Meso-scale deformation and damage in thermally bonded nonwovens

Thermal bonding is the fastest and the cheapest technique for manufacturing nonwovens. Understanding mechanical behaviour of these materials, especially related to damage, can aid in design of products containing nonwoven parts. A finite element (FE) model incorporating mechanical properties related to damage such as maximum stress and strain at failure of fabric’s fibres would be a powerful design and optimisation tool. In this study, polypropylene-based thermally bonded nonwovens manufactured at optimal processing conditions were used as a model system. A damage behaviour of the nonwoven fabric is governed by its single-fibre properties, which are obtained by conducting tensile tests over a wide range of strain rates. The fibres for the tests were extracted from the nonwoven fabric in a way that a single bond point was attached at both ends of each fibre. Additionally, similar tests were performed on unprocessed fibres, which form the nonwoven. Those experiments not only provided insight into damage mechanisms of fibres in thermally bonded nonwovens but also demonstrated a significant drop in magnitudes of failure stress and respective strain in fibres due to the bonding process. A novel technique was introduced in this study to develop damage criteria based on the deformation and fracture behaviour of a single fibre in a thermally bonded nonwoven fabric. The damage behaviour of a fibrous network within the thermally bonded fabric was simulated with a FE model consisting of a number of fibres attached to two neighbouring bond points. Additionally, various arrangements of fibres’ orientation and material properties were implemented in the model to analyse the respective effects.     

Experimental and FEM analysis of the compressive behavior of 3D random fibrous materials with bonded networks

Three-dimensional (3D) random fibrous (RF) materials with bonded networks exhibit unique mechanical properties. In this paper, an inorganic RF material is fabricated and its compressive behavior is studied by experiment. Based on the experimental characterization, a 3D numerical model is constructed using a series of geometrical algorithms. Afterward, the statistical distribution of fiber segment aspect ratio is calculated using the numerical model, which is consistent with that measured from SEM photos. The compressive behavior of this RF material is simulated using finite element method (FEM). In the FEM model, the effect of fiber–fiber contact is realized by contact spring. The calculated results agree well with experimental data. Moreover, the predicted failure mechanism is verified by SEM observation. Finally, the FEM model is employed to investigate the influences of porosity on the elastic modulus and compressive strength of the RF material.     

Non-uniformity of deformation in low-density thermally point bonded non-woven material: effect of microstructure

One of the most important characteristic features of a low-density thermally bonded non-woven material is its discontinuous and non-uniform microstructure, resulting in a complicated and unstable deformation mechanism of the material. In order to estimate the effects of such microstructure on the overall mechanical properties of the non-woven material, tensile tests are carried out for specimens with different systems of marks for both two principle directions—machine direction and cross direction—with images being captured with high-speed camera. The non-uniform strain fields are analysed based on the obtained images. Discontinuous finite-element models are developed to study the deformation mechanism of non-woven specimens in both principle directions, and the effects of the discontinuous and non-uniform fibrous network and different arrangements of bond points are analysed numerically.     

Emergence of fibrous fan morphologies in deformation directed reformation of hyperelastic filamentary networks

Recently, the authors generalized a theory for modelling the scission and reforming of crosslinks in isotropic polymeric materials to include materials in which elastic fibers are embedded in an elastic matrix. The fibers were assumed to dissolve with increasing deformation and then to immediately reassemble in a direction defined as part of the model. The model was illustrated in detail for uniaxial stretching along the direction of the fibers. Fiber reassembly was along the original fiber direction and did not result in a change in fiber alignment. The present work examines the implications of this model when the direction of reassembly is uncorrelated with the original fiber direction. In particular, the fibers are assumed to reassemble in the direction of maximum principal stretch of the matrix. The specific case is treated when the deformation is simple shear and the initial fiber direction is perpendicular to the direction of shear. The resulting fiber elongation with increasing shear results in fiber dissolution over a constitutively determined interval of the amount of simple shear. Newly formed fibers align in the current principal direction of maximum stretch, which is a direction that changes with the amount of simple shear. The resulting interval of alignment angles generates a fan-like fiber morphology at each material point. The formation and structure of the fan is described. In addition, the relation between the shear and normal stresses and the amount of shear is discussed, both during loading and unloading. It is shown that there can be a state of permanent set that is related to the original shape by triaxial extension and shear.     

