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Fatigue life

Overview of fatigue

Fatigue can be defined as progressive loss of a material resistance due to repeated stress or strain. With time, even the most resilient materials develop cracks or breaks due to constant load cycles. Therefore, understanding these processes is of extreme importance for predicting the durability and reliability of materials used in various engineering applications, where structural integrity is a must.

Forms of fatigue

  1. Mechanical fatigue: Mechanical fatigue is the consequence of the application of repeated mechanical load, whether compressive,  tensile, or shear.
  2. Vibration fatigue: Vibration fatigue is experienced due to oscillatory motion or high speed rotation of parts within a short span of time. It is usually caused by poor design, lack of support (or dampeners), or excessive support or stiffness.
  3. Corrosion fatigue: Corrosion fatigue is exemplified by enhanced progressive damage in mechanisms that are periodically loaded together with aggressive fluids. As corrosion develops, the area of damage serves as a point of stress concentration and results in the initiation of a crack.
  4. Thermal fatigue: Thermal fatigue develops when a material repeatedly undergoes cycles of heat treatment and subsequent cooling. This form leads to expansion and contraction that induces stress within the material. Examples include turbine blades and brake discs.
  5. High cycle fatigue (HCF): High cycle fatigue relates to designs that are expected to undergo millions of loading cycles at relatively low stress levels. Even though the stress is low, the effect of many such cycles can, over time, lead to fatigue failure, which is always encountered in components such as heat exchangers.
  6. Low cycle fatigue (LCF): In low cycle fatigue, the number of cycles is lower and the stress levels are much higher, and often happens in structural beams or parts of heavy machines. The stress involved typically exceeds the yield strength of the material.

What is fatigue life?

Fatigue life is the measure of the number of load cycles that a substance can endure before a disruption occurs. Understanding fatigue life is essential for predicting the durability and reliability of materials used in various engineering applications.

Causes of fatigue

  • Cyclic loading: The main loads leading to fatigue are the cycles of loading. It is the repeated application of stress or force to material or structural components over time. Every such cycle reduces the strength of the material by some amount. Load spectrum, load sequence and load history are the key elements in cyclic loading. 
  • Stress range and amplitude: The stress variations or their amplitude add more load to a material and speed up the prevention of this effect. When the limits of stress are further apart, fatigue takes effect faster.
  • Material properties: While some materials may have a higher fatigue resistance owing to their ductility, strength, or even grain structure, others tend to have a faster rate of deterioration under similar conditions.
  • Environmental factors: Agents such as corrosive environments, moisture, thermal variations etc tend to heighten fatigue effects especially when subjected to other forms of stress. 
  • Design flaws: Apparent differences in a material including sharp corners, grooves or sudden changes in contour can lead to a concentration of stress thereby inhibiting fatigue life.

Calculating and predicting fatigue

Engineers use specific equations and methods to estimate fatigue life, ensuring materials last longer without failure.

Fatigue equations

  • Goodman diagram: This representation illustrates the stress-rupture timeline of a material indicating the safe stress values for different number of load cycles.
  • Soderberg equation: This parameter expresses the amount of load that can be applied for a period of time before the part is subjected to cyclic loading.
  • Miner’s rule: Miner’s Rule depicts various fatigue phenomena by calculating a fatigue damage curve as a summation of damages caused by several stress cycles. It is based on uniaxial testing data.

Fatigue testing

Fatigue testing proves essential in assessing the expected service behavior of a material, particularly metals and alloys in real-world scenarios. Common testing techniques include Crack growth testing, Thermal-fatigue testing, High-frequency resonance testing etc.

How to prevent fatigue failure

Design considerations

  • Material selection: High fatigue strength and other environmental protective materials are very important especially in critical applications.
  • Stress reduction: Fatigue life can be greatly improved by relieving stress concentrations through the removal of sharp corners or reducing load spectra.
  • Quality control: Quality of manufacturing is so significant that in most cases material defects, which usually initiate fatigue cracks, are absent.
  • Non-destructive testing (NDT): Routine inspections assist in detecting defects in the structures without destruction of elements enabling timely corrective measures.

Maintenance and inspection

  • Regular inspection: Regular inspections can help identify the development of fatigue.
  • Monitoring: Deploying systems of continuous monitoring assists in tracking the changes in the behavior of materials under loads. Use sensors to monitor the stress level and vibrations.
  • Predictive maintenance: Informed by inspection and monitoring data, predictive maintenance uses advanced analytics to preemptively address wear and tear and damage. It extends the asset life, minimizes downtime, and repair costs by addressing the issues before they result in failure.

Proper fatigue analysis is required to prevent any catastrophic failures that can arise from fatigue-related mechanisms, particularly in critical components subjected to cyclic loads. Fatigue life has no value unless it adds into design opportunities intent on the structure mitigation measures. This is how Strata Global respects this concern in all the projects carried out.

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