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

Explanation:

Fatigue Failure:

• Fatigue failure is one of the most critical failure mechanisms observed in engineering materials and structures. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. This failure mechanism is particularly dangerous because it can occur well below the ultimate tensile strength or even the yield strength of the material. Fatigue failure is a leading cause of failure in mechanical components such as shafts, gears, and bridges.
• When a material undergoes repeated loading and unloading (cyclic loading), microscopic cracks may form at stress concentration points, such as sharp corners, holes, or surface imperfections. Over time, these cracks propagate and grow, eventually leading to catastrophic failure. The phenomenon is influenced by several factors, including the magnitude of cyclic stress, the number of cycles, material properties, and environmental conditions.

Additional Information

Stages of Fatigue Failure:

1. Crack Initiation: Fatigue failure begins with the initiation of a small crack at a point of high stress concentration, such as a surface defect, notch, or inclusion in the material.

2. Crack Propagation: Under cyclic loading, the initiated crack grows incrementally with each load cycle. The crack growth rate depends on the magnitude of the applied stress and the material's resistance to crack propagation.

3. Final Fracture: When the crack grows to a critical size, the remaining cross-sectional area of the material becomes insufficient to support the applied load, leading to sudden and catastrophic fracture.

Factors Influencing Fatigue Failure:

• Stress Concentrations: Features such as sharp corners, notches, and holes act as stress concentrators and significantly reduce fatigue life.
• Surface Finish: A rough surface finish can reduce fatigue resistance because it provides sites for crack initiation.
• Material Properties: Materials with higher toughness and ductility generally exhibit better fatigue resistance.
• Environment: Corrosive environments can accelerate fatigue failure through a process known as corrosion fatigue.
• Load Magnitude: Higher cyclic stress amplitudes result in faster crack growth and shorter fatigue life.

Explanation:

Stainless Steel and Its Alloying Elements

• Stainless steel is a corrosion-resistant alloy of iron that contains significant amounts of alloying elements to enhance its properties, such as durability, strength, and resistance to oxidation. The primary alloying elements in stainless steel typically include chromium, nickel, and carbon. These elements play critical roles in imparting the unique characteristics of stainless steel, making it suitable for a wide range of applications, including construction, automotive, medical equipment, and kitchen utensils.

Lead

• Lead is NOT a primary alloying element in stainless steel. This is because lead does not contribute to the corrosion resistance, strength, or durability of stainless steel. In fact, lead is generally avoided in stainless steel due to its toxicity and inability to bond effectively with the alloy matrix. Lead is primarily used in applications requiring lubrication or machining ease, such as in free-cutting steels, but it is not suitable for stainless steel compositions.
• Stainless steel relies on elements such as chromium and nickel for its corrosion resistance and mechanical properties. Lead does not provide any of these benefits and would compromise the integrity and performance of the alloy. Therefore, it is excluded from the formulation of stainless steel.

Chromium

• Chromium is the most critical alloying element in stainless steel. It imparts corrosion resistance by forming a thin, stable oxide layer on the surface of the steel, known as the "passive layer." This layer prevents further oxidation and protects the material from rust and environmental damage. Typically, stainless steel contains at least 10.5% chromium to achieve its signature corrosion-resistant properties.

Carbon

• Carbon is another important element in stainless steel, although its percentage is kept low to prevent carbide precipitation, which can lead to corrosion. In some grades of stainless steel, carbon is intentionally added in controlled amounts to enhance strength and hardness. For example, high-carbon stainless steels are used in applications requiring wear resistance, such as knives and cutting tools.

Nickel

• Nickel is a key alloying element in austenitic stainless steels. It stabilizes the austenitic crystal structure, which provides excellent toughness and ductility, even at low temperatures. Nickel also enhances corrosion resistance, especially in environments containing acids or chlorides. Common grades of stainless steel, such as 304 and 316, contain significant amounts of nickel.

Additional Information

Stainless steel is classified into several types based on its microstructure:

• Austenitic Stainless Steel: Contains high chromium and nickel, providing excellent corrosion resistance and ductility.
• Ferritic Stainless Steel: Contains chromium but little or no nickel, offering good corrosion resistance and magnetic properties.
• Martensitic Stainless Steel: Contains chromium and higher carbon levels, providing strength and hardness but lower corrosion resistance.
• Duplex Stainless Steel: A mix of austenitic and ferritic structures, offering a balance of strength and corrosion resistance.
• Precipitation-Hardening Stainless Steel: Contains alloying elements like aluminum or copper, which enable heat treatment for enhanced strength.

