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1、PEC 材料工程英语证书考试材料工程英语证书考试-材料特性术语材料特性术语Density Units: SI: Mg/m3; cgs: g/cm3; Imperial: lb/ft3 The density is the weight per unit volume. We measure it today as Archimedes did: by weighing the material in air and in a fluid of known density.Atomic VolumeUnits: SI: m3/kmol; cgs: 106cm3/kmol; Imperial: i
2、n3/kmol The atomic (or molecular) volume Vm is the average volume per 103N0 of atoms in the structure, where N0 is Avogadros number (6.022 x 1023/mol). For a pure element, it is simply:where A is the atomic weight in kg/kmol and r is the density in kg/m3. For compounds the average atomic volume iswh
3、ere M is the molecular weight and n is the number of atoms in the molecule. Thus for a compound with the formula AxBy it iswhere AA is the atomic weight of element A, and AB is the atomic weight of element B. For a polymer (CxHyOz)n it is thereforewhere AC is the atomic weight of carbon, and so on.
4、The atomic volume is involved in many property correlations (and thus is crucial for checking and estimating properties) and, together with the density, it gives the atomic weight.Energy ContentUnits: SI: MJ/kg; cgs: kcal/g; Imperial: kcal/lb The energy content of a material is an approximate estima
5、te of the energy used to make it from its naturally-occurring ores, feed stocks or sources, plus the energy content of the source material itself. (Usually the energy content of the source material is small, except, for example, when the source is oil.) Thus the energy content of Aluminium is domina
6、ted by the electric power absorbed in its extraction from Bauxite; that for polymers, for which the feed stock is crude oil is the energy contained in the oil itself plus that of the subsequent processing; and that for wood is the energy content of wood plus the energy required to harvest it.Youngs
7、ModulusUnits: SI: GPa; cgs: 1010dyne/cm2; Imperial: 106psi Youngs modulus, E, is the slope of the initial, linear-elastic part of the stress-strain curve in tension or compression. For isotropic materials it is related to the bulk modulus K and to the shear modulus G bywhere n is Poissons ratio. Com
8、monly n = 1/3, and hence E = K, and E = (8/3)G.Bulk ModulusUnits: SI: GPa; cgs: 1010dyne/cm2; Imperial: 106psi The bulk modulus, K, measures the elastic response to hydrostatic pressure, p:where v is the volume. For isotropic solids it is related to Youngs modulus E and to the shear modulus G bywher
9、e n is Poissons ratio. When n = 1/3, E = K, and K = (8/3)G.Shear ModulusUnits: SI: GPa; cgs: 1010dyne/cm2; Imperial: 106psi The shear modulus is the initial, linear elastic slope of the stress-strain curve in shear. For isotropic materials it is related to Youngs modulus E and to the bulk modulus K
10、and Poissons ratio byWhen n = 1/3, G = (3/8)E, and G = (3/8)K.Poissons RatioUnits: DimensionlessPoissons ratio n is the negative of the ratio of the lateral strain to uniaxial strain, in axial loading. Its value for many solids, is close to 1/3. For elastomers it is just under 0.5.Elastic Limit/Yiel
11、d StrengthUnits: SI: MPa; cgs: 107dyne/cm2; Imperial: 103psi The elastic limit sel, of a solid requires careful definition. For metals, the elastic limit is defined as the 0.2% offset yield strength. This represents the stress at which the stress-strain curve for uniaxial tensile loading deviates by
12、 a strain of 0.2% from the linear-elastic line. It is the same in tension and compression. It is the stress at which dislocations move large distance through the crystals of the metal. For polymers, the elastic limit is the stress at which the uniaxial stress-strain curve becomes markedly nonlinear:
13、 typically, a strain of 1%. This may be caused by shear yielding (irreversible slipping of molecular chains) or by crazing (formation of low density, crack-like volumes which scatter light, making the polymer look white). For fine ceramics and glasses, the database entry for the elastic limit is an
14、estimate, based on the tensile strength (which is low due to brittle fracture). When based on direct measurements at high pressures, or on hardness measurements, of the stress required to cause plastic flow, it is very high: higher than the compressive strength, which is lowered by crushing. For com
15、posites, the elastic limit is best defined by a set deviation from linear-elastic uniaxial behaviour: 0.5% is taken in the database. Elastic limit depends on the mode of loading. For modes of loading other than uniaxial tension, such as shear and multiaxial loading, the strength is related to that i
16、n simple tension by a yield function. For metals, the Von Mises yield function works well. It specifies the relationship between the principal stresses s1, s2, s3 and the yield strength sy(elastic limit):The Tresca function is sometimes more convenient, because it is less complicated:For ceramics, a Coulomb flow law is used:Tensile StrengthUnits: SI: MPa; cgs: 107dyne/cm2; Imperial: 103psi The Tensile strength is the nominal stress at which a round b
