STRENGTH OF MATERIALS

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ENGINEERING SERVICES GOVT EXAMINATION NOTES

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Strength of Materials IES ESE Exam Ace Academy Handwritten Notes free download PDF

STRUCTURAL MECHANICS

The branch of physics called mechanics concerns the actions of forces on physical bodies. Most of engineering design and investigation is based on applications of the science of mechanics. Statics is the branch of mechanics that deals with bodies held in a state of unchanging motion by the balanced nature (called static equilibrium) of the forces acting on them. Dynamics is the branch of mechanics that concerns bodies in motion or in a process of change of shape due to actions of forces. A static condition is essentially unchanging with regard to time; a dynamic condition implies a time-dependent action and response.

When external forces act on a body, two things happen. First, internal forces that resist the actions of the external forces are set up in the body. These internal forces produce stresses in the material of the body. Second, the external forces produce deformations, or changes in shape, of the body. Strength of materials, or mechanics of materials, is the study of the properties of material bodies that enable them to resist the actions of external forces, of the stresses within the bodies, and of the deformations of bodies that result from external forces. Taken together, the topics of applied mechanics and strength of materials are often given the overall designation of structural mechanics or structural analysis. This is the fundamental basis for structural investigation, which is essentially an analytical process. On the other hand, design is a progressive refining process in which a structure is first generally visualized; then it is investigated for required force responses and its performance is evaluated; finally—possibly after several cycles of investigation and modification—an acceptable form is derived for the structure.

Strength

Strength is probably the most obvious requirement for a structure. Even though it is stable, the plank is not strong enough to hold the weight of ten people. This has to do partly with the material—if the plank were made of steel, it might do the job. It also has to do with the form and orientation of the plank cross section—if the wood plank were turned on its edge, like a floor joist, it would probably also support ten people. Material strength often depends on the type of stress that the material must sustain. Steel is adaptable and capable of major resistance to tension, compression, shearing, twisting, and bending with equal dexterity. Wood, however, has different strengths depending on the direction of the stress with reference to the wood grain.

Stiffness

All structures change shape and move when subjected to forces. The relative magnitude of these changes determines a quality of the structure called rigidity or stiffness. The degree of stiffness depends on the material of the of the structure, on the configuration of its parts, and—for assemblages—on the arrangement of the assembled members. It may also depend on the connections between parts and on the type of restraint offered by supports. The presence or absence of bracing may also be a factor.