Fatigue limit

Fatigue strength is a term from the field of strength of materials and refers to the load limit that can endure a dynamic ( eg swinging ) contaminated material without significant fatigue or failure symptoms. See also fatigue strength.

Haigh diagram

The fatigue strength is dependent on the type of load encountered. Depending on whether the type of load only of pressure, and pressure train, train only or additionally also from bending and torsion is, their relative size changes dramatically. The dependence of the fatigue strength of the type of load is shown in the Haigh diagram. and mentions the

The tension of the upper and lower voltage curve of the oscillating voltage waveform on the timeline. Furthermore, the medium-voltage referred. represents the fatigue strength indicates the tensile strength, the yield strength.

Materials with face-centered cubic crystal lattice, such as copper not show this limit. This is to be expected "over time " with fatigue even at low stress amplitudes.

Smith chart

Another diagram, the so-called " fatigue strength diagram to Smith " (short- Smith chart ), is used in materials technology for strength calculation for dynamically stressed components. It shows the dependence of the medium-voltage, amplitude voltage, and fatigue strength. The fatigue strength of the field is characterized by a pentagon. If the amplitudes of the upper or lower voltage are for the present medium voltage outside of this labeled pentagon, fatigue strength is not given.

Effects

When voltage excursions above the fatigue strength show distinct signs of fatigue and damage, it will bear only a certain number of cycles to fracture (fracture orbital speed ). This dependence is shown in the Wöhler diagram, a design guide for engineering interpretations on the principle of structural durability. To determine the SN curve, see also SN test.

When the material is so highly polluted that it can withstand long exposure only a certain time, then one speaks of fatigue strength. This is usually indicated in a predetermined number of load cycles that can be then converted into a time ( that is, a desired service life ).

The fatigue strength is defined differently. Materials, bear their samples in an attempt to swing games without break, apply with appropriate load level than endurance.

To the fatigue strength for high volume parts (number greater than 1 million) to be able to determine with sufficient accuracy despite the relatively small number of samples in the trial, numerous methods have been developed to determine the statistically sufficiently hedged characteristics. Alternatively, to determine the material properties fatigue strength can also be the fatigue strength of a whole assembly with the help of a test bed to be determined. At the break, the number of cycles in N is measured. A fatigue test series consists of about 10 individual experiments with samples of the same material. The stress amplitude cyclic loading, starting from the reduced yield strength Re from trial to trial. With 10 samples, however, one can only determine a mean SN curve with the probability of breakage of 500,000 ppm. If you want to obtain the fatigue strength at 1 ppm or 10 ppm, one has to at least 50 samples with the Weibull distribution to evaluate. Before, but you have to project all measurement values ​​to a fatigue life.