Factors Affecting the Structural Integrity of Wood-Based Composites: Elevated Temperature and Adhesive Bonding
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This study focuses on factors that affect the structural integrity of wood-based composites. Wood-based composites exposed to fire may decompose due to the elevated temperatures, resulting in a degradation in performance. Thermal modelling can only predict the structural integrity of construction materials in fire if it is given accurate inputs. Consequently, methods for the characterization of the thermal, physical, and mechanical behaviors of wood and wood-based composites are selected, designed, and benchmarked. The relevant thermal and physical responses characterized includes porosity, permeability and thermal diffusivity. Common construction materials (white pine board, medium density fiberboard and spruce 24) are characterized from room temperature to complete decomposition. The characterization techniques and processes are based on existing literature and relevant ASTM standards. To reduce the number of experiments required for future material characterization, estimates based upon the degree of decomposition and the measured values for the virgin and charred materials are used. For porosity and thermal diffusivity, these models allow values at intermediate temperatures to be estimated with measurements at room temperature and complete decomposition and thermogravimetric analysis (TGA). We find that permeability depends heavily on the microstructure of materials and should be measured independently at the conditions of interest. An additional important aspect of the performance of wood-based composites is the fracture behavior of wood/adhesive systems. Adhesive bonding enables many engineered wood products such as furniture and structural wood joints and the adhesive fracture toughness often determines the durability. The conventional characterization method for wood/adhesive fracture resistance relies on samples with machined grain angles designed to funnel cracks to the adhesive interface. This method of sample preparation is difficult and time-consuming for certain wood species. In this work, a practical and efficient method is developed to characterize adhesive fracture energy of adhesively bonded veneer systems. In the method, auxiliary aluminum adherends are bonded to the veneers in an effort to drive the crack to the wood/adhesive interface. The method is applied to rotary-peeled veneers and saw-cut veneers produced from three species of wood bonded with three commonly used adhesives. The new tests method yields a high interfacial failure rate and successfully identifies differences in the performance of the three adhesives. SPG (one species of the rotary-peeled veneers) demonstrates a rising R-curve behavior (an increase in the fracture toughness with crack length) when bonded on the loose side. This increase in fracture toughness is observed to be a result of adhesive-substrate interaction, which is a developing process zone behind the crack tip consisting of bridged wood ligaments.
General Audience Abstract
Construction materials exposed to elevated temperatures from fires may reach temperatures where the material decomposes from the original material to a char. Protected and unprotected structural timber products exposed to fires may exhibit this behavior resulting in a degradation of performance. Understanding the thermal and physical responses of these materials is crucial in evaluating the materials behavior in fire. Additionally, many wood-based products (such as furniture) rely on adhesive bonds. Consequently, their usefulness is determined by the performance of those bonds. In this work, methods are developed to measure key properties impacting the behavior of wood-based systems at elevated temperatures, such as that experienced in fires and when they are subjected to forces attempting to debond one wood material from another. These techniques are demonstrated on common building materials (white pine board, medium density fiberboard and spruce 24) and wood veneers from three different species bonded with three different adhesives. Mathematical models are developed to expand the use of the data beyond the specific conditions for which it is measured.
- Doctoral Dissertations