Strength and stress

Part II

If a metal bar with a square cross section 2 in. by 2 in. breaks when we pull it with a force of 200,000 lb, its breaking strength is the same as the unit stress or:

Breaking strength = P/A

= 200,000 lb / 2 x 2 sq in.

= 50,000 lb per sq in.

The deformation that we can allow in the bar depends on its use. Therefore, failure depends upon the purpose for which we use the material. Beams which support a roof can withstand any unit stress that does not break them; but if the beams support a plaster ceiling, they fail at a unit stress that causes plaster cracking.

Designers determine the unit stress that can cause failure for various materials and their uses. However, they don’t design to stress a material to the point where it will fail. Instead, they select a lower unit stress (the allowable unit stress), and this is the maximum which they allow.

In order to determine unit stress engineers divide the force by the original area upon which it acts. Tensile and compressive unit stresses act on the cross-sectional area perpendicular to the direction of the force. Shear unit stresses act on the cross-sectional area parallel to the direction of the force.

Shear unit stress does not act equally over an area, and the engineer must consider the unit stress at the location of the highest unit stress. The action of shear unit stresses is complex compared to that of the axial unit stresses, tension and compression.

Since forces change the cross sections in size, they influence the unit stresses. If a force remains constant, the actual unit stress changes when the cross section changes. Engineers determine unit stress on the basis of the area as it is before any force acts on it.

Notes:

breaking strength предел прочности

shear unit stress единичное напряжение сдвига (среза), единичное касательное напряжение

Ответьте на вопросы к тексту.

1. What is the breaking strength of the metal bar?

2. How is it determined?

3. What does failure depend upon?

4. What unit stress can beams support?

5. What can happen in the case of a plaster ceiling?

6. Which unit stress do designers determine?

7. What do engineers do in order to determine unit stress?

8. How do tensile and compressive unit stresses act?

9. How do shear unit stresses act?

10. Why do forces influence the unit stresses?

11. What takes place if a force remains constant?

12. How do engineers determine unit stress?