Fatigue Failure in Structures

Fatigue Mechanism

• Fatigue is a failure mechanism that occurs in structures subjected to dynamic and fluctuating stresses.
• It can lead to failure at a stress level lower than the yield strength for a static load.

Characteristics of Fatigue

• Fatigue has a brittle morphology.
• It is catastrophic and insidious.
• The process occurs by the initiation and propagation of cracks.
• It is the single largest cause of failure in metals (about 90%).

Cyclic Stresses

• The applied stress can be axial, flexural, or torsional in nature.
• Three types of fluctuating stress-time modes are possible: reversed stress cycle, random cycle, and repeated stress cycle.

Stress Cycle Parameters

• Mean stress: σm = (σmax + σmin) / 2
• Range of stress: σr = σmax - σmin
• Stress amplitude: σa = (σmax - σmin) / 2
• Stress ratio: R = σmin / σmax

S-N Curve (Wöhler Curve)

• The fatigue stress can be applied to samples using special machines.
• The S-N curve represents the relationship between stress amplitude and the number of cycles to failure.

Types of Fatigue Behavior

• Ferrous and titanium alloys have a fatigue limit (also known as endurance limit).
• Non-ferrous alloys do not have a fatigue limit, instead, they have a fatigue strength.

S-N Curve Characteristics

• The S-N curve represents 'best fit' curves through average-value data points.
• There is considerable scatter in fatigue data.
• Fatigue S-N curves have a statistical meaning.

Fatigue Probability Curves

• One convenient way of representing fatigue data is with a series of constant probability curves.

Low-Cycle and High-Cycle Fatigue

• Low-cycle fatigue: high loads, low cycles (< 10^4 - 10^5)
• High-cycle fatigue: low loads, high cycles (> 10^4 - 10^5)

Crack Initiation and Propagation

• Fatigue failure is characterized by three distinct steps: crack initiation, crack propagation, and final failure.
• Fatigue cracks almost always initiate on the surface of a component at a point of stress concentration.