What is a Fatigue Analysis?
Fatigue is the failure of a part that is subjected to a repeated load. The failure is caused by the initiation and growth of a crack (or growth from a pre-existing defect) until it reaches a critical size.
The aim of a fatigue analysis is to determine if a part will survive the large number of load cycles experienced in its lifetime. A fatigue analysis will determine the durability or the fatigue life of a part.
Why do Engineers perform Fatigue Analyses?
Many failures in the field are due to fatigue. Failures in the field are always very costly and often dangerous so Engineers are obviously keen to avoid them. However it is easy to miss out on predicting a fatigue failure because the failure is commonly at loads much lower than the ultimate strength of the part.
How Do Engineers assess the Fatigue Life or Durability of their designs?
Engineers will often use the results from a stress analysis and predict durability using hand calculations or a spreadsheet or even an Engineering Calculations tool such as Mathcad.
Can durability be assessed from only a stress analysis?
Sometimes it can. Some designs are driven by factors other than fatigue. When that is the case, a satisfactory fatigue life may be proven using a simple stress limit. Some parts only experience very simple loading conditions. Here again, a simple fatigue calculation may be satisfactory. However, when a structural component has multiple loads that vary with time, a simple stress limit can lead to incorrect critical locations and/or an over designed part. Therefore, more and more engineers are using Fatigue Analysis software to assess durability.
- Methods
-
Methods for Assessing Fatigue Life
In a general sense, a Fatigue Analysis has three main methods- Stress Life
- Strain Life
- Fracture Mechanics
Stress-Life approaches are used for high-cycle fatigue. High-cycle fatigue is when the number of cycles (or repetitions) of the load is high (e.g. between 10,000 - 1,000,000,000 cycles). The stresses are usually low compared with the material’s ultimate strength.
Strain-Life approaches are best suited for low-cycle fatigue evaluation Low-cycle fatigue occurs when the number of cycles is relatively low. Plastic deformation often accompanies low-cycle fatigue.
Fracture Mechanics starts with an assumed flaw of known size and determines the crack’s growth. It is sometimes referred to as “Crack Life” and is widely used to determine inspection intervals.
- Loads & Effects
-
- Loading Type
o Constant amplitude, proportional loading
o Constant amplitude, non-proportional loading
o Non-constant amplitude, proportional loading
o Non-constant amplitude, non-proportional loading
- Mean Stress Effects
o For Stress Life Approach
§ Gerber, Goodman and Soderberg theories
§ Mean Stress Curves
o For Strain Life Approach
§ No Mean Stress Effects
§ Morrow and Smith-Watson-Topper (SWT) theories
- Multiaxial Stress Correction
o Multi-axial safety factor (the Dang Van criterion)
- Modifications & Results
-
- Fatigue Modification Factor
o Value of Infinite Life
o Fatigue Strength Factor
o Loading Scale Factor
o Stress Life Interpolation
- Types of Results
o Fatigue life contour plots
o Fatigue damage at a specified design life
o Fatigue factor of safety at a specified design life
o Stress biaxiality
o Fatigue sensitivity chart
o Rainflow matrix output
o Damage matrix output
o Haigh and Smith diagrams
o Dang Van plots
o And more …
- Industries
-
LEAP sees engineers in Australia and New Zealand concerned with fatigue when designing components in the following areas:
- Vehicle and vehicle accessories Design
- Train and Railway Equipment Design
- Mining Equipment Design
- Conveyor Design
- Manufacturing Plant Equipment Design
- Minerals Processing Equipment Design
- Design of any product with rotating parts
- Design of welded structures subjected to vibration
- LEAP Products
-
FE-Safe
Creo Fatigue Advisor
ANSYS Fatigue