Papers by Janis Andersons
Macromolecular Symposia
Rigid low‐density closed‐cell polyurethane foams with different crosslink densities have been pro... more Rigid low‐density closed‐cell polyurethane foams with different crosslink densities have been produced using biobased polyols derived from tall oil. Mechanical and dilatometric tests reveal that the transverse stiffness and tensile strength of the foams increased with growing crosslink density both at the room and cryogenic temperatures, while the coefficient of linear thermal expansion exhibited an opposite trend. Cryogenic safety factors, estimated based on the foam properties obtained, reveal no consistent trend with changing crosslink density of the polyurethane polymer.
CRC Press eBooks, Dec 20, 2021
ABSTRACT
An Improved Method For Identification Of The Interfacial Shear Strength By Tensile Tests Of Short... more An Improved Method For Identification Of The Interfacial Shear Strength By Tensile Tests Of Short-Fiber Composites
Key Engineering Materials, 2019
Rigid low-density closed-cell polyurethane (PU) foams are used primarily as a thermal insulation ... more Rigid low-density closed-cell polyurethane (PU) foams are used primarily as a thermal insulation material. The foams have to possess a sufficient strength and stiffness in order to ensure their mechanical integrity and dimensional stability in service. The mechanical characteristics of foams are enhanced by adding cellulose nanofibers to the polyol system, which both modify the foaming process and act as a reinforcement of cell struts and walls. A model of composite foam strength is developed based on a regular unit cell and assuming the onset of strut failure as the foam fracture criterion. The load-bearing capacity of foam struts is estimated by the modified Fukuda and Chou model considering the orientation of nanofibers along the strut axis. The model developed is shown to provide a reasonably accurate prediction for the nanofiber loading effect on the strength of composite foams.
Polymer Composites, 2016
In order to fully realize the efficiency of bast fibers or their yarns as reinforcement of polyme... more 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/acrylated epoxidized soy oil resin composite specimens of different sizes. The theoretical prediction is found to be in a reasonable agreement with strength test results. POLYM. COMPOS., 2016. © 2016 Society of Plastics Engineers
Theoretical and Applied Fracture Mechanics, 2015
In order to elucidate the effect of stress concentration on the tensile strength of rigid polyure... more 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.
Mechanics of Composite Materials, 1998
Experimental results and theoretical prediction of the response of gia~fibel'reinforced polyester... more Experimental results and theoretical prediction of the response of gia~fibel'reinforced polyester under quasi-static, static (creep), and cyclic (fatigue) loading are presented. The nonlinear strain component at static loading and the strain amplitude rate at cyclic off.'is loading of an orthotmpic composite are shown to follow the associated flow rule with a sing~paramster quadratic potential function. The influence of fatigue damage on deformation is considerable due to the reduction in the elastic modulus of the composite and is apparently neglib~e with respect to its effect on the parameters of the creep kernel.
Adhesion of the constituents in fiber-reinforced polymers, frequently characterized by the interf... more Adhesion of the constituents in fiber-reinforced polymers, frequently characterized by the interfacial shear strength (IFSS), is an important characteristic of the composite material determining to a large extent its strength and toughness. A number of experimental methods have been developed for evaluation of the IFSS. However, it has been argued that adhesion should be determined by tests reflecting the actual stress state the fiber/matrix interface is subjected to, rather than that achieved in model single-fiber composites. Such an adhesion evaluation method for short-fiber composites has been proposed by Bowyer and Bader based on the mechanical response of composite material in tension. In the current study, we apply this method to short-flax-fiber reinforced polypropylene to evaluate the effect of fiber volume fraction on the apparent IFSS. Experimental tension curves of unmodified and modified polypropylene matrix composites reinforced with short flax fibers with three differe...
Application of plant-derived natural fibers as the reinforcement of polymer-matrix composites is ... more Application of plant-derived natural fibers as the reinforcement of polymer-matrix composites is driven by economic and environmental reasons. In order to fully exploit the reinforcing effect of the fibers, they have to be aligned in the composite material. When using such UD composites in load-bearing applications, the dependence of strength on the material volume subjected to loading, i.e. the scale effect of strength, should be taken into account in design. UD-reinforced flax fiber/epoxy matrix composites, produced from prepregs, were tested in tension in fiber direction in order to elucidate the scale effect of the tensile strength. The strength distribution at a fixed specimen size and the dependence of the mean strength on the size were shown to agree reasonably well with the Weibull strength statistics, corroborating the previous results for flax-fabric-reinforced composites. A probabilistic model of the strength in tension along the reinforcement direction, using fiber stren...
It is shown that the tensile strength of cellulose fibers at a fixed length follows two-parameter... more It is shown that the tensile strength of cellulose fibers at a fixed length follows two-parameter Weibull distribution. However, due to non-linear behaviour of cellulose fibers, size effect of strength is better captured by the three-parameter Weibull distribution, Single fiber as well as bundle tensile tests are performed, effect of moisture and strain rate on fiber strength is estimated. INTRODUCTION Natural fibers, such as flax and hemp for example, are widely used as reinforcement in polymer composites. Although there is a large potential in using these fibers for creating high performance composites, there are still some problems associated with these reinforcements which delay wider usage of such materials for structural applications. Some of the problems to mention are: non-uniform quality of filaments (not only within the batch but also from batch to batch), limited fiber length (often fibers are too short to be effective), poor fiber/matrix adhesion etc. One of the weakest ...
