专业英语拉伸试验

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 fig.1 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 diagram we observe a linear stress-strain relationship which extends up to some point, and shortly thereafter one can observe an increasing deformation without a proportional increase in the load and the stress. This roughly corresponds to the point that we have a substantial yielding of the material and we call this the” yield point”. The corresponding stress (σy) is called the “yield strength” of the material.We have been intentionally vague in the precise definition of the yield point where we have the transition from elastic to plastic behavior. In fact, in most engineering materials this transition is not abrupt and it is a matter of definition to specify the yield point. Usually we define the yield point as the point where a certain percentage of plastic deformation remains after loading.

Ultimate tensile strength (σu) is the maximum nominal stress which can be observed in the stress-strain diagram, which, corresponds to the maximum nominal stress that the material can sustain, the radio of maximum load to the original cross-sectional area.

In ductile materials, there is a substantial difference between yield and tensile strength.In high-strength materials, the difference between these two values decreases.