Stress durability curve

FIGURE 12. Stress-durability curves for aluminum joints with a variety of surface pretreatments when exposed to 100% RH at 52°C. [From J. D. Minford, in Adhesives in Manufacturing (G. L. Schneberger, ed.), Chapter 23, Marcel Dekker, New York (1983).]... 

Strength versus log of exposure time, the resulting plot is a straight line similar to the stressed durability curves as shown in Figure 11.4 [18]. 

Figure 10.5 shows the data obtained on an epoxy-nitrile film adhesive on 5052-H34 aluminum alloy after immersion in hot water for 50, 100, 300, 500, and 1000 hours. This test is very useful because it permits a large number of adhesive-bonded specimens with different adhesives, adher-ends, and surface pretreatments to be tested at the same time with a relatively small investment in man hours and equipment. Figure 10.6 shows a comparison of the stressed-durability data and unstressed hot-water-soak data on the same epoxy-nitrile film adhesive, using 2024-T3 aluminum alloy. Note the parallelism of the plots. The curve in the lower left was obtained when lap-shear specimens were subjected to various levels of stress and then exposed to an environment of 60°C and 95% RH until failure. The failure time is plotted as a log function. The curve in the upper right portion is a plot of the data when the same types of lap-shear specimens were subjected to 60°C water for specified periods of time and then tested for their residual strength. In the first case, failure time was recorded. In the latter case, residual strength was determined. The same type of data is obtained with both curves. ... 

The joint action of the thermal fluctuations and mechanical stresses results in a gradual decrease in the time required for the sample to fracture with an increase in stress. This was established by Zhuikov et al. for various types of solids [62], In the case of contact with the active medium, such as in the case of the contact of a metal with a melt, high stresses can cause a sharp decrease in durabUity, while low stresses will have practically no impact on it The durability curve reveals a kink related to the same mechanism as the brittle-to-plastic transition upon active elongation with a given deformation rate (Figure 7.26). 

FIGURE 3.2 Calculated dependences of the relative durability W of cyclic loaded rubber-bitumen binder samples on the sizes of the rubber particles introduced into the bitumen. Curve 1 - at an average mechanical stress in the pavement a = 2.5 MPa. 

Cyclic tensile testing was done after exposure durability testing [16]. The as-received and an exposed curve (similar time and stress of exposed fiber push out sanqile) are shown in Figure 10. As can be seen from the cyclic tensile tests performed at room temperature both as received and post exposed that the exposed sample has a smaller hysteresis consistent with scenario 2 above. 

Slow Crack Growth. Many thermoplastics exposed to constant and moderate stresses over extended periods of time, as for instance pipes imder internal pressure, fail in either of two different modes, in a ductile or in an apparently brittle manner. The durability is often represented as a stress-lifetime a-t) diagram (Fig. 29). The simultaneous action of two failure mechanisms gives in this case rise to two different branches of the lifetime curves. At moderate stresses (above 50% of cTy) the HDPE pipes fail in a ductile manner because of plastic instability of the creeping material (Fig. 2, which corresponds to Point 1 in Fig. 29). The ductile failures are strongly stress-activated = 307 kJ/mol) giving rise to the flat portions of the cr-t curves. Fracture at smaller stresses and after more extended time periods often occurs in an apparently brittle manner by thermally activated slow crack growth (SCG) (steep branches in Fig. 28, = 181 kJ/mol). Such a... 

USCAR Fuel Cell Tech Team, Cell component accelerated stress test and polarization curve protocols for PEM fuel cells (Revised May 26, 2010), see http //wwwl.eere.energy.gov/hydrogenandfuelcells/pdfs/component durabllity may 2010.pdf... 

Upon the transformation from exponent notation back to decimal logarithm, one observes a linear decay, that is, log tf,= a- bo, where a = (l/2.3)[/o/kT + log Tq, and the coefficient b includes the reciprocal of temperature, b = (l/2.3)y/kT At very high values of stress, o UJy, the fracture time reduces to the period of oscillations in the lattice, Tq. This extrapolation does not realistically apply to the experimental data but helps one in illustrating an important point. In particular, in the log tf, plots at different temperatures, this dependence corresponds to a family of curves that converge to the pole with coordinates o = t/o/y and log t, = log Tq. The higher the temperature, the lower the position of the curves, that is, the lower the durability. 

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