3 Continued on next page... the testing machine divided by the original cross-sectional area of the specimen. The force registered at the instant of breakage, divided by the final cross-section area of the specimen at the point of breakage, is termed the fracture strength. In steel, fracture strength, while of little practical significance, almost always has a higher value than ultimate tensile strength. Permanent deformation of steel increases its unit tensile strength. That’s why steel wire, which is repeatedly deformed as it is drawn, is stronger (in terms of breaking force per unit of cross-section) than a steel bar from the same heat of steel, and why cold-rolled steel is stronger than hot-rolled steel. Let’s now try to define more precisely the several terms just introduced in describing the tensile testing of a steel bar: Fig. 8-2. Tensile strength of a weld coupon can be determined by pulling the coupon until it breaks. Fig. 8-3. This coupon has started to ”neck down” outside the weld zone. The weld is good.
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