Resistance to alternating loads
The resistance of a mechanical component to fatigue is described by its resistance to alternating loads. This can be described and plotted by the material-specific S-N curve. In an S-N test, a material sample is loaded cyclically – usually sinusoidally – with a constant amplitude. The test runs until the material fails (e.g. breaks). If this test is now repeated with different amplitudes, and the number of cycles to failure noted in each case, representing the points in a graph produces the S-N curve. The number of cycles to material failure is entered logarithmically on the x axis, the associated amplitude is entered as mechanical stress or strain, on the y axis.
The following diagram shows the S-N curves for different materials. The high resistance to alternating loads of fiber composites is clear to see.