creep

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creep

1. Geology the gradual downwards movement of loose rock material, soil, etc., on a slope
2. a slow relative movement of two adjacent parts, structural components, etc.
3. slow plastic deformation of metals

creep

[krēp]
(electronics)
A slow change in a characteristic with time or usage.
(engineering)
The tendency of wood to move while it is being cut, particularly when being mitered.
(geology)
A slow, imperceptible downward movement of slope-forming rock or soil under sheer stress.
(graphic arts)
A forward movement of the blanket during offset printing.
(mechanics)
A time-dependent strain of solids caused by stress.
(mining engineering)

Creep (materials)

The time-dependent strain occurring when solids are subjected to an applied stress. See Stress and strain

Some of the different kinds of creep phenomena that can be exhibited by materials are shown in the illustration. The strain ε = ΔL/L0, in which L0 is the initial length of a body and ΔL is its increase in length, is plotted against the time t for which it is subjected to an applied stress. The most common kind of creep response is represented by the curve A. Following the loading strain ε0, the creep rate, as indicated by the slope of the curve, is high but decreases as the material deforms during the primary creep stage. At sufficiently large strains, the material creeps at a constant rate. This is called the secondary or steady-state creep stage. Ordinarily this is the most important stage of creep since the time to failure tf is determined primarily by the secondary creep rate s. In the case of tension creep, the secondary creep stage is eventually interrupted by the onset of tertiary creep, which is characterized by internal fracturing of the material, creep acceleration, and finally failure. The creep rate is usually very temperature-dependent. At low temperatures or applied stresses the time scale can be thousands of years or longer. At high temperatures the entire creep process can occur in a matter of seconds. Another kind of creep response is shown by curve B. This is the sort of strain-time behavior observed when the applied stress is partially or completely removed in the course of creep. This results in time-dependent or anelastic strain recovery.

Typical creep curves for materialsenlarge picture
Typical creep curves for materials

Creep of materials often limits their use in engineering structures. The centrifugal forces acting on turbine blades cause them to extend by creep. In nuclear reactors the metal tubes that contain the fuel undergo creep in response to the pressures and forces exerted on them. In these examples the occurrence of creep is brought about by the need to operate these systems at the highest possible temperatures. Creep also occurs in ordinary structures. An example is found in prestressed concrete beams, which are held in compression by steel rods that extend through them. Creep and stress relaxation in the steel rods eventually leads to a reduction of the compression force acting in the beam, and this can result in failure. See Prestressed concrete

The mechanism of creep invariably involves the sliding motion of atoms or molecules past each other. In amorphous materials such as glasses, almost any atom or molecule within the material is free to slide past its neighbor in response to a shear stress. In plastics, the long molecular chains can slide past each other only to a limited extent. Such materials typically show large anelastic creep effects (curve B in the illustration).

For crystalline materials, creep deformation also involves the sliding of atoms past each other, but here the sliding can occur only within the cores of crystal dislocations. Thus, creep of metals and ceramics is usually governed by the motion of dislocations.

It is possible to design materials with superior creep resistance. When solute atoms are added to metals, they are attracted to the strain fields of the dislocations. There they inhibit dislocation motion and in this way improve the creep resistance. Many of the aluminum alloys used for aircraft structures are strengthened in this way. The addition of second-phase particles to alloys is another way to improve the creep resistance. The most effective strengthening phases are oxides, carbides, or intermetallic phases, because they are usually much stronger than the host metal and therefore create strong obstacles to dislocation motion. Materials containing finely dispersed, strong particles of a stable phase are usually very creep-resistant. Nickel-based superalloys, used in gas-turbine engines, derive their creep resistance from these effects. See High-temperature materials, Metal, mechanical properties of

creep

1. The continuing, time-dependent part of strain resulting from stress; the permanent and continuing dimensional deformation of a material under a sustained load, following the initial instantaneous elastic deformation.
2. Slow movement of rock debris or soil, usually imperceptible except in observations of long duration.
3. In structures, particularly of concrete, permanent deflection of structural framing or structural decking resulting from plastic flow under continued stress.
4. In roofing, permanent elongation or shrinkage of a roofing membrane resulting from thermal or moisture changes.
5. The flow of water along the interface between a structure and the surrounding soil or rock foundation.

creep

creep
Normal position of creep marks.
creep
Tire needs replacement.
creepclick for a larger image
i. The gradual rotation of a tire around a wheel. To detect creeping, white index marks are made on tires and wheels and are called creep marks. Some tires have two molded arrows in between which creep marks are painted.
ii. The property of a metal that allows it to be permanently deformed when subject to stress (e.g., turbine blades that operate at temperatures at which creep can become serious). Creep increases with temperature, duration, and magnitude of force.
Primary creep, stage 1, is a period of decreasing creep rate. Primary creep is a period of primarily transient creep. During this period deformation takes place and the resistance to creep increases until stage 2. Secondary creep, stage 2, is a period of roughly constant creep rate. Stage 2 is referred to as steady state creep. Tertiary creep, stage 3, occurs when there is a reduction in cross sectional area due to necking or effective reduction in area due to internal void formation.
References in periodicals archive ?
Putting this extra revenue from Obamacare together with other additional revenue derived from a combination of overall bracket creep and this January's tax increase, and the picture becomes clear: even as spending rises as a share of the economy, so will revenue.
Bracket creep in the individual income tax, which in the past brought government more revenue, has been curtailed by indexing of tax brackets for inflation since 1984.
He lamented that the failure to adjust tax brackets to inflation resulting to the so-called "bracket creep" phenomenon and the failure of a large number of the self-employed to pay the right amount of taxes resulting to "low tax participation" made the income tax system grossly unfair and inequitable.
A diluted version of the superannuation surcharge reform was allowed through only because it was cynically understood that the minor tax cut involved would soon be negated by bracket creep. It is perverse that Vladimir Putin, a former head of the K.G.B., has given Russia a liberal, incentive-building revenue law while Australia still suffers under a tax regime based on class warfare ideology.
Fifth, the tax bracket creep due to inflation that is compounded over long periods of time would drastically increase the dollar income to retirees.
We may mask the issue by defining less and less severe mood states as pathology, in effect saying, 'If it responds to an anti-depressant, it's depression.' Already, it seems to me, psychiatric diagnosis has been subject to 'diagnostic bracket creep'--the expansion of categories to match the scope of relevant medications....How large a sphere of human problems we choose to define as medical is an important social decision."
This ends bracket creep for Manitoba taxpayers, ensuring individuals and families keep more of their income.
Thanks to bracket creep, the state income tax in particular was a very elastic tax for the state treasury.
More important for the fiscal outlook at the end of the decade, taxes were on a path of continued increase, since they were not then indexed for inflaction (as they are now) and high inflation rates produced "bracket creep.'
The Medicare levy low?income thresholds have been increased in line with growth in the consumer price index to address bracket creep and ensure the thresholds keep pace with consumer prices so that low-income taxpayers are protected from having to start paying the levy.
Similarly, as the inflation of the 1970s automatically pushed Americans into higher tax brackets, more and more Americans sought to shelter their earnings in real estate Between 1965 and 1980, "bracket creep' increased the marginal tax rate for a family earning the median income from 14 to 24 percent.