Engineering Materials Science MCQ Quiz - Objective Question with Answer for Engineering Materials Science - Download Free PDF

Explanation:

Compressive Strength of Materials

• Compressive strength is the capacity of a material to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. It is measured by applying a force to the material until it fails and recording the amount of force per unit area.

Cast Iron:

• Cast iron is an alloy of iron that contains 2-4% carbon, along with varying amounts of silicon and manganese and traces of impurities such as sulfur and phosphorus. The high carbon content makes cast iron very brittle, but it also significantly enhances its compressive strength. Cast iron has a compressive strength in the range of 600 MPa (megapascals) to 700 MPa, making it suitable for applications where high compressive loads are present. This high compressive strength is why cast iron is widely used in the construction of columns, bases, and other load-bearing structures.

Additional InformationOption 1: Copper

Copper is a ductile metal with excellent electrical conductivity, thermal conductivity, and corrosion resistance. However, its compressive strength is significantly lower than that of cast iron. Copper has a compressive strength of approximately 210 MPa. While copper is used in various engineering applications, particularly in electrical components and plumbing, its compressive strength is not comparable to that of cast iron.

Option 2: Mild Steel

Mild steel, also known as low carbon steel, contains approximately 0.05-0.25% carbon. It is known for its ductility, weldability, and relatively low cost. Mild steel has a compressive strength of about 250 MPa to 400 MPa, which is higher than that of copper but still lower than that of cast iron. While mild steel is widely used in construction, automotive, and manufacturing industries due to its versatility, it does not match the compressive strength of cast iron.

Option 3: Rubber

Rubber is a highly elastic material commonly used in applications requiring flexibility and resilience. However, rubber has a very low compressive strength compared to metals and alloys. The compressive strength of rubber varies depending on its formulation but typically ranges from 10 MPa to 20 MPa. Rubber's primary applications include seals, gaskets, and flexible joints, where its low compressive strength is not a limiting factor.

The correct answer is 4) Electrical insulators for high-voltage transmission lines.

Concept:

• High Mechanical Strength: High-voltage transmission lines require insulators that can support the weight of the conductors and withstand strong winds, ice loads, and other mechanical stresses. Mineral insulators like porcelain and glass possess excellent mechanical strength.
• Electrical Insulating Properties: These materials have high dielectric strength, meaning they can withstand very high voltages without allowing current leakage or breakdown. This is crucial for preventing short circuits and ensuring the safe and efficient transmission of power.

Additional Information

• Batteries and electrodes: While some minerals are used in batteries, they are chosen for their electrochemical properties rather than their bulk mechanical strength and electrical insulation. Insulation within batteries uses different materials.
• Electrical wires for household use: The insulation on household wires is typically made of polymers (plastics or rubber) which offer flexibility and adequate insulation for lower voltages. While some mineral-insulated cables exist for specialized applications (like fire survival circuits), they are not the typical choice for general household wiring.
• Low-voltage electronic devices: Low-voltage electronics often use plastic or ceramic materials for insulation due to their ease of manufacturing and suitability for smaller components. The high mechanical strength of mineral insulators is usually not a primary requirement in these applications.

The correct answer is 4) Copper.

Concept:

• High Conductivity: Copper is well-known for its excellent electrical conductivity, second only to silver among pure metals. This allows for efficient transfer of electrical current with minimal energy loss.  
   

   
• Resistance to Corrosion: Copper has good resistance to corrosion in various environments. It forms a protective layer of copper oxide or copper carbonate (patina) when exposed to the atmosphere, which inhibits further corrosion.  
   

   
• Cost-Effective: While gold also has high conductivity and excellent corrosion resistance, it is significantly more expensive than copper, making copper the preferred choice for most electrical conductor applications.
   

Option Analysis:

• Manganin: Manganin is an alloy known for its stable electrical resistance over a range of temperatures, making it suitable for precision resistors, not general conductors.  
   
• Steel: Steel has relatively low electrical conductivity compared to copper and is prone to corrosion (rusting) unless specially treated or alloyed.  
   
