Finally, the technician may examine the fracture surfaces to determine the “% shear.” Shear technically implies ductility at both macro and micro scales, although the test method does not require anything more than a visual check using the eyeballs. Shear is evaluated by the presence of shear lips. The shear lips on the brittle coupons are non-existent. The % shear would be zero. Those on the ductile coupons are very large. They take up around 35-60% of the fracture cross-section.
On Figure 2, The protruding (up) shear lips are matched by recessed (down) shear lips on the left of each pair of fracture surfaces. The far-right image is difficult to interpret. The second-from-the-left actually has a shear lip that is half up and half down on the right side. The coupons tested at a warm temperature have about 25% shear, while those on the left have about 10%.
Charpy tests are one type of mechanical tests. Mechanical behavior overall is defined by the numerical outputs of this and other strength-measuring tests we subject any structural material to. Like the Charpy, the tensile test provides numerical values that characterize both the strength (pounds per square inch, or MPa) and the ductility (measured in % elongation or % reduction in area for cylindrical test bars). Tensile-test results are easier to use in a design because the tensile strength in pounds per square inch, or MPa, directly helps engineers calculate the loads that the structure will be able to support.
If something goes wrong (such as a loading spike) and if the specified structural steel had minimum Charpy requirements (assuming the material met the requirements), a bit of a safety factor is ensured. This was not the case, it was determined, for a very long series of castings made for the U.S. Navy. The metallurgist at the foundry decided that she knew better. When the test bars failed to meet the requirements, she lied and said that the castings met all requirements. She is now facing prison time, and she and her employer will have to come up with a chunk of change to cover the fines associated with this criminal activity.
Understanding whether a broken part displayed ductile or brittle behavior is key to understanding the job of mechanical and civil engineers involved with design of structures. Materials that display ductile behavior surprise us with disaster less frequently than those that are inherently brittle.
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