Testing of materials are necessary for many reasons. The tests are:- 1. Tensile Test 2. Compression Test 3. Ductility Testing 4. Impact Testing 5. Creep Testing 6. Hardness Testing 7. Non-Destructive Testing.
1. Tensile Test:
The main principle of the tensile test is denotes the resistance of a material to a tensile load applied axially to a specimen. It is very important to the tensile test to be considered is the standard dimensions and profiles are adhered to. The typical progress of tensile test can be seen in figure.
Let’s now look at another figure. In this figure, the gauge length (L0) is the length over which the elongation of the specimen is measured. The minimum parallel length (Lc) is the minimum length over which the specimen must maintain a constant cross- sectional area before the test load is applied. The lengths L0, Lc. Li and the cross- sectional area (A) are all specified in BS 18.
The elongation obtained for a given force depends upon the length and area of the cross-section of the specimen or component, since-
Elongation = Applied Force × L/E × A
where, L = Length, A = Cross-sectional area E = Elastic modulus.
ADVERTISEMENTS:
Therefore if the ratio [L/A] is kept constant (as it is in a proportional test piece), and E remains constant for a given material, then comparisons can be made between elongation and applied force for specimens of different sizes.
Tensile Test Results:
The load applied to the specimen and the corresponding extension can be plotted in the form of a graph, as shown in figure.
From A to B the extension is proportional to the applied load. Also, if the load is removed the specimen returns to its original length. Under these relatively lightly loaded conditions the material is showing elastic properties.
ADVERTISEMENTS:
From B to C it can be seen from the graph that the metal suddenly extends with no increase in load. If the load is removed at this point the metal will not spring back to its original length and it is said to have taken a permanent set. Therefore, B is called “limit of proportionality”, and if the force is increased beyond this point a stage is reached where a sudden extension takes place with no increase in force. This is known as the “yield point” C.
The yield stress is the stress at the yield point; that is, the load at B divided by the original cross-section area of the specimen. Usually, a performer works at 50 percent of this figure to allow for a ‘factor of safety’.
From C to D extension is no longer proportional to the load, and if the load is removed little or no spring back will occur. Due to this relatively greater loads the material is showing plastic properties.
ADVERTISEMENTS:
The point D is referred to as the ‘ultimate tensile strength’ when referred extension graphs or the ‘ultimate tensile stress’ (UTS) when referred to stress-strain graphs. The ultimate tensile stress is calculated by dividing the load at D by the original cross-sectional area of the specimen. Although a useful figure for comparing the relative strengths of materials, it has little practical value since engineering equipment is not usually operated so near to the breaking point.
From D to E the specimen appears to be stretching under reduced load conditions. In fact the specimen is thinning out (necking) so that the ‘load per unit area’ or stress is actually increasing. The specimen finally work hardens to such an extent that it breaks at E.
In general, values of load and extension are of limited use since they apply to one particular size of specimen and it is more usual to plot the stress-stain curve.
Stress and strain are calculated as follows:
Therefore ductility is usually expressed, for practical purposes, as the percentage; Elongation in gauge length of a standard test piece at the point of fracture when subjected to a tensile test to destruction.
The increase in length is determined by fitting the pieces of the fractured specimen together carefully and measuring the length at failure.
Increase in length (elongation) = Length at failure – Original length
2. Compression Test:
Because of the presence of submicroscopic cracks, brittle materials are often weak in tension, as tensile stress tends to propagate those cracks which are oriented perpendicular to the axis of tension. The tensile strengths they exhibit are low and usually vary from sample to sample.
These same materials can never be quite strong in compression. Brittle materials are chiefly used in compression, where their strengths are much higher. A schematic diagram of a typical compression test is shown in figure.
Figure shows a comparison of the compressive and tensile strengths of gray cast iron and concrete, both of which are brittle materials.
Because the compression, it increase the cross-sectional area of the sample, necking never occurs. Extremely ductile materials are seldom tested in compression because the sample is constrained by friction at the points of contact with the plants of the apparatus. This constraint gives rise to a complicated stress distribution which can only be analyzed in an approximate fashion.
3. Ductility Testing:
The percentage elongation, as determined by the tensile test is a measure of ductility test can also be performed by is a simple bend test.
There are several ways in which this test can be done, as shown in figure. The test chosen will depend upon the ductility of the material and the severity of the test required.
This also applies to the following tests:
i. Close Bend Test:
The specimen is bent over on itself and flattened. No allowance is made for spring back, and the material is satisfactory if the test can be completed without the metal tearing or fracturing.
ii. Angle Bend Test:
The material is bent over a former and the nose radius of the former and the angle of bend (θ°) are fixed by specification. Again no allowance is made for spring back.
iii. 180° Bend Test:
This is a development of the angle bend test using a flat former as shown. Only the nose radius of the former is specified.
4. Impact Testing (Toughness Testing):
Impact tests consist of striking a suitable specimen with a controlled blow and measuring the energy absorbed in bending or breaking the specimen. The energy value indicates the toughness of the material under test.
Below figure shows how a piece of high carbon steel rod will bend when in the annealed condition, after hardening and tempering, the same piece of steel will fracture when hit with a different hammer.
There are several types of the impact tests and the mostly famous type is the Izod test.
In the Izod test, a 10mm square, notched specimen is used, it is preferred to use a specimen that have a more than one or two and even three notched in the same specimen. The striker of the pendulum hits the specimen with a kinetic energy of 162.72 J at a velocity of 3.8 m/s. Figure shows details of the specimen and the manner in which it is supported.
Since test use a notched specimen, useful information can be obtained regarding the resistance of the material to the spread of a crack which can originate from a point of stress concentration such as sharp corners, undercuts, sudden changes in section, and machining marks in stressed components. Such points of stress concentration should be eliminated during design and manufacture. Izod test suitable for room and high temp. Angle of (α): α < 90°, normally 90°.
It needs adjustment in location of specimen. A second type of impact test is the Charpy test. While in the Izod test the specimen is supported as a cantilever, but in the Charpy test it is supported as a beam. It is struck with a kinetic energy of 298.3 J at a velocity of 5m/s. The Charpy impact test is use for testing the toughness of polymers. Figure shows details of the Charpy test- manner in which it is supported.
Charpy test is better for low temperature test (-29°C strike to – 186°) α < 90° , normally 160°, α = angle of strike.
The Effect of Temperature on the Materials Mechanical Properties:
i. The embrittlement of low-carbon steels at refrigerated temperatures, and hence they are unsuitable for use in refrigeration plant and space vehicles.
ii. The nature of ductile material to behave as brittle material is called notch sensitivity. It cause due to stress concentration, and lowers the properties of material.
iii. The impact test is also useful as a production tool in comparing manufactured materials with others which have shown satisfactory in service.
iv. Steels, like most other BCC metals and alloys, absorb more energy when they fracture in a ductile fashion rather than in a brittle fashion.
v. Due to this the impact test is often used to assess the temperature of the transition from the ductile to brittle state which occurs as the temperature is lowered.
vi. The transition temperature is also dependent on the shape of the notch in the specimen. For identical materials, the sharper the notch, the higher the transition temperature.
5. Creep Test:
Creep testing is done in the tensile mode, and the type of test-piece used is similar to the normal tensile test-piece. Generally, creep testing is carried out under constant- load conditions and utilizes dead weights acting through a simple lever system.
In the creep testing an extensometer readings are noted at regular time intervals until the required amount of data has been obtained, or until the test-piece fractured, depending on whether the object of the test is to determine the creep rate or to determine the total creep strain.
One of the difficulties in creep testing is that a single test take a very long time to complete (10000 hours is 417 days), and there are serious difficulties in attempting to extrapolate from the results of comparatively short-term tests to evaluate the probable behavior of a material over a 10 or 20 year period of service.
Creep is sensitive to both the applied load and the testing temperature, as shown in figure- increasing stress raises the level of the creep curve, and increasing temperature, which accelerates recovery processes, increase the creep rate.
Creep deformation strength: highest stress that a material can bear for a specific duration at a certain temperature without excessive deformation which is predecided.
Creep rupture strength- Highest stress that a material can bear for a specific duration at a certain temp, without rupture.
