Fully reversed stress

When smooth specimens of PS are cycled at 26.7 Hz in a tensile mode at a maximum stress of 17.2 MPa discontinuous crack growth bands similar to those shown in Fig. 8 are seen in the mirror region near the fracture source. These bands are also similar in morphology to those observed in testing notched PS specimens at a minimum stress that is maintained at one-tenth the maximum stress However, DCG bands were not seen when tests are carried out under a fully reversed stress of 17.2 MPa at... 

As already explained in section 10.3, many ceramics do not exhibit any cyclic effects and can thus bear infinitely many cycles of any load that is smaller than the static strength (e.g., under tension or bending). In these ceramics, the S-N curve is simply a horizontal line at cruax = or uniaxial load). For a fully reversed stress, this results in... 

Fig. 1.22 Variation in the surface hardness measured by Kemsley on cold-worked and annealed copper at two different fully reversed stress amplitude [27]...

For instance, in the case of the aluminum alloy considered in Fig. 5.31, for R — 0.6 and N — 10 cycles the mean stress is 262.5 MPa so that the term between parenthesis in (5.48) is equal to 0.45. Since for / = —1 the fatigue limit oy is equal to 135 MPa, the new fatigue limit oy will be 61 MPa, in practice the one experimentally obtained. Note that in this way Eq. (5.48) defines a limit oyor any other stress amplitude as the equivalent fully reversed stress amplitude OequiN) that produces the same fatigue effect, i. e., results in fatigue fracture after the same N cycles of a stress amplitude (Ta,m(N) having a mean value... 

While fatigue data collected in the laboratory are generated using a fully reversed stress cycle, actual loading applications usually involve a nonzero mean stress. The mean stress can be compressive or zero and it affects the strain-life curve as shown schematically in Eig. 1.33. Mean stress has its largest effects in the high-cycle regime. Compressive means extend life and tensile means reduce it. 

Fracture surface morphology varies with stress amplitude and test frequency as well as with composition and molecular weight. Even under fully reversed cycling, discontinuous crack growth bands have been observed on the fatigue fracture surface in PS, SAN and ABS but they have not been seen in HIPS. 

Stability is an essential condition for PFC emulsions to be of practical use. The principal mechanism for irreversible droplet growth in submicronic PFC emulsions during storage is molecular diffusion (also known as Ostwald ripening or isothermal distillationj.P Coalescence may contribute to instability when mechanical stress is applied and at higher temperatures, as during heat sterilization. Sedimentation and flocculation are fully reversible and pose no problem. 

Structure Breakdown. When a linearly elastic material is deformed and then allowed to relax, the stress-strain curve is fully reversible, as illustrated in Figure 17.5, frame (a). For a larger deformation, the curve is generally not linear, and perceptible hysteresis tends to occur, as depicted in frame (b). Nevertheless, the deformation is fully reversible. This means that deformation/relaxation has left the structure unaltered repeating the test on the same specimen leads to an identical result. The hysteresis is due to energy dissipation, caused by flow of solvent through the gel network if it concerns a gel, as mentioned in Section 5.1.3. 

In summary, the fully reversible cyclic stress-strain loops are due to the growth and shrinkage of basal plane dislocation loops associated with the IKBs. The values of Wj achieved for the MAX phases are some of the highest ever reported for crystalline... 

Mesangial cells start to change their phenotype as soon as they face NO (Sandau and Brune 2000). When they cope with NO, most stress fibres of their cytoskeleton vanish and only a few contacts to neighbouring cells persist. This phenomenon is fully reversible and does nor result in a detachment of the cells. The mechanism leading to F-actin dissolution is very sensitive to NO as 500 nM or 1 iM of S-nitrosoglutathione was sufficient to achieve these alterations. 

It should be stressed that we are discussing here numerically exact results obtained by the solution of the time-dependent Schrodinger equation for an isolated system. No assumptions or approximations leading to decay or dissipation have been introduced. The time evolution of the wave function (t) is thus fully reversible. The obviously irreversible time evolution of the electronic population probabilities in Figs. 2 and 3 arises from the reduction process, that is, the integration over part of the system [in this case, the nuclear degrees of freedom, cf. Eqs. (12) and (13)]. 

Fi 7.11 An electric displacement D as a function of the mechanical stress Ti (schematically), (a) Fully reversible twinning, (b) Partially stable twins... 

Initially the pseudo-elastic material is in its austenitic phase at room temperature. Initially the material in the austenitic phase deforms like a conventional material linear elastic under load. With increasing loads a stress-induced transformation of the austenitic to the martensitic phase is initiated at the pseudo-yield stress Rpe- This transformation is accompanied with large reversible strains at nearly constant stresses, resulting in a stress plateau shown in Fig. 6.53. At the end of the stress plateau the sample is completely transformed into martensite. Additional loading passing the upper stress plateau causes a conventional elastic and subsequently plastic deformation of the martensitic material. If the load is decreased within the plateau and the stress reaches the lower stress level a reverse transformation from martensite to austenite occurs. Since the strains are fully reversible the material and the sample respectively is completely recovered to its underformed shape. These strains are often called pseudo-elastic because the reversible deformation is caused by a reversible phase transformation and is not only due to a translation of atoms out of their former equilibrium position [74]. 

A further study [81] examined the development of internal stress as a function of temperature for an anode-supported cell based on a Sc stabilized zirconia (ScSZ) electrolyte with a NiO-3 mol%-YSZ anode, finding that there were significant changes in the internal stress in both NiO and ScSZ that were not fully reversible, Fig. 19.10. Residual stress in the electrolyte was measured as a function of the reduction cycle and found to be 400 MPa in air, but decreased to 200 MPa on reduction of the NiO, and concluded that this redox cycle would be detrimental to SOFC performance. 

Two additional observations may provide clues as to the origin of the compressive stress. First of all, if the growth is interrupted while the stress magnitude is at its plateau value, the stress magnitude falls off rapidly. Then, upon resumption of the deposition flux, the falloff in stress magnitude is fully reversed and the same compressive stress plateau is eventually re-established (Shull and Spaepen (1996), Floro et al. (2001)). The second observation concerns the role of grain boundaries. It has been demonstrated that the stress in Pd films deposited on polycrystalline Pt substrates becomes compressive while the stress in otherwise identical films deposited onto single crystal Pt surfaces remains tensile (Ramaswamy et al. 2001). 

Fully reversible elastic behavior is characterized by a direct proportionality between stresses and deformations and is described by Hook s law ... 

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