Testing of Materials MCQ Quiz - Objective Question with Answer for Testing of Materials - Download Free PDF

Explanation:

Charpy Impact Test

• The Charpy impact test, also known as the Charpy V-notch test, is a standardized high strain-rate test that determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of the material's toughness and acts as an indicator of its ability to resist brittle fracture.

Working Principle: In the Charpy impact test, a notched specimen is struck by a swinging pendulum hammer at a fixed speed. The specimen is typically supported at both ends, and the hammer impacts the specimen at the notch. The energy absorbed by the specimen during fracture is measured by the height to which the pendulum rises after breaking the specimen. This energy is directly related to the toughness of the material.

Procedure:

• A rectangular bar specimen, usually with a V-notch or U-notch in the middle, is prepared according to standardized dimensions.
• The specimen is placed horizontally between two supports in the testing machine.
• The pendulum hammer is released from a known height to strike the specimen at the notch.
• The pendulum swings through the specimen, breaking it and rising to a height determined by the energy absorbed during fracture.
• The difference in the height of the pendulum before and after the impact is used to calculate the absorbed energy.

Explanation:

Factor of Safety (FoS) Analysis

• The Factor of Safety (FoS) is a measure used in engineering to describe the load-carrying capacity of a system beyond the expected or actual loads. It is a dimensionless number that represents the ratio of the maximum load a component can withstand to the actual load applied during normal operation.

The formula for the Factor of Safety is given by:

FoS = (Maximum Load) / (Actual Load)

In practical terms, a Factor of Safety is used to provide a buffer against unexpected loads, material defects, and uncertainties in the design and manufacturing processes. It ensures that the component will perform reliably under specified conditions.

When a component is designed with a certain Factor of Safety, it means that the component can handle a load that is a multiple of the actual load it is expected to carry. For example, if a component is designed with a FoS of 2, it can theoretically handle twice the actual load it will experience in operation. This approach helps account for unknown factors that could affect the component's performance.

Concept:

The Vickers Hardness Number (VHN) is used to quantify the hardness of materials based on the size of an indentation left by a diamond-shaped indenter.

Formula:

\( \text{VHN} = \frac{1.854 \cdot F}{d^2} \)

Where:

• F = Load in kgf = 20 kg
• d = Diagonal of indentation in mm = 0.3 mm

Calculation:

\( \text{VHN} = \frac{1.854 \cdot 20}{(0.3)^2} = \frac{37.08}{0.09} = 412 \)

Explanation:

Impact Strength:

• Impact strength is a measure of a material's ability to withstand a suddenly applied load or force without breaking. It is a critical property for materials that are subjected to dynamic loading conditions, such as in automotive, aerospace, and construction applications. The higher the impact strength, the more resistant the material is to sudden impacts or shocks.
• Impact strength is typically measured in units of energy, as it represents the amount of energy a material can absorb before fracturing. The standard unit of energy in the International System of Units (SI) is the joule (J). During impact testing, a material sample is subjected to a controlled impact, and the energy absorbed by the sample is measured in joules. This measurement helps determine the material's toughness and its ability to resist fracture under sudden loading conditions.

Measurement Methods:

There are several standardized methods for measuring impact strength, including the Charpy and Izod impact tests:

• Charpy Impact Test: In the Charpy test, a notched sample is struck by a pendulum hammer, and the energy absorbed in breaking the sample is measured. The test provides valuable information about the material's toughness and its behavior under dynamic loading conditions.
• Izod Impact Test: Similar to the Charpy test, the Izod test involves striking a notched sample with a pendulum hammer. However, in the Izod test, the sample is clamped vertically, and the impact is delivered to the upper part of the sample. The energy absorbed in fracturing the sample is measured in joules.

Applications of Impact Strength Measurement:

Understanding the impact strength of materials is crucial in various industries, including:

• Automotive: Ensuring that materials used in vehicle components can withstand sudden impacts during collisions.
• Aerospace: Selecting materials that can endure high-impact forces during takeoff, flight, and landing.
• Construction: Choosing materials for building structures that can resist dynamic loads, such as earthquakes or heavy machinery impacts.