6. Hardness Testing:
It is done by indentation. A hard indenter is pressed into the specimen by a standard load, and the magnitude of the indentation (either area or depth) is taken as a measure of hardness. Hardness tests are in practice used for assessing material properties because they are quick and convenient. The most well known hardness tests are Brinell and Rockwell.
i. Brinell Hardness Test:
In this test, hardness is measured by pressing a hard steel ball into the surface of the test piece, using a known load. It is important to choose the combination of load and bail size carefully so that the indentation is free from distortion and suitable for measurement. The relationship of the Brinell hardness [HB] which is between load P (kg), the diameter D (mm) of the hardened ball indenter and the diameter d (mm) of the indentation on the surface is given by the expression-
For different materials, the ratio P/D2 has been standardized in order to obtain accurate and comparative results such as-
K = P/D2
where K is a constant; typical values of K are-
Ferrous metals K = 30
Copper and copper alloys K = 10
Aluminum and aluminum alloys K = 5
Lead, tin and white-bearing metals K = 1
Figure shows how the Brinell hardness value is determined. The diameter of the indentation is measured in two directions at right angles and the average taken. The diameter is measured either by using a microscope scale, or by a projection screen with micrometer adjustment.
To ensure consistent results, the following precautions should be made:
a. To thickness of the specimen should be at least seven times the depth of the indentation to allow unrestricted plastic flow below the indenter.
b. The edge of the indentation should be at least three times the diameter of the indentation from the edge of the test piece.
c. The test is unsuitable for materials whose hardness exceeds 500 HE, as the ball indenter tends to flatten.
d. There are a definite relationship between strength and hardness so it is possible to measure the tensile strength from the hardness test.
Drawback:
a. Sinking effect found in manganese steel and austenitic steel.
b. Piling-up effect- in lead, Sn, Mg
ii. Vickers Hardness Test:
This test is preferable to the Brinell test where hard materials are concerned, as it uses a diamond indenter. (Diamond is the hardest material known-approximately 6000 HB). The diamond indenter is in the form of a square-based pyramid with an angle of 136° between opposite faces. Since only one type of indenter is used the load has to be varied for different hardness ranges. Standard loads are 5, 10, 20, 30, 50 and 100 kg.
It is necessary to state the load when specifying a Vickers hardness number. For example, if the hardness number is found to be 200 when using a 50 kg load, then the hardness number is written as HV (50) = 200.
Hardness number (HD) is calculated by dividing the load by the projected area of the indentation-
Where, P is the load in Kg and d (mm) is the diagonal of the impression made by the indenter made by the diamond.
iii. Rockwell Hardness Test:
The Rockwell test is used in industry as it is quick, simple and direct reading. Universal electronic hardness testing machines are now used at large scale which, at the turn of a switch, can provide either Brinell, Vickers Or Rockwell tests and show the hardness number as a digital readout automatically.
They also give a “hard copy” printout of the test result together with the test conditions and date. In principle the Rockwell hardness test compares the differences in depth of penetration of the indenter when using forces of two different values. That is, a minor force is first applied (to take up the backlash and pierce the skin of the component) and the scale are set to read zero.
Then a major force is applied over and above the minor force and the increased depth of penetration is shown on the scales of the machine as a direct reading of hardness without the need for calculation or conversion tables. The standard Rockwell test cannot be used for very thin sheet and foils and for these the Rockwell superficial hardness test is used.
Shore Scleroscope:
The scleroscope is an instrument that measures the rebound height of a hammer dropped from a certain distance above the surface of the material to be tested. The hammer consist of a weight with diamond indenter attached to it. The scleroscope therefore measures the mechanical energy absorbed by the material when the indenters strikes the surface. The energy absorbed gives an indication of resistance to penetration, which matches out definition of hardness.
The primary use of the scleroscope seems to be in measuring the hardness of large parts of steel, large, rolls, casting and gears. Since the scleroscope can be carried to the work piece, it is useful for testing large surfaces and other components which cannot easily be placed on the testing tables of any other testing machines.
7. Non-Destructive Testing (NDT):
NDT is the method of detection and measurement of properties or condition of materials, structures, machines without damaging or destroying their operational capabilities.
