Jessica A. Schroeder

Structure/property relationships for a series of crosslinked aromatic/aliphatic epoxy mixtures

Nine thermoset networks consisting of an aromatic epoxy novolac and an aliphatic epoxy, crosslinked with an imidazole catalyst, were investigated with the aim of correlating macroscopic behaviour and structure as network composition varied. The networks had properties ranging from the two extremes of the brittle, highly crosslinked aromatic, to the rubbery, less crosslinked aliphatic epoxy. The relationships between crosslink density, a measure of network microstructure, and both primary (Tg) and secondary (Tβ) thermal transitions were investigated. These properties were also correlated with tensile toughness (stress/strain behaviour), strength and modulus. It was found that Tg varied linearly with crosslink density whereas Tβ depended on the chemical composition of the network. Although no dependence of the room temperature modulus on crosslink density was found, both tensile strength and toughness were shown to change with crosslink density. Tensile toughness also exhibited a dependence on the temperature and frequency of the β-transition.

Photoelastic Stress Analysis of Bonded Lap Shear Joints Having Thermoplastic Adherends

Abstract The effects of substrate stiffness and modulus on joint strength and stress distribution were investigated for a series of nylon substrates bonded with an epoxy adhesive. Substrate stiffness and modulus were controlled by the level of glass filler in the resin. Single lap shear samples having both identical (“self-bonded”) and dissimilar (“cross-bonded”) substrates were investigated. For the self-bonded samples, lap shear strength was found to increase with increasing substrate modulus and stiffness. The strengths of the cross-bonded samples were intermediate to the strengths of the corresponding self-bonded samples. Photoelastic techniques were used to observe stress patterns in the lap joints during testing. One type of stress pattern was observed for all self-bonded samples regardless of substrate stiffness. Two patterns, one for the stiff substrate and one for the more flexible substrate, were observed for cross-bonded samples. The photoelastic analysis agreed qualitatively with predictions o...

Fracture toughness and molecular structure of unfilled epoxy adhesives

The cohesive, mode I (tensile cleavage) fracture energy (or fracture toughness), GIc, of bulk tapered double cantilevered beam (TDCB) samples of a series of three epoxy thermoset networks was determined using a linear elastic fracture mechanics (LEFM) analysis. Networks of different crosslink density were obtained by mixing various amounts of an aromatic epoxy novolac and an aliphatic epoxy and crosslinking with an imidazole catalyst. Brittle, stick-slip fracture was observed for all formulations, with GIc increasing as the amount of aliphatic epoxy increased. However, fracture surface morphologies exhibited evidence of increasing plastic deformation as GIc increased. In the investigation of structure-property relationships for this series of thermoset networks, GIc was found to be inversely related to both network crosslink density and glass transition temperature (Tg). It was also found that the room temperature frequency of the glassy state transition (β-transition) increased as fracture toughness increased.