0000000000006313
AUTHOR
Janis Andersons
showing 38 related works from this author
Bio-based rigid high-density polyurethane foams as a structural thermal break material
2020
Abstract Sustainable development of building industry implies increasing usage of green materials. With this aim and for the intended application as a structural thermal break material, rigid high-density polyurethane foams have been manufactured using polyols derived from renewable resources - tall oil fatty acids. Thermal conductivity, compressive strength and stiffness of the foams of density ranging from ca. 100 to 680 kg/m3 have been determined. Comparison of the bio-based foams with reference foams derived from petrochemical resources demonstrated similar performance characteristics thus suggesting that bio-based foams can also serve as structural thermal break materials. Analytical m…
The Effect of Damage and Geometrical Variability on the Tensile Strength Distribution of Flax Fibers
2010
Natural fibers of plant origin are finding non-traditional applications as reinforcement of composite materials. The mechanical properties of fibers exhibit considerable scatter, being affected by the natural variability in plant as well as the damage accumulated during processing. For bast fibers, the primary damage mode is kink bands – zones of misaligned cellulose microfibrils extending across the fiber and oriented roughly perpendicularly to its axis. Another feature typical for natural fibers and contributing to the scatter of fiber strength is the variability of diameter along a fiber length and among the fibers. An analytical expression for the distribution of the longitudinal tensil…
Mechanical performance of thermoplastic matrix natural-fibre composites
2008
Publisher Summary Natural-Fibre Composites (NFCs) are being considered as high-tech materials meant to replace conventional composites in high-performance applications. There are a number of advantages that natural fibres can offer compared with synthetic materials. Thermoplastic natural-fibre composites can be classified by the polymer used as a matrix as well as by the type of the fibre reinforcing the polymer. Currently, the main areas of application of thermoplastic NFCs are packaging, transportation and building industries. The packaging industry requires materials that possess specific properties, but that can also be easily discarded after use. This chapter discusses the mechanical p…
Evaluation of interfacial shear strength by tensile tests of impregnated flax fiber yarns
2011
Adhesion of flax fibers and polymer matrix as well as mutual bonding of elementary fibers in a technical fiber are among the principal factors governing the mechanical response of flax fiber-reinforced polymer–matrix composites. A method for evaluation of adhesion is proposed based on tension tests of impregnated fiber yarns, with subsequent characterization by optical microscopy of length distribution of fibers pulled out of the yarn fracture surfaces. An elementary probabilistic model is derived relating aspect ratio distribution of the pulled-out fibers to the fiber tensile strength distribution and the effective interfacial shear strength (IFSS). The method was applied to flax fiber/vi…
Anisotropy of the stiffness and strength of rigid low-density closed-cell polyisocyanurate foams
2016
The cells of polymer foams are usually extended in the foam rise direction, causing a geometrical anisotropy, the degree of which, characterized by the cell aspect ratio R, depends on foam density and production method. Such elongated cell shape translates into anisotropy of the mechanical properties of foams. Rigid low-density closed-cell polyisocyanurate foams of apparent density ranging from ca. 30 to 75 kg/m3, containing polyols derived from renewable resources, have been produced and tested for the stiffness and strength in the foam rise and transverse directions in order to experimentally characterize their mechanical anisotropy. Analytical relations for foams with rectangular paralle…
Modeling the nonlinear deformation of flax-fiber-reinforced polymer matrix laminates in active loading
2015
In an attempt to fully utilize the mechanical properties of bast fibers in polymer-matrix composites, unidirectional (UD) or quasi-UD flax-fiber-reinforced composites are being developed and characterized. Their response in tension is markedly nonlinear both in on- and off-axis loading. A semiempirical tensor-linear model is applied to describe such deformation nonlinearity in active combined loading. The deformation model of UD ply, combined with an elementary laminate theory, is used to predict the stress–strain curves of laminated composites in tension. Reasonable accuracy of prediction is demonstrated for fiber-dominated layups.
Dependence of fracture toughness of composite laminates on interface ply orientations and delamination growth direction
2004
A critical review has been performed of the published experimental research concerning delamination onset and growth in composite laminate interfaces of different lay-ups under single-mode loadings. It was found that, depending on the loading mode and interface lay-up, the traditional fracture toughness characterization by unidirectionally reinforced composite tests can lead to marked under- or overestimation of material resistance to crack growth. Empirical models of fracture toughness as a function of delamination front orientation with respect to reinforcement directions of the adjacent laminae have been validated and their applicability range established.
Strength and Damage of Elementary Flax Fibers Extracted from Tow and Long Line Flax
2009
Flax fibers possess high specific strength and stiffness, and thus are competitive in terms of mechanical properties with traditional reinforcing fibers used in polymer-matrix composite materials. For environmental and economical benefit, it would be preferable to apply nontextile grade fibers in composites provided their mechanical characteristics are acceptable. Elementary fibers have been extracted from long line flax, used as high-quality raw material for textile industry, and flax tow, and their strength distribution and damage level determined. It is shown that the elementary flax fibers coming from short flax fiber are not inferior to those of textile-quality flax in terms of streng…
Progressive cracking mastercurves of the transverse ply in a laminate
2009
In this study, progressive cracking of a transverse layer in a cross-ply composite laminate subjected to tensile loading is considered. Using the results of a probabilistic cracking model, approximate relations for crack density as a function of stress are derived for initiation-controlled and propagation-controlled cracking. It is shown that the crack density evolution in the transverse ply can be represented by a mastercurve in suitably normalized coordinates. The mastercurve approach is applied to progressive cracking in glass/epoxy laminates. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers
Analysis of the effect of a stress raiser on the strength of a UD flax/epoxy composite in off-axis tension
2014
The effect of stress raisers in the form of a slit-like notch and an open circular hole on the tensile strength of a quasi-UD flax-fiber-reinforced composite is studied experimentally. A finite fracture mechanics approach is applied to determine the intralaminar fracture toughness of the composite and to predict the strength in the presence of stress concentration. Reasonably good agreement of the notch effect predicted using finite fracture mechanics with a coupled strength and toughness fracture criterion and test results is demonstrated.
Modeling the non-linear deformation of a short-flax-fiber-reinforced polymer composite by orientation averaging
2013
Abstract The growing usage of short-flax-fiber-reinforced polymer composites in such applications as automotive industry necessitates the prediction of their mechanical response up to and beyond the limit of elasticity. Due to the imperfect, mechanical interlocking-dominated adhesion of natural fibers to most polymers, both fiber debonding and matrix yielding contribute to the non-linear deformation. In the present study, the deformation under an active loading of a short misaligned fiber composite is modeled by the orientation averaging approach, employing an analytical description of the behavior of a unit cell (UC), the parameters of which are determined using a FEM analysis of UC respon…
Evaluation of toughness by finite fracture mechanics from crack onset strain of brittle coatings on polymers
2008
Crack onset strain measurements of a confined layer in tension provide the means for layer toughness estimation. The procedure can be simplified if steady-state conditions prevail starting from the commencement of crack propagation, an assumption frequently employed in energy release rate evaluation. It is demonstrated, by numerical analysis of experimental data, that an estimate of the defect size in the film is needed in order to reliably evaluate its fracture toughness from the crack onset strain. Only if microcracks of sufficient size are present in the brittle layer, the steady-state energy release rate at the crack onset strain can be identified with layer toughness. Otherwise, the to…
Finite fracture mechanics analysis of crack onset at a stress concentration in a UD glass/epoxy composite in off-axis tension
2010
The presence of stress concentrations at holes and notches is known to reduce the strength of composite materials. Due to complexity of the damage processes at a stress raiser in a composite, different modeling approaches have been developed, ranging from empirical point and average stress criteria to involved damage mechanics or cohesive zone-based models of failure. Finite fracture mechanics approach with a coupled stress and energy failure criterion, recently developed and applied mainly to cracking in homogeneous isotropic materials, allows predicting the appearance and propagation of a crack using material strength and toughness characteristics obtained from independent tests. The pres…
Coating fragmentation by branching cracks at large biaxial strain
2007
The fragmentation behaviour of a thin brittle coating attached to a ductile substrate subjected to equibiaxial quasi-static in-plane tension is studied. The experimentally observed cracking patterns are related to repetitively branching coating cracks. The fragmentation process is modelled by the rate equation approach. It is established that fragmentation by branching cracks leads to a qualitatively different fragment distribution compared to binary fragmentation. The fragmentation model is applied to identify crack branching and coating/substrate stress transfer parameters.
Apparent interfacial shear strength of short-flax-fiber/starch acetate composites
2016
Abstract The paper deals with an indirect industry-friendly method for identification of the interfacial shear strength (IFSS) in a fully bio-based composite. The IFSS of flax fiber/starch acetate is evaluated by a modified Bowyer and Bader method based on an analysis of the stress–strain curve of a short-fiber-reinforced composite in tension. A shear lag model is developed for the tensile stress–strain response of short-fiber-reinforced composites allowing for an elastic-perfectly plastic stress transfer. Composites with different fiber volume fractions and a variable content of plasticizer have been analyzed. The apparent IFSS of flax/starch acetate is within the range of 5.5–20.5 MPa, de…
Statistical model of the transverse ply cracking in cross-ply laminates by strength and fracture toughness based failure criteria
2008
Cross-ply laminate subjected to tensile loading provides a relatively well understood and widely used model system for studying progressive cracking of the transverse ply. This test allows to identify material strength and/or toughness characteristics as well as to establish relation between damage level and the composite stiffness reduction. The transverse ply cracking is an inherently stochastic process due to the random variability of local material properties of the plies. The variability affects both crack initiation (governed by the local strength) and propagation (governed by the local fracture toughness). The primary aim of the present study is elucidation of the relative importance…
The effect of mechanical defects on the strength distribution of elementary flax fibres
2009
Flax fibres are finding non-traditional applications as reinforcement of composite materials. The mechanical properties of fibres are affected by the natural variability in plant as well as the damage accumulated during processing, and thus have considerable variability that necessitates statistical treatment of fibre characteristics. The strength distribution of elementary flax fibres has been determined at several fibre lengths by standard tensile tests, and the amount of kink bands in the fibres evaluated by optical microscopy. Strength distribution function, based on the assumption that the presence of kink bands limits fibre strength, is derived and found to provide reasonable agreemen…
Interfacial shear strength of flax fibers in thermoset resins evaluated via tensile tests of UD composites
2012
Abstract A method of interfacial shear strength evaluation, based on the length distribution of fibers pulled out from the tensile fracture surface of an oriented flax-reinforced composite, is applied to composites with vinyl ester and acrylated epoxidized soy oil resin matrices. Two approaches for characterizing the strength of fibers with modified Weibull distribution, fiber fragmentation tests and fiber tension tests, are compared in the analysis of pull-out data. Interfacial shear strength is found to increase by a few percent when loading rate is increased from 1.33% to 8%/min.
Evaluation of thin film adhesion to a compliant substrate by the analysis of progressive buckling in the fragmentation test
2009
The interface toughness of a thin coating/compliant substrate system is estimated based on the evolution of coating buckle patterns in the fragmentation test. The linear density of coating buckles as a function of applied strain is determined experimentally for a SiOx coating deposited on a polyethylene terephthalate film. A three-dimensional non-linear finite element model is developed to simulate the process of buckle formation in a single narrow coating strip. The elastic energy released during buckling-driven delamination is obtained from the energy balance in the system before and after the buckling event. Both the interface adhesion and the total energy release rate, which includes th…
Modeling strength scatter of elementary flax fibers: The effect of mechanical damage and geometrical characteristics
2011
Elementary bast fibers, apart from acceptable specific mechanical properties, possess a marked variability in geometrical and damage characteristics, which affects their axial tensile strength. A strength distribution function is derived that allows for the effect of kink bands and the scatter of fiber diameter. The distribution function is validated by applying it to the experimental strength data of both intact, carefully hand-decorticated, and damaged elementary flax fibers obtained by standard processing. The results suggest that the presence of kink bands is a limiting factor for the fiber strength.
Glass fibre strength distribution determined by common experimental methods
2002
The tensile strength of brittle fibres is routinely described by the Weibull distribution. The parameters of the distribution can be obtained from tests on single fibres and fibre bundles or from model composite tests. However, there is growing evidence that the distribution parameters obtained by different experimental techniques differ systematically. In order to investigate the possible causes of such discrepancies, single-fibre tension, fibre bundle, and single-fibre fragmentation tests are employed in this study to obtain strength distribution of commercial E-glass fibres. The results reveal parameter dependence on the approach used to extract the distribution parameters from experimen…
Estimation of the effective diffusivity of blowing agents in closed-cell low-density polyurethane foams based on thermal aging data
2021
Abstract Low-density closed-cell polyurethane (PU) foams are applied as thermal insulation materials due to their low thermal conductivity imparted by that of the physical blowing agent (PBA) used in foam production. However, foam conductivity tends to gradually increase with time, primarily due to changes in the gas composition in foam cells brought about by gas diffusion. To enable predicting the variation of conductivity during the service life of foam insulation, gas diffusivities are usually determined by measuring the gas composition in foams at different aging times. This study considers an alternative approach of estimating the effective diffusivities of gases in PU foams, which is …
The effect of defect location on coating fragmentation patterns under biaxial tension
2005
Fragmentation of a coating possessing orthogonal preferential crack propagation directions is modeled for equibiaxial tensile loading. Two plausible cracking scenarios are compared, caused by flaws randomly distributed over the area of the coating or along the coating fragment edges. The two fragmentation scenarios considered are shown to yield qualitatively different fragment patterns.
