How do fatigue-testing machines produce repeatedly changing loads?

In the context of materials testing, fatigue-testing machines generate repeatedly changing loads primarily by employing a rotating eccentric weight. This mechanism creates a cyclically varying load that shifts in a regular cycle between a maximum and a minimum value.1

While these machines traditionally rely on cyclic stress to measure fatigue, modern laboratories often utilize techniques that apply random fatigue stresses to better simulate real-world conditions, as actual stresses are rarely perfectly cyclic. These tests are critical for understanding phenomena such as atomic slip, crack initiation, and crack propagation, which ultimately determine the lifespan of a material.1