9.8 Wöhler Diagrams

In fatigue failure mechanics, the number of load cycles to failure can be calculated using the classical Wöhler diagram. This diagram is assumed to be a straight line in the diagram of stress amplitude S as function of the logarithmic number of load cycles log N [Fig.9.15].

Figure 9.15: Wöhler diagram

The line is defined by two points, respectively on the S -axis and the N -axis: (0, S1) and (N1, 0) . Sometimes the Wöhler diagram depends on the average stress level in the model.

The SWOEHL and NWOEHL input items in table 'MATERI' as described below, together specify Wöhler diagrams for fatigue failure analysis [Fig.9.16].

Figure 9.16: Input of Wöhler diagrams

    (syntax)

SWOEHL

sig1 is the norm of the stress value for static failure at the first load cycle.

NWOEHL

n1 is the number of load cycles to fatigue failure for the asymptotic zero-stress situation. DIANA assumes that the Wöhler diagram is a straight line in the diagram of stress amplitude S as a function of the logarithmic number of load cycles log N . The line is defined by two points (N, S) : the first point is (0, sig1) the second is (n1, 0) . If you specify only one set of values, i.e, sig1 and n1, then DIANA applies the Wöhler diagram for all types of load cycles: tension-to-tension, pressure-to-tension, and pressure-to-pressure [Fig.9.16a]. Alternatively you may specify three values for both N and S . In that case sig1 and n1 apply for tension-to-tension load cycles, sig2 and n2 for pressure-to-tension, and sig3 and n3 for pressure-to-pressure [Fig.9.16b].