An Accurate and Efficient Fitness-For-Service Assessment Method of Pipes with Defects under Surface Load

With continued urbanization in China, the construction of urban gas pipelines is increasing, and the safety of gas pipelines are also increasingly affected by urban development and the increased scope of buildings and roads. Pipes with defects are more likely to fail under the surface loads. In this study, uniaxial tensile tests of high-density polyethylene (HDPE) pipes were carried out to obtain the real material parameters of pipe. A pipeline-soil interaction finite element model of HDPE pipeline with defects under surface load was established. The failure mechanism of the urban gas pipeline was studied and the influence of parameters such as internal pressure, defect position, defect depth on the mechanical behavior, and failure of pipelines were analyzed. A failure criterion for HDPE pipes with defects under surface load was proposed based on the limit-state curves obtained under different working conditions. Furthermore, an accurate and efficient fitness-for-service assessment procedure of pipes with defects under surface load was proposed. The results showed that maximum Mises stress of the pipeline gradually increased with increasing surface load and the position of maximum stress changed from the top and bottom of the pipe to the defect position and both sides of the pipe. Finally, when Mises stress of the HDPE pipe exceeds the yield limit, failure will occur. Internal pressure, defect location, and defect depth were found to influence the failure process and critical surface load of the pipeline. Safety evaluation curves of the gas pipeline with defects under surface load were obtained by calculating the critical failure load of the pipeline under various working conditions. Finally, a nonlinear fitting method was used to derive a formula for calculating the critical surface load under different defect parameters. The proposed method provides a useful reference for urban gas pipeline safety management.

Defect Detection of Printed Fabric Based on RGBAAM and Image Pyramid

To solve the problem of defect detection in printed fabrics caused by abundant colors and varied patterns, a defect detection method based on RGB accumulative average method (RGBAAM) and image pyramid matching is proposed. First, the minimum period of the printed fabric is calculated by the RGBAAM. Second, a Gaussian pyramid is constructed for the template image and the detected image by using the minimum period as a template. Third, the similarity measurement method is used to match the template image and the detected image. Finally, the position of the printed fabric defect is marked in the image to be detected by using the Laplacian pyramid restoration. The experimental results show that the method can accurately segment the printed fabric periodic unit and locate the defect position. The calculation cost is low for the method proposed in this article.

Experimental Investigation of the Mechanical Behavior of NC Linings in consideration of Voids and Lining Thinning

The presence of voids or lining thinning directly affects the mechanical behavior of linings, and these defects threaten the safety of tunnel operation. In this study, a series of 1/5-scale model tests was used to investigate the mechanical behavior of normal concrete (NC) linings in consideration of voids and combined defects. Test results showed that the void and combined defects substantially reduced the load-bearing capacity and deformation properties of the linings. The inelastic mechanical behavior of the linings was also significantly affected by the defects. The effects of lining defects located at the spandrel were slightly weaker than those of lining defects located at the crown. As the void size or degree of combined defects increased, the tensile strain at the location of the lining defects also increased. Therefore, the defect position of the linings was easily damaged. The defects considerably reduced the overall deformation of the linings but increased the local deformation. The distribution of lining cracks was concentrated at the defect position. In addition, different failure characteristics of the lining were observed due to the differences in defects.

The potential of detecting flaws in an experimental dam at Älvkarleby, Sweden, using P-wave traveltime tomography

<p>A large number of dams located in Sweden, built in the second half of 20<sup>th </sup>century, are earth embankment dams. Seepages and internal erosion represent safety issues, which are difficult to detect and predict. There are indirect methods to detect seepages, but these do not provide their location. The hydropower operator Vattenfall has initiated a research project to assess geophysical methods as a decision support and asset management tool for this type of structure. The project consists of detecting built-in flaws in the core of a 20 m long and 4 m high experimental dam using geophysical (seismic and resistivity) and temperature measurements taken at the top of and inside the dam structure for a period of approximately 18 months. The behaviour of the dam itself will be monitored by geotechnical instrumentation.</p><p>This work focuses on testing P-wave traveltime tomography for detecting defects and supporting the interpretation of P-wave reflection seismic data. Synthetic traveltime studies were performed using the dam structure, constant P-wave velocities for each material, and the seismic acquisition design. Five parallel lines of hydrophones were used, three at the top and two at the bottom of the dam. The central hydrophone line at the top of the dam coincides with the position of the seismic sources. In addition, four boreholes to 4 m depth are positioned on each side of the central hydrophone line in both edges of the dam. Within these boreholes shots and receivers were positioned at every 0.5 m depth. The initial velocity model of the dam considers that the dam is filled with water up to a height of 3.5 m. A series of defects (low velocity zones with varying size and position) were inserted. Other factors, like noise or error in the acquisition geometry, were also considered. The defects may be cavities or permeable/loose layers.</p><p>Preliminary results show, in general, that the defect position can be identified by tomography. The velocity and size of the defects, however, are not well recovered by the method. Recovery of the defects using traveltime tomography is greatly influenced by the defect position, as the seismic ray coverage is limited in some parts, such as the central lower part of the dam. In the case of a defect located closer to the top hydrophone lines or one of larger size, the anomalies are better identified. We note that the amplitudes of the anomalies are very small, which may complicate identifying defects using real data. The anomaly signatures depend on the shape of the defect, for example a cubic defect compared to a horizontal permeable layer, which could help to identify and characterize the defect. Although the primary focus lies on identifying the presence of defects, information about their dimension and type is also important.</p><p>Future work will be focused on processing repeated seismic fieldwork campaigns at the experimental dam, in order to investigate the dam integrity using time-lapse seismic measurements, including comparing the seismic data with other types of data.</p>