Mechanical Properties of Materials Viva Questions

Mechanical Properties of Materials Viva Questions

Mechanical Properties of Materials Viva Questions, Viva Questions on Mechanical Properties of Materials, Material Science Engineering Viva Questions, Short Question answer on Mechanical Properties of Materials, Engineering Viva Questions, Mechanical Properties of Materials Viva Questions, Mechanical Properties of Materials Viva Questions, Interview question on Mechanical Properties of Materials

Crystal Structure of Materials Viva Questions

Questions with Answer

Q.1. Explain the importance of mechanical properties in the design.

Ans. Mechanical properties are the most important requirements of materials from the engineering point of view in selecting them for design purposes. Mechanical properties of materials describe their behavior under mechanical usage. A designer must have considerable knowledge of materials and their properties. Cast iron, wrought iron, mild steel, copper, brass, aluminum, and their alloys are the materials more commonly used in engineering industries. Complete specifications of properties and compositions of all these materials have been standardized by the Bureau of Indian Standards. The importance of mechanical properties is easy to appreciate in many of those load-bearing applications.

The most important mechanical properties of materials are strength, elasticity, plasticity, ductility, brittleness, hardness, toughness, resilience, creep, etc.

Q.2. Write short notes on the following.

1. Elasticity
2. Plasticity
3. Ductility
4. Brittleness
5. Hardness
6. Hardenability

Ans.

Elasticity: It is the property of the material which enables it to regain its original shape after deformation within the elastic limit. This property is desirable in materials used in tools and machines. It is defined as non-permanent deformation.

Each body will more or less deform after applying load on it. The deformation will be elastic when the body recovers its original shape completely after removing the load.

Examples – Steel, rubber.

Steel is said to be more elastic than rubber.

Plasticity: It is the property of the material which enables the formation of permanent deformation. When each and every deformation, without fracture, remains after removing the load, the material is called plastic.

Examples – Clay, lead.

Ductility: It is the ability of the material to be permanently deformed without fracture even after the removal of the force. It is the property of the material which enables it to be drawn out or elongated to an appreciable extent before rupture occurs. The ductility of material can be measured by the percentage of elongation and the percentage of reduction of the area before rupture of a test piece.

The property depends very largely upon hardness. The ductility of a metal is usually much less when hot than when cold, hence wires are drawn cold. Glass, on the other hand, is extremely ductile when hot, and may be drawn out into a very fine thread.

Brittleness: The property fracturing a material without warning or appreciable deformation is called brittleness. It is the property of the material, which is opposite to ductility. Materials, having very little property of deformation, either elastic or plastic are called brittle. Although they may resist a pretty heavy load when smoothly applied, they will readily break at any point.

The lack of ductility is commonly called brittleness. Therefore, a non-ductile material is said to be brittle material. Usually, the tensile strength of brittle materials is only a fraction of their compressive strength.

Example – Cast iron (especially white), glass.

Hardness: Hardness of a material is defined as the resistance of a material to scratch, wear, or penetration of its surface by harder bodies. It is a property of the material which enables it to resist abrasion, indentation, machining, and scratching. It is usually expressed in relation to the hardness of other materials. The hardness decreases by heating. Glass is harder than other materials and diamond is harder than glass

In the workshop, one calls a steel “hard” when it cannot be removed with a file.

The word hardness has been used to define many things. Even in technology, hardness has been measured in various ways, and depending upon the field of science, or engineering considered, hardness measures different properties.

Hardenability: It refers to the degree of hardness that can be imparted to a metal particularly steel, by the process of hardening. It determines the depth and distribution of hardness induced by quenching.

Hardness is associated with strength, while hardenability is connected with the transformation characteristics of steel. Hardenability may be increased by the transformation kinetics, by the addition of alloying elements, while the hardness of steel with given transformation kinetics is controlled primarily by the carbon content.

Q.3. Explain the elastic and plastic behavior of solids.

Ans. When a material is subjected to some external force or load, it undergo some deformation which may be classified as

1. Elastic deformation (or Recoverable)
2. Plastic deformation (or non-recoverable)

Deformation is the change in dimension or form of matter under the action of applied forces. Deformation is caused by:

1. Mechanical action of external forces
2. Various physical and Physiochemical processes.

Elastic deformation: When a metal is loaded within certain limits, a temporary deformation of crystal takes place through the displacement of atoms. As the deforming load is removed, the atoms return to their original stable position and the crystal recovers its original shape as depicted in figure shown below.

Elastic deformation occurs with comparatively smaller deforming loads so that working stresses are within the elastic range.

Plastic deformation: Plastic deformation follows elastic deformation and even persists after the removal of deforming load. Plastic deformation takes place when the stress exceeds the elastic limits.

The Plastic deformation is a function of applied stress, temperature, and rate of straining and is intentionally carried out in metal forming of shaping processes such as bending, stamping, drawing, etc. Thus, various mechanical properties of metals and alloys are improved.

Q.4. Explain briefly the following.

