FATIGUE TESTING

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Introduction

Fatigue testing is like bending a paper clip back and forth and counting the number of times you’re able to bend it before it breaks.

Thus, a fatigue test involves subjecting a test sample to repeated loading and unloading to evaluate how it will perform over time in its intended usage environment

Fatigue tests are made with the object of determining the relationship between the stress range and the number of times it can be applied before causing failure.

However, when the stresses occurs a sufficient number of times, it causes failure by fatigue. Fatigue is an interesting phenomenon in that load-bearing components can fail while the overall stress applied may not exceed the yield stress

Theory

When the stress occur a sufficient number so times, fatigue failure happened although these stresses are often below the yield strength of the material.

Example of fatigue failure that happened in load-bearing components in cars, airplanes, turbines blades, spring, crankshafts, and shoes that are subjected constantly to repetitive stresses in form of tension, compression, bending, vibration, and thermal expansion.

The term ‘fatigue’ is being used because this type of failure usually happened after a long period of repeated stress. Almost 90% of metallic materials, polymers and ceramics are vulnerable to this type of failure.

Even though this failure is slow in coming but the disastrous fatigue failure can happen very sudden.

Theory

For instance, a rotating-beam fatigue machine, fitted with a fatigue specimen hung by a weight in the middle.

Specimen rotating action is driven by a motor on the right results in tensile stress in the lower fibrous and compressive stress in the upper fibrous of the specimen gauge length.

Therefore, along the gauge length, specimen will be subjected to alternating tensile and compressive stresses similar to the reversed cyclic loading.

The specimen will be fatigue loaded until failure. The number of cycles to failure according to the cyclic stress applied will then be recorded.

Theory

Theory

The fatigue specimen is gripped on to a motor at one end to provide the rotational motion whereas the other end is attached to a bearing and also subjected to a load or stress.

When the specimen is rotated about the longitudinal axis, the upper and the lower parts of the specimen gauge length are subjected to tensile and compressive stresses respectively.

Therefore, stress varies sinusoially at any point on the specimen surface. The test proceeds until specimen failure takes place. The revolution counter is used to obtain the number of cycles to failures corresponding to the stress applied.

Theory

Theory

Increasing of the weight applied to the fatigue specimen results in a reduction in number of cycles to failure.

We can then use the experimental results to construct an S-N curve

The fatigue test is normally conducted using at least 8-12 specimens in order to provide sufficient information for the interpretation of fatigue behaviour of the tested material.

The S-N curve shows a relationship between the applied stress and the number of cycles to failure, which can be used to determine the fatigue life of the material subjected to cyclic loading.

High applied cyclic stress results in a low number of cycles to failure

Theory

For example, the fatigue testing of specific steel provides a small number of cycles to failure at a high cyclic stress.

As the cyclic stress reduces the number of cycles to failure increases at the fatigue endurance limit, there will be a certain value of the cyclic stress where specimen failure will not occur.

This cyclic stress level is called the fatigue strength however; nonferrous alloys such as some alloys of aluminium, magnesium and copper will not normally show the fatigue endurance limit.

The slope can be found gradually downwards with increasing number of cycles to failure and shows no horizontal line. In such a case, the fatigue strength will be defined at a stress level where the number of cycles to failure reaches 107 or 108 cycles.

Theory

The fatigue strength of engineering materials is in general lower than their tensile strength. A ratio of the fatigue strength to the tensile strength is called the fatigue ratio.

It is normally observed that, in the case of steels, the fatigue strength increases in proportional to the tensile stress.

Therefore, improving the tensile strength by hardening or other heat treatments normally increases the fatigue strength of the material.

However for nonferrous metals such as aluminium alloys, the fatigue ratio are found approximately 0.3 and the improvement of the tensile strength do not necessary increases the fatigue strength of the material.

Theory

Fatigue failure occur in three stages, first stage, small crack initiate at the surface where the stress is at maximum, and include surface defects such as scratches or pits, sharp corners due to poor design or manufacture, grain boundaries, or dislocation concentration.

In the second stage, the crack gradually propagates as the load continues to cycle.

Finally, a sudden fracture of the material occurs when the remaining cross-section of the material is too small to support the applied load.

So, components fail by fatigue because even through the overall applied stresses may remain below the yield strength.

Fatigue limit Fatigue limit is the level below where the structures subjected to

dynamic and fluctuating stresses without having fatigue failure.

Fatigue limit also known as endurance limit. In this experiment we want to know the fatigue limit of a specimen is when a smooth cylindrical specimen is bending with a specific load and we take the number of cycle for that load before it fail.

It is determine that the lower stress on a specimen will have a higher number of cycles before specimen fails.

Importance of fatigue limit

The fatigue limit is important for machinery in industrial or engineering design to avoid any hazard. This is because if we know what is fatigue limit of one material, we will know what is the maximum allowable load per infinite cycle of that can be sustained by the material .

Fatigue limit also can be a reference for material choosing in engineering design. Using fatigue limit we can approximately determine other mechanical property like ultimate tensile strength.

