Formation et propagation des microfissures au sein d’un joint collé. Influence de la longueur de recouvrement

Application à l’assemblage par collage de tubes de cuivre

Influence of the lap-length on initiation and propagation of microcracks in an adhesively bonded joint: case of bonded co-axial copper cylinders

^* Professeur des Universités, URCA de Reims
^** Professeur des Universités, Université Paris VI

Abstract

Civil engineers use pipes made of hardened copper to carry various fluids. These are generally joined by a brazed sleeve : during that procedure, adherends are put at relatively high temperature (600°C), which alte rates mechanical properties such as hardening. On the other hand, joining with an adhesive procedure uses only a rather low temperature (180°C) and does not affect the thermal properties of the metallic adherends. Moreover .bonding provides waterproofness and uniform distribution of stresses along the lap, with mechanical or thermal solicitations (differentia! shear).

Former works are connected with experimental and theoretical studies of mechanical complex behaviour for axisymmetric specimen. Comparison between the elastic model and the reed behaviour are made with the help of extensometric electrical gauges and acoustic emission.

Our aim in this paper is to precize, on one hand, the boudaries of the modelization, and on the other hand, to predict -as far as possible- the non linear procedures such as initialisation, growth and spreading of microcracks in stable conditions, their growth in instable conditions to give cracks in the neighbourhood of the collapse of the specimen.

In a bonded joint, the interaction between end singularities is related with the geometry of the whole structure and with the micro-geometry of the subjectiles, especially with the length of the lap, 1, this last parameter varying from a very small value to an optimate one, where the better strength is obtained. So, the main kinds of mechanical behaviour are put in evidence.

Six values of the parameter 1 are studied : 10, 15, 25, 35, 45 and 60 mm, for two pipes with the same axis and with an internal diameter of 12 and 14 mm and a 1 mm thickness. The adhesive is an epoxy resin with a polymerisation temperature of 180°C and whose bulk behaviour is slightly viscoelastic and brittle.