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Tuesday, 29 September 2015

TENSILE TEST

               The tensile test is one of the most widely used mechanical test.  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 mild steel test piece is described below:

STRESS - STRAIN DIAGRAM:

Consider a rod of initial length L0 and area A0 which is subjected to a load F. The stress σ is the force per unit area, and strain is the change in length (δ) divided by the initial length. Thus,

Stress      σ  = F / A0
Strain      ε   = δ / L0

The σ-ε curve for a material (say mild steel) is shown in the Fig. 1.1. Up to the proportionality point A, the stress-strain variation is linear. Up to this point Hooke's law holds good.
i.e.,  σ α ε
       Or            
                        σ= E ε
Where, 
               E is Young's modulus commonly called modulus of elasticity

               Beyond point A and up to point B, material remains elastic i.e., the material remains to its original condition of the force acting on it is removed.

              If the specimen is stressed beyond point B, permanent set takes place and we enter plastic deformation region. In the plastic deformation region, the strain does not get fully removed even with the removal of the force causing it. If the force is increased further, point 'C' is reached, where the test specimen streches even when the stress is not increased. This point is called Yield point. In fact, there are two Yield points C and D which are called upper and lower yield points respectively.
             
               With further straining, the effect of this phenomenon called strain hardening or work hardening takes place. The material becomes stronger and harder and its load bearing capacity increases. The test specimen is therefore able to bear more stress. On progressively increasing the force acting on the specimen, point E is reached. This point is the highest point in the stress-strain curve and represents the point of maximum stress. It is, therefore, called Ultimate Tensile Strength (UTS) of the material. It is equal to the maximum load applied divided by the original cross-sectional area (A0) of the test specimen.

              Here, we must consider the effect of increasing load on the cross-sectional area of the test specimen. As plastic deformation increases, the cross-sectional area of the specimen decreases. However for calculation of the stress in the stress-strain graph, the original cross-sectional area is considered. It is for this reason, that the point of breakage F seems to occur at a lower stress level than the UTS point E. After UTS point E, a sharp reduction in cross-sectional area of the test specimen takes place and a "neck" is formed in the centre of the specimen. Ultimately the test specimen breaks in two pieces as the neck becomes thinner and thinner. The actual breaking stress is much higher than the UTS, if the reduced cross-sectional area of the test specimen is taken into account.
               The measure of the strength of a material is the ultimate tensile strength (σ at point E). However, from the point of view of a design engineer, the yield point is more important as the structure designed by him should withstand forces without yielding. Usually yield stress (σ at point D) is two-thirds of the UTS and this is referred to as yield-strength of the material.
                  In actual practice, to determine UTS, a tensile test is carried out on a tensile testing or a universal testing machine. In order that tests conducted in different laboratories on the same material may give identical test results, the test piece used for the tensile test has been standardized. A standard test piece is shown in Fig. 1.2.



             A stress-strain curve for brittle material is obtained by subjecting a test bar of such material in a tensile testing machine. The tensile load is gradually increased and the extension of the test piece is recorded. The stress-strain curve for a brittle material shows some marked differences as compared to the curve obtained for a ductile material. A typical stress-strain curve for a brittle material is shown in Fig.1.3.



             This curve displays no yield point, and the test specimen breaks suddenly without any appreciable necking or extension. In the absence of a yield point, concept of "proof-stress" has been evolved for measuring yield strength of a brittle material. For example, 0.2% proof-stress indicates the stress at which the test specimen 'suffers' a permanent elongation equal to 0.2% of initial gauge length and is denoted by σ 0.2.
              The tensile test and the stress-strain curve has been described above in some detail, because a lot of useful information with regard to other properties of material can be cleaned from it. It may be noted that most tensile testing machines are provided with equipment to carry out a compressive strength test as well.



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POISSON'S RATIO

                 If a body is subjected to a load its length changes, the ratio of change in length to the original length is called as linear or primary strain. Due to this load, the dimension of the body in all directions right angle to its line of application change; the strains thus produced are called lateral or secondary strains and are of nature opposite to that of primary strain. For example, if tensile load is applied on a body, there will be increase in length and corresponding decrease in cross-sectional area of the body. In this case tensile strain is primary and compressive strain is lateral or secondary strain.
                It is defined as the ratio of the contraction strain normal to the applied load divided by the extension strain in the direction of the applied load. Since most common materials become thinner in cross section when stretched, Poisson's ratio for them is positive.

i.e., poisson's ratio,        μ or 1/m

             Where, m is called a constant and its value varies between 3 and 4 for different materials. For a perfectly incompressible materials, the poisson's ratio would be exactly 0.5. Most practical engineering materials have 'V' between 0 and 0.5. Cork is close to 0 (zero), most steels are around 0.3, and rubber is almost 0.5. A poisson's ratio greater than 0.5 cannot be maintained for large amounts of strain because at a certain strain the material would reach zero volume, and any further strain would give the material negative volume.

Poisson’s ratio for different materials: 
Material
Poisson's ratio(μ )
rubber
0.4999
gold
0.42–0.44
saturated clay
0.40–0.49
magnesium
0.252-0.289
titanium
0.265-0.34
copper
0.33
aluminium-alloy
0.32
clay
0.30–0.45
stainless steel
0.30–0.31
steel
0.27–0.30
cast iron
0.21–0.26
sand
0.20–0.45
concrete
0.20
glass
0.18–0.3
foam
0.10–0.50
cork
~ 0.00

Tuesday, 22 September 2015

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 strength 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.