For example, fatigue limit for tool steel is usually 0.4 to 0.5 times the tensile strength.

By this information, we can estimate the value of the tensile strength when the fatigue limit of the tool steel is known.

Types of Fatigue

There are three commonly recognized forms of fatigue:

High cycle fatigue (HCF) (>1,000 cycles) Low cycle fatigue (LCF) (<1,000 cycles) Thermal mechanical fatigue (TMF). 

The failure occurs in three phases

Crack initiation Crack propagation Catastrophic overload failure.

The principal distinction between HCF and LCF is the region of the stress strain curve where the repetitive application of load (and resultant deformation or strain) is taking place.HCF is characterized by low amplitude high frequency elastic strains. An example would be an airfoil subjected to repeat bending.

 

High cycle fatigue (HCF)

The tuning fork can endure tens of millions of cycles under these conditions but eventually it will fail due to HCF.

A typical plot of HCF test data is shown in Figure below. The example of how a turbine, fan or compressor airfoil can become elastically stressed describes one source of HCF excitation

High cycle fatigue (HCF)

The study of high cycle fatigue concerns about fatigue behaviour of the materials which is controlled by the applied load or stress and where the gross deformation taking place is elastic.

However highly localized plastic deformation can also be observed for example at the crack tip. The number of cycles to failure in this case is normally determined at higher than 105 cycles.

The S-N curve in the high cycle fatigue region can be expressed using the Basquin equation as follow;

High cycle fatigue (HCF)

Low cycle fatigue (LCF)

LCF is the mode of material degradation when plastic strains are induced in an engine component due to the service environment.

The results of a typical LCF comparison are shown in Figure below. LCF is characterized by high amplitude low frequency plastic strains

Low cycle fatigue (LCF) In the case of low cycle fatigue, the fatigue behaviour is controlled

by elastic and plastic strains and the number of cycles leading to failure is lower than 104 or 105 cycles.

Gross plastic deformation is due to high levels of the applied stresses and leads to difficulties for stress interpretation.

The low cycle fatigue data is generally presented as a relationship between plastic strain (_εp) and the number of cycles to failure (N).

When plotted in a log-log scale, the relationship can be expressed following the Coffin-Manson relationship

Low cycle fatigue (LCF)

Thermal Mechanical Fatigue  

In the case of TMF (present in turbine blades, vanes and other hot section components) large temperature changes result in significant thermal expansion and contraction and therefore significant strain excursions.

These strains are reinforced or countered by mechanical strains associated with centrifugal loads as engine speed changes. The combination of these events causes material degradation due to TMF.

  Fatigue Data

Engineers and technicians obtain fatigue data much as they do tensile data. The test machines are similar to that shown for tensile tests and similar specimens are used.

The chief difference lies in the application of load. In an HCF specimen test, the load is applied to the specimen at 30 to 60 cycles per second and often at much higher frequencies.

In engine components where HCF is a concern, turbo machinery designers observe what is referred to as a material's fatigue strength. This is determined by running multiple specimen tests at a number of different stresses. The objective is to identify the highest stress that will produce a fatigue life beyond ten million cycles. This stress is also known as the material's endurance limit.

Gas turbines are designed so that the stresses in engine components do not exceed this value including an additional safety factor.

LCF testing is conducted in a similar fashion, the chief difference being the need for higher (plastic) loading and lower frequencies

Factors influencing fatigue

There are a range of factors which are found to significantly influence the fatigue properties of engineering materials.

These are for example, stress concentration, size effect, surface effect, combined stresses, cumulative fatigue and sequence effect, metallurgical variables, corrosion and temperature.

Generally, the fatigue crack initiations are observed near the surface. Rough surfaces are therefore undesirable due to stress concentration which accounts for further fatigue crack propagation and eventually lead to global failure.

Corrosive environment and high service temperatures are reckoned to have negative effects on fatigue properties of the materials as they accelerate faster rates of both fatigue initiation and propagation.

Procedure

A cyclic load is applied to the specimen until it breaks in order to measure the fatigue resistance of the material. The fatigue life is indicated by the number of cycles to failure, N. The fatigue testing is conducted by using a rotating beam fatigue tester MT 3012. The machine can record the number of cycles to failure with the counter. The number of cycles to failure indicates the lifespan of the specimen.

The loading device is lowered to the same height as the drive shaft in order to secure the specimen onto the machine.

The lock nut is slide over the specimen. In the right end, the narrow shaft of the specimen is inserted into the bearing of the loading device.

In the left end, the conical part is attached to the motor shaft and the lock nut is tightened thoroughly using wrenches. (Note: Make sure the lock nut is properly tightened to avoid any loosening during the rotation).

Procedure The counter is reset to zero and the loading control is adjusted to 27 kg on

the scale. (Note: Always RESET the micro switch before turning on the electrical supply for every specimen).

The power is switched on and the switch button is pressed to start the test.

The number of load cycles (x10) on the counter is recorded once fracture occurs.

The lock nut and spring load is loosened. The specimen is removed from the tester and the fracture surfaces are examined.

The procedures are repeated for the next specimen with the loading control adjustment