Fatigue behavior of adhesive joints under modes I and II fracture in carbon-epoxy composites, influence of exposure time in a saline environment

This work investigates the fatigue crack growth behavior of adhesive joints under pure modes I and II within epoxy matrix composites reinforced with unidirectional carbon fibers. Experimental tests are made using Double Cantilever Beam (DCB) and End-Notched Flexure (ENF) setups for modes I and II respectively, considering exposure periods of one week and twelve weeks in a salt spray chamber. Control specimens are also studied for comparison.

Static tests were conducted to securely establish the levels of Energy Release Rate (ERR) that were subsequently used to obtain the fatigue initiation curves (G-N) and fatigue crack growth curves (G-da/dN). A probabilistic model based on a Weibull distribution is applied to analyze fatigue initiation data.

The fatigue limit in mode I, for all aging periods, is around 25 % of the static strength, while in mode II, it is around 20 %. These results are very close at all aging levels (0, 1, and 12 weeks). From this, it is inferred that aging in a saline environment of the studied joints does not have a significant impact on the fatigue limit.

In the crack growth zone, for mode I, the velocity is higher in the specimens aged in both periods than in the unaged specimens. The same cannot be said for mode II, where a clear trend cannot be appreciated.