Answer Construction Question 17

Question 17: With reference to fatigue of engineering components:

A. Explain the influence of stress level and cyclical frequency on expected operating life;

B. Explain the influence of material defects on the safe operating life of an engineering component;

C. State the factors which influence the possibility of fatigue cracking of a bedplate transverse girder and explain how the risk of such cracking can be minimized.

Answer:-A. Fatigue is the failure of a material under a repeatedly applied stress than its breaking stress, when its breaking stress, when subjected to thousands or million of cyclic loads.

A series of identical specimens are tested to fracture, starting at a high stress and progressively reducing the stress on successive specimens. Stress amplitude S is plotted against the number of cycles to fracture, N, using semi-log or log-log scale. The S-N diagram shows that the fatigue strength or endurance strength decreases with increasing number of cycles.

B) Factors affecting fatigue strength are 
(i) Surface roughness, porosity or foreign inclusion (slag, oxides), abrupt change in section due to manufacturing or machining defects causes severe stress raisers and reduces fatigue strength.
ii) Coarse grain, chemical composition, and cold working causes residual stress to remain in the material and reduces the fatigue strength of the material.
iii) Corrosion environment, erosion during service and decarbonisation during processing or heat treatment reduces fatigue strength.

C) Bed plate forms the foundation of the engine. It supports the engine static load, running load as well as provides the alignment of the engine. It transfers the loads on to the tank top through cast iron or resin chocks.
Load and stresses on bed-Plate –
1) The filing load (gas pressure) acting downwards and tie-rod stress acting upwards.
2) Primary and secondary forces and external moments due to rotating and reciprocating masses.
3) H-Type and X-type guide force moments.
4) Axial vibration due to in-plane bending of crank webs.
5) Torsional stress due to varying torque and propeller thrust. Causes of fatigue cracking —
(i) Cylinder overload due to excess power or high cylinder pressure.
(ii) Incorrect crankshaft alignment.
(iii) Manufacturing fault — (a) Small material defect, (b) High residual stresses in welded joints (c) Hardening of heat affecting zone, (d) presence of dissolved hydrogen, (e) poor welding joints,
(iv) Tank top deformation due to pressurization of double bottom Tank top or overheating of tank fluid.
Minimization of Cracking risks —
Bed plate are made from mild steel plates and steel castings which are assembled and welded together so that the bed plate is strong longitudinally (fore and aft) and transversely (port and starboard) with good resistance to twisting along its length. The longitudinal strength is obtained by making each side of the bed plate in the form of box girder formed with two flanges and two webs. The cast steel cross girders in which the main bearings are placed give the bed plate its transverse strength and resistance against twisting along its length.
Besides the above resin cast chocks are used between bed plate and double bottom tank top to absorb the shocks and cyclic stresses which are almost twice as efficient compared to conventional cast iron chocks.