3.4.2 Analysis of the eye-bar steel
Many sections were taken of the steel near the fracture to examine its microstructure, and were compared with different parts of the same eye bar as well as with other eye bars. The sections showed a steel core surrounded by a zone that could be identified as being of higher strength due to the presence of martensite.
Martensite is a strong, hard phase of steel usually formed by rapid quenching from a high temperature.
XPS, X-ray photoelectron spectroscopy, gives information about the elements on the surface of a material. It does this by analysis of the X-ray spectrum emitted by the surface when impinged by the electron beam.
The fracture surface was also analysed by XPS for trace elements that might give a clue to the corrosion processes at work over the 39-year lifetime of the bridge. In addition to small traces of manganese present in the original metal, the researchers found significant traces of sulphur present within the cracks; this is an element not present in the metal itself, indicating an unknown, external source. The sulphur concentration was greatest at the mouth of the crack. The steel had a carbon content of 0.6%, slightly higher than the normal content of mild steel (which is up to about 0.3%).
It was possible that the tiny cracks present on the inner surface of eye bar 330 initiated the collapse by causing brittle fracture. It therefore became important to determine the strength of the steel and, also, its fracture toughness. Steel from eye bar 330 was tested, as well as from other eye bars from the Silver Bridge. Hardness tests across a section though an eye bar showed a soft outer zone, followed by a harder zone and then a softer core (Figure 41). The hard zone extended from about 2.5 mm to 9 mm inside the section. This represented the hardened zone produced by quenching the steel during manufacture. The outer layer of the bar showed loss of carbon due to the heat treatment during manufacture.
Charpy impact tests at several laboratories showed the toughness to fall with lowering temperature, with a low value at or near the freezing point of 0 °C (Figure 42), a temperature close to that experienced at the bridge at the time of the accident.
Small samples cut from eye-bar material were also tested in simple tension at 25 °C, and gave the following results:
yield strength of outer layers = 590 MPa
tensile strength of outer layers = 835 MPa
yield strength of inner layers = 490 MPa
tensile strength of inner layers = 810 MPa.
All the samples showed high ductility with a reduction in cross-sectional area of nearly 50%.