Creep (deformation)

Creep ( also retardation ) referred to in the materials on time and temperature dependent plastic deformation under load. A measure of the creep is the creep modulus or the creep (English creep coefficient).

Must creep for structural tasks such as mechanical engineering or be taken into account in the construction industry and affects the behavior of the respective objects in some cases considerably. It generally applies to all metal materials, as well as polymers (plastics, rubber ) and a number of ceramics, including concrete, but also for wood.

Creep of concrete

Creep refers to the deformation of the concrete increases with time at a constant voltage. It is a property of the concrete, which manifests itself especially when pressure is applied by a structural transformation and volume reduction.

The creep is made possible by the water contained in the cement stone. An external load leads to the place exchange of water molecules in the hardened cement gel. These compression and sliding processes come between the gel particles. The pressing is not chemically bound water from the cement pores in the capillaries and evaporates, resulting in a shrinkage of the gel result. The increase in creep with time dwindling and comes after several years almost to a standstill.

The creep is composed of two parts. The reversible deformation component, which goes back after discharge with a time lag, also called the back crawl, is influenced by the age of the concrete bit and reached its final value after a short time. The dominant irreversible deformation component is fully retained after discharge, he is also known as flow, however, is strongly dependent on age of the concrete and reaches its value only after a long time. Under unfavorable conditions, the final creep can reach a value of about 3.0, ie the deformations of concrete creep are three times as large as from the elastic deformation.

Course and extent of creep, in addition to load size and age of the concrete, particularly influenced by the cement paste volume and the water - cement ratio. Other parameters are humidity, cross-sectional geometry of the component of hardening of the cement and concrete compressive strength. The creep values ​​are determined in the laboratory with the creep test.

The information in the DIN 1045-1 apply to the linear creep under compressive stress, ie the creep values ​​are independent of the load level. This is true up to a voltage of about 45% of the cylinder strength of the concrete. For higher concrete compressive stress occurs due to increased micro cracking of the concrete on the nonlinear creep. Here, take the creep deformation with increasing load disproportionately.

In the calculation of prestressed concrete parts ( prestressed concrete ) the creep of the concrete is an important parameter that it is important to note that large concrete compressive stresses are always present by the bias voltage. The resulting creep strain of the prestressed concrete part reduce the tension steel strain and hence the bias force. The creep of the concrete can also be a determining element in the ultimate limit state of slender reinforced concrete columns or in the deformation analysis of thin blankets.

Creep of plastics

Since plastics ( entangled in the case of thermoplastics and elastomers ) of large molecular chains exist, slide or entknäueln this under external load, resulting in a strain results. Depending on the production, base polymer, filler and filling of the plastic can be several 100 % elongation.

Creep in metallic materials

The properties of metallic materials are above the so-called transition temperature time-dependent, since it run all structural mechanisms thermally activated. The transition temperature is dependent on the material and is about 40 % of the melting temperature in Kelvin. At these high temperatures () a number of different material state changes from running, which can be traced back to influences of temperature, mechanical stress, time and ambient atmosphere. Here metallic materials learn even at low stresses below the yield point an irreversible plastic deformation, which is progressing slowly but steadily. This progressive plastic deformation under static load is called creep and is temperature -, voltage-, time-and material-dependent. Creep is always associated with damage of the metallic material. Creep is mainly due to transgranular processes such as the movement of dislocations and vacancy diffusion. But intergranular processes such as grain boundary sliding and grain boundary diffusion are involved in the creep.

Thus, while as a rule at room temperature, a static load below the yield point leads exclusively to elastic deformation and may be practically endlessly long endure components, creep leads high-temperature stress in addition to an elastic elongation in addition to a time- progressive plastic strain ( creep ) with a material is connected and limited damage to the component life. A distinction is made ( with increasing load):

• Diffusional creep: vacancies or interstitial atoms to diffuse through the crystal lattice.
• Dislocation creep: by thermally activated processes (diffusion) to overcome obstacles such as vacancies or interstitials ( on climbs ).
• Dislocation slip: The dislocations moving on slip planes and overcome obstacles by thermally activated processes.
• Grain boundary creep / slide: In polycrystalline materials, these processes may instead occur at grain boundaries with lattice sites.

