Strengthening of timber structures with flax fibres

Abstract: The natural defects present in timber are the source of large variations in mechanical properties. This drawback has been partially counteracted with the use of Engineered Wood Products EWP (glulam, composite I-beam, parallel strand lumber, etc.) instead of solid wood. A few decades ago, fiber/polymer composite materials made their entrance in the civil engineering arena. They are used mostly as strengthening devices. The content of this thesis is related to the use of natural composite materials to strengthen glulam. The weak mechanical properties of wood in tension perpendicular to the grain are often the origin of catastrophic brittle failure. In order to enhance the design value of the tension perpendicular to grain strength, decrease the mechanical variation and provide the structure a more ductile failure, flax fibres and glass fibres reinforced polymer (FRP) composites have been used to strengthen glulam timber specimens. Three series of specimen of glued-laminated timber (flax fibre reinforced, glass fibre reinforced and unreinforced), with a grand total of 28 specimens, have been tested in tension perpendicular to the grain. Epoxy resin has been used in the composite and for bonding glulam to composite. For an approximate amount of FRP reinforcement of 1.2 % in volume (thickness ~ 0.7 mm), an increase of the tensile strength was shown by +23% using glass fibre reinforced polymer (GFRP-250 g/m2) , +25% using flax fibre reinforced polymer (FFRP-185 g/m2) and +74% using flax fibre reinforced polymer (FFRP- 230 g/m2). Regarding the modulus of elasticity, the previous reinforcement devices led to an increase by respectively +35%, +32% and +41%. For all specimen reinforced with fibre composites, semi-ductile failures were observed. An analytical model describing the mechanical behaviour of the glulam reinforced FRP was developed. It was found that the theoretical results from the model are comparable to those obtained experimentally for the flax fiber reinforced glulam specimens. However, debonding was observed for the glass fibre reinforced glulam specimen and the model does not include this type of failure. A parametric study was carried out using both the Monte Carlo method (MC) and the First Order Second Moment method (FOSM). It was shown that the mean values obtained during experiments where in agreement with those from the MC simulation. However, the standard deviations from the MC simulation are much higher. From the FOSM analysis, it was demonstrated that the variation within the stiffness perpendicular to grain of the glulam is not the first parameter driving the variation for the reinforced system. The variation within the mechanical properties of the flax fibres appeared to be the driving parameters for the design value of the system. A Finite Element Analysis was carried out to model the small prismatic glulam specimens and curved glulam beams. Two- and three-dimensional models were used to study first the elastic response and then the softening response of the specimen. Damage and crack opening was modeled based on the "fictitious crack model". Cohesive elements together with a traction separation law were used. A glulam model where high tensile stresses perpendicular to grain are expected should take into account the cylindrical orthotropy (annual rings) assumption. For both prismatic glulam specimen and curved glulam beams, the tensile stresses perpendicular to grain obtained with FEA are comparable to those from experiments.