Abstract
The estimation of the size and location of defects in multi-layered composite structures by ultrasonic non-destructive testing using guided waves has attracted the attention of researchers for the last few decades. Although extensive signal processing techniques are available, there are only a few studies available based on image processing of the ultrasonic B-scan image to extract the size and location of defects via the process of ultrasonic non-destructive testing. This work presents an image processing technique for ultrasonic B-scan images to improve the estimation of the location and size of disbond-type defects in glass fiber-reinforced plastic materials with 25-mm and 51-mm diameters. The sample is a segment of a wind turbine blade with a variable thickness ranging from 3 to 24 mm. The experiment is performed by using a low-frequency ultrasonic system and a pair of contact-type piezoceramic transducers kept apart by a 50-mm distance and embedded on a moving mechanical panel. The B-scan image acquired by the ultrasonic pitch-catch technique is denoised by utilizing features of two-dimensional discrete wavelet transform. Thereafter, the normalized pixel densities are compared along the scanned distance on the region of interest of the image, and a −3 dB threshold is applied to the locations and sizes the defects in the spatial domain.
Highlights
Composite structures made of glass fiber-reinforced plastic (GFRP) or carbon fiber-reinforced plastic (CFRP), such as a wind turbine blade (WTB) or a wing of an aircraft, are often subjected to cyclic loading, which leads to defects and delamination usually initiated at the microscopic or macroscopic level on the surface of the structure [1,2]
guided waves (GWs) are useful for non-destructive testing (NDT) and structural health monitoring (SHM) applications, because large areas can be interrogated with relatively few numbers of transducers [3,4,5,6]
The interaction of GWs with multi-layered composite materials such as GFRP and CFRP may lead to the scattering, reflection, mode conversion, etc. of GWs
Summary
Composite structures made of glass fiber-reinforced plastic (GFRP) or carbon fiber-reinforced plastic (CFRP), such as a wind turbine blade (WTB) or a wing of an aircraft, are often subjected to cyclic loading, which leads to defects and delamination usually initiated at the microscopic or macroscopic level on the surface of the structure [1,2]. The interaction of GWs with multi-layered composite materials such as GFRP and CFRP may lead to the scattering, reflection, mode conversion, etc. Of GWs. the detection of defects is possible by experimental scanning, identifying the size and location of defects requires the post-processing of experimental results. The possible mechanisms that may occur during the interaction of ultrasonic GWs with the layered structure of composites are reflection or refraction of wave modes, scattering, and mode conversions. This increases the complexity of the received ultrasonic signals, so that the extraction of defect-related information becomes very difficult. The post-processing of experimental results is required for the estimation of the size and location of defects in composite material
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