Examples of NDT are: magnetic dust method, penetrating liquid method, ultrasonic test and radiography. All NDTs are used to detect various types of flaws on the surface of material or internal inclusions of impurities and these techniques are also very useful during preventive maintenance and repair. There are few techniques which do not require any special apparatus and are quite simple to handle and only a moderate skill being required.
Some of the applications of NDTs are detecting:
(i) Surface cracks
(ii) Material composition
(iii) Internal inclusions
(iv) Internal voids and discontinuities and
(v) Condition of internal stresses.
Ultrasonic Test:
High frequency ultrasonic (sound) waves are applied to the test piece by a piezoelectric crystal. If the test piece is free from cracks, or flawless, then it reflects ultrasonic waves without distortion.
If there are any flaws in the specimen, the time taken by the ultrasonic waves will be less as the reflection of these waves will be from flaw points and not from the bottom of the specimen.
Cathode ray oscilloscope (CRO) is used to receive the sound signals, whose time base circuit is connected to it. This test is a very fast method used to test aerospace components and automobiles. This test is generally used to detect internal cracks like shrinkage cavities, hot tears, zones of corrosion and non-metallic inclusions.
Liquid-Penetration Test:
This test is used for detection of small defects which are very small to detect with the naked eye. This test is used to detect surface cracks or flaws in non-ferrous metals. This test employs a visible colour contrast dye penetrant technique for the detection of open surface flaws in metallic and non-metallic objects. The penetrants are applied by spraying over the surface of material to be inspected. The excess penetrant is then washed or cleaned. Absorbent powder is then applied to absorb the penetrants in the cracks, voids which reveals the flaws.
This test reveals flaws such as shrinkage cracks, porosity, fatigue cracks, grinding cracks, forging cracks, seams, heat treatment cracks and leaks etc., on castings, weldings, and machined parts, cutting tools, pipes and tubes. If the fluorescent penetrant is used, the developed surface must be examined under ultra violet light to see the presence of defects. This technique is used for non-porous and non-absorbent materials.
Radiography:
Done in/on radiographic film by x-rays to know about crack or flaws in welding etc. to have better results. Radiography technique is based upon exposing the components to short wavelength radiations in the form of X-rays (wave length less than 10-11 cm to about 40 × 10-8 cm) or gamma (γ) rays (wavelength about 0.005 × 10-8 to 3 × 10-8 cm) from a suitable source such as an X-ray tube or cobalt 60.
These tests are used to detect defects such as blow holes, cracks, shrinkage cavities and slag inclusions. These defects are of special importance in components designed to withstand high temperatures and pressure employed in power plant atomic reactors, chemical and pressure vessels and oil refining equipments, because then (i.e., defects) cause stress concentration which may frequently lead to part failure.
In X-ray radiography, the portion of the casting where defects are suspected is exposed to X-rays emitted from the X-ray tube. A cassette containing X-ray film is placed behind and in contact with the casting perpendicular to the rays. During exposure, X-rays penetrate the casting and thus affect the X-ray film.
Since most defects (such as blow holes, porosity, cracks, etc.) possess less density than the sound metal of the casting, they transmit X-rays better than the sound metal does; therefore film appears to be more dark where defects are in line of X-ray beam. The exposed and developed X-ray film showing light and dark areas is termed as Radiograph (or Exograph).
Gamma Radiography:
The principle of detecting defects is same as X-ray radiography. Gamma rays are emitted during the disintegration of radio-active material and like X-rays are electromagnetic radiations. Gamma rays are shorter in wavelength and consequently are most penetrating.
Apparatus necessary for gamma-ray radiography is very simple and less costly than X-ray units. Gamma rays are used for detecting defects in castings thicker than those inspected by X-rays.
Owing to less scatter, gamma rays are most satisfactory than X-rays for examining objects varying thickness, whereas X-rays provide better result for casting of uniform thickness. X-rays are better than gamma rays for detecting small defects in casting sections less than about 50 mm. X-rays method is much more rapid than gamma-ray method, because unlike gamma- ray method, it requires seconds or minutes instead of hours.