Sharp Yield

Very little or no work on this subject has been reported in the field of ceramics. Therefore, only minor consideration will be given in this section to the above topics. 

Johnson and Gilman reported that, in order to obtain a sharp yield drop, the two necessary criteria are (a) an increase in the number of moving dislocations and (b) a direct relation between the stress and the velocity of the dislocations. By knowing the strain rate, given as  

all processes which impede dislocation movement are prone to induce yield point pinning and drop (softening). 

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For stainless steel, the stress-strain curve (see Fig. 26-37) has no sharp yield point at the upper stress limit of elastic deformation. Yield strength is generally defined as the stress at 2 percent elongation. 

A corroboration of the key role of the intermediate cation 291 is the observation of a sharp yield increase of disproportionation products 297 and 298, compared to their direct formation from salt 280 on treatment of dimer 281 with catalytic amounts of triethylammonium perchlorate, which acts as the protonating agent generating intermediate 293. The formation of unsaturated dimers 296 is possible not only by the direct hydride transfer, followed by deprotonation of dimeric salt 294 (pathway a, Scheme 16), but also by equivalent 1,5-hydrogen transfer in the ortho-quinonoid intermediate 295 formed by deprotonation of the acidic methine group in intermediate 291 (pathway b). 

The stress-strain behavior of a mUd carbon steel is characterized by a sharp yield point and large ultimate strains (percent elongation >20 %). High-strength low-aUoy steels exhibit higher yield points (345-480 MPa) and somewhat larger tensile strengths (450-600 MPa) without an appreciable drop in ductility. AUoy steels are heat-treated steels (mostly quenched and tempered) with yield points of 550-760 MPa, but lower tensile to yield ratios and ductility than for other structural steels. 

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