Impact tests:

             A pendulum type impact testing machine is generally used for conducting notched bar impact tests. The following type of impact tests are performed on these machines.
    
                  1) IZOD Impact Test                            2) CHARPY Impact Test


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IZOD TEST

             The 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 a specimen is the function of the angle of rise.

Fig shows pendulum type impact testing machine.  

CHARPY TEST

               This test is more common than Izod test and it uses simply supported test piece 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 rupture the specimen in both Charpy and Izod tests is calculated as follows:

Initial energy = WH = W(R-R cos α) = WR (1- cos α )
Energy after rupture = WH1  = W(R-R cos β) = WR(1- cos β)
Energy used to rupture specimen = WH- WH1
                                                       WR (1-cos α) - WR (1-cos β)
                                                       = WR [(1-cos α) - (1-cos β) ]
                                                       = WR [cos β - cos α ]

Where, W = Weight of pendulum/strike
               H = Height of fall of center of gravity of pendulum/strike
               H1 = Height of rise of center of gravity of pendulum/strike
               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°
  2. Shape of the notch. As the sharpness of the notch increases the energy required to rupture the specimen deceases.
  3. Dimensions of the specimen. By decreasing the dimensions of the specimen the energy of rupture decreases.
  4. Velocity of Impact. The important 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 failure will be brittle, ductile or mixed character.


Monday, 21 September 2015

HARDNESS TEST

            The hardness of material is its resistance to penetration under a localized pressure or resistance to abrasion. The hardness can be determined by any one of the following tests.

(a) Indentation or Penetration test:
  1. Brinell
  2. Vicker's
  3. Rockwell.

(b) Rebound test.
(c) Scratch test.


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BRINELL HARDNESS TEST

This test is used: (1) to determine hardness of metallic materials, 
                             (2) to check the quality of the product,
                             (3) for uniformity of samples of metals, and 
                             (4) for uniformity of results of heat treatment.
              In this test, a standard hardened steel ball is pressed into the surface of the specimen by a gradually applied load which is maintained on the specimen for definite time. The impression so obtained is measured by a microscope and the Brinell Hardness Number (B.H.N) is found out by following equation:

B.H.N = Applied load in KG/Area of impression or indentation of steel ball in m2
             = P/A


Brinell Hardness Tester: Brinell hardness tester is shown in below Fig.




The hardness test is carried out as follows:
  • Place the test sample on the top of the test table and rise it with the elevating screw, till the test sample just touches the ball.
  • Apply the desired load (about 30' diameter of the ball in mm) either mechanically or by oil pressure.
  • The steel ball during this period moves to the position of the sample and makes an impression or indentation.
  • Measure the indentation diameter at two places, either on the screen provided with the machine or by coinciding the two points of a reading microscope.
  • Using above equation, one can calculate BHN after substituting the values of P, D, d.

Precautions:

(1) The Brinell test should be performed on smooth, flat specimen from which dirt and scale have been cleaned.
(2) Successive impressions made too close to one another tend to produce high readings because of work hardening.
(3) The test should not be made on the specimen so thin that the impression shows through the metal nor should impressions be made too close to the edge of a specimen.

Advantages:
  • It is widely applied in the industry due to the rapidity and simplicity with which they may be performed.
  • High Accuracy achieved and due to small size of the impression produced.
  • Also Rockwell hardness number can be converted to Brinell number using special table or chart.
  • It can be used both for hard and soft material.

Disadvantages:
  1. The test machine is very heavy.
  2. The area of indentation is quite large that it affects the surface quality. This is why, some it is considered as a destructive test.
  3. The thickness of the test sample also limits its use, e.g., thin sheets will bulge or be destroyed during the test.
  4. For very hard materials, the test results are unreliable. The ball gets flattened on hard surfaces.
  5. One faces difficulty in measuring the indentation diameter accurately.



VICKER'S HARDNESS TEST

This is similar to Brinell Hardness test, but in this method the drawback of the flattening of the steel ball in testing harder materials is eliminated. It uses a similar relationship and most of the errors and limitations of Brinell Hardness test are eliminated. In this method of hardness testing a diamond square based pyramid indentor with 136° angle between opposite faces is used. The load varies from 5 kg to 120 kg in increments of 5 kg. Similar to Brinell and Rockwell hardness measuring methods, this method also uses the indentation produced by the indentor (Diamond pyramid). The indentor gives geometrically similar impressions under different loads.
A piston and a dash pot of oil is used for controlling the rate and duration of loading. The specimen is placed on the anvil, which is then raised to indentor. Load is applied and then removed. The value of Vicker's Hardness can be obtained by the following relation:

VPN=DPN=
             
                

Where VPN = Vicker's Pyramid Number and
             DPN = Diamond Pyramid Number

Let P is the load applied,
       d is the average length of two diagonals, in mm, and 
       q is the angle between opposite faces of diamond pyramid (136°). Then


              This method is used for the determination of hardness of very thin and very hard materials. This method also facilitates the ease of measurement of a diagonal of the indentation area ( Fig ), as compared to circular dimensions, which are difficult to measure. This method is rapid, accurate and suitable for metals as thin as 0.15 mm. The indentor is capable of giving geometrically similar impression with different loads. Obviously, the hardness number is independent of the load applied. Some typical values of VPN are:

Material
VPN Value (Kg/mm2)


Diamond
8400
Steel
210
Aluminium
22
Lead
12
Tungsten carbide
2100


             The values of Brinell and Vickers hardness are practically the same up to 300. We must note that Vicker's test can be carried out accurately on polished surfaces but does not give accurate results when used for rough surfaces.


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