1. Mechanism of fatigue
   1. Creep fractions
   2. Wear and abrasion  

Ans.

Mechanism of fatigue: A fatigue fracture always starts as a small crack that under repeated loading of stress, grows in size. As the cracks expand, the loud carrying cross-section of the metal component is reduced with the result that stress on this section rises.

Ultimately, a point will reach where the remaining cross-section is no longer strong enough to carry the load and finally the result is a fracture. Most cracks that are responsible for fatigue failure start as visible discontinuity such as design and other details (e.g., holes, fillets. inclusions, blow holes, etc.)

The events that result in fatigue fracture include

1. Crack nucleation
2. Crack growth
3. Fracture

Creep fracture: Sometimes the materials are required to sustain steady loads for long periods of time. The material may continue to deform and a stage will reach when the deformation will result in the fracture of the material. Due to the loading of material by tension, there is a deformation of the material, if this stress loading is maintained for a long time some additional deformation is produced in it. This is known as creep.

ASTM defines creep as the time-dependent part of the strain resulting from the stress. For creating creep in the material, time is an important factor.

Because the cross-section reduces with elongation, viscous creep in tension generally ends in a fracture if allowed to continue for a long time. At high stress and moderate temperature involved for a short time fracture, it is the same as in simple tension.

Wear and abrasion: Wear may be defined as the undesirable removal of solid material from rubbing surfaces. The following types of wear are usually found when any rubbing action takes place

1. Adhesive wear
2. Abrasive wear

Adhesive wear is characterized by an intensive interaction between two bearing surfaces resulting from the mutual adhesion of metals at the junction. While abrasive wear is frequently encountered during industrial machining and may be defined as removal, by plowing or gouging out from the surface of the material by another body much harder than the abraded surface.

Abrasion wear may be due to the following:

1. The presence of hard particles i.e., dust or impurities between two sliding surfaces.
2. The rubbing of a hard surface against a soft one.

The extent of wear depends upon the size, shape, and hardness of particles.

Q.5. Define the term creep and its type.

Ans. It is the property of the material which enables it under constant stress to deform slowly but progressively over a certain period. Creep occurs in steel at high temperatures. A material subjected to constant tensile load at an elevated temperature will creep and undergo a time-dependent deformation. This property is considered in designing I.C. engines, boiler,s and turbines.

Types of Creep: Creep occurs at stresses well below the elastic limit at elevated temperatures. Whether a given temperature is “elevated” or not depends on the material since one material may creep more at room temperature than another, say at 1000°C. It is, therefore, best to study the behavior of creep and define their types at a different ranges of temperature. The creep may, therefore, be classified, depending on the temperature, as logarithmic creep, recovery creep, and diffusion creep or plastic creep.

Q.6. Differentiate between Slip and Twining.

Ans.

Slip	Twining
The orientation of the crystal above and below the slip plane is the same after deformation as before.	Twining results in orientation differences across the twin Plane
Slip occurs in discrete multiples of the atomic distance	The atom’s displacement is much less than atomic distances.
Slip occurs on widely separate planes	In the twinned region of the crystal, every atomic plane is involved in deformation.
Under the microscope, slip appears as thin lines	Under the microscope, it appears as broad lines or Planes.
It requires less shear stress for deformation	It requires more shear stress than slip.

Q.7. Explain the various factors affecting creep.

Ans. Creep is much more affected by grain size, microstructure, and previous strain history, for instance, cold work and many other factors.

Grain size is a major factor in creep. Coarse-grained materials exhibit better creeps resistance than fine-grained ones since fine-grained materials have a greater amount of grain boundary and grain boundaries behave as a quasi- viscous material with a high tendency to flow at elevated temperature. It is due to the reason that single crystals show a higher creep resistance than polycrystalline materials. Since tungsten filaments of electrical bulbs are made of single crystals, they can withstand very high temperatures. It is due to this reason that single crystals show a higher creep resistance than polycrystalline materials. Since tungsten filaments of electrical bulbs are made of single crystals, they can withstand very high temperatures.

The next important factor is the thermal stability of the microstructure of alloys and its resistance to oxidation at high temperatures. An annealed specimen, for example, for having greater thermal stabilities is far superior in its creep resistance to a quenched steel for its poor thermal stability.

Cold working has also a strong effect on creep which is accelerated in some metals during recrystallization following cold water. This accelerated creep is followed by a period of relatively slow creep in the recrystallized metal. Pure metals which have high melting points and a compact atomic structure generally exhibit good creep resistance at high temperatures. The creep resistance of pure metals can be considerably increased by alloying them with suitable elements.

Q.8. Describe the term fatigue. What are the different types of stresses causing fatigue?

Ans. The term fatigue is generally referred to the effect of cyclically repeated stresses. It has been found by experience that the metals used in construction ultimately fracture by cyclically repeated stresses which are very much lower than their ultimate statical strength. It is also found that if the stresses are not merely repeated, but reversed the resistance of fracture is less than if the same intensity of only one kind of stress were repeated. In such cases, the material is often said to have become fatigued.

