Creep deformation defined

When a fiber is stressed, the instantaneous elongation that occurs is defined as instantaneous elastic deformation. The subsequent delayed additional elongation that occurs with increasing time is creep deformation. Upon stress removal, the instantaneous recovery that occurs is called instantaneous elastic recovery and is approximately equal to the instantaneous elastic deformation. If the subsequent creep recovery is 100%, ie, equal to the creep deformation, the specimen exhibits primary creep only and is thus completely elastic. In such a case, the specimen has probably not been extended beyond its yield point. If after loading and load removal, the specimen fails to recover to its original length, the portion of creep deformation that is recoverable is still called primary creep the portion that is nonrecoverable is called secondary creep. This nonrecoverable elongation is typically called permanent set. 

There has been some controversy over the definition of creep which should be used. Traditionally, creep was defined in the rubber industry as the increase in deformation after a specified time interval expressed as a percentage of the test piece deformation at the start of that time interval. In other industries creep is normally defined as the increase in deformation expressed as a percentage of the original unstressed dimension of the test piece. Consequently, care has to be taken when comparing creep values obtained from different sources. ISO 8013 has both definitions, calling them creep index and creep increment respectively. The definition of creep increment in the standard refers to the original dimension as thickness, which would not apply to tension. ISO 8013 also defines a compliance index which is the ratio of the increase in strain to the constant applied stress. 

Creep, yielding, and post-yielding plastic deformation (drawing) as well as flow are brought about by the stress-biased deformation and displacement (jumps) of molecular groups and chain segments. Creep is defined as the time-... 

This property is an important consideration in the design of parts from polytetrafluoroethylene. PTFE deforms substantially overtime when it is subjected to load. Metals similarly deform at elevated temperatures. Creep is defined as the total deformation under stress after a period of time, beyond the instantaneous deformation upon load application. Significant variables that affect creep are load, time under load, and temperature. Creep data under various conditions in tensile, compressive, and torsional modes can be found in Figs. 3.12 through 3.19. 

Viscoelastic creep manifests itself in the time-dependent deformation of a material. Experimental data obtained from a laboratory creep test under constant applied stress for a viscoelastic solid is shown in Fig. 12.1. Traditionally, a creep curve consists of three stages. In the first stage, also known as primary creep, the creep strain rate decreases with time until it reaches a constant value. The second stage, known as steady state creep, is defined as the region where the slope of the creep strain is a constant with respect to time. In the third and final stage, termed tertiary creep, the creep strain rate increases with time through progressive failure and terminates with the rupture of the specimen. 

Creep is defined as a long-term property measured by deforming a sample at a constant stress and monitoring the flow (strain) over a period of time. Viscoelastic materials flow or deform when subjected to loading (stress). In a creep experiment, a constant stress is applied and the resulting deformation is measured as a function of temperature and time. Just as stress relaxation is an important property to structural engineers and polymer scientists, so is creep behaviour. 

According to the generally accepted hypothesis, shrinkage and creep are independent and additive. It means that shrinkage is the time-dependent deformation in an unloaded concrete at constant temperature and creep is defined as the change of deformation since the application of load, corrected for shrinkage, and is related to the stress state and its intensity. Consequently,... 

Products subjected to a given load develop a corresponding predictable deformation. If it continues to increase without any increase in load or stress, the material is said to be experiencing creep or cold flow. Creep is defined as increasing strain over time in the presence of a constant stress (see, for instance. Figs. 3-28 through 3-32). The rate of creep for any given plastic, steel, or wood material depends on the basic applied stress, time, and temperature [1, 2, 5, 11-14, 62-68, 268-69, 282-301]. 

Engineering design often requires engineers to predict material properties at high temperatures where no experimental data are available. The creep deformation rate can be so slow that it might require 10 years test time to reach 1% deformation. Reliable predictions based on accelerated test data obtained over a shorter period of time are essential. Several theoretical parameters were proposed to predict long-term metal creep or stress rupture life based on short-term test data. One of the most utilized parameters is the Larson-Miller parameter, as defined by Equation 4.20 ... 

Linear viscoelasticity Linear viscoelastic theory and its application to static stress analysis is now developed. According to this theory, material is linearly viscoelastic if, when it is stressed below some limiting stress (about half the short-time yield stress), small strains are at any time almost linearly proportional to the imposed stresses. Portions of the creep data typify such behavior and furnish the basis for fairly accurate predictions concerning the deformation of plastics when subjected to loads over long periods of time. It should be noted that linear behavior, as defined, does not always persist throughout the time span over which the data are acquired i.e., the theory is not valid in nonlinear regions and other prediction methods must be used in such cases. 

Plastics, both thermoplastic and thermosetting, will deform under static load. This is known as creep. For this reason those materials whose prime function is mechanical are generally reinforced with mineral filler or short fibres, or else oriented by drawing. Many components have a limit on acceptable deformation, and the predicted creep strain at the end of life will be fed back to define either a maximum load, or mechanical dimensions large enough for the component to remain within the limitations on strain. Creep becomes more pronounced at higher temperatures. 

Fatigue is an example of the influence of time on the mechanical properties of a material. Another example of a time-dependent mechanical property is creep. Creep, sometimes called viscoplasticity, is defined as time-dependent deformation nnder constant stress, usually at elevated temperatures. Elevated temperatures are necessary because creep is typically important only above Tmp % where T p is the absolute melting point of the material. 

The volume fraction o in Eqn. (2) defines a percolation limit for creep.69,70 Above this threshold, most particles within the slurry are part of an extended cluster that dominates the flow process. Two-dimensional experiments on slurries demonstrate that, at the threshold, 60-80% of the particles within the slurry form part of the cluster.71,72 When this happens, particle motion within the slurry is controlled by the cluster, and further deformation of the slurry is dominated by deformation of the cluster itself. 

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