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the use of microprobe and x-ray diffraction techniques to identify the nature of the deposits on the crack surfaces. The principal conclusions of this work were as follows:
(1) Cracks appeared to have initiated in the rather hard material on the surface of the pinhole.
(2) Microprobe analyses indicated the presence of more than normal amounts of sulfur compounds in the crack surface. This indicates that sulfur bearing gases such as sulfur dioxide or hydrogen-sulfide in the atmosphere may have penetrated into the cracks and caused them to grow by the stress-corrosion cracking process.
(3) The cracks propagated along the carbide structures within the material, probably by a combination of stress-corrosion and mechanical loading effects. Figures 11a and 11b indicate the marked difference in sulfur levels on the surface of the pre-existent crack and on the surface of steel in the interior which was freshly exposed.
(4) During the conduct of the studies of the mechanism of crack propagation, the Bureau of Standards also made some limited tests to examine the stress-corrosion cracking susceptibility of the eyebar steel. A portion from eyebar No. 330 was used for this test. This specimen was approximately 2" x 2-3/4" x 3/16" and had one concave face, since it came from the end of the specimen block next to the pinhole. This specimen was subjected to a static bending moment by a three point loading fixture so as to produce a maximum tensile stress of 113,000 psi on the outer fiber. After 60 hours of exposure to a solution in which H₂S gas was dissolved, a black corrosion product (presumably iron-sulfide) formed, most intense at the area of highest stress. The specimen was returned to the corrosion cell for an additional 49 days and, after this exposure, exhibited extensive cracking. These cracks were most numerous in the area of highest stress and were found to extend to a depth of at least 0.033 inches below the surface.
d. The investigations of the chemical composition and mechanical properties of the A7-24 steel of the trusses, hangers, and chain bent posts are covered in Part 4 (Reference 20). Specimens were cut from the 13/16-inch thick gusset plate at joint U7N and from the 5/8-inch thick web of the channel from the north Ohio chain bent post. The