On the crushing behavior and microstructure of closed-cell polymeric foams : experiments and phenomenological investigation




Skeens, Joe Wayne

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This thesis investigates the compressive behavior and crushing of Divinycell H100 under combined axial compression and external pressure. Divinycell H100 is a closed-cell polymeric foam with a relative density of 0.077 that is used extensively in sandwich composites for marine, transportation, energy, and infrastructure applications. The study starts with characterization of the microstructure using micro-computed tomography. It has a nearly monodisperse polyhedral microstructure with mean cell size and wall thickness of 0.50 mm and 0.0348 mm respectively. A custom triaxial apparatus is used to compress cylindrical specimens at different levels of external pressure. A typical axial stress-displacement response exhibits a stiff elastic branch that terminates into a maximum beyond which deformation localizes into a horizontal axisymmetric, radially contracted band of crushed cells. The band then propagates axially with the stress remaining essentially constant. Both the initial stress maximum and the plateau stress decrease linearly as the pressure increases. X-ray imaging of the microstructure of a specimen crushed axially at zero pressure confirmed that during the stress plateau, a highly crushed zone of cells with an average strain of about 50% coexists with zones of essentially undeformed cells. Postmortem images of specimens tested under triaxial loading reveal a similar evolution of crushing. Above a critical pressure, the mode of instability switches to predominantly lateral contraction that evolves into a neck. This is also the mechanism of failure under pure pressure. The localized crushing behavior observed is similar to that of low-density open-cell foams and the results should inform and guide further development of homogenized models for this class of materials.


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