Mechanical behaviour of thermally bonded bicomponent fibre nonwovens: experimental analysis and numerical modelling

In contrast to composites and woven fabrics, nonwoven materials have a unique web structure, which is composed of randomly oriented fibres bonded in a pattern by mechanical, thermal or chemical techniques. The type of nonwovens studied in this research is a thermally bonded one with polymer-based bicomponent fibres. Such fibres have a core/sheath structure with outer layer (sheath) having a lower melting temperature than that of the core. In thermal bonding of such fibres, as the hot calender with an engraved pattern contacts the fibrous web, bond points are formed thanks to melting of the sheath material. Molten sheath material acts as an adhesive while core parts of the fibres remain fully intact in the bond points. On the other hand, web regions, which are not in contact with the hot engraved pattern, remain unaffected and form the fibre matrix that acts as a link between bond points. With two distinct regions, namely, bond points and fibre matrix, with different structures, nonwovens exhibit a unique deformation behaviour. This research aims to analyse the complex mechanical behaviour of thermally bonded bicomponent fibre nonwoven materials using a combination of experimental and numerical methods. A novel approach is introduced in the thesis to predict the complex mechanical behaviour of thermally bonded bicomponent fibre nonwovens under various threedimensional time-dependent loading conditions. Development of the approach starts with experimental studies on thermally bonded bicomponent fibre nonwovens to achieve a better understating of their complex deformation characteristics. Mechanical performance of single bicomponent fibres is investigated with tensile and relaxation tests since they are the basic constituents of nonwoven fabrics. The fabric microstructure, which is one of the most important factors affecting its mechanical behaviour, is examined with scanning electron microscopy and X-ray micro computed tomography techniques. At the final part of experimental studies, mechanical response of thermally bonded bicomponent fibre nonwovens is characterised with several mechanical tests. (Continues...)

Characterization of input parameters for numerical modeling of nonwoven materials

Characterization of input parameters for numerical modeling of nonwoven material

Micro-indentation based study on steel sheet degradation through forming and flattening: Toward a predictive model to assess cold recyclability

In Cold Roll Forming (CRF) process sheet material undergo a complex set of deformation that entail complicated through thickness residual deformation. This paper, focuses on material behaviour in CRF processes, with regarding damage and material degradation. A roll forming process is taken as case study and experimental investigation using extensive microhardness mapping alongside FE simulation of the process are the basis of material damage study. Indentation on different cross-sectional cutting angles -45, 0, 45 has been performed to study the sensitivity to orientation and crystallographic texture. A 3D Finite Element simulation with emphasis on through-thickness variation of the plastic deformation was carried out, using multiple layers of solid elements representing the sheet. A smart approach to reduce computational cost was employed in MSC.Marc by implementing simulation of a master model of complete material with shell elements, followed by partial sub-modelling comprising solid elements in regions of interest. This cross-sectional hardness map was then converted to the corresponding equivalent plastic strain in the cross section for validation. The correlation factor between Hardness and yield stress was discussed

Computational assessment of residual formability in sheet metal forming processes for sustainable recycling

This paper introduces a new computational scheme addressing a problem of cold recyclability of sheet–metal products based on the assessment of their post-manufacture residual formability. Formability of sheet metals has been studied for several decades, and various techniques were suggested since a Forming Limit Diagram was first introduced in the 1960s. At the same time, cold recycling, or re-manufacturing, of sheet metals is an emerging area studied mostly empirically; in its current form, it lacks theoretical foundation. In order to address the challenge of residual formability for sheet-metal products, a reformability index is introduced in this study. The proposed method takes advantage of the latest developments in the area of evaluating multiple-path formability and introduces a quantitative reformability index for the manufactured material. This index represents possible levels of strains for deformation along different paths, based on Polar Effective Plastic Strain (PEPS). PEPS provides robustness against non-linear strain-path effects, thus making a reliable basis for such analysis. Based on residual formability, a predictive model was sought to assess a degrading effect of the flattening process. Taking advantage of extensive numerical simulation, a wide range of geometrical parameters in an unbending process, as a predominant mechanism in flattening, was studied. The reformability index alongside prediction of degradation in flattening allows evaluation of prospective re-manufacturing. The significance of this research is its advancement towards recycling of sheet-metal products without melting them by facilitating design for sustainability. The proposed scheme also provides a subroutine friendly framework for numerical simulations

Variability and anisotropy of mechanical behavior of cortical bone in tension and compression

