എങ്ങനെയാവണം നിർമ്മാണ മേഖലയിലെ ഘടകങ്ങളുടെ ദൃഢത

STRENGTH OF A MATERIAL

INTRODUCTION

Nowadays the building of structures, machines and other engineering structures is impossible without projects previously drawn. The project consists of the drawings and explanation notes presenting the dimensions of the construction elements, the materials necessary for their building and the technology for their building. The dimensions of the elements and details depend on the characteristics of the used materials and the external forces acting upon the structures and they have to be determined carefully during the design procedure. The structure must be reliable as well as economical during the exploitation process. The reliability is guaranteed when the definite strength, stiffness, stability and durability are taken in mind in the structure. The economy of the construction depends on the material’s expenditure, on the new technology introduction and on the cheaper materials application. It is obvious that the reliability and the economy are opposite requirements. Because of that, the Strength of Materials relies on the experience as well as the theory and is a science in development.  

BASIC CONCEPTS
Strength is the ability of the structure to resist the influence of the external forces acting upon it. Stiffness is the ability of the structure to resist the strains caused by the external forces acting upon it. Stability is the property of the structure to keep its initial position of equilibrium. Durability is the property of the structure to save its strength, stiffness and stability during the exploitation time. Strength of Materials widely relies on the Theoretical Mechanics, Mathematics and Physics. Besides, it is the basis of the other subjects in the engineering practice.

BASIC PROBLEM OF THE STRENGTH OF MATERIALS

The basic problem of the science is development of engineering methods to design the structure elements applying the restraining conditions about the strength, stiffness and stability of the structure when the definite durability as well as economy is given.

REAL OBJECT AND CORRESPONDING COMPUTATIONAL SCHEME

To examine the real object a correct corresponding computational scheme must be chosen. The computational scheme is a real body for which the unessential attributes are eliminated. To choose the correct computational scheme the main hypotheses of Strength of materials have to be introduced.

MAIN HYPOTHESES

A. Hypotheses about the material building the body - Hypothesis of the material continuity The material is uniformly distributed in a whole body volume. - Hypothesis of the material homogeneity All points of the body have the same material properties. - Hypothesis of the material isotropy The material properties are the same in each direction of a body. - Hypothesis of the deformability of the body Contrary to the Theoretical Mechanics studying the rigid bodies, Strength of Materials studies the bodies possessing the ability to deform, i.e. the ability to change its initial shape and dimensions under the action of external forces. The deformations at each point are assumed to be small relative to the dimensions of construction. Then, their influence onto the mutual positions of the loads can be neglected (the calculations will be made about the undeformed construction). - Hypothesis of the elasticity Elasticity is the ability of the body to restore its initial shape and dimensions when the acting forces have been removed. B. Hypotheses about the shape of the body - The basic problem of Strength of Materials is referred to the case of the beam type bodies.

The beam is a body which length is significant bigger than the cross-sectional dimensions. - Hypothesis of the planar cross-sections (Bernoulli’s hypothesis) Each planar cross-section normal to the axis of the beam before the deformation remains planar and normal to the same axis after deformation. 5 C. Hypotheses about the applied forces - The distributed upon a small area loads are assumed to be concentrated. - Principle of Saint-Venant 1 of the deformable body with other setWIf we replace a set of forces acting upon an area  2 of the same body, theWof forces equivalent to the first one, but acting upon the area  2 ,W1 and W , 

containing Wreplacing will influence on the stresses and deformations in the area  1 and W where the influence’s magnitude will correspond to the size of the bigger area between  2 .W - H 6 - Principle of superposition The final magnitude of a quantity considered (stress, strain, displacement, rotation) caused by the set of external forces can be obtained as an algebraic sum of the quantity magnitudes caused by the particular forces composing the set. Principle of hardening A body has a definite shape and dimensions before loading. The same body has the definite shape and dimensions after loading, again, but they are different than the first ones. 7 Rigid body – a body consisting of particles the distances between which do not change. Deformable body – a body consisting of particles the distances between which change.