FATIGUE-CRACK GROWTH IN EPOXY ALUMINUM AND EPOXY STEEL JOINTS

The fatigue crack growth rate within epoxy/aluminum and epoxy/steel jo ints was evaluated as a function of a) type of surface pretreatment, b ) water soak, c) fatigue cycle rate (Hz), d) adhesive thickness and e) type of epoxy adhesive.For both adherends, aluminum and steel, a sign ificant improvement in the fatigue behavior was obtained by use of a m ercaptoester coupling agent. After an 8-day, 57-degrees-C water soak, the metal surfaces which were pretreated with coupling agent (CA) or b y phosphoric acid anodization (PAA) still resulted in cohesive failure , while the controls had higher crack growth rate and showed greater s catter. The room-temperature cure matrix with CA-treated aluminum show ed a less dramatic improvement, probably because of a known difference in the application procedure. For the steel joints and room-temperatu re adhesive the improvement in the fatigue behavior of CA-treated samp les was maintained after the 8-day hot water soak. No significant chan ge was found in the fatigue crack growth rate over a frequency range o f 1 to 5 Hz, but a significant change was found as a function of the b ondline thickness. The room temperature curing adhesive evaluated here in exhibited a much lower fatigue resistance than a heat-cured commerc ial structural adhesive FM-73.