Design of Machine Elements - short answer questions from AMIE exams (Winter 2019)

Explain briefly and draw the suitable diagram if required (10 x 2)

What do you understand by a factor of safety?

While designing a component, it is necessary to provide sufficient reserve strength in case of an accident. This is achieved by taking a suitable factor of safety (fs). The factor of safety is defined as fs = failure stress/allowable stress or  fs = failure load/working load

Define ductility and malleability.

The ductility of the material enables it to draw out into thin wire on the application of the load. Mild steel is a ductile material. The wires of gold, silver, copper, aluminium, etc. are drawn by extrusion or by pulling through a hole in a die due to the ductile property. The ductility' decreases with the increase in temperature. The per cent elongation and the reduction in area in tension are often used as empirical measures of ductility.

The malleability of a material is its ability to be flattened into thin sheets without cracking by hot or cold working. Aluminium, copper, tin, lead, steel, etc. are malleable metals.
Lead can be readily rolled and hammered into thin sheets but can not be drawn into wire. Ductility is a tensile property, whereas malleability is a compressive property. Malleability increases with an increase in temperature.

Define the toughness and hardness of a material.

  • The toughness of a material is its ability to withstand both plastic and elastic deformations. It is a highly desirable quality for structural and machine parts to withstand shock and vibration. Manganese steel, wrought iron, mild steels are tough materials.
  • Example: If a load is suddenly applied to a piece of mild steel and then to a piece of glass the mild steel will absorb much more energy before failure occurs. Thus, mild steel is said to be much tougher than glass.
  • Toughness is a measure of the amount of energy a material can absorb before acmal fracture or failure takes place. "The work or energy a material absorbs is called modulus of toughness”
  • Toughness is also called resistance to shock loading. 
  • It is measured by a special test on Impact Testing Machine.
  • Hardness is closely related to strength. It is the ability of a material to resist scratching, abrasion, indentation, or penetration.
  • It is directly proportional to tensile strength and is measured on special hardness testing machines by measuring the resistance of the material against penetration of an indentor of special shape and material under a given load. 
  • The different scales of hardness are Brinell hardness. Rockwell hardness. Vicker's hardness, etc.
  • The hardness of a metal does not directly relate to the hardenability of the metal. Hardenability is indicative of the degree of hardness that the metal can acquire through the hardening process, i.e. heating or quenching.

In between rubber and steel which is more elastic, and why?

The strain produced in rubber is much larger compared to that in steel. This means that steel has a larger value of Young’s modulus of elasticity and hence, steel has more elasticity than rubber.

Draw an S-N Curve for bearing life.

The results of these tests are plotted by means of an S-N curve. The S-N curve is the graphical representation of stress amplitude (Sf versus the number of stress cycles (N) before the fatigue failure on a log-log graph paper. The S-N curve for steels is illustrated in the given figure. 

S-N curve

Each test on the fatigue testing machine gives one failure point on the S-N diagram. In practice, the points are scattered in the figure and an average curve is drawn through them. 

Which type of threads are used in Screw Jack and at which angle of thread its efficiency be maximum. Write its formula.

Square and trapezoidal threads are the most commonly used threads.
  • Square thread: In this thread, the flanks are perpendicular to the axis of the thread. This is used for transmitting motion or power. Example Fly presses, Screw jack, vice handles, cross-slide and compound slide etc.
  • Trapezoidal threads: These threads have a profile that is neither square nor V thread form and have a form of a trapezoid. Different forms of trapezoidal threads are ACME thread, Buttress thread, Saw-tooth thread, Worm thread

What do you understand by stress concentration?

In practice, discontinuities and abrupt changes in cross-section are unavoidable due to certain features of the component such as oil holes and grooves, keyways and splines, screw threads and shoulders. Therefore, it cannot be assumed that the cross-section of the machine component is uniform. Under these circumstances, the ‘elementary’ equations do not give correct results.

Let us consider a  plate with a small circular hole. 

Stress concentration

When the plate is loaded at the edges, It is observed that there is a sudden rise in the magnitude of stresses in the vicinity of the hole. The localized stresses in the neighbourhood of the hole are far greater than the stresses obtained by elementary equations.

