文档介绍：Strength and ductility of materials
Capacity of machine component is related to the most severe service condition it can sustain without a change which will prevent the component from continuing its intended function. In most cases, loads are the main manifestation of capacity. To assess the load-carrying capacity of a machine component, the maximum unit load (stress) has to be compared with the appropriate material property. Here, we shall discuss the capacity of engineering materials to sustain loads.
A most informative material test is the simple tensile test. A specimen in the form of a cylindrical bar, machined to a certain specification, is slowly loaded in a tensile testing machine and load F and displacement (extension) ΔL are recorded. The resulting load-displacement curve is shown in for a typical mild steel. Assuming constant, cross-section of the rod, the same curve relates stress and strain. The curve of this figure does not correspond exactly to the real stress-strain relationship because of the way it is made; that is, we measure force and displacement and interpret them as stress and strain based on the initial length and the initial cross-section of the test specimen. These properties continuously change during the experiment therefore the results of such a test will have only a formal value.
For most material used in machine design, from this diagra