A probabilistic model of the tensile strength of a UD flax-fabric-reinforced polymer composite
2016
In order to fully realize the efficiency of bast fibers or their yarns as reinforcement of polymer composites, they should be aligned. Tensile strength of such composites in the direction of reinforcement has been found to exhibit size effect commensurable with that of unidirectionally reinforced (UD) composites with inorganic fibers, necessitating a probabilistic modeling approach. Strength distribution of a polymer matrix composite reinforced with aligned twisted bast fiber yarns is considered allowing for the strength scatter of elementary fibers, the presence of defects in the form of adjacent fiber breaks, and twist of the yarns. The model is applied to predicting the strength of flax/…
Strength distribution of elementary flax fibres
2005
Abstract Flax fibres, along with a number of other natural fibres, are being considered as an environmentally friendly alternative of synthetic fibres in fibre-reinforced polymer composites. A common feature of natural fibres is a much higher variability of mechanical properties. This necessitates study of the flax fibre strength distribution and efficient experimental methods for its determination. Elementary flax fibres of different gauge lengths are tested by single fibre tension in order to obtain the stress–strain response and strength and failure strain distributions. The applicability of single fibre fragmentation test for flax fibre failure strain and strength characterization is co…
Scale effect of the tensile strength of flax-fabric-reinforced polymer composites
2011
The development of UD natural fiber composites, considered for application as structural materials, necessitates evaluation of the scale effect of their strength. Alignment of the fibers in flax bast fiber composites can be achieved by employing textile reinforcement, such as yarns and fabrics. Cutting specimens for mechanical tests out of such textile-reinforced composite plates results in a complex non-uniform reinforcement structure at their edges, which may affect the strength of specimens. Scale effect of the tensile strength in the fiber direction of flax-fabric-reinforced composites is studied in the current work. A model accounting for both volume and edge effect of the specimens o…
Ultimate strain and deformability of elementary flax fibres
2011
Flax fibres possess high specific strength and stiffness, and thus are competitive in terms of mechanical properties with the traditional reinforcing fibres used in polymer-matrix composite materials. The mechanical properties of fibres have considerable variability that needs to be characterized and allowed for in the analysis of mechanical response of composites. In this study, the distribution of ultimate strain of elementary flax fibres and its dependence on gauge length is considered. The applicability of the modified Weibull distribution, used for fibre strength, to fibre ultimate strain is evaluated. A simplified relation of ultimate strain and fibre strength distributions is propos…
Estimation of interfacial fracture toughness based on progressive edge delamination of a thin transparent coating on a polymer substrate
2010
Evaluation of interfacial toughness of sub-micron-thickness layers deposited on a ductile substrate is a challenging task which has motivated different experimental approaches Fragmentation testing was used in the present study as a means of interface characterization of a silicon-nitride-coated polyimide substrate. During the test, after an initial rapid segmentation-cracking phase, the coating fragments developed edge delaminations which propagated in a stable manner with further increase in the applied strain The debonding process was modelled by the finite element method Incorporating a cohesive zone at the front of the interfacial crack The edge cracks were found to be dominated by mod…
Evaluation of interfacial stress transfer efficiency by coating fragmentation test
2007
Probabilistic model of coating fragmentation under uniaxial tensile loading is developed. Analytical expressions of the crack spacing evolution are obtained for small-strain and large-strain fragmentation regimes. The model is applied for coating and interface property identification of several thin brittle coating/polymer substrate systems. An estimate of the stress transfer length, derived from the fragmentation data, is found to correlate with the interfacial shear strength thus suggesting that both parameters reflect an intrinsic property related to the mechanical efficiency of coating/substrate interface.
Model of the mechanical response of short flax fiber reinforced polymer matrix composites
2011
Abstract Natural-fiber-reinforced short-fiber composites are finding more applications lately, therefore there is a need for estimation of the mechanical properties of such composites based on the properties of the constituents. The fibers themselves also possess complicated internal structure, resulting in anisotropy of their properties. Taking into account the internal structure of bast fiber, we evaluate the elastic properties of a composite unit cell, consisting of a fiber of average length and matrix according to the fiber volume fraction in the composite. The unit cell properties are used to estimate the stiffness of a misaligned short-fiber composite by means of orientation averaging…
The effect of a circular hole on the tensile strength of neat and filled rigid PUR foams
2015
Abstract In order to elucidate the effect of stress concentration on the tensile strength of rigid polyurethane (PUR) foams, specimens with open circular holes and different ratios of hole diameter to specimen width were tested in tension. The reduction in the net-section strength of the specimens with a center hole ranged from 1% to 18% for neat foams and from 18% to 28% for foams produced from a nanoclay-filled PUR. The finite fracture mechanics approach, based on simultaneous application of the strength and fracture mechanics criteria of failure, yielded a reasonably accurate prediction of foam strength in the presence of stress concentration.
