The most common test of building materials is the strength test to destruction. This is partly because strength is a very important property of a building material, even a material in a " non-load-bearing" part of the building; partly because strength tests are comparatively simple to carry out; and partly because they offer a guide to other properties, such as durability.

The strength of a ductile material such as steel, aluminum, or plastics is usually determined by applying a tensile load.?A?compression test is used for brittle materials such as concrete, stone, and brick because their tensile strength is low and thus harder to measure accurately.

The method of testing and the dimensions of the test pieces are laid down in the appropriate standards published by the American Society for Testing Materials (ASTM), the British Standards Institution (BSI), the Standards Association of Australia (SAA), etc.

The size and shape of the test specimen are particularly important for brittle materials because they influence the number of flaws that are likely to occur in the test specimen. For concrete tests, the standard American and Australian test specimen -a cylinder 150 mm (6 in. ) in diameter and 300 mm (12 in. ) long gives a lower result than the standard British test specimen a 150-mm cube because the former contains more concrete.

The speed of testing is also specified.?A passage of time is?required for both plastic deformation and the formation of cracks, and?a faster rate of testing thus gives a higher result.

?For tests on concrete and timber, it is necessary to specify moisture content because this affects the strength.

A test on a single specimen is unreliable because we do not know whether it is an average test specimen or whether it has fewer or more than the average number of minute flaws. Standard specifications lay down how many specimens shall be tested and how they are to be selected.

Tests of factory-made materials carried out by the manufacturer are usually accepted by the user of the building material unless he has reason to doubt their veracity. Since concrete is made on the building site or brought from a ready-mix concrete plant, its testing becomes the responsibility of the building contractor. This is therefore a more frequent testing activity than that for other materials. Concrete cylinders or cubes are normally tested in a hydraulic press, which may be used exclusively for this purpose. A universal machine for tension, compression, and bending is a more expensive machine based on the same principle.

Timber differs from other building materials in that it is produced from growing trees and is thus more variable. Cut timber from virgin forests may consist of a variety of different species. Even timber cut from a planted forest containing trees of the same species all planted at the same time may show appreciable variation between pieces because of knots, or other flaws.

A substantial proportion of timber is used in domestic construction where it is not highly stressed; in such cases, "visual?grading" (that is, merely looking at it) may be sufficient. Because of the imperfections in individual pieces, a stress grading" is usually more reliable than even accurate testing of selected test pieces. A?stress grading machine tests every individual piece of timber by a method that is very fast and relatively cheap. The machine is based on an empirical relation between the strength and the deflection of timber. Each piece of timber is deflected ( but not stressed to its limit) at several points along its length, and the deflection category marked by means of a spot of dye. The timber is then classified visually by its color markings.

The strength of metals is reduced if they are repeatedly loaded alternately in tension and in compression. This is called repeated loading if it is applied several hundred or thousands of times, and fatigue loading if it is applied millions of times. Fatigue loading is a major problem in machines but rarely in buildings. Wind loads, however, can cause repeated loading in roof structures. There are special machines for testing the strength of materials under repeated loading.

Other special tests are for ductility and for hardness. Ductility is tested by bending a bar around a pin over a wide angle. Hardness is tested by indentation with a diamond or a hardened steel ball. The hardness test is carried out only if an accurate result is required because there is a good correlation between the tensile strength test and the various hardness tests for metals. If the tensile strength has been tested, then the hardness of the metal can be deduced from that with reasonable tolerance.

The toughness of a metal can also be deduced from the tension test. Toughness is defined as the energy required to break a material.?Energy is force multiplied by distance, that is, the integral of force in relation to length, or the area contained under a force-deformation curve. Stress is force per unit area, and strain is deformation per unit length, so that the area contained under the stress-strain diagram represents the energy per unit volume. The greater the area contained?under a stress-strain curve up to failure, the greater the toughness of the material. Consequently, ductile materials that deform plastically are much tougher than brittle materials that show little plastic deformation.