Two-stress level fatigue of unidirectional fiber–metal hybrid composite: GLARE 2

Effects of the change in amplitude on the off-axis fatigue strength of a unidirectional hybrid laminate (GLARE 2) have been studied. Tension–tension fatigue tests with a single change in stress amplitude are performed at room temperature for step-down and step-up sequences, respectively. History-dependence of the fatigue strength of GLARE 2 under two-stress level cycling conditions is examined through comparisons with the results of constant amplitude fatigue tests. The two-stress level fatigue strength is affected in a complicated manner by the sequence of fatigue load, the number of prior cycles and the off-axis angle of specimen. The subsequent fatigue life for the step-down sequence is governed by the decrease in stress amplitude, while for the step-up sequence it is done by both an increase in stress amplitude and the number of prior cycles. A characteristic nonlinear accumulation of the fatigue damage in GLARE 2 under two-stress level conditions is also highlighted by a comparison between the off-axis residual lives observed by experiments and those predicted using conventional linear and nonlinear fatigue damage accumulation theories. Finally, mechanisms of the two-stress level off-axis fatigue life of GLARE 2 are qualitatively discussed on the basis of the state of ply stress predicted using a modified classical laminated plate theory. It is suggested that the load-history dependence of the off-axis fatigue strength of GLARE 2 is due to the plastic deformation of the aluminum–alloy layers and to the variable-amplitude fatigue properties of the constituents.