In some circumstances, the prolonged application of constant stress produces deterioration in materials. The failure occurs by the reason of fatigue at stresses which would be safe for static loading. Much attention is to be given to the fatigue failure along with the lightness, economy, and use of materials at higher speeds. It may be noted that the treatment, to which metals are subjected to the repeated cyclical variations of stress, is quite distinct from the blows or impacts. The different types of stresses causing fatigue are as follows:

1. Fluctuating stresses are those stresses which vary from a minimum value to a maximum value of the same nature (i e. tensile or compressive).
2. Repeated stresses are those stresses which vary from zero to a certain maximum value.
3. Alternating stresses are those stresses which vary from a minimum value to a maximum value of the opposite nature.
4. Reversed stresses are those stresses which vary from one value of compressive to the same value of tensile stresses or vice versa.

Q.9. What is effects of stress concentration and temperature on fatigue?

Ans. Most of the machine parts contain key ways, screws, threads, hole press fits, fillets, etc. All these geometrical irregularities act as stress raisers and the fatigue strength is greatly reduced due to the presence of such stress raisers. These geometrical irregularities must be reduced by careful design. Surface roughness also causes stress concentration. There is also another class of stress raisers. These is called metallurgical stress raisers. They consist of inclusion, decarburization, local overheating due to grinding porosity, etc.

To study the effect of stress raiser on fatigue the specimen containing a V-notch or circular notch is prepared. When the specimen is loaded, the notch has the following effects:

1. A triaxial state of stress is produced.
2. A stress gradient is set up from the root of the notch to the center of the specimen.
3. There is stress concentration at the root of the notch.

Due to stress concentration at the root, a crack is developed.

Low-temperature fatigue: Below room temperature, fatigue strength increases with decreasing temperature. There is no sudden change in fatigue properties at temperature below ductile to the brittle transition temperature. Fatigue failure at room temperature is associated with vacancy formation and condensation.

High-temperature fatigue: Fatigue strength decreases with an increase in temperature above room temperature. But, only for steel, fatigue strength is maximum at 400°C to 600°C. This maximum tensile strength at high temperature is due to the strain aging of steel. When the temperature is increased above room temperature creep comes into play. Creep increases with increasing stress. Some materials which show a sharp fatigue limit at room temperature do not show a fatigue limit at high temperature. At high temperatures, the fatigue properties are dependent upon the frequency of stress application. The finer the grain size, the better the fatigue properties at low temperatures. At high temperatures where creep predominates, coarse grain materials have high strength. The materials which have creep strength will have fatigue strength at high temperatures. Compressive residual stresses improve fatigue properties at room temperature. These stresses will be annealed out at high temperatures. Hence, there will be a reduction in fatigue properties.

Q.10. What are the major mechanisms available to increase the strength of metal?

Ans. The following are the major mechanisms available to increase the strength of metals:

1. Grain boundary strengthening: Grain boundaries obstruct the movement of dislocations in metal and thereby make it difficult to deform the metal. This increases the strength of the metal. Smaller grains result in more grain boundaries than higher strength.

The relationship between grain size and the yield strength a is expressed by Hall-petch equation,  \sigma =\sigma _{i}+\frac{k}{\sqrt{d}}

This indicates that the smaller the grain size (hence more the grain boundaries), the greater is the strength. Grain size can be reduced during the casting stage or by heat treatment.

• Strain aging: If metal is deformed plastically, unloaded and. after a little heating, reloaded, its yield strength is increased. This method is called strain aging.
• Solid solution strengthening: If solute atoms are introduced into solvent atoms, (parent matrix of an alloy) strength of the parent metal is increased. This is due to the interaction between the solute atoms and the dislocations.
• Precipitation hardening or age hardening: This is done by solution treating and quenching an alloy in which a second phase is in solid solution at the elevated temperature but precipitates upon quenching and aging at a lower temperature. This results in increased hardness and strength. Aluminium-copper alloys, and duralumins are the most common examples of this type of strengthening.
• Fiber strengthening: Fibres of high strength such as those of alumina, boron, graphite, or tungsten are incorporated in a ductile and low strength matrix to increase their strength. Such materials are called composites.
• Strain hardening: Metals and alloys that do not respond to hardening by heat treatment are strengthened by cold working. Hardness and strength increase by cold working (plastically deforming at low temperatures). Higher, this is also accompanied by a decrease in ductility and increased tendency to corrosion. This may need a compromise between the two or a remedial measure like annealing.

Q.11. Explain the term ‘Fracture Toughness‘.

Ans. In all engineering materials defects, cracks or flaws are inevitably present. They may be introduced during the solidification, fabrication, or heat treatment stages of the material. The fracture resisting capability of a machine component or an engineering structure, therefore, must be evaluated in the presence of cracks. The fracture resistance of a material in the presence of cracks or discontinuities is known as its fracture toughness.

Mechanical Properties of Materials Viva Questions