J.J.L. Morais

Characterisation of the bending stiffness components of MDF panels from full-field slope measurements

This paper deals with the characterisation of bending stiffness components of medium density fibreboard (MDF) by carrying out a single plate bending test. The approach couples full-field slope measurements with an inverse identification method. MDF panels manufactured with different fractions of Eucalyptus fibres and sugarcane bagasse particles were used. The slope maps generated across the plate surface were measured by the deflectometry technique. The curvature fields of the deformed plate were reconstructed by numerical differentiation afterwards. The virtual fields method was then implemented for material parameter identification under the framework of Kirchhoff–Love plate bending theory. The elastic properties obtained from the proposed data reduction (i.e. simultaneous identification of modulus of elasticity, Poisson’s ratio and shear modulus) were compared with values determined from classical three-point bending tests and reported in the literature. The set of properties were found in relatively good agreement.

Estimate of resistance-curve in wood through the double cantilever beam test

Abstract Numerical and experimental studies involving the double cantilever beam test to estimate the resistance-curve under mode I loading in wood (Pinus pinaster Ait. – RL fracture system) are presented. Two data reduction schemes based on the specimen compliance and crack equivalent concept are proposed to obtain accurate values of fracture energy. The methods do not require crack length monitoring and mitigate the influence of scatter of wood elastic properties on the measured fracture energy. A finite element analysis to validate the proposed methodologies was performed. Good agreement was achieved in both cases, especially for one case. Experimental tests were also carried out to determine the fracture energy of this wood species. Application of both data reduction schemes provided consistent values.

Response of CFRP laminates under high strain rate compression until failure

This work describes an experimental characterization the non-linear rate-dependent mechanical behavior of a composite material under compression. Fiber reinforced polymer matrix composites exhibit non-linear mechanical behavior, except in fiber direction, which is rate-dependent. In this work the Texipreg® HS160 REM material system was used, comprising high strength carbon fiber and epoxy resin. Unidirectional laminates were tested under uniaxial compression tests on a universal testing machine. The stress/strain curves of several specimens were obtained at three different strain rates of 0.07, 0.001 and 0.0001/s. In all cases tests were continued until failure was reached to measure the strain rate effect on strength. A 3-parameter constitutive viscoplastic model [6,7] was used to describe the mechanical behavior. This model was developed based on data for strain rate between 0.0001 and 0.07/s. In transverse direction the viscoplastic model was able to predict the high strain rate experiments conducted on a Split Hopkinson Pressure Bar.

Mechanical behaviour of wood T-joints. Experimental and numerical investigation

Results of a double-shear single-dowel wood connection tested under monotonic quasi-static compression loading are presented and discussed in this paper. The wood used in this study was a pine wood, namely the Pinus pinaster species, which is one of the most important Portuguese species. Each connection (specimen) consists of three wood members: a centre member, loaded in compression along the parallel-tograin direction and two simply supported side members, loaded along the perpendicular-to-grain direction (Tconnection). The load transfer between wood members was assured by means of a steel dowel, which is representative of the most common joining technique applied for structural details in wooden structures. The complete load-slip behaviour of the joint is obtained until failure. In particular, the values of the stiffness, the ultimate loads and the ductility were evaluated. Additionally, this investigation proposed non-linear 3D finite element models to simulate the T-joint behaviour. The interaction between the dowel and the wood members was simulated using contact finite elements. A plasticity model, based on Hill’s criterion, was used to simulate the joint ductility and cohesive damage modelling was applied to simulate the brittle failure modes (splitting) observed in the side members of the joint. The simulation procedure allowed a satisfactory description of the non-linear behaviour of the T-joint including the collapse prediction.

Monitoring of non-homogeneous strains in wood glued joints with embedded FBG optical sensors in mode I delamination tests

In this work it is presented a study of the reflection spectra yielded by a Fiber Bragg Grating sensor embedded into an epoxy glue line between two wood arms, in a double cantilever beam (DCB) Mode I delamination test. The reflection spectra were obtained using a Spectral Analyzer Fibersensing Bragmeter FS2200SA in regular time intervals, as the stress applied to the laminates is continuously increased until fracture occurs. They initially show a typical Bragg grating reflection spectrum, which gradually changes into more complicated, multiple-peak spectra, resulting from a non-homogenous strain distribution along the board line. Based on these results, a model was derived for the variation of the grating effective index which fits the observed spectra when the irregular strain distribution is observed. This model consists of usual cosine description of Bragg grating effective index with linear phase variation, plus a logarithmic phase change along the fiber length, resulting in the increment of the grating wavelength with increasing distance from the load application point. Moreover, from this model the strain distribution along the grating is found, yielding the expected result.

Identification of transverse elastic properties of the diaphysis of cortical bone

An inverse identification method is proposed to determine transverse elastic properties of cortical bone of diaphysis of long bones. Compression tests on tubular diaphysis disks are carried out. The approach couples the digital image correlation technique with the finite element updating method. An optimisation algorithm is used to minimize the numerical-experimental response for the elastic properties of the model. Diaphysis of tibia, femur and humerus is tested. In a first approximation, cortical bone tissue is considered transversely isotropic. A transverse elastic modulus of 8.2, 8.9 and 7.0 GPa is identified for tibia, femur and humerus tissue, respectively, with a typical scatter for biological materials. The response of the diaphysis disk to compression loading is however less sensitive to the Poisson’s ratio; hence, this parameter is more difficult to evaluate in practice for the proposed configuration. DOI: https://doi.org/10.24243/JMEB/2.5.172

Fracture Properties of Pine and Spruce in Mode I

The fracture behaviour of a quasibrittle material such as wood, characterized by the development of a large fracture process zone (FPZ), is nowadays well-known to be efficiently described by cohesive crack models. The most common applications of fictitious or cohesive crack models used to simulate nonlinear fracture mechanics of quasibrittle materials are considered as variations [1] of a model proposed by Hillerborg and co-workers [2]. These applications need to introduce a cohesive zone at the crack tip, i.e., a fictitious line crack transmitting normal stress dependent on the corresponding opening displacement w. Cohesive crack models are typically applied through finite elements calculations. Interface elements or springs [3] with prearranged strain-softening properties are integrated into the structural model along the most probable crack path. Since the pioneering application of cohesive crack models to wood due to Bostrom [4], a so called bilinear strain-softening model was applied by Stanzl-Tschegg et al. [5] to obtain wood load-deflection curves according to a developed wedge-splitting test protocol.