Unit 13

Question Bank

Question 1) Explain polymer?

Answer 1) Polymers have chain molecule structure of carbon as back bone atoms. They are mainly made up of tough organic materials. They are low density materials and also flexible. In some cases polymers are not flexible.

Polymers are not only used as structural materials, they can be used as fiber and resins in the matrix of composite materials.

e.g.: polyester as fibers, phenolic and epoxides as resins.

Elastomers are also polymers but they are considered separately due to their specific design for certain purposes like shock and vibration absorption.

Natural polymers:

Eg : wool, silk, DNA, cellulose, proteins, etc.

Synthetic polymers:

1. Thermo plastics
2. Thermosetting plastics

Eg: nylon, polyethylene, polyester, Teflon, epoxy, Bakelite, etc.

Applications:

1. Polyethylene is used for making carry bags.
2. Polypropylene is used for making high temperature resistance products like feeding bottle.
3. Polyether ether ketone and polyethylene ketone are used in mineral water bottle concept.
4. Poly carbonate is used to make high performance polymers like transparent polymers
5. Polyaniline is a conducting polymer.
6. Bakelite used for making insulating materials. 

Question 2) Explain Cutting Tools and Wear Components

Answer 2) The properties exploited by these applications are hardness, strength, low thermal expansion coefficient, low friction coefficient and chemical resistivity. Some of the products in this area include oil drilling bits, rock drill cutters, wire drawing dies, extrusion dies, cutting tool inserts, optical grinding tools, coatings for computer hard discs and coatings for ball bearings.

Either polycrystalline diamond or diamond coatings can be used in this area (see figure 1). When using coatings, manufacturers must pay attention to coating adhesion and ensure the coating is uniform and follows the component contours for successful use. The coating cannot be applied to ferrous materials as it will react and dissolve.

Question 3) Explain Synthetic Graphite?

Answer 3)

Synthetic graphite can be produced from coke and pitch. Although this graphite is not as crystalline as natural graphite, it is likely to have higher purity. There are basically two types of synthetic graphite. One is electro graphite; pure carbon produced from coal tar pitch and calcined petroleum coke in an electric furnace. The second is synthetic graphite, produced by heating calcined petroleum pitch to 2800 °C.

Essentially, synthetic graphite has higher electrical resistance and porosity, and lower density. Its enhanced porosity makes it unsuitable for refractory applications.

Synthetic graphite contains mainly graphitic carbon that has been attained by graphitization, heat treatment of non-graphitic carbon, or chemical vapor deposition from hydrocarbons at temperatures over 2100 K.

Question 4) Draw a classification tree of engineering materials?

Answer 4)

Question 5) Explain the difference between cast iron and carbon steel?

Answer 5)

One of the most popular ways to make lasting and quality components is through castings. Casting allows for a high level of detail, which results in not needing and additional fabrication or assembly. While many different materials can be cast, steel and iron are the two most popular due to their excellent mechanical properties for a wide range of applications.

CAST IRON usually refers to gray iron, ductile iron and malleable iron. Which is an iron casting with carbon content higher than 2%.

CAST STEEL usually refers to normal carbon steel and alloy steel. Which is a steel casting with carbon content lower than 2%.

Therefore, no big difference from the chemical content and raw materials for cast iron vs cast steel.

Corrosion Resistance

When it comes to corrosion, iron has better corrosion resistance than steel. That doesn't mean that either is impervious to corrosion though. When left unprotected, both metals will oxidize in the presence of moisture. Eventually, they will completely decompose. To prevent this, the coating is recommended for both steel and iron castings.

Cost

Cast iron is often cheaper than cast steel because of the lower material costs, energy, and labor required to produce a final product. While raw steel is more expensive, there are, however, prefabricated forms of steel. Those include sheets, rods, bars, tubes, and beams.

Castability

Cast iron is relatively easy to cast, as it pours easily and doesn't shrink as much as steel. This flowability makes cast iron an ideal metal for architectural or ornate ironwork structures such as fencing and street furniture.

Question 6) Explain the advantage and disadvantage of cast iron and cast steel?

Answer 6) The advantage and disadvantage of cast iron are given below:

The grey cast iron's good casting properties are: good vibration damping, good wear resistance, good machinability and low notch sensitivity. However, its tensile strength and elongation are very low. So, it can only produce some metal parts with low physical requirements. Requirements such as protective cover, cover, oil pan, hand wheels, frame, floor, hammer, small handle, base, frame, box, knife, bed, bearing seat, table, wheels, cover, pump, valve, pipe, flywheel, motor blocks etc. As for the higher grades, grey cast iron can withstand greater load and a certain degree of tightness or corrosion resistance. This allows for some of the more important castings such as cylinder, gear, base, flywheels, bed, cylinder block, cylinder liner, piston, gear box, brake wheel, coupling plate, medium pressure valve, etc

The ductile iron and malleable iron have high strength, ductility and heat-resistance and toughness. So a wider application, in some cases, can replace the carbon steel. However, its production technology is high. The production process is more complex. This makes the production cost higher than normal grey cast iron and cast steel. Therefore, there are more casting defects for ductile iron. There are many fields that use ductile iron, such as pressure pipes and fittings, automotive applications, agriculture, road and construction applications and general engineering applications.

The advantage and disadvantage of cast steel

The main advantage of cast steel is the design flexibility. The designer of the casting has the greatest freedom of design choices. This allows for complex shapes and hollow cross-section parts.

Cast steel has the metallurgy manufacturing flexibility and strongest variability. One can choose a different chemical composition and control that is adapted to the various requirements of different projects. This offers different heat treatment choices in the larger context of the mechanical properties and performance. Also offering good weld-ability and workability.

Cast steel is a kind of isotropic material and can be made into the overall structural strength steel castings. This improves the reliability of the project. Coupled with the design and weight, the advantages of short delivery time, price and economy gives cast steel a competitive advantage.

The weight range of steel castings is larger. Little weight can be only a few dozen grams of molten mould precision castings. The weight of large steel castings goes up to several tons, dozens of tons or hundreds of tons.

Steel castings can be used for a variety of working conditions. Its mechanical properties are superior to any other casting alloys and a variety of high-alloy steel for special purposes. To withstand high tensile stress or dynamic load of components, it is important to consider pressure vessel castings. In low or high temperature, large and important part load key parts should give priority to steel castings.

However, cast steel has comparatively bad shake-suction, wear resistance and mobility. The casting performance, compared to cast iron, is bad. Also, the costs are higher than normal cast iron.

Question 7) What is Stress?

Answer 7) when the deforming force is applied to an object. The object deforms. In order to bring the object back to the original shape and size, there will be an opposing force generated inside the object.

This restoring force will be equal in magnitude and opposite in direction to the applied deforming force. The measure of this restoring force generated per unit area of the material is called stress.

Thus, Stress is defined as “The restoring force per unit area of the material”. It is a tensor quantity. Denoted by Greek letter σ. Measured using Pascal or N/m2. Mathematically expressed as –

σ=F / A

Where,

• F is the restoring force measured in Newton or N.
• A is the area of cross-section measured in m2.
• σ is the stress measured using N/m2 or Pa.

Stress Units

Stress can be expressed using multiple units. Refer to the table given below for Stress units.

Types of Stress

There are several types of stress in physics but mainly it is categorized into two forms that are Normal Stress and Tangential or Shearing Stress. Some stress types are discussed in the points below.

Normal Stress:

As the name suggests, Stress is said to be Normal stress when the direction of the deforming force is perpendicular to the cross-sectional area of the body. The length of the wire or the volume of the body changes stress will be at normal. Normal stress can be further classified into two types based on the dimension of force-

• Longitudinal stress
• Bulk Stress or Volumetric stress

Longitudinal Stress:

Consider a cylinder. When two cross-sectional areas of the cylinder are subjected to equal and opposite forces the stress experienced by the cylinder is called longitudinal stress.

Longitudinal Stress = Deforming Force / Area of cross-section = F/A

As the name suggests, when the body is under longitudinal stress-

• The deforming force will be acting along the length of the body.
• Longitudinal stress results in the change in the length of the body, Hence thereby it affects slight change in diameter.

The Longitudinal Stress either stretch the object or compress the object along its length. Thus, it can be further classified into two types based on the direction of deforming force-

• Tensile stress
• Compressive stress

Tensile Stress

If the deforming force or applied force results in the increase in the object’s length then the resulting stress is termed as tensile stress. For example: When a rod or wire is stretched by pulling it with equal and opposite forces(outwards) at both ends.

Compressive Stress

If the deforming force or applied force results in the decrease in the object’s length then the resulting stress is termed as compressive stress. For example: When a rod or wire is compressed/squeezed by pushing it with equal and opposite forces(inwards) at both ends.

Bulk Stress or Volume Stress

When the deforming force or applied force acts from all dimension resulting in the change of volume of the object then such stress in called volumetric stress or Bulk stress. In short, when the volume of body changes due to the deforming force it is termed as Volume stress.

Shearing Stress or Tangential Stress

When the direction of the deforming force or external force is parallel to the cross-sectional area, the stress experienced by the object is called shearing stress or tangential stress. This results in the change in the shape of the body

Question 8) What is Strain?

Answer 8)

The strain is the amount of deformation experienced by the body in the direction of force applied, divided by initial dimensions of the body. The relation for deformation in terms of length of a solid is given below.

ϵ= δl / L

Where,

ϵ is the strain due to stress applied
δl is the change in length
L is the original length of the material.

The strain is a dimensionless quantity as it just defines the relative change in shape.

Depending on stress application, strain experienced in a body can be of two types. They are:

● Tensile Strain: It is the change in length (or area) of a body due to the application of tensile stress.

● Compressive Strain: It is the change in length (or area) of a body due to the application of compressive strain

When we study solids and their mechanical properties, information regarding their elastic properties is most important. These can be obtained by studying the stress-strain relationships, under different loads, in these materials.

Question 9) Writes the types of alloy steel?

Answer 9) Types of alloy steel

There are multiple subcategories of alloy steel. These include:

Question 10) Explain Stress Concentration?

Answer 10)

When an axial load is applied to a piece of material with a uniform cross-section, the norm al stress will be uniformly distributed over the cross-section. However, if a hole is drilled in the material, the stress distribution will no longer be uniform. Since the material that has been removed from the hole is no longer available to carry any load, the load must be redistributed over the remaining material. It is not redistributed evenly over the entire remaining cross-sectional area but instead will be redistributed in an uneven pattern that is highest at the edges of the hole as shown in the image. This phenomenon is known as stress concentration.