AIM OF THE EXPERIMENT: Determination of fatigue strength / endurance limit of a mild steel specimen.

APPARATUS REQUIRED:

i. Fatigue testing machineii. Load pan

iii. Different sizes of loadiv. Collectsv. Digital Tachometer

vi. Specimen

THEORY: A metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than required to cause fracture on a single application of load. Failures occurring under conditions of dynamic loading are called fatigue failures .The failure of a metal under repeated pulsating or reversing stresses is called fatigue. Fatigue has become progressively more prevalent as technology has developed a greater amount of equipment, such as automobiles, aircraft, compressors, pumps, turbines etc., subjected to repeated loading and vibrations, until today it is often stated that fatigue accounts for at least 90 % of all service failures due to mechanical causes.

So in selecting a metal for manufacturing a m/c component it is necessary to take into consideration its resistance to fatigue failure, i.e. its ability to withstand a large no. of repeated varying stresses. The resistance of a metal to fatigue failure is determined by its fatigue limit /endurance limit. The fatigue limit is obtained on the basis of 5 millions cycles for steel and on a basis of 20 millions cycles for light cast alloys.

To carry out fatigue test many machines have been developed. But for laboratory investigations a rotary bending test is often used .The m/c is meant for bending test of a cantilever test piece in rotation .The test is conducted by a steady bending moment to a rotating shaft via ball bearings fitted with load hanger . When the shaft rotates the test piece passes through tension and compression stresses alternately. The maximum stress always occurs at the surface, halfway along the length of the specimen where cross section is minimum. For each complete rotation of the specimen, a point in the surface originally at the top center goes alternately from a maximum in compression to maximum in tension and then back to the same maximum in compression are tested to failure using different t loads, and the no. of cycles before failure is noted for each load. The results of such tests are plotted as graphs of applied stress against the logarithms of the no. of cycles to failure. The curve is known as S.N.

DESCRIPTION OF MACHINE :The fatigue m/c consists

Of a motor,which carries the collect .This holds the spe-

Cimen .Two ball bearings fitted with load hangers are

Provided from the square mettalic frame. The specimen

Passes through these ball bearings .Additional load and

Loadpans are also provided with the m/c.A digital tacho-

Meter is kept fixed for recording the no. of cycles of rotation

The input for speed sgnal of the tachometer is taken from the

rotation of specimen. The wholesystm is placed on a platform.

PROCEDURE : Take the collect .Fix it on the spindle of motor where keyways are privided.Insert the mild steel specimen I it .Put the collect nut through the specimen and tighten them in position .For this operation , the load pans has to be kept at proper heights for easy insertion of the ,this may be done by the hand .Put the designed weight at the pan. Put the plug into the mains and be sure of the earth connections .Switch on the mains . Reset tachometer at zero.Start the motor.The m/c will run still the breaks .Calculate the stress applied in each test . Plot F verses N curve.

OBSERVATION :

Load(w) Av.Dia. of specimen(d) No.of cycles(N) StressfN/mm 2

Endurance limit =__________________cycles

PRECAUTIONS :

1)The load should be applied after the desired speed has been attained.

2)The load should be applied incrementally.