Mechanical Design Fundamentals – Types of Stress

 🔧 Mechanical Design Fundamentals – Types of Stress

In mechanical design, every component experiences forces. How these forces act determines the type of stress inside the material. A good designer must understand these before creating CAD/CAE models.

1️⃣ Stress (σ)

📌 Definition → Internal resistance offered by a material per unit area when an external force is applied.

📌 Formula → σ = F / A

📌 Units → N/m² (Pascal or MPa)

✔ Why it matters? Stress analysis ensures that parts don’t fail under real-world loads.

2️⃣ Tensile Stress (Tension)

Occurs when forces try to stretch or pull apart a material.

👉 Example → A rope in a crane lifting a load, or a bolt under axial pulling force.

✔ Designers must ensure the ultimate tensile strength (UTS) of the material is higher than working stress.

3️⃣ Compressive Stress (Compression)

When forces push or squeeze a material, reducing its length.

👉 Example → Columns, pistons, or press-fit joints.

⚠ Risk → Buckling (sudden sideways failure in long slender members).

4️⃣ Shear Stress (τ)

When forces act parallel to the surface, causing layers to slide over each other.

📌 Formula → τ = F / A (parallel force / shear area)

👉 Example → Rivets, bolts under shear, scissors cutting paper.

✔ Important in fastener and joint design.

5️⃣ Torsional Stress (Twisting)

When torque (rotational force) is applied, causing shear stress around the axis.

👉 Example → Shafts in engines, gearboxes, propellers.

📌 Formula → τ = T·r / J

T = Torque

r = Radius

J = Polar moment of inertia

✔ Key design factor → Angle of twist must be within limits to avoid misalignment.

6️⃣ Bending Stress (Flexural)

When a beam or shaft is subjected to a load that causes it to bend.

👉 Combination of tension (at one side) and compression (at the other).

📌 Formula → σ = M·y / I

M = Bending moment

y = Distance from neutral axis

I = Moment of inertia

👉 Example → Bridges, cantilever beams, levers.

✅ Why This Matters in CAD/CAE?

Every CAD design will eventually be checked in CAE. Knowing these stress types allows engineers to:

Select the right material

Apply the correct load conditions in FEA

Ensure safety & reliability of designs

source : Technocraft Academy