Answer EKG Question 10

Q10. A. Explain Creep; Brenelling; Fretting; Fretting corrosion; State, with reasons, where these may occur in ship propulsion System.

Answer:  A. Explain Creep; Brinelling; Fretting; Fretting corrosion

1. Creep:- 

Creep may be defined as the slow plastic deformation of a material under a constant stress. A material may fail under creep conditions at a much lower stress and elongation than would be ascertained in a straight tensile test. Hence tests have to be conducted to determine a limiting creep stress with small creep rate.

Creep test:-

The creep test consists of applying a fixed load to a test piece which is maintained at a uniform temperature. The test is a long term one and a number of specimens of the same material are subjected to this test simultaneously, all at different stresses but at the same temperature. In this way the creep rate and limiting stress can be determined, these values depend upon how the material is going to be employed.

Figure shows a typical curve for a metal, to obtain a minimum uniform creep rate V (i.e slope of line AB). It is necessary that the test be conducted long enough, in order to reach the second stage of creep. Hence, for a time t greater than that covered by the test, the total creep or plastic strain is given approximately by

$\displaystyle \mathrm{e=e_o+Vt}$ ,

Where e, is the plastic strain which would be expected at the end of the first stage, this is important to the designer when considering tolerances, t is the time usually in hours.

Fine grained materials creep more readily than coarse grained because of their greater amorphous metal content, i.e. the structure-less metal between the grains. 

2. Brinelling:- 

Brinelling is the permanent indentation of a hard surface. It is named after the Brinell scale of hardness.

Brinell Test: This test consists of indenting the surface of a metal by means of a 10 mm diameter hardened steel ball under load.

The Brinell number is a function of the load applied and the area of indentation, thus:

Brinell number =Load in Newtons /Area of indentation in mm-sq.

Only the diameter of the indentation is required and this is determined by a low powered microscope with a sliding scale.

Tables have been compiled to avoid unnecessary calculations in ascertaining the hardness numeral. Loads normally employed are 30,000 N for steels, 10,000 N for copper and brasses and 5,000 N for aluminum.

Duration of application of the load is usually 15 seconds. (Industry is still using the old system of calculating Brinell numbers, i.e. load in kilograms/area of indentation in mm2. Hence, their Brinell numbers will be less by a factor of 10.)

3. Fretting:-  

Fretting can take place whenever low-amplitude vibratory sliding takes place between two surfaces. It is a common occurrence because most machinery is subject to vibration, both in transit and in operation. Examples of vulnerable components are shrink fits, bolted parts and rolling bearings.

Fretting can combine many of the wear processes described earlier. The oscillatory motion causes fatigue wear, which can be enhanced by adhesion. The wear can also be combined with corrosion – principally oxidation – and the corrosion products can be abrasive. The fact that no macroscopic sliding takes place often means that wear debris cannot escape, but is trapped between the surfaces.

4. Fretting corrosion

Fretting is a phenomenon of wear which occurs between two mating surfaces subjected to cyclic relative motion of extremely small amplitude of vibrations. Fretting appears as pits or grooves surrounded by corrosion products. The deterioration of material by the conjoint action of fretting and corrosion is called 'Fretting Corrosion.' Fretting is usually accompanied by corrosion in a corrosive environment. It occurs in bolted parts, engine components and other machineries.

Factors affecting fretting

1 Contact load: - Wear is a linear function of load and fretting would, therefore, increase with increased load as long as the amplitude is not reduced.

2 Amplitude: - No measurable threshold amplitude exists below which fretting does not occur. An upper threshold limit, however, exists above which a rapid increase in the rate of wear exists. Amplitude oscillations as low as 3 or 4 nm are sufficient.

3 Number of cycles: - The degree of fretting increases with the number of cycles. The appearance of surface changes with the number of cycles. An incubation period is reported to exist during which the damage is negligible. This period is accompanied by a steady-state period, during which the fretting rate is generally constant. In the final stage, the rate of fretting wear is increased.

4 Temperature:- The effect of temperature depends on the type of oxide that is produced. If a protective, adherent, compact oxide is formed which prevents the metal-to-metal contact, fretting wear is decreased. For example, a thick layer of oxide is formed at 650° C on titanium surface. The damage by fretting is, therefore, reduced at this temperature. The crucial factor is not the temperature by itself, but the effect of temperature on the formation of oxide on a metal surface. The nature and type of the oxide is the deciding factor.

5 Relative humidity:- The effect of humidity on fretting is opposite to the effect of general corrosion where an increase in humidity causes an increase in the rate of corrosion, and an increase in dryness causes a decrease in corrosion. Fretting corrosion is increased in dry air rather than decreased for metals which form rust in air. In case of fretting, in dry air, the debris which is formed as a consequence of wear on the metal surface is not removed from the surface and, therefore, prevents direct contact between two metallic surfaces. If the air is humid, debris becomes more mobile and it may escape from the metal surface, providing sites for metal-to-metal contact.

Fretting proceeds in three stages:

(a) The first stage is the metallic contact between two surfaces. The surfaces must be in close contact with each other. The contact occurs at few sites, called asperities (surface protrusions). Fretting can be produced by very small movements, as little as 10–8 cm.

(b) The second stage is oxidation and debris generation. There is a considerable disagreement between the workers on whether the metal is oxidized prior to its removal or after its removal. It is possible that both processes may occur, each process being controlled by conditions which lead to fretting. In either case, the debris is produced as a result of oxidation.

(c) Initiation of cracks at low stresses below the fatigue limit.

B. State, with reasons, where these may occur in ship propulsion System.

1.Common example of creep failure is the cracking of the expansion bellows after a long period of operation of the main engine. The failure occurs because of the alternate conditions of working state and idle state. During working condition the bellow is at about 350⁰ C and in idle state it is at about 35⁰ C . This situation leads to creep failure.

2. Brinelling in the propulsion systems is the damage to the fuel pump, exhaust valve rollers and cam profiles.

3. fretting and fretting corrosion can be seen in the deformation or reduction in the shank diameter of the bottom end bolts of medium speed auxiliary engines. The wear in the shank diameter is caused by fretting action due to the variation in the stresses the bolt is subjected to and this wear is called fretting wear. Similarly the coupling bolts of the propeller shafting is subjected to fretting caused by the variation in transmission of torque due to heavy seas during bad weather.