The following points highlight the seven main tests conducted for identifying metals and alloys.
1. Tensile Test:
The tensile test is one of the most widely used of the mechanical tests. There are many variations of this test to accommodate the widely differing character of materials such as metals, elastomers, plastics and glasses. The tensile test on a mild steel test piece is described below-
The tensile test is carried out on a bar of uniform cross-section throughout the gauge length. The specimen is mounted in the jaws of a testing machine with which a gradually increasing load can be applied. The extension or elongation of the gauge length is recorded continuously and finally a graph is drawn between the loads and extensions or between the stress and strain; which is of the type shown in Fig. 2.38.
Upto the point M. Hooke’s law holds good and this point is known as “limit of proportionality”. Beyond the point M Hooke’s law is not obeyed although the material remains elastic i.e., strain completely disappears after the removal of load. At the point N elastic limit is reached.
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If the material is loaded or stressed upto this point the material will regain its original shape on the removal of the load. Up to the point P strain increases more quickly than stress, at this point the metal yields. In the mild steel yielding commences immediately and two points P and Q, the upper and lower yield points respectively are obtained.
On further increasing the load slightly, the strain increases rapidly till R when neck or waist is formed. When this point (R) is reached the deformation or extension continues even with lesser load and ultimately/fracture occurs.
The various properties connected with this test are given more elaborately as follows:
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(i) Proportional Limit:
It is the maximum stress at which stress remains directly proportional to strain. The proportional limit is determined from the stress strain curve by drawing straight line tangent at the origin and noting the first deviation of the plot from the line.
The proportional limit has limited engineering significance because of its great dependence upon the precision available for its determination.
(ii) Elastic Limit:
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The elastic limit is the maximum stress which the material can withstand without causing permanent deformation which remains after removal of stress. For engineering usage the elastic limit has little significance.
(iii) Yield Strength:
The yield strength is the stress at which a material exhibits a specified limiting permanent set.
The yield strength of a metal is a property of considerable significance. The tensile yield strength indicates resistance to permanent deformation produced by tensile loads. It is related to resistance to permanent deformation by shearing, bending, compressive, and complex combination of forces.
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Because of this and the ease of its measurement the tensile yield strength is used widely as a factor of design; it is preferable in most instances to the use of tensile strength. The yield strength also is indicative of the ease of forming or shaping metals by mechanical means.
(iv) Yield Point:
The yield point is the stress at which there first occurs a marked increase in strain without an increase in stress. The yield point can be determined by noting the first load at which there is visible increase in the distance between two gauge marks on a tensile specimen. This is conveniently accomplished by checking the length with a pair of dividers.
If an extensometer is used, the length can be observed to increase rapidly without an increase in load. Still a third method is to coat the specimen with lacquer which cracks when the yield point is reached. The yield point most commonly is observed in mild steels, although it has been detected in a few other alloys as well.
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(v) Tensile Strength (Ultimate or Maximum Strength):
It is calculated by dividing the maximum load carried by the specimen during a tension test by the original cross-sectional area of the specimen.
Tensile strength is widely used design factor, although there is more justification for yield strength.
(vi) Rupture Strength:
It is determined by dividing the load at the time of fracture by the original cross-sectional area. If the rupture load is divided by the actual cross-section at the time of fracture, the time rupture strength is obtained.
The rupture strength is of indirect and limited interest to engineers. It provides the terminal point of the stress-strain curve and makes possible a computation of static toughness.
(vii) Elongation:
Elongation of a specimen after fracture may be determined by placing the parts of the broken specimen closely together and holding them in place by a vice. The distance between gauge marks may be measured by means of dividers.
Elongation has considerable engineering significance because it indicates ductility or the ability to deform appreciably without rupture. Ductility is essential in forming operation for metals, where it is desirable to achieve as much deformation as possible in one operation without danger of causing rupture. Ductility is also essential to avoid local failures leading to overall failures in metal members which are locally highly stressed as a result of design or fabricating techniques.
(viii) Reduction of Area:
After the metal has fractured the percentage reduction in area is calculated by measuring the test piece diameter at the point of fracture, calculating the cross-sectional area at this point, and expressing it as a percentage of original area.
(ix) Modulus of Elasticity:
Below the proportional limit stress and strain are related to one another by a constant of proportionality known as modulus of elasticity.
The value of E is determined from the stress strain curve by measuring the slope of initial straight line portion of the plot.
E indicates resistance to elastic deformation. Resistance to elastic deformation is more commonly called stiffness.
