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Materials Testing

Measurement of the characteristics and behaviour of such substances as metals, ceramics, or plastics under various conditions. The data thus obtained can be used in specifying the suitability of materials for various applications—e.g., building or aircraft construction, machinery, or packaging. A full- or small-scale model of a proposed machine or structure may be tested. Alternatively, investigators may construct mathematical models that utilize known material characteristics and behaviour to predict capabilities of the structure.

Materials testing breaks down into five major categories: mechanical testing; testing for thermal properties; testing for electrical properties; testing for resistance to corrosion, radiation, and biological deterioration; and nondestructive testing. Standard test methods have been established by such national and international bodies as the International Organization for Standardization (ISO), with headquarters in Geneva, and the American Society for Testing and Materials (ASTM).


Materials testing tests the load capacity and accuracy of materials in different environmental conditions.
Through material characteristic values it delivers a clear definition of the material properties, which in turn allows for the comparison between materials. Materials testing is not only performed at research institutes, it also helps companies obtain valuable knowledge for the development of new products, and the improvement of existing products.
The fundamental principle of materials testing is mechanical loading of a material or standard-compliant specimen up to break or up to a certain deformation. The resulting material properties are expressed through material characteristics. and machines, or their components, fail because of fracture or excessive deformation. In attempting to prevent such failure, the designer estimates how much stress (load per unit area) can be anticipated, and specifies materials that can withstand expected stresses. A stress analysis, accomplished either experimentally or by means of a mathematical model, indicates expected areas of high stress in a machine or structure. Mechanical property tests, carried out experimentally, indicate which materials may safely be employed.

Tension, Compression, Bending

When subjected to a tension (pulling apart), a material elongates and eventually breaks. A simple static tension test determines the breaking point of the material and its elongation, designated as strain (change in length per unit length).
A static tensile strength test is commonly used to determine two things: whether a material can withstand a specified load, and, what is the ultimate breaking (tensile) strength of a material. These are not necessarily the same thing. These tests are often used to determine the acceptable working loads of component materials, or entire product assemblies.
It defines the point at which a material will no longer elastically deform (return to original state), plastically deform (does not return to original state), and fail. The ultimate tensile strength of a material will usually far exceed that of the working load of a material. This difference helps to establish a safety margin to offset any variance in the material during manufacture
The most common testing machine used in tensile testing is the universal testing machine. This type of machine has two crossheads; one is adjusted for the length of the specimen and the other is driven to apply tension to the test specimen. There are two types: hydraulic powered and electromagnetically powered machines.

Dynamic testing, Durability

Materials that survive a single application of stress frequently fail when stressed repeatedly. This phenomenon, known as fatigue, is measured by mechanical tests that involve repeated application of different stresses varying in a regular cycle from maximum to minimum value. 
“Fatigue” testing gives data to predict the in-service life of materials.  stresses acting upon a material in the real world are usually random in nature rather than cyclic. 
It usually uses higher speeds and frequencies, and up to millions of cycles when compared to static testing.
Material fatigue involves a number of phenomena, among which are atomic slip (in which the upper plane of a metal crystal moves or slips in relation to the lower plane, in response to a shearing stress), crack initiation, and crack propagation. Thus, a fatigue test may measure the number of cycles required to initiate a crack, as well as the number of cycles to failure.
A cautious designer always bears the statistical nature of fatigue in mind, for the lives of material specimens tested at a common stress level always range above and below some average value. Statistical theory tells the designer how many samples of a material must be tested in order to provide adequate data; it is not uncommon to test several hundred specimens before drawing firm conclusions.

Drop and Pendulum
Charpy, Izod, Components

Many materials, sensitive to the presence of flaws, cracks, and notches, fail suddenly under impact. The most common impact tests employ a swinging pendulum to strike a notched bar; heights before and after impact are used to compute the energy required to fracture the bar and, consequently, the bar’s impact strength. In nonmetal tests, however, the striking hammer falls vertically in a guide column, and the test is repeated from increasing heights until failure occurs.
Some materials vary in impact strength at different temperatures, becoming very brittle when cold. Tests have shown that the decrease in material strength and elasticity is often quite abrupt at a certain temperature, which is called the transition temperature for that material. Designers always specify a material that possesses a transition temperature well below the range of heat and cold to which the structure or machine is exposed. Thus, even a building in the tropics, which will doubtless never be exposed to freezing weather, employs materials with transition temperatures slightly below freezing.