This material damage leads to a reduction in the strength values ​​, their dependence is very complex on the parameters of temperature, mechanical stress, time and material. Creep during the high temperature stress is a significant problem in the art, since it may lead to component failure, for example, by collision of the turbine blades on the housing, deformation of high-load notches on turbine shafts or leakage of boiler tubes. Therefore, the knowledge of the creep behavior of the material under cyclic loading is essential for the design and operation of high -temperature components. In high-temperature creep strain can not be prevented. Through selective alloying techniques, the creep process can be influenced, however. Therefore, for high temperature components, special materials (eg martensitic and austenitic steels and nickel -based alloys ) are used.

The determination of the creep behavior occurs ( standardized according to DIN EN 10 291 and ISO 204) with so-called creep tests. In the creep test, a sample is statically loaded under constant high temperature, thereby extending the sample by plastic deformation over the time measured. Out of this plastic deformation, the creep strain can be determined. This results in the creep curve shown in the image that is divided into the technical Kriechbereiche I, II and III.

Another important result of the creep test, the stress rupture time. By creep tests at different stresses thus different stress times are up to fracture. For the interpretation of the components determined from this creep strength is essential. The creep rupture strength is the stress level of the material at the temperature T for the stress time t until failure. The technically relevant exposure times often be several years so that creep tests are usually carried out very long time.

Mathematical Description

For the mathematical description of the creep Norton'sche creep law is often used. From studies of the secondary creep 1929 first purely voltage-dependent Kriechbeschreibung was developed by Norton, which describes the minimum creep rate as a power function of voltage:

In this formula the stress exponent and the factor temperature-dependent material constants dar. The stress exponent is paid as an indicator of the deformation mechanism. In the literature it is assumed for a stress exponent for dislocation creep and grain boundary sliding. At very low stresses and creep stress exponent may occur by describing the here based solely on diffusion creep deformation mechanism. This Norton'sche creep law comes up today because of its ease of application and is often used for rough estimation of creep or stress redistribution, and use of stress in the component. However, it is only valid for medium and low voltages in the secondary creep and an identification of the parameters, and must be done separately for each application temperature.

For more accurate calculations of time-and temperature-dependent deformations due to high temperature stress much more powerful descriptions are needed. A distinction is phenomenological equations, which are mathematical descriptions of the measured creep and constitutive equations based on continuum mechanics or micro- structural approaches and pair deformation and damage. A powerful type of equation represents eg the phenomenological "modified Garofalo equation " represents or the constitutive " Chaboche model." Both types of descriptions are very expensive in the parameter identification in the rule and require a lot of mechanical- Light Packaging knowledge.

Light metal alloys

In light alloys such as aluminum and magnesium alloys, which are widely used in automotive, aerospace, creep already occurs at temperatures of about 80-100 ° C. The increased number of sliding planes in the face centered cubic lattice structure of the aluminum also provide the plastic Kriechverformungsprozess less resistance, which limits the use of these alloys at elevated temperatures.

Prestressing steel

Wherein the high-strength prestressing steel creep is also possible at room temperature and high constantly acting use stresses below the yield point. Their use for prestressing reinforced concrete structures causes a creep and thus voltage losses due to relaxation (decrease in tension at constant strain ). These voltage drops may have up to 10 % of the initial voltage of magnitude.

Creep in floors

Soils deform under time-dependent compressible and shear stresses. It plays in addition to the consolidation, in which cohesive soils with low permeability, the pore water can absorb only delayed in time or leave, the creep due to the viscosity of the soil an important role.

For compressible loads in one-dimensional ( ödometrischen ) or hydrostatic ( isotropic ) tensions the density of a base element with a constant effective voltage continues to increase. The increase in the density follows this for the one-dimensional compression of a simple empirical law.

In the one-dimensional expansion as a result of creep, an empirical creep coefficient, the initial number of spores, and a reference time.

Semantics

The substantive meaning (semantics) of the term creep is often used very blurred in the technical practice and often equated with the concept of relaxation. A simplified distinction is possible by:

• Creep: constant voltage, strain variable
• Relaxation: voltage variable, constant strain