Tensile behaviour of thermally bonded nonwoven structures: model description

Nonwovens are complex three-dimensional anisotropic structures and consisting of fibres orientated in certain directions, which are bonded by thermal, chemical, mechanical entanglement or a combination of these techniques. Thermally bonded are further classified in two categories, i.e. through-air and calendared nonwoven structures. In this study, a modified micromechanical model describing the tensile behaviour of thermally bonded nonwovens is proposed by incorporating the effect of fibre re-orientation during the deformation. The anisotropic behaviour of through-air bonded structures is demonstrated through theoretical stress–strain curves and the relationship between the fibre re-orientation and fabric strain is also analysed. Furthermore, the failure criterion of thermally bonded nonwovens is analysed using pull-out behaviour of fibres in the system. A parametric study revealing the dependencies of various structural and geometrical characteristics of fibres on pull-out behaviour of fibres in thermally bonded nonwovens is also discussed.     

Effect of fiber orientation on the non-affine deformation of random fiber networks

The study of fiber networks is essential in understanding the mechanical properties of many polymeric and biological materials. These systems deform non-affinely, i.e. the local deformation is different than the applied far-field. The degree of non-affinity increases with decreasing scale of observation. Here, we show that this relationship is a power law with a scaling exponent independent of the type of applied load. Preferential fiber orientation influences non-affinity in a significant way: this parameter generally increases upon increasing orientation. However, some components of non-affinity, such as that associated with the normal strain in the direction of the preferential fiber orientation, decrease. In random networks, the nature of the far-field has little influence on the level of non-affinity. This is not the case in oriented networks.     

Finite element modelling of thermally bonded bicomponent fibre nonwovens: Tensile behaviour

Nonwovens are polymer-based engineered textiles with a random microstructure and hence require a numerical model to predict their mechanical performance. This paper focuses on finite element (FE) modelling the elastic–plastic mechanical response of polymer-based core/sheath type thermally bonded bicomponent fibre nonwoven materials. The nonwoven fabric is treated as an assembly of two regions having distinct mechanical properties: fibre matrix and bond points. The fibre matrix is composed of randomly oriented core/sheath type fibres acting as load-transfer link between bond points. Random orientation of individual fibres is introduced into the model in terms of the orientation distribution function (ODF) in order to determine the material’s anisotropy. The ODF is obtained by analysing the data acquired with scanning electron microscopy (SEM) and X-ray micro computed tomography (CT). On the other hand, bond points are treated as a deformable bicomponent composite material composed of the sheath material as matrix and the core material as fibres having random orientations. An algorithm is developed to calculate the anisotropic material properties of these regions based on properties of fibres and manufacturing parameters such as the planar density, core/sheath ratio and fibre diameter. Having distinct anisotropic mechanical properties for two regions, the fabric is modelled with shell elements with thicknesses identical to those of the bond points and fibre matrix. Finally, nonwoven specimens are subjected to tensile tests along different loading directions with respect to the machine direction of the fabric. The force–displacement curves obtained in these tests are compared with the results of FE simulations.     

Processes of microstructural evolution during superplastic deformation

Microstructural changes occurring at the surface and in the bulk of superplastically deformed materials have been considered. Surface studies showed formation of macroscopic surface steps and fibers, which determine surface roughness and can affect corrosion properties, respectively. Bulk microstructural changes include morphological and crystallographic changes, as well as defect accumulation. Phase/grain growth, phase spatial distribution, and phase/grain shape changes control the morphology of the phase constituents. Weakening of preexisting texture and an increase in the ratio of high angle grain boundaries determine crystallographic changes. Defect accumulation is related to cavity formation; density of lattice dislocation is superplastically deformed materials is low. Various explanations proposed for these processes of microstructural evolution in superplastic materials are considered. It is shown that these processes are closely related to cooperative grain boundary sliding—that is, the sliding of grain groups.     

