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In this work, reduced order nonlinear state of Lamb wave propagation due to stress-relaxation of composites was experimentally observed. Residual stresses in the composites are developed under tensile-tensile fatigue loading, which reduce over time during relaxation process due to viscoelastic behavior of the polymer matrix. To investigate reduction in nonlinearity of Lamb wave during stress-relaxation, fatigue loading on the composite specimens were conducted at an interval of 75k until 225k cycles for different cyclic frequencies (i.e., 2Hz, 5Hz and 10Hz), and relaxation experiments were conducted for a duration of 8hrs between two successive fatigue loading sequence. Experimental results show a 6-20% reduction acoustic nonlinearity of Lamb wave during relaxation. Reduction in nonlinearity is mainly contributed by stress redistribution at fibers and recovery of plastic strain during relaxation. This technique is imperative to explore long-term performances and conditions of advanced composite structures.
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