Q.6 Explain with neat sketch and suitable example, the stress-strain diagram of : (i) Ductile material and (ii) Brittle material. (AKTU - 2009-10)
Ans.
Q.7 Write short note on fatigue. (AKTU - 2008-09, 09-10)
Related Questions-
Q. Due to which type of loading the fatigue failure occurs? (AKTU - 2011 - 12)
Ans.
Q.7 Write short note on fatigue. (AKTU - 2008-09, 09-10)
Related Questions-
Q. Due to which type of loading the fatigue failure occurs? (AKTU - 2011 - 12)
Ans. Fatigue: - (AKTU - 2010 - 11)
When the element of machine or structure is applied load, number of times the element may become brittle. The irregularities occur on the surface of the metals. This phenomenon is called fatigue.
This property of a material decides its behaviour under a particular type of loading, in which a much smaller load than the one required for material failure in a single application is repeatedly applied innumerous times. Thus, the material is subjected to repetitive cycles, in very large number, of fluctuating stress. Under such conditions the material fails at a much lower stress than the one required for its failure through fracture under a single application of steady loads. This phenomenon of material failure, under the conditions described above, is known as fatigue. The stress at which the material fails due to fatigue is known as fatigue strength. The fatigue failure always shows a brittle fracture with no appreciable deformation of the material at the fracture. In almost all metals there is a well defined value of stress below which the material will not tail due to fatigue even if there is a repeated application of load in the above manner. This value of stress, which is much below the normal yield stress, is known as the fatigue limit or endurance limit of the material.
The phenomenon of fatigue failure is very important from the point of view of design and manufacture of various components which are supposed to be subjected to repeated or cyclic loading continuously. Some examples of such components are rotating machine parts, motor shafts, gears, components of high speed turbines and aero engines, aircraft wings, etc. The factors which generally govern the fatigue strength of a material are its chemical composition, extent of cold working and grain size.
Q.8 Write short notes on creeping. (AKTU - 2009-10)
Ans. Creep: - (AKTU - 2010 - 11, 11 - 12) Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature.
The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function - for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. Moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.
Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Creep deformation is “time-dependent” deformation.
The temperature range in which creep deformation may occur differs in various materials. For example, Tungsten requires a temperature in the thousands of degrees before creep deformation can occur while ice formations will creep in freezing temperatures. As a rule of thumb, the effects of creep deformation generally become noticeable at approximately 30% of the melting point for metals and 40-50% of melting point for ceramics. Virtually any material will creep upon approaching its melting temperature. Since the minimum temperature is relative to melting point, creep can be seen at relatively low temperatures for some materials.
Q.9. Discuss the creep curve. (AKTU - 2011 - 12)
Ans. A creep test is conducted under the conditions of fixed tensile load and fixed temperature and measuring the change in length over a period of time. A creep curve curve plotted between elongation (strain) and time is shown in fig. The curve contains three distinct stages:
1. A short lived initial stage, called primary stage. This stage usually occurs at very low temperatures, hence it is also called cold creep.
2. A long stage, called secondary stage, where the elongation is somewhat linear. This stage dominates at high temperatures, hence also known as hot creep. The steady rate of creep in the second stage determines the useful life of the material, and
3. A short-lived thrid stage, called tertiary stage which leads to fracture. During this stage, the straining is too fast due to neck formation in the material.
The creep rate increases with increase in temperature and applied load.
When the element of machine or structure is applied load, number of times the element may become brittle. The irregularities occur on the surface of the metals. This phenomenon is called fatigue.
This property of a material decides its behaviour under a particular type of loading, in which a much smaller load than the one required for material failure in a single application is repeatedly applied innumerous times. Thus, the material is subjected to repetitive cycles, in very large number, of fluctuating stress. Under such conditions the material fails at a much lower stress than the one required for its failure through fracture under a single application of steady loads. This phenomenon of material failure, under the conditions described above, is known as fatigue. The stress at which the material fails due to fatigue is known as fatigue strength. The fatigue failure always shows a brittle fracture with no appreciable deformation of the material at the fracture. In almost all metals there is a well defined value of stress below which the material will not tail due to fatigue even if there is a repeated application of load in the above manner. This value of stress, which is much below the normal yield stress, is known as the fatigue limit or endurance limit of the material.
The phenomenon of fatigue failure is very important from the point of view of design and manufacture of various components which are supposed to be subjected to repeated or cyclic loading continuously. Some examples of such components are rotating machine parts, motor shafts, gears, components of high speed turbines and aero engines, aircraft wings, etc. The factors which generally govern the fatigue strength of a material are its chemical composition, extent of cold working and grain size.
Q.8 Write short notes on creeping. (AKTU - 2009-10)
Ans. Creep: - (AKTU - 2010 - 11, 11 - 12) Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature.
The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function - for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. Moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.
Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Creep deformation is “time-dependent” deformation.
