Category: Interview Questions

Type of Material	Definition	Examples
Metals	Materials with high strength, ductility, and good electrical/thermal conductivity. Commonly used in structural and mechanical applications.	Steel, Aluminum, Copper
Polymers	Long-chain organic materials with low density, good corrosion resistance, and easy manufacturability. Generally weaker than metals.	PVC, Nylon, Polyethylene
Ceramics	Hard, brittle, heat-resistant inorganic materials. Excellent in high-temperature and wear applications.	Glass, Porcelain, Silicon Carbide
Composites	Combination of two or more materials to get superior properties. High strength-to-weight ratio.	CFRP, GFRP
Semiconductors	Materials with electrical conductivity between conductors and insulators. Used in electronic and computing devices.	Silicon, Germanium
Smart Materials	Materials that change properties with temperature, stress, or magnetic field. Used in advanced systems.	Shape Memory Alloys, Piezoelectrics

Mechanical Property	Simple Definition
Strength	Ability of a metal to withstand an applied load without failure. Includes tensile, compressive, and shear strength.
Hardness	Resistance to indentation, scratching, or wear. Indicates surface durability.
Ductility	Ability to deform plastically without breaking. Measured by % elongation.
Malleability	Ability to be shaped or rolled into thin sheets without cracking.
Toughness	Ability to absorb energy before fracture. Combination of strength and ductility.
Elasticity	Ability to return to original shape after removing the load. Governed by Young’s modulus.
Plasticity	Property that allows permanent deformation under load. Useful in forming processes.
Creep	Time-dependent slow deformation under constant load at high temperature.
Fatigue Strength	Ability to resist failure under repeated or cyclic loading.
Resilience	Ability to store energy and release it when the load is removed (elastic energy).

Parameter	Static Stress	Fluctuating Stress
Definition	Stress that remains constant with time.	Stress that varies with time (changes in magnitude and sign).
Load Type	Steady, unchanging load.	Repeated, alternating, or cyclic load.
Failure Type	Produces immediate or static failure.	Causes fatigue failure over time.
Design Basis	Yield strength (Sy).	Endurance limit (Se), fatigue theories.
Examples	Columns under constant load, beams with static weight.	Rotating shafts, connecting rods, springs.

Type of Stress	Definition	Stress Range	Example
Fluctuating Stress	Stress varies between two unequal values.	σmin to σmax (both ≠ in magnitude)	Shaft with variable torque
Completely Reversed Stress	Stress changes from equal tension to equal compression.	+σ to –σ	Rotating beam test
Alternating Stress	Stress varies symmetrically between +σ and –σ; used in fatigue.	+σa to –σa	Fatigue analysis of rods
Repeated Stress	Stress varies between zero and a maximum value.	0 to +σ	Springs in machines
Variable Stress	Stress changes continuously with time due to varying load.	Irregular	Machine components under dynamic load

Type of S–N Curve	Definition	Materials	Key Feature
Finite Life Curve	Shows failure at high stresses within limited cycles.	Most materials	Steep drop in life as stress increases.
Endurance Limit Curve	Curve becomes horizontal after a point; below this stress, failure won’t occur.	Ferrous materials (steel)	Has endurance limit (Se).
No Endurance Limit Curve	No horizontal region; failure occurs at any stress if cycles are high enough.	Non-ferrous materials (Al, Cu)	Only fatigue strength at specific cycles.
Low Cycle Fatigue Curve	Represents high stress + low cycles (<10⁴).	Heavy load components	Plastic deformation dominates.
High Cycle Fatigue Curve	Represents low stress + high cycles (>10⁴–10⁶).	Steel, Aluminum	Elastic deformation dominates.

Theory / Criterion	Description	Used For	Nature
Soderberg Line	Very safe; uses yield strength with endurance limit.	Ductile materials, conservative design.	Linear & most conservative.
Goodman Line	Uses ultimate strength with endurance limit.	General fatigue design.	Linear; less conservative than Soderberg.
Gerber Curve	Uses a parabolic curve between endurance limit and ultimate strength.	Ductile materials under fluctuating loads.	Nonlinear; more accurate, less conservative.
ASME Elliptic Theory	Combines shear, yield, and endurance limits in elliptical form.	Shafts & machine members.	Moderate conservatism, realistic.
Modified Goodman	Similar to Goodman but includes factor of safety.	General purpose, safer than Goodman.	Linear with safety factor.
Goodman–Soderberg Comparison	Not a theory, but used to compare how conservative each is.	Design selection.	Soderberg < Goodman < Gerber (conservative → less conservative).

