ME-372 Mechanical Design IIFatigue Experimental Lab

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FATIGUE TEST

OBJECTIVE:

The main objectives of this experiment are:

1) Perform the fatigue test on the given specimens using the Fatigue tester MT 3012 to predict the fatigue life.

2) Determine the safe stress level for the specimens if a fatigue life of 1,000,000 reversals had to be withstood.

APPARATUS REQUIRED:

Fatigue tester MT3012, Vernier caliper, Aluminum specimens.

THEORY:

Fatigue of materials is a well known situation whereby rupture can be caused by a large number of stress variations at a point even though the maximum stress is less than the proof or yield stress. The fracture is initiated by tensile stress at a macro or microscopic flaw. Once started the edge of the crack acts as a stress raiser and thus assists in propagation of the crack until the reduced section can no longer carry the imposed load. While it appears that fatigue failure may occur in all materials, there are marked differences in the incidence of fatigue. For example, mild steel is known to have an 'endurance limit stress' below which fatigue fracture does not occur, this is know as the fatigue limit. This does not occur with non-ferrous material, such as aluminum alloys, however, there is no such limit. This can be seen in the figures below. In figure 1, it can be seen that the steel sample has a fatigue limit of 414 MPa.

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As a consequence of these differences there are two design methods. With a material like mild steel the actual stress range can be kept below the endurance limit. Alternatively one can design for a specified number of stress variations, on condition that the part will be replaced at that stage. The latter method is quite common with aircraft where the use of aluminum is widespread. When designing a part for fatigue strength, an engineer uses results from a fatigue test. However, when designing for infinite life (millions of cycles), such results may not exist and will take too long to reproduce. In such a case, the interpolation from the fatigue data will be used instead. This procedure will be performed in this experiment.

If design is to be based on a possibility of fatigue failure then test data must be produced to this end. The subject of fatigue testing is extensive, and is complicated by important factors like the surface condition of the specimen, the type of stress variation, and the influence of the shape of the specimen on the stress flow. It is known that highly polished specimens withstand fatigue better than normally machined ones. The most damaging type of stress variation is the complete reversal that is between the limits +_ for which the stress range is 2σ. Fluctuating stresses are less damaging, the standard case being between the limits of 0 and +σ. The shape of a machine part is very important, since it is known that at corners and notches the local stress can be several times more than the calculated average value. Evidently since fatigue is a localized stress phenomenon any form of stress raiser must affect performance.

To introduce this very complex subject in a simple way, the apparatus demonstrates the classical fatigue experiments carried out by Wohler. He selected the method of reversing the stress on a part by employing a cantilever rotated about its longitudinal axis. Hence the stress at any point on the surface of the cantilever varied sinusoidally.

DESCRIPTION OF THE APPARATUS:

Fatigue tester MT 3012 shown in Fig.1.is driven by an induction squirrel cage motor at 3000rpm. Power supply provided is 220V single phase. The motor is connected on one side to a counter mechanism, which can record 7 figure numbers. Attached to the shaft at the

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other end is a fixture. The loading device consists of a spherical ball bearing and a micro switch, which automatically switches off the motor when the fracture occurs.

The apparatus is supplied with a recommended standard specimen. The bending stress for a

load P (N) is:

Where, L…distance from neck to specimen’s contact point with bearing d…Diameter of the neck

P…Load applied (measured by digital read out)

By turning the loading wheel clockwise the loading on the test piece can be increased. A cell load which a digital read out measures the loading value. The fatigue tester, which is designed to be placed on a bench, is very stable on 8 feet, weighing 24kg. Dimensions are 980x280x460 mm.

Figure 3. MT3012 Fatigue tester.

Figure 4. MT3012 Fatigue tester with the load cell integrated.

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EXPERIMENTAL PROCEDURE:

As fatigue fracture experiments may run on for half an hour or so the usual procedure is for each group in a class to set up and start two aluminum specimens and for all the results to be shared at the end. The load sets will be provided in the lab session.

1. Measure the diameter at the neck of the specimen and inspect the surface roughness.2. Slide one end of the specimen into the adapter at the shaft end and slide the other end

into the adapter at the load end.3. Measure the distance from the neck to the specimen’s contact surface with the

bearing.4. Apply the given load. Check with the lab instructor about loading the specimen in

order to have a precise bending loading condition.

Don't put any excessive force on the loading arm!!! It will damage the specimen.

Results from other load cases will be collected an made available to each group after all groups have completed the experiment.

4. Set the revolution counter to zero and start the motor.

5. Normally the test terminates itself through the fracture of the specimen opening the micro switch and hence stopping the motor. As the onset of fracture approaches the specimen will bend more, and this may open the micro switch before complete fracture occurs. In this case move the micro switch down slightly and restart the motor.

6. Collate the results and plot them as they occur on a graph of stress range, , against logl0 number of reversals N. Note that in the case of a rotating cantilever the stress range is twice the applied bending stress.

RESULTS:

After obtaining the results for your load cases and getting the results of the remaining cases from other groups, plot σ against logl0 N on a suitable graph paper and look for best-fit lines and also determine the safe stress level if a fatigue life of 1,000,000 reversals had to be withstood. Also discuss the ruptured cross-section and identify the cause of the rupture and analyze the factors, which will affect the results.