Explanation:

In the context of hardness testing, the Rockwell C scale is a widely used method for determining the hardness of materials, particularly metals. The Rockwell C hardness test, also known as the Rockwell Hardness Test, employs a diamond cone indenter, also known as a Brale indenter, to measure the hardness of a material. This scale is particularly suitable for harder materials, such as steels and other hard alloys.

Rockwell C Scale (Correct Option 4):

Definition: The Rockwell hardness test is a method for measuring the hardness of a material by determining the depth of penetration of an indenter under a large load compared to the penetration made by a preload. The Rockwell C scale specifically uses a diamond cone indenter with a 120-degree angle and applies a major load of 150 kgf (kilogram-force).

Working Principle: The Rockwell C hardness test involves two main steps. First, a minor load of 10 kgf is applied to the material using the diamond cone indenter, establishing a zero reference position. Then, a major load of 150 kgf is applied, causing the indenter to penetrate the material further. After holding the major load for a specific dwell time, the major load is removed while maintaining the minor load. The depth of the indentation is measured, and the hardness value is calculated based on the difference in depth between the minor and major loads.

Advantages:

• Quick and straightforward testing process.
• Non-destructive, as the indentation is usually small.
• Suitable for a wide range of materials, including hard metals.
• Provides direct reading of hardness value on the dial or digital display.

Disadvantages:

• Limited to testing materials with flat and smooth surfaces.
• Not suitable for very soft materials, as the diamond cone may cause excessive deformation.

Applications: The Rockwell C scale is widely used in industrial applications to determine the hardness of steel, tool steel, and other hard alloys. It is commonly employed in quality control, material selection, and research and development.

Let's now analyze the other options provided in the question:

Option 1: Mohs Scale

The Mohs scale of mineral hardness is a qualitative ordinal scale that characterizes the scratch resistance of various minerals by comparing their ability to scratch softer materials. It was created by the German geologist and mineralogist Friedrich Mohs in 1812. The Mohs scale ranges from 1 (talc) to 10 (diamond), with diamond being the hardest known natural material. The Mohs scale does not use an indenter; instead, it relies on the ability of a material to scratch another material. Therefore, the Mohs scale is not suitable for determining the hardness of metals and does not use a diamond cone indenter.

Option 2: Brinell Scale

The Brinell hardness test is a method used to determine the hardness of materials, primarily metals and alloys. The test involves pressing a hard steel or carbide ball indenter into the material's surface under a specified load. The diameter of the resulting indentation is measured, and the Brinell hardness number (BHN) is calculated. The Brinell test does not use a diamond cone indenter; instead, it uses a spherical indenter, typically made of hardened steel or tungsten carbide. The Brinell test is more suitable for materials with coarse or uneven surfaces and is often used for softer metals and alloys.

Option 3: Vickers Scale

The Vickers hardness test is another method for measuring the hardness of materials, particularly metals and ceramics. The test involves pressing a diamond pyramid indenter with a square base and an angle of 136 degrees between opposite faces into the material's surface under a specified load. The Vickers hardness number (VHN) is calculated based on the diagonal length of the resulting indentation. While the Vickers test uses a diamond indenter, it is not a cone-shaped indenter like in the Rockwell C scale. The Vickers test is known for its high accuracy and is suitable for a wide range of materials, including very hard and very thin materials.

In conclusion, the correct option for the hardness scale that uses a diamond cone indenter is the Rockwell C scale (Option 4). This scale is widely used for measuring the hardness of metals, particularly harder alloys, and provides quick and straightforward results. The Mohs scale, Brinell scale, and Vickers scale each have their specific applications and indenter types, but none of them employ a diamond cone indenter as used in the Rockwell C scale.

Explanation:

Ductility:

• The property of a material by virtue of which, it can be drawn into the wire with the application of tensile force is known as ductility. It is the property of a structure that indicates yield deformation before the fracture in the structure. It is an indication of how much plastic strain a material can withstand before it breaks. A ductile material can withstand large strains even after it has begun to yield. Common measures of ductility include percent elongation and reduction in area, as discussed in this section.

