Fig. 1.2. The major groups of engineering materials

 

The service properties characterize the behavior of the materials in exploitation. They are service life, reliability (safety), cold and heat resistance, etc.

 

Mechanical Properties

 

1.2.1. Stress and Strain

 

The mechanical properties are of special importance for engineering materials. Knowledge of the mechanical properties allows design and process engineers to determine the limiting loads which a machine part can be exposed to under certain work conditions (load and media) without damage and failure.

The mechanical properties are usually characterized by the following quantities:

- strength, which is a resistance of a material to deformation and failure;

- plasticity, i.e. the ability to withstand permanent deformation, remaining after removal of the deforming forces without failure; low plasticity or its deficiency is called brittleness;

- hardness, which is the capability of external layers of a material to resist plastic deformation upon penetration from side of a harder body with definite shape;

- impact strength, which is the work, required to fracture the specimen by the impact load;

- fatigue strength, which is the resistance to failure under repeating loads a few millions of times.

Deformation, or strain, is the change in size and shape of a solid body due to the influence of applied forces. The Fig. 1.3 illustrates the simplest case of an axial tension for cylindrical body by forces P.

The stress s developed in the body:

 

                                                  (1.1)

where F is the area of body's section which is perpendicular to the forces P.

 

Fig. 1.3. Axial tension of cylindrical body

 

Strain (deformation) may be either elastic or plastic. Elastic deformation (Fig. 1.4b) disappears completely when the external force causing it is removed. Elastic strain does not lead to any appreciable changes in the structure and properties of material; the applied load causes only a slight relative displacement of the atoms. Plastic deformation (Fig. 1.4 d) leads to the slip of crystal parts relatively one another and to change of specimen's shape. When the load is removed, the shape, structure and properties of the body are not restored, the body acquires residual deformation.

Fig. 1.4. Elastic and permanent strain in shear:

a - unstressed crystal; b - elastic strain; c - elastic and plastic strain; d - plastic (residual) strain

 

The main purpose of mechanical tests is to get knowledge about machine parts' workability under the certain conditions of work. Due to this reason mechanical properties are determined in static and dynamic tests, at variable loads, low and elevated temperatures, in vacuum and chemically active media. Static tests include tension, compression, bending, shear, torsional and hardness ones (Fig 1.5). These tests are carried out by slowly increasing load P and are referred to as static tests.

Fig. 1.5. Schemes of tests: a - tensile, b - compressive, c - bending, d - shear and e - torsional tests

 

1.2.2. Tension Tests

 

This type of tests is widely used for structural steels, cast irons, nonferrous metals, their alloys as well as nonmetallic materials.

Tension tests are conducted on standard specimens (mainly of round or square cross section). Testing machine has an instrument that records the stress - strain diagram. The applied load P is plotted against the elongation D l of the specimen.

Fig 1.6 illustrates the specimen before and after the tests as well as stress-strain diagram. After the tests the length of the specimen increases, and its cross section decreases. The local reduction of cross section is formed on the specimen. It is referred to as necking. The diameter d₁ is measured at this spot, and then the cross-sectional area F₁ is calculated.

The diagram has typical points A, B, C, D, E, F, which allow us to specify the characteristics of metal resistance to deformation. These characteristics are known as stresses, expressed in megapaskals (MPa).

 

                          а                             b                                c

Fig. 1.6. (a) The Specimen for tests, (b) stress-strain diagram obtained after tension test

of a ductile metal; (c) principle of a conventional yield point determination

 

We can see, that the relationship between the stress and strain remains linear up to the point A, or up to load P_p. According to Hooke’s law of proportionality, stress s and relative strain e=Dl/l₀ may be written as the relationship:

                                                     (1.2)

where E is the constant of proportionality, or modulus of elasticity, or Young’s modulus, MPa.

