Research article

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In this study, thermal and mechanical properties of nanocomposites containing SC-15 epoxy resin and polyhedral-oligomeric-sil-sesquioxanes (POSS) have been studied. Dynamic Mechanical Analysis (DMA) results show that the addition of 5 wt% of POSS yielded a 13% increase in the storage modulus and a 16°C enhancement in T_g. Thermo gravimetric Analysis (TGA) results show that the thermal stability of epoxy increased with higher POSS content. Tension tests were used to evaluate the mechanical properties of materials. Both modulus and tensile strength are linear functions of POSS content. Scanning Electric Microscopy (SEM) pictures of fracture surfaces show that the roughness of the fracture surfaces of epoxy increased after adding POSS. Based on experiment results, a three-parameter nonlinear constitutive equation was developed to describe the strain-softening stress-strain relationship behavior of materials. The parameters in this model are the elastic modulus, a strain exponent, m, and a compliance factor, β. Their relationships to the POSS weight fraction were obtained from the experiment results. The simulated stress-strain curves from the model agree with the test data. Analysis of the model shows that both the strain exponent, m, which controls the strain softening and hardening effect of the material, and the compliance parameter, β, which controls the flow stress level of the material, increase with higher POSS content.