Axial Stress Measurement of Small Diameter Aluminum Alloy Tubes Using a Conformal Oblique Incidence-Guided Wave Probe

Abstract

Quantitative evaluation of axial stress in small diameter thin-walled aircraft tubing is important for characterizing its mechanical state during assembly and service. However, restricted circumferential access necessitates a compact local excitation configuration, for which geometric mismatch between conventional planar acoustic sources and curved tube surfaces can lead to circumferentially nonuniform excitation and interfere with stable time of flight (TOF) estimation. This study proposes a reference-based axial stress measurement method using an L(0,2)-dominated guided wave response generated by a conformal oblique incidence probe. Based on acoustoelastic-guided wave analysis and transient response characteristics, 500 kHz was adopted as an appropriate operating frequency for the present sensing configuration. Three-dimensional finite element simulations showed that the conformal loading configuration produced a more regular circumferential wavefield distribution. Experimental comparisons showed that the amplitude of the strong subsequent response was approximately 91% of the target wave packet amplitude for the conventional planar probe but below 31% for the conformal probe, indicating greater time domain prominence of the target wave packet relative to subsequent responses and providing a more stable waveform basis for TOF estimation. Repeated calibration established an approximately linear relationship between TOF shift and axial tensile stress. Validation tests on tubes from the same batch, together with repeated measurements after probe reinstallation and recoupling, yielded an overall mean absolute deviation from the reference stress of 1.86 MPa and a mean absolute relative deviation of 3.17%. The results demonstrate the feasibility of reference based quantitative axial stress evaluation using an established zero-load reference state in small diameter thin-walled aluminum alloy tubes under restricted circumferential access.