Stress creep

Creep stresses used for design purposes are usually determined based on two criteria the stress for a given acceptable creep deformation after a certain number of hours, which ranges from 0.01 to 1% deformation in 1000 hours and the nominal... 

The creep strength of steels is a factor limiting the maximum temperatures for such high-pressure equipment as shells and stirrers of high temperature reactors. Table 3.10 presents creep data for temperatures ranging from 400 to 600°C. The stress for 1% creep in 100,000 hours (which is a design criterion) is accepted to be not less than two-thirds of the creep stresses. 

Lead is the softest and most easily worked metal used in plant construction. The main difficulty in design is that the metal has a very low creep stress. 

The Fig. 2-30 shows the curves of a family of TPs describe as failure that is fairly typical of the behavior of certain TPs. The time-dependent strains resulting from several levels of sustained or creep stress are shown,... 

DiCarlo, J. A., Creep Stress Relaxation Modeling of Polycrystalline Ceramic Fibers, NASA, 1994. 

Manson, S. S., et al., A linear Time-Temperature Relation of Creep Stress Rupture Data, NACA, TN 2890,1953. 

Although creep, stress relaxation, and constant-rate tests are most often measured in tension, they can be measured in shear (19-22), compression (23,24), flexure (19), or under biaxial conditions. The latter can be applied... 

Many other data in the literature show a strong dependence of creep compliance on the applied load, although in some cases the authors did not discuss this aspect of creep. Stress dependence is found with all kinds of plastics. For example, the creep of polyethylene has been studied by... 

During the creep of PET and PpPTA fibres it has been observed that the sonic compliance decreases linearly with the creep strain, implying that the orientation distribution contracts [ 56,57]. Thus, the rotation of the chain axes during creep is caused by viscoelastic shear deformation. Hence, for a creep stress larger than the yield stress, Oy,the orientation angle is a decreasing function of the time. Consequently, we can write for the viscoelastic extension of the fibre... 

Fig. 59 Comparison of the calculated creep rate with the observed data of a series PpPTA yarns with different moduli for a creep stress of 0.9 GPa...

For crb 0 the lifetime fb °°,so Eq. 115 presents a non-linear relation between log( b) and the creep stress crb, which is different from the Coleman relation. According to Eq. 115, at constant load the lifetime of a fibre decreases with increasing orientation parameter. Figure 61 compares the observed data for a PpPTA fibre by Wu et al. with the calculated lifetime curve using the parameter values /J=0.08, tan =0.1483, g= 1.6 GPa, j O.032 (GPa)-1, which implies a fibre with a sonic modulus of 91.8 GPa [30]. As shown by Wu et al., fibres that were tested at high stresses had shorter lifetimes than those calculated from the experimental lifetime relation. 

The maximum shear strain criterion is now applied for the calculation of the creep curve up to fracture for increasing creep stress. The total creep strain of the fibre, q(f), is the sum of the elastic strain, cf, and the viscoelastic plus plastic strain, cj(f),... 

Assuming logarithmic creep, j(f)=j110log(f), Eqs. 118 and 119 allow the calculation of the creep curves up to fracture. The results are depicted in Fig. 62. Note that for increasing creep stress the slope of these curves decreases. 

Olefin fiber production, economic aspects of, 11 242-243 Olefin fibers, 11 224-246 applications of, 11 243-244 creep, stress relaxation, and elastic recovery in, 11 227-228 extrusion of, 11 231-234 hard-elastic, 11 242 high-strength, 11 241-242 manufacture and processing of,... 

Mechanical stress Fatigue, creep, stress-relaxation, set, abrasion... 

Static Load Creep, Stress-Relaxation and Creep-Rupture... 

In contrast to creep, stress relaxation is rarely measured on plastics and there are no standard methods. 

The creep stress was assumed to be shared between the polymer structure yield stress and the cell gas pressure. A finite difference model was used to model the gas loss rate, and thereby predict the creep curves. In this model the gas diffusion direction was assumed to be perpendicular to the line of action of the compressive stress, as the strain is uniform through the thickness, but the gas pressure varies from the side to the centre of the foam block. In a later variant of the model, the diffusion direction was taken to be parallel to the compressive stress axis. Figure 10 compares experimental creep curves with those predicted for an EVA foam of density 270 kg m used in nmning shoes (90), using the parameters ... 

Creep, stress relaxation and set are all methods of investigating the result of an applied stress or strain as a function of time. Creep is the measurement of the increase of strain with time under constant force stress relaxation is the measurement of change of stress with time under constant strain and set is the measurement of recovery after the removal of an applied stress or strain. It is important to appreciate that there are two distinct causes for the phenomena of creep, relaxation and set, the first physical and the second chemical. The physical effect is due to rubbers being viscoelastic, as discussed in Chapter 9, and the response to a stress or strain is not instantaneous but develops with time. The chemical effect is due to ageing of the rubber by oxidative chain scission, further crosslinking or other reaction. 

Creep, Stress Relaxation, Elastic Recovery. Olefin fibers exhibit creep, or time-dependent deformation under load, and undergo stress relaxation, or the spontaneous relief of internal stress. High molecular weight and high orientation reduce creep. 

Mechanical properties of polyethylenes vary with density and melt index. Low-density polyethylenes are flexible and tough high-density products arc quite rigid and have creep resistance under load. Toughness is the primary mechanical property affected by melt index, with lower-melt-index polyethylenes having greater toughness. Under loads, polyethylene is subject to creep, stress relaxation, or a combination of both,... 

For the studied materials D0 amounts typically to 28 to 150 nm. A plot of smax as a function of creep stress a at a given temperature reveals three distinct regimes each with a straight slope (Fig. 12) the slope is related to the energy r necessary to create the craze fibril surface [60,61] ... 

Fig. 12 Scattering vector smax of SAN specimens uni-axially deformed to a total creep strain of 3.5% as a function of creep stress o showing three distinct regimes I, II, III (after [54])...

The derivation of fundamental linear viscoelastic properties from experimental data obtained in static and dynamic tests, and the relationships between these properties, are described by Barnes etal. (1989), Gunasekaran and Ak (2002) and Rao (1992). In the linear viscoelastic region, the moduli and viscosity coefficients from creep, stress relaxation and dynamic tests are interconvertible mathematically, and independent of the imposed stress or strain (Harnett, 1989). 

More recently, Yoon and Chen68 developed a theory to predict the deformation behavior of particulate composites. Their theory treats the case of rigid particles embedded in a nonNewtonian matrix. The relative deformation rate, e/k0, is related to the volume fraction of particles, , the creep stress exponent of the matrix, n, and the stress concentration factor, k, of the inclusion in the matrix ... 

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