 

The modulus of elasticity equals the tangent of the angle that line OA makes with the axis of abscissas. For the most typical metals the magnitude of this modulus ranges between
4.5×10⁴ MPa for magnesium and 40.7×10⁴ MPa for tungsten. The modulus of elasticity depends only slightly on the structure of the metal and is determined by the forces of the interatomic bonds.

Taking into consideration the magnitude of the load P_p, the limit of proportionality is defined as:

                                            (1.3)

 

At the beginning of the test up the to point B or load P_e deformation is only elastic one. It means that deformation disappears completely when load P is removed. The stress calculated for the load P_e is called elastic limit:

                                               (1.4)

 

The elastic limit is frequently defined as the stress at which residual strain reaches a certain value (from 0.005 to 0.05 %of the initial gauge length of the specimen). Hence, the elastic limits are denoted by s_0.005, s_0.01, s_0.05.

At an increase in load above P_p the relationship between the load and the elongation deviates and can no longer be linear. The straight line on the stress-strain diagram becomes a curve and, a horizontal step CD is sometimes observed at a certain load P_y. This step indicates that the metal is elongated (yields) without any increase in the load. The stress corresponding to the load P_y is called the yield limit or yield point:

                                                 (1.5)

 

Yield step CD is observed only in testing of high plastic metals. For brittle materials the conventional yield limit is defined as the stress at which the specimen receives permanent elongation equaled to 0.2 percent of the initial gauge length lo (Fig. 1.6 c). It is denoted by s_0.2.

The quantifies s_P, s_e, s_0.05 and s_y(s_0.2) characterize the resistance of a material to small plastic deformation. A further increase in load leads to more essential deformation of the whole volume of the metal. The stress corresponding to the maximum load preceding failure of the specimen is called the ultimate strength or tensile strength (the point E):

 

                                                      (1.6)

 

In ductile metals, deformation is concentrated at one part of the cross-section starting with the stress s_u. A localized reduction of the cross-sectional area, called necking, appears on the specimen, the load drops and, at a certain moment, failure occurs.

In determining s_p, s_y and s_u the corresponding loads P_p, P_y and P_u were considered to be applied to the initial cross-sectional area F₀ of the specimen. Hence, we don't take into consideration the substantial reduction in cross-sectional area of the specimen in testing, thus s_P, s_y and s_U are conditional values. The true, instantaneous, stress S_f is found by dividing the load P at the failure instant by the cross-sectional area of the specimen F₁ at the same instant. A true stress-strain diagram is shown in Fig. 1.7.

Fig. 1.7. True stress-strain diagram

 

The failure stress (true stress):

                                    (1.7)

 

Tension tests are also used to determine ductility indices: the relative elongation, in percents and the reduction in area. The relative elongation d is defined as the ratio of the increase in the gauge length of the specimen after fracture to its initial gauge length l₀, expressed in percent:

                                                 (1.8)

 

The reduction in the area j is defined as the ratio of the reduction in area of the fractured specimen’s necked portion to the initial cross-sectional area, also expressed in percent:

                                     (1.9)

 

1.2.3. Hardness of Metals

Hardness is the property of materials to resist plastic deformation (less frequently, brittle fracture) upon penetration. Due to the rapidity and simplicity of the tests as well as the possibility of assessing the properties of metal without destroying it, hardness measurement has found exceptionally wide application in the quality control of metals and alloys. There are various methods of testing metals for hardness. The most popular of them are briefly described below.

Brinell Hardness Tests. This method is based on forcing a hardened steel ball 2.5, 5.0 or 10.0 mm in diameter, at constant load P into flat surface of metal on the Brinell press
(Fig. 1.8 a). When the load is removed, an impression (indentation) remains on the surface of the metal (Fig. 1.8. b).

The Brinell hardness number BHn, or Brinell hardness HB, is the ratio of the load P, applied to the ball in the test to the area of the impression obtained F:

                                (1.10)

 

Fig. 1.8. Principle of Brinell hardness test: