Abstract
A high-precision evaluation of ultrasonic detection sensitivity for a micro-crack can be restricted by a corroded rough surface when the surface microtopography is of the same order of magnitude as the crack depth. In this study, a back-surface micro-crack is considered as a research target. A roughness-modified ultrasonic testing model for micro-cracks is established based on a multi-Gaussian beam model and the principle of phase-screen approximation. The echo signals of micro-cracks and noises corresponding to different rough front surfaces and rough back surfaces are obtained based on a reference reflector signal acquired from a two-dimensional simulation model. Further comparison between the analytical and numerical models shows that the responses of micro-cracks under the effects of different corroded rough surfaces can be accurately predicted. The numerical and analytical results show that the echo signal amplitude of the micro-crack decreases significantly with an increase in roughness, whereas the noise amplitude slightly increases. Moreover, the effect of the rough front surface on the echo signal of the micro-crack is greater than that of the rough back surface. When the root-mean-square (RMS) height of the surface microtopography is less than 15 μm, the two rough surfaces have less influence on the echo signals detected by a focused transducer with a frequency of 5 MHz and diameter of 6 mm. A method for predicting and evaluating the detection accuracy of micro-cracks under different rough surfaces is proposed by combining the theoretical model and a finite element simulation. Then, a series of rough surface samples containing different micro-cracks are fabricated to experimentally validate the evaluation method.
Highlights
Micro-cracks are one of the most common defects in industrial fundamental components such as pipes, plates, and bars
It has been found that the surface microtopography has a great influence on the ultrasonic echo signals and signal-to-noise ratio of the micro-cracks, owing to the reflection, transmission, and scattering of the incident acoustic wave [4]
This study mainly focuses on the effects of corroded rough surfaces as caused by internal operating conditions and chemical conditions of the inner and outer walls of vessels and pipelines
Summary
Micro-cracks are one of the most common defects in industrial fundamental components such as pipes, plates, and bars. Vessels and Pipeline, Hefei General Machinery Research Institute Co., Ltd, Full list of author information is available at the end of the article providing a high-precision inspection and evaluation of defects [2]. The crack sizes are generally much larger than the fluctuation height of the surface microtopography in the existing ultrasonic inspection. The microtopography characteristics of rough surfaces generally show differences owing to a variety of forming processes, such as casting, wire cutting, shot blasting, and corrosion. [21, 22] measuring the surface microtopography of corroded pipes have indicated that the height probability distribution of general uniform corrosion tends to have a Gaussian distribution. The surface roughness is a comprehensive evaluation parameter, and different methods have been proposed for describing rough surfaces in a considerable number of studies. A simple expression of the probability distribution function of the microscopic profile height of a Gaussian random surface can be written as Eq (1) [22]
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