Tensile Strength, Elastic Modulus and Elongation at Break in Polymers - A434

 

Tensile Strength, Elastic Modulus and Elongation at Break in Polymers
Foundational knowledge article
Document Type	Article
Document Identifier	434
Tags	Physical aging Chemical degradation Thermal degradation
Prerequisites	Materials science Material properties Polymer properties

Introduction

The mechanical behavior of polymers—characterized by tensile strength, elastic modulus, and elongation at break—is governed by their molecular structure and environmental conditions. Their properties are highly sensitive to strain rate and temperature, although thermoplastic and thermoset properties are affected differently, therefore experimental testing under realistic conditions to capture temperature- and rate-dependent effects is recommended.

Significance

Understanding the physical phenomena behind tensile measurement of polymers is essential because they reflect the fundamental deformation and failure mechanisms. Tensile properties — such as modulus, yield strength, and elongation at break — are variously governed by molecular structure, chain entanglement, and intermolecular forces. For thermoplastics, these properties depend on crystallinity and glass transition temperature, while thermosets are influenced by crosslink density. Furthermore, as viscoelastic materials, the mechanical properties of polymers are strain rate and temperature dependent.

Scope

This document describes the measurements accessible though tensile testing of coupons (elastic modulus, yield strength, and elongation at break) with a view towards the physical phenomena that impact their value for both thermoplastic and thermoset polymers.

Definitions and Fundamental Material Properties

Tensile Strength is the maximum stress a polymer can withstand while being stretched before fracturing. Given that thermal energy increases chain mobility in polymers, in general, tensile strength for ductile materials decreases as temperature increases. Yielding is an onset of permanent deformation: beyond this point, the material undergoes irreversible molecular rearrangement, leading to necking or microcracking.

Elastic Modulus (Young’s Modulus) measures a polymer’s stiffness, representing the ratio of stress to strain within the elastic region. A high modulus indicates rigidity, while a low modulus suggests flexibility.

Elongation at break measures the polymer's ductility, expressed as the percent increase in length before fracture. Highly ductile polymers can elongate several hundred percent, while brittle materials may break at less than 5% strain.

Unlike metals, polymers exhibit viscoelastic behavior, meaning they experience both flow and energy storage phenomena simultaneously, and their mechanical properties are strain rate and temperature dependent. Faster loading increases apparent modulus and tensile strength while elevated temperatures reduce modulus and yield strength by enhancing molecular mobility and relaxation processes.

Polymers are typically divided into thermoplastics, which can be melted and reshaped, and thermosets, which are infusible. Thermoplastic polymers consist of linear or branched chains held by weak intermolecular forces; they offer recyclability and toughness with generally lower mechanical strength. Thermoset polymers undergo irreversible curing by forming a rigid 3D network though molecular bonds; they offer high thermal and chemical resistance and dimensional stability at the cost of reprocess ability.

Experimental tensile testing at varying temperatures and strain rates as close as possible to the expected real-world values provides essential data for reliable design, helping predict performance and meet application demands.

Thermoplastics

• Tensile Strