Study of effects of surface preparation on carbon-fiber-reinforced-polymer (CFRP) Single Lap Joints (SLJ) using positioning of gates in ultrasonic signals

ABSTRACT

Ultrasonic inspection of adhesively bonded Single Lap Joint (SLJ) with composite material as adherend is quite challenging due to very close and overlapping reflections of ultrasonic waves from multiple interfaces across the joint. This study aims to precisely identify gate positions for these reflections in the pulse-echo technique. Further, investigation of surface roughness effects in the bonded region of SLJ is attempted using reflection coefficient R. For a real-valued A-scan signal in ultrasonic pulse-echo mode, a complex-valued analytic signal with amplitude and phase was obtained from the Hilbert transform. A sudden phase change of the analytic signal and a trough in the analytic signal’s envelope was observed at the reflection plane in A-scan. Analysis of spectra of reflected signals shows variation in magnitude at peak frequencies with the surface roughness of adherend. This paper provides a technique for identifying precise gate placement for the reflected signal and R-values evaluation for composite SLJ. For each surface roughness category, reflection coefficient R was experimentally evaluated at the adherend-adhesive interface of SLJ. These R values are correlated with the shear strength of the SLJ obtained from standard destructive tests.

KEYWORDS:

• Single lap adhesive joint
• ultrasonic testing
• CFRP composite
• Hilbert transform
• reflection coefficient

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