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(1) The microstructure and hardness of the steel varied markedly with distance from the faces of the bar, as would be expected for material of this composition, size and heat treatment.
(2) The material in eyebar No. 330 did not appear to differ in any important respect from other eyebar material removed from the structure.
(3) The tests of mechanical properties and chemical composition showed that the materials in eyebar No. 330 and in the eyebar from panel C9-C11, north chain, north bar, were similar in composition and strength to the sample bars that were tested by the American Bridge Company at the time of manufacture (See Appendix B). The mechanical properties fall within a range that satisfies the requirements stipulated in the material specification for the structure.
(4) Results of Charpy V-notch and drop weight tests indicate that this material had limited fracture toughness (2 to 4 foot pounds in the Charpy tests) at 32°F., the approximate temperature at the time of the bridge collapse (See Appendix B). The 15 ft. lb. transition temperature measured in the Charpy tests was about 220 degrees Fahrenheit, and the NDT (Nil-Ductility Temperature) measured by crack-starter drop weight tests was between 130°F. and 170°F. Figures 8a, 8b, and 8c, illustrate the typical microstructure found in the decarburized area near the surface of a bar, the martensite structure at slightly greater depth, and the somewhat softer material found in the interior of the bar.
(5) The burr on the north face of the bearing section of the eyebar hole in the C13 end of eyebar No. 33 was sectioned through the thickness of the bar and along a plane in the longitudinal axis. Figure 9 shows the general geometry of this section and Figure 10 shows a detail of a magnification of the burr itself after polishing and etching. This detail indicates that this burr was formed in a single step as the bar slid off the end of the pin, with no indication that a series of terraces was formed during progressive movements of the bar relative to the pin.
c. Part 3 of the National Bureau of Standards report (Reference 19) discusses the results of fractographic analyses of the fracture in eyebar No. 330 performed with the scanning electron microscope and