Mechanical properties are physical properties that a material exhibits upon the application of forces. Examples of mechanical properties are the modulus of elasticity, tensile strength, elongation, hardness, and fatigue limit.
An engineer must have an intimate knowledge of the properties and behavioral characteristics of the materials that he intends to use. While designing a product you need to select materials to create the product.
For selecting materials, you must assess the properties of each material to ensure that the selected material is appropriate for manufacturing the desired product.
The understanding of the properties of materials is highly essential because, without this information & knowledge, the designing of the manufacturing process may be an expensive & complex task.
The few important and useful mechanical properties of engineering materials are explained below.
It is defined as the property of a material to regain its original shape after deformation when the external forces are removed.
It can also be referred to as the power of the material to come back to its original position after deformation when the stress or load is removed. It is also called the tensile property of the material.
2. Proportional limit
It is defined as the maximum stress under which material will maintain a perfectly uniform rate of strain to stress.
Though its value is difficult to measure, yet it can be used as an important application for building precision instruments, springs, etc.
3. Elastic limit
Many metals can be put under stress slightly above the proportional limit without taking a permanent set.
The greatest stress that a material can endure without taking up some permanent set is called the elastic limit. Beyond this limit, the metal does not regain its original form and the permanent set will occur.
4. Yield point
At specific stress, ductile metals particularly cease, offering resistance to tensile forces. This means the metals flow and a relatively large permanent set takes place without a noticeable increase in load. This point is called the yield point.
Certain metals such as mild steel exhibit a definite yield point, in which case the yield stress is simply the stress at this point.
Strength is defined as the ability of a material to resist the externally applied forces with breakdown or yielding. The internal resistance offered by a material to an externally applied force is called stress.
The capacity of bearing load by metal and to withstand destruction under the action of external loads is known as strength.
The stronger the material the greater the load it can withstand. This property of material, therefore, determines the ability to withstand stress without failure.
Strength varies according to the type of loading. It is always possible to assess tensile, compressive, shearing, and torsional strengths. The maximum stress that any material can withstand before destruction is called its ultimate strength. The tenacity of the material is its ultimate strength in tension.
It is defined as the ability of a material to resist deformation under stress. The resistance of a material to elastic deformation or deflection is called stiffness or rigidity.
The modulus of elasticity is the measure of stiffness.
A material that suffers slight or very little deformation under load has a high degree of stiffness or rigidity. For instance, suspended beams of steel and aluminum may both be strong enough to carry the required load but the aluminum beam will “sag” or deflect further.
That means the steel beam is stiffer or more rigid than an aluminum beam. If the material behaves elastically with a linear stress-strain relationship under Hooks law, its stiffness is measured by Young’s modulus of elasticity (E).
The higher is the value of Young’s modulus, the stiffer is the material. In tensile and compressive stress, it is called modulus of stiffness or “modulus of elasticity”; in shear, the modulus of rigidity, and this is usually 40% of the value of Young’s modulus for commonly used materials; in volumetric distortion, the bulk modulus.
Plasticity is defined as the mechanical property of a material that retains the deformation produced under load permanently. This property of the material is required in forging, in stamping images on coins, and ornamental work.
It is the ability or tendency of the material to undergo some degree of permanent deformation without its rupture or its failure. Plastic deformation takes place only after the elastic range of material has been exceeded.
Such property of a material is important in forming, shaping, extruding, and many other hot or cold working processes. Materials such as clay, lead, etc. are plastic at room temperature and steel is plastic at forging temperature. This property generally increases with an increase in the temperature of materials.
Ductility is termed as the property of a material enabling it to be drawn into the wire with the application of tensile load.
A ductile material must be strong and plastic. The ductility is usually measured by the terms, percentage elongation, and percent reduction in the area which is often used as empirical measures of ductility.
The materials that possess more than 5% elongation are called as ductile materials. The ductile material commonly used in engineering practice in order of diminishing ductility is mild steel, copper, aluminum, nickel, zinc, tin, and lead.
Malleability is the ability of the material to be flattened into thin sheets under applications of heavy compressive forces without cracking by hot or cold working means.
It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong.
The malleable materials commonly used in engineering practice in order of diminishing malleability are lead, soft steel, wrought iron, copper, and aluminum. Aluminum, copper, tin, lead, steel, etc. are recognized as highly malleable metals.
Hardness is defined as the ability of a metal to cut another metal. A harder metal can always cut or put an impression on the softer metals under its hardness.
It is a very important property of metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation, and machinability, etc.
It also means the ability of a metal to cut another metal.
The hardness is usually expressed in numbers which are dependent on the method of making the test.
The hardness of a metal may be determined by the following tests:
(a) Brinell hardness test, (b) Rockwell hardness test,
(c) Vickers hardness test and (d) Shore scleroscope.
Brittleness is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. The materials having less than 5% elongation under loading behavior are said to be brittle materials.
Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Glass, cast iron, brass, and ceramics are considered brittle materials.
When a metal part when is subjected to high constant stress at a high temperature for a longer period, it will undergo a slow and permanent deformation (in the form of a crack which may further propagate towards creep failure) called creep.
It is the property of metals that denotes the ease in its forming into various shapes and sizes. The different factors that affect the formability are the crystal structure of the metal, the grain size of metal hot and cold working, an alloying element present in the parent metal.
Metals with small grain sizes are suitable for shallow forming while metal with the size is suitable for heavy forming. Hot-working increases formability. Low carbon steel possesses good formability.
Castability is defined as the property of metal, which indicates the ease with it can be cast into different shapes and sizes. Cast iron, aluminum, and brass are possessing good castability.
Weldability is defined as the property of a metal that indicates the two similar or dissimilar metals are joined by fusion with or without the application of pressure and with or without the use of filler metal (welding) efficiently.
Metals having weldability in descending order are iron, steel, cast steels, and stainless steel.
Toughness is the ability of a material to absorb energy without rupturing. The rubbers and most plastic materials do not shatter (break), therefore they are tough. For example, if a rod is made of high-carbon steel then it will be bend without breaking under the impact of the hammer, while if a rod is made of glass then it will break by impact loading.
The toughness of the material decreases when it is heated.
It is measured by the amount of energy that a unit volume of the material has absorbed after being stressed up to the point of fracture
This property is desirable in parts subjected to shock and impact loads
When a part is subjected to constant stress at a high temperature for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing internal combustion engines, boilers, and turbines.
It is the property of a material to absorb energy and to resist shock and impact loads. It is measure by the amount of energy absorbed per unit volume within the elastic limit. This property is essential for spring materials.
It is measured by the amount of energy absorbed per unit volume within the elastic limit.
This property is essential for spring materials.
19. Thermal conductivity
This is the ability of the material to transmit heat energy by conduction.
A material fails at stresses below the yield point stresses when it is subjected to repeated tensile and compressive stresses. This type of failure of material is known as fatigue.
This property is considered in designing shafts, connecting rods, gears, springs, etc.
The failure is caused by means of a progressive crack formation which is usually fine and of microscopic size.
21. Electrical resistivity
It is the property of a material due to which it resists the flow of electricity through it.
22. Electrical conductivity
It is the property of a material due to which it allows the flow of electricity through it.