Lesson 1 MECHANICAL PROPERTIES OF METALS
Mechanical properties are measures of how materials behave under applied loads. Another way of saying this is how strong is a metal when it comes in contact with one or more forces. If you know the strength properties of a metal, you can build a structure that is safe and sound. Hence strength is the ability of a metal to withstand loads (forces) without breaking down.
Strength properties are commonly referred to as tensile strength, bending strength, compressive strength, torsional strength, shear strength, fatigue strength and impact strength.
1. Stress is the internal resistance a material offers to being deformed and is measured in terms of the applied load.
2. Strain is the deformation that results from a stress and is expressed in terms of the amount of deformation per centimeter.
3. Elasticity is the ability of a metal to return to its original shape after being elongated or distorted, when the forces are released. A rubber band is the best example of what is meant by elasticity. If the rubber is stretched, it will return to its original shape after you let it go. However, if the rubber is pulled beyond a certain point, it will break. Metals with elastic properties act in the same way.
4. Elastic limit is the last point at which a material may be stretched and still return to its undeformed condition upon release of the stress.
5. Modulus of elasticity is the ratio of stress to strain within the elastic limit. The less a material deforms under a given stress the higher the modulus of elasticity is. By checking the modulus of elasticity the comparative stiffness of different materials can readily be ascertained. Rigidity or stiffness is very important for many machine and structural applications.
6. Tensile strength is that property which resists forces acting to pull the metal apart. It is one of the most important factors in the evaluation of a metal.
7. Compressive strength is the ability of a material to resist to be 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 moderate tensile strength.
8. Bending strength is that quality which resists forces to cause a member to bend or deflect in the direction which the load is applied. Actually a bending stress is a combination of tensile and compressive stress.
9. Torsional strength is the ability of a metal to withstand forces that cause a member to twist.
10. Shear strength refers to how well a member can withstand two equal forces acting in opposite directions.
11. 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.
12. Impact strength is the ability of a metal to resist loads that are applied suddenly and often at high velocity. The higher the impact strength of a metal is, the greater the energy is required to break it. Impact strength may be seriously affected by welding since it is one of the most sensitive properties of structure.
13. Ductility refers to the ability of metal to stretch, bend, or twist without breaking or cracking. A metal of high ductility, such as copper or soft iron, will fail or break gradually as the load on it is increased. A metal of low ductility, such as cast iron, fails suddenly by cracking when subjected to a heavy load.
14. Hardness is the 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.
15. Cryogenic properties of metals represent behavior characteristics under stress in environments of very low temperatures. In addition to being sensitive to crystal structure and processing conditions, metals are also sensitive to low and high temperatures. Some alloys which perform satisfactorily at room temperatures may fail completely at low or high temperatures. The changes from ductile to brittle failure occurs rather suddenly at low temperatures.
alloy [ə'lɔi] n. 合金
alternate ['ɔ:ltənit] v. 交替, 轮流
ascertain [ˌæsə'tein] vt. 确定, 查明
axle ['æksl] n. 轮轴, 车轴
behavior [bi'heivjə] n. 性能
bend [bend] v. 弯曲
brittle ['britl] adv. 易碎的
characteristic [kærəktə'ristik] n. 特性
compression [kəm'preʃən] n. 压缩
compressive [kəm'presiv] a. 压缩的
crack [kræk] v.（使）破裂, 裂纹
crush [krʌʃ] vt. 压碎
cryogenic [ˌkraiə'dʒenik] a. 低温学的
crystal ['kristl] a. 结晶状的
deflect [di'flekt] vt. 使弯曲
deform [di'fɔ:m] v. (使)变形
New Words
distort [dis'tɔ:t] vt. (使)变形, 扭曲
ductility [dʌk'tiliti] n. 延展性, 韧性
elasticity [ilæ'stisiti] n. 弹性
elongate [i'lɔŋget] vt. (使)伸长, 延长
evaluation [i'væljuˌeiʃən] n. 评估
fail [feil] n. 损坏
fatigue [fə'tig] n. 疲劳
hardness ['ha:dnis] n. 硬度
impact ['impækt] n. 冲击
indentation [ˌinden'teiʃən] n. 压痕
