Large deformation of thermally bonded random fibrous networks: microstructural changes and damage

A mechanical behaviour of random fibrous networks is predominantly governed by their microstructure. This study examines the effect of microstructure on macroscopic deformation and failure behaviour of random fibrous networks and its practical implication for optimisation of its structure by using finite-element simulations. A subroutine-based parametric modelling approach-a tool to develop and characterise random fibrous networks-is also presented. Here, a thermally bonded polypropylene nonwoven fabric is used as a model system. Its microstructure is incorporated into the model by explicit introduction of fibres according to their orientation distribution in the fabric. The model accounts for main deformation and damage mechanisms experimentally observed and provides the meso- and macro-level responses of the fabric. The suggested microstructure-based approach identifies and quantifies the spread of stresses and strains in fibres of the network as well as its structural evolution during deformation and damage. Its simulations also predict a continuous shift in the distribution of stresses due to structural evolution and progressive failure of fibres. © 2014 Springer Science+Business Media New York

Strength of fibres in low-density thermally bonded nonwovens: an experimental investigation

Mechanical properties of nonwovens related to damage such as failure stress and strain at that stress depend on deformation and damage characteristics of their constituent fibres. Damage of polypropylene-fibre commercial low-density thermally bonded nonwovens in tension was analysed with tensile tests on single fibres, extracted from nonwovens bonded at optimal manufacturing parameters and attached to individual bond points at both ends. The same tests were performed on raw polypropylene fibres that were used in manufacturing of the analysed nonwovens to study quantitatively the effect of manufacturing parameters on tenacity of fibres. Those tests were performed with a wide range of strain rates. It was found that the fibres break at their weakest point, i.e. bond edge, in optimally bonded nonwovens. Additionally, failure stress and strain in tension of a fibre extracted from the fabric were significantly lower than those of virgin fibre. Since damage in nonwovens occurs by progressive failure of fibres, those experiments were used to establish damage initiation and propagation in thermally bonded nonwovens based on polypropylene fibres. Moreover, the results obtained from the experiments are useful to simulate the damage behaviour of nonwoven fabrics

Modelling of cyclic plasticity and crack-tip deformation in a single crystal nickel-based superalloy

In this paper, cyclic plasticity in a single crystal nickel-based superalloy CMSX4 at elevated temperature was modelled using crystal plasticity (CP) and discrete dislocation dynamics (DDD). The CP modelling was carried out at the continuum level based on shear deformation along the octahedral slip systems. The DDD approach was implemented for a representative volume element with periodical boundary conditions. The DDD model parameters were calibrated using the predicted response by the CP model. The obtained numerical results by the two models compared with each other under strain-controlled monotonic and cyclic loading conditions in both and orientations. The CP model was further applied to study crack-tip deformation under fatigue loading. Stress-strain responses, as well as stress and strain distributions, near the crack tip were particularly monitored to assess the role of cyclic plasticity in controlling fatigue-crack propagation. The work has applications in life assessment of nickel-based superalloy components for high-temperature application

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

Numerical analysis of dynamic out-of-plane loading of nonwovens

This paper presents finite element (FE) modelling of deformation behaviour of thermally bonded bicomponent fibre nonwovens under out-of-plane dynamic loading. Nonwoven fabric was treated as an assembly of two regions with distinct mechanical properties. Bond points were treated as composite material having a matrix of the sheath material reinforced with fibres of the core material. Elastic-plastic and viscous properties of the constituent fibres, obtained with tensile and relaxation tests were implemented into the FE model. The mechanical behaviour of the material under out-of-plane dynamic loading was observed with visual techniques. The deformation behaviour of nonwoven under out-of-plane dynamic loading computed with the numerical model was compared with that observed in the tests

Nonwovens modelling: a review of finite-element strategies

This paper reviews the main strategies used to simulate the mechanical behaviour of nonwoven materials that is defined by a structure of their fibrous networks and a mechanical behaviour of constituent fibres or filaments. The main parameters influencing the network structure of nonwoven materials are discussed in the first part. The second part deals with two main strategies employed in the analysis of mechanical behaviour of nonwoven materials using finiteelement models based on continuous and discontinuous techniques. Both strategies have further sub-types, which are critically reviewed, and future trends in this area of research are discussed

Fatigue crack growth in a nickel-based superalloy at elevated temperature - experimental studies, viscoplasticity modelling and XFEM predictions

