What is factor of safety

In machine design, while designing the component, it is necessary to provide sufficient reserve strength in case of an accident so this is achieved by taking a suitable factor of safety.

fs = Failure stress / Allowable stress

fs = Failure load / Working load

The magnitude of the factor of safety depends upon the following factors:
  • Effect of failure 
  • Types of load
  • A degree of accuracy in force analysis
  • Material of component
  • Reliability of component
  • Cost of component
  • Testing of the machine element
  • Service conditions
  • Quality of manufacture
Points mention below the following condition where a higher factor of safety is chosen :
  • Magnitude and nature of external forces acting on the machine component cannot be precisely estimated.
  • The material of the machine component has a non-homogeneous structure.
  • The component of the machine is subject to the force of impact in service.
  • There is a possibility of residual stresses in a machine component.
  • The machine part is subjected to a high temperature during operation.
  • In applications such as aircraft components, higher reliability is required.
  • There is a possibility if abnormal variation in external load on some occasions.
  • The machine part's manufacturing quality is poor.
  • There is stress concentration in a machine component.
A higher factor of safety increases the component's reliability.

Factors to be considered during machine design

In machine design, there are so many factors to consider when designing the machine because the small amount of machine error leads to a high amount of loss so it is better to take care of some factor when designing the machine.
The list of these factors is given below :
  • Cost
  • High output and efficiency
  • Strength
  • Stiffness or rigidity
  • Wear resistance
  • Lubrication
  • Operational safety
  • Ease of assembly
  • Ease and simplicity of disassembly
  • Ease and simplicity of servicing and control
  • Lightweight and minimum dimensions
  • Reliability
  • Durability
  • Economy of performance
  • Accessibility
  • Processability
  • Compliance with state standards
  • The economy of repairs and maintenance
  • Use of standard parts
  • Use of easily available materials
  • The appearance of the machine
  • Number of machines to be built

Types of thermodynamic process

Introduction of thermodynamic process : 


Before going to study the thermodynamic process and types of thermodynamic processes, let us understand the meaning of the thermodynamic state of the system. The system has a certain temperature, pressure, volume, etc. characteristics. The present values of the system property are called the thermodynamic state of the system. 


Thermodynamic process :


When the system undergoes a change from one thermodynamic state to final state due change in properties such as temperature, pressure, and volume etc the system is said to have undergone the thermodynamic process. Types of the thermodynamic process described below. 


In simple word, a thermodynamic process occurred when the system changes from initial state to the final state.

  • Process - Adiabatic 
Properties held constant - Heat energy 
  • Process - Isenthalpic 
Properties held constant - Enthalpy
  • Process - Isentropic 
Properties held constant - Entropy, Heat energy, Equilibrium 
  • Process - Isobaric 
Properties held constant - Pressure 
  • Process - Isochoric 
Properties held constant - Volume 
  • Process - Isothermal   
Properties held constant - Temperature
  • Process - Isotropic 
Properties held constant - Direction 
  • Process - Polytropic 
Properties held constant - PVn = C
  • Process - Reversible 
Properties held constant - Entropy, Equilibrium 

Adiabatic process:  


An adiabatic process occurs when no heat can flow between a thermodynamic system and its surroundings. 


In this process Q = 0.
Adiabatic Process

Example - Vertical flow of air in the atmosphere, Air expands and cools as it rises, and contracts and grows warmer as it descends. 

Isenthalpic process : 


An isenthalpic process is also called isoenthalpic process. It is a thermodynamic process in which enthalpy is constant. 

In this process H = 0. 

Example - Throttling process, consider the lifting of a relief valve or safety valve on a pressure vessel.


Isentropic process :


An isentropic process is an idealized thermodynamic process in which both adiabatic and reversible. 

It is a process in which entropy remains constant. 

In this process ΔS  = 0.




Example - Some isentropic thermodynamics device such as pumps, gas compressors, turbines, nozzles, diffusers.


Isothermal process :  


An isothermal process is a change of a system, in which the temperature of the system stays constant but heat may flow in or out of the system during an isothermal process. 
In this process ΔT = 0.
Isothermal Process

Example - Condensation, All the reactions going on in the refrigerator as a constant temperature is maintained in it, Melting of ice at zero degrees, and heat pump. 



Isochoric process :  


An isochoric process as the name suggests iso means same and choric means volume also called constant-volume process or isovolumetric process or isometric process. 

It is a thermodynamic process during which the volume of the closed system is kept constant.

In this process ΔV = 0.
Isochoric process

Example - Heating of a gas in a closed cylinder.


Isobaric process : 


An isobaric is a thermodynamic process where the pressure of the system stays constant. 
In this process  ΔP  = 0.
Isobaric process


Example: Heating of water in an open vessel and the expansion of a gas in a cylinder with a freely moving piston.


Isotropic process : 


The isotropic process is one that the permittivity ε and permeability μ of the medium is uniform in all directions of the medium. 

Example - Glass and metals are examples of isotropic materials. 


Reversible processes :  


A reversible process is a process whose direction can be reversed by including infinitesimal changes to some property of the system via its surroundings. In thermodynamics, throughout the entire process, the system is in thermodynamic equilibrium with its surroundings.
Reversible Process


Example - Frictionless relative motion, and expansion and compression of spring.


Polytropic Process :


A polytropic is a thermodynamic process that obeys the relation where p is the pressure, V is volume, n is the polytropic index and C is a constant. The equation of this process describes multiple expansion and compression processes which include heat transfer. 


PVn = C
From this relationship, we can arrive at relationships for several other types of a thermodynamic process.

  • When n = 0 the process is isobaric
  • When n = 1 the process is isothermal
  • When n = k the process is isentropic
  • When n = ∞ the process is isochoric
Example - Expansion of the combustion gasses in the cylinder of a water-cooled reciprocating engine.

Properties of a system in thermodynamics

Introduction of various properties :


Thermodynamic property is a point function and defines the state of a system. It is independent of the path followed. 

Generally, a thermodynamic property is two types one is macroscopic and another one is microscopic property.

The word microscopic means something like so small that it can only be seen with the use of microscope while macroscopic means either to something that can be seen with the naked eye or large in scale. 

If a system contains a large number of chemical species such as atoms, ions, and molecules, called macroscopic system and the properties which are associated with this system are called macroscopic properties.

Examples: pressure, volume, temperature, composition, density, viscosity, surface tension, refractive index, colour etc.


Extensive properties: 


Extensive properties depend upon the quantity of matter which is contained in the system. 

Extensive property is dependent on mass.

Examples: mass, volume, heat capacity, internal energy, enthalpy, entropy, Gibb's free energy. 


Intensive properties:  


Intensive properties depend upon the amount of the substance which is present in the system.

The intensive property is not dependent on mass.

Examples: temperature, refractive index, density, surface tension, specific heat, freezing point, and boiling point.

What is Entropy

Definition of Entropy :

Entropy is a thermodynamic quantity representing the unavailability of a system's thermal energy for conversion into mechanical work and interpreted as the molecular disorder in the system. 

In other words, entropy is the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work. OR Entropy is also the measure of the number of possible arrangements the atoms in a system can have. 

SI unit for entropy is J / K ( joules/degree Kelvin ).

Example: 

Spraying perfume in the corner of the room and we all know what happens next. The perfume will not just stay in the corner of the room but the perfume molecule eventually fills up the room. The perfume went an ordered state to a state of the disorder so the system gets disorder so is called the higher entropy.