Metals yield strengths

Creep Rupture. Metals and their alloys lose appreciable strength at elevated temperatures. For most materials, the ultimate tensile and yield strengths fall off regularly as the temperature iacreases, as illustrated ia Figure 2 (2). The exceptions are some iatermetaUics, eg, nickel aluniinide(3 l)... 

The design of load-bearing structures for service at room temperature is generally based on the yield strength or for some appHcations on the tensile strength. The metal is expected to behave essentially in an elastic manner, that is, the stmcture undergoes an elastic deformation immediately upon load apphcation and no further deformation occurs with time. When the load is removed, the stmcture returns to its original dimensions. 

Metal Tensile strength, MPa Yield strength, MPa Elongation, %... 

Cemented carbides possess high compressive strength but low ductihty at room temperature, but at temperatures associated with metal-cuttiag these materials exhibit a small but finite amouat of ductihty. Measuremeat of yield strength is therefore more appropriate at higher temperatures. Like hardness, the compressive yield strength of cemented carbide decreases monotonicaHy with increa sing temperatures. 

ANSI/ADA specification no. 38 requires a minimum yield strength (0.2% offset) of 250 MPa (36,000 psi) and a minimum elongation of 2% for ceramo—metallic alloys. 

Specification P no. Grade Temper Size range, in Minimum tensile strength, kip/in" Minimum yield strength, kip/in" Notes Minimum temperature, (IS) Metal temperature, F (22) ... 

Local reduction in size or wall thickness or local use of a material having reduced yield strength (for example, girth welds of substantially lower strength than the base metal)... 

Structural Properties at Low Temperatures It is most convenient to classify metals by their lattice symmetiy for low temperature mechanical properties considerations. The face-centered-cubic (fee) metals and their alloys are most often used in the construc tion of cryogenic equipment. Al, Cu Ni, their alloys, and the austenitic stainless steels of the 18-8 type are fee and do not exhibit an impact duc tile-to-brittle transition at low temperatures. As a general nile, the mechanical properties of these metals with the exception of 2024-T4 aluminum, improve as the temperature is reduced. Since annealing of these metals and alloys can affect both the ultimate and yield strengths, care must be exercised under these conditions. 

Sufficient tensile stress. Sufficiency here is difficult to define since it depends on a number of factors such as alloy composition, concentration of corrodent, and temperature. In some cases, stresses near the jdeld strength of the metal are necessary. In other cases, the stresses can be much lower. However, for each combination of environment and alloy system, there appears to be a threshold stress below which SCC will not occur. Threshold stresses can vary from 10 to 70% of yield strength depending on the alloy and environment combination and temperature (Fig. 9.6). 

The level of stress may be, and generally is, much less than the yield strength of the metal. However, in general, higher stresses increase crack growth rate and the number of cracks initiated. 

All metals, on the other hand, have yield strengths far below the levels predicted by our calculation - as much as a factor of 10 smaller. Even ceramics, many of them, yield at stresses which are as much as a factor of 10 below their ideal strength. Why is this ... 

The result is work-hardening the steeply rising stress-strain curve after yield, shown in Chapter 8. All metals and ceramics work-harden. It can be a nuisance if you want to roll thin sheet, work-hardening quickly raises the yield strength so much that you have to stop and anneal the metal (heat it up to remove the accumulated dislocations) before you can go on. But it is also useful it is a potent strengthening method, which can be added to the other methods to produce strong materials. 

In this chapter we show that k = Oy/2, and use k to relate the hardness to the yield strength of a solid. We then examine tensile instabilities which appear in the drawing of metals and polymers. 

How are these metals used in the traction engine The design loads in components like the wheels and frames are sufficiently low that mild steel, with a yield strength Cy of around 220 MPa, is more than strong enough. It is also easy to cut, bend or machine to shape. And last, but not least, it is cheap. 

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