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INTRODUCTION AND DESCRIPTION OF THE PROBLEM

Cyclic stress-strain curves describe the stress-strain behavior under cyclic loads. Usually, the cyclic stress amplitude Δσ/2 is plotted as a function of the cyclic strain amplitude Δε/2 for a defined cycle. The fact that the stress-strain response of a material is usually cycle-dependent requires a reference cycle which can be considered as representative for the cyclic stress-strain curve.

In contrast to monotonic stress-strain curves which deliver a unique relationship between the stress and strain of a material, the cyclic stress-strain relationships may undergo cycle-dependent changes. The material can be:

• Cyclic hardening
• Cyclic softening
• Cyclic stable
• Combinations of cyclic softening and cyclic hardening

This means that the stress-strain relationship determined in strain fatigue tests is usually cycle dependent. Figure 1-1 shows the change of stress amplitude with a number of cycles for a low carbon steel as an example [1].