Tensile strength

Tensile strength is the maximum force a material withstands while being stretched before breaking. For a single hair fiber, tensile strength is measured in centinewtons (cN) by pulling the fiber to break at a controlled extension rate. The number is a direct measurement of the cortex's structural integrity.

Background

The standard method for hair tensile-strength testing: 1. Mount a single fiber between two clamps. 2. Measure the cross-sectional area (often via laser micrometry — important because force/area = stress, the unit-normalized quantity). 3. Extend the fiber at a controlled rate (typical: 25 mm/min) until break. 4. Record the maximum force achieved (breaking force, in cN). 5. Calculate stress (cN / cross-sectional area in μm²) and strain (extension at break / original length).

Tensile strength is not the same as breaking force. Tensile strength = breaking force / cross-sectional area, and is the unit-normalized comparison across fibers of different thickness. Some published claims report only breaking force (cN), which is the raw instrument reading; others report the normalized tensile strength.

What tensile testing measures: - Cortex integrity (the load-bearing structure) - Effect of damage (bleach, heat, mechanical) - Effect of treatments on damage recovery

What it does not measure: - Cuticle smoothness or shine - Color retention - Subjective "feel" or softness

ANATOMY's published result (SGS Proderm 24.0172-96): 135% increase in breaking force on bleached hair, from 15.2 cN to 35.8 cN. The substrate is bleached human hair; the method is single-fiber tensile testing with laser cross-section measurement. The number is an instrument reading, not a beauty tagline. Compared to a control of unreconstructed bleached hair, treated fibers carry 2.35x the load before breaking.