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Transcript of engineering materials
1.2 Selection of materials
There are reasons for selecting materials:
Ease of manufacture
Engineering properties of materials
Ease of forming by extrusion, forging and casting
Corrosion resistance, etc.
1.4 Physical properties of materials
1.1 Engineering materials
materials satisfy our human requirements
in the past humans used:
simple weapons - for hunting food and for territorial defense.
tools to cultivate the lands.
tools to manufacture the artifacts (i.e. cooking utensils)
woven clothes took place of animal skin.
in the present we use materials for:
1.3 Mechanical properties of materials
The mechanical properties are concerned mainly with strength. The strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive or shear.
1.3.1 Tensile strength
This is the ability of a material to withstand tensile (stretching) loads without breaking; stress is divided by the original cross-sectional area of the material. Tensile strength measures the force required to pull something such as rope wire, or a structural beam to the point where it breaks. Tensile strengths have dimensions of force per unit area and in the English system of measurement are commonly expressed in units of pounds per square inch, or N/mm2
1.3.2 Compressive strength
This is the ability of a material to withstand compressive (squeezing) loads without being crushed or broken. Therefore the component needs to be made from a material with adequate compressive strength to resist the load. Stress is the applied force (load) divided by the cross- sectional area of the component measured at right angles to the direction of application of the force that is, force per unit area. The units may be N/mm2 or MN/m2. The numerical value is the same in both cases.
1.3.3 Shear strength
1.3.4 Toughness (impact resistance)
1.3.11 Rigidity (stiffness)
1.4.2 Melting temperature of materials
1.4.3 Electrical Conductivity
1.4.6 Magnetic Properties
Hard magnetic materials
Soft magnetic materials
1.4.7 Thermal properties
1.4.9 Temperature stability
The strength of the material to resist deformation under "sliding" forces is called shear strength. This is the ability of a material to withstand offset loads, or transverse cutting (shearing actions). Again we can use stress but this time it will be shear stress and again it is force per unit area. The units are the same as those in tensile and compressive strength. The value for the shear stress for any given material is not the same as the value for tensile stress or for compressive stress.
This is the ability of a material to withstand shatter. If a material shatters it is brittle (e.g. glass). Rubbers and most plastic materials do not shatter, therefore they are tough. Toughness should not be confused with strength. If a rod is made from a piece of high-carbon steel — for example, silver steel in the annealed (soft) condition (as normally supplied) it will have only a moderate tensile strength, but under the impact of the hammer it will bend without breaking, therefore it is rough. If a similar specimen is made hard by making it red hot and quenching it (cooling it quickly in water), it will now have a very much higher tensile strength. However, although it is now stronger it will prove to be brittle and will break off easily when struck with a hammer. Therefore it now lacks toughness. A material is brittle and shatters layers tend to stretch and arc in tension.) Therefore, any material in which the spread of surface cracks does not occur or only occurs to a small extent is said to be tough. It is also defined as the resistance to fracture of a material when stressed. Toughness requires a balance of strength and ductilit
Toughness (impact resistance)
This is the ability of a material to deform under load and return to its original size and shape when the load is removed. If it is made from an elastic material it will be the same length before and after the load is applied, despite the fact that it will be longer whilst the load is being applied.
This property is the exact opposite to elasticity. It is the state of a material which has been loaded beyond its elastic limit so as to cause the material to deform permanently. Under such conditions the material takes a permanent set and will not return to its original size and shape when the load is removed. When a piece of mild steel strip is bent at right angles into the shape of a bracket, it shows the property of plasticity since it does not spring back straight again.
This is the term used when plastic deformation occurs as the result of applying a tensile load. A ductile material allows a useful amount of plastic deformation to occur under tensile loading before fracture occurs. Such a material is required for manipulation by such processes as wire drawing, tube drawing, and cold pressing low-carbon steel sheets into motor car body panels.
Malleability is the ability of a metal to be hammered, rolled, or pressed into various shapes without rupture or fracture.
Hardness is the ability of a metal to resist penetration and wear by another metal or material. It takes a combination of hardness and toughness to withstand heavy pounding. The hardness of a metal limits the ease with which it can be machined, since toughness decreases as hardness increases. The hardness of a metal can usually be controlled by heat treatment.
Brittleness is the tendency of a material to fracture or break with little or no deformation, bending, or twisting. Brittleness is usually not a desirable mechanical property.
Rigidity is the extent to which it resists deformation in response to an applied force. The complementary concept is flexibility or pliability: the more flexible an object is the less stiff it is.