Stress concentration is defined as the localization of high stresses due to the irregularities present in
the component and abrupt changes of the crosssection.

Draw the stress-strain curve for ductile and brittle material.

Stress-Strain Curves for Ductile Materials
If a mild steel bar of uniform cross-sectional area is subjected to gradually increasing axial tensile
force (generally is done in Universal Testing Machine) till failure of the bar occurs, and if we plot
the graph for stress and strain, the following curve may be obtained.

Stress-strain curve for ductile material

The curve may be divided into the following parts:
  • Portion OA: This portion is absolutely straight, where the stress is proportional to strain and the material obeys Hooke’s law. The value of stress at point A is called proportional limit.
  • Portion AB: In this portion, Hook’s law is not obeyed, although the material may still be elastic. Point B indicates the elastic limit.
  • Portion BC: In this portion, the metal shows an appreciable strain even without further increase
  • in stress and the strain is not fully recoverable when the load is removed.
  • Portion CC': Yielding commences in this portion and there is a drop of stress at the point C' immediately after yielding commences at C. The point C' is termed as lower yield point and C is called the upper yield point.
  • Portion C'D: After yielding has taken place at C', further straining takes place at this portion by increasing the stress and the stress-strain curve continues to rise up to points D. Strain in this portion is about 100 times that of portion O to C. At point D, the bar begins to form a local neck. Point D is termed as an ultimate tensile stress point. Ultimate stress is calculated at this point.
  • Portion DE: In this portion, the load falls off from the maximum until fracture at E takes place. Point E is termed as fracture or breaking point and the corresponding stress is called breaking stress.
Stress-Strain Curves for Brittle Materials
Materials that show very small elongation before they fracture are called brittle materials. The shape of
the curve for high carbon steel is shown in the following figure and is typical of many brittle materials such as G.I., concrete and high strength light alloys. For most brittle materials the permanent elongation (i.e. increase in length) is less than 10%.

Stress-Strain Curves for Brittle Materials

Define plain carbon steel. How it is designated according to Indian standards?

Carbon steel or plain-carbon steel is a metal alloy. It is a combination of two elements, iron and carbon. Other elements are present in quantities too small to affect their properties.
The plain carbon steels varying from 0.06% carbon to 1.5% carbon are divided into the following types depending upon the carbon content.
  • Dead mild steel — up to 0.15% carbon
  • Low carbon or mild steel — 0.15% to 0.45% carbon
  • Medium carbon steel — 0.45% to 0.8% carbon
  • High carbon steel — 0.8% to 1.5% carbon
Steels are designated by a group of letters or numbers indicating any one of the following three properties.
  • tensile strength;
  • carbon content; and
  • composition of alloying elements.
Steel, which is standardized based on their tensile strength without detailed chemical composition, are specified in two ways- a symbol Fe followed by the minimum tensile strength in N/mm². Another method is FeE steel followed by the yield strength in N/mm2.

  • Fe350 – This indicates steel with a tensile strength of 250 newtons per mm square.
  • FeE 250- yield strength of 250 N/mm².
BIS Designation of Plain Carbon Steels

This consist following three quantities:
  • figure and indicating 100 times the average percentage of carbon.
  • a letter C
  • a figure indicating 10 times the average percentage of manganese.
  • 55C4 indicates plain carbon steel with 0.55% carbon and 0.4 % manganese.

What is the difference between modulus of elasticity and modulus of rigidity?

Direction of Force
  • Modulus of elasticity is used to calculate the deformation of an object when a deforming force acts at right angles to the surface of the object.
  • Modulus of rigidity is used to calculate deformations when a deforming force acts parallel to the surface of an object.
Change in Shape
  • Where modulus of elasticity is calculated, the object under the deforming force either gets lengthened or shortened.
  • Where modulus of rigidity is calculated, one of the surfaces of the object becomes displaced with respect to another surface.
Relative Size
  • For most materials, the modulus of elasticity is larger than the modulus of rigidity. The exceptions to this rule are the so-called “auxetic” materials which have negative Poisson’s ratios, but these materials are less common.
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