Lesson 1 Mechanical Properties of Metals
Mechanical properties are the characteristic responses of a material to applied forces. These properties fall into five broad categories: strength, hardness, elasticity, ductility, and toughness.
Strength is the ability of a metal to resist applied forces.
Strength properties are commonly referred to as tensile strength, bending strength, compressive strength, torsional strength, shear strength and fatigue strength.
Tensile strength is that property which resists forces acting to pull the metal apart. It is one of more important factors in the evaluation of a metal.
Compressive strength is the ability of a material to resist being crushed. Compression is the opposite of tension with respect to the direction of the applied load. Most metals have high tensile strength and high compressive strength. However, brittle materials such as cast iron have high compressive strength but only a moderate tensile strength.
Bending strength is that quality which resists forces from causing a member to bend or deflect in the direction in which the load is applied. Actually a bending stress is a combination of tensile and compressive stresses.
Torsional strength is the ability of a metal to withstand forces that cause a member to twist.
Shear strength refers to how well a member can withstand two equal forces acting in opposite directions.
Fatigue strength is the property of a material to resist various kinds of rapidly alternating stresses. For example, a piston rod or an axle undergoes complete reversal of stresses from tension to compression. Bending a piece of wire back and forth until it breaks is another example
of fatigue strength.
Hardness is that property in steel which resists indentation or penetration. Hardness is usually expressed in terms of the area of an indentation made by a special ball under a standard load, or the depth of a special indenter under a specific load.
Elasticity is the ability to spring back to original shape. Auto bumpers and all springs should have this quality.
Ductility is the ability to undergo permanent changes of shape without rupturing. Modern, deep-formed auto bodies and fenders, and other stamped and formed products must have this property.
Toughness is the ability to absorb mechanically applied energy. Strength and ductility determine a material’s toughness. Toughness is needed in railroad cars, automobile axles, hammers, rails, and similar products.
New Words学习
1.Mechanical 力学的，机械的
2.Response 反应，响应，答复
3.Broad 主要的，概括的
4.Category 种类，类别
……
Notes
1.Tensile strength is that property which resists forces acting
to pull the metal apart.
抗拉强度是金属抵抗外力把它拉断的能力。

句中关系代词which 引导一定语从句修饰property。

在从句中which 作主语。

此外，在该从句中，现在分词短语acting to pull the metal apart 作定语修饰forces。

Pull……apart意为“把……拉开，把……拉断”。

pression is the opposite of tension with respect to the direction of the applied load.
压缩时载荷的方向与拉伸时的载荷方向相反。

该句中介词短语with respect to the direction of the applied load 作全句的状语。

3.Hardness is usually expressed in terms of the area of an indentation made by a special ball under a standard load, or the depth of a special indenter under a specific load.
硬度通常用在标准载荷作用下特制球产生的压痕面积来表示，或用特定载荷下专门压头所形成的深度来表示。

句中the area…a standard load和the depth…a specific load为并列的两个名词性短语，通过等立连词or连接，它们都是短语介词in terms of 的宾语。

介词短语in terms of…a specific load作全句的状语。