IMPACT TEST

Significance of Impact test:

            An impact test signifies the toughness of material i.e., 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 kinds 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

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 workhardening.

(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.
• Due to the small size of the impression produced we can achieve high accuracy.
• Also Rockwell hardness number can be converted to Brinell number using special table or chart.
•  For both hard and soft material it can be used.

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.

TORSION TEST

              The torsion test is carried out to determine the value of modulus of rigidity and ultimate shear strength of a metallic specimen. A schematic diagram of a torsion testing machine is shown in fig. The torsion test is carried out to determine the value of modulus of rigidity and ultimate shear strength of a metallic specimen. A schematic diagram of a torsion testing machine is shown in fig.

ROCKWELL HARDNESS TEST

FATIGUE TEST

                This failure occurs as a result of repeated application of small loads (cyclic stresses) which are individually incapable of producing 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. The maximum stress that a material that can withstand without failure for a specific large number of cycles of stress is termed its fatigue or endurance limit. Fatigue is distinguished by 3 main features: 
                                                       (a) loss of strength

                                                       (b) loss of ductility and 

                                                       (c) increased certainty in strength and service

Fatigue failure takes place due to the following reasons: 

(1) Corrosion: Corrosion reduces the number of cycles required to reach for every stress amplitude.

(2) Surface finish: surface finish , such as tool marks  or scratches.

(3) Temperature: as a consequence of oxidation or corrosion of the metal surface increasing, increase in temperature can lead to a reduction in fatigue properties.

(4) Internal voids such as shrinkage cracks and cooling cracks in castings and weldments.

(5) Defects, stresses introduced by electroplating.

(6) stress concentration points like notches, key ways, screw threads and machining under cuts.

Fatigue Failure

One can recognize  by the appearance of fracture. Fatigue failure has a number of specific features compared with failure under static loads:

1. It occurs at lower stress than the failure at static load, i.e., lower than the yeild strength or ultimate strength.
2. Failure starts on the surface (or near it) locally, in places of stress (strain) concentration. Local stress concentrations are formed by surface defects appearing on cyclic loading or notches as traces of surface treatment or the effect of the surrounding medium.
3. Failure occurs in number of stages; accumulation of defects in the material: nucleation of fatigue cracks; gradual propagation and joining of some cracks into single main crack; and rapid final destruction.
4. Failure has the typical structure which reflects the sequence of fatigue processes. A failure usually has the initial zone of destruction (the zone of nucleation of micro cracks), the fatigue zone, and the final failure zone (above Fig.). The initial zone of failure is usually near the surface and has small size and smooth surface. The fatigue zone is the zone where a fatigue crack gradually develops. It has typical concentric ripple lines which are an evidence of jump wise propagation of fatigue cracks. The fatigue zone develops until the increasing stresses in the gradually diminishing actual section attain a level at which instantaneous destruction takes place and forms the zone of final failure.   
   

The main basic reasons for taking place of fatigue failures are:-

(i)-Surface imperfections like machining marks and surface irregularities.

(ii)-Stress concentrations like notches, keyways, screw threads and matching undercuts.

(iii)- At low temperature the fatigue strength is high and decreases gradually with rise temperature.

(iv) Fatigue strength reduces by corroding environments. Following surface treatments like polishing, coating, carburizing, nitriding, etc., their effect can be reduced.

Wohler’s Fatigue Test:

              Figure shows a diagrammatic sketch of Wohler fatigue testing machine. In this machine, the specimen in the form of cantilever forms the extension of a shaft which is driven by an electric motor. Through a ball bearing, dead loading is applied to the specimen. When the machine is in action i.e., it runs, the specimen rotates and the fibres of the specimen are subjected to reversed stresses. In some instances the specimen is tapered or a two point loading is applied to obtain a uniform surface stress over a considerable length of specimen.

                To cause failure the number of cycles vary with applied stress. When stress is higher, fewer are the cycles required for causing the fracture. Obviously, a stress is reached below which fracture would not take place within the limits of a standard test and this is termed as 'endurance limit'. The length of such a standard test depends on the material being tested and types of loading. Usually it is of the order of 5,00,000 cycles for very hard steels, 50,00,000 for soft steels; 100,00,000 for cast steel and cast iron; and for non-ferrous metals and alloys from 10,00,000 to about 5,00,00,000. If the fracture does not take place within these limits, then it is understood that it will not take place at all.

                There are certain well defined characteristics for fatigue failures of metallic materials. The fractured surface frequently exhibit two distinct zones. One can find the cause of the failure by careful examination of such a failure. There is a smooth part usually showing concentric markings starting from a nucleus stress raisers, and rougher part often presenting crystal line faces.

MECHANICAL PROPERTIES AND TESTING QUESTIONS

QUESTIONS:

1) Draw stress-strain curve for a ductile material. In what respect, can we expect an identical curve for a brittle material to be different from that of a ductile material?

2) Explain weldability and machinability of metals?

3) What do we understand by the subsequent terms?

(i) Limit of proportionality        (ii) Yeild-point       (iii) Ultimate tensile strength

4) Differentitate between Brittle fracture and Ductile fracture. How they're caused.

5) Explain the meaning of the subsequent terms:

(i) Stiffness,           (ii) Hardness,  and     (iii) Toughness

6) What are the variuos non-destructive tests? Explain briefly their fields of application.

7) Differentiate between failure of material due to fatigue and Creep?

8) What is the effect of stress and temperature on a creep curve?

9) What do we understand by percentage elongation? What does a high percentage elongation value signify?

10) Name three common "Hardness tests". Describe anyone of them?

11) Do all metals have endurance limits? Explain.

12) Explain the role of fatigue failure behaviour of metals?

13) Why are brittle materials used more often in compression than in tension in structural design?