Impact testing

7.1.4 Impact testing

Tensile specimens can also give information on the type of fracture exhibited. Usually in polycrystalline

A material may have a high tensile strength and yet metals transgranular fractures occur (i.e. the fracture

be unsuitable for shock loading conditions. To deter- surface cuts through the grains) and the ‘cup and cone’

mine this the impact resistance is usually measured by type of fracture is extremely common in really duc-

means of the notched or un-notched Izod or Charpy tile metals such as copper. In this, the fracture starts

impact test. In this test a load swings from a given at the centre of the necked portion of the test piece

height to strike the specimen, and the energy dissi- and at first grows roughly perpendicular to the tensile

pated in the fracture is measured. The test is partic- axis, so forming the ‘cup’, but then, as it nears the

ularly useful in showing the decrease in ductility and outer surface, it turns into a ‘cone’ by fracturing along

impact strength of materials of bcc structure at mod-

a surface at about 45 ° to the tensile axis. In detail erately low temperatures. For example, carbon steels the ‘cup’ itself consists of many irregular surfaces at

have a relatively high ductile–brittle transition tem- about 45 ° to the tensile axis, which gives the fracture a

perature (Figure 7.1c) and, consequently, they may be fibrous appearance. Cleavage is also a fairly common

used with safety at sub-zero temperatures only if the type of transgranular fracture, particularly in materi-

transition temperature is lowered by suitable alloy- als of bcc structure when tested at low temperatures.

ing additions or by refining the grain size. Nowadays, The fracture surface follows certain crystal planes (e.g.

increasing importance is given to defining a fracture f1 0 0g planes), as is shown by the grains revealing

toughness parameter K c for an alloy, since many alloys large bright facets, but the surface also appears gran-

contain small cracks which, when subjected to some ular with ‘river lines’ running across the facets where

critical stress, propagate; K c defines the critical com- cleavage planes have been torn apart. Intercrystalline

bination of stress and crack length. Brittle fracture is fractures sometimes occur, often without appreciable

discussed more fully in Chapter 8. deformation. This type of fracture is usually caused by a brittle second phase precipitating out around the

7.1.5 Creep testing

grain boundaries, as shown by copper containing bis- muth or antimony.

Creep is defined as plastic flow under constant stress, and although the majority of tests are carried out under

7.1.3 Indentation hardness testing

constant load conditions, equipment is available for reducing the loading during the test to compensate

The hardness of a metal, defined as the resistance to for the small reduction in cross-section of the spec- penetration, gives a conveniently rapid indication of

imen. At relatively high temperatures creep appears to

200 Modern Physical Metallurgy and Materials Engineering tension or compression, but all involve the same prin-

ciple of subjecting the material to constant cycles of stress. To express the characteristics of the stress sys- tem, three properties are usually quoted: these include (1) the maximum range of stress, (2) the mean stress, and (3) the time period for the stress cycle. Four dif- ferent arrangements of the stress cycle are shown in Figure 7.4, but the reverse and the repeated cycle tests (e.g. ‘push–pull’) are the most common, since they are the easiest to achieve in the laboratory.

The standard method of studying fatigue is to pre- pare a large number of specimens free from flaws, and to subject them to tests using a different range of stress, S, on each group of specimens. The num-

Figure 7.3 Typical creep curves . ber of stress cycles, N, endured by each specimen at

a given stress level is recorded and plotted, as shown occur at all stress levels, but the creep rate increase

in Figure 7.5. This S–N diagram indicates that some with increasing stress at a given temperature. For the

metals can withstand indefinitely the application of a accurate assessment of creep properties, it is clear that

large number of stress reversals, provided the applied special attention must be given to the maintenance of

stress is below a limiting stress known as the endurance the specimen at a constant temperature, and to the mea-

limit. For certain ferrous materials when they are used surement of the small dimensional changes involved.

in the absence of corrosive conditions the assumption This latter precaution is necessary, since in many mate-

of a safe working range of stress seems justified, but rials a rise in temperature by a few tens of degrees is

for non-ferrous materials and for steels when they are sufficient to double the creep rate. Figure 7.3, curve a,

used in corrosive conditions a definite endurance limit shows the characteristics of a typical creep curve and

cannot be defined. Fatigue is discussed in more detail following the instantaneous strain caused by the sud-

in Section 7.11.

den application of the load, the creep process may be divided into three stages, usually termed primary or

7.1.7 Testing of ceramics

transient creep, second or steady-state creep and ter- Direct tensile testing of ceramics is not generally tiary or accelerating creep. The characteristics of the

favoured, mainly because of the extreme sensitivity of creep curve often vary, however, and the tertiary stage

ceramics to surface flaws. First, it is difficult to apply of creep may be advanced or retarded if the tempera-

a truly uniaxial tensile stress: mounting the specimen ture and stress at which the test is carried out is high

in the machine grips can seriously damage the surface or low respectively (see Figure 7.3, curves b and c).

and any bending of the specimen during the test will Creep is discussed more fully in Section 7.9.

cause premature failure. Second, suitable waisted spec- imens with the necessary fine and flawless finish are

7.1.6 Fatigue testing

expensive to produce. It is therefore common practice to use bend tests for engineering ceramics and glasses.

The fatigue phenomenon is concerned with the prema- (They have long been used for other non-ductile mate- ture fracture of metals under repeatedly applied low

rials such as concretes and grey cast iron.) In the three- stresses, and is of importance in many branches of

and four-point bend methods portrayed in Figure 7.6, a engineering (e.g. aircraft structures). Several differ-

beam specimen is placed between rollers and carefully ent types of testing machines have been constructed

loaded at a constant strain rate. The flexural strength in which the stress is applied by bending, torsion,

at failure, calculated from the standard formulae, is

Figure 7.4 Alternative forms of stress cycling: (a) reversed; (b) alternating (mean stress 6D zero), (c) fluctuating and (d) repeated .

Mechanical behaviour of materials 201 centre of an electric furnace heated by SiC elements.

A similar type of hot-bend test has been used for the routine testing of graphite electrode samples and gives a useful indication of their ability to withstand accidental lateral impact during service in steel melting furnaces.

Proof-testing is a long-established method of test- ing certain engineering components and structures. In a typical proof test, each component is held at a certain proof stress for a fixed period of time; load- ing and unloading conditions are standardized. In the case of ceramics, it may involve bend-testing, inter- nal pressurization (for tubes) or rotation at high speed (‘overspeeding’ of grinding wheels). Components that withstand the proof test are, in the simplest analysis, judged to be sound and suitable for long-term service at the lower design stress. The underlying philosophy has been often questioned, not least because there is a

Figure 7.5 S–N curve for carburized and decarburized iron . risk that the proof test itself may cause incipient crack- ing. Nevertheless, proof-testing now has an important role in the statistical control of strength in ceramics.