Metal Fatigue Definition
The unexpected failure of metallic components by progressive fracturing while in operating condition is known as metal fatigue. Local overstressing of a component sometimes generates a small crack (cracks can be present in the component due to manufacturing defect as well) that slowly keeps on growing with subsequent operating cycles and the metal part keeps weakening. When the crack grows to a critical size the component fails catastrophically without any warning sign. Such failure is known as metal fatigue failure. Metal fatigue failure occurs in three stages:
- Crack initiation
- Crack propagation, and
- metal failure
Depending on the metal type, the failed surface appears different and does not follow specific rules similar to ductile fractured surfaces.
Factors affecting Metal Fatigue
Metal fatigue of a part is directly related to the number of stress cycles and the value of stress imposed on it. If the local stresses are kept below a defined value, there would not be any fatigue failure on the metal and the part will operate satisfactority for infinite period of time. That limiting value is known as the endurance limit of that material.
Metal Fatigue is greatly affected by the presence of stress raisers like holes, notches, and keyways. Stress concentration increases locally in presence of stress raisers. A metal’s ultimate tensile strength, hardness and its ability to handle fatigue loads are related to some extent. In general, the higher the tensile strength and hardness, the more is the likelyhood of metal fatigue when subjected to high fluctuating loads.
Surface finish of the component also plays a great role in metal fatigue failure. Smooth surfaces increases fatigue life.
Types of Metal Fatigue Failure
Depending on how fatigue failure occurs in a metal part they can be grouped into various types as mentioned below:
- Thermal fatigue failure: Temperature changes imposes such type of metal fatigues in components. Temperature change can be because of operating parameters or environmental factors.
- Fatigue failure due to temperature and pressure cycles: Both temperature and pressure cycles combinedly can induce this type of fatigue failure in a material.
- Corrosion fatigue failure: Highly corrosive environment can cause such type of metal fatigue. Initial crack is normally created by corrosion, which also deteriorates the metal surface and the metal fatigue tendency increases.
- Vibration fatigue failure: Vibration fatigue is generated in a component due to vibration. Continuous vibration of mechanical equipment can lead to crack generation which subsequently results in fatigue failure. This type of metal fatigue is because of the stresses occurring over time and includes corrosion and vibration fatigue failure.
How to identify Metal Fatigue?
Several methods are available for determining that a metal is starting to become fatigued:
- Visual inspection: Detection of cracks or other deformations can be checked visually after a certain period of time.
- Noise analysis: Probability of metal fatigue can also be understood by the noise analysis. Usually, damaged metal makes a specific rattling noise.
- Ultrasonic and X-ray inspection: Non Destructive Ultrasonic and X-ray inspection is the best method to find out evidence of any crack.
- Fluorescent dyes: They make cracks visible and this provides a hint on fatigue initiation.
- Magnetic powders: Magnetic particle inspection can be used for ferrous materials to find out the crack initiation.
How to increase Fatigue Life?
Proper engineering considerations can reduce metal fatigue by increasing its life. While designing a component, various factors need to be considered to increase its fatigue life-
- Avoiding sharp corners: Use of generous radii will reduce stress concentration levels that in turn will increase metal fatigue life.
- Avoiding abrupt Cross-sectional changes: Fatigue life of a metal can be increased by making smooth transition between cross-sections.
- Fatigue life of materials increases with decrease in surface roughness. Mirror polished surfaces provides a very good fatigue life as polished surfaced removes stress raisers.
- Good quality welding with no inclusions, gas holes (porosity) or worm holes improves fatigue life.
- Selecting materials having very good fatigue resistant properties.
- Treating the surfaces those will by loaded cyclically.
Determining the fatigue strength of a metal
To determine the fatigue strength of a material in laboratories, test specimen is prepared following standard guidelines. Then a constant known bending stress is applied on the rotating specimen in the fatigue testing equipment. As the test specimen rotates, the stress applied to any outside fibre of the sample varies from maximum-tensile to zero to maximum-compressive and back. The test mechanism counts the number of rotations (cycles) until the specimen fails to produce the fatigue strength results.
A curve is plotted by performing fatigue tests on various stress levels and finding the number of cycles to failure. This curve is popular as S-N curve. Various codes (ASME, BS, CEN, etc) provides the S-N curve in logarithmic scale from where for a specified stress level the number of cycles till failure can be easily determined. The following figure shows a typical S-N curve from ASME Code.
How to Prevent Metal Fatigue?
Metal fatigue can be prevented by following proper engineering design considerations and by performing a fatigue analysis during design phase. In most of the situations, the parts those will be subjected to cyclic loadings are known beforehand during design phase. So, using those data fatigue analysis can be performed to find out metal fatigue capabilities. various software have the capabilities to perform fatigue analysis of different components like
- COMSOL Multiphysics
- nCode Design Life
- FRANC 2D/3D
- LIMIT stress evaluation software for fatigue analysis
- MSc fatigue
- Caesar II