Characterization Methods for Elastic Properties of Wood Fibers from Mats for Composite Materials

Wood fibers offer excellent specific properties at low cost and are of interest as reinforcement in composites. This work compares two alternative test methods to determine the stiffness of wood fibers from simple macroscopic tests on fiber mats. One method is compression of the fiber mat in the thickness direction, which uses a statistical micromechanical model based on first-order beam theory to describe the deformation. The other method is tensile testing of fiber mats and back calculation of the fiber stiffness with a laminate model. Experiments include compression tests and tensile stiffness index tests as well as determination of fiber content, orientation, and dimensional distribution. For mats with unbleached softwood kraft fibers, an effective value of the Young's modulus of 20.1 GPa determined by the compression method can be compared with values of 17.4-19.0 GPa obtained from tensile tests. These are in agreement with values for similar cellulosic fibers found in literature. The compression method is more appropriate for low-density fiber mats, while the tensile test works better for well-consolidated high-density fiber mats. The two methods have different ranges of applicability and are complementary to one another. Limitations of the methods are also discussed. The main advantage of the methods is that they are quantitative. The potential as stiffening reinforcement of various types of fibers can be systematically investigated, even if the fiber mat microstructures are different

Fibre-matrix debonding in transverse cyclic loading of unidirectional composite plies

COMPTEST 2006 — 10,11,12 abril 2006 OportoFatigue of composite materials is of great concern in load-carrying structures. In fact, most failures of composite structures can be attributed to fatigue. Due to the heterogeneity of composite materials at different scales, a large variety of interacting mechanisms contribute to fatigue failure. If the incipient mechanisms at the onset of damage accumulation could be better understood, bases for a physically based fatigue law may be built and measures could be taken in order to extend the lifetime of the material

Creep in oak material from the Vasa ship: verification of linear viscoelasticity and identification of stress thresholds

Creep deformation is a general problem for large wooden structures, and in particular for shipwrecks in museums. In this study, experimental creep data on the wooden cubic samples from the Vasa ship have been analysed to confirm the linearity of the viscoelastic response in the directions where creep was detectable (T and R directions). Isochronous stress-strain curves were derived for relevant uniaxial compressive stresses within reasonable time spans. These curves and the associated creep compliance values justify that it is reasonable to assume a linear viscoelastic behaviour within the tested ranges, given the high degree of general variability. Furthermore, the creep curves were fitted with a one-dimensional standard linear solid model, and although the rheological parameters show a fair amount of scatter, they are candidates as input parameters in a numerical model to predict creep deformations. The isochronous stress-strain relationships were used to define a creep threshold stress below which only negligible creep is expected. These thresholds ranges were 0.3-0.5 MPa in the R direction and 0.05-0.2 MPa in the T direction

Mechanical performance of yew (Taxus baccata L.) from a longbow perspective

Yew (Taxus baccata L.) longbow was the preferred weapon in the Middle Ages until the emergence of guns. In this study, the tensile, compression, and bending properties of yew were investigated. The advantage of yew over the other species in the study was also confirmed by a simple beam model. The superior toughness of yew has the effect that a yew longbow has a higher range compared with bows made from other species. Unexpectedly, the mechanical performance of a bow made from yew is influenced by the juvenile-to-mature wood ratio rather than by the heartwood-to-sapwood ratio. A yew bow is predicted to have maximized performance at a juvenile wood content of 30-50%, and located at the concave side (the compressive side facing the bowyer). Here, the stiffness and yield stress in compression should be as high as possibl

Experimental and numerical analysis of cellulosic insulation failures of continuously transposed conductors under short circuits and thermal ageing in power transformers

