Failure mode and ageing of steelrepoxy joints

Torsional adhesive joints were made using a filled DGEBA based epoxy resin cured with dicyandiamide DDA.. Ageing, both of the joints and the bulk adhesive, was effected at 40, 55 and 708C at ca. 98% relative humidity. Joint strength was monitored after various ageing periods. Auger electron spectroscopy AES.and X-ray photoelectron spectroscopy XPS. were used to analyze the adhesive and joint fracture surfaces. It was found that the adhesive composition near the adherent zinc-electrocoated and bare steel.was modified compared with the bulk material. Before ageing there appeared to be an enrichment in hardener DDA. near the adherentradhesive interface and apparently adhesive failures were indeed interfacial. During ageing, the formation of ester groups subsequently transformed into carboxylates was observed. After ageing for up to 11 000 h, joints using a zinc electrocoated steel adherent showed fracture near the interface brought about by the modification of the adhesive. For longer ageing periods, failure occurred in a corrosion layer mixed with adhesive. A slightly better performance observed with the zinc electrocoated adherents has been attributed to the migration of Zn or Zn species into the adhesive during ageing. q1998 Elsevier Science B.V. All rights reserved