Toughness

• It is the ability of a material to absorb energy and gets plastically deformed without fracturing. Its numerical value is determined by the amount of energy per unit volume. Its unit is Joule/m³. 
• For example brittle materials, having good strength but limited ductility are not tough enough. Conversely, materials having good ductility but low strength are also not tough enough. Therefore, to be tough, the material should be capable to withstand both high stress and strain.

Hardness

• It is the ability of a material to resist to permanent shape change due to external stress. There are various measures of hardness – Scratch Hardness.

Brittleness

• The brittleness of a material indicates how easily it gets fractured when it is subjected to a force or load. When a brittle material is subjected to stress it observes very less energy and gets fractures without significant strain.
• Brittleness is converse to the ductility of the material. The brittleness of the material is temperature-dependent. Some metals which are ductile at normal temperatures become brittle at low temperatures.

Malleability

• Malleability is a property of solid materials which indicates how easily a material gets deformed under compressive stress.
• Malleability is often categorized by the ability of a material to be formed in the form of a thin sheet by hammering or rolling.
• This mechanical property is an aspect of the plasticity of the material. The malleability of material is temperature dependent. With the rise in temperature, the malleability of material increases.

Creep and Slip

• Creep is the property of a material that indicates the tendency of a material to move slowly and deform permanently under the influence of external mechanical stress. It results due to long time exposure to large external mechanical stress with in limit of yielding.

Resilience

• Resilience is the ability of a material to absorb the energy when it is deformed elastically by applying stress and release the energy when stress is removed. Proof resilience is defined as the maximum energy that can be absorbed without permanent deformation. The modulus of resilience is defined as the maximum energy that can be absorbed per unit volume without permanent deformation. It can be determined by integrating the stress-strain cure from zero to elastic limit. Its unit is joule/m3.

Fatigue

• Fatigue is the weakening of a material caused by the repeated loading of the material. When a material is subjected to cyclic loading, and loading greater than certain threshold value but much below the strength of material (ultimate tensile strength limit or yield stress limit), microscopic cracks begin to form at grain boundaries and interfaces. 

Explanation:

Additional Information

Ductility:

• It is the property of the material that enables it to be drawn out or elongated to an appreciable extent before rupture occurs.
• Ductility is the ability of a metal to exhibit large deformation or plastic response when being subjected to a tensile force.
• Tension test is done to check or measure the ductility.
• 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, aluminum, mild steel are typical ductile materials.

Toughness:

• Toughness is defined as the ability of the material to absorb energy before fracture takes place.
• This property is essential for machine components that are required to withstand impact loads.
• Tough materials have the ability to bend, twist or stretch before failure takes place.
• Toughness is measured by a quantity called modulus of toughness. Modulus of toughness is the total area under the stress-strain curve in a tension test.
• Toughness is measured by Izod and Charpy impact testing machines.
• When a material is heated it becomes ductile or simply soft and thus less stress is required to deform the material and the stress-strain curve will shift down and the area under the curve decreases thus toughness decreases.
• Toughness decreases as temperature increases.

Endurance limit:

• Endurance limit is defined as the maximum value of the completely reversed bending stress that a material can withstand for the infinite number of cycles without fatigue failure.
• This endurance limit is less than the yield strength and ultimate strength of the material.
• Creep is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material.
• Impact loads are sudden loads and their variation is very fast with respect to time, unlike the static loads whose variation is very low with respect to time. This is in no way related to the endurance limit.

Hardness Test:

• Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. 
• Hardness Testing measures a material’s strength by determining resistance to penetration.
• There are various hardness test methods, including Rockwell, Brinell, Vickers, Knoop and Shore Durometer testing.

Transmission refers to the passage of sound waves through a material, which is an acoustical property of construction material. This property is measured to determine how much sound the material can absorb or reflect.

Important PointsThermal resistivity: This is a thermal property. It refers to a material's resistance to conductive heat transfer.Creep: It's a mechanical property. It refers to the tendency of a solid material to slowly move or deform permanently under the influence of mechanical stresses.
Hygroscopicity: This property refers to a substance's ability to absorb moisture from the air, which is a water interaction property.

Explanation:

Hardness 

• It is the measure of materials localized plastic deformation.
• Hardness test are performed by forcing a small indentor into the surface of a material to be tested and under controlled conditions of load and rate of application.
• The size of the resulting indentation is related to hardness numbers.

Rockwell hardness test:

• Rockwell hardness testers use a number of indenters in combination with a variety of loads.
• It has 1/16, 1/8, 1/4, 1/2  inch diameter spherical steel indenter as well as conical diamond indenter which is used for the hardest material.
• Each of the indenters can be used with a major load of 60, 100, or 150 kg, and a minor load of 10 kg.
• The hardness number is the difference between the penetrations caused by major and minor load applications.
• Based on these load and indenters scales are developed,
  • Rockwell A scale, 60 kg with diamond indenter is used for steels and similar hard alloys.
  • Rockwell B scale, 100 kg with 1/16 inch diameter sphere indenter are used for aluminum alloys.
  • Rockwell C scale, 150 kg with the diamond pyramid indenter are used for steel and similar hard alloys.
  • Copper alloys are measured in k scales.
  • Polymers are measured in the Rockwell E and M scales. M scales are used for hard polymers.

Brinell hardness Scale:

• In Brinell hardness test a hard spherical indenter of 10 mm diameter is used, and the load varies between 500 and 3000 kg.
• Harder materials require greater applied load.

In Brittle materials under tension test undergoes brittle fracture i.e their failure plane is 90° to the axis of load and there is no elongation in the rod that’s why the diameter remains same before and after the load. Example: Cast Iron, concrete etc

But in case of ductile materials, material first elongates and then fail, their failure plane is 45° to the axis of the load. After failure cup-cone failure is seen. Example Mild steel, high tensile steel etc.

Explanation:

UTM:

A Universal Testing Machine is a machine that is used to perform standard tensile and compressive tests on materials, components, and structures.

It can be used to perform the following tests:

• Tensile Test
• Compressive Test
• Shear Test

Working Principle:

It is a load-controlled machine and works on the principle of elongation/deformation of material on the application of load.

Concept:

Fatigue:

• The deformation of a material due to t repeated cycle of stress & strain results in progressive cracking.
• In a typical fatigue failure, a microscopic crack developed at a point of high stress and gradually enlarges as the loads are applied repeatedly, when the crack becomes large sudden failure occurs.

Endurance limit 

• It is the stress level below which even a large number of stress cycles can not produce fatigue failure.

Hence, The maximum stress at which even a billion reversals of stress can not cause the failure of the material is called as endurance limit.

Explanation:

Isotropic Material:

If the response of the material is independent of the orientation of the load axis of the sample, then we say that the material is isotropic.

A material is said to be isotropic when it exhibits the same elastic properties in any direction at a given point.

Homogenous Material:

A material is homogenous if it has the same composition throughout the body. Hence the elastic properties are the same at every point in the body.

Isotropic material can be either homogeneous or non-homogeneous.

Mistake Points A material is said to be isotropic when it exhibits the same elastic properties in any direction at a given point. When the elastic properties is same at all locations, then the material is Homogenous.

Explanation:

Distress:

• It is a collective term for the physical manifestation of problems such as cracks, spalls, pop-outs, staining, decay or corrosion.
• Distress can be thought of as the symptoms indicating that the defects are present.

Additional InformationDefects:

• The defects are the flaws those creeps into structure because of design mistakes or poor workmanship during manufacturing, fabrication and construction, before it begins its service life, or by inappropriate operation and maintenance during its service life . The flaw that has a potential to lead to a failure, becomes a defect. 

Explanation:

Plasticity:

• Plasticity of a material is its ability to undergo some degree of permanent deformation without rupture or failure.
•  This property is important in forming, shaping, extruding, and many other hot and cold working processes.

Elasticity:

• It is the property of a material to regain its original shape after deformation when the external forces are removed.
• All materials are plastic to some extent but the degree varies, for example, both mild steel and rubber are elastic materials but steel is more elastic than rubber.

Durability:

• It is defined as the ability of a product to perform its required function over a lengthy period under normal conditions of use without excessive expenditure on maintenance or repair.