 nanoparticles

This study investigated the effect of Nb 3+ substitution on the magnetic and structural properties of CoFe 2 O 4 nanoparticles (NPs) synthesized by hydrothermal approach. The formation of a single phase of spinel ferrite was confirmed through X-ray powder diffraction, and crystallite sizes in the range 18–30 nm were observed. Moreover, it found that the Fourier transform infrared (FT-IR) spectra of the NPs included the main vibration bands of the spinel structure. The partially cubic structure was confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The energy band gaps for CoNb x Fe 2-x O 4 were estimated to be in the range 0.48–0.53 eV for Nb 3+ content x = 0.0–0.10. Magnetization measurements at room temperature (RT; 300 K) and at 10 K were performed on spinel CoNb x Fe 2-x O 4 (0.00 ? x ? 0.10) NPs using a vibrating sample magnetometer (VSM). Nb 3+ doping significantly changed the magnetization and coercivity of the Co ferrite samples. RT hysteresis curves indicated well-defined ferrimagnetic behavior for all prepared NPs with saturation magnetization (M s ) in the range 44.45 – 49.40 emu/g and remanent magnetization (M r ) in the range 12.16 – 17.90 emu/g. The coercive field (H c ) is found to be equal 936 Oe and is decreased with Nb 3+ substitutions. However, hysteresis curves at 10 K showed finite remanent specific magnetization (1.90–6.70 emu/g) but significant asymmetric coercivity (715–2810 Oe), particularly for the Nb 3+ -doped samples. At 10 K, the magnetization values were 4–6 times smaller but symmetric coercivity field values were 2–3 times larger compared with the RT-VSM curves. The obtained magnetic parameters indicated the semi-hard magnetic character of the Co ferrite samples at low temperatures. © 2019 Elsevier Ltd and Techna Group S.r.l