机械工程材料第二章

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Chapter 2 The Mechanical Properties of Metals
2.2 Properties of Materials
• Application Properties：Properties performed when the materials were applied, including mechanical properties, physical properties and chemical properties. • Processes properties：Properties performed when the materials were machined, including casting, forging, jointing, heat treatment and cutting properties.
Melting point: Properties under high temperature
Thermal expansibility: Precision, stability Thermal conductivity: Stability Electric conductivity Magnetic properties
E is called the modulus of elasticity (Young's modulus); σ is on the internal force per unit area, call it stress；ε is strain, resistance to deformation of the internal force of object.
2.1 Introduction
Mechanical tests are those in which specially prepared specimens (test pieces) of standard form and size (and less frequently, the articles and structures) tested by special machines to obtain the srength, ductility, and toughness characteristics of the metal.
thermal conductivity of tool;
Welding performance: tendency to produce welding defects; Heat treatment performance: hardenability, temper resistance, two secondary hardening and temper brittleness.
segregation tendency of liquid metals;
Forging performance: formability and deformation resistance; Cutting performance: wear, chip breaking ability and
after tension test
•
Standard specimen of tension test (GB 228.1-2010)
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Chapter 2 The Mechanical Properties of Metals
2.3 Static mechanical properties
2.3.1 Tension tests
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Chapter 2 The Mechanical Properties of Metals
2.3 Static mechanical properties
2.3.1 Tension tests
• Tension tests are of the static type, i.e., they are tests in which the load increases comparatively slowly, from zero to a certain final
value.
before tension test
Normal tension testing machines have two chief parts:
• Units for applying a load to the specimen (with a hydraulic of mechanical drive); Units for measuring the applied load (lever with adjustable weight, penduium-type lever, presure gauge and hydraulic capsule).
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Chapter 2 The Mechanical Properties of Metals
2.2 Properties of Materials
2.2.2 Chemical properties Corrosion resistance: the ability of materials to resist corrosion in medium;
Tension stress and strain curve
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Chapter 2 The Mechanical Properties of Metals
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Chapter 2 The Mechanical Properties of Metals
2.2 Properties of Materials
2.2.3 Processes properties
Casting properties: fluidity, filling, shrinkage and
Antioxidant activity: the ability of materials to resist
oxidation at high temperature; chemical stability: comprehensive evaluation of corrosion resistance and oxidation resistance of materials.
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Chapter 2 The Mechanical Properties of Metals
2.2 Properties of Materials
• • • Deformation is the alternation of shape and size of materials under external force. Elastic deformation could recover after the external force is moved away. Plastic deformation could not recover after the external force is
Static， when the load increases slowly and gradually and the metal is loaded by tension, compression, torsion, or bending;
Test types
Dynamic， when the load increases rapidly as an impact;
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•
Rigidity：Young’s modulus E, the
material’s ability to resist plastic deformation.
Hooke's Law:
E / tan
(MPa) Tension stress and strain curve
of low carbon steel
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Chapter 2 The Mechanical Properties of Metals
2.3 Static mechanical properties
2.3.2 Elesticity and stiffness
ε is strain, resistance to deformation of the internal force of object, l / l0 , Δl is the elongation change rate, the total length (l) of the stretch minus the original length ( l0)， that is Δl = l-l0. σ is on the internal force per unit area, and P / A , p is the load of the limit of prportionality, in units of newtons (N), and A is the original cross-section area with no load applied (mm2). Stresses not exceeding the proportional limit ( σp) practically result in elastic deformation (σe), and strain disappears whe the load is removed. Actually any structure or machine component must be designed with working etresses less than σp and σe.
Young’s modulus is mainly determined by the nature of the material, the parts ’ rigidity could be improved by increasing the cross-section area or altering the shape of the crosssection areas.
Mechanical Engineering Materials
School of Mechanical and Electrical Engineering,
Guilin University of Electronic Technology
2017~2018
Chapter 2 The Mechanical Properties of Metals
Repeated or fatigue (both static and dynamic type)， when the load repeatedly varies in the course of tests either in value or in both value and direction.
moved away.
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Chapter 2 The Mechanical Properties of Metals
2.2 Properties of Materials
2.2.1 Physical properties Density and specific strength: σb/ρ， aerospace materials
Tension tester
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Chapter 2 The Mechanical Properties of Metals
2.3 Static mechanical properties
2.3.2 Elesticity and stiffness
• Elasticity: Elastic limit R e (e)，the stress when material is bearing the maximum deformation.