Evaluation of the imaging performance of a CFRP-adapted TFM algorithm

Grager, Jan-Carl; Siemens AG; Germany

Grager, J-C.; Siemens AG; Germany
Mooshofer, H.; Siemens AG; Germany
Grosse, C.U.; Technical University of Munich; Germany

ID: ECNDT-0183-2018
Download: PDF
Session: Composite Material - UT 4
Room: G1
Date: 2018-06-15
Time: 09:00 - 09:20

Structural composite components of several centimeters thickness are increasingly used in the energy industry. Consequently, the demand for more reliable and sensitive non-destructive testing techniques for such structures is also rising. In the case of ultrasonic testing, lower test frequencies are often used to temper prominent effects like scattering or absorption in thicker polymer matrix composites. This facilitates the inspection but also reduces the sensitivity to small reflectors. Novel array reconstruction algorithms such as the total focusing method (TFM) counteract this issue.
Recent studies of our group demonstrated that modifications of the TFM are necessary to reconstruct full matrix capture (FMC) acquired data for carbon fiber-reinforced polymers (CFRP). First, constructive superposition of reflector signals is achieved by considering the anisotropic sound velocity within the material. Second, the noticeably smaller divergence angle of a single-element transmission, compared to that in isotropic metals, should be taken into account. As a result, the signal-to-noise ratio (SNR) in the ultrasonic image and the detectability in the near-subsurface region (close to the transducer) is increased.
This paper describes the successful application of an adapted TFM algorithm to image small artificial defects in a 20 mm thick CFRP laminate. Of particular concern is the evaluation of its imaging performance to reconstruct side-drilled holes (1.0 mm and 3.0 mm in diameter) located at different depths. The SNR and the spatial resolution of the reconstructed images are evaluated and optimized. Ultrasonic imaging artifacts that occur in the TFM images are discussed. This contribution also addresses the challenge of inspecting thick and highly attenuative composites using the total focusing method. To this end, two similar arrangements of side-drilled holes for aluminum and CFRP were examined and their TFM results compared.