The temperature range in which creep deformation may occur differs in various materials. For example, Tungsten requires a temperature in the thousands of degrees before creep deformation can occur while ice formations will creep in freezing temperatures. As a rule of thumb, the effects of creep deformation generally become noticeable at approximately 30% of the melting point for metals and 40-50% of melting point for ceramics. Virtually any material will creep upon approaching its melting temperature. Since the minimum temperature is relative to melting point, creep can be seen at relatively low temperatures for some materials.
Q.9. Discuss the creep curve. (AKTU - 2011 - 12)
Ans. A creep test is conducted under the conditions of fixed tensile load and fixed temperature and measuring the change in length over a period of time. A creep curve curve plotted between elongation (strain) and time is shown in fig. The curve contains three distinct stages:
1. A short lived initial stage, called primary stage. This stage usually occurs at very low temperatures, hence it is also called cold creep.
2. A long stage, called secondary stage, where the elongation is somewhat linear. This stage dominates at high temperatures, hence also known as hot creep. The steady rate of creep in the second stage determines the useful life of the material, and
3. A short-lived thrid stage, called tertiary stage which leads to fracture. During this stage, the straining is too fast due to neck formation in the material.
The creep rate increases with increase in temperature and applied load.
Q.10 What are the objectives of tempering ? (AKTU - 2012 - 13)
Sol. Tempering is carried out to serve the following purposes :
(i) To increase toughness.
(ii) To relieve internal stresses
(iii) To reduce hardness and increase ductility.
(iv) To reduce brittleness.
Choose Correct Answers
1. The strength is the property of a material to resist
(a) breaking in the material
(b) externally applied forces with breakdown or yielding
(c) fracture due to high impact loads (d) none of these
2. Fatigue failure occurs when a part is subjected to (AKTU -2008-09)
(a) compressive stress (b) tensile stress
(c) fluctuating stress, (d) uniform stress.
3. The ability of a material to resist fracture due to high impact loads, is called
(a) strength (b) stiffness (c) toughness (d) brittleness
4. The property of a material due to which it can undergo permanent deformation without failure or rupture is known as
(a) Plasticity (b) Elasticity (c) Stiffness (d) None of the above
5. Ductility is the property of a material due to which it
(a) can be drawn into wires
(b) breaks with little permanent distortion
(c) can be rolled or hammered into thin sheets
(d) can resist fracture due to high impact loads
6. Which of the following property is desirable for materials used in tools and machines?
(a) Elasticity (b) Plasticity (c) Ductility (d) Malleability
7. Due to hardness a material
(a) can be drawn into wires
(b) breaks with little permanent distortion
(c) can cut another metal
(d) can be rolled or hammered into thin sheets
8. In the following which material has maximum malleability?
(a) Lead (b) Soft steel (c) Wrought iron (d) Copper
9. The property of a material to absorb energy in the plastic range is called
(a) resilience (b) creep (c) fatigue strength (d) toughness
10. The ductility of a piece of chalk is ............ (AKTU - 2008 - 09)
(a) 0 (b) 2
(c) 5 (d) None of the above
11. Metals having ............... property can be easily drawn into wires. (AKTU - 2010-11)
(a) compressive stress (b) tensile stress
(c) fluctuating stress, (d) uniform stress.
3. The ability of a material to resist fracture due to high impact loads, is called
(a) strength (b) stiffness (c) toughness (d) brittleness
4. The property of a material due to which it can undergo permanent deformation without failure or rupture is known as
(a) Plasticity (b) Elasticity (c) Stiffness (d) None of the above
5. Ductility is the property of a material due to which it
(a) can be drawn into wires
(b) breaks with little permanent distortion
(c) can be rolled or hammered into thin sheets
(d) can resist fracture due to high impact loads
6. Which of the following property is desirable for materials used in tools and machines?
(a) Elasticity (b) Plasticity (c) Ductility (d) Malleability
7. Due to hardness a material
(a) can be drawn into wires
(b) breaks with little permanent distortion
(c) can cut another metal
(d) can be rolled or hammered into thin sheets
8. In the following which material has maximum malleability?
(a) Lead (b) Soft steel (c) Wrought iron (d) Copper
9. The property of a material to absorb energy in the plastic range is called
(a) resilience (b) creep (c) fatigue strength (d) toughness
10. The ductility of a piece of chalk is ............ (AKTU - 2008 - 09)
(a) 0 (b) 2
(c) 5 (d) None of the above
11. Metals having ............... property can be easily drawn into wires. (AKTU - 2010-11)
Answer -
1. (b) 2. (c) 3. (c) 4. (d) 5. (a) 6. (b) 7. (c) 8. (a) 9. (a) 10. (a) 11. ductility.
1. (b) 2. (c) 3. (c) 4. (d) 5. (a) 6. (b) 7. (c) 8. (a) 9. (a) 10. (a) 11. ductility.