Non-Linear Signature in Ultrasonic Pulse-Echo Signal for Structural Health Monitoring of Fatigue Induced Composite Structures

Experimental evaluation of material state damage due to frequency varied fatigue loading has been reported in this article by estimating the non-linearity in ultrasonic signals in CFRP composite structures. In recent years, researchers have been utilizing the non-linearity in Lamb wave propagation for structural health monitoring, and thus it is becoming a powerful technique for non-destructive evaluation. Unlike isotropic and homogeneous materials, the damage state modeling of composite structures is complex due to their heterogeneous arrangement of material constituents, especially in fatigue induced loading cycles. Although established for metals and alloys, the quantification of material state of carbon fiber reinforced viscoelastic polymer composites due to dynamic loading environment is still under an active research area. This research evaluates the non-linearity effect in stress relaxation using high frequency (~25 MHz) ultrasonic pulse-echo signals. A set of tensile test specimen as per ASTM standard is prepared and fatigue loads are applied at constant time intervals. During these intervals, scanning acoustic microscope is used to generate and capture pulse-echo signals. The change of second and third order non-linearity parameter of ultrasonic pressure waves are correlated with the fatigue induced internal stresses and damage accumulations. Initial investigation indicates that the higher order non-linearity is a function of applied fatigue loading frequency and the remaining life of the composite structure.