Methods to quantification of precursor damage in carbon fiber reinforced polymer (CFRP) composite structures are reported herein. These techniques include Coda Wave Interferometry (CWI) and non-linear ultrasonics (NLU). Since low frequency Lamb wave propagation is insensitive to the early-stage material degradation, it is shown that decoding the information in coda wave can overcome this well-known limitation. To conclude this possibility, CWI technique is cross verified with traditional high frequency ultrasound method. To achieve this goal a tensile-tensile fatigue experiment was designed for CFRP composite specimens. By inducing controlled fatigue damage in these structures, material states are assessed using low frequency (<500 kHz) ultrasonic guided wave and high frequency (>10 MHz) P-wave. Stretching guided Coda wave is utilized to quantify the precursor damage as a unique approach in this article. However, such method could be illuded by the changes in the signals due to bonds and contacts. To verify if the CWI is successful, and to evaluate the precursor damage in composite structures, additional non-linear analysis of ultrasonic signals from both guided waves and P-waves are performed. Higher order non-linearities in both low frequency guided wave and in high frequency P-wave propagation demonstrate the growth of precursor damage in CFRP composite structures. So does the CWI of low frequency guided wave data. Accuracy of these ultrasonic techniques are validated with experimentally obtained remaining strengths of the fatigue specimens. With this verification it is envisioned that both CWI and NLU together could quantify the precursor damage in composite structures.

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