E. Kristofer Gamstedt

Stiffness Contribution of Various Wood Fibers to Composite Materials

Wood pulp fibers can serve as useful reinforcement of plastics for increased stiffness. To assess the potential of various wood fibers as reinforcement, a method has been developed to determine the contribution of the fibers to the elastic properties of the composite. A micromechanical composite model and classical laminate mechanics are used to relate the elastic properties of the fibers to the elastic properties of the composite. A large variety of composites made of various wood pulp fibers in an epoxy vinyl ester matrix was manufactured. From the tensile test results of the composites, the contributing Young’s moduli of the fibers in the longitudinal direction are back-calculated and summarized. One finding is that there is an optimum in fiber stiffness as a function of lignin content. It is also found that industrially pulped hardwood fibers have higher stiffness than the corresponding softwood fibers. One example is kraft-cooked Norway spruce fiber, for which a Young’s modulus of 40 GPa is found. The effects of hornification, prehydrolysis, and sulfite processing are also investigated. The results indicate that mild defibration process should be used, that does not damage the cell wall structure so that the inherent high stiffness of the native fibers can be retained. It can be concluded that the proposed method works well to rank the wood fiber candidates in terms of their contribution to the composite stiffness.

Fatigue mechanisms in unidirectional glass-fibre-reinforced polypropylene

Polypropylene (PP) and polypropylene modified with maleic anhydride (MA-PP) reinforced by continuous longitudinal glass fibres have been investigated. The most prominent effect of the modification with maleic anhydride in the composite is a stronger fibre/matrix interface. The effects of interfacial strength on fatigue performance and on the underlying micromechanisms have been studied for these composite systems. Tension–tension fatigue tests (R=0.1) were carried out on 0∘ glass-fibre/PP and glass-fibre/MA-PP coupons. The macroscopic fatigue behaviour was characterized in terms of stiffness reduction and fatigue-life curves. The results showed that the longitudinal Youngs modulus degraded more rapidly for glass-fibre/PP, which was caused by a higher degree of damage growth and accumulation. The improvement in monotonic strength was negligible, but the fatigue life was prolonged by about one decade for the composite with the stronger interface by use of the maleic-anhydride grafted polypropylene matrix. During the fatigue testing, the microscopic mechanisms were monitored intermittently by a surface replication technique. From microscopic observations, it could be concluded that the better fatigue resistance of glass-fibre/MA-PP can be attributed to the greater interfacial strength and the resistance to debond propagation.

Dynamic–Mechanical Properties of Wood–Fiber Reinforced Polylactide: Experimental Characterization and Micromechanical Modeling:

Wood-fiber reinforced polylactide is a biodegradable composite where both fibers and matrix are from renewable resources. When designing new materials of this kind, it is useful to measure the influence of fiber–matrix interface properties on macroscopic mechanical properties. In particular, a quantitative measure of the dynamic stress transfer between the fibers and the matrix when the material is subjected to cyclic loading would simplify the development of wood-fiber composites. This is obtained by comparing the mechanical dissipation of the composite with a value predicted by a viscoelastic micromechanical model based on perfect interfacial stress transfer. The loss factors predicted by the model are 0.12 and 0.16 at dry and humid conditions, respectively, which amount to 63 and 66% of the experimentally determined values. For Youngs moduli the predicted values are 1.01 and 0.88 GPa, which correspond to 92% of the experimentally determined values. The mismatch between the predicted and experimental values may be attributed to imperfect interfaces with restrained stress transfer. Loss factors are also determined for specific molecular bonds using dynamic Fourier transform infrared (FT-IR) spectroscopy. These values show the same trends with regard to moisture content as the macroscopically determined loss factors.

Ultrastructural features affecting mechanical properties of wood fibres

Abstract The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure–property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure–property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.

Effects of voids on quasi-static and tension fatigue behaviour of carbon-fibre composite laminates

The effect of voids on quasi-isotropic carbon-fibre reinforced plastic laminates under quasi-static loading is compared with that under cyclic tension loading. Emphasis is placed on following damage development at the non-crimp fabric ply-level by investigating the influence of voids on damage accumulation, most notably transverse cracking and delamination. Details from experiments include micrographs of voids taken in both scanning-electron and light microscopy, measurements of void content and crack density using light microscopy, and stiffness plots from both quasi-static and cyclic tests. The stiffness results are compared with theoretical predictions accounting for transverse cracks. Voids have a significantly more detrimental effect on the mechanical properties in cyclic loading compared with quasi-static loading. Specifically, the stiffness reduction development, the underlying transverse cracking in layers and the number of cycles to failure are affected. Quality control by only quasi-static testing for void-containing composite materials to be used in components subjected to fatigue cannot therefore be recommended.

