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NDT for the Lincoln Park Zoo Animal Cages

by Stuart Kleven*

 

Have you ever been to the zoo when one of the dangerous animals was trying to get out of its cage? It is reassuring that some of the welds in such cages have been checked by an NDT inspector. It also makes me feel good that NDT has reached into such an unusual application. Reading this article recalls some basics about testing of galvanized steel welds - which also makes me feel good. Frank Iddings Tutorial Projects Editor

INTRODUCTION

Nondestructive testing (NDT) affects every aspect of our life. Almost everything we touch or use has been tested in some way. Airplanes require numerous nondestructive tests such as radiographic, liquid penetrant, magnetic particle, ultrasonic, eddy current and visual testing. Automotive parts are tested using radiographic, eddy current and magnetic particle testing. Railroad wheels, engines, tanks and frames are tested with radiography, penetrant, magnetic particles, acoustic emission, ultrasonics, lasers and visually. It has been said that nondestructive testing saves more lives in one day than many other professions do in a lifetime. The following article is a short description of a less common NDT application: zoo animal cages.

 

Testing of Animal Cages at the Lincoln Park Zoo
A new animal facility, the Regenstein Center for African Apes, was constructed at the Lincoln Park Zoo in Chicago, Illinois. Public safety is of primary concern to zoo officials. Therefore, they recently required testing under the requirements of their own specification for metal caging systems. Construction of the cages was performed under the requirements of the American Welding Society's AWS D1.1, Structural Welding Code (2000), as well as the workmanship standards of the National Association of Architectural Metal Manufacturers and the American Institute for Steel Construction.

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Visual and magnetic particle testing were used to verify the acceptability of the caging system welds.

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One aspect of the fabrication that presented a challenge during the construction of the caging system was the requirement for the cage materials to be hot dip galvanized. This is necessary since the cage system requires frequent cleaning with water to remove any waste. Cage sections were prefabricated at the cage manufacturer's facility and were hot dip galvanized after welding. Fabrication of the assembled cage and transfer system was performed using the galvanized sections of cage material. Welding on galvanized members is not extremely uncommon; however, welding in confined spaces at elevated heights out of position can present problems. Zinc from the galvanized coating process can readily be drawn into the welding arc and cause intergranular penetration of the zinc in the weld metal. The steel is embrittled and can cause cracking in the weld. Welding in the horizontal and vertical positions frequently causes undercutting of the base metal as well.

 

NDT of Zoo Cages
Visual and magnetic particle testing were used to verify the acceptability of the caging system welds. Dry red powder and continuous, alternating current yoke magnetic particle testing of fillet and full penetration welds did not reveal any cracking problems. Alternating current yokes were verified for usability by lifting a 4.5 kg (10 lb) weight at the required pole spacing. Use of the red colored powder provided sufficient contrast with the bright silver color of the galvanized components. The welding of tube steel to tube steel members utilized the corners of the square tubes that formed a flare bevel groove weld. This joint configuration required the tubes to be in contact to maintain cage configurations. This normally does not cause a problem in welding. With the zinc coating, however, some of the welds tended to crack, particularly at the beginnings and ends (Figure 1). The specification for welds on galvanized materials indicates that a 1.5 mm (0.06 in.) minimum gap be maintained (American Welding Society, 1982). This allows the zinc to fume away from the weld metal.

Figure 1  - A circled indication found through dry powder magnetic particle testing.

 

Visual testing was performed to test for weld discontinuities, such as undersize welds, undercuts, overlaps, weld length and skip patterns and lack of weld in areas specified to be welded. Undercut gages and fillet weld gages, mirrors and flashlights were used to verify weld leg and throat size and quality. Many welding contractors fail to supply these types of gages to welders and inspectors. This can lead to unexpected surprises if the weld criteria are not well understood by all involved in the project. Along with the visual weld test, all areas of the cages required smooth surfaces with a 1.5 mm (0.06 in.) radius to prevent any cutting or injury to animals due to sharp edges. Proper cleaning and touch up painting of welds with zinc rich paint were required to ensure the absence of corrosion initiation sites during cage cleaning. To allow door latches to freely engage, some latches were raised up off the cage frame and welded with the minimum weld size required. This resulted in a weld leg and throat that was significantly smaller than required to hold the member together and maintain the weld integrity under stress. Welds that had this condition required the addition of more filler metal to increase the weld size to compensate for the gap. This is permitted up to 6.4 mm (0.25 in.) for fillet welds in AWS D1.1. After this gap of 6.4 mm (0.25 in.) is exceeded, a filler plate must be welded in place to fill the gap. Then the item to be welded in place must be welded on top of it. Proper alignment of doors without large, uneven gaps was also verified during visual testing.

 

Conclusion
Satisfactory completion of the weld test has provided zoo officials with confidence that the caging system will perform acceptably and contain the animals as it was designed to do. Public safety will be ensured and a zoo visit will continue to be an enjoyable experience. Application of visual testing in conjunction with magnetic particle testing provided added assurance that the fabrication process was within specification limits and was fit for the purpose intended. Nondestructive testing will continue to be used as a valuable tool to protect the public and affect almost every aspect of our lives.

 

References
American Welding Society, Welding Handbook, seventh edition, Vol. 4, AWS, Miami, Florida, 1982.

American Welding Society, AWS D1.1, Structural Welding Code, AWS, Miami, Florida, 2000.

 

* Alloyweld Inspection Company, Inc., 796 Maple Lane, Bensenville, IL 60106; (630) 595-2145; fax (630) 595-2128; e-mail <alloyweldinsp@sbcglobal.net>.

 

Copyright © 2004 by the American Society for Nondestructive Testing, Inc. All rights reserved.