Strength and ductility of materials
Capacity of machine component is related to the most vere rvice condition it can sustain without a change which will prevent the component from continuing its intended function. In most cas, loads are the main manifestation of capacity. To asss the load-carrying capacity of a machine component, the maximum unit load (stress) has to be compared with the appropriate material property. 睛字怎么组词Here, we shall discuss the capacity of engineering materials to sustain loads.
孕期检查时间表
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-ction 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 becau of the way it is made; that is, we measure force and displacement and interpret them as stress and strain bad o
n the initial length and the initial cross-ction of the test specimen. The properties continuously change during the experiment therefore the results of such a test will have only a formal value.
弥漫意思
For most material ud in machine design, from this diagram we obrve a linear stress-strain relationship which extends up to some point, and shortly thereafter one can obrve an increasing deformation without a proportional increa 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 preci 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 (σ大和锦多肉u) is the maximum nominal stress which can be obrved in th
e stress-strain diagram, which, corresponds to the maximum nominal stress that the material can sustain, the radio of maximum load to the original cross-ctional area.
In ductile materials, there is a substantial difference between yield and tensile strength. In high-strength materials, the difference between the two values decreas.
In most materials the strength is the same in tension and in compression. Some materials, however, have very different values of strength in tension and compression, such as cast iron for example. Micro-cracks exist in the structure of this material which give ri to high stress concentration during tensile leading while in compressive loading, for geometrical reasons the Micro-cracks are ineffective. Therefore the material can sustain much higher loads. In such materials the strength in tension and in compression have to be recorded independently.
In material property tables, especially for design purpos, one will also obrve different strength in tension and bending. Similar differences might be obrved in shear and torsion. Although it appears that no matter what loads cau the stress, the strength to
particular type of stress musts be the same, and this is not always the ca. Take for example the strength in pure tension and in bending. In both cas the direction of stress is the same, namely tensile stress. In flexure, however, only the outer fibers of the material have high stress while the stress diminish as we move towards the neutral line. Since, the Micro-cracks already mentioned are uniformly distributed in the material, the probability of having a Micro-crack in the area of high stress is smaller in flexure than in pure tension, therefore the strength in flexure is, in general, greater than that in pure tension. A similar situation exists for strengths in torsion and shear.
Another deviation of the stress-strain curve from reality must be pointed out: the horizontal scale is usually arbitrarily nonlinear since elastic deformation at small strains is very small compared with plastic deformation. For this reason, the strain scale is enlarged for small strains.
苏州林园
This yield point in ductile materials is usually well defined. In cas where there is no pronounced yield point in the diagram, the yield strength is defined as the stress at which
the permanent t εp1=0.002or 0.2% (fig.27.2). In some cas the yield strength is established for右脑控制什么ε五句话p1=0.5%.
To distinguish between the yield point in tension and in compression, an additional subscript ”t” or “c” is introduced in the notion when it is necessary in some materials. Thus we obtain the symbolsσyt andσyc for the yield point.
