Tensile Behavior of Low Density Thermally Bonded Nonwoven Material

A discontinuous and non-uniform microstructure of alow-density thermally bonded nonwoven materialdisplays in a complicated and unstable tensilebehavior. This paper reports uniaxial tensile tests of alow density thermally bonded nonwoven toinvestigate the effect of the specimen size and shapefactor, as well as the cyclic tensile loading conditionsemployed to investigate the deformational behaviorand performance of the nonwoven at differentloading stages. The experimental data are comparedwith results of microscopic image analysis and FEmodels

Mechanical Stimuli in Prediction of Trabecular Bone Adaptation: Numerical Comparison

Adaptation is the process, with which bone responds to changes in loading environment and modifies its properties and organisation to meet the mechanical demands. Trabecular bone undergoes significant adaptation when subjected to external forces, accomplished through resorption of old and fractured bone and formation of a new bone material. These processes are assumed to be driven by mechanical stimuli of bone-matrix deformation sensed by bone mechanosensory cells. Although numerous in vivo and in vitro experimental evidence of trabecular bone morphology adaptation was obtained, the exact nature of mechanical stimuli triggering biological responses (i.e., osteoclastic resorption and osteoblastic formation) is still debated. This study aims to compare different mechanical stimuli with regard to their ability to initiate the load-induced adaptation in trabecular bone. For this purpose, a 2D model of two trabeculae, connected at their basement, with bone marrow in the intertrabecular space was developed. The finite-element method was implemented for the model loaded in compression to calculate magnitudes of several candidates of the bone-adaptation stimuli. A user material subroutine was developed to relate a magnitude of each candidate to changes in the shape of trabeculae

Theoretical analysis on needle-punched carbon/carbon composites

© 2019, Springer Nature B.V. Needle-punched carbon/carbon composites (NP-C/Cs) are advanced materials widely used in aerospace applications. The needle-punching technique improves the integrality of carbon-fibre plies, however, it also introduces many defects, affecting the mechanical behavior of NP-C/Cs. A theoretical model of irregular beams is suggested to investigate the mechanical behavior of unidirectional needle-punched carbon/carbon composites. Stress distributions in punched and squeezed fibres and an effect of the needle-punching technology are assessed

Hybrid equilibrium finite element formulation for composite beams with partial interaction

Thanks to their various benefits, composite beams have been increasingly used in various applications. This study will focus on two-layer composite beams with a flexible shear interface between layers. The finite element method, in particular its displacement-based formulation, has been recognized as the most popular method for numerical analysis of composite beams. However, when applied to Timoshenko beams with partial interaction, the displacement-based formulation may suffer from the so-called shear-locking and slip-locking phenomena, leading to erroneous solutions. Hybrid and mixed finite element formulations have been viewed as competitive alternatives, since they naturally avoid locking effects. Special types of these formulations are the so-called equilibrium-based formulations, producing statically admissible solutions. This work introduces for the first time an equilibrium-based finite element formulation for the analysis of Timoshenko composite beams with partial interaction. The formulation relies on a variational principle of complementary energy involving only force/moment-like variables as fundamental unknown fields. The approximate field variables are selected such that all equilibrium equations hold in strong form. The inter-element equilibrium is enforced by resorting to the Lagrangian multiplier method. Unlike traditional displacement-based finite element formulations, the proposed scheme is naturally free from both shear- and slip-locking phenomena. The accuracy and effectiveness of the new formulation is numerically assessed through the analysis of several numerical examples. In particular, the ability of the formulation to model accurately both very flexible and very stiff shear connections is numerically shown

Numerical assessment of residual formability in sheet metal products : towards design for sustainability

A new computational scheme is presented to addresses cold recyclability of sheet- metal products. Cold recycling or re-manufacturing is an emerging area studied mostly empirically; in its current form, it lacks theoretical foundation especially in the area of sheet metals. In this study, a re-formability index was introduced based on post-manufacture residual formability in sheet metal products. This index accounts for possible levels of deformation along different strain paths based on Polar Effective Plastic Strain (PEPS) technique. PEPS is strain-path independent, hence provides a foundation for residual formability analysis. A user- friendly code was developed to implement this assessment in conjunction with advanced finite- element (FE) analysis. The significance of this approach is the advancement towards recycling of sheet metal products without melting them

Finite element analysis of hypervelocity impact behaviour of CFRP-Al/HC sandwich panel

The mechanical response of CFRP-Al/HC (carbon fibre-reinforced/epoxy composite face sheets with Al honeycomb core) sandwich panels to hyper-velocity impact (up to 1 km/s) is studied using a finite-element model developed in ABAQUS/Explicit. The intraply damage of CFRP face sheets is analysed by mean of a user-defined material model (VUMAT) employing a combination of Hashin and Puck criteria, delamination modelled using cohesive-zone elements. The damaged Al/HC core is assessed on the basis of a Johnson Cook dynamic failure model while its hydrodynamic response is captured using the Mie-Gruneisen equation of state. The results obtained with the developed finite-element model showed a reasonable correlation to experimental damage patterns. The surface peeling of both face sheets was evident, with a significant delamination around the impact location accompanied by crushing HC core

