Early fatigue damage in carbon fiber composites observed by electrical resistance measurement

Early fatigue damage during the first tenth (or less) of the fatigue life was observed in carbon fiber composites by DC electrical resistance measurement. The damage was most severe in the first loading cycle and the incremental damage in each subsequent cycle diminished cycle by cycle. For the continuous carbon fiber carbon-matrix composite, the resistance increased irreversibly during early fatigue due to matrix damage and possibly fiber fracture as well. For the short carbon fiber polymer-matrix and cement-matrix composites, the resistance decreased irreversibly during early fatigue due to matrix damage near the junction of adjacent fibers and the resulting increase in the chance that adjacent fibers touched one another.