Experimental studies and computational modelling of crack deformation and growth in a nickel-based superalloy at elevated temperature have been carried out for a threepoint bending specimen subjected to fatigue loading condition. In order to remove the influence of oxidation which can be considerable at elevated temperature, crack growth was particularly tested in a nominal vacuum/minimal oxidation environment with a focus on dwell effects. For simulation, the material behaviour was described by a cyclic viscoplastic constitutive model with nonlinear kinematic and isotropic hardening rules. Computational analyses of a stationary crack showed the progressive accumulation of plastic strain near the crack tip, which has been subsequently used as a fracture criterion to predict crack growth using the extended finite element method (XFEM). The crack was assumed to grow when the accumulated plastic strain ahead of the crack tip reached a critical value which was calibrated from crack growth test data in vacuum. During the simulation, the crack length was recorded against the number of loading cycles, and the results are in good agreement with the experimental data which proves the model’s capability to predict fatigue crack growth in nickelbased superalloys at high temperature. It is also shown, both experimentally and numerically, that an increase of dwell period leads to an increase of crack growth rate due to the increased creep deformation near the crack tip, but this effect is marginal when compared to the dwell effects under fatigue-oxidation conditions

Mechanical behaviour of nonwovens: analysis of effect of manufacturing parameters with parametric computational model

A deformation behaviour of, and damage in, polymer-based thermally bonded nonwovens was studied with a parametric finite-element model. Microstructure of the studied nonwoven was modelled by direct introduction of fibres and bond points, employing a subroutine-based parametric technique. This technique helped to implement variations in dimensional characteristics of structural entities related with manufacturing of these materials. Following experimental observations, a realistic orientation distribution of fibres and single-fibre failure criteria were included into the model. The developed model was demonstrated to be a very useful tool not only for predicting effects of parameters related to manufacturing of nonwovens or of specimen’s size on a macroscopic response of the nonwoven but also for getting an insight into deformation mechanisms and damage localization in its structure

Damage mechanisms of random fibrous networks

Fibrous networks are ubiquitous: they can be found in various engineering applications as well as in biological tissues. Due to complexity of their random microstructure, anisotropic properties and large deformation, their modelling is challenging. Though, there are numerous studies in literature focusing either on numerical simulations of fibrous networks or explaining their damage mechanisms at micro or meso-scale, the respective models usually do not include actual random microstructure and failure mechanisms. The microstructure of fibrous networks, together with highly non-linear mechanical behaviour of their fibres, is a key to initiation of damage, its spatial localization and ultimate failure [1]. Numerical models available in literature are not capable of elucidating actual microstructure of the material and, hence, its influence on damage processes in fibrous networks. To emulate a real-life microstructure in a developed finite-element model, an orientation distribution function for fibres obtained from X-ray micro computed-tomography images was considered to provide actual alignment of fibres. To validate the suggested model, notched and unnotched rectangular specimens were experimentally tested. A good correlation between the experimental data and simulation results was observed. This study revealed a significant effect of a notch on damage evolution

Mechanical analysis of bi-component-fibre nonwovens: finite-element strategy

In thermally bonded bi-component fibre nonwovens, a significant contribution is made by bond points in defining their mechanical behaviour formed as a result of their manufacture. Bond points are composite regions with a sheath material reinforced by a network of fibres’ cores. These composite regions are connected by bi-component fibres — a discontinuous domain of the material. Microstructural and mechanical characterization of this material was carried out with experimental and numerical modelling techniques. Two numerical modelling strategies were implemented: (i) traditional finite element (FE) and (ii) a new parametric discrete phase FE model to elucidate the mechanical behaviour and underlying mechanisms involved in deformation of these materials. In FE models the studied nonwoven material was treated as an assembly of two regions having distinct microstructure and mechanical properties: fibre matrix and bond points. The former is composed of randomly oriented core/sheath fibres acting as load-transfer link between composite bond points. Randomness of material’s microstructure was introduced in terms of orientation distribution function (ODF). The ODF was obtained by analysing the data acquired with scanning electron microscopy (SEM) and X-ray micro computed tomography (CT). Bond points were treated as a deformable two-phase composite. An in-house algorithm was used to calculate anisotropic material properties of composite bond points based on properties of constituent fibres and manufacturing parameters such as the planar density, core/sheath ratio and fibre diameter. Individual fibres connecting the composite bond points were modelled in the discrete phase model directly according to their orientation distribution. The developed models were validated by comparing numerical results with experimental tensile test data, demonstrating that the proposed approach is highly suitable for prediction of complex deformation mechanisms, mechanical performance and structure-properties relationships of composites