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

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

Thermal and mechanical properties of chitosan nanocomposites with cellulose modified in ionic liquids

In this paper, ionic liquid treatment was applied to produce nanometric cellulose particles of two polymorphic forms. A complex characterization of nanofillers including wide-angle X-ray scattering, Fourier transform infrared spectroscopy, and particle size determination was performed. The evaluated ionic liquid treatment was effective in terms of nanocrystalline cellulose production, leaving chemical and supermolecular structure of the materials intact. However, nanocrystalline cellulose II was found to be more prone to ionic liquid hydrolysis leading to formation larger amount of small particles. Each nanocrystalline cellulose was subsequently mixed with a solution of chitosan, so that composite films containing 1, 3, and 5% mass/mass of nanometric filler were obtained. Reference samples of chitosan and chitosan with micrometric celluloses were also solvent casted. Thermal, mechanical, and morphological properties of films were tested and correlated with properties of filler used. The results of both, tensile tests and thermogravimetric analysis showed a significant discrepancy between composites filled with nanocrystalline cellulose I and nanocrystalline cellulose II

Stiffness of Aligned Wood Fiber Composites: Effect of Microstructure and Phase Properties

The effect of wood fiber anisotropy and their geometrical features on wood fiber composite stiffness is analyzed. An analytical model for an N-phase composite with orthotropic properties of constituents is developed and used. This model is a straightforward generalization of Hashin’s concentric cylinder assembly model and Christensen’s generalized self-consistent approach. It was found that most macro-properties are governed by only one property of the cell wall which is very important in attempts to back-calculate the fiber properties. The role of lumen (whether it filled by resin or not) has a very large effect on the composite shear properties. It is shown that several of the unknown anisotropic constants characterizing wood fiber are not affecting the stiffness significantly and rough assumptions regarding their value would suffice. The errors introduced by application of the Hashin’s model and neglecting the orthotropic nature of the material behavior in cylindrical axes are evaluated. The effect of geometrical deviations from circular cross-section, representing, for example, collapsed fibers, is analyzed using the finite element method (FEM) and the observed trends are discussed

A micro-CT investigation of densification in pressboard due to compression

As a non-destructive inspection method, micro-computed tomography has been employed for determining local properties of a cellulose-based product, specifically pressboard. Furthermore, by utilizing the determined properties in a detailed numerical model, by means of a finite element analysis, we demonstrate a continuum anisotropic viscoelastic-viscoplastic model. Through such a combination of non-invasive experiments with accurate computations in mechanics, we attain a better understanding of materials and its structural integrity at a pre-production stage increasing the success of the first prototype. In detail, this combination of micro-computed tomography and finite element analysis improves accuracy in predicting materials response by taking into account the local material variations. Specifically, we have performed indentation tests and scanned the internal structure of the specimen for analysing the densification patterns within the material. Subsequently, we have used a developed material model for predicting the response of material to indentation. We have computed the indentation test itself by simulating the mechanical response of high-density cellulose-based materials. In the end, we have observed that pressboard, having initially a heterogeneous density distribution through the thickness, shows a shift in the densification to the more porous part after indentation. The densification maps of the simulated results are presented by comparing with the experimental results. A reasonable agreement is observed between the experimental and the simulated densifications patterns, which suggests that the proposed methodology can be used to predict densification also for other fibre-based materials during manufacturing or in service loading