Effect of patch length and elevated temperature on fatigue behaviour of repaired aluminium panels with a CFRP patch

Thin panels of aluminium alloy 6061-T6 with a centre pre-crack were repaired with a one-sided asymmetrical CFRP patch through co-curing of epoxy at the room temperature to have minimal residual stresses. The specimens were tested with a tension-tension fatigue load. The numerical simulation as well as the experiments showed that the fatigue life was improved considerably with the increasing patch length when tested at room temperature. In the asymmetrical repair with a single patch, a bending moment was induced whose magnitude decreased with the increasing patch length, resulting into longer fatigue life. The specimens failed through the growth of the pre-crack in the skin. However, the crack in the skin did not grow in case of the specimen with the longest patch length and the failure occurred at substantially higher number of fatigue cycles through a different mechanism in the bare portion of the aluminium alloy skin. Experiments were also conducted at the elevated temperature of 80 degrees C. Thermal compressive stresses were developed in the aluminium alloy panel due to the vast difference in the coefficient of thermal expansion of the aluminium alloy and the CFRP patch. Consequently, the stress intensity factor of the crack in the aluminium panel was reduced considerably, resulting into the increase of the fatigue failure life significantly.