Introduction to structural integrity
Introduction to structural integrity

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Introduction to structural integrity

3.4.1 Fracture surface

One half of the eye at the joint is shown in Figure 38(a), and it shows two breaks in the limbs either side of the pin-hole. Although both appear brittle in this picture, in fact one side showed signs of ductile deformation. The way it had fractured was unique when compared with the other eye bars collected. The missing part of the eye bar was located and examined (Figure 38b). It corresponded well to the main part, although it had been damaged in one corner – presumably when it fell off the pin and impacted with the deck or another part of the bridge, which must have been still standing at that point in time. This second part shows more clearly the ductile portion on the left, where the limb has broken with a large lip projecting from one side of the component. This surface is seen in oblique view in Figure 38(c), a view that also shows a secondary crack or branch away from the main path of the crack. The surface on this ductile part of the eye bar was much coarser than the brittle fracture side. A thin layer of rust covered all the surfaces, as would be expected from their immersion in the river for several days.

Figure 38
Figure 38 The brittle fracture in eye bar 330

Exercise 8

Suggest why it was important to find the missing portion of the eye bar.

Answer

It is always best if a corresponding part to a fracture surface is examined, because it can corroborate features present on the half of the surface already found. It is especially important where subsequent damage such as corrosion has occurred. If eye bar 330 had fractured at an early stage in the disaster, it would be vital to determine the cause of the brittle fracture.

When the thin coating of recent red-brown rust was removed gently in the laboratory, the original state of the surface on the lower part of the eye-bar hole, the part showing the brittle fracture, was revealed. Citric acid, present in citrus fruits like lemons, was used to remove the rust. It is a very weak acid, and so its dissolution of red-brown rust is slow. This allowed more control of the cleaning process, minimising damage to the underlying surface. The overall fracture surface showed very little sign of ductility, except for a small shear lip along a short length of the outer edge of the fracture.

Part of the fracture surface is shown in Figure 39. It was noticed that one corner of the inner side of the fracture, i.e. the side next to the pin, showed two curved features of different colour and texture from the rest of the fracture. These zones were very small, measuring only 1.5 mm and 3 mm in diameter respectively; the origin of the larger zone is shown in the figure. They were dark grey, almost black, a tone probably representing Fe3O4, the iron oxide formed in low concentrations of oxygen or air: see Box 6: Rusting. Remnants of the recent red-brown rust were visible in pockets on the rest of the surface. The lines on the curved features pointed back to the inner surface of the eye bar. It was feasible to suggest that the two zones represented brittle cracks present before the final failure that reached a critical size just before the catastrophe.

Figure 39
Figure 39 Close-up of the critical defect in the inner edge of the eye bar

Box 6: Rusting

The reactions of iron and water include several end products, depending on the presence or absence of air, the temperature and the concentrations of salts in solution. The chemical reactions here are for illustration: you don't have to remember them.

The most common product is red-brown rust, formed by the reaction:

4Fe + 3O2 → 2Fe2O3

Note that in the Fe2O3 produced, the ratio of iron to oxygen is 1:1.5. The volume of red-brown rust is about 50 per cent greater than that of the metal, and so can enhance crack growth.

Hydration of the oxide is usual in the presence of water:

Fe2O3 + 2H2O → Fe2O3.2H2O

The hydrated oxide is a very weakly protective film because it tends to spall away from the underlying surface in lamellar flakes, exposing a fresh surface to further attack. The volume change associated with producing the hydrated oxide is larger than for the oxide itself owing to the water molecules in the atomic structure.

The reaction of iron with water can also form hydroxides, producing hydrogen gas:

Fe + 2H2O → Fe (OH)2 + H2

2Fe + 6H2O → 2Fe (OH)3 + 3H2

The hydrogen gas may represent a danger if the reactions occur in an enclosed environment, such as a steel tank, for example. Many welders have been injured and killed by explosions when the welding torch penetrates to the interior: the hydrogen is released to mix with air and then explodes.

If the concentration of oxygen is low, then different oxides are formed:

2Fe + O2 → 2FeO

where the ratio of iron to oxygen is 1:1, and:

FeO + Fe2O3 → Fe3O4

where the ratio of iron to oxygen is 1:1.3.

Both products are black and form preferentially at high temperatures, such as during forging of hot metal, when they are known as ‘black scale’. They are usually removed by treatment with sulphuric acid in large-scale manufacture, a process known as pickling. Black oxide is also formed in central heating systems, since the system is closed to the outer air and oxygen is depleted in the closed water supply by reaction. Hydrogen gas accumulates at the top of the system, and is liberated when the system is bled.

To explore the problem further, the eye bar was examined for signs of further cracks. The mechanism that caused the critical crack was probably at work at other points on the inner surface of the eye bar, so could be tested by several techniques.

Many such sub-critical cracks were found (Figure 40), showing that there was a single mechanism at work. The interior of many of the cracks was filled with iron oxides, often present in a lamellar form showing successive and intermittent phases of formation. An adjacent eye bar on the next joint down along the chain was also found to be cracked in a similar way at roughly the same point.

Figure 40
Figure 40 (a) The inner surface showing sub-critical cracks; (b) micrograph of cracks propagating from the hole surface, 8 mm depth

SAQ 5

  • (a) Describe the construction of eye bar 330 and the position of the critical crack in relation to the stresses on the joint.

  • (b) Explain why a crack may have formed at that location.

    (Hint: bear in mind that a hole in a component represents a stress concentration factor of about three.)

  • (c) If corrosion is an important failure mechanism, explain why the lower part of the eye is more susceptible than the upper.

Answer

  • (a) The joint consists of four eye bars fitted onto a steel pin, with covers bolted on either side to prevent the outer bars falling off. There is a set of two inner bars and a set of two outer bars. At eye-bar joint 330, the north-facing outer bar cracked in a brittle fashion at the lower part of the eye, from a defect present on the inside surface of the eye itself. Both of the upper bars will have been slightly more heavily loaded than the two lower eye bars because they will have borne the combined tension of the lower chain and the weight of the deck transmitted from the hanger. The load path leads from the hanger to the centre pin holding the joint together, to the bearing surfaces of the upper eye bars.

  • (b) The bearing surfaces will be on the inside surface of the eye, one at the shank side and one 180° away on the inner surface. Because there was a clearance of about 3 mm between the pin and the eye, there will have been a gap along the lower edge of the fit of a maximum of 3 mm. The maximum tensile load will therefore have been at two points opposite one another, and at roughly 90° to the bearing points. They are at the lowest and highest parts of the inner surface of the eye. The maximum tensile load will have been about three times the nominal applied load owing to the stress-concentrating effects of a hole, the eye of the bar.

  • (c) The gap between the pin and eye will be exposed to the environment, and will tend to fill with rain water. Rusting will tend to occur there, and if there are any pre-existing cracks, crevice corrosion will develop. Red-brown rust will be formed first, followed by black oxide where the oxygen gradient is low, i.e. at the deepest parts of the crack or cracks.

The other eye bar of the same joint was located, and showed damage to the hole consistent with having been pulled off the pin. A large burr existed on one side of the hole only, showing that the end had been subjected to a large force in the accident.

T357_1

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