In this chapter, we introduce some basic terms used in fatigue analysis and briefly describe the basics.

## Load

**Load** is defined as any physical quantity that reflects the excitation or the behavior of a system or component over time. The most typical loads are:

- forces,
- torques,
- stresses,
- strains,
- displacements,
- velocities,
- accelerations etc.

An example of a load signal is depicted in the figure below, showing vertical load measured on a truck transporting gravel. The changes in the mean originate from a loaded and unloaded truck, whereas the changes in the standard deviation derived from different road qualities.

Image 9: Example of a load signal

## Cycle

A **half-cycle** is a pair of two consecutive extrema in the load signal, going from a minimum to a maximum or vice versa, as depicted in the image 10 below. A **full-cycle** is a cycle consisting of two consecutive half-cycles, as depicted in image 11 below.

Image 10: Half-cycle | Image 11: Full-cyle |

The most important cycle characteristics are **Range**, **Amplitude **and **Mean**, defined as follows:

- Amplitude = (Maximum - Minimum) / 2,
- Range = Maximum - Minimum,
- Mean = (Maximum + Minimum) / 2.

## Cycle Counting Methods

**Cycle counting methods** are used to calculate the load spectrum of a load signal, i.e., number of cycles corresponding to each range in a load signal. An example of a load spectrum is depicted in the figure below. Typical cycle counting methods are **rainflow counting** and **Markov counting** described in the following chapters.

Image 12: Cycle counting methods

## Fatigue

**Fatigue** is the failure mechanism that is caused by repeated load cycles with amplitudes well below the ultimate static material strength. Formally, the fatigue process is divided into three stages:

**crack initiation**,**crack propagation**,- unstable rupture and
**final fracture**.

A repeated load applied to a particular object under observation will sooner or later initiate microscopic cracks in the material that will propagate over time and eventually lead to failure. Fatigue damage is typically cumulative and, therefore, unrecoverable.

Fatigue behavior depends on many factors such as:

- load type,
- object size
- stress/strain concentration and distribution,
- mean stress/strain,
- environmental effects,
- metallurgical factors and material properties,
- load rate and frequency effects.

## Fatigue Life Prediction

**Fatigue life prediction** is the process of predicting fatigue life of a particular object under observation. According to ASTM fatigue life is defined as number of stress cycles that a specimen sustains before failure. Fatigue life prediction is of vital importance in order to assure product quality and safety.

## Durability

**Durability** is the capacity of an item to survive its intended use for a suitable long period of time. Good durability leads to good quality, company profitability and customer satisfaction.