Izzard, Vanessa (2012) Dynamic performance and recovery properties of polyamide foams. (PhD thesis), Kingston University, .
Abstract
This research focuses on development of novel methods and techniques to measure the mechanical properties of very low density polymer cellular solids (foams) particularly their response to dynamic impact. Two foams are extensively studied; Zotefoams Nylon based ZOTEK N BSD and N A30 with nominal densities of 50 and 30 kg/m3 respectively. This work has developed a verified adaptation of the traditional Split Hopkinson Pressure Bar. This method no longer uses a stress pressure pulse to load the sample. This has greatly simplified and eliminated many of the technical problems presented by the SHPB method for low stiffness/density foams. Compression set (fixed strain) were undertaken on these foams at various temperatures (-5°C to 90°C) and the foam recovery monitored over time periods in excess of those dictated by standard methods. This data produced a strong log relationship of strain recovery as a function of time. This recovery dependency was also found to be a function of temperature. Based on the results, it was possible to fit the data to produce a useful surface fitted, constitutive equation that described compression set as a function of both initial compressive strain and temperature. Visual observation during compressive tests showed that lateral expansion 01 the materials was non zero. A novel method was developed to determine Poisson's ratio as a function of compressive strain. For N BSD and N A30 Poisson's ratio was found to be less than 0.014 and to be dependent on the compressive strain. Unusual behaviour was observed during tensile tests for N A30 as a function of temperature. The results indicated that over the strain range 0.01-0.10 the modulus of elasticity is not a constant and increased. This behaviour is very unusual and believed to be a result of the base polymer being a blend of two polymers. Compressive tests showed that for low density foams the supporting gas pressure has a dominating effect on the stress strain behaviour of the materials. During heating, proportionally, the change in gas pressure within the foam effects the stress strain curve to such an extent that this affect must be included in the Nagy and William Landel Ferry constitutive equations.
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