In ultrasonic structural health monitoring (SHM), environmental and operational conditions, especially temperature, can
significantly affect the propagation of ultrasonic waves and thus degrade damage detection. Typically, temperature
effects are compensated using optimal baseline selection (OBS) or optimal signal stretch (OSS). The OSS method
achieves compensation by adjusting phase shifts caused by temperature, but it does not fully compensate phase shifts
and it does not compensate for accompanying signal amplitude changes. In this paper, we develop a new temperature
compensation strategy to address both phase shifts and amplitude changes. In this strategy, OSS is first used to
compensate some of the phase shifts and to quantify the temperature effects by stretching factors. Based on stretching
factors, empirical adjusting factors for a damage indicator are then applied to compensate for the temperature induced
remaining phase shifts and amplitude changes. The empirical adjusting factors can be trained from baseline data with
temperature variations in the absence of incremental damage. We applied this temperature compensation approach to
detect volume loss in a thick wall aluminum tube with multiple damage levels and temperature variations. Our specimen
is a thick-walled short tube, with dimensions closely comparable to the outlet region of a frac iron elbow where flow-induced
erosion produces the volume loss that governs the service life of that component, and our experimental sequence
simulates the erosion process by removing material in small damage steps. Our results show that damage detection is
greatly improved when this new temperature compensation strategy, termed modified-OSS, is implemented.
The pulse-echo method is widely used for plate and pipe thickness measurement. However, the pulse echo method does not work well for detecting localized volumetric loss in thick-wall tubes, as created by erosion damage, when the morphology of volumetric loss is irregular and can reflect ultrasonic pulses away from the transducer, making it difficult to detect an echo. In this paper, we propose a novel method using an inductively coupled transducer to generate longitudinal waves propagating in a thick-wall aluminum tube for the volumetric loss quantification. In the experiment, longitudinal waves exhibit diffraction effects during the propagation which can be explained by the Huygens-Fresnel principle. The diffractive waves are also shown to be significantly delayed by the machined volumetric loss on the inside surface of the thick-wall aluminum tube. It is also shown that the inductively coupled transducers can generate and receive similar ultrasonic waves to those from wired transducers, and the inductively coupled transducers perform as well as the wired transducers in the volumetric loss quantification when other conditions are the same.
Guided wave ultrasound is a very powerful and reliable nondestructive testing technique. The emerging smart structure health monitoring strategies demand a wireless sensor for most applications. Passive sensor interfacing with wireless technology is advanced due mainly to the mW power requirements of such sensors. Guided wave sensors, on the other hand, are active sensors that require orders of magnitude more power than the typical passive sensor. Consequently, the design of the sensor, embedded electronics, and adjacent power source become more complicated. Sensor accessories can be minimized by locating zones on the phase dispersion curves where modes are efficiently generated. In this paper, this concept formulated via the source influence phenomenon. Experimentation focuses on quantifying the activation power requirements in different zone of the dispersion curves.