Journal of Materials Science, 2001
Crack patterns of 53 nm and 103 nm thick silicon oxide coatings on poly(ethylene terephthalate) f... more Crack patterns of 53 nm and 103 nm thick silicon oxide coatings on poly(ethylene terephthalate) films are analyzed under equibiaxial stress loading, by means of a bulging cell mounted under an optical microscope with stepwise pressurization of film specimens. The biaxial stress and strain are modeled from classical elastic membrane equations, and an excellent agreement is obtained with a finite
SID Symposium Digest of Technical Papers, 2008
Flexible displays present a challenging problem in terms of mechanical integrity, a result of the... more Flexible displays present a challenging problem in terms of mechanical integrity, a result of the considerable hygro-thermomechanical contrast between the inorganic, brittle device layers and the compliant polymer substrates. This paper reviews the main approaches to study and identify the key factors, which control the mechanical stability of this class of displays. Focus is put on the analyses of residual stress and damage under tensile loading. Novel electro-mechanical methods are used for accurate insight into critical phenomena. An important result is that the thickness and stiffness of the substrate control the critical strain for failure of the device layers.
International Journal of Solids and Structures, 2008
A solution for buckling of a stiff strip of finite width bonded to a compliant elastic half-space... more A solution for buckling of a stiff strip of finite width bonded to a compliant elastic half-space and subjected to uniform axial compression is presented. Approximate semi-analytical and finite element solutions are obtained and compared with a two-dimensional case of a plate on elastic foundation. The comparison demonstrates that the two-dimensional solution can be applied to predict the buckling wavelength and critical compressive strain when the width of the strip is equal to or larger than the buckling wavelength. For narrow strips, the wavelength is smaller and critical strain is higher than that of a plate on foundation.
Mechanics of Composite Materials, 2013
The application of hemp fibers as a reinforcement of composite materials necessitates the charact... more The application of hemp fibers as a reinforcement of composite materials necessitates the characterization of fiber strength scatter and the effect of fiber length on its strength. With this aim, elementary hemp fibers were tested in tension at two different gage lengths. Due to the similar morphology of hemp and flax fibers, the probabilistic strength models derived and verified for the latter were applied to the former. The fiber strength was found to agree with the modified Weibull distribution. The modeling approaches developed for describing the variability of the strength and failure strain of elementary flax fibers are shown to be also applicable to hemp fibers.
Key Engineering Materials, 2007
Application of coating fragmentation test for determination of coating properties is considered. ... more Application of coating fragmentation test for determination of coating properties is considered. A probabilistic model of coating fragmentation under uniaxial tensile loading is applied for coating and interface property identification of thin brittle coating/polymer substrate system. A finite element model is developed to simulate the process of buckle formation in coating strips during fragmentation test. The measured buckle geometry parameters and buckle density evolution as a function of the applied strain is used to estimate the interface toughness.
Thin Solid Films, 2009
The interface toughness of a thin coating/compliant substrate system is estimated based on the ev... more 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 SiO x 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 the plastic dissipation in the substrate during debonding, are evaluated. The apparent interfacial toughness, equal to 15 J/m 2 at the onset of buckling, is found to increase with strain. This is tentatively explained by the probabilistic features of the buckle accumulation process, reflected also in the random locations of buckles evolving towards a log-normal distribution of buckle spacings at high strains.
Thin Solid Films, 2003
Crack patterns of 100-nm thick silicon oxide coatings on poly(ethylene terephthalate), polypropyl... more Crack patterns of 100-nm thick silicon oxide coatings on poly(ethylene terephthalate), polypropylene and polyamide films are analyzed under equibiaxial stress loading. 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 in the early stages of the fragmentation process is modeled using Weibull statistics to describe the coating strength. Fragment areas are found to follow the exponential distribution at low strains close to the cracking onset. With the increase of equibiaxial strain, the distribution changes reflecting stress transfer and crack propagation features.
Theoretical and Applied Fracture Mechanics, 2008
Crack onset strain measurements of a confined layer in tension provide the means for layer toughn... more 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 toughness estimate obtained by such a procedure is likely to be non-conservative.
Procedia Engineering, 2011
Natural-fiber-reinforced short-fiber composites are finding more applications lately, therefore t... more 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 results obtained are compared with the experimental stress-strain diagrams of short flax fiber/polypropylene matrix composite. Usually the mechanical response of short-fiber/polymer matrix composite is non-linear due to the inelastic matrix behavior. Accounting for the non-linear deformation of the matrix in the unit cell by modeling it as an elastic-plastic material, we also estimate its non-linear response under uniaxial loading.
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Papers by Janis Andersons