Strain permanent

Fig. 4. Paitial hysteresis loop for a feiioelastic material. After the appHed stress is removed a permanent strain - 0.0064 remains (see eq. 1).

Proof stress (F/Aq at a permanent strain of 0.1%) (0.2% proof stress is often quoted instead. Proof stress is useful for characterising yield of a material that yields gradually, and does not show a distinct yield point.)... 

The stress which produces a permanent strain equal to a specified percentage of the specimen length. A common proof stress is one corresponding to 0.1% permanent strain. 

Elastic limit The elastic limit of a material is the greatest stress at which it is capable of sustaining an applied load without any permanent strain remaining, once stress is completely released. 

The relation between matter and ether was rendered clearer by Lord Kelvin s vortex-atom theory, which assumed that material atoms are vortex rings in the ether. The properties of electrical and magnetic systems have been included by regarding the atom as a structure of electrons, and an electron as a nucleus of permanent strain in the ether— a place at which the continuity of the medium has been broken and cemented together again without fitting the parts, so that there is a residual strain all round the place (Larmor). 

T1 under two different compressive stresses. Phase fraction of martensite is proportional to the permanent strain which can be determined by the stress-free specimen length. From Burkart and Read [16],... 

Tf the load should be released after reaching ft, the load deformation relationship will follow die line BE, or a curve line terminating between O and E. Thus the permanent strain will be e or a somewhat smaller value. When the final or permanent strain is specified, the stress is known as the proof stress. 

The usual way in which the deformation changes with time, has been dealt with in 6.1. The best representation appeared to be a Maxwell element with a Kelvin-Voigt element in series the deformation is then composed of three components an immediate elastic strain, which recovers spontaneously after removal of the load, a delayed elastic strain which gradually recovers, and a permanent strain. Moreover, we noticed that a single retardation time (a single Kelvin-Voigt element) is not sufficient we need to introduce a spectrum ... 

Misfit strain, Cl Bilayer distortion Permanent strain, ep Residual crack opening 0-2 x 10 ... 

Several experimental procedures can be used to measure the residual stresses. The three preferred methods involve diffraction (X-ray or neutron), beam deflection, and permanent strain determination. X-ray diffraction measurements have the limitation that the penetration depth is small, such that only near-surface information is obtained. Moreover, in composites, residual stresses are redistributed near surfaces.47 Consequently, a full stress analysis is needed to relate the measured strains to either q or a. ... 

Beam deflection and permanent strain measurements have the advantage... 

Finally, the permanent strains that arise following tensile plastic deformation also relate to ft. Measurement of these strains allows ft to be assessed.18 The relevant formulae are presented in Section 1.5. 

Analyses of the plastic strains caused by matrix cracks, combined with calculations of the compliance change, provide a constitutive law for the material. The important parameters are the permanent strain, e0 and the unloading modulus, E. These quantities, in turn, depend on several constituent properties the sliding stress, r, the debond energy, T, and the misfit strain, il. The most important results are summarized below. 

Small Debond Energy. For SDE, when cr< crs, the unloading modulus E depends on r0, but is independent of T, and Cl. However, the permanent strain e0 depends on T, and Cl, as well as r0. These differing dependencies of E and e0 on constituent properties have the following two implications. (1) To simulate the stress-strain curve, both e0 and E are required. Consequently, r0, T, and Cl must be known. (2) The use of unloading and reloading to evaluate the constituent properties has the convenience that the hysteresis is dependent only on tq. Consequently, precise determination of r0 is possible. Moreover, with t0 known from the hysteresis, both T,- and Cl can be evaluated from the permanent strain. The principal SDE results are as follows. 

Fig. 6.11 Changes in the hysteresis behavior during the fatigue of unidirectional SiCf/CAS-II. The number of cycles (in thousands) is shown above each curve. Note that the average modulus, area of the hysteresis loops, and the permanent strain offset all change during fatigue. Failure took place at 3.21 x 106 cycles. After Holmes and Cho.12...

Repeated Load Triaxial Tests for Permanent Strain Evaluation. In accordance with the procedure in the VESYS IIM Users Manual, the same sample used in the creep tests was also used to measure permanent strain, an important indicator of the rutting potential of a pavement. Repetitive loads of the haversine type were applied to the specimen at a magnitude equal to the applied vertical stress used in the creep tests. The load duration was 0.1 sec followed by a rest period of 0.9 sec. 

Data from the repeated load triaxial test are used to calculate permanent strain which can be plotted vs. load repetitions on log-log scale. Figure 5 shows a typical plot of accumulated axial strain vs. number of load repetitions. The confining stress was zero for all triaxial tests. 

If it is assumed that the resilient strain, cr, is large compared with the increase of the permanent strain with each load repetition, then the following approximation can be made ... 

Most crystalline materials which can undergo a large permanent strain without fracture deform in a complex manner that is neither viscous nor perfectly plastic. At low temperatures, such materials deform by a process... 

It follows from these arguments that any piece of glass will be permanently strained if part of the material is heated to some temperature at which the glass molecules are free to move relative to one another. Should this strain result in stresses-above the breaking stress then the material will fail. 

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