Fatigue (material)

The fatigue describes a slowly progressive damage process in a material under ambient influences such as changing mechanical load, changing temperature, UV radiation, radioactive radiation, possibly the additional action of a corrosive medium.

Fatigue means that a static non-critical load can lead (still in the elastic range, ie still below the yield point of the material ) to a functional failure (fatigue cracking) or even total failure ( fatigue fracture ) of a component when it often applied to the component.

Cyclically loaded parts are therefore in principle a limited life. Therefore, one has to perform, allow an assessment of the durability of the component on critical components prior to using a life evaluation, calculation or experiments. Components that bear theoretically unlimited number of cycles (because they consist of certain suitable materials ), is referred to as endurance.

Simple Example: The holder of a pen can be repeatedly elastically bent back and forth. With the number of bending operations, however, increases the likelihood of fragmentation. The phrase " Constant dripping wears away the stone " or the philosophical law of " turning quantitative changes into qualitative " describes this phenomenon.

The exact observation of the fracture pattern of a component indicates whether a violent or a stress fracture is present. This conclusion must be drawn in order to avoid component failure in the future.

Subspecies

A distinction

• Isothermal mechanical fatigue (eg fatigue under cyclic mechanical loading at constant temperature s, a Wöhler test, vibration)
• Thermal fatigue
• Thermo- mechanical fatigue
• Creep fatigue
• Tribological fatigue

Isothermal mechanical fatigue

As mechanical fatigue of metallic materials is the process that ultimately leads to the failure of a component or material by forming a surface scratch, reaching a certain crack length or fatigue fracture. The process begins with local dislocation movements that occur already at stresses below the yield point, especially on the component surface in cross-section transitions and surface notches or in the volume of Werkstoffinhomogenitäten as inclusions, pores, precipitates, dispersions, etc. due to local stress peaks. Repeated stress form statistically randomly distributed regions of local plastic deformations. In the further course of the stress from it form dislocation configurations that can have damaging effects by concentration of plastic deformation to very small areas. The behavior under cyclic loading is strongly dependent on the material and its history. Most are formed in near-surface material regions Ermüdungsgleitbänder, so-called persistent slip bands, from which then form at 45 ° to the direction of loading (highest shear stress and therefore preferred direction of dislocation motion, Mohr's stress circle ) called Ex and intrusions on the component surface. These act as sharp notches and initiate microcracks which extend in parallel to the slide strips. After a few micrometers ( often about double grain diameter of the structure ) to pivot about the cracks and extend at 90 ° to the direction of stress. Upon reaching a certain crack length then one speaks of macrocracks or so-called technical cracks that propagate as a function of the crack geometry, type of loading (crack modes) and the height. Achieved the scribe, the so-called critical crack length, the component failed by unstable crack propagation ( fracture force ) in the remaining cross section.

Historical events

Fatigue strength

The fatigue strength ( fatigue behavior engl. ) denotes the deformation and failure behavior of materials under cyclic loading.

The doctrine of the fatigue strength has been the first of August Wöhler studied from the year 1858. Until then, just load tests were carried out under static load. Static load here means the burden of a body ( object) with a constant force from a constant direction. An example is mentioned on a table parking a stone. The stone has a constant mass by the acceleration of gravity and exerts a constant force on the table. Exceeds the force of a certain value, the table will collapse.

Unexpectedly With the advent of the railroad, problems arose. According to static calculations have wheels of the car must absorb the stresses of the trip easily without getting damaged. Increasingly, however, fell from railroad cars because the wheels were broken. Wöhler studied this phenomenon and discovered that swinging loads can also damage a component when it is obviously not damaged by a one-time charge with the same force.

The maximum number of power cycles for a given deflection is shown in the SN curve. It depends on the material properties (cyclic consolidating / cyclic entfestigend ), the force or the resultant voltage and the type of load (alternating / swelling ( pressure or the train ) ). With the same displacement amplitude, the changing load damages the component most.

For example, a screw which serves for fastening an exhaust system on the vehicle, for example, can break because of the oscillating load movement of the vehicle, even though the actual yield strength was not achieved. This effect may be exacerbated by corrosion and / or temperature.

Vibrophores uses it to determine the fatigue strength of materials and components in the time and fatigue strength range, for example in the fatigue test according to DIN 50100 ( Wöhler curve), in the train, pressure, and alternating load range.

Fatigue strength

Fatigue strength is a term from the field of strength and indicates the maximum load that can tolerate a dynamic ( eg swinging ) contaminated material without significant fatigue or failure symptoms.

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.

In addition, the static dead load to observe the so-called medium voltage. It influences the material behavior tremendously. Depending on the medium voltage is called fatigue strength or fatigue limit:

• The fatigue strength is the fatigue strength value, wherein the medium-voltage is zero.

Theoretically there is an infinite number of each material fatigue strength values ​​- consisting of a medium voltage and a voltage swing to -. In the so-called Haigh diagram, the fatigue strength is plotted versus the stress ratio R. R denotes the ratio of lower -to upper voltage.

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

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 specified in a certain number of load cycles, which in a period are translated then.

The fatigue strength is defined differently. Materials, bear their samples in Experiment 2 to swing games without breaking, are considered endurance.

In order to determine the fatigue strength ( > 1 million pieces) with sufficient accuracy despite the low number of parts in the experiment for large series, numerous methods have been developed to identify sufficiently statistically reliable indicators. Alternatively, to determine the material properties, the fatigue strength of the machine can also be determined by means of a test bench.

Wöhler test

The SN test the fatigue strength of materials or components (component Wöhler test) is determined. For this, the test specimens are cyclic, usually loaded under a sinusoidal load-time functions. The load amplitude and the voltage ratio under load to the upper load ( the so-called peace degree [R ] ) are constant.

To determine the values ​​of the test specimens are tested for several load horizons. The test continues until a defined failure (breakage, crack initiation ) occurs or a specified limit number of cycles is reached. Test specimens that reach the limit number of cycles without any apparent failure are referred to as completers.

The results of the experiment you carry into a doppellogarithmisches diagram. Usually, the nominal stress amplitude Sa is plotted against the tolerable number of cycles in the Wöhler diagram. The resulting curve is called the Wöhler curve or Wöhler curve. In the adjacent SN curve the three regions K, Z and D are registered.

• K is the portion of the short-term stability or short- cycle fatigue ( LCF and = Low Cycle Fatigue ) below about 104 to 105 load cycles. This type of fatigue occurs at high plastic Dehnamplituden that lead to early failure. To represent this area in more detail, the Coffin - Manson plot is used in the rule. At a load that leads within a quarter of cycles to failure, it is called the static strength, which is also determined in the tensile test.

• Z is the range of fatigue strength or fatigue life of between 104 and depending on the material about 2 x 106 load cycles in which the SN curve nearly straight runs at doppellogarithmischer representation.

• D is the subsequent area of ​​the so-called fatigue strength. For ferritic- pearlitic steels of the fatigue strength at about 1-5 × 106 begins When austenitic steels and fcc -based materials (eg, aluminum, gold, copper ) falls tolerable amplitude further. A "real" endurance limit does not exist here. Therefore, the tolerable amplitude at 107 load cycles is here usually referred to as fatigue. Where a permanent component corrosion or highly elevated temperatures, so can not be expected to fatigue strength.

Below the fatigue strength SaD can endure a lot of load cycles a component in principle arbitrary. Stresses above the fatigue strength cause failure of the component after a certain number of load cycles. The number of endured load cycles of a component under operating load ( variable load amplitudes) to failure can be predicted within the framework of statistical accuracy with the help of the SN curve. For this we use the methods of linear damage accumulation according to Palmgren, Langer and Miner. This is known as fixed operational assessment of a component. Fatigue is employed in almost all areas of technology for the purpose of lightweight construction.

Cyclic deformation behavior at room temperature

In cyclic stress of steels can be distinguished four stages of fatigue after Macherauch: The elastic-plastic cyclic deformation stage, the micro-cracking stage, the stage of stable crack propagation and finally the fatigue fracture. In hardened steels, the cyclic deformation stage and micro-cracking predominates only occurs shortly before the stress fracture. When normalized or quenched and tempered steels yet highly stable crack propagation can comprise a significant part of life depending on the stress level.

For elastic-plastic cyclic deformation provides the stress- strain relation Total hysteresis loops, which at sufficiently stabilized material behavior different parameters may be taken in accordance with Figure 17. In voltage- controlled trial management can be the total strain amplitude εa, t and determine the plastic strain amplitude εa, p as a function of number of load cycles N. Cyclic hardening ( softening ) is associated with a decrease (increase) of εa, p, and therefore also of εa, t connected. In total strain controlled trial leadership to face, however, the stress amplitudes σa and the plastic strain amplitude εa, p as the dependent variables. A cyclic hardening ( softening ) is an increase (decrease) in σa and a decrease (increase) of εa, p linked. If the dependent variables as a function of the logarithm of the number of cycles at a given stress amplitude, the result is called cyclic deformation curves. Taking out these related pairs of values ​​of σa and εa, p and εa, t and transmits them against each other, we obtain the cyclic stress - strain curve. This may such as a stress-strain curve of the tensile test, cyclical stretching and proof stress are removed.

The cyclic deformation curves allow conclusions on the material behavior during cyclic loading. Normalized steels usually show after a quasi- elastic incubation period a number of load cycles interval strong Wechselentfestigung to which a lifetime range connects with exchange consolidation. The observed Wechselentfestigung is due to the occurrence of Dehnungsinhomogenitäten that run as fatigue Lüders bands on the test section.

Even tempered steels show after an incubation period a strong Wechselentfestigung that continues to crack initiation. With increasing voltage amplitude thereby decreases both the incubation time as well as the service life. Since a formation of dislocations is very unlikely because of the present high dislocation density, which occur plastic deformation to be generated by rearrangement of the existing dislocation structure. For hardened material states offer enhanced opportunities for the dislocations to the elastic interaction with the solute in an elevated non-equilibrium concentration of carbon atoms, which leads to a change of solidification. Since it is low by tempering the proportion of the dissolved carbon, the interaction possibilities of the dislocations with the carbon atoms and the dislocation structure transformations reduce lead to Wechselentfestigungen.

For stable crack propagation, the cyclic plastic deformation at the crack tip are crucial. The crack propagation is determined by the stress range of stress intensity DELTA.K. The crack length growth per load cycle is regulated by Law of Paris

Described, c and n are constants. For double- logarithmic plot of da / dN on DELTA.K results in a linear relationship. Below a threshold value of DELTA.K occurs no more crack growth. At very high values ​​DELTA.K unstable crack propagation leads to breakage.