Question 1

What is material stress?

Accepted Answer

Stress (σ) is the internal force per unit area within a material resisting an applied load. SI unit: Pascal (Pa = N/m²). Common engineering unit: MPa (= N/mm²).

Question 2

Why is N/mm² the same as MPa?

Accepted Answer

1 N/mm² = 1 N / (10⁻³ m)² = 1 N / 10⁻⁶ m² = 10⁶ N/m² = 1 MPa. The two notations are numerically identical; N/mm² is common in structural engineering, MPa in physics and materials science.

Question 3

How do I convert psi to MPa?

Accepted Answer

1 psi ≈ 6894.76 Pa = 0.006895 MPa. To convert: divide psi by 145.038 to get MPa, or multiply MPa by 145.038 to get psi.

Question 4

What is ksi and where is it used?

Accepted Answer

ksi = kilopounds per square inch = 1000 psi ≈ 6.895 MPa. Used in the United States for material specifications, structural steel, and aerospace components.

Question 5

What is the difference between stress and pressure?

Accepted Answer

Pressure is an external compressive force per unit area applied to a fluid or surface. Stress is the internal force per unit area inside a solid material, and can be tensile (+), compressive (−), or shear. Both share the same unit (Pa or MPa), but describe different physical phenomena.

Question 6

What is yield strength and in what units is it measured?

Accepted Answer

Yield strength (σ_y) is the stress at which a material begins to deform plastically — i.e., it will not return to its original shape when the load is removed. It is measured in MPa (or N/mm²) in SI, and in ksi or psi in US customary units.

Bar to Pound-force per square foot

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