What is the difference between stress and strain?
Quick Answer
Stress (σ) is force per unit area (F/A), measured in N/m² or Pascal—it's the internal resistance in material. Strain (ε) is deformation ratio (ΔL/L), dimensionless—it's the resulting change. Related by Hooke's Law: σ = Eε, where E is Young's Modulus (material stiffness). Stress is the cause, strain is the effect.
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Why Interviewers Ask This
Most fundamental concept in Strength of Materials
Tests understanding of material behavior under load
Critical for design and failure analysis
Foundation for understanding mechanical properties
Required knowledge for structural analysis
Concept Explanation
Simple Explanation (Start Here)
Stress is the "cause" (force applied), strain is the "effect" (deformation produced). If you pull a rubber band, stress is how hard you pull (force per area), strain is how much it stretches (change in length divided by original length). Stress has units (N/m² or Pascal), strain is dimensionless (ratio).
Real-World Analogy
Imagine hanging a weight on a spring: Stress is like the "burden" the spring feels (force/area)—a property of loading. Strain is the "visible effect"—how much the spring stretches. A stiffer spring (higher Young's Modulus) will show less strain for the same stress.
Detailed Technical Explanation
Stress (σ): Internal resistance developed within a material when external force is applied. Defined as force per unit cross-sectional area. σ = F/A [N/m² or Pascal] Types: Tensile (pulling), Compressive (pushing), Shear (sliding)
Strain (ε): Measure of deformation. Ratio of change in dimension to original dimension. ε = ΔL/L (for linear strain) [Dimensionless] Types: Linear (length change), Lateral (width change), Volumetric, Shear
Hooke's Law: Within elastic limit, stress is proportional to strain. σ = E × ε, where E = Young's Modulus (material property)
Key Facts to Remember
- Stress: Force per unit area, has units (N/m² = Pa), internal resistance
- Strain: Deformation ratio, dimensionless, external effect
- Hooke's Law: σ = Eε (within elastic limit)
- Young's Modulus (E): Ratio of stress to strain, material stiffness property
- Types of Stress: Tensile, Compressive, Shear, Bending, Torsional
- Types of Strain: Linear, Lateral, Volumetric, Shear
Quick Comparison Table
Use this table to quickly understand the key differences:
| Aspect | Stress (sigma) | Strain (epsilon) |
|---|---|---|
| Definition | Force per unit area (F/A) | Deformation ratio (deltaL/L) |
| Unit | N/m^2 or Pascal | Dimensionless |
| Nature | Cause (internal resistance) | Effect (resulting change) |
| Formula | sigma = F/A | epsilon = deltaL/L |
| Relationship | sigma = E * epsilon | epsilon = sigma/E |
| Measurement | Requires force sensor | Requires length measurement |
| Material Property | Depends on area | Independent of dimensions |
Formulas & Code
Stress: σ = F/A [N/m² or Pa]Strain: ε = ΔL/L [Dimensionless]Hooke's Law: σ = E × εYoung's Modulus: E = σ/ε [Pa]Poisson's Ratio: ν = Lateral Strain / Linear StrainVisual Explanation
Draw a stress-strain curve for mild steel showing: Proportional limit, Elastic limit, Yield point (upper and lower), Ultimate stress, Breaking point. Mark elastic region (Hooke's Law applies) and plastic region. Show the formula E = σ/ε as slope in elastic region.
Pro tip: Draw this diagram while explaining to leave a strong impression.
Common Mistakes to Avoid
- ✗Giving units to strain (it's dimensionless)
- ✗Confusing stress (internal) with pressure (external)
- ✗Forgetting Hooke's Law only applies within elastic limit
- ✗Not mentioning Young's Modulus as the connecting property
- ✗Confusing tensile and compressive stress
Pro Tips for Success
- ✓Remember: Stress = Cause (force), Strain = Effect (deformation)
- ✓Draw the stress-strain diagram—it shows comprehensive understanding
- ✓Know typical E values: Steel ≈ 200 GPa, Aluminum ≈ 70 GPa, Rubber ≈ 0.01 GPa
- ✓Be ready to explain why strain is dimensionless (ratio of same quantities)
Expected Follow-up Questions
Key Takeaways
- Stress = F/A [Pa], Strain = ΔL/L [dimensionless]
- Stress is cause (internal), strain is effect (deformation)
- Hooke's Law: σ = Eε (elastic region only)
- Young's Modulus = slope of stress-strain curve
- Steel E ≈ 200 GPa, Aluminum ≈ 70 GPa
Related Questions You Should Know
Explain the four strokes of an IC engine
A four-stroke engine works like breathing deeply: (1) Inhale = Suction stroke (take in air-fuel), (2) Hold breath = Compression stroke (squeeze the mixture), (3) Cough = Power stroke (spark ignites, pushes piston), (4) Exhale = Exhaust stroke (push out burnt gases). One complete cycle = 2 crankshaft rotations = 720°.
What is the difference between one-way and two-way slab?
Imagine carrying a rectangular tray: If it's very long and narrow (like a ruler), it bends mainly along the length—that's one-way slab behavior. If it's nearly square, it bends in both directions equally—that's two-way slab behavior. The aspect ratio (Ly/Lx) determines which type it is.
Research Foundations
Our Mechanical Engineering interview guides are built on established pedagogical research and industry best practices. Here are the key sources that inform our approach:
Dr. HC Verma
Concepts of Physics (1992)
“Understanding fundamentals deeply enables solving complex problems by breaking them into basic principles.”
How We Apply This:
When answering technical questions, always start from first principles. Interviewers value candidates who understand WHY, not just WHAT.
Gayle Laakmann McDowell
Cracking the Coding Interview (2022)
“Technical interviews test problem-solving process, not just memorized answers.”
How We Apply This:
Think out loud, explain your reasoning, and show how you approach unfamiliar problems systematically.
Richard Feynman
The Feynman Technique
“If you cannot explain something simply, you do not understand it well enough.”
How We Apply This:
Practice explaining complex concepts in simple terms. Use analogies and real-world examples to demonstrate mastery.
NPTEL Faculty
National Programme on Technology Enhanced Learning
“Strong fundamentals in core subjects differentiate exceptional engineers from average ones.”
How We Apply This:
Revisit core subjects from your curriculum. Most technical questions test fundamental concepts, not advanced topics.
George Pólya
How to Solve It (1945)
“A systematic approach to problem-solving works across all engineering domains.”
How We Apply This:
Use a structured approach: Understand → Plan → Execute → Verify. Interviewers notice methodical thinking.
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