Graduate Thesis Or Dissertation
 

On the phenomena of fracture in particle-board

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  • Nonlocal theories of continuum mechanics are used to model the fracture behavior of particleboard in tension perpendicular to the plane of the board. Classical theories of fracture are examined in comparison to a nonlocal crack solution which has removed the stress singularity at the crack tip. With the removal of the classical stress singularity a fracture criterion based on a maximum stress hypothesis results. The nonlocal fracture equations are in terms of a material's internal characteristics such as the dimensions of internal structures (characteristic length) and the bonding between them (intrinsic strength). Examination of the fracture phenomena using the nonlocal equations provides an insight of the actual mechanism of failure. Many materials, including wood, consist of internal structures which are, themselves, composites of smaller units of structure. The level of internal structure that is important to the fracture process is a function of the relative size of internal structures in comparison to the flaw where fracture initiates and the bond strength at the various levels of internal structure. In order to test the nonlocal fracture model it was essential to work with a composite material for Which the dimensions of its internal structures may be varied. This Was possible with particleboard, a composite formed of wood particles and resin. If the nonlocal model is able to accurately predict the fracture for particleboard, it should be able to describe the manner in which a change in particle Size affects fracture resistance. Experimental results in this study give good correlation with the nonlocal prediction. For the particleboard formed in this study the characteristic length of the nonlocal relationship is shown to correspond to particle thickness at low resin contents (5% for a high efficiency laboratory blender). Fracture toughness increases with the square root of particle thickness as predicted by the nonlocal solution. There should be a limit in which an increase in particle thickness can no longer increase fracture resistance. This limit will depend on the relative intrinsic bond strength between particles and within particles. By increasing resin content, which increases the number of bonds between particles, the limiting process was shown to exist. This study showed that a particleboard formed from relatively long thin particles has an average intrinsic flaw size of approximately 0.34 inch. Because this intrinsic flaw size changed only slightly with change in resin content it was postulated that the resin-particle bond causes stress concentration due to a dissimilar response to load. Hence, the compatibility of the resin and wood particles found in particleboard could be as important to the final board properties as was the amount of resin used. If the orientation of substructures in a material can be expected to change the properties of the material, the material is best described as a micropolar continuum. The micropolar crack solution predicts couple stress (rotational stress) in the vicinity of a flaw. Conceivably, particles in the vicinity of a flaw could rotate to align their strongest axis with the direction of maximum stress. This interpretation may explain the initiation of the crazing phenomena in amorphous glassy polymers. For particleboard a flexible resin and short particles may allow such rotation and a large increase in strength could be expected. Caution should be taken in using the approach until the combined nonlocal micropolar crack problem is solved.
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