Fatigue Test

fatigue test many everyday items are not loaded uniformly but rather nonuniformly with periodically changing loads this is quite obvious in off-road biking frames handlebars and wheels are subjected to high alternating mechanical loads it is just as obvious in the case of a garage door spring the spring is unloaded when opening the door and loaded when closing the door the connection rods of this old compressor have endured a large number of rapidly changing tensile and compressive forces even when cycling quite moderately in the city many components are exposed to periodically varying loads without us being aware

of it the crank arms are alternately underload then swing back almost load free are under load again with the rhythm of pedaling the spokes the pedals even the individual links in the chain are stressed periodically also the Railway Bridge Over the River Ry seemingly totally addressed alternately has to Bear the additional burden of a heavy freight train and then only its own Mass aircraft experience quite different stresses during take off and in turbulence than on ground this type of loading is generally termed fatigue loading and the material response to it fatigue the term fatigue is

used because it takes some time for the failure to occur under repeated loads the problem with materials is that they normally endure less at repeated loading than with a once only load the wire does not break when first bent but bending it back and forth repeatedly leads to an early fracture this bicycle crank arm had only been able to endure the cyclic loading for a certain time before it broke the fate of of the garage door spring was the same it had borne the opening and closing of the garage door about 60,000 times before it

finally fractured the broken exercise equipment a torn blade of a fan a Banda Blade full of cracks the broken screw a defective universal joint and many other examples show how important this phenomenon is each day thousands of items break due to cyclic often repeated loading how can we make sure that all these components withstand the cyclic loads to achieve this one has to test the materials appropriately we intend to measure the fatigue strength of a frequently used steel in our laboratory hot rolled steel rods made from the structural steel s235 Jr are used as base

material in order to eliminate the influence of the surface and to stress the material in a well- defined manner suitable specimens have to be machine from the rods this specimen type would be ideal the shoulder shaped ends have been designed for safe gripping the actually tested region is cylindrical with a diameter of 6 mm the material tester goes to the workshop to get 10 specimens manufactured from the rods not just one all of the specimens are as identical as possible back in the lab he has all specimens ready for use and picks out the first

one he mounts the specimen carefully and securely into the fatigue testing machine together with two half shells he inserts one end of the specimen into the right specimen grip with a union nut he locks the right end of the specimen likewise he mounts the Left End of the specimen into the left specimen grip and tightens the nuts the preparation is now complete the material tester then selects the appropriate test parameters on the control computer he intends to load the first specimen alternately with a tensile force of 7 kons and a compressive force of 7 kons

the mean force is zero a click on the start button and the testing machine begins to stress the specimen periodically our fatigue testing machine is equipped with a mechanical resonance Drive [Music] the specimen is connected at its left grip via a load cell to the machine frame at the right grip a spring is mounted and there in turn a mass an imbalance stimulates the mass to vibrate longitudinal guides prevent the Mass from vibrating in the transverse Direction via the spring the test Force acts on the specimen by varying the excitation frequency of the imbalance the

test force can be controlled in our case the specimen is loaded with about 25 cycles per second meanwhile the first specimen has already endured over 300,000 load cycles and still seems to be intact in reality however several cracks have already developed starting from the surface the so-called fatigue cracks with each load cycle they grow a bit further into the specimen the biggest crack gains the upper hand grows the fastest and finally leads to fracture after 381,000 load Cycles the first test is finished the material tester removes the fragments cautiously and inspects the fracture surfaces at

first glance the fracture surface looks unimpressive a close look reveals two regions the rather dull predominantly flat fatigue crack propagation area and the rough residual fracture area the residual fracture area results from the final catastrophic rupture the fatigue crack propagation area has developed during the cyclic growth of the fatigue cracks starting at the surface of the specimen what does the fatigue crack propagation area look like at higher magnification one of the fragments is carefully placed on the specimen stage of our scanning electron microscope now we can investigate the fatigue crack propagation area in detail in

some places but not everywhere parallel grooves and ridges can be seen the so-called fatigue striations each Groove and each Ridge has been formed by an individual load cycle a typical phenomenon of course now we want to know how many load Cycles this material can endure with other loads for this purpose the material tester picks the next specimen from his Supply and installs it into the fatigue testing machine he chooses a higher alternating load for the second specimen a tensile force of 7.5 kons and a compressive force of 7.5 kons under the increased load the specimen

is only able to withstand 43,600 Cycles to failure the third specimen is only tested with a cyclic load of 6.5 K under this reduced load it does not break even after 3 million load Cycles additional specimens complete the series of measurements some of the specimens are tested with the same load we've already used in this way we get an impression of the range of scattering to compare materials in a Fair Way the different cyclic loads are converted to the respective stresses acting in the specimen in this case the stress alternates sinusoidally around the mean value

which is termed mean stress Sigma m in this series of tests the mean stress is zero the stress amplitude is labeled with a symbol Sigma a for graphical representation the SN diagram is used also termed vula diagram in an unusual manner the predetermined variable is plotted upwards on the y- AIS it is the stress amplitude Sigma a and the result of the test the number of Cycles to failure n is plotted to the right on the x-axis to fit small and large numbers into the diagram the xaxis is not scaled linearly but logarithmically the first

specimen was loaded with 7 kiltons this corresponds to a stress amplitude of 248 Newt per square mm under this stress amplitude it broke after 381,000 load Cycles the second specimen was loaded with 7.5 kons this corresponds to a stress amplitude of 265 Newtons per square mm under this stress amplitude it broke after 43,600 load cycles and these are the results of the other specimens which have been tested by us when additional specimens are tested and the results are displayed as a curve it becomes clear that this material will not break below a certain level of

stress amplitude even after a very large number of Cycles this stress level is termed fatigue limit Sigma capital A for most Steels with body centered cubic structure the fatigue limit is reached after 10 million load Cycles at the latest it must be noted that the experimental results scatter considerably therefore instead of a Sharp curve a statistically defined scatter band is commonly used of course most users don't load their components with a constant cyclic load but rather in an irregular manner therefore these components have to be tested under this type of loading termed operational loading in

this test rig critical bicycle components are loaded similarly to practical use stem and handlebar must Ure the typical bending and torsional loads for a sufficient number of Cycles only then they are approved for serial production moreover the fatigue strength does not only depend on the material the mean stress has an important influence the shape the surface quality environmental conditions and much more not an easy story before aircraft is allowed to take off for the first time it has to go through extensive stress tests the wings and many other elements are cyclically loaded in large test

rigs this later ensures the safe operation of the aircraft nevertheless particularly important components are repeatedly examined for fatigue cracks in the course of operation the tram for instance has to go into the test Hall at regular intervals experienced staff use ultrasound to examine special component onon especially the wheel set axles to check whether they are still in perfect condition aircraft are also inspected for fatigue cracks at reasonable intervals this helps us to depart for air travel without worry and land again safely [Music]