Compensation for temperature-dependent phase and velocity of guided wave signals in baseline subtraction for structural health monitoring

Abstract

Baseline subtraction is commonly used in guided wave structural health monitoring to identify the signal changes produced by defects. However, before subtracting the current signal from the baseline, it is essential to compensate for changes in environmental conditions such as temperature between the two readings. This is often done via the baseline stretch method that seeks to compensate for wave velocity changes with temperature. However, the phase of the signal generated by the transduction system is also commonly temperature sensitive and this effect is neglected in the usual compensation procedure. This article presents a new compensation procedure that deals with both velocity and phase changes. The results with this new method have been compared with those obtained using the standard baseline stretch technique on both a set of experimental signals and a series of synthetic signals with different coherent noise levels, feature reflections, and defect sizes, the range of noise levels and phase changes being chosen based on initial experiments and prior field experience. It has been shown that the new method both reduces the residual signal from a set baseline and enables better defect detection performance than the conventional baseline signal stretch method under all conditions examined, the improvement increasing with the size of the temperature and phase differences encountered. For example, in the experimental data, the new method roughly halved the residual between baseline and current signals when the two signals were acquired at temperatures 15°C apart.

Location Specific Temperature Compensation of Guided Wave Signals in Structural Health Monitoring

“…4 using the known T(0, 1) velocity. Each signal was normalized to the reflection from the end of the pipe and was compensated for the temperature-dependent wave speed and transducer phase shift using the PSC procedure [6] described in the introduction, using the first measurement as the baseline.…”

“…Theoretically, the generation of flexural modes can be prevented by ensuring that the number of elements in the ring is greater than k, where F(k, 1) is the highest order flexural mode whose cutoff frequency is within the bandwidth of the excitation signal [5]. However, in practical systems, some nonuniform transduction sensitivity of the transducers around the circumference is inevitable and will break the desired axisymmetry, hence generating (and receiving) flexural modes and enhancing the amplitude of the circumferential modes [6].…”

“…Because these unwanted signal components are deterministic, they cannot be eliminated through averaging, and hence they set a background noise level which is referred to as coherent noise. Defects must produce a reflection somewhat larger than this noise for reliable detection in a one-off inspection [6]. For this reason, the defect "call level" is typically set to reflection amplitudes corresponding to anomalies (e.g., defects) that present approximately 5% change in the cross-sectional area (CSA) of the pipe [7].…”

“…Finally, some applications of the guided-wave-based monitoring systems are affected by strong signal attenuation, which is usually temperature dependent, for example, monitoring of a pipe coated with a viscoelastic material such as bitumen. Recently, Mariani et al [6] proposed a phase compensation procedure that concurrently targets wave speed and transducer phase response changes. In this article, this will be denoted as the phase and stretch compensation (PSC) procedure.…”

“…Measurement #1 acquired at 19.1 • C and used as baseline for the PSC temperature compensation procedure[6].…”