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Title: Deformation and fracture of PMMA with application to nanofoaming and adhesive joints
Author: Van Loock, Frederik
ISNI:       0000 0004 7968 5638
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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This thesis contributes to the understanding of the deformation and fracture of methyl methacrylate (MMA)-based polymers in the context of void growth. The first part of the thesis focuses on the prediction of void growth during solid state nanofoaming of polymethyl methacrylate (PMMA). These predictions may contribute to the development of polymeric foams with a thermal conductivity close to that of air. The second part of the thesis explores the fracture behaviour of structural adhesive (e.g. MMA)-based joints. An adhesive layer within such a joint is prone to defects such as (micro)voids and (micro)cracks. The ability to accurately predict the failure strength of adhesive joints as a function of pore or crack size is essential in order to design reliable structures based on adhesive bonding technology. A one dimensional void growth model is developed to simulate cavity expansion during solid-state nanofoaming of PMMA by CO₂ in the first part of the thesis. To that end, tensile tests on two PMMA grades of markedly different molecular weight are conducted close to the glass transition temperature and over two decades of strain rate. The void growth model makes use of fitted constitutive laws for each PMMA grade and the effect of dissolved CO₂ is accounted for by a shift in the glass transition temperature of the PMMA. Solid-state nanofoaming experiments are performed on the two PMMA grades to validate the void growth model. The morphology of the foams (and the limit in attainable porosity) is found to be sensitive to the molecular weight. The measured porosity versus foaming time curves are in good agreement with those predicted by the model, for porosities below the maximum observed porosity. The observed limit of achievable porosity is interpreted in terms of cell wall tearing; it is deduced that the failure criterion is sensitive to cell wall thickness. The tensile strength of a centre-cracked elastic layer, sandwiched between two elastic substrates, and subjected to remote tensile stress, is predicted in the second part of the thesis. An analytical theory is developed by making use of a cohesive zone at the crack tip to predict the strength of the joint as a function of the relative magnitude of crack length, layer thickness, plastic zone size, specimen width, and elastic modulus mismatch ratio. Joint design maps are constructed, revealing competing regimes of fracture. The analytical theory is verified by finite element calculations, and validated by means of two experimental case studies.
Supervisor: Fleck, Norman Sponsor: EPSRC ; SABIC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Polymethyl methacrylate ; Nanofoams ; Deformation maps ; Failure maps ; Tensile tests ; Solid-state foaming ; Gas dissolution foaming ; Molecular weight ; Void growth model ; Cavity expansion ; Adhesive joint ; Finite element analysis ; LEFM ; Cohesive zone ; Stiffness mismatch ; Cellulose acetate