Damage detection of steel tube with spherical joints using ultrasonic guided wave

Damage detection of steel tube in in-service steel grid structures (SGSs) is essential for maintaining structural safety. Ultrasonic guided waves (UGWs) enable non-destructive detection of micro-damage using singlepoint excitation. UGWs hold significant potential for inspecting tube members. However, tube members in SGSs are typically connected by spherical joints, and echoes reflected from the joint can potentially obscure damage identification using UGW signals. To address this issue, an enhanced UGW-based damage detection method for steel tube with spherical joints (STSJs) is proposed. First, the low-frequency torsional T(0,1) mode UGW is selected as the excitation signal. Next, a bidirectional control strategy for UGW excitation and reception is proposed and incorporated into the UGW-based damage detection method. This strategy employs the pulse-echo method, enabling the same set of transducers to both excite and receive signals. Two transducer rings are positioned a quarter wavelength apart, with a phase shift of π/2 between their signals. This configuration not only suppresses excitation signals in non-detection direction, but also attenuates reception signals from that direction, effectively mitigating interference caused by spherical joints and significantly enhancing the signal-to-noise ratio for damage detection. Subsequently, the reflection coefficient method is used to locate damage in STSJs. This strategy facilitates UGW-based damage detection in STSJs without the need for reference signals. The effectiveness of the proposed method is validated through an experiment on a notched STSJ specimen. Results indicate that by eliminating the interference signals, the proposed method achieves a damage localization error of 0.08%, demonstrating potential applicability of the method for practical damage detection in STSJs.