A – Brinell press; b – principle of testing; 1 – table for specimen; 2 – fly-wheel; 3 – ball (indentor); 4 – electrical motor; 5 – weight

 

The ball diameter D and the constant load P are selected to suit the composition of the metal, its hardness and thickness of the test specimen. The load P=300D² is taken for steel and cast iron;

 P=100D² for copper and its alloys;

 P=25D² for very soft metals (aluminium, babbitts), where P is taken in N, D in mm.

So maximum one is P=300·100=30000 N (=3000 kgf=3t). Minimum one is P=25·6,25=
=156 N (=15,6 kgf=0,0156t)

In practice the diameter of impression is measured and used to find the Brinell hardness directly from specially compiled tables. When hardness number is calculated by formula 1.9 (in kgf/mm²) it is denoted HB250, HB320; when calculations are made in MPa it is denoted HB2500MPa, HB3200MPa.

Disadvantages of the Brinell tests are:

- the ball may be deformed, when the hardness of testified material exceeds
HB 4500 MPa, or HB450;

- significant size of the impression;

- the test may be used only for specimens with comparatively high thickness
(t>2...6 mm). But now very hard balls from WC (tungsten carbide) may be used and scale of Brinell Hardness may be prolongeted.

Rockwell Hardness Tests. In Rockwell tests the hardness is determined by the depth of the indentation, without taking into consideration its diameter.

The indenter, or penetrator, is a diamond cone (brale) with an apex angle of 120°, or hardened steel ball 1.5875 mm in diameter. The diamond cone is used for testing hard metals, and the steel ball is used for softer ones. The cone or ball is indented by two consecutive loads
(Fig. 1.9): the minor P₀ equal to 100 N and additional load P₁ equal to:

- 900 N for ball (B scale) - soft-materials;

- 1400 N for cone (C scale) - middle hardness ones;

- 500 N for cone (A scale) - very hard materials.

    

Fig. 1.9. Principle of Rockwell hardness test:

a- Rockwell hardness machine; b – principle of testing; 1 – fly wheel; 2 – table for specimen;

3 – indenter; 4 – scale; 5 – level; 6 – weight

 

Rockwell hardness is measured in conventional units. The hardness number is read directly on the dial of the instrument. Rockwell hardness is indicated: HRA40, HRC35, HRB70. The Rockwell hardness number is conventionally calculated by the formulas:

 

,                                           (1.11)

,                                                  (1.12)

where e is found by the formula:

,                                                (1.13)

where h₀ is depth of penetration of the indenter when preliminary load P₀ is applied;

h₁ is depth of penetration of the indenter due to the total load P.

 

Rockwell method doesn't have the disadvantages typical for Brinell method: soft and hard metals may by tested; the impression size is small; finished parts may be tested without destruction.

Vickers Hardness Tests (diamond pyramid tests). This method is used for measuring hardness of thin components (t=0.25…3.0 mm) or of thin and hard surface layers as well as of very soft metals and alloys.

It consists in forcing a square-based diamond pyramid (with angle of 136° between opposite faces) into the ground, or even polished surface being tested. A square indentation is obtained
(Fig. 1.10).

Fig. 1.10. Vickers hardness machine (a) and principle of testing (b)

 

The Vickers hardness number HV is the ratio of the load P applied to the diamond pyramid to the permanent indentation:

 

                              (1.14)

where P is the load applied (50, 100, 200, 300, 500, 1000, 1200 N),

a is angle between opposite faces of the pyramid indenter (a=136°),

d is arithmetic average of the two diagonals of the indentation in mm, measured after the load is removed with a micrometer microscope.

 

The thinner the specimen, the less the load required in the Vickers hardness tests.

Vickers and Brinell hardness numbers have the same dimensions (MN/m², MPa) and coincide for materials with hardness up to about 4500 MPa (Fig 1.11).

The minimum thickness of specimen for hardness testing:

- by Brinell method is 2...6 mm;

- by Rockwell method is 0,4...2 mm;

- by Vickers method is 0,25...3 mm.

Fig. 1.11. Comparison of several hardness scales

 

Hardness tests are performed more frequently than any other mechanical test for several reasons:

- they are simple and inexpensive - usually no special specimen need to be prepared;

- the test is nondestructive - the specimen is neither fractured nor excessively deformed; a small indentation is the only deformation;

- other mechanical properties may often be estimated from hardness data, such as tensile strength (Fig. 1.12).

Fig. 1.12. The relationship between hardness and tensile strength for steel, brass, and cast iron

 

1.2.4. Mechanical Properties Determined in Dynamic Tests

 

Many parts and structures in normal operational conditions undergo impact loading which promotes brittle fracture. Susceptibility to brittle fracture is enhanced by increase in grain size of metal structure, loading rate, decrease in temperature and presence of notches. Chemical content also influences on impact strength: it is reduced by increase of carbon content as well as detrimental impurities, such as sulphur, phosphorus, hydrogen, nitrogen and oxygen. Static tests do not reveal the resistance of materials to brittle fracture. Therefore the dynamic tests are used.

The dynamic impact bending tests (GOST 9454-78) have gained the widest application. They are of the most severe type and promote brittle fracture. Notched-bar test specimens are commonly used in these tests. The impact strength KC is determined as a ratio of the work required to fracture the specimen to its cross-sectional area and, hence, represents the specific work required to fracture. The area is gauged at the notched section before fracture (specimens of brittle materials do not require notches). The impact strength is normally denoted (signed) in accordance with the form of the notch (U-notched specimen with the notch radius of 1 mm,
V-notched with the radius of 0.25 mm (Charpy) and crack-notched) as KCU, KCV and KCT correspondently. The work required to get the fracture decreases with the decrease in notch radius.

The pendulum-type machine is used for impact testing (Fig. 1.13). The specimen is placed on two supports of the machine so that the blow of the striker is opposite to the notch. The pendulum of mass G is then raised to height H, from where it is released to fracture the specimen and rise again to height h. The work required to fracture the specimen is:

 

                         (1.15)

where g is intensity of gravity, or free-fall acceleration;

m is the mass of the pendulum;

t is the length of the pendulum rising;

H, h are the heights of the pendulum rising;

a₁, a₂  are the angles of the pendulum rising.

 

Impact strength can be found by the formula:

                                                   (1.16)

where F is the cross-sectional area of the specimen at the notch before fracture.

 

Fig. 1.13. Pendulum-type impact testing machine (a), notched-bar test specimen (b);

principle of the impact test (c): 1 – base; 2 – pendulum (striker); 3 – scale; 4 – specimen;

5 – belt of the brake; 6 – handle of the brake.

 

1.2.5. Fatigue

 

Fatigue is a kind of failure that occurs in structures subjected to dynamic and fluctuating stresses (e.g., bridges, aircraft and machine components). Under these circumstances it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for a static load. The term “fatigue” is used because this type of failure normally occurs after a lengthy period of repeated stress or strain cyclings. Fatigue strength is important property, because fatique practure is a cause of failure in metals, estimated to comprise approximately 90% of all metallic failures. Polymers and ceramics (except for glasses) are also susceptible to this type of failure.

Fatigue failure is brittlelike in nature even in normally ductile metals, in which there is very little (if any) gross plastic deformation associated with failure. The process occurs by the initiation and propagation of cracks. Usually the fracture surface is perpendicular to the direction of an applied tensile stress.

In a fatigue failure the fracture consists of two distinct zones (Fig. 1.14). The first zone (1)has a smooth, rubbed-over surface and is called the fatigue zone. This zone is gradually formed. At first a microcrack (fatigue crack) is initiated. It then develops into a macrocrack due to the repeated action of the forces on the metal. When the fatigue crack occupies a considerable part of the cross-section, failure occurs in the other part. The second, after-fracture zone (2)has a coarse-crystalline surface on brittle metals and a fibrous surface on ductile metals.

 

Fig. 1.14. Fracture obtained in failure: 1 – fatigue zone; 2 – after-fracture zone;

3 – point of initiation of a crack

 

The crack is most often formed at the surface of the first zone The resistance of a metal to failure from cyclic loading is characterized by its endurance limit, i.e. the maximum stress that the metal can withstand without failure when subject to a great number Nof cycles ( Nis stipulated by the specification and may range from 10⁵ to 10⁸ or even higher).

The endurance limit is commonly determined by subjecting a rotating specimen (plain or notched) to flexural loads which developreversing stresses (tension-compression) in a symmetrical cycle where .

At least ten specimens are tested to find the endurance limit. The first specimen is tested at the stress s₁(s₁=0.6s_t for steel), determining the number of cycles N when failure occurs. The stresses s₂, s₃, etc. for the second and subsequent specimens are increased or reduced each time by 20 or 40MPa,depending upon the number of cycles which caused the failure of the first specimen.

Fig. 1.15. Fatigue curve

 

The results of the tests are plotted on a diagram with the coordinates stress s vsnumber of cycles Nin either rectilinear or logarithmic scales (Fig. 1.15). The horizontal portion is the maximum stress at which failure will not occur even at an infinite number of loading cycles. It corresponds to the endurance limit. Endurance limits are expressed in terms of the nominal stresses and denoted by s_-1, MPa.