Wood is one of the oldest construction materials in the world and has been widely used to build a variety of civil infrastructures. Today there is a renewed interest on structural applications of wood due to environmental reasons and energy shortages. In fact, wood is the only construction material that is readily renewable and it is energy efficient in production, processing and use.
The issue of repair and strengthening of timber structures has a critical importance, because of conservation of historic heritage, ecological concerns about the more rational utilization of forest resources, and economic competitiveness of wood relatively to other structural materials. The primary reasons for repairing timber structures are the deterioration from moisture fluctuations (checks and horizontal splits), decay fungi, and insects attack. Repair of timber structures may also be necessary for remedy inadequate initial design or construction, and for accidental reasons such as overloading or fire damage. The changing of function, more stringent design standards, and increased safety requirements may result in wood structures that are structurally deficient and need to be upgraded through strengthening.
Over the last decades, a number of repair technologies were developed, aimed at extending the useful life of timber structures while maintaining or rising the load-carrying capacity and the structural safety. The most recent technology involves the use of fibre reinforced polymers (FRP) patches, as is the case of glass-epoxy (GFRP) or carbon-epoxy (CFRP) laminates. However, the repair procedures are often based on empirical knowledge. Moreover, at this time, there are no codes or design guidelines for FRP repair of timber structures. Consequently, a more fundamental materials science and engineering research is needed in order to exploit the full potential of FRP.
The main objective of this project is to establish design guidelines in order to optimize wood beams repair. Because of the crucial role of the FRP/wood bonded interface on the performance of the repair, a complementary objective is to define suitable tests to obtain the strain energy release rates of bonded interface in mode I and II. It must be pointed out that the problem of fracture behaviour of bonded joints between dissimilar materials is one of the major topics of Fracture Mechanics.
A previously developed cohesive mixed-mode damage model will be used to simulate several fracture tests, in order to select the most appropriate ones to obtain the mode I and II strain energy release rates of FRP/wood bonded interface. This model allows simulating damage initiation and growth. A further goal of the numerical simulations will be to verify the aptitude of the cohesive damage model to adjust different adherend materials. The selected fracture tests will be performed experimentally. The surface inspection of failed specimens by scanning electron microscopy will be performed in order to obtain additional information about the failure process. This part of the project will be based on the experience of the research team on the measurement of fracture properties of wood.
The cohesive mixed-mode damage model will be used also to simulate the mechanical behavior of repaired specimens. This model must be calibrated using experimental data, obtained by performing mechanical tests on the following physical models: (1) a double lap repair loaded in tension; (3) a defect-free beam strengthened with patches bonded on the tension and compression faces; (3) a defect-free beam reinforced with shear patches bonded on the side faces. The digital image correlation (DIC) technique will be used to evaluate the displacement field in the reinforced structure so as to observe the progressive load transfer from the wooden substrate to the composite patch. The DIC measurements will be supplemented with conventional point-wise measurements using electrical resistance strain gauges and liner variable differential transducers.
The above mentioned calibrated finite element model will be essential to analyse the performance of different types of repair, which are relevant for timber structures: (1) beams with longitudinal splits, repaired with external or embedded patches; (2) beams with circular or rectangular openings, strengthened with external patches. Beyond the geometrical aspects characterizing the repair, the influence of patch stiffness and adhesive properties will also be evaluated. Special attention will be paid to design repair in order to minimize stress concentrations leading to premature failure of the repaired structure. Experimental tests will be performed to validate the numerical results, and the efficiency of the repair will be evaluated by comparison with the behavior of the beam with and without the notch. The final objective will be to define design practical rules of wood beams repair by bonding composite patches.