Parameter	Conduction	Convection	Radiation
Definition	Heat transfer through direct contact of molecules in a solid.	Heat transfer due to fluid (liquid or gas) motion.	Heat transfer through electromagnetic waves without any medium.
Medium Required	Solid medium required.	Fluid medium required.	No medium required (can occur in vacuum).
Heat Transfer Mechanism	Molecule-to-molecule vibration.	Bulk movement of fluid particles.	Emission and absorption of thermal radiation.
Example	Heating one end of a metal rod.	Boiling water circulations.	Sun’s heat reaching Earth.
Rate of Transfer	Slow.	Moderate.	Fast.

Coordinate System	General Heat Conduction Equation
Cartesian (x, y, z)	$\frac{\partial T}{\partial t} = α (\frac{\partial^{2} T}{\partial x^{2}} + \frac{\partial^{2} T}{\partial y^{2}} + \frac{\partial^{2} T}{\partial z^{2}})$
Cylindrical (r, θ, z)	$\frac{\partial T}{\partial t} = α (\frac{1}{r} \frac{\partial}{\partial r} (r \frac{\partial T}{\partial r}) + \frac{1}{r^{2}} \frac{\partial^{2} T}{\partial θ^{2}} + \frac{\partial^{2} T}{\partial z^{2}})$
Spherical (r, θ, φ)	$\frac{\partial T}{\partial t} = α (\frac{1}{r^{2}} \frac{\partial}{\partial r} (r^{2} \frac{\partial T}{\partial r}) + \frac{1}{r^{2} \sin θ} \frac{\partial}{\partial θ} (\sin θ \frac{\partial T}{\partial θ}) + \frac{1}{r^{2} \sin^{2} θ} \frac{\partial^{2} T}{\partial ϕ^{2}})$

Property	Definition	Formula	Unit
Thermal Conductivity (k)	Ability of a material to conduct heat. Higher k means better heat conduction.	$q = - k A \frac{d T}{d x}$	W/m·K
Thermal Resistance (R)	Opposition offered by a material to heat flow. Higher R means lower heat transfer.	$R = \frac{L}{k A}$	K/W
Thermal Diffusivity (α)	Rate at which heat spreads through a material. Indicates how quickly temperature changes.	$α = \frac{k}{ρ C_{p}}$	m²/s

Type of Fluid	Definition
Ideal Fluid	No viscosity and no frictional losses; imaginary fluid for theory.
Real Fluid	Has viscosity; actual fluids we see in real life.
Newtonian Fluid	Viscosity remains constant; follows Newton’s law of viscosity (e.g., water, air).
Non-Newtonian Fluid	Viscosity changes with applied shear (e.g., toothpaste, blood).
Incompressible Fluid	Density remains constant during flow (e.g., liquids).
Compressible Fluid	Density changes significantly with pressure (e.g., gases).

Fluid Property	Simple Definition (2–3 lines)
Density (ρ)	Mass per unit volume of a fluid. Indicates how heavy the fluid is.
Specific Weight (γ)	Weight per unit volume. Shows how strongly gravity acts on the fluid.
Specific Gravity (SG)	Ratio of fluid density to water density. No units.
Viscosity (μ)	Internal resistance to flow. Higher viscosity → thicker fluid.
Kinematic Viscosity (ν)	Ratio of viscosity to density. Represents flow behavior without gravity effect.
Pressure (p)	Force applied by the fluid per unit area.
Temperature (T)	Measure of fluid heat energy affecting viscosity and density.
Vapor Pressure	Pressure at which fluid starts to vaporize.
Surface Tension	Force acting on the fluid surface causing it to behave like a stretched film.
Capillarity	Rise or fall of fluid in a narrow tube due to surface tension.

Property	Dynamic Viscosity (μ)	Kinematic Viscosity (ν)
Definition	Resistance offered by a fluid to shear or flow.	Ratio of dynamic viscosity to fluid density.
Formula	μ = τ / (du/dy)	ν = μ / ρ
Units	N·s/m² or Pa·s	m²/s
Depends on	Fluid’s internal friction.	Viscosity and density both.
Meaning	Indicates how “thick” or sticky the fluid is.	Indicates how easily the fluid flows under gravity.
Example	Honey has high μ, water has low μ.	Kinematic viscosity of oil > water because of higher μ/ρ.

