Fatigue is a property of metal which when be subjected to thousands or millions of cyclic load applications in which the maximum stress developed in each cycle is well within the elastic range of material.
The fatigue test determines the stress which a sample of material can safely endure for a given number of cycles.
Specimens are tested to failure using different loads and the number of cycles before failure is noted for each each load. the results are plotted as graphs of applied stress against the logarithm of the number of cycles of failure. The curve is known as S-N curve.
The number of cycles for failure reduces with increase in maximum stress and vice-versa.
Explain the influence of material defects on the safe operating life of an engineering component
Any material defect increases the stress in the material which drastically reduces the operating life.
Stress raisers like sharp corners decreases operating life and hence must be eliminated.
Sharp surface resulting from punching, stamping, shearing results in stress raiser which reduces operating life.
Surface irregularities during manufacture. Inherent residual stress during manufacture, cracks, fractures on surface increases the maximum stress to which it is subjected which seriously affects the operating life.
State the factors which influence the possibility of fatigue cracking of a bed plate transverse girder and explain how the risk of such cracking can be minimized.
The transverse girder of a slow speed engine incorporates the main bearing pocket.
It is subjected to bending moment as the gas loads force the crankshaft down into the bearing pocket.
This bending is resisted by TIE BOLTS. In addition forces are trying to flex the transverse girder as the gas loads alternate on either side.
The most obvious place for cracking to occur in the transverse girder is under the bearing pocket. These cracks can be radial or circumferential.
Cracking can also occur at the joining welds between transverse and longitudinal girders and at very very sharp change in section.
E.g. :- At root of stiffening webs
The older box section fabricated transverse girder is susceptible to cracking. This cracking will be due to fatigue i.e. action of a varying tensile stress.
RISK OF SUCH CRACKING CAN BE MINIMISED
1. Prevention of excessive vibrations
2. To check Tie bolts tightness at regular intervals. to Re-tighten at correct tightness if found slack. REFRAIN from operating the engine with slack tie-rods.
3. Do not overload the engine.
4. Reduce the power imbalance within the engine cylinders by taking performance curves at regular intervals.
5. Bed-plate inspection to check for cracks, welding, castings and corrosion etc.
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