mechanical properties of materials

What are the Mechanical Properties of Materials ?

The mechanical properties of materials define the behaviour of materials under the action of external forces, called loads. They are a measure of the strength and lasting characteristic of a material in service and are of great importance in the design of tools, machines and structures.

Mechanical properties are structure sensitive in the sense that they depend upon the crystal structure and its bonding process and specially upon the nature and behaviour of the imperfects which exist within the crystal itself or at the grain boundaries.

The most important and useful Mechanical properties of Materials are briefly explained below to ensure that the readers will be able to choose the proper material for a given design quickly and wisely.

1. Strength

The strength of a material is its capacity to be tested and destruction under the action of external loads. The stronger the material the greater the load it can withstand. It therefore determines the ability of a material to the stand stress without failure. Since strength varies according to the type of loading. It is possible to cancel tensile, compressive, shearing or torsional strengths.

The maximum stress that any material will withstand before destruction is called its ultimate strength. The tendency of a material is its ultimate strength in tension.

2. Elasticity

Elasticity is that mechanical properties of materials by virtue of which deformation caused by applied load disappears upon removal of the load. In other words, elasticity of a material is its power of coming back to its original position after deformation when the stress or load is removed. Elasticity is a tensile property of the material.

  • Proportional limit :- It is the maximum stress under which a material will maintain a perfectly uniform rate of strain to stress. Even though this value is difficult to measure, it is used in important application such as precision instruments, spring, etc.
  • Elastic limit :- Most materials can be stressed 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 elastic limit. Beyond elastic limit , therefore, the material in its original form and permanent change occurs.
  • Yield point :- At a certain stress, ductile materials particularly cease offering resistance to tensile forces , i.e., they flow and a relatively large permanent change takes place without a noticeable increase in load. This point is called yield point. Some materials exhibit a definite yield point, in which case the yield stress or yield strength is simply the stress at this point. Mild steel is an instance of this.
  • Proof stress :- Most ductile materials exhibit progressive yield and another measure of yield stress, usually known as proof stress. Proof stress is defined as the amount of stress a material can withstand without taking more than a small amount of set, common measure being, 0.1 or 0.2 % of the original gauge length.

3. Stiffness

The resistance of a material to elastic deformation or deflection is called stiffness or rigidity. A material which suffers slight deformation under load has a high degree of stiffness or rigidity. For instance suspended beams of Steel and Aluminium may both be a strong enough to carry the required load but the aluminium will ”sag”or deflect further. In other words, the Steel beam is stiffer or more rigid than aluminium Beam.

If the material follows hook’s law, i.e., has a linear stress- strain relation, its stiffness is measured by the young’s modulus E. The higher the value of the Young’s modulus, the stiffer 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 the Young’s modulus for commonly used materials ;in volumetric distortion , the bulk modulus.

The term flexibility is sometimes used as the opposite of stiffness. However flexibility usually has to do with flexure or bending. Also it may imply use of bending in the plastic range.

4. Plasticity

The plasticity of a material is its ability to undergo some degree of permanent deformation without rupture of failure. Plastic deformation will take place only after the elastic range has been exceeded.

Plasticity 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 the Steel is plastic when at bright heat. In general, plasticity increases with increasing temperature.

5. Ductility

Ductility is one of the mechanical properties of a materials which enables it to draw out into thin wire. Mild steel is a ductile material. The per cent elongation and the reduction in area in tension is often used as a empirical measures of the ductility.

6. Malleability ( Mechanical properties of materials )

Malleability of a material is its ability to be flattened into thin sheets without cracking by hot or cold working. Aluminium, copper, tin, lead, steel, etc. are malleable metals.

It is important to note that some materials may be malleable and ductile. Lead for example, can be readily rolled and hammered into thin sheets but cannot be drawn into wire. Although ductility and malleability are frequently used interchangeably, ductility is thought of as a tensile quality, whereas the malleability is considered as a compressive quality.

The words ductility and malleability makes it almost synonymous with workability or formability which is clearly related to plastic deformation.

7. Resilience

Resilience is a mechanical properties of materials which has the capacity of a material to absorb energy loss on removal of the load. The energy stored is given as exactly in a string if the load is removed.

