Fatigue Failure in Structures

What type of stresses can lead to fatigue in structures?

What percentage of failures in metals is attributed to fatigue?

What is a characteristic of fatigue failure?

What is the purpose of a rotating bending test?

What is the formula for the mean stress?

What is the term for the difference between the maximum and minimum stresses in a cycle?

What type of waveform is used to approximate the stress cycle?

What is the name of the curve that shows the relationship between the applied stress and the number of cycles to failure?

What is the purpose of repeating the test procedure on other specimens at progressively decreasing stress amplitudes?

What is the main difference between ferrous and titanium alloys in terms of S-N behavior?

What is the purpose of representing fatigue data with a series of constant probability curves?

What are the two domains of fatigue behaviors?

What is the characteristic of crack initiation in fatigue failure?

What is the equation for the total number of cycles to failure?

What is the characteristic of high-cycle fatigue?

What is the typical location of crack initiation in a component?

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.

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. Learn about the typical features of fatigue and its effects.