The mechanical properties of cortical bone vary not only from bone to bone; they demonstrate a spatial viability even within the same bone due to its changing microstructure. They also depend considerably on different loading modes and orientations. To understand the variability and anisotropic mechanical behavior of a cortical bone tissue, specimens cut from four anatomical quadrants of bovine femurs were investigated both in tension and compression tests. The obtained experimental results revealed a highly anisotropic mechanical behavior, depending also on the loading mode (tension and compression). A compressive longitudinal loading regime resulted in the best load-bearing capacity for cortical bone, while tensile transverse loading provided significantly poorer results. The distinctive stress-strain curves obtained for tension and compression demonstrated various damage mechanisms associated with different loading modes. The variability of mechanical properties for different cortices was evaluated with two-way ANOVA analyses. Statistical significances were found among different quadrants for the Young’s modulus. The results of microstructure analysis of the entire transverse cross section of a cortical bone also confirmed variations of volume fractions of constituents at microscopic level between anatomic quadrants: microstructure of the anterior quadrant was dominated by plexiform bone, whereas secondary osteons were prominent in the posterior quadrant. The effective Young’s modulus predicted using the modified Voigt-Reuss-Hill averaging scheme accurately reproduced our experimental results, corroborating additionally a strong effect of random and heterogeneous microstructure on variation of mechanical properties in cortical bone

Numerical study of strain-rate effect in cold rolls forming of steel

Cold roll forming (CRF) is a well-known continuous manufacturing process, in which a flat strip is deformed by successive rotating pairs of tools, without changing the material thickness. In the past decades, to lessen the process-development efforts, finite-element simulations have been increasingly employed to improve the process design and predict the manufacturing-induced defects. One of the important aspects in design of the CRF process is consideration of resulting strains in the final product as the material passes through several complex forming stands. Sufficient knowledge of longitudinal strain in the workpiece is required to set various process parameters. Increasing a process speed in a roll forming operation can bring cost advantages, but the influence of the forming speed on the strain distribution should be explored. This study is focussed on a strain-rate effect in the CRF process of steel sheets. The strain-rate dependency of a plastic behaviour observed in most metals can affect the finished product’s quality as well as process parameters. This paper investigates the influence of the strain rate on longitudinal strains induced in the roll forming operation by incorporating a phenomenological Johnson-Cook constitutive model, which allows studying the impact of the process speed on the output product. Taking advantage of 3D finite element analysis, a roll forming process was simulated using MCS.Marc, comprising a complete set of forming stations. Through the changing of the process speed, the strain rate impact on longitudinal peak strains and forming length was investigated. The results highlight the effect of the strain rate on edge thinning and subsequent undesirable distortions in the product

Developing 3D fully parametric multi-scale computational model for nonwoven simulations

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Cellular response to cyclic compression of tissue engineered intervertebral disk constructs composed of electrospun polycaprolactone

There is lack of investigation capturing the complex mechanical interaction of tissue engineered IVD (intervertebral disc) constructs in physiologically-relevant environmental conditions. In this study, mechanical characterisation of anisotropic eletrospinning (ES) substrates made of polycaprolactone (PCL) was carried out in wet and dry conditions and viability of human bone marrow derived mesenchymal stem cells (hMSCs) seeded within double layers of ES PCL was also studied. Cyclic compression of IVD-like constructs composed of an agarose core confined by ES PCL double-layers was implemented using a bioreactor and the cellular response to the mechanical stimulation was evaluated. Tensile tests showed decrease of elastic modulus of ES PCL as the angle of stretching increased and at 60° stretching angle in wet, maximum ultimate tensile strength was observed. Based on the configuration of IVD-like constructs, the calculated circumferential stress experienced by the ES PCL double layers was 40 times of the vertical compressive stress. Confined compression of IVD-like constructs at 5% and 10% displacement dramatically reduced cell viability, particularly at 10%, although cell presence in small and isolated area can still be observed after mechanical conditioning. Hence, material mechanical properties of tissue-engineered scaffolds, composed of fibril structure of polymer with low melting point, are affected by the testing condition. Circumferential stress induced by axial compressive stimulation, conveyed to the ES PCL double-layer wrapped around an agarose core, can affect the viability of cells seeded at the interface, depending on the mechanical configuration and magnitude of the load

Effect of morphological state of graphene on mechanical properties of nanocomposites

In the last decade, graphene has emerged as one of the best-performing reinforcement materials for nanocomposites. Incorporation of graphene into polymer results in a nanocomposite with a new microstructure responsible for its enhanced features. A morphological state of graphene flakes is one of the factors that govern formation of microstructure. Studies showed that graphene-oxide (GO) flakes can be found either as fully exfoliated or intercalated in polymer-based nanocomposites. While traditional parameters are commonly taken into consideration in theoretical assessment of properties of composites by means of micromechanical models, the morphological state is often ignored. This research aims to investigate the effect of morphological state of GO flakes on stiffness of nanocomposites with widely used micromechanical models, e.g., rule of mixtures, Hui-Shia and Halpin-Tsai. Pure sodium alginate and nanocomposites on its basis reinforced with 1.0 and 2.5 wt% GO were used in the study. Parameters required for modelling were quantified with microstructural characterisation. Micromechanical models were adapted to account for the morphological state of intercalation observed in the characterisation study. Tensile experiments were employed to assess the adopted models, and the effect matrix stiffness, GO thickness, spacing of intercalates as well as the Poisson’s ratio and stiffness of inter-flake polymer layers was studied

Multiscale modelling of mechanical and flow performance of nonwovens

Multiscale modelling of mechanical and flow performance of nonwoven