Plastic behaviour of microstructural constituents of cortical bone tissue: a nanoindentation study

A mechanical behaviour of bone tissues is defined by mechanical properties of its microstructural constituents. Also, those properties are important as an input for finiteelement models of cortical bone to simulate its deformation and fracture behaviours at the microstructural level. The aim of this study was to investigate a post-yield behaviour of osteonal cortical bone’s microstructural constituents at different loading rates, maximum load levels and dwell times; nanoindentation with a spherical-diamond-tip indenter was employed to determine it. The nanoindentation results revealed significant difference in stiffness values of cortical bone’s microstructural features − interstitial matrix and osteons. Similarly, interstitial matrix exhibited a stiffer post-yield behaviour compared to that of osteons that reflects the relationship between the post-yield behaviour and collagen maturity. In addition, both osteons and interstitial matrix demonstrated a time-dependent behaviour. However, in order to assess elastic-plastic behaviour accurately, an effect of viscosity on nanoindentation results was reduced by using a time-delay method

Deformation mechanisms in advanced Ti-based alloy in instrument-workpiece interaction

Industrial applications of Ti-based alloys especially in aerospace, marine and offshore industries have grown significantly over the years primarily due to their high strength, light weight as well as excellent temperature- and corrosion-resistance properties. Since these alloys are hard to machine, there is an obvious demand to develop simulation tools in order to analyze the material's behavior in machining processes, such as a turning, and to optimize process parameters. High levels of strains and strain rates accompanied by generated high temperatures characterize the deformation process in turning. The character of realisation of deformation mechanisms as well as a spatial distribution of their parameters in turning has some similar features with other tool-workpiece interaction processes under certain conditions. One example is dynamic indentation, in which an indenter penetrates into a workpiece. Comparison of a dynamic indentation technique with quasi-static one enables us to measure time-resolved depth and load responses. Hence, it can serve as a tool for understanding the spatio-temporal realization of deformation mechanisms in Ti-based alloy. In parallel with studies of the macroscopic (global) response of the material to indentation, micro-indentation is also studied to elucidate the effect of crystallographic texture on the material's behaviour at micro-scale. This study, based on combination of various experimental techniques with finite-element simulations of instrument-workpiece interaction, provides a comparative analysis of characteristic features of deformation processes at micro and macro scales in Ti-based alloy and investigates the effect of various factors on their realization

Nonlinear Compression Behavior of Warp-Knitted Spacer Fabric: Effect of Sandwich Structure

Compressibility of warp-knitted spacer fabrics is one of their important mechanical properties with regard to many special applications such as body protection, cushion and mattresses. Due to specific structural features of the fabric and a non-linear mechanical behavior of monofilaments, the compression properties of this kind of fabrics are very complicated. Although several studies have been performed to investigate their compression behavior, its mechanism has not well been understood yet. This work is concerned with a study of compression mechanism of a selected warp-knitted spacer fabric with a given sandwich structure. Both experimental and numerical methods are used to study the effect of the material's structure on the overall compression mechanism. Compression tests are conducted to obtain force-displacement relationships of the fabric. A micro-computed tomography system is used to analyze specimens under different levels of compression displacement to investigate the change in material's structure during the compression process. At the same time, finite element models are developed separately to simulate the initial geometric structure and the compression behavior of the fabric. Three finite element models based on beam elements are firstly developed to simulate the effect of manufacturing process on shapes of monofilaments within the fabric and to determine their morphologies, which are used to assemble a geometry part of the finite element model of the overall fabric. Then the finite-element model is developed using beam and shell elements to describe the compression behavior of the fabric by introducing the effect of its complex microstructure and real non-linear mechanical properties of the monofilaments. A comparison of the obtained experimental and CT data, and results of simulation is carried out, demonstrating a good agreement. With this study, a compression mechanism of the warp-knitted spacer fabric can be better understood

Numerical modelling of impact fracture of cortical bone tissue using X-FEM

A cortical bone tissue is susceptible to fracture that can be caused by events, such as traumatic falls, sports injuries and traffic accidents. A proper treatment of bones and prevention of their fracture can be supported by in-depth understanding of deformation and fracture behaviour of this tissue in such dynamic events. Parameters such as damage initiation under impact, damage progression and impact strength can help to achieve this goal. In this paper, Extended Finite-Element Method (X-FEM) implemented into the commercial finite-element software Abaqus is used to simulate the actual crack initiation and growth in a cantilever beam of cortical bone exposed to quasi-static and impact loading using the Izod loading scheme. Izod tests were performed on notched bone specimens of bovine femur to measure its impact strength and to validate simulations. The simulation results show a good agreement with the experimental data