Abstract
An expansion joint is mainly composed of bellows and other components; it is attached on a container shell or pipe to compensate for the additional stress caused by temperature differences and mechanical vibrations. In China, the expansion joint fatigue tests are often used to assess the quality of products. After fatigue tests, convolution pitch will be changed. The amount of change is an important index that can be used to evaluate bellows expansion joints. However, the convolution pitch detection is mainly done manually and randomly by inspection agencies before shipping to the end users. This common practice is not efficient and is often subjective. This paper introduced a novel method for automatically detecting the change of the convolution pitch based on a laser line scanner and data processing technology. The laser line scanner is combined with a precision motorized stage to obtain the point cloud data of the bellows. After denoising and fitting, a peak-finding algorithm is applied to search for the crest of a convolution. The method to find the convolution pitch and the decision that needs to be made to ensure product eligibility are described in detail. A DN500 expansion joint is used as a sample to illustrate the efficiency of the system. The application of the technique intuitively allows a higher precision and relative efficiency in quality inspection of bellows expansion joints. It has also been implemented in the Special Equipment Safety Supervision and Inspection Institute of Jiangsu province with great success.
Highlights
An expansion joint is a connector that can expand under different forms of loading while remaining structurally sound [1,2,3]
It is composed of bellows and other components and can compensate for thermal and mechanical deformations, absorb mechanical vibrations, reduce stress, and increase the average service lives of pipes and tubes
The convolution pitch of a bellows expansion joint is an important parameter in quality inspection; it is a key consideration when making the decision of whether bellows expansion joints meet quality requirements
Summary
An expansion joint is a connector that can expand under different forms of loading while remaining structurally sound [1,2,3]. After thousands of tensile and compression fatigue tests, the method of detecting the convolution pitch deformation is usually done manually. It is typically found with a caliper that measures the distance from crest to crest. The collected data is processed to determine the quality of the bellows expansion joint. This type of inspection is overly dependent on manual work, which leads to low efficiency and accuracy. Brazhkin and Mirotvorskii [4] introduced a method for detecting a curved surface using the tangent point of a curve They used the Coordinate Measuring Machine (CMM) to collect data and extract features.
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