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Mechanics of materials also called strength of materials or resistance of materials is that branch of engineering science which treats the relations between the external forces acting upon a body, the resulting forces and the changes in shape and size of the body. Mechanics of materials along with engineering mechanics is the basis for many branches of engineering design particularly machine design and structural design. The principles of mechanics of materials are employed to determine the proper dimensions of each member, so that it will be not only safe but also economical in proportion.

An internal force set up within a body which resists or holds in equilibrium the externally applied forces is known as a stress. The stress may be expressed in terms of any suitable units of force such as pounds (lb), tons, kips (1000 lb), kilograms. The stress per unit of area is called the unit stress or intensity of stress and may be expressed in various units such as pounds per square inch (lb per sq. in.; lb/in. ; or psi), tons per square foot.

The term stress is used rather loosely to indicate both total stress and unit stress. Unless absolutely necessary which meaning is intended the qualifying term “total” or “unit” should be used. The definition of unit stress is valid for any of the three types of stress: tension, compression or shear. The bar is said to be in simple tension when it is acted upon by two equal, collinear and oppositely directed forces. If the line of action of the forces is assumed to coincide with the axis of the bar the load is said to be axially applied.

If the forces acting on that bar are reversed in direction, the bar is said to be in compression and the internal force is called a compressive stress. External loads causing compressive stress also tend to produce a shortening of the bar. Stresses directed normal or perpendicular to the area upon which they act are called normal stress. Thus a normal stress may be either tension or compression.

A shearing stress is directed parallel to the area upon which the force acts and tends to cause a sliding action of the surface upon the adjacent section.

There are other terms in common use to designate various stresses or combinations of stresses. Flexural stress refers to the tensile and compressive stresses set up in the fibers of a beam by bending action or flexure. Torsion stress is a shearing stress developed as the result of torsion or twisting action applied to a bar.

A change of dimension or shape resulting from the external loading is known as strain or deformation. A strain that disappear when the load is removed is called an elastic strain, whereas a strain that remains after the load is removed is called a plastic strain.

An internal force set up within a body which resists or holds in equilibrium the externally applied forces is known as a stress. The stress may be expressed in terms of any suitable units of force such as pounds (lb), tons, kips (1000 lb), kilograms. The stress per unit of area is called the unit stress or intensity of stress and may be expressed in various units such as pounds per square inch (lb per sq. in.; lb/in. ; or psi), tons per square foot.

The term stress is used rather loosely to indicate both total stress and unit stress. Unless absolutely necessary which meaning is intended the qualifying term “total” or “unit” should be used. The definition of unit stress is valid for any of the three types of stress: tension, compression or shear. The bar is said to be in simple tension when it is acted upon by two equal, collinear and oppositely directed forces. If the line of action of the forces is assumed to coincide with the axis of the bar the load is said to be axially applied.

If the forces acting on that bar are reversed in direction, the bar is said to be in compression and the internal force is called a compressive stress. External loads causing compressive stress also tend to produce a shortening of the bar. Stresses directed normal or perpendicular to the area upon which they act are called normal stress. Thus a normal stress may be either tension or compression.

A shearing stress is directed parallel to the area upon which the force acts and tends to cause a sliding action of the surface upon the adjacent section.

There are other terms in common use to designate various stresses or combinations of stresses. Flexural stress refers to the tensile and compressive stresses set up in the fibers of a beam by bending action or flexure. Torsion stress is a shearing stress developed as the result of torsion or twisting action applied to a bar.

A change of dimension or shape resulting from the external loading is known as strain or deformation. A strain that disappear when the load is removed is called an elastic strain, whereas a strain that remains after the load is removed is called a plastic strain.

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