Wood versus Plant Fibers: Similarities and Differences in Composite Applications

The work on cellulose fiber composites is typically strictly divided into two separated research fields depending on the fiber origin, that is, from wood and from annual plants, representing the two different industries of forest and agriculture, respectively. The present paper evaluates in parallel wood fibers and plant fibers to highlight their similarities and differences regarding their use as reinforcement in composites and to enable mutual transfer of knowledge and technology between the two research fields. The paper gives an introduction to the morphology, chemistry, and ultrastructure of the fibers, the modeling of the mechanical properties of the fibers, the fiber preforms available for manufacturing of composites, the typical mechanical properties of the composites, the modeling of the mechanical properties with focus on composites having a random fiber orientation and a non-negligible porosity content, and finally, the moisture sensitivity of the composites. The performance of wood and plant fiber composites is compared to the synthetic glass and carbon fibers conventionally used for composites, and advantages and disadvantages of the different fibers are discussed.

Fatigue Dissipation and Failure in Unidirectional and Angle-Ply Glass Fibre/Carbon Fibre Hybrid Laminates

The tensile fatigue behaviour of unidirectional 0degrees, [+/-10](4S) and [+/-45](4S) carbon fibre/glass fibre hybrid composite has been investigated. The dissipation was measured by the stiffness, hysteresis loss and temperature field of the specimen surface in an insulated testing chamber. The hysteresis loss correlates well with the increase temperature. Microscopic studies show frictional sliding of longitudinal crack faces between carbon and glass fibre bundles to be the main source of dissipation for on-axis specimens. With increasing off-axis angle the primary loss mechanism became cyclic shear deformation of the polymer matrix. With a finer dispersion of the constituents of the hybrid, the growth of these longitudinal cracks or of zones of inelastic matrix shear deformation. would be suppressed, which would result in a more fatigue resistant material. A localisation of heat generation sets in just prior to final failure. Damage and heat localisation lead to impending failure. If the parameters that control localisation were better understood, it would be possible to improve the fatigue resistance of the material by sensible microstructural design.

State of degradation in archeological oak from the 17th century Vasa ship: substantial strength loss correlates with reduction in (holo)cellulose molecular weight.

In 1628, the Swedish warship Vasa capsized on her maiden voyage and sank in the Stockholm harbor. The ship was recovered in 1961 and, after polyethylene glycol (PEG) impregnation, it was displayed in the Vasa museum. Chemical investigations of the Vasa were undertaken in 2000, and extensive holocellulose degradation was reported at numerous locations in the hull. We have now studied the longitudinal tensile strength of Vasa oak as a function of distance from the surface. The PEG-content, wood density, and cellulose microfibril angle were determined. The molar mass distribution of holocellulose was determined as well as the acid and iron content. A good correlation was found between the tensile strength of the Vasa oak and the average molecular weight of the holocellulose, where the load-bearing cellulose microfibril is the critical constituent. The mean tensile strength is reduced by approximately 40%, and the most affected areas show a reduction of up to 80%. A methodology is developed where variations in density, cellulose microfibril angle, and PEG content are taken into account, so that cell wall effects can be evaluated in wood samples with different rate of impregnation and morphologies.

Toward an alternative compatibilizer for PLA/cellulose composites: Grafting of xyloglucan with PLA

Poly(L-lactic acid) (PLLA) chains were grafted on xyloglucan substrates via ring-opening polymerization of the L-lactide monomer. Different parameters such as the nature of the substrate (native or modified xyloglucan) and the substrate/monomer ratios were varied in the synthesis to achieve different lengths of the grafted chains. A range of experimental techniques including infrared spectroscopy and nuclear magnetic resonance were used to characterize the final product. Thermal analysis showed that the glass transition temperature of xyloglucan was decreased from 252 °C to 216 °C following the grafting of PLLA. The grafting of less hydrophilic chains from xyloglucan also affected the interaction with water: the PLLA-grafted xyloglucan was insoluble in water and the moisture uptake could be decreased by about 30%. Xyloglucan adsorbs strongly to cellulose; therefore such a graft copolymer may improve the compatibility between cellulose fibers and PLLA. The PLLA-grafted xyloglucan may be useful as a novel compatibilizer in fiber-reinforced PLLA composites.

Hierarchical modelling of microstructural effects on mechanical properties of wood. A review COST Action E35 2004-2008 : Wood machining - micromechanics and fracture

Abstract Wood exhibits a hierarchical architecture. Its macroscopic properties are determined by microstructural features at different scales of observation. Recent developments of experimental micro-characterisation techniques have delivered further insight into the appearance and the behaviour of wood at smaller length scales. The improved knowledge and the availability of increasingly powerful micromechanical modelling techniques and homogenisation methods have stimulated rather comprehensive research on multiscale modelling of wood. Linking microstructural properties to macroscopic characteristics is expected to improve the knowledge of the mechanical behaviour of wood and to serve as the basis for the development of innovative wood-based products and for biomimetic material design. Moreover, understanding fundamental aspects of wood machining requires multiscale approaches which can take into account the heterogeneity, anisotropy and hierarchies of wood and wood composites. In this review, recent developments in the field of hierarchical modelling of the hygroelastic behaviour of wood are discussed, and a short outline of the theoretical background is given. Much focus is placed on composite micromechanical models for the wood cell wall and on multiscale models for wood resting upon hierarchical finite element models and on the application of continuum micromechanics, respectively. These models generally lead to the specification of equivalent homogeneous continua with effective material properties. Finally, current deficiencies and limitations of hierarchical models are sketched and possible future research directions are specified.