· Table of Contents
· Fundamental & Applied Research
Ultrasonic testing for spatial grid structure evaluationZ. Q. Zhou, C. H. Zhu
Suzhou Nuclear Power Research Institute, State Power Corporation, China
Phone: 86-512-8287256, Fax: 86-512-8262502,
C. X.. XU
Southeast spatial grid group, Hangzhou zhejiang. China
In this paper, the development history and present status of spatial grid structures in our country are reviewed, and the advantages and shortages of the utilization of the spatial grid structures are shown. Upon this base, the future and development trend of latticed shell structures are discussed in the aspects such as structural span, joint type, fabrication and erection etc. and then some advanced research subjects and ultrasonic testing technical problems to be solved are proposed.
Keywords: ultrasonic testing, spatial grid structure, evaluation
This paper gives a comprehensive review on the achievements and experience in the field of large-span structures in China. The Chinese spatial frame enterprises are technically supported by the civil Engineering Departments of Zhejiang University, Tongji University and Tianjin University. Based on the strong support from the universities and research institutes, the enterprises concentrate on developing and innovating the large-span spatial grid, steel structure, grid structure, tension structure, hybrid structure, movable roof structure and steel megastructure, it makes continuously expanding the application fields and improving the process technique.
Dalian Jinzhou stadium is in stalled with the closed thread-ball joints with an area of over 17 thousand square meters. The complete plane is composed of 10-block lattice frames with maximum overhanging span of 30m.
Jiaxing power plant, its coal-drying is equipped with the 3-core column latticed shell. They are supported on the floor in pairs. Its span reaches 103.5m. For the latticed shell design, the thread-ball joints are combined with the welded ball joints so as to successfully solve the technical problems as its axial strength is more than the super-strength screws.
The roof of coal storage in Zhanjiang power plant is orthogonal square pyramid spatial grids with welded hollow sphere joints. It is supported by four columns. The thickness of the roof varies from 4m to 6m,which forms eight slopes for drainage. It is covered by triangular prefabricated ceramic concrete with ribs. The structure was made up on site and installed by integral lifting. It has three features: the roof has eight slopes, and the mean thickness is relatively small; the central part of the roof is fabricated in checkerboard pattern, which improves the mechanical property of the structure; for convenient of making up, inner column cap is adopted.
The coal-drying of Ya He Kou power plant is equipped with the 3-core column lattice shell. Its span reaches 108m which is the longest one in Asia.
However, the quality control system plays an important role for ensuring the good quality and the effective application of the spatial grid structures. The quality control is carried out from the material selection to structure installation.
Ultrasonic testing is one of the inspection methods widely used for control the quality of spatial grid structure, but it has two major difficulties as follows:
To solve these problems, the methods for ultrasonic testing and the classification of welds for steel spatial grid structures shall be studied.
Frequencies and many other parameters for ultrasound generation and reception can be selected. To generate a specific mode, the angle of incidence and the frequency should be carefully tuned. The theoretical dispersion curves for the guided waves in welds for steel spatial grid structures and latticed shell with bolted joints are only approximate due to model assumption to steel tubing welds section.
Experimental simulations were carried out based on previously developed theoretical results on heat exchanger and steam generator tubing. Three double crystal probe with different focal distance of 4, 6 and 8mm were tested over a wide rang of frequencies. A 70 degree angle of refraction for the double crystal probe is most effective in detecting various sizes of circumferential cracks, wear scars, and holes in the welds of latticed shell.
Recently developed ultrasonic testing techniques are very effective in civil Engineering inspection over large spatial grid structures. A specially designed double crystal probe and instrument measuring system is used. Pulse echo technique is used for flaw detection at frequency of 5MHz. Excellent result on flaw detection is obtained.
The physics of ultrasonic guided waves is developed for many applications including tubing and piping inspection. Excellent results for application of guided waves in the field for steam generator and heat exchanger inspection were obtained from SNPI lab experiments.
The ultrasonic testing technique , adopting special made-up probe and test piece as well as the method for acceptance of defective welds on grading basis, which can serve as a ground for examination of welds at tubular-section elements of F48~F159 mm in size and 3.5~8.0 mm in wall thickness. An angle of refraction of 70 degrees is selected for the final guided wave probe design. This results in shorter inspection times with even greater sensitivities over larger distances. Theoretical modeling results for different materials and sizes of tubing will be useful. It is recommended that the experimental studies on small diameter tubing samples with axial cracks and corrosion be carried out.
The suggested methods and the produced devices make it possible to detect defects of steel spatial structures. Experimental results show that the developed ultrasonic testing technique has high detecting sensitivity and resolution for the holes and cracks. Detecting sensitivity will depend on the ability to discriminate defect scattered signals from the background. The probe with the frequency of 5MHz has the better sensitivity for testing the spatial grid structure welds. Accordingly, the focus with depth of 4mm, 5mm and 6mm is applied, the crystal is made of type PZT i.e. lead zirconate titanate. The scanner is equipped with handles for manual control to move in axial direction. Furthermore, its signal-to-noise ratio was favorable since the echo heights were high.
The combination of the modern technique with spatial grid structures is the new achievement of spatial structures, through the design and construction of curved surface latticed shells, the fabric and structural behaviors of this new type of spatial structure are discussed. The economical analysis shows that the new type of spatial structure at Jiaxing power plant is worthy to be applied widely.
By using ultrasonic testing to guarantee the internal quality of the tube-weld is practicable tool. UT-OK doesn't mean that there haven't flaws, but exist acceptable flaws in welds. The different UT specification has different sensitivity for doing the test and get the different test result.
This work was supported by the National standard council of China under the contract No. JG3034.