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

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 (K_IC ) respectively.     

Influence of Residual Thermal Stresses on the Fracture Behavior of Hybrid Bonded Joints

The objective of this work is to study the influence of the residual thermal stresses on the fracture behavior of hybrid composite-aluminium bonded joints. A modified DCLS specimen is designed and the strain energy release rate is determined using both an analytical fracture mechanics based method and a finite element method. The residual thermal stresses, which appear in the adhesive because of the difference between the curing and the service temperatures, are evaluated with a two-ply laminate specimen. Some rupture tests are performed on the DCLS specimens at different temperatures and for different geometries and materials. The results show a good agreement between analytical calculations, numerical analysis and experiments. The residual thermal stresses are never negligible in the considered hybrid bonded joints and do influence their fracture behaviors. Although the designed specimen is not adapted to study fatigue crack propagation, it provides a simple way to study the crack initiation and thus to characterize the rupture properties of the joint.