Effect of various hygrothermal environments on the failure of adhesively bonded composite single-lap joints

The effect of moisture and temperature on the strength of composite single-lap bonded joints were extensively investigated. Secondary bonded-joint specimens were immersed in tap water and 3.5% salt water at 71°C for different durations up to a maximum of 300 days. A total of 160 specimens were tested under six different moisture contents and two different temperature environments. Test results showed that while the strengths of the joints with 2.1% or more tap water moisture content decreased compared with those of dry joints, the strengths of pre-saturated joints with up to 1.0% moisture content increased at both room and elevated temperature. However, the strengths of saturated joints at 2.5% moisture content decreased by 34% (room temperature) and 44% (71°C) compared with those of the dry joints. In the joints with 1.0% or lower moisture content, the higher temperature environments resulted in slightly increased strengths, whereas the elevated temperature degraded the strengths in the joints with 2.1% or greater moisture content. The major failure modes of the joints were delamination and intralaminar failure of the composite adherend; the joints with high moisture content tested under elevated temperature showed cohesive failure of adhesive. The strengths and failure modes of the joints that were immersed in salt water did not show a remarkable difference from the joints that absorbed tap water.