Note:
Proof stress is the stress at which the stress strain curve departs from a straight line by not more than 0.1 percent of length of the test piece. The material is said to have passed the proof stress test if application of certain load for 15 seconds does not produce more than 0.1 percent elongation.
Testing Machines:
Two types of testing machines commonly employed are:
(i) Manually operated
(ii) Power operated.
(i) The manually operated testing machine is used where low capacity is necessitated; it works on the principle of screw and lever system.
(ii) The power operated type machine type is used where large capacity is required. It consists of a cylinder in which a piston moves under the action of certain pressure. This pressure depends on pressure of oil which enters the cylinder. The force on the piston can be increased by increasing the oil pressure. The oil pressure and hence the load can be read from the gauge calibrated in N or kg.
Extensometer:
An extensometer is an instrument or device by which the changes in length of specimen under test can be precisely measured. Extensometer with different designs are available: here we shall discuss only one of the types i.e. Ewing’s extensometer.
The Ewing’s extensometer is fitted to a test piece L by two parts of set screws attached to the clamping pieces M and N. The top horizontal piece passes through the point N and is fitted with a micrometer screw at left end and on the right end a rod is pivoted.
When the tensile test is carried on the test piece, it would be stretched and the movement of the rod is measured by means of a microscope fixed in the line with clamp M and focused upon a fine cross wire on the rod. The displacement is read on the micrometer scale in the microscope eye piece. The movement of the rod is twice the extension produced in the test piece.
True Stress-Strain Curves:
The conventional stress strain curves represent the nominal stress (load divided by the original area) neglecting the increase in stress due to the decrease in cross-sectional area and also the stress concentration effect in areas of local necking. In elastic deformation the strain is negligible and there is no objection to the use of nominal stress-strain curve tor the evaluation of elastic properties.
During plastic deformation the information revealed by the nominal stress strain curves is very limited. The curve shows an upward trend beyond the yield point due to work or strain hardening, but for several metals the load at the time of fracture shows an essential fall and the actual strain hardening effect is not revealed.
In recent years due to the use of metals for sophisticated applications the true stress-strain approach is found to be highly useful. “True stress” is the load at any elongation divided by the cross- sectional area at that elongation and “true strain” is the change in length with reference to the instantaneous gauge length rather than the original length.
Fig. 2.42 shows the shape of nominal and true stress-strain curves. It can be seen that when true stress is plotted against true strain the curve uniformly rises till fracture and it shows the real strain hardening characteristics of the material. The gradient of the straight part of the true stress strain curve beyond maximum elastic stress is called the strain hardening or work hardening coefficient.
Stress Strain Curve for ‘Brittle Materials’:
Structural steel is the only material that exhibits a marked yield point. Most of the other materials show a gradual change from linear to the non-linear range. Brittle materials have a very low proportional point and do not show the yield point.
Note:
The stress-strain curves for compression can similarly be plotted to determine the characteristic stress such as proportional stress, yield stress, and ultimate stress. In case of steel these stresses are the same both in tension and in compression.
Poisson’s Ratio:
If a body is subjected to a load its length changes; ratio of this change in length to the original length is known as linear or primary strain. Due to this load, the dimensions of the body in all directions right angles to its line of application change: the strains thus produced are called lateral or secondary or transverse strains and are of nature opposite to that of primary strains.
For example if the load its tensile, there will be increase in length and corresponding decrease in cross-sectional area of the body (Fig. 2.44). In this case, linear or primary strain will be tensile and secondary or lateral or transverse strain compressive.
The ratio of lateral strain to linear strain is known as Poisson’s ratio.
Where m is a constant and its value varies between 3 and 4 for different materials.
Table 2.7 gives the average values of Poisson’s ratio for common materials.
Relations between the Elastic Moduli:
Relations exist between the elastic constants for any specific material and these relations hold good for all materials within the elastic range. The relations result from the fact that the application of any particular type of stress necessarily produces other types of stresses on other places in the material. Further, each of the stresses produces its corresponding strain and all the strains produced must be consistent.
(i) Relation between E and C:
Refer to Fig. 2.45. LMST is a solid cube subjected to a shearing force F. Let be the shear stress produced in the faces MS and LT due to the shearing forces. The complementary shear stress consequently produced in the faces ML and ST is also . Due to the shearing load the cube is distorted LMST and as such the edge M moves to M’, S to S ‘and the diagonal LS to LS’.