Explanation:

Electroplating

• Electroplating is a process that uses an electric current to reduce dissolved metal cations so that they form a coherent metal coating on an electrode. This process is widely used for decorative purposes, corrosion resistance, and improving surface properties. Electroplating involves the movement of metal ions from the anode to the cathode through an electrolytic solution.

The rate of anodic dissolution is found equal to the rate of deposition of metal onto the substrate.

• This statement accurately describes a fundamental principle of electroplating. During the electroplating process, the metal ions from the anode dissolve into the electrolyte solution at a rate equal to their deposition onto the cathode (the substrate to be plated). This ensures a steady-state process, where the amount of metal lost from the anode equals the amount of metal deposited on the cathode.

The process is governed by Faraday's laws of electrolysis, which state that the mass of a substance deposited or dissolved during electrolysis is directly proportional to the amount of electric charge passed through the electrolyte.

Key Steps in Electroplating:

• The substrate to be plated (cathode) and the metal to be deposited (anode) are submerged in an electrolytic solution containing metal ions of the anode.
• An electric current is passed through the electrolyte, causing the metal anode to dissolve into the solution as metal cations.
• These cations migrate toward the cathode, where they gain electrons and are deposited as a metal layer.

Explanation:

Mild Steel

• Mild steel, also known as low-carbon steel, is a widely used material due to its excellent balance of mechanical properties and cost-effectiveness. One of the critical factors influencing the mechanical properties of mild steel is its carbon content. The carbon content of mild steel typically ranges from 0.15% to 0.30%. When the carbon content increases within this range, it has a profound effect on the mechanical properties such as strength, hardness, and ductility.

When the carbon content in mild steel increases, the following changes occur:

• Strength: The strength of mild steel increases with an increase in carbon content. This happens because carbon atoms enhance the lattice structure of the steel by creating solid-solution strengthening and forming carbides that obstruct dislocation movement. Dislocations are responsible for the deformation of materials, and their hindrance leads to higher strength. Therefore, higher carbon content improves the tensile strength and yield strength of mild steel.
• Ductility: Ductility, which is the ability of a material to undergo plastic deformation before fracture, decreases as carbon content increases. This reduction in ductility occurs because the increased strength and hardness of the steel reduce its ability to deform plastically. In essence, the steel becomes more brittle with higher carbon content.

Explanation:

Nitriding Process:

• Nitriding is a surface-hardening heat treatment process that introduces nitrogen into the surface of a material, typically steel or other ferrous alloys. This process increases surface hardness, wear resistance, and fatigue strength while maintaining the material's core properties. Nitriding is widely used in industries such as automotive, aerospace, and manufacturing for components like gears, crankshafts, and dies.
• The nitriding process can be performed through various methods, each employing a different medium to introduce nitrogen into the material. These methods include plasma nitriding, gas nitriding, and liquid nitriding. The choice of method depends on the specific application, material type, and desired properties.

Vacuum nitriding

• Vacuum nitriding is NOT a type of nitriding process. While the term "vacuum nitriding" might imply a nitriding process carried out in a vacuum, this is not a recognized or standard method in the field of nitriding. Nitriding processes generally require a medium (such as gas, plasma, or liquid) to introduce nitrogen into the surface of the material. A vacuum environment, on the other hand, is typically used for processes like vacuum carburizing or vacuum heat treatment, where the absence of oxygen prevents oxidation and ensures precise control of the process atmosphere. Therefore, the inclusion of "vacuum nitriding" as a nitriding method is incorrect.

Additional Information

Option 1: Plasma nitriding

• Plasma nitriding, also known as ion nitriding, is a nitriding process that uses plasma (a state of matter consisting of charged particles) to introduce nitrogen into the surface of the material. In this process, the material is placed in a vacuum chamber, and a high-voltage electrical field is applied, creating plasma from a nitrogen-containing gas. The plasma ions bombard the material's surface, causing nitrogen to diffuse into the material. Plasma nitriding offers precise control over the process and is suitable for complex geometries and materials.

Option 2: Gas nitriding

• Gas nitriding is a commonly used nitriding process where the material is exposed to a nitrogen-rich gas, such as ammonia (NH₃), at elevated temperatures. The ammonia gas dissociates into nitrogen and hydrogen, and the nitrogen diffuses into the material's surface. Gas nitriding is widely used for its simplicity and ability to create a uniform and controlled nitrided layer.

Option 3: Liquid nitriding

• Liquid nitriding, also known as salt bath nitriding, involves immersing the material in a molten salt bath containing nitrogen-rich compounds. The nitrogen diffuses into the material's surface, creating a hardened layer. Liquid nitriding is known for its rapid processing time and ability to treat complex shapes uniformly. However, environmental concerns related to the disposal of the salt bath have limited its use in some regions.

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.

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

(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: