Question

(a) Discuss EACH of the following terms in connection with steel.

(i) Brittle fracture

(ii) Creep

(iii) Fatigue

State, with reasons, where EACH of the mechanisms in (a) may occur in a ship's hull.

SOLUTION.

(a)

(i) Some metals which are normally considered quite ductile can fracture catastrophically in a brittle manner. Mild steel can behave in this manner. Normally, ductile materials undergo 'slip' within the structure so that substantial plastic deformation takes place before fracture. However, in some cases slip can be suppressed such that the fracture occurs suddenly with very little prior plastic deformation. Brittle fracture is characterised by very high speed propagation giving a bright, coarse crystalline fracture surface, which often exhibits chevrons ('V markings) which point back to the source of fracture.

- The tendency for brittle fracture to occur increases with:-

The presence of tri-axial stresses. Hence thicker plate is more prone than thin plate.

High strain rates, usually associated with the presence of notches or stress concentrations.

Low temperature.

The effect of low temperature can be simulated by a notched bar impact test, e.g. Charpy or Izod. If energy to fracture is plotted against temperature there may be a sudden change from ductile to brittle behaviour. This is known as the ductile brittle transition.

(ii) Creep is a strain-time phenomena, at constant stress. If a constant tensile load below the elastic limit is applied to steel, particularly at high temperatures, then the material may undergo plastic (non recoverable) deformation. The classic strain-time curve shows three-regions: primary, secondary and tertiary regions where the 'rate' of strain decreases, is.constant and then increases to eventual fracture.

(iii) Many mechanical failures originate from fatigue. The classic case, say a shaft, shows a smooth 'beach marked' surface radiating from a point of initiation on the surface together with a coarse crystalline region.

Failure occurs suddenly when the crack has grown to a size so that remaining material is insufficient to carry the load.

The basic factors necessary for fatigue failure are:-tensile stress must be present, sufficiently large fluctuations in stress, fluctuations occur sufficiently in number.

The stress necessary to cause failure reduces as number of cycles increase, this can clearly be shown by S - N curves, i.e. fluctuating stress versus number of cycles. (Steels have a minimum stress value -called fatigue limit).

(b) Brittle fracture could be initiated by hogging, sagging and torsional stresses by wave action, particularly in the mid ship 40% length in longitudinal structure, furthest from the neutral axis, such as bottom shell, tank top plating, side shell at the top (the sheerstrake) deck, girders and longitudinals.

In the fore end high stresses are generated from the deflections of the members due to varying hydrostatic and air pressures (panting) and the impact stresses (pounding).

Discontinuities in the ships structure may cause a stress concentration, leading to the possibility of brittle failure of the material. This problem particularly arises at the hatch corners due to the rigid construction (thicker steel) and discontinuity effect.

Creep failure is not a normal problem with ships hulls. In the first instance the stresses that the hull is subject to are normally fluctuating, whilst high temperatures are usually not experienced (although liquid bitumen may be carried at 200°

C)

Opinions have differed as to the importance of fatigue in ships structures. The evidence seems to indicate that complete fatigue failure is rare. It is more likely that small cracks initiated by fatigue could lead to brittle fracture. Discontinuities or cyclic stresses promote fatigue initiation, together with the presence of any resonant vibrations. The midship 40% of the length, fore end and after end are the problem areas.