Checking the Validity of Properties in the Material Library
The following section lists points to consider about the definition, error estimate, and conditions for some of the Material Library properties listed in Table 2-4.
The property functions listed below have a literature reference where you can find more details about the conditions and validity range for that specific property.
Coefficient of Thermal Expansion
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The coefficient is defined as (ΔL/L)T/(T − Tref) and in most cases, it is calculated from the ΔL/L values.
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The error is expected to be in the range of 10–15%, but it might be higher near room temperature due to the small value of T − Tref.
Elastic and Initial Shear Modulus
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The data accuracy is approximately 5–10%.
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For solder alloys the literature reports a wide spread of values. Data from several sources (when available) are evaluated, and representative values are given; the error is estimated to be 10–25%.
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For some polymers the flexural modulus is used as the elastic modulus, and it is typically within 10% of the elastic modulus.
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Typically, values measured with a strain gauge are approximately 10% lower than those measured with a dynamic technique.
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Values measured by a dynamic technique are preferred over those measured by strain gauge techniques.
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For cubic materials where the elastic and shear modulus are calculated from the elastic constants (C11, C12, and C44), the Material Library uses the average of the Reuss and Voigt equations (see R.F.S. Hearmon, Advances in Physics, vol. 5, 1956, p. 232).
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For isotropic solids (glasses), it uses methods from L.D. Landau and E.M. Lifshitz, Theory of Elasticity, Addison-Wesley, New York, 1966.
Poisson's Ratio and Initial Bulk Modulus
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Calculated from the elastic modulus and the shear modulus using standard relationships, and in this sense they are self-consistent and accurate.
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Data accuracy is approximately 10–20%. Because these are derived quantities the error can be significantly higher.
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The curves for these properties often show improbable shapes that are most likely due to their derived nature and are not believed to be real. If the elastic and shear modulus were determined in a self-consistent manner, the curves would likely be much better behaved. However, all of the data are presented “as is” from the original references and are self-consistent within the Material Library.
Thermal Conductivity
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Can be very sensitive to impurities, heat treatment, and mechanical worked state, especially at very low temperatures.
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The sensitivity is somewhat decreased above room temperature and decreases as the amount of alloying increases. Compare 4340-QT (quenched and tempered) and 4340-NT (annealed).
Thermal Diffusivity
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For metals this property can be very sensitive to impurities, heat treatment, and mechanical worked state, especially at very low temperatures.
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This sensitivity is somewhat decreased above room temperature and decreases as the amount of alloying increases. To see an example of this, compare the data for elemental (high purity) Fe and Armco iron (commercial purity).
Electric Resistivity
This property is very sensitive to impurities, heat treatment, and mechanical worked state, especially at very low temperatures.
Electrical Conductivity
This property is very sensitive to impurities, heat treatment, and mechanical worked state, especially at very low temperatures.
Surface Emissivity (εT)
This property is the measured emissivity over all wavelengths and 2π radians. This is the emissivity used in the Stefan-Boltzmann law.
Normal Total Emissivity (εT,n)
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The measured emissivity is over all wavelengths at a direction normal to the surface. This is the most commonly reported value.
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For polished metal, this assumption is valid: εT/εT,n = 1.15–1.20.
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Both emissivities are sensitive to the surface condition (roughness and oxide thickness).
Density (ρ)
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The density for solids is calculated from the room-temperature density and the linear expansion coefficient and is given by ρ/(1 + ΔL/L)3.
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The data for oxides, carbides, and nitrides depend on the material’s porosity.
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For gases the ideal gas law is used.
Tensile Strength, Yield Strength Level, and Elongation
Most of the data for tensile strength, yield strength level, and elongation is from supplier product brochures. When using this data, remember it is only representative of the actual material properties.
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The variation with temperature is usually not smooth. Many of these materials are precipitation hardening alloys, and the temperature affects the aging processes in different ways at different temperatures.
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Unless otherwise stated, the data are for “short” times at the indicated temperatures and not for the equilibrium structure.
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These properties are very sensitive to the details of the processing and heat treatments. Comparison of data from different suppliers indicate that the spread in the published values is approximately 20% for materials with similar processing. The spread in the elongation data can be as high as 50–100%.
Fatigue S-N Curve
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Fatigue data is given as the maximum stress, σmax, as function of the number of cycles. The stress amplitude, maximum stress, and minimum stress are related through the stress ratio, R.
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The maximum stress, σmax, is given together with the stress ratio for all fatigue data. Then calculate the stress amplitude as:
Creep Strength and Stress-Rupture Curves
This property is very sensitive to the test atmosphere as well as the microstructure and heat treatment of the material.
Polymers and Polymer-Based Composites
Properties of polymers and polymer-based composites are sensitive to moisture and processing conditions, and they can show time-dependence at higher temperatures. The errors/uncertainties can be large compared to those of other materials. Keep these aspects in mind when using the properties of these materials.
General
The magnitude of the errors reported by authors for a given property is usually smaller by a factor of 2–3 than the error between different sources for the same data. This is especially true for materials such as ceramics.