2. Analyze the principal notions of the text.

Statics. Strenth of Materials

Strenth of materials is a branch of engi­neering that deals with the behavior of materials in response to applied forces. Strength-of-materials data are widely used in designing machinery, buildings, bridges, dams, and other structures. The parts of a bridge, for example, must be strong enough to withstand the various kinds of forces placed on the bridge by traffic, wind, and other agents. These applied forces are resisted by internal forces, called stresses, in the beams, columns, cables, and other parts of the bridge, and in the bolted, riveted, or welded joints be­tween parts. As a result, the structural parts are deformed in various ways, including stretching, compressing, shearing, twisting, and bending. Such deformations, or changes of shape, are known as strains.

If the stresses and strains in some parts are too great, the parts

would break or give way, possibly leading to the collapse of the bridge.

Hence bridge designers rely on strength-of-materials data to specify suitable materials for the various parts of the bridge. High-tensile-strength steel, for example, would be specified for cables, a fatigue-resistant steel for beams and plates subject to repeated flexing, and high-compressive-strength concrete for piers and abutments.

Stress. Stress in a body can be defined as resistance to external forces. It is measured in terms of the force exerted per unit of area. Generally, the force is given in pounds and the resisting area in square inches. Consequently, stress usually is expressed in pounds per square inch (psi). Sometimes it is expressed in kilo-pounds per square inch, which is

commonly ab­breviated as kips.

Stress is produced in all bodies that are sub­jected to external forces. The three main kinds of stress are tensile stress, compressive stress, and shearing stress. Other stresses are similar to these basic stresses or are a combination of them. For instance, a bending stress in a beam actually is a combination of tensile, compressive, and shearing stresses.

Tensile Stress. Consider a vertical rod of uni­form cross-sectional area A subjected to equal and opposite pulls F at the ends of the rod.

These pulls tend to lengthen the rod, which is said to be in tension. The rod is in equilibrium under the action of the two external forces. Thus a cross section A of the rod is acted on by a down­ward force equal to the force F exerted by the part of the rod below it and an upward force equal to the force F exerted by the part of the rod above it. The tensile stress at that cross sec­tion in the rod is defined as

st = F/A

where st is the tensile stress in pounds per square inch, F is the external tensile force (or applied load) in pounds, and A is the stressed cross-sectional area in square inches. A tensile stress is also called a normal stress because the external and internal forces are perpendicular (normal) to the cross-sectional area under con­sideration.

Compressive Sfress is caused by equal and opposite pushes F at the ends of the rod. These pushes tend to shorten the rod, which is said to be in compression. The compressive stress in the rod is s_c = F/A

Shearing Sfress occurs because of the sliding of the above and below layers of the solid. The shearing stress is defined as s_s = F/A.

Strain is defined as the deformation of a body that results from an applied force. The deformation may be a change of length, a change of cross-sectional area, a change of volume, or a change of angle as when a rectangular face of a solid is distorted into a parallelogram.

Total strain is the total change in the size or shape of a body. Unit strain

is the total change in the size or shape of the body divided by its original size or shape. When the term "strain" alone is used, it invariably means unit strain. Each type of stress is associated with a cor­responding type of strain.

Notes. inch ( дюйм)

psi – pound (фунт) per square inch

kip – kilo-pound per square inch