Properties of engineering materials and methods of testing them

While using engineering materials in practice we must know their properties because they affect manufacture and application of materials. All engineering materials have definite characteristics which determine their abilities to assume external loads because of which materials change their shape. When a metal is treated or when machine parts and tools are in the process of work¹ the metals which they are made of are subjected to the influence of external forces. These forces are called "loads" and may have different characteristics: according to their value they may be small or large; according to the duration and character of their action they may be constant and impact. According to the influence of the loads upon the metal causing different changes of its shape, loads are distinguished as compression, tensile, torsional, shearing, and bending ones. By testing a metal under a load one can define what mechanical properties it has. In other words, one can determine strength, elasticity, plasticity, hardness and other properties of the metal. In order to have a clear conception² of the metal properties it is subjected to tests on special devices and machines. The determination of these properties is made in the laboratory using a specimen from the metal to be tested. Let us consider some of the mechanical properties of metals, such as:

Strength of metals is the property of hard materials to be subjected to the influence of external forces without incurring damage and without changing their shape. The ultimate tensile strength of a material is that unit stress developed in the material by maximum slowly applied load that the material can resist without rupturing in a tensile test. A stress is the force within a body which resists deformation due to an externally applied load. If this load acts upon a surface of unit area, it is called a "unit force", and the stress resisting it is called a "unit stress". An external force acting upon an elastic material, the material is deformed and the deformation is in proportion to the load. This distortion or deformation is called "strain".

Special machines, called "rupture machines", are used to test metals for strength. Fig. 1 shows one of these machines. When testing a specimen, the upper clamp remains fixed and the lower one is being slowly lowered, thus causing the extension of the specimen. The load upon the specimen may be easily determined at any moment by means of pointer indications on the dial.

Elasticity is the ability of a material to change its shape under the influence of external loads and return to its original form upon removal of the loads. All materials are elastic but the range of elasticity varies for different materials. Elasticity is evaluated by means of the modulus of elasticity. The modulus of elasticity is the ratio of the unit stress S to the unit deformation Δl within the proportional limit of the material to be tested.

E=S/ Δl

For determining the elasticity of metals a rupture machine may be used.

Fig. 1. Rupture Machine:

1 - stand; 2 - clamps; 3 - specimen to be tested; 4 – dial

Plasticity is that property of a material when under the influence of loads, specimens of different materials may elongate while their cross-section decreases. Plasticity is the opposite of elasticity. So, plasticity is the ability of material to change its form without breaking under the influence of load and preserve this changed form after removal of the load. For determining the plasticity of metals a rupture machine may be used too.

Hardness of material is the property to resist deformation under applied load. Hardness is the most important mechanical property of metals. Hardness may also be defined as the ability of metals to resist penetration of other harder materials or as resistance to wear.

1. are in the process of work – обрабатываются

2. to have a clear conception – иметь ясное представление

Дата добавления: 2014-12-27