0000000000082342
AUTHOR
E. Spārniņš
The Effect of Damage and Geometrical Variability on the Tensile Strength Distribution of Flax Fibers
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…
Evaluation of interfacial shear strength by tensile tests of impregnated flax fiber yarns
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…
Modeling the nonlinear deformation of flax-fiber-reinforced polymer matrix laminates in active loading
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.
Strength and Damage of Elementary Flax Fibers Extracted from Tow and Long Line Flax
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…
Analysis of the effect of a stress raiser on the strength of a UD flax/epoxy composite in off-axis tension
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.
Finite fracture mechanics analysis of crack onset at a stress concentration in a UD glass/epoxy composite in off-axis tension
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…
Statistical model of the transverse ply cracking in cross-ply laminates by strength and fracture toughness based failure criteria
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
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…
Fracture Toughness of PIR Foams Produced from Renewable Resources
Rigid low-density closed-cell polyisocyanurate (PIR) foams are used primarily as a thermal insulation material. Traditionally, they are manufactured from constituents produced by petrochemical industry. Introducing renewable materials in PIR formulation brings definite economical and environmental benefits. Fracture toughness of PIR foams obtained from renewable resources (with the polyol system comprising up to 80% of rapeseed oil esters) and petrochemical PIR foams has been characterized experimentally, by compact tension tests, for mode I crack propagation along the rise direction of the foams.
Interfacial shear strength of flax fibers in thermoset resins evaluated via tensile tests of UD composites
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.
Modeling strength scatter of elementary flax fibers: The effect of mechanical damage and geometrical characteristics
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.
Ultimate strain and deformability of elementary flax fibres
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…
Prediction of crack onset strain in composite laminates at mixed mode cracking
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
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