• Gold: While having excellent conductivity and corrosion resistance, its high cost limits its use to specialized applications where these factors outweigh the expense, such as in electronics for critical contacts.

The Correct Answer is 2) The maximum voltage a dielectric material can withstand without breaking down

Explanation:

Dielectric Strength is a key property of insulating (dielectric) materials and is defined as:

• The maximum electric field (or voltage per unit thickness) that a material can withstand without electrical breakdown.

• It is typically expressed in kV/mm or V/mil.

When the dielectric strength is exceeded:

• The material loses its insulating properties

• It allows current to pass, resulting in dielectric breakdown

Option Analysis

• 1) Electrical conductivity under stress – Conductivity is the opposite of insulation.

• 3) Amount of heat tolerated before melting – Refers to thermal properties, not dielectric.

• 4) Resistance to thermal expansion – Describes mechanical/thermal property, not electrical insulation.

Explanation:

Brittleness in Materials

Definition: Brittleness is a property of materials that causes them to break or shatter without significant deformation when subjected to stress. Brittle materials absorb very little energy before fracture, making them susceptible to sudden catastrophic failure.

Correct Option Analysis:

The correct option is:

Option 2: In tools that need to withstand heavy impact.

Brittleness would be most undesirable in tools that need to withstand heavy impact. Tools such as hammers, chisels, or any equipment used in construction and manufacturing processes are frequently subjected to heavy impacts and stresses. If these tools were made of brittle materials, they would be prone to breaking or shattering upon impact, posing a significant safety risk to users and potentially causing damage to workpieces. Therefore, materials with high toughness and the ability to absorb impact without fracturing are preferred for such applications.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: In materials used in high-speed applications.

In high-speed applications, materials are required to have high strength and durability, but brittleness is not necessarily the most undesirable property. Depending on the specific application, materials can be designed to accommodate stresses and strains at high speeds. For instance, in some high-speed machinery, components might not experience heavy impacts but rather continuous and uniform loads, where brittleness might not be as critical a concern as in tools subject to heavy impact.

Option 3: In structural beams under static load.

Structural beams under static load primarily need to have high strength and the ability to bear loads without excessive deformation. While brittleness can be undesirable, it is not the most critical factor in this context. Ductility and strength are more critical properties for structural beams to prevent sudden failure and to ensure the structure can support loads safely over time. The primary concern would be the beam's ability to withstand static loads without excessive deflection or failure, rather than the impact resistance.

Option 4: In materials exposed to high temperatures.

High temperatures can affect the mechanical properties of materials, but brittleness is not the sole concern. Materials exposed to high temperatures must maintain their strength, thermal stability, and resistance to thermal expansion. While brittleness at high temperatures can be problematic, it is typically addressed by selecting materials that resist thermal degradation and maintain ductility and toughness even at elevated temperatures.

Conclusion:

Understanding the importance of material properties such as brittleness in different applications is crucial for ensuring safety and functionality. In the context of tools that need to withstand heavy impact, brittleness is highly undesirable because it can lead to sudden and catastrophic failure, posing significant safety risks. In other applications, such as high-speed machinery, structural beams under static load, and materials exposed to high temperatures, brittleness can be managed or is less critical compared to other material properties like strength, ductility, and thermal stability.

Concept:

• The hardness of the mineral is defined on Moh's scale of hardness. On this scale, a mineral is rated between 1-10 on the basis of its strength.
• It is used to rate the hardness of a variety of substances and elements, not only metals. The softest materials it rates are assigned a rating of 1; the hardest earn a rating of 10.​ 

Explanation:

The Moh's scale of different minerals shown below -

• Tungsten is the hardest metal. ∴ Option 4 is correct.
• Platinum is a less hard metal. That's why it is used in Jewellery. It can make intricate designs. It is highly ductile.
• The name Tungsten originates from the Swedish name tungsten meaning heavy stone.
• Hardness is the ability to scratch making a dent on the surface of the metal. It is just a number measured using (Rockwell, Brinell, Vickers test)  Of which Brinell is most accurate.
• Gold: 25 Mpa
• Platinum: 40 Mpa
• Tungsten: 310 Mpa
• Iron: 150  Mpa
• Diamond: 10000 Mpa (Non-metal)
• It is a chemical element with atomic number 74 that has the highest tensile strength of all the metals present in the world. Its symbol is  "W"
• When combined with carbon, tungsten becomes stronger and even more durable. Tungsten carbide is the end product of mixing tungsten with carbon. Tungsten carbide is 4 times stronger than platinum with a hardness rating of 9 on the Mohs scale, softer only than diamond.
• From the above, 310 > 40, So, Tungsten is harder than Platinum.

Additional Information

• The Youngs Modulus value of Tungsten is 34.48 × 1010 Pa and 
• The Youngs Modulus value of Platinum is 14.48 × 1010 Pa 

Explanation:

• An alloy is a homogeneous mixture of two or more metals or nonmetals.
• Alloys are metal mixtures with other elements and the combination of both is governed by the properties required.
• The following table shows some metals with there alloys.

Important Points

Duralumin: It is an aluminium alloy. It contains 3.5 to 4.5% copper, 0.4 to 0.7% manganese, 0.4 to 0.7% magnesium and the remaining being aluminium. It is widely used in the aircraft industry for forging, stamping, bars, sheets, rivets, and so on.

Hindalium: It contains 5% copper and the rest aluminium. It is used for containers, utensils, tubes, rivets, etc.

Ductility

• Ductility is the property of the material that enables it to be drawn out or elongated to an appreciable extent before rupture occurs.
• The percentage elongation or percentage reduction in the area before rupture of a test specimen is the measure of ductility. Normally if the percentage elongation exceeds 15% the material is ductile and if it is less than 5% the material is brittle.
• Lead, copper, aluminium, mild steel are typical ductile materials.

Brittleness

• Brittleness is the opposite of ductility. Brittle materials show little deformation before fracture and failure occur suddenly without any warning i.e. it is the property of breaking without much permanent distortion. Normally if the elongation is less than 5% the material is brittle. E.g. cast iron, glass, ceramics are typical brittle materials.

Malleability

• Malleability is the property by virtue of which a material may be hammered or rolled into thin sheets without rupture. This property generally increases with the increase of temperature.
• Malleability is the ability of a metal to exhibit large deformation or plastic response when being subjected to compressive force.
• Lead, soft steel, wrought iron, copper and aluminium are some materials in order of diminishing malleability.
• A material that can be beaten into thin plates is said to possess the property of malleability.

Elasticity: 

• When an external force acts on the body, the body tends to undergo some deformation.
• If the external force is removed, then the body comes back to its original shape and size, the body is known as elastic body and this property is called elasticity.

Plasticity: 

• A plastic material does not regain its original shape after removal of load. An elastic material regains its original shape after removal of load.

Ductility: 

• A property by virtue of which the substance can be drawn into a wire, is called ductile substance.

Explanation:

If the atoms or atom groups in the solid are represented by points and the points are connected, the resulting lattice will consist of an orderly stacking of blocks or unit cells.

• The orthorhombic unit cell is distinguished by three lines called axes of twofold symmetry about which the cell can be rotated by 180° without changing its appearance.
• This characteristic requires that the angles between any two edges of the unit cell be right angles but the edges may be any length.

Important Points

There are 7 types of crystal systems:

• Ductility is when a solid material stretches under tensile stress. If ductile, a material may be stretched into a wire.
• Malleability, a similar property, is a material's ability to deform under pressure (compressive stress).
• If malleable, a material may be flattened by hammering or rolling.

Explanation:

Elasticity is the ability of a body that resists body to distort under any force and try to return to its original shape and size when that force is removed.

Elasticity is measured from the modulus of elasticity which is defined as the ratio of stress to strain up to the elastic limit.

The modulus of elasticity or Young’s modulus is the slope of the stress-strain curve in the elastic region.

\(E = \frac{\sigma }{\epsilon}\)

The modulus of elasticity is highest for steel among the given materials and is taken as 200 GPa.

Explanation:

Steel is an alloy of iron and carbon, along with small amounts of other alloying elements or residual elements as well. The plain iron-carbon alloys (Steel) contain 0.002 - 2.1% by weight carbon. For most of the materials, it varies from 0.1-1.5%.

There are 3 types of plain carbon steel:

(i) Low-carbon steels: Carbon content in the range of < 0.3%

(ii) Medium carbon steels: Carbon content in the range of 0.3 – 0.6%.

(iii) High-carbon steels: Carbon content in the range of 0.6 – 1.4%.

Resistance to corrosion: Is the ability of a material that resists against reaction with caustic elements that corrode or degrade the material.

Ultimate Strength: The maximum strength the material can withstand without breaking.

Hardness is defined as the resistance of a material to penetration or permanent deformation. It usually indicates resistance to abrasion, scratching, cutting or shaping.

Ductility is the ability of a material to withstand tensile force when it is applied upon it as it undergoes plastic deformation, this is often characterized by the material's ability to be stretched into a wire. 

With the increase in carbon content, the strength, hardness, and brittleness increase but the ductility and toughness decrease.

Because with an increase in carbon the cementite phase in the material increases and since cementite is hard and brittle so the ductility decreases with an increase in carbon.

The correct answer is Graphite.

Key Points

Graphite is not used as a coolant in nuclear reactors.

• A coolant in a nuclear reactor is used to remove heat from the machine core and transfer it to the environment. 
• Almost all currently operating nuclear power plants are light water reactors (LWRs) using ordinary water under high pressure as coolant.
• Heavy water reactors use deuterium (isotope of Hydrogen) oxide which has identical properties to ordinary water but much lower neutron capture.

Additional Information

Parameters for a good coolant:

Frenkel defect:

• It is a type of point defect in a crystal lattice when an atom or ion leaves its own lattice site vacant and instead of that, it occupies a normally vacant site.
• It is also called a Dislocation defect.
   

Schottky defect:

• It was named after Walter H. Schottky.
• It is a type of point defect in a crystal lattice that occurs when oppositely charged ions or atoms leave their lattice sites, creating vacancies.
   

The radius ratio for AgBr is intermediate. Thus, it shows both Frenkel and Schottky's defects.

Soft magnetic materials have a small area of the hysteresis loop.

Hysteresis Loop (B.H Curve):

• Consider a completely demagnetized ferromagnetic material (i.e. B = H = 0)
• It will be subjected to the increasing value of magnetic field strength (H) and the corresponding flux density (B) measured the result is shown in the below figure by the curve O-a-b.
• At point b, if the field intensity (H) is increased further the flux density (B’) will not increase anymore, this is called saturation b-y is called solution flux density.
• Now if field intensity (H) is decreased, the flux density (B) will follow the curve b-c. When field intensity (H) is reduced to zero, flux remains the iron this is called remanent flux density or remanence, it is shown in fig. O-C.
• Now if the H increased in the opposite direction the flux density decreases until the point d here the flux density (B) is zero.
• The magnetic field strength (points between O and d) require to remove the residual magnetism i.e. reduce B to zero called a coercive force.
• Now if H is increased further in the reverse direction causes the flux density to increase in the reverse direction all the saturation point e.
• If H is varied backwords OX to O-Y, the flux Density (B) follows the curve b-c-d-d.
• From the figure the clear that flux density changes ‘log behind the changes in the magnetic field strength this effect is called hysteresis.
• The closed figure b-c-d-e-f-g-b is called the hysteresis loop.

• The energy loss associated with hysteresis is proportional to the area of the hysteresis loop.
• The area of the hysteresis loop varies with the type of material.
• For hard material: hysteresis loop area large → hysteresis loss also more → high remanence (O-C) and large coercivity (O-d).
• For soft material: hysteresis loop area small → hysteresis loss less → large remanence and small coercivity.

Note:

The difference between soft magnetic materials & hard magnetic materials is as shown: