3. Read and translate the text. Strength Theories

The mechanical properties of structural materials are normally determined by tests which subject the specimen to comparatively simple stress conditions. For example, most of our information concerning the strength of metals was obtained from tensile tests. Similarly, compression tests have been used for studying brittle materials like stone and concrete. In addition, a small amount of information is available concerning the strength of materials under shear loading. However, the strength of materials under yielding will be higher than the stress in simple tension. This result is obtained because the elongation in each direction is decreased by the tension in the perpendicular direction. However, this conclusion is not supported by experiments. Tests of materials under uniform hydrostatic pressure also contradict the theory.

Experiments show that homogeneous materials under uniform compression can withstand much higher stresses and remain elastic. The maximum shear theory gives better agreement with experiments, at least for ductile materials.

The theory assumed that yielding begins when the maximum shear stress in the material becomes equal to the maximum shear stress at the yield point in a tension test.

In machine design the maximum shear theory is usually for ductile materials. The theory is in good agreement with experiments and is simple to apply.

In the maximum energy the quantity of strain energy per unit volume of the material used as the basis for determining failure.

To compare the preceding strength theories let us consider the case of pure shear. For this special case of two dimensional stress the maximum tensile, compressive and shearing stresses are all numerically equal.

To obtain better agreement between theory and experiment, Huber proposed that the total strain energy be resolved into two parts, the strain energy of uniform tension or compression, and the strain energy of distortion. Then he proposed that only the strain energy be used to determine yielding and failure of the material. The condition for yielding based on the distortion energy theory is equivalent to statement that yielding begins when the octathedral shear stress reaches a critical value.

Such criterion is furnished by the strength theory developed by Mohr, in which not only yielding but also fracture is considered. In developing this theory Mohr made use of the graphical representation of the stress conditions on an element of a body by the use of circles.

Mohr’s theory may be recommended in the case of brittle materials, although the assumption that only the outer stress circle must be considered is not always supported by experiments.

4. Find the words and expressions of the active vocabulary in the text. Translate the sentences they are use in.

5. Find the words with the -ed endings in the text. State their origin. Translate them into Russian.

6. Form the adjectives from the nouns using the suffixes -al, -able, -ive:

mechanics, structure, norm, exception, practice, element, compression, critics, graphics.

7. Match the words with their meanings.

criterion	- something you consider likely to be true even though no one has told you directly or even though you have no proof.
quantity	- a quality or feature of something.
assumption	- a scientific test to find out what happens to someone or something in particular conditions.
distortion	- a situation in which something such as a machine or an organ in your body stops working correctly.
stress	- a change in the way something looks, sounds, or behaves so that it becomes strange or difficult to recognize.
experiment	- standards that are used for judging something or making a decision about something
property	- the amount of something
failure	- physical pressure put on something that can make it change its shape or break.