The Effects of Cure Temperature and Time on the Stress-Whitening Behavior of Structural Adhesives. Part I. Analysis of Bulk Tensile Data

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Abstract
The phenomenon of stress whitening in rubber-modified thermosetting polymers has been observed and reported by several researchers. It is necessary to define and characterize the stress-whitening stress [sgrave]* and strain ε* values for modified structural adhesives since they define localized material failure more accurately (in comparison with the tensile strength) from a fracture point of view. Such characterization should include variation with respect to cure parameters since they are known to affect bulk tensile properties. For this purpose, an analytical method of defining and calculating the [sgrave]* and ε* values is reviewed. The analytical method consists of a modified bilinear form of the Ramberg–Osgood equation which is used in conjunction with bulk tensile data. As a novel approach, the inverses of the Ramberg–Osgood exponents n 1 and n 2 are related to material strength and failure behaviors as a function of cure temperature. The stress-whitening stress levels are also related to the cure temperature. It is shown that the variations of 1/n 1 and 1/n 2 with respect to the cure temperature are similar to the variations of the tensile strength ([sgrave] y ) and the fracture toughness (KIC ) respectively.