Mechanical properties of thermally cycled nylon bonded Nd-Fe-B permanent magnets

The effect of thermal cycling on the stress-strain behavior of polyamide (nylon) and polyphenylene-sulfide (PPS) based injection molded Nd-Fe-B magnets was investigated after test specimens were cycled between –40 and 150°C for 50, 500, or 5000 repetitions. It was found that PPS based magnets exhibit higher ultimate strengths, higher modulus and lower toughness than nylon based magnets. Furthermore, formulations containing platelet morphology particles exhibited higher strengths and modulus than those containing spherical morphology particles, with increases in particle volume fraction leading to a decrease in strength. Differences in strength, modulus, and toughness were attributed to the degree of bonding between the matrix and the magnet powder in the various formulations, the degree of crosslinking, along with the effects of powder morphology. Additionally, it was found that while the stiffness of these materials increased with thermal cycling, their toughness decreased significantly, by as much as 99%. The extent of these effects was found to be dependent on the polymer matrix, powder morphology, and volume fraction of powder in the magnet. Finally, it was found that the PPS composites showed less relative change due to thermal cycling than the Nylon composites.     

Direct visualization of microstructural deformation processes in polyethylene

The deformation morphology of high density polyethylene was investigated by electron microscopy of oriented thin films. A melt-drawing process was used to produce chain axis oriented and planar textured thin film, which was subsequently deformedin situ at room temperature in a scanning transmission electron microscope. Both as-drawn and annealed films were studied. Deformation along the orientation direction is initially accomodated by the interlamellar regions, which cavitate and form microfibres. For annealed films it is possible to directly observe strain-induced crystallization at about 300% strain in the fibrils. With increased deformation, suitably oriented lamellar crystals deform by two clearly visualized chain slip systems: {1 0 0}, 0 0 1 and {0 1 0}, 0 0 1. Thesec axis shear processes could be further distinguished as fine slip or as block shear. Still higher deformation causes more breakup of blocks by shear; when the block size is less than some critical size, the blocks decrystallize. The deformation leads toward a fibrillar morphology consisting of oriented crystals from crystallized amorphous material at high elongations, crystal blocks broken out of lamellae, and chains drawn out of lamellae and recrystallized. The deformation behaviour of the as-drawn films is somewhat different from that of the annealed films. Initially as for the annealed films the lamellae are separated with increasing strain as the interlamellar regions are deformed but there is less voiding. Higher deformation causes the lamellae in the as-drawn films to shear apart at significantly lower strain levels (50% as opposed to 300%) than in the annealed films. At about 100% deformation, the as-drawn film no longer has a recognizable lamellar structure. Although generalization is tempered by the simplicity of this model texture, these deformation results are highly relevant to the current microstructural understanding of lamellar deformation in different regions of a spherulite, to the morphology of commercial extruded and blown films, and to specially prepared textured polymers, such as rolled and annealed films or capillary melt flow and solidification methods which can produce texture approaching that of a single crystal.     

2D finite element analysis of thermally bonded nonwoven materials: Continuous and discontinuous models

Due to a random structure of nonwoven materials, their non-uniform local material properties and nonlinear properties of single fibres, it is difficult to develop a numerical model that adequately accounts for these features and properly describes their performance. Two different finite element (FE) models – continuous and discontinuous – are developed here to describe the tensile behaviour of nonwoven materials. A macro-level continuum finite element model is developed based on the classic composite theory by treating the fibrous network as orthotropic material. This model is used to analyse the effect of thermally bonding points on the deformational behaviour and deformation mechanisms of thermally bonded nonwoven materials at macro-scale. To describe the effects of discontinuous microstructure of the fabric and implement the properties of polypropylene fibres, a micro-level discontinuous finite element model is developed. Applicability of both models to describe various deformational features observed in experiments with a real thermally bonded nonwoven is discussed.     

Microstructure,deformation and failure of polymer bonded explosives

Polymer bonded explosives (PBXs) are highly particle filled composite materials comprised of explosive crystals and a polymeric binder (ca. 5–10% by weight). The microstructure and mechanical properties of two pressed PBXs with different binder systems were studied in this paper. The initial microstructure of the pressed PBXs and its evolution under different mechanical aggressions were studied, including quasi-static tension and compression, ultrasonic wave stressing and long-pulse low-velocity impact. Real-time microscopic observation of the PBXs under tension was conducted by using a scanning electron microscope equipped with a loading stage. The mechanical properties under tensile creep, quasi-static tension and compression were studied. The Brazilian test, or diametrical compression, was used to study the tensile properties. The influences of pressing pressures and temperatures, and strain rates on the mechanical properties of PBXs were analyzed. The mesoscale damage modes in initial pressed samples and the samples insulted by different mechanical aggressions, and the corresponding failure mechanisms of the PBXs under different loading conditions were analyzed.     

