Stress–Strain Behaviour and Yield Point
topic
PET fibre stress–strain curve (at 20°C, 65% RH, 20 mm gauge length, 20%/min strain rate): linear elastic region (modulus = slope, 50–120 cN/dtex for FDY) → yield point (0.5–1.5 cN/dtex, chain slip in amorphous regions) → post-yield plateau (drawing of amorphous chains) → strain hardening → break. Breaking tenacity: commodity FDY 3.5–4.5 cN/dtex; HT yarn 7–9 cN/dtex. Fracture morphology: fibrillar fracture with progressive fibrillation.
Role
The stress–strain curve is the primary mechanical property descriptor for polyester fibres, encoding tenacity, modulus, work of rupture, and yield behaviour. Its interpretation in terms of fibre structure is a central topic in all textile materials science courses.
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