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.

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 places 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 fatigue failures 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 dtresses. In some instancex 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.

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MECHANIACL PROPERTIES AND TESTING QUESTIONS

QUESTIONS

1) Draw a stress-strain curve for a ductile material. In what respect, a similar curve for a brittle material will be different?

2) Explain weldability and machinability of metal

3) What do you understand by the following terms?

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

4) Differentiate between brittle fracture and ductile fracture. How they are caused.

5) Explain the meaning of the following terms :

(I)Stiffness,                                    (ii)Toughness, and                         (iii)Hardness.

6) What are the different 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 you understand by percentage elongation? What does a high percentage elongation value signify?

10) Name three common “hardness” tests. Describe anyone of them?State advantages and disadvantages?

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?

CREEP TEST

        Creep is the slow plastic deformation of metals under constant stresses or under prolonged loading usually at high temperatures. Creep is specially taken care of while designing I.C Engines, Boilers, and Turbines.

           Creep at lower temperature is known as low temperature creep and can occur in load pipes, roofings, glass as well as in metal bearings. Creep at high temperature is known as high temperature creep. 

Where creep is important?

In design, we seek materials that will carry the design loads without failure for the design life at the service temperature. Creep is an important consideration in design in three types of high temperature applications:

1. Displacement-limited applications in which precise dimensions or small clearances must be maintained such as in turbine rotors in jet engines. (Figure 1a).

2. Rupture-limited applications in which precise dimensions are not essentials but fracture must be avoided such as in high-pressure steam tubes and pipes. (Figure 1b).

3. Stress-relaxation-limited applications  in which an initial tension relaxes with time such as in suspended cables and tightened bolts (Figure 1c).

In these types of applications, design engineers must consider creep deformation and its dependence on time and temperature. Many mechanical systems and components like turbines , steam boilers, and reactors operate at high temperatures and creep properties for the materials used must be determined.  

Figure  A creep test setup

                  The main objective in creep test is to measure how a given metal or an alloy will perform under constant load, at elevated temperatures. In a creep test, a tensile specimen (with similar dimensions as a tensile test specimen) is subjected to a constant load inside a electric furnance where the specimen is heated to specific temperature and the temperature is maintained constant. Figure illustrate a simple setup for creep testing. The resulting deformation or strain is measured and plotted as a function of elapsed time. Figure shows a schematic creep curve for a metal tested at constant load until rupture. Metals, polymers, and ceramics all show similar strain-time behaviors. The instantaneous strain is purely elastic and can be calculated by, equation 1 with E as the modulus at the testing, temperature. The creep curve in Figure demonstrates three regions of strain-time behavior:

1. Primary creep  where the rate of change of strain (creep rate=∆ε/∆t) decreases with       time due to strain hardening of the material.

2. Steady-State creep  where the strain increases linearly with time. From design point of view, this region is the most important one for parts designed for long service life because it comprises the longest creep duration. The main creep test result is the slope of this region which is known as the steady state creep rate (′εs). During this stage of creep, thee is a balance between strain hardening due to deformation and softening due to recovery processes similar to those occurring during the annealing of metals at elevated temperature.   

3. Tertiary-creep where the strain increases rapidly until failure or rupture. The time to failure is often called as the time to rupture lifetime  (tr). This parameter is an important consideration in designing against creep for parts intended for short-life applications. To determine the rupture lifetime, the creep test must be conducted to the point of failure. Such test is also known as the stress rupture test or creep rupture test.

Testing either at higher stresses or higher temperature will increase the steady state creep rate (′εs ) and reduces the rupture lifetime (tr) as illustrated. Note that the strain is constant and independent of time for temperatures below 0.4 Tm. Experiments suggest that the combined influence of applied stress and temperature on the steady state creep rate can be represented as

ε′s = K σn exp (-Qc/RT) --(2)  

where,  K is the creep constant,
             Qc is the activation  energy for creep, 
              n is the creep exponent ( lies b/n 3 & 8 ) and,
              R is the gas constant.
The values of the three constants K,  Qc and n describe the creep of a given material and if they are known, you can calculate the steady state creep rate at any temperature and stress using equation 2. However, these parameters vary from material to material, and have to be determined experimentally. 

Factors affecting creep:

(1) load: Creep strain varies with the applied load. With applied stress(load) creep  strain rate increases.

(2) Temperature: High temperature increases creep rate. At higher temperatures  materials undergoes more creep strain compared to one at lower temperatures.

(3) Composition: Pure metals with high melting points and compact atomic structure generally exhibit more creep resistance at high temperature. By allowing the pure metals with suitable elements, the creep resistance can be increased considerably.

(4) Grain size: The major factor in creep is Grain Size. Usually coarse grained materials exhibit better creep resistance than fine grained. At lower temperatures a material with a smaller grain size has a slower creep rate. Coarse grains show higher creep strain.


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Profit and loss

Cost Price:

The price, at which an article is purchased, is called its cost price, abbreviated as C.P.

Selling Price:

The price, at which an article is sold, is called its selling prices, abbreviated as S.P.

Profit or Gain:

If S.P. is greater than C.P., the seller is said to have a profit or gain.

Loss:

If S.P. is less than C.P., the seller is said to have incurred a loss.

IMPORTANT FORMULAE

1. Gain = (S.P.) - (C.P.)

2. Loss = (C.P.) - (S.P.)

3. Loss or gain is always reckoned on C.P.

4. Gain Percentage: (Gain %)

Gain % =		Gain x 100
		C.P.

5. Loss Percentage: (Loss %)

Loss % =		Loss x 100
		C.P.

6. Selling Price: (S.P.)

SP =		(100 + Gain %)	x C.P
		100

7. Selling Price: (S.P.)

SP =		(100 - Loss %)	x C.P.
		100

8. Cost Price: (C.P.)

C.P. =		100	x S.P.
		(100 + Gain %)

9. Cost Price: (C.P.)

C.P. =		100	x S.P.
		(100 - Loss %)

10. If an article is sold at a gain of say 35%, then S.P. = 135% of C.P.

11. If an article is sold at a loss of say, 35% then S.P. = 65% of C.P.

12. When a person sells two similar items, one at a gain of say x%, and the other at a loss of x%, then the seller always incurs a loss given by:

Loss % =		Common Loss and Gain %		2	=		x		2	.
		10					10

13. If a trader professes to sell his goods at cost price, but uses false weights, then

Gain % =		Error	x 100	%.
		(True Value) - (Error)