A New Method for Detecting Surface and Subsurface Defects in Carbon Fiber- Reinforced Polymer (CFRP) Unidirectional Laminates

Defects in carbon fiber-reinforced polymer (CFRP) sheets greatly affect their performance and can pose serious safety hazards during service. Methods, such as radiography and infrared, are widely used but have limitations such as safety and cost. Eddy current methods allow nondestructive testing at low cost and fast response times. Due to the sheet properties of unidirectional laminate, it is more difficult to detect defects at lower frequencies using eddy current methods. At lower frequencies, defect data values are small and susceptible to interference. In this article, a new method for defect detection based on differential coils at lower frequencies is proposed. The main idea of the new method is to use a vertical differential eddy current sensor (DECS) so that it operates near the resonant frequency and uses the impedance imaginary part difference to detect surface and subsurface defects. The method enhances the immunity to interference, improves the sensitivity, and is more favorable for defect detection. The analysis results show that the new method can not only effectively detect surface and subsurface defects but also predict the height of defects by fitting curves. For the same type of defect height prediction, the error is within 0.1 mm at low defect heights, and the error extends to 0.3 mm when the defect height increases to 2 mm. The method proposed in this article achieves sensitive, accurate, and interference-resistant detection of surface and subsurface defects in plates.