What is the difference between stress and strain?

Stress is the internal force per unit area within a material (σ = F/A), measured in pascals or psi. Strain is the resulting dimensional change per unit length (ε = ΔL/L), a dimensionless ratio. Stress is the cause and strain is the effect; the stress-strain curve plots one against the other to characterize a material.

What is the difference between the yield strength and the ultimate tensile strength?

Yield strength is the stress at which a material begins to deform permanently (plastically) — below it, deformation is elastic and recoverable. Ultimate tensile strength (UTS) is the maximum stress the material can withstand, the peak of the engineering stress-strain curve. Designers usually keep working stresses well below the yield strength.

What does Young's modulus tell you?

Young's modulus (the modulus of elasticity, E) is the slope of the elastic portion of the stress-strain curve, E = σ/ε. It measures stiffness — resistance to elastic deformation. A high modulus (steel ≈ 200 GPa) means the material barely stretches under load; a low modulus (rubber, polymers) means it deflects easily. It is a material property, independent of part geometry.

How is toughness different from strength or hardness?

Strength is resistance to deformation or fracture, and hardness is resistance to localized surface indentation. Toughness is the total energy a material absorbs before fracturing — the area under the stress-strain curve. A material can be very strong but brittle (low toughness), or moderately strong but very tough. Toughness combines both strength and ductility.

What is Poisson's ratio?

Poisson's ratio (ν) is the ratio of lateral (transverse) strain to axial strain when a material is stretched — when you pull a bar longer, it gets thinner. Most metals have ν around 0.3, meaning a material strained 1% lengthwise contracts about 0.3% across. It ranges from near 0 (cork) to nearly 0.5 (rubber, which conserves volume).

Engineering·13 min read·February 4, 2025

📈 Stress, Strain & Mechanical Properties of Materials

Master the stress-strain curve and the mechanical properties of materials — elastic vs plastic behavior, yield and ultimate strength, ductility, toughness, hardness, Young's modulus, and Poisson's ratio.

📖Materials Science & Metallurgy Handbook (Full Access)

View →

Why Mechanical Properties Matter

Every load-bearing component — a bridge girder, an engine crankshaft, a phone casing — must carry forces without breaking or deforming unacceptably. The mechanical properties of a material quantify exactly how it responds to those forces. Engineers extract these properties from a single, elegant experiment: the tensile test, which produces the famous stress-strain curve.

Topics covered

stress strain curvemechanical propertiesYoung's modulusyield strengthultimate tensile strengthductilitytoughnesshardnessPoisson's ratioelastic deformationplastic deformationmodulus of elasticitytensile testengineering stress