The integrity of the cellulosic insulation in power transformers is considered one of the most relevant parameters that affects their performance and reliability. Electric faults, such as short circuits, have thermal and mechanical effects that degrade the paper and can eventually produce the end-of-life of the transformer. The evolution of the properties of the paper insulation of a commercial continuously transposed conductor due to thermal ageing was characterised through the degree of polymerisation and tensile testing. Failure initiation and propagation in the paper was analysed macroscopically and microscopically using scanning electron microscope. A finite element numerical mechanical model of the conductor was implemented to reproduce the experiments and to obtain the load level and strain state that produce failure at each ageing state, aiming at developing a failure model for the insulation. This model may contribute to an improvement in manufacturing processes and management of the electrical system

Comportamiento de las grietas de interfase en materiales compuestos fibrosos ante carga cíclica de tracción-tracción y tracción-compresión

El origen del daño ante carga cíclica en materiales compuestos suele encontrarse en las láminas off-axis, en forma de grietas transversales. Este mecanismo de rotura está dominado por la aparición y crecimiento de despegues entre la matriz y las fibras (grietas de interfase), cuya posterior coalescencia da lugar al fallo transversal. En este trabajo se evalúa el crecimiento de los despegues ante la aplicación de ciclos de carga transversal de tracción-tracción (T-T) y tracción-compresión (T-C), ya que existen referencias al hecho de que la presencia de ciclos de compresión puede afectar a la resistencia a fatiga del laminado. Experimentalmente, mediante probetas de fibra única, se observa que los ciclos T-C resultan más perjudiciales que los ciclos T-T, pudiendo medir la extensión del daño. Numéricamente, mediante un modelo de elementos de contorno, se estudia el diferente efecto de las partes de tracción y compresión de los ciclos, aportando resultados que ayudan a explicar la evidencia experimental.The initiation of damage under cyclic loading in composite materials generally appears as transverse cracks in off-axis plies. The mechanism of damage is dominated by the appearance of debonds between the fibres and the matrix (interface cracks). The transverse cracks forms when debonds coalesce. Based on the references that accounts for the effect of the presence of compression in the laminate strength under cyclic loading, the growth of the interface cracks is studied in this work under transverse tension-tension (T-T) and tension-compression (T-C) cyclic loading, Experimentally, a more detrimental effect of the T-C cycles than the T-T cycles is observed and quantified for single fibre specimens. The numerical study of the effect of the tensile and compressive part of the loading cycle by means of a boundary elements model contributes to the explanation of the experimental evidence

Flexural response of polypropylene/E-glass fibre reinforced unidirectional composites

This paper presents a study of the flexural response of continuous E-glass fibre reinforced polypropylene composites. Experiments were designed to investigate monotonic and cyclic flexural response using three point bending test for laminates with different angle-ply and cross-ply arrangements. Results show that the monotonic and cyclic flexural response of the composites are influenced by the plastic deformation of the matrix. The study observed that increasing numbers of cyclic loads led to significant energy dissipation, stiffness reduction and micro-damage accumulation within the composite and especially at the matrix-fibre interface. Significant energy dissipation and damage were observed to dominate the first load-unload cycle. With subsequent cycles, the magnitude of energy dissipation and global damage reduces to a threshold value which is cycle independent. This study has also developed a phenomenological model to predict the dependence of energy dissipation with number of cycles. The experimental data generated here will be useful in the development of holistic macroscale constitutive models and finite element studies of the chosen test composite

Comparison of the structure and flexural properties of Moso, Guadua and Tre Gai bamboo

Bamboo is an underutilized resource widely available in countries with rapidly developing economies. Structural bamboo products, analogous to wood products, allow flexibility in the shape and dimensions of bamboo structural members. Here, the ultrastructure, microstructure, cell wall properties and flexural properties of three species of bamboo (Moso, Guadua and Tre Gai) are compared. At a given density, the axial modulus of elasticity of Guadua is higher than that of Moso or Tre Gai, which are similar; ultrastructural results suggest that Guadua has a higher solid cell wall stiffness. At a given density, their moduli of rupture are similar.National Science Foundation (U.S.) (OISE-1258574