Thermodynamics

1. What is thermodynamics?

Thermodynamics is the study of heat, energy, and their transformations.
It explains how energy flows between systems and how it affects work and temperature.

2. Explain the laws of thermodynamics ?

Zeroth law defines temperature equality, first law is energy conservation, second law explains entropy, and third law states entropy becomes zero at absolute zero. Together, they describe how energy behaves in all systems.

3. What is the difference between heat and work?

Heat is energy transfer due to temperature difference, while work is energy transfer due to force or motion. Both are boundary phenomena and not stored in a system.

4. Define system, surroundings, and boundary ?

Term	Definition	Key Points
System	The part of the universe selected for study.	Can be open, closed, or isolated depending on mass/energy exchange.
Surroundings	Everything outside the system that can interact with it.	Interacts with the system through heat, work, or mass (in open systems).
Boundary	The real or imaginary surface that separates the system from surroundings.	Can be fixed or movable; determines what enters or leaves the system.

5. What is entropy?

Entropy is a measure of disorder or randomness. Higher entropy means more energy is unavailable for useful work.

6. What is the Zeroth Law of Thermodynamics?

If two bodies are each in thermal equilibrium with a third body, they are in thermal equilibrium with each other. It forms the basis of temperature measurement.

7. Explain enthalpy ?

Enthalpy is the total heat content of a system. It is useful for studying heat changes at constant pressure.

8. What is a thermodynamic process?

A thermodynamic process is any change in the state of a system. Examples include isothermal, adiabatic, isobaric, and isochoric processes.

9. Difference between open, closed, and isolated systems.

Type of System	Definition	Mass Transfer	Energy Transfer (Heat/Work)	Example
Open System	A system that exchanges both mass and energy with its surroundings.	Yes	Yes	Boiler, human body, turbine
Closed System	A system that exchanges only energy but not mass with surroundings.	No	Yes	Piston–cylinder with fixed mass
Isolated System	A system that exchanges neither mass nor energy with surroundings.	No	No	Thermos flask (ideal), universe

10. What is steady-state and unsteady-state?

In steady-state, properties remain constant with time. In unsteady-state, properties change with time.

December 1, 2025

Solid Mechanics

1.What are the key assumptions made in Strength of Materials analysis, and why are they important for simplifying the study of material behavior under stress?

The key assumptions in Strength of Materials are:

Homogeneity – Material properties are uniform throughout.
Isotropy – Properties are the same in all directions.
Linear Elasticity – Stress is proportional to strain (Hooke’s Law).
Small Deformations – Deformations are minimal, ensuring linear behavior.
Plane Sections Remain Plane – Cross-sections remain flat during bending.

These assumptions simplify the analysis by allowing linear models and ignoring complexities like material nonlinearity or large deformations.

2.Engineering stress/strain and True stress/strain ?

Aspect	Engineering Stress/Strain	True Stress/Strain

Definition

Based on original dimensions (area/length).

Accounts for current dimensions during deformation.

Formula

Stress = Force / Original Area, Strain = ΔLength / Original Length

Stress = Force / Instantaneous Area, Strain = ln(1 + ΔLength / Original Length)

Accuracy

Less accurate for large deformations.

More accurate for large strains and plastic deformation.

Application Range

Valid in elastic region, small deformations.

More valid in plastic region, for large deformations.

Representation

Assumes constant original dimensions throughout the process.

Considers changing dimensions (area/length) during deformation.

Measurement Focus

Initial length and area.

Instantaneous length and area.

3.What are the different elastic constants in Strength of Materials?

Elastic Constant	Symbol	Definition
Young’s Modulus	E	Ratio of normal stress to normal strain. Measures stiffness of a material. Higher E → more rigid.
Shear Modulus / Modulus of Rigidity	G	Ratio of shear stress to shear strain. Indicates resistance to shear deformation.
Bulk Modulus	K	Ratio of volumetric stress to volumetric strain. Shows how incompressible a material is.
Poisson’s Ratio	ν	Ratio of lateral strain to longitudinal strain. Indicates how a material contracts laterally when stretched.

4. What are thermal stress and thermal strain?

Parameter	Definition	Formula
Thermal Strain	Change in length due to change in temperature. It occurs even without external load.	εₜ = α ΔT
Thermal Stress	Stress developed when thermal expansion or contraction is restricted. No restriction → no thermal stress.	σₜ = E α ΔT