Quasi-static yield stress

Figure 7.3. Dislocation densities required to fit the precursor curves as a function of the initial quasi-static yield stress.

Figure 6.8. Plot of the quasi-static reloaded yield stress of shock-loaded copper versus the natural logarithm of residual strain for a 10 GPa symmetric shock with 1 /is pulse duration.

Gray and Follansbee [44] quasi-statically tested OFE copper samples that had been shock loaded to 10 GPa and pulse durations of 0.1 fis, 1 /rs, and 2 fus. The quasi-static stress-strain curves are shown in Fig. 7.10 with the response of annealed starting copper included for comparison. The yield strength of shock-loaded copper is observed to increase with pulse duration, as the work-hardening rate is seen to systematically decrease. 

Figure 3.38. Examples of dynamic stress leading to failure at approximately 1 000000 cycles at20°C versus quasi-static stress at yield...

Besides impact testing, quasi-static measurements are carried out to assess the Young modulus, E, the yield stress, cry, and the elongation at break, break> as the most current parameters. They follow international standards (e.g. ISO 527 for tensile tests, ISO 178 for bending measurements). 

In the quasi-static case, effective frequency dependent moduli and loss factors may be calculated from Equation 8. With respect to Equation 29, a lossy matrix material implies that k is now a complex number. The new expressions for c and a differ from Equations 31 and 32, but follow straightforwardly. Equation 30 is usually cited only for elastic matrix materials, but, of course, it need not be used to interpret a. The potential problem (also with viscoelastic inclusions) is that the derivation of Equation 30 is based on homogeneous stress waves, whereas in viscoelastic materials one should, strictly speaking, consider inhomogeneous waves. The results obtained from Equation 29 are reasonable in the sense of yielding the expected superposition of scattering and dissipation effects. 

Quasi-static Young s modulus measured by Hertzian indentation (b) Data taken from ref [5] (c) Measured by Dynamic Mechanical Thermal Analysis (D.M.T.A) at 1 Hz (T is taken as the temperature of the maximum in tan 5) (d) (7y and Oy are the yield stress under uniaxial and plane strain compression, respectively, for an equivalent strain rate of 5x10" s" (see ref... 

Quasi-static tensile test - tensile properties without yield point - data Polymer Solids and Polymer Melts C. Bierdgel, W. Grellmann The following Table 4.2 shows a summary of available tensile properties of thermoplastics according to stress-strain diagrams of type a and d ( Fig. 4.3). Table 4.2 Tensile properties of thermoplastics without yield point. ... 

Fig. 27. Temperature dependence of the resolved yield stress at three pressures in single-crystal Ta, as obtained using the legacy lattice-based DD code at a quasi-static strain rate of 10 s. Experimental data at ambient pressnre and the same strain rate (sohd sqnares) are from Ref. [78].

Fig. 29. Resolved yield stresses of single-crystal Mo obtained at ambient pressure (solid circles and solid lines) and at 225 GPa (solid squares and dashed lines). Except for the estimated point at 225 GPa and 300 K (see text), the remaining results were obtained from the stress-strain curves in Fig. 28 obtained at a strain rate of Is . For comparison, ambient temperature and pressure experimental data points from the Seeger group [70,71] and the Aono group [79] at lower quasi-static strain rates are also shown.

The absence of yield-point maxima on stress-strain plots obtained at room temperature and the early mechanical failures observed with all samples studied were typical of the stretching of brittle glassy polymers far below their glass transition temperatures, T. As expected, the quasi-static Young s moduli E (Table 1) were always lower (by about 20-30%) than the dynamic moduli E nevertheless, their composition dependence was essentially similar. The stresses, 0), and strains, at break for the PNC (Table 1) also turned out comparable to (albeit slightly smaller than) those for the pristine... 

In Figure 8.7 the dependences of the crystallinity degree K and the yield stress o obtained in quasi-static and impact tests, on epoxy polymer contents c p for the studied nanocomposites is adduced. Despite the sharp difference of conditions of mechanical tests, both dependences Oy(c p) showed a similar extreme increase in Oy with increase in c p. It is particularly important that such a Oy change is accompanied by a monotone linear reduction of crystallinity degree that excludes the possibility of linear correlation Cy K). 

Figure 8.7 The dependences of yield stress Oy in impact (1) and quasi-static (2) tests and crystallinity degree K (3) on epoxy polymer contents c p for nanocomposites HDPE/EP [31]...

Relationship 9.35 implies that the pressure applied in microhardness tests under an indentor is higher than the yield stress in quasi-static tests because of the restriction... 

Since polymers are viscoelastic materials it is necessaiy to specify both the temperature and the time scale under which the result was obtained. This protocol is meant for low speed ( quasi-static ) displacement contrd mode of loading and as a bade test condition, it is recommended that 23°C and a load-point displacement rate of 10 mm/min be used, but other conditions are possible. Li all cases, the time-D-ffacture should be quoted and all related determinations (yield stress and indentation correction, see below) should be obtained at comparable loading times as for the fracture test... 

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