Oluschinski at 07:00, 15 December 2025

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	<span style="font-size:1.2em;font-weight:bold;">Fatigue strength or continuous fatigue strength</span>		<span style="font-size:1.2em;font-weight:bold;">Fatigue strength or continuous fatigue strength</span>

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	<span style="font-size:1.2em;font-weight:bold;">Fatigue strength or continuous fatigue strength</span>		<span style="font-size:1.2em;font-weight:bold;">Fatigue strength or continuous fatigue strength</span>

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{{Language_sel|LANG=eng|ARTIKEL=Dauerfestigkeit}}
{{PSM_Infobox}}
Fatigue strength or continuous fatigue strength
__FORCETOC__

==Determination of fatigue strength==

The aim of a [[Vibration Test|vibration test]] or [[Fatigue|fatigue test]] is to determine the vibration strength, or fatigue strength ''σ''D for short. ''σ''D characterises the maximum stress amplitude ''σ''a that a test [[Test Specimen for Fatigue Tests|specimen]] can withstand an infinite number of times without unacceptable deformation. At all stress amplitudes above ''σ''D, the test [[Test Specimen for Fatigue Tests|specimen]] [[Fracture|breaks]]. Since this destruction of the plastic test specimen occurs in the linear-elastic or [[Linear-viscoelastic Behaviour|linear-viscoelastic deformation range]], the term [[Fatigue|fatigue]] is used in this context. The fractures that occur are referred to as fatigue fractures.

Indexing is done with capital letters in order to distinguish between the stress parameters to be set and [[Material Value|characteristic values]] of the fatigue strength. The type of [[Stress|stress]] is indicated by ‘z’ for tension, ‘d’ for compression and ‘b’ for bending.

Example: ''σ''zD... Fatigue strength in the alternating tension range

Frequently used special cases of fatigue strength are:

* Alternating strength ''σ''W = ''σ''a = ''σ''o = |''σ''u| for ''σ''m = 0.
* Threshold strength ''σ''Sch = 2 ''σ''a for ''σ''m = ''σ''a.
* The time-dependent fatigue strength ''σ''(N) characterises the stress amplitudes above ''σ''D at which fatigue fracture of the test specimen occurs.
* ''σ''z(6) means time-dependent strength in the alternating tensile range of 106 load cycles.

N is always the number of cycles or load cycles and ''N''C is the limit number of cycles that is reached without [[Fracture|fracture]] of the test specimen.

==Performing WÖHLER tests==

Fatigue strength is determined using the WÖHLER test. This consists of a series of single-stage vibration tests, i.e. with stress cycles of constant amplitude ''σ''a at a constant mean stress ''σ''m or constant stress ratio ''R''.

The [[Stress|stress]] should be selected so that at least one test specimen breaks at a low number of cycles and another test specimen runs through to the limit number of cycles ''N''C. The selected stress amplitudes ''σ''a are plotted on a double logarithmic scale as a function of the number of cycles ''N'' endured until [[Fracture|fracture]]. Connecting the individual measurement points results in the Wöhler curve ('''Fig. 1'''), which is referred to as the S–N curve.

[[File:Fatigue Strength-1.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Schematic WÖHLER curve (S–N curve) for plastics compared to metallic materials
|}

As expected, the S-N line shows an increase in the number of cycles ''N'' with decreasing stress amplitude. In metallic materials, especially structural steels, the WÖHLER line approaches a horizontal line above a certain number of cycles ''N''D. This stress limit, at which no [[Fracture|fracture]] occurs even after an infinite number of cycles, is the fatigue vibration strength, often referred to simply as fatigue strength ''σ''D. To determine this value in practice, the WÖHLER test must be carried out until a limit number of cycles ''N''G is reached. Values for ''N''G derived from experience are 2•106 for steels and 10 to 50•106 for light metals. For non-metallic materials and metallic materials under corrosive stress, the WÖHLER line (S–N line) continues to decline even at very high fatigue cycles. For [[Plastics|plastics]], a fatigue strength based on 7•107 fatigue cycles is determined. However, fatigue fractures are to be expected here even at higher fatigue cycles.

==Test equipment for fatigue testing==

[[Servo-hydraulic Testing Machine|Servo-hydraulic universal testing machines]] ('''Fig. 2''') can be used to perform dynamic tests at medium frequencies, while pulsators (electrodynamic principle) or cyclic bending machines can be used at high frequencies. These machines must always have closed control loops for force and deformation (see also: tensile test control). When measuring temperature dependence, a temperature control chamber must also be connected.

[[File:Fatigue Strength-2.jpg|300px]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |[[Servo-hydraulic Testing Machine|Servo-hydraulic testing machine]] MTS 319.25 from [https://www.mts.com/de/products/materials MTS Systems GmbH], Berlin, for performing WÖHLER tests
|}

==See also==

* [[Vibration Test|Vibration test]]
* [[Fatigue]]
* [[Test Specimen for Fatigue Tests|Test specimen for fatigue tests]]
* [[Vibration Fracture|Vibration fracture]]
* [[Vibration-induced Creep Fracture|Vibration-induced creep fracture]]

'''Reference'''

* DIN 50100 (2022-12): Load Controlled Fatigue Testing – Execution and Evaluation of Cyclic Tests at Constant Load Amplitudes on Metallic Specimens and Components
* DIN 53442 (1990-09): Flexural Fatigue Testing of Plastics using Flat Specimens
* ISO 3385 (2014-07): Flexible Cellular Polymeric Materials – Determination of Fatigue by Constant-load Pounding

[[Category:Fatigue]]

Fatigue Strength - Revision history

Oluschinski at 07:00, 15 December 2025

Oluschinski at 06:59, 15 December 2025