Table of Contents
What Is Mechanical Stress?
Mechanical stress is the internal force per unit area within a material that arises in response to an externally applied load. Measured in Pascals (Pa) or megapascals (MPa), stress determines whether a structural member can safely carry its design load. When stress exceeds the material's yield strength, permanent deformation occurs; beyond the ultimate tensile strength, fracture follows.
Understanding stress is fundamental to all structural and mechanical engineering. Every bridge, building, aircraft, and machine component is designed so that the maximum stress remains well below the material's allowable stress, with an appropriate safety factor. The ratio of allowable stress to working stress is the factor of safety, typically ranging from 1.5 to 4 depending on the application.
Stress, Strain & Hooke's Law
Where σ is stress (Pa), F is force (N), A is area (m²), ε is strain (dimensionless), E is Young's modulus (Pa), L is original length, and ΔL is the change in length. These relationships hold within the elastic region (Hooke's Law).
Material Yield Strengths
| Material | Yield Strength (MPa) | E (GPa) |
|---|---|---|
| Mild Steel (A36) | 250 | 200 |
| High-Strength Steel | 350-700 | 200 |
| Aluminum 6061-T6 | 276 | 69 |
| Titanium Ti-6Al-4V | 880 | 114 |
| Copper (annealed) | 70 | 117 |
| Concrete (compression) | 20-40 | 30-45 |
| Nylon 6/6 | 70 | 3.5 |
Types of Stress
- Tensile stress: Pulling forces that tend to elongate the material.
- Compressive stress: Pushing forces that tend to shorten the material.
- Shear stress: Forces acting parallel to a cross-section (τ = V/A).
- Bending stress: Combination of tension and compression from bending moments (σ = My/I).
- Torsional stress: Shear stress from twisting (τ = Tr/J).
Frequently Asked Questions
What is the difference between stress and pressure?
Both have units of force per area (Pa), but stress refers to internal forces within a solid material due to applied loads, while pressure typically refers to a force distributed over a surface by a fluid. Stress can be tensile (pulling apart); pressure is always compressive.
What is a factor of safety?
Factor of safety (FoS) = Material Strength / Working Stress. An FoS of 2 means the component can withstand twice the expected load before failure. Higher FoS values are used for critical applications (bridges: 3-5, aircraft: 1.5, consumer products: 2-3).
What happens when stress exceeds yield strength?
Beyond the yield point, the material deforms plastically—it will not return to its original shape when unloaded. The material continues to deform under increasing stress until it reaches its ultimate tensile strength, after which necking begins and fracture follows.