EFFECT OF HIGH-STRAIN RATE ON 2 STRUCTURAL ADHESIVE JOINTS TESTED IN SHEARING

The introduction of adhesives in the car industry will be dependent gr adually on the increase of experience and confidence in such joining t echnique. The behaviour of adhesive joints under high strain rate need s to be clearly understood. A new experimental set-up (the inertia whe el) has proved to be useful to finely define the behaviour under impac t load of two steel sheets bonded with two epoxy systems (a modified e poxy resin and a unmodified epoxy resin) tested in shearing. Concernin g the modified epoxy, the analysis of each mechanical parameter as a f unction of the strain rate (The energy, the stress or the strain at fr acture) has pointed out the existence of a critical strain rate (epsil on(c)). This critical value might be linked to the presence of a secon dary transition beta in the range-of the explored strain rate (10(4)-1 0(5) s(-1)). Below epsilon(c), the strain would be ruled by the a alph a relaxation (principal relaxation), whereas beyond epsilon(c) the alp ha processus would be first subordinated to tile liberation of the loc al modes of the beta relaxation (secondary relaxation). Effectively, t he unmodified epoxy resin which does not show a beta transition does n ot show a critical strain rate either.