(ii) Relation between E and K:
If the solid cube in question is subjected to σn (normal compressive stress) on all the faces, the direct strain in each axis = σn / E (compressive) and later strain in other axes = σn / mE (tensile).
Note:
When a square or rectangular block subjected to a shear load is in equilibrium, the shear stress in one plane is always associated with a complementary shear stress (of equal value) in the other plane at right angles to it.
2. Impact Test:
Significance of Impact Test:
An impact test signifies toughness of material that is ability of material to absorb energy during plastic deformation. Static tension tests of unnotched specimens do not always reveal the susceptibility of a metal to brittle fracture. This important factor is determined by impact test. Toughness takes into account both the strength and ductility of the material.
Several engineering materials have to withstand impact or suddenly applied load while in service. Impact strengths are generally lower as compared to strength achieved under slowly applied load. Of all types of impact tests, the notched bar tests are most extensively used. The impact test measures the energy necessary to fracture a standard notch bar by applying an impulse load.
The test measures the notch toughness of material under shock loading. Values obtained from these tests are not of much utility to design problems directly and are highly arbitrary. Still it is important to note that it provides a good way of comparing toughness of various materials or toughness of the same material under different conditions. This test can also be used to assess the ductile brittle transition temperature of the material occurring due to lowering of temperature.
Impact Tests:
A pendulum type impact testing machine is generally used for conducting notched bar impact tests.
The following types of impact tests are performed on this machine:
1. Izod test
2. Charpy test
1. Izod Test:
Refer to Fig. 2.46.
This test uses a cantilever test piece of 10 mm x 10 mm section specimen having standard 45° notch 2 mm deep. This is broken by means of a swinging pendulum which is allowed to fall from a certain height to cause an impact load on the specimen. The angle rise of the pendulum after rupture of the specimen or energy to rupture the specimen is indicated on the graduated scale by a pointer. The energy required to rupture the specimen is the function of the angle of rise.
Fig. 2.47 shows pendulum type impact testing machine.
2. Charpy Test:
This test is more common than Izod test and it uses simply supported test piece (Fig. 2.48) of 10 mm x 10 mm section. The specimen is placed on supports or anvil so that the blow of striker is opposite to the notch.
The energy used in rupturing the specimen in both charpy and Izod tests is calculated as follows:
Refer to Fig. 2.49.
Initial energy = WH = W(R-R cos α) = WR (I – cos α)
Energy after rupture = WH’= W(R-R cos β) = WR (1 – cos β)
Energy used to rupture specimen
= WH- WH’- WR [(1 – cos α) – (1 – cos β)] = WR (cos β – cos α)
where, W = Weight of pendulum/striker,
H = Height of fall of centre of gravity of pendulum/striker,
H’ = Height of rise of centre of gravity of pendulum/striker,
α = Angle of fall,
β = Angle of rise, and
R = Distance from C.G. of pendulum/striker to axis of rotation O.
Effect of Important Variables on Impact Strength:
1. Angle of Notch:
There is no appreciable effect of notch angle until its value exceeds 60°. For material like mild steel the impact values amply improve if the angle exceeds 60°.
2. Shape of the Notch:
As the sharpness of the notch increases the energy required to rupture the specimen decreases.
3. Dimensions of the Specimen:
By decreasing the dimensions of the specimen the energy of rupture decreases.
4. Velocity of Impact:
The impact resistance decreases above certain critical velocity, this varies from metal to metal.
5. Specimen Temperature:
The temperature of specimen for a particular metal, determines whether the failures will be brittle, ductile or mixed character.
3. Hardness Testing:
The hardness of a material is its resistance to penetration under a localised pressure or resistance to abmsion.
The hardness can be determined by any one of the following tests:
(a) Indentation or Penetration Test-
(i) Brinell
(ii) Vicker’s
(iii) Rockwell
(b) Rebound test
(c) Scratch test.
4. Torsion Test:
The torsion test is carried out to determine the value of modulus of rigidity and ultimate shear strength of a metallic specimen.
In the testing machine the ends of the specimen are held in suitable grips through one of which the torque is applied, the other is connected to the torque arm, by means of which the torque on the specimen is measured. The torque arm is attached to the indicating unit through intermediate levers housed in the cabinet.
The levers are so arranged that, the indicator moves in clockwise direction, irrespective of the direction of torsion in the specimen. The chart range (60,000 kg-cm or 20,000 kg-cm) is selected by means of a hand lever at the front of the cabinet, identical face plates provided with attachment holes and tenon slot are fitted to the straining spindle and torque arm spindle. The angular moment of straining spindle and holder is indicated on a larger diameter protractor and venier which record deflections down to 0.1 degree.
The test is conducted as follows:
I. Select the driving dogs to suit the size of the specimen and clamp it in the machine by adjusting the length of the specimen by means of a sliding spindle. Measure the diameter at about three places and take an average value. Choose the appropriate range by capacity. Change lever.
Set maximum load pointer to zero. Set the protractor to zero for convenience and clamp it by means of a knurled screw. Carry out straining by rotating the hand wheel in either direction. Load the machine in about 200 kg cm increment, observing and recording strain readings. Then load out to failure so as to cause equal increments of strain reading.
II. Plot a torque-twist (T – θ) graph. Read off co-ordinates of a convenient point from the straight line portion of the T – θ graph and calculate the value of C, modulus of rigidity, from the torsion equation as follows –
Where, T = Torque applied (kg cm),
IP = Polar moment of inertia (cm4),
C = Modulus of rigidity (kg/cm2),
θ = Angle of twist in radians, and
= Shear stress in material at radius ‘r’.
The highest point on the T – θ curve corresponds to torque for ultimate shear strength.
5. Fatigue Test:
Fatigue Failure:
Fatigue failure occurs as a result of repeated application of small loads which are individually incapable of producing detectable plastic deformation. Eventually these repeated loads cause a macro-crack to open and spread across the piece. Stress intensification occurs and ultimately a sudden, brittle fracture results.
Ferrous metals and alloys have a limiting value of repeated stress which can be applied and reversed for an indefinitely large number of cycles without causing failure. This stress is known a fatigue limit. Non-ferrous metals and alloys do not have known limiting stress values below which failure will not occur if the cycle is repeated often enough.
For these materials, fatigue strengths are usually given as stresses for which failure can be expected to occur at 107, 108 or some other specified number of cycles of loading. The results of fatigue tests are usually plotted on semilog plots.
Fig. 2.54 shows the different arrangement of fatigue loadings, causing reversed stress, fluctuating stress and irregular stress.
Fatigue failures can be recognized by the appearance of fracture; a typical fracture is shown in Fig. 2.55.
Fatigue failure starts at the point of highest stress.
Fatigue takes places due to the following reasons:
(i) Surface finish, such as tool marks or scratches.
(ii) Internal voids such as shrinkage cracks and cooling cracks in castings and weldments.
(iii) Stress concentration points like notches, key ways, screw threads and machining under-cuts.
(iv) Defects, stresses introduced by electroplating.
It must be remembered that surface and internal defects are stress raisers, and point of highest actual stress may occur at these rather than at minimum cross-section of highest nominal stress. Thus processing methods are extremely important as they affect fatigue behaviour.
Fatigue Test:
To carry out fatigue test various machines have been developed; the tests are conducted for long periods under varying loads and at high frequencies.
For laboratory investigations a rotary bending test based on “wholer system” is often used. The machine is meant for bending test of a cantilever test piece in rotation. The test is conducted by a steady bending moment to a rotating shaft via ball bearings fitted with load hangers. When the shaft rotates the test piece passes through tension and compression stresses alternately.
The main advantage of this type of machine is its simplicity whereas serious drawback is that the maximum stresses are concentrated on a very short section of the test piece length near the fillet. Fig. 2.56 illustrates the testing arrangement.
After performing the fatigue test its results are drawn as a fatigue curve or the S/N (maximum stress/number of cycles to failure) fatigue strength. It shows the magnitude of stress causing failure as a function of number of cycles. Fig. 2.57 shows a typical S/N curve.
Several metals and alloys, such as iron, mild steel, titanium and magnesium alloys show a fatigue limit in their S/N relationship, but curves for most other metals fall continuously. Most of the failures in actual practice in iron and steel occur in less than 10 million cycles, for steel of many grades the endurance limit varies between 15 to 80 x 107 N/mm2 and these values are about 0.4 to 0.5 of the ultimate tensile strength.
The following procedures may be employed to avoid fatigue failures:
(i) Precise control of the surface finish.
(ii) Modification of the design to avoid stress concentration.
(iii) Surface treatment of the metal.
6. Creep Test:
Creep is the slow plastic deformation of metals under constant stresses or under prolonged loading usually at high temperature. It can take place and lead to fracture at static stresses much smaller than those which will break the specimen by loading it quickly. Creep is specially taken care of while designing I.C. engines, boilers and turbines.
The creep at room temperature is known as low temperature creep and can occur in load pipes, roofings, glass as well as in white metal bearings. The creep at high temperature is known as high temperature creep. It mainly depends upon the metal, service temperature to be encountered and the stress involved. For studying its effects, the specimens are put under a constant load; the creep is measured during various time intervals and results then plotted to get a creep curve.
Creep Test:
Refer to Fig. 2.58. The installation for creep test is shown in the figure. The specimen to be tested is placed in the electric furnace where it is heated to a given temperature and is constantly subjected to a load. The strain variations are measured with strain gauges. The plot with time will be as shown in Fig. 2.59.
The various stages are explained as follows:
i. Primary Stage:
Primary creep represents a transient stage in which the resistance of the metal increases due to its own deformation. It forms part of the total extension reached in a given time affecting clearance. This stage is of great interest to the designer.
ii. Secondary Stage:
This state of the curve indicates the period of extension during which the creep occurs at a more or less constant rate having its minimum creep rate. Secondary creep is the result of balance between processes of strain hardening and recovery. It is the important part of the curve and covers the whole of estimated service life of the alloy.
iii. Tertiary Stage:
In this stage the rate of extension or strain rate accelerates, rapidly leading to rupture finally. In this stage, the use of alloys should be avoided. Tertiary creep is considered to be due to necking.
Factors Affecting Creep:
Following factors influence the creep deformation:
(i) Load:
Creep strain varies directly with the applied stress. With a higher applied stress creep strain rate increases i.e., material creeps more in the same period of loading.
(ii) Temperature:
Higher temperatures increase the creep rate. A machine part loaded at a constant stress but at a higher environmental temperature undergoes more creep strain compared to one at a lower temperature.
(iii) Composition:
Pure metals have a higher creep rate as compared to alloys. Different phases in alloys or dispersed impurities in the metal decrease the rate of deformation.
(iv) Grain Size:
At lower temperatures a material with a smaller grain size has a slower creep rate. Coarser grains show higher creep strain. At higher temperature, however, the behaviour is reversed. Above a certain equicohesive temperature, fine grained metal shows more creep strain than a coarse grained metal.
Effect of grain size, strain hardening, heat treatment and alloying additions on creep:
1. Grain Size:
At temperature below the lowest temperature of re-crystallisation a fine grained steel possesses the greater resistance against creep, whereas at temperatures above that point, a course grained structure is superior.
2. Strain Hardening:
The magnitude of the creep resulting from the application of a given stress over given time period appears to depend on the opposing effects of the yielding of the material and the strain hardening caused by such yielding.
At or below the equicohesive temperatures (at which the manner of fracture changes from intra-to-inter-crystalline, which is about 450°C for mild steel), strain hardening tends to predominate and continuous measurable creep will not occur unless the stresses are large enough to overcome the resistance caused by strain hardening.
If the strain hardening predominates, the diagram representing the second stage becomes a horizontal line. At temperatures above the equicohesive temperature, however, the yielding rate exceeds the strain hardening rate and creep will proceed even under low stresses.
3. Heat Treatment:
The creep resistance of steel is also influenced by heat treatment. At temperatures of 550°C or higher, the maximum resistance is usually produced by normalising, provided the drawing temperature is 100°C above the test temperature; the lowest resistance is produced by quenching and drawing.
4. Alloying Additions:
The desired operating temperature decides the most suitable composition of the metal. At temperatures below the lowest temperature of recrystallisation, creep resistance of steel may be improved either by certain elements that largely enter into solid solution in ferrite, such as nickel, cobalt and manganese, or by the carbide-forming elements such as chromium, molybdenum, tungsten and vanidum.
At temperatures above the lowest temperature of recrystallisation, however, the carbide forming elements are the most effective in increasing the creep strength. Small additions of titanium and columbium to chromium- nickel stainless steel appreciably reduce creep characteristics over a considerable range of high temperatures.
Creep Curve Equations:
Refer to Fig. 2.60.
Creep curve can be considered to consist of a combination of two different creep processes, which occur after the sudden strain.
These are:
(i) Transient creep having decreasing creep rate, and
(ii) Viscous creep having constant creep rate.
The equation of the curve is given by the following empirical relation:
“Creep curve” is a function of stress and temperature. If temperature is kept constant, the shape of the creep curve will be different for different stresses. The three stages of creep will be clearly defined for certain stresses while for other stresses only two stages will be visible.
The series of creep curve according to different stresses, all at constant temperature are shown in Fig. 2.61. Similar results are observed when stress is kept constant and temperature is changed, as shown in Fig. 2.62.
