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strength of materials

strength of materials, measurement in engineering of the capacity of metal, wood, concrete, and other materials to withstand stress and strain. Stress is the internal force exerted by one part of an elastic body upon the adjoining part, and strain is the deformation or change in dimension occasioned by stress. When a body is subjected to pull, it is said to be under tension, or tensional stress, and when it is being pushed, i.e., is supporting a weight, it is under compression, or compressive stress. Shear, or shearing stress, results when a force tends to make part of the body or one side of a plane slide past the other. Torsion, or torsional stress, occurs when external forces tend to twist a body around an axis. Materials are considered to be elastic in relation to an applied stress if the strain disappears after the force is removed. The elastic limit is the maximum stress a material can sustain and still return to its original form. According to Hooke's law, the stress created in an elastic material is proportional to strain, within the elastic limit (see elasticity). In calculating the dimensions of materials required for specific application, the engineer uses working stresses that are ultimate strengths, or elastic limits, divided by a quantity called factor of safety. In laboratories materials are frequently “tested to destruction.” They are deliberately overloaded with the particular force that acts against the property or strength to be measured. Changes in form are measured to the millionth of an inch. Static tests are conducted to determine a material's elastic limit, ductility, hardness, reaction to temperature change, and other qualities. Dynamic tests are those in which the material is exposed to a combination of expected operating circumstances including impact (e.g., a shell against a steel tank), vibration, cyclic stress, fluctuating loads, and fatigue. Polarized light, X rays, ultrasonic waves, and microscopic examination are some of the means of testing materials.

Bibliography

See H. E. Parker, Simplified Mechanics and Strength of Materials (rev. ed. 1961); S. Timoshenko and D. H. Young, Elements of Strength of Materials (5th ed. 1968); M. G. Bassin, Statics and Strength of Materials (4th ed. 1988).

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Strain

 

a pure culture of a species of microorganism in which its morphological and physiological characteristics are studied.

Strains can be isolated from a variety of sources, for example, soil, water, or food, or they can be isolated from a single source at different times. Hence, the same species of bacterium, yeast, or microscopic fungus may have a great many strains, differing from one another in several characteristics, such as sensitivity to antibiotics and capacity to produce toxins and enzymes. Commercial strains of microorganisms developed as a result of selection and used in industry for the microbiological synthesis of proteins (including enzymes), antibiotics, vitamins, and organic acids are much more productive than wild strains.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

strain

[strān]
(biology)
An intraspecific group of organisms that possess only one or a few distinctive traits and are maintained as an artificial breeding group.
(cell and molecular biology)
A population of cells derived either from a primary culture or from a cell line by the selection or cloning of cells having specific properties or markers.
(mechanics)
Change in length of an object in some direction per unit undistorted length in some direction, not necessarily the same; the nine possible strains form a second-rank tensor.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

strain

A change in the form or shape of a body or material which is subjected to an external force.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

strain

Any deformation or deflection in a body caused by stress. The strain is directly proportional to the stress, as a load is applied until the proportional limit is reached. Beyond that point, the strain may increase at a changing rate until the yield stress is reached, but the part will return to its original size and shape when the load is removed and the strain is zero. The body will be deformed if the yield stress is exceeded. The strain is recorded as the change of size over the original size.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved

strain

1
1. Music a theme, melody, or tune
2. a feeling of tension and tiredness resulting from overwork, worry, etc.; stress
3. a particular style or recurring theme in speech or writing
4. Physics the change in dimension of a body under load expressed as the ratio of the total deflection or change in dimension to the original unloaded dimension. It may be a ratio of lengths, areas, or volumes

strain

2
1. a group of organisms within a species or variety, distinguished by one or more minor characteristics
2. a variety of bacterium or fungus, esp one used for a culture
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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