Curve of Wöhler

The curve (or diagram) of Wöhler is called curved SN ( Stress - Number off cycles ) in the Anglo-Saxon countries.

Universally known, the curve of Wöhler is oldest and the only one which allows to visualize the behavior of the part or materials in the field of tiredness. It defines a relation between the Contrainte applied σ ( sigma sometimes noted S) and the number of cycles to the rupture NR (in fact many cycles for which one observes P% of ruptures). In practice, the curve of Wöhler is generally given for a probability of rupture p = 0.5.

To trace it, one generally carries out simple tests which consist in subjecting each test-tube with cycles of periodic efforts, constant amplitude of loading fluctuating around a fixed median value, and to note the number of cycles to the end of which starting of a crack is observed, called here many cycles with rupture NR. For more convenience, it number NR is deferred in X-coordinate on a logarithmic scale, and the amplitude of constraint σa is deferred in ordinate on a linear scale. Thus, to each part tested, corresponds thus a point of the plan (NR, σa) and starting from a certain number of tests with generally decreasing constraint, one can establish the curve of Wöhler.

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