From these two figures, we may observe the following:
A. High stresses and high temperatures reduce the primary stage upto some extent and also eliminate secondary stage. This results in acceleration creep rate at the beginning of test.
B. For intermediate stress and intermediate temperature the primary and secondary stages are more clearly defined.
C. At low temperature and low stress, the secondary stage becomes more pronounced with very slow creep rate.
Stress-Rupture Test:
In a stress-rupture test, creep test is extended upto failure.
From this test, the following can be calculated:
(i) Time required to cause failure at a given stress and at constant temperature.
(ii) Minimum creep rate by measuring elongation as a function of time.
The test is carried at higher loads, in order to have higher creep rates (In creep tests, lesser load is applied to avoid 111 stage). Creep tests are generally carried out even upto 10000 hours, whereas stress rupture tests are carried upto 1000 hours only. In creep test, the strain is lesser, i.e., upto 0.5% whereas in stress rupture test the strain is about 50%.
Stress-rupture data are plotted on log-log plot.
The following relation holds good:
tr = α σn
Where, tr = Time of rupture,
α = Stress, and
a = Constant.
Creep Resisting Materials:
At high temperatures, metals having high melting points and a compact atomic structure (high density) show good creep resistance.
The temperature (T) at which material starts to creep depends on its melting temperature (Tm) in K.
As a general rule it is found that creep starts when:
T > 0.3 to 0.4 Tm for metals and > 0.4 to 0.5 Tm for ceramics.
In general alloying the metals with suitable elements increases creep resistance considerably (Y-alloys is a very useful group of creep-resisting light alloys).
To develop creep resistant alloys there should be:
(i) Increased resistance to grains and grain boundaries to flow.
(ii) Minimum softening or recovery effects.
Most of the creep resistant alloys are developed by the following hardening methods:
(i) Hardening by cold work.
(ii) Solid solution hardening.
(iii) Precipitation hardening.
Some of the creep resistant materials are:
(i) Low Alloys Ferritic Steels:
a. Iron containing upto 4% chromium, molybdenum and vanadium deriving most of their creep resistance from carbide precipitates.
b. Good upto 600°C.
(ii) High Alloy Ferritic Steels:
a. 304, 316, 321 stainless steel-iron containing a solid solution (mainly nickel and chromium) and precipitates.
b. Good upto 950°C.
(iii) Nickel Based Super Alloys:
a. Alloys of nickel containing a solid solution (mainly chromium, tungsten, cobalt) and precipitates.
b. Good upto 950°C.
(iv) Refractory Oxides and Carbides:
a. Like alumina, glass ceramic based on silicon carbide, silicon nitride and sialen-alloys of silicon nitride and alumina.
b. Good upto 1300°C.
Materials used in petrochemical, fertilizer industry for ammonia and methanol furnace reformer tubes—High temperature reforming alloys are used for high temperature service.
7. Tests for Pipes and Tubes:
In order to prove the ductility of certain tabular products following tests are carried out:
(i) Flattening test
(ii) Flange test
(iii) Flaring test
(iv) Bend test.
(i) Flattening Test:
The test on specimens cut from tabular products is conducted by subjecting rings from the tube or pipe to a prescribed degree of flattening between parallel plates and shall show no cracks of flow.
The severity of the test is measured by distance between the parallel plates and is varied according to the dimensions of the tube or pipe.
The specimen used for the flattening test should not be less than 63.5 mm in length and should be flattened cold to the extent required by the applicable specifications.
(ii) Flange Test:
This test is conducted to determine the ductility of boiler tubes and their ability to withstand the operation of bending into a tube sheet.
The test is carried out on a ring cut from a tube usually not less than 10 mm long and consists of having a flange turned over at right angles to the body of the tube to the width required by the applicable material specifications.
(iii) Flaring Test:
This test is carried out for certain types of pressure tubes as an alternate to flange test.
In this test a tapered mandrel having a slope of 1 in 10 or a 60° included angle is driven into a section cut from the tube approximately 100 mm in length and thus expanding the specimen until the inside diameter has been increased to the extent required by the applicable material specifications.
(iv) Bend Test:
A bend test is carried out for pipes used for coiling in sizes 50 mm and below to determine ductility and the soundness of weld.
In this test a sufficient length of full size pipe is bent cold through 90° around a cylindrical mandrel having a diameter 12 times the nominal diameter of the pipe. For close coiling the pipe is bent cold through 180° around a mandrel having a diameter 3 times the nominal diameter of the pipe.