indenter [in'dentə] n. 压头
internal [in'tə:nl] a. 内部的
measure ['meʒə] n. 大小, 度量
moderate ['mɔdərit] a. 适中的
modulus ['mɔdjuləs] n. 系数
opposite ['ɔpəzit] n. 反面
penetration [peni'treiʃən] n. 穿透
piston ['pistən] n. 活塞
property ['prɔpəti] n. 特性
quality ['kwaliti] n. 质量
rapidly ['ræpidli] adv. 迅速地
ratio ['reiʃiəu] n. 比, 比率
refer [ri'fə:] v. 指, 涉及, 谈及
release [ri'li:s] vt. 释放
resist [ri'zist] vt. 抵抗
resistance [ri'zistəns] n. 阻力, 抵抗
reversal [ri'və:sl] n. 反向
rigidity [ri'dʒiditi] n. 刚度
rod [rɔd] n. 杆, 棒
satisfactorily [ˌsætis'fæktrili] adv. 满意地
sensitive ['sensitiv] a. 敏感的
seriously ['siəriəsli] adv. 严重地
shear [ʃiə] n. 剪, 切
sound [saund] a. 结实的, 坚固的
stiffness ['stifnis] n. 刚度
strain [strein] n. 应变
strength [streŋθ] n. 强度
stress [stres] n. 压力, 应力
stretch [stretʃ] v. 伸展
subject ['sʌbdʒikt] vt. 使受到
tensile ['tensl] a. 拉紧的, 张紧的
tension ['tenʃən] n. 拉紧, 张紧
torsional ['tɔ:ʃənl] a. 扭转的
twist [twist] v. 扭曲, 扭转
undergo [ʌndə'gəu] vt. 经受
various ['vεəriəs] a. 不同的, 各种的
velocity [vi'lɔsiti] n. 速度
welding ['weldiŋ] n. 焊接
withstand [wið'stænd] vt. 经受, 经得起
applied loads作用力
in terms of依据
in contact with接触
break down破坏
with respect to相对于
Phrases and Expressions
Notes
1. Mechanical properties are measures of how materials behave under applied loads. Another way of saying this is how strong is a metal when it comes in contact with one or more forces.
机械性能是材料在外加载荷作用下所呈现的特性的量度。换句话说，机械性能是金属在一个力或几个力的作用下，所具有的强度。
2. Strength properties are commonly referred to as tensile strength, bending strength, compressive strength, torsional strength, shear strength, fatigue strength and impact strength.
强度特性通常指的是抗拉强度、抗弯强度、抗压强度、抗扭强度、抗剪强度、抗疲劳强度和抗冲击强度。
3. Elastic limit is the last point at which a material may be stretched and still return to its undeformed condition upon release of the stress.
弹性极限是材料在拉伸时，所加应力去掉后还能回到未变形前状态的最大应力。
4. 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.
硬度通常是用在标准载荷作用下特制钢球的压痕面积来表示，或是用在特定载荷作用下，专门的压头所形成的深度来表示。
5. The changes from ductile to brittle failure occurs rather suddenly at low temperatures.
在低温下从塑性到脆性破坏，变化的发生是相当突然的。
Reading Material
THE NATURE OF MATERIALS SCIENCE
During the last generation we have witnessed (证明) and benefited from the development of numerous new technological systems, such as nuclear power plants, satellites, computers, lasers, etc. Each of these has been advanced by the development of materials with new and exotic (奇异的) properties.
The properties of materials, have dictated nearly every design and every useful application that the engineer could devise. But with the present sophistication (复杂化) of our engineering science, it is no longer simply a question of being satisfied to design with existing materials. We are now requiring new materials with new properties to fit our designs. This is true in all fields of engineering, whether it be the mechanical engineer trying to design high-strength, light-weight casings (包装) for rocket hulls (外壳), the electrical engineer trying to design a solid state electronic device that will operate at temperatures above a few hundred degrees Celsius (摄氏的), or the nuclear engineer concerned with the materials needed to contain, control, and utilize a nuclear reaction. This search for new materials with improved properties now occupies an important position in the engineering world.
Along with the search for new materials has come to the realization, that effective usage of materials can be realized only when the engineer fully understands the various properties of materials. The reason for this is that practically all of the useful properties of materials are strongly dependent on their internal (内部的) structure. The rather broad term internal structure is defined as the arrangement (排列) of electrons and atoms within a material. We shall see shortly that for a material of given chemical composition, the internal structure is not constant, and can vary greatly, depending on (1) how the material was manufactured (exactly what processing conditions were involved) and (2) under what conditions (temperature, pressure, exposure (暴露) to radiation, etc. ) the material is placed into service. By altering the internal structure of a material in a controlled manner it is possible to effectively control the properties of the material. And because a single material may be treated to have different internal structures and correspondingly different properties, one maternal may be used for many applications, each calling for different physical properties.