The maximum energy which can be stored in a body of elastic limit is called the proof resilience, and the proof resilience per unit volume is called modulus of resilience. In other words, the modulus of resilience is defined as the amount of energy required to stress unit volume of a material to its proportional limit. The quantity gives capacity of the material to wear shocks and vibrations.

8. Toughness

The toughness is a measure of the amount of energy a material can absorb before actual fracture or failure takes place. For example, if a load is suddenly applied to a piece of mild steel and then to a piece of glass, the mild steel will absorb much more energy before failure occurs. Thus a mild steel is said to be much tougher than a glass.

The toughness of a material is its ability to withstand both plastic and elastic deformations. It is, therefore, a highly desirable quality for structural and machine parts which have to withstand shock and vibration. Manganese steel, wrought iron, mild steel, etc. are tough materials.

The work or energy a material absorbs is sometimes called modulus of toughness. Toughness is related to impact strength, I e., that means resistance to soch loading.

9. Hardness ( Mechanical properties of materials )

Hardness is a fundamental property which is closely related to strength. Hardness is usually defined in terms of the ability of a material to resist to a scratching, abrasion, cutting, indentation, or penetration. It is important to note that the hardness of a metal does not directly related to the hardenability of the metal.

Many methods are now in use for determining the hardness of material. They are Brinell, Rockwell and Vickers .

10. Hardenability

Hardenability indicates the degree of hardness that can be imparted to metal particularly Steel, by the process of hardening. It determines the depth and distribution of hardness induced by quenching. The hardenability of a metal is determined by a Jominy test to determine how well a metal hardness from the center of the metal to the interface of the metal. The Jominy test (ISO 642:1999 ) involves heating a test piece from the Steel ( 25mm diameter and 100mm long ) to an austenitising temperature and quenching from one end with a controlled and standardized Jet of water. A metal that is capable of being hardened throughout its structure is said to have a higher hardenability.

11. Brittleness ( Mechanical properties of materials )

The brittleness of a material is the property of breaking without much permanent distortion. There are many materials which break or fail before much deformation takes place.

Such materials are brittle, e.g., glass, cast iron. Therefore, a non-ductile material is said to be the brittle material.

Usually the tensile strength of brittle materials is only a fraction of their comprehensive strength.

12. Machinability

Machinability is not an intrinsic mechanical properties of materials, but rather the result of complex interaction between the workpiece and various cutting devices operated at different rates under different lubricating conditions. As a result, machinability is measured empirically, with result applicable only under similar conditions.

However, simply stated, it is the ease with which a metal can be removed in various machining operations. Good machinability implies satisfactory results in machining.

The machinability of metal is indicated by percentage what is machinability index. All machines metals are compared to a basic standard. The standard metal used to for 100 per cent machinability rating is free-cutting steel. Machinability index of carbon steels generally range from 40 to 60 per cent ,and that of cast iron from 50 to 80 per cent.

13. Creep

Creep is the mechanical properties of materials. The slow and progressive deformation of a material with time at constant stress is called creep. The simplest type of creep deformation is viscous flow.

Depending on temperature, stress even below the elastic limit and cause some permanent deformation. It is most generally defined as a time-dependent strain occurring under stress. Metals generally exhibit creep at highest temperatures, whereas plastic, rubber, and similar amorphous materials are very temperature-sensitive to creep.

There are three stages of creep. In the first one the material elongates rapidly but at a decreasing rate. In the second stage, the rate of elongation is constant. In 3rd stage, the rate of elongation increases rapidly until the material fails. The stress for a specified rate of strain at constant temperature is called creep strength.

14. Fatigue ( Mechanical properties of materials )

The fatigue properties of a material determine its behaviour when subjected to thousands or even millions of cyclic load applications in which the maximum stress developed in each cycle is well within the elastic range of the material. Under these conditions failure may occur after a certain number of load applications ,or the material may continue to give service indefinitely. In many instances a component is designed to give a certain length of service under a specified loading cycle; many components of high speed aero and turbine engines are of this type.

So these were all the different Mechanical Properties of Materials which is helpful in getting insights about which type of material should be chosen according to the requirement.

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