Estimation of the elastic constants of highly porous cellular plastics reinforced with fibres embedded in foam struts
2015
In order to enhance the mechanical properties of polymer foams, fillers of different materials and sizes are being applied. Fibrous fillers shorter than the characteristic foam cell dimensions have the potential of efficient reinforcement, due to their high aspect ratio, without detrimentally interfering with foam structure. A theoretical model is developed for evaluation of the effect of filler on foam stiffness. The elastic response of rigid polymer foams filled with short fibres, of length commensurable with that of foam struts, is modelled by using the orientational averaging technique. Explicit expressions for components of the stiffness tensor of composite foams are derived in terms …
Prediction of crack onset strain in composite laminates at mixed mode cracking
2009
Failure process of continuous fiber reinforced composite laminates in tension usually starts with appearance of intralaminar cracks. In composite laminates with complex lay-ups and/or under combined loading, intralaminar cracks may develop in plies with different reinforcement directions. A necessary part of mixed mode cracking models is the criterion of failure. For propagation-controlled fracture it is usually formulated in terms of energy release rates and their critical values of the particular composite material. Intralaminar fracture toughness of unidirectionally reinforced glass/epoxy composite was experimentally determined at several mode I and mode II ratios. It is found that the c…
Stiffness and strength of flax fiber/polymer matrix composites
2006
Flax fiber composites with thermoset and thermoplastic polymer matrices have been manufactured and tested for stiffness and strength under uniaxial tension. Flax/polypropylene and flax/maleic anhydride grafted polypropylene composites are produced from compound obtained by coextrusion of granulated polypropylene and flax fibers, while flax fiber mat/vinylester and modified acrylic resin composites are manufactured by resin transfer molding. The applicability of rule-of-mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers is considered. POLYM. COMPOS. 27:221–229, 2006. © 2006 Society of Plastics Engineers
Fibre fragment distribution in a single-fibre composite tension test
2001
Abstract Single fibre fragmentation tests are performed for brittle fibres with Weibull strength distribution and different surface treatments. The fragmentation process is modelled and closed-form expressions for break spacing distribution are obtained. The model accounts for the effect of finite fibre length on the initial fragmentation as well as for break interaction on the advanced fragmentation stage. It is assumed that the exclusion zone due to fibre–matrix interface failure and stress recovery in the fibre is linearly dependent on the applied load. This assumption is validated experimentally. The derived theoretical average fragment length dependence on applied load is used to deter…
Model of delamination propagation in brittle-matrix composites under cyclic loading
2001
A model of interlaminar fatigue crack growth based on damage accumulation ahead of the crack is proposed. Linear cumulative assumption is used for damage estimation, and a quadratic failure criterion is applied for complex interlaminar loading. Model parameters are determined from mode I and mode II fatigue tests, and used to predict mixed-mode delamination propagation rate. Comparison of theoretical prediction with mixed-mode test results for different brittle graphite FRP at several mode- and load ratios show reasonable agreement.
Advanced fragmentation stage of oxide coating on polymer substrate under biaxial tension
2005
Crack patterns of 100-nm-thick silicon oxide coating on polypropylene film subjected to equibiaxial stress loading are studied experimentally. The loading is achieved by means of a bulging cell mounted under an optical microscope with stepwise pressurization of film specimens. The evolution of the coating fragment area distribution at relatively high strains is modeled using Weibull statistics to describe the coating strength. The fragment area distribution at an advanced fragmentation stage is shown to scale with the average fragment area, the latter being a power function of the applied biaxial strain.
Modeling elastic properties of short flax fiber-reinforced composites by orientation averaging
2010
Abstract Natural fibers of plant origin, used as reinforcement in polymer matrix composite materials, exhibit highly anisotropic elastic properties due to their complex internal structure. Mechanical properties can be evaluated not only by tests but also by mechanical models reflecting the principal morphological features of fibers. Such a FEM model is applied to estimate the elastic properties of a unit cell of a short-fiber-reinforced composite, an elementary flax fiber embedded in a polymer matrix. Orientation averaging approach is used for prediction of the stiffness of short flax fiber reinforced polymer matrix composite. The numerical estimates of Young’s modulus are compared to the t…