3.4.5 Fretting fatigue
An additional possibility was considered. It was known that there was significant movement of the bridge during passage of traffic, because users had noticed it many times when crossing. The joints would thus have been subjected to rotary motion around the pin in order to accommodate such vibrations. Could these have caused fatigue crack growth at the bearing surfaces?
Contact between a circular and a flat plate creates so-called Hertzian stresses at the contact zone: compressive at the centre, and surrounded by a tensile zone. A similar effect will occur at a circular pin joint, provided there is some clearance between the two parts. In addition, there could be considerable wear caused by corrosion. Rusting would create particles of Fe2O3.2H2O, which, being harder than the steel, would act as an abrasive powder as the surfaces moved against one another. The fact that the rust particles had a larger volume than the metal they replaced would also stimulate wear. The inner surfaces of the eye-bar holes showed deep grooves (Figure 40a), indicating fretting action. Could fretting have initiated critical cracks?
To test this hypothesis, pin and collar shapes were machined from eye bar 330 (away from the region of the actual failure), fitted together and then rotated so that the pin acted against the collar. The results are shown in Figure 43. Even with the effects of fretting, the material around the eye still showed a higher fatigue life than the material in the shank.
SAQ 6
Suggest how fretting fatigue could occur at a pin joint in the main chains of the Silver Bridge. Indicate the most likely place for such a problem, and compare the actual position of the critical and sub-critical cracks on eye bar 330, drawing any appropriate conclusions.
Answer
Fretting wear occurs owing to repeated cyclical movement at a joint and was caused in the Silver Bridge pin joints by corrosion producing particles of Fe2O3.2H2O that were harder than the underlying steel, and of greater volume. The action will have been most severe at the upper joints on the main chains, where the loads were largest. Tension cracks through fatigue could have formed at either side of the contact zone between the edge of the eye bar and the central pin. Although fretting fatigue had been shown in the tests to be a possible failure mode, the mechanism demands that fatigue cracks could grow only very near the points of contact between the eye-bar hole and the pin. Since the main load will occur along the chain, the contact zones will be on the long axis and not at 90° to the axis. The critical crack was found on the lower edge of the pin-hole at 90° to the axis, so is unlikely to have been formed by fretting fatigue.
There is no doubt that fretting action on the inner surface of eye-bar joint 330 occurred during its 39-year life. The surface next to the critical crack is very rough indeed, showing deep corrugations aligned circumferentially: that is, at right angles to the sub-critical cracks seen in Figure 40(a). There appears to be no obvious correlation between the crack positions and the corrugations, however. Fretting action will have been most severe on the highest joints of the chain where the load on the joint was greatest.