Nonlinear mechanics of soft fibrous networks

Mechanical networks of fibres arise on a range of scales in nature and technology, from the cytoskeleton of a cell to blood clots, from textiles and felts to skin and collageneous tissues. Their collective response is dependent on the individual response of the constituent filaments as well as density, topology and order in the network. Here, we use the example of a low-density synthetic felt of athermal filaments to study the generic features of the mechanical response of such networks including strain stiffening and large effective Poisson ratios. A simple microscopic model allows us to explain these features of our observations, and provides us with a baseline framework to understand active biomechanical networks.     

Nonlinear random vibrations of thermally buckled skew plates

Random vibrations in the postbuckling range have chaotic properties superposed. For hard and simply supported polygonal plates a multi-modal projection by the Galerkin-procedure renders as a result of a proper non-dimensional formulation a set of nonlinearly coupled ordinary differential equations. Exact unifying solutions of the stationary F-P-K equation are constructed for that class of problems where the nonlinear restoring forces are derived from a potential function. Assuming an effective white noise excitation, the probability of first occurrence of dynamic snap-through is determined for a single mode approximation. Using a two-mode expansion the probability distribution of the asymmetric snap-buckling is also evaluated.     

Overall behavior and microstructural deformation of R-CNT/polymer composites

Carbon nanotubes (CNTs) are an excellent candidate for the reinforcement of composite materials owing to their distinctive mechanical and electrical properties. Reticulate carbon nanotubes (R-CNTs) with a 2D or 3D configuration have been manufactured in which nonwoven connected CNTs are homogeneously distributed and connected with each other. A composite reinforced by R-CNTs can be fabricated by infiltrating a polymer into the R-CNT structure, which overcomes the inherent disadvantages of the lack of weaving of the CNTs and the low strength of the interface between CNTs and the polymer. In this paper, a 2D plane strain model of a R-CNT composite is presented to investigate its micro-deformation and effective stiffness. Using the two-scale expansion method, the effective stiffness coefficients and Young’s modulus are determined. The influences of microstructural parameters on the micro-deformation and effective stiffness of the R-CNT composite are studied to aid the design of new composites with optimal properties. It is shown that R-CNT composites have a strong microstructure-dependence and better effective mechanical properties than other CNT composites.     

Hot deformation behavior and microstructural evolution of Ag-containing 2519 aluminum alloy

The hot deformation behaviors of Ag-containing 2519 aluminum alloy were studied by isothermal compression at 300–500 °C with strain rates from 0.01 s⁻¹ to 10 s⁻¹. The microstructural evolution of the alloy was investigated using Polyvar-MET optical microscope and Tecnai G² 20 transmission electron microscope (TEM). It has been shown that the flow stress of the alloy increases with increasing the strain rate and decreasing the deformation temperature. When the strain rate is lower than 10 s⁻¹, the flow stress increases with increasing strain until the stress reached the peak value, after which the flow stress remains almost constant. This result indicates that dynamic recovery happens during deformation. When the strain rate is 10 s⁻¹ and the temperature is higher than 300 °C, serrated flow behavior is generally observed with the stress decreasing with increasing strain, a typical phenomenon of dynamic recrystallization.     

Optimization of fibrous laminates undergoing progressive damage

Laminates made of fibrous composites are optimized with the objective of maximizing the remaining stiffness at the end of some given load history, during which damage and deterioration of the material have occurred. The material behaviour is thus both highly non‐linear and history dependent. The design variables are the fibre orientations of each of the plies. The progressive and anisotropic damage is modelled by a continuum damage model in which a set of internal damage variables describes the decrease of the elastic moduli and Poisson ratios and yet another set describes the reduction of the strength properties in each of the plies. The evolution of the deterioration is based on a principle of maximum energy dissipation. The numerical model and the formulation of a consistent tangent operator is thoroughly described. The involved design sensitivity analysis is performed by direct differentiation in a manner fully compatible with the numerical model. The importance of taking the damage into consideration in the design process is clearly demonstrated by some computational examples where also the advantages of the optimization process become evident. Copyright © 2000 John Wiley & Sons, Ltd.