What Is Fatigue Testing?
Fatigue testing is a test in which a load is repeatedly applied to a test sample to determine at what point this test sample is destroyed.
Even if a metal is subjected to a small load that does not break in one time, an invisible crack is generated by repeated application of the load. This is called “cracking.”
If further load is applied while cracks have formed, the metal will eventually fracture. This is called “fatigue failure.” Fatigue testing is used to investigate the number of times until fatigue failure occurs.
Uses of Fatigue Testing
Fatigue testing is performed on almost all products in which metal is used. It is no exaggeration to say that fatigue testing is indispensable, especially for products that are expected to be used repeatedly, such as automobiles, cables, and overpasses.
Fatigue testing is also used to determine durability against bending operations. This is because metallic materials are often processed in long and narrow shapes and are prone to small cracks as a result of repeated bending.
In addition, although not to the same extent as metals, rubber and resins also have a lower rupture stress when subjected to repeated loading. Depending on the material and application, fatigue testing is recommended, as with metals.
Principle of Fatigue Testing
Fatigue testing is performed in the following sequence:
- A sample is cut from the metal material used for the product to be tested.
- The specimen is placed in a special machine and subjected to a load.
- A graph is created by plotting the load and the number of times a rupture occurs.
- The fatigue limit point is calculated from the curve obtained by the graph.
The S-N diagram can be created by plotting the “magnitude of the load (stress)” on the vertical axis and the “number of times the load has ruptured” on the horizontal axis.
The S-N diagram shows that the load is horizontal after a certain stress. This stress is called the fatigue limit, which is the force at which fatigue failure does not occur, no matter how many times the load is repeated. By multiplying this fatigue limit by the safety factor, the allowable stress of the material can be calculated.
In order to create an accurate S-N diagram, it is important to test with a well-balanced set of large and small loads. It is recommended that at least six different loads be given and that as many points as possible be shaken to perform the test.
Types of Fatigue Testing
There are many types of fatigue testing. The type of test to be evaluated and the magnitude of the load will depend on the product and operating environment in which the test sample will be used.
- Rotating Bending Fatigue Testing
Tests to evaluate whether a rotating shaft can withstand its own weight, etc. - Tensile-Compressive Fatigue Testing
Testing of materials in tension or compression - Giga-Cycle Fatigue Testing
Test to fracture by applying low stress 10 times - Plane Bending Fatigue Testing
Tests to evaluate the durability of steel sheets under bending loads. - Thermal Fatigue Testing
Repeated heating and cooling tests with fixed displacement - Torsional Fatigue Testing
Tests in which torque is repeatedly applied to a specimen - Ultrasonic Fatigue Testing
Tests in which specimens are vibrated by ultrasonic waves - Internal Pressure Fatigue Testing
A test to evaluate the durability of a pressure medium by increasing the pressure in the specimen. - Fatigue Crack Propagation Testing
Fatigue testing to evaluate fatigue crack growth rate
Torsional fatigue testing is essential for automobile shafts because they are subjected to repeated torsional loading. In addition, there are two types of tension-compression fatigue testing: high-cycle fatigue testing, in which the entire surface of the specimen is subjected to loading, and low-cycle fatigue testing, in which loading is applied only to areas where stress is concentrated, such as steps.
For products that may be subjected to localized stress, it is recommended that both tests be performed.
Other Information on Fatigue Testing
Background on Fatigue Testing
The background of fatigue testing is that “metals are not always destroyed only when they are subjected to strong loads.” Machines and structures are often broken by “repetitive loading” in which force is applied periodically, which is said to be about 80% of the time.
If fatigue testing is not used to make the correct assessment, serious accidents may result. For example, the 2007 “ExpoLand roller coaster rollover accident” is said to have been caused by destruction due to axle fatigue.
To prevent such a situation, it is important to conduct fatigue testing appropriate for the material and uses of the product, and to calculate the upper limit of stress (allowable stress) that can be used for the material.