Abstracts:The aim of the present work is to study the hydrodynamic behavior of the single phase water flow through three metallic foam samples (i.e. Copper, NiFeAlC, and Inconel) in a plate heat exchanger. These samples have a same pore diameter (1200 µm) and grade (20 PPI), however, the Microscopic images show that the ligament diameter and the surface roughness are different. The effect of the metallic foam surface roughness and the ligament diameter on the pressure drop behavior is analyzed, and the regime transitions from pre-Darcy to turbulent are identified. It is found that more, the metallic foam surface is rough, more the pressure drop is important and the turbulent regime is reached rapidly. The Permeability and Forchheimer parameters were calculated for each flow regime. It is noticed that each fluid regime can exhibit different permeability and Forchheimer coefficient for the same foam sample. The present data were confronted to those available in the literature and a good qualitative and quantitative agreement was obtained.

Abstracts:A multi-scale model, based on the concept of Representative Volume Element (RVE), is proposed linking a classical continuum at RVE level to a macro-scale strain-gradient theory. The multi-scale model accounts for the effect of body forces and inertia phenomena occurring at the micro-scale. The Method of Multiscale Virtual Power recently proposed by the authors drives the construction of the model. In this context, the coupling between the macro- and micro-scale kinematical descriptors is defined by means of kinematical insertion and homogenisation operators, carefully postulated to ensure kinematical conservation in the scale transition. Micro-scale equilibrium equations as well as formulae for the homogenised (macro-scale) force- and stress-like quantities are naturally derived from the Principle of Multiscale Virtual Power – a variational extension of the Hill-Mandel Principle that enforces the balance of the virtual powers of both scales. As an additional contribution, further insight into the theory is gained with the enforcement of the RVE kinematical constraints by means of Lagrange multipliers. This approach unveils the reactive nature of homogenised force- and stress-like quantities and allows the characterisation of the homogenised stress-like quantities exclusively in terms of RVE boundary data in a straightforward manner.

Abstracts:Localized deformation within energetic materials under impact loading may lead to the formation of hot spots, which can cause initiation or detonation of energetic materials. In this work, the thermal-mechanical response of cyclotetramethylene-tetranitramine (HMX) based granular explosives (GXs) and polymer-bonded explosives (PBXs) under impact loading has been investigated using finite element software ABAQUS. A series of three-dimensional mesoscale calculations is performed at impact velocities from 100m/s to 500m/s using a crystal plasticity constitutive model for HMX crystals that accounts for nonlinear, anisotropic thermoelasticity and for crystal plasticity. For PBX simulations, a viscoelasticity model is used for the polymer binder. Results show that the average and localized stress and temperature field, which are greatly affected by crystal anisotropy and polymer binder, of GXs are larger than those of PBXs. Qualitative agreement with Pop Plots from the experiments shows that GXs are more sensitive than PBXs.

Abstracts:This paper develops an experimentally calibrated and validated 3D finite element model for simulating strain-rate dependent deformation and damage behavior in representative volume elements of S-glass fiber reinforced epoxy-matrix composites. The fiber and matrix phases in the model are assumed to be elastic with their interfaces represented by potential-based and non-potential, rate-dependent cohesive zone models. Damage, leading to failure, in the fiber and matrix phases is modeled by a rate-dependent non-local scalar CDM model. The interface and damage models are calibrated using experimental results available in the literature, as well as from those conducted in this work. A limited number of tests are conducted with a cruciform specimen that is fabricated to characterize interfacial damage behavior. Validation studies are subsequently conducted by comparing results of FEM simulations with cruciform and from micro-droplet experiments. Sensitivity analyses are conducted to investigate the effect of mesh, material parameters and strain rate on the evolution of damage. Furthermore, their effect on partitions of the overall energy are also explored. Finally the paper examines the effect of microstructural morphology on the evolution of damage and its path.

Abstracts:The residual stress distribution introduced by shot peening (SP) in the deformed surface layer of titanium matrix composite (TiB+TiC)/Ti-6Al-4V was investigated via three-dimensional (3D) finite element dynamic simulation and experimental validation. The program of ANSYS/LS-DYNA was utilized in the 3D finite element dynamic analysis of SP process, and the 3D homogeneous and inhomogeneous models were set up. The results showed that the compressive residual stresses (CRS) were introduced in the matrix, but the tensile residual stresses appeared in the reinforcements. The maximum CRS and tensile residual stress were -1511 and +1155MPa respectively, which revealed the higher yield strength of reinforcements. This type of stress distribution revealed the effect of reinforcements, keeping the adverse tensile stresses in the reinforcements and retarding the damage to the matrix during deformation. In terms of experiments, after SP, the residual stresses along the depth from the surface were measured using X-ray diffraction (XRD) method. The experimental results indicated that the CRS formed in the surface layer and the maximum appeared on the subsurface. The range of residual stresses found in experiments supported the simulated results, which verified the validity of 3D finite element dynamic analysis.

Abstracts:This study aims at understanding the constitutive relation and critical condition for the shock compression of cellular solids. A 2D virtual foam is constructed from the cross-section of a closed-cell aluminium foam imaged by micro X-ray computed tomography, which enables the realistic consideration of meso-scale structural effect in numerical modelling. Quasi-static and shock compressions of the 2D foam are simulated. A series of Hugoniot relations between shock speed (and other mechanical quantities) and impact speed are determined from the finite element (FE) simulations. It is found that the shock speed increases approximately linearly with impact speed, similar to that observed for condensed solids, but the related material constants for cellular solids have different physical implications, whereas the shock strain, stress and energy increase with impact speed nonlinearly, due to shock-enhanced cell compaction and cell-wall plastic deformation. Based on conservation laws in continuum mechanics, other Hugoniot relations are derived from the basic linear one, which agree well with those obtained from the FE simulations. It is thus demonstrated that the unique linear Hugoniot relation can be used to characterise the shock constitutive behaviour which is distinct from the quasi-static one. Furthermore, a new analytical method based on the linear Hugoniot relation is proposed to estimate the critical impact speed for shock initiation, which has reasonable agreement with the present FE simulations and previous experimental and numerical results, and outperforms the existing methods.

Abstracts:In the present paper, we put forward some experimental results on the local thermomechanical behavior of a wet PA6.6 matrix subjected to cyclic loading. We used photomechanical techniques to assess the creep and cyclic strain rate fields, as well as the associated dissipation and coupling heat source fields. This thermomechanical analysis enabled us, from the very beginning of the cyclic test, to track the onset of strain concentration zones and note their limited development. Moreover, this analysis helped us to highlight the co-existence of glassy, rubber and mixed thermal responses. This revealed that wet PA6.6 was a graded material whose property variations surely promoted the development of strain concentration zones.