06~chapter 06 - Materials: engineering, science, processing...

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Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
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Yielding Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Yield strength is the stress beyond which a material becomes plastic – deformation is permanent Determined by standard tensile testing procedures Units MPa – MN/m 2 psi – lb/in 2 1 MPa = 145.04 psi
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Stress-Strain Curve: Metal Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Figure 6.1 Yield strength σ y is defined by a 0.2% offset from the linear elastic region When strained beyond σ y , most metals work harden, causing the rising part of the curve Maximum stress is defined as the tensile strength σ ts
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Stress-Strain Curve: Polymer Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Figure 6.2 σ y is identified as the stress at which the stress-strain curve becomes markedly non-linear, typically around a strain value of 1% The behavior of the polymer beyond the yield point depends on its temperature relative to the materials glass transition temperature T g
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Stress-Strain Curve: Ceramics Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Figure 6.3 Glasses and ceramics have a yield strength; however, it is so large that it is never reached during a tensile test – the material fractures first The elastic limit σ el is defined by the end of the elastic region of the stress-strain curve – this is the value generally used to compare the strength of ceramics with other materials
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Strain and Elongation Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Plastic strain, ε pl , is the permanent strain resulting from plasticity Ductility is a measure of how much plastic strain a material can tolerate and is measured by the elongation ε f ε f is dependant on the sample dimensions and is therefore not a material property – handbook values are based on standard test geometries
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Hardness Tests Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Tensile and compression tests require a large sample and are destructive – hardness tests require only a small volume and are non-destructive Figure 6.4 In a hardness test, a diamond or ball shaped indenter is pressed into the surface of a material The hardness of the material is determined by its resistance to the indentation
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Figure 6.5 Two most common types of hardness tests are Brinell and Rockwell Figure 6.5 shows conversions to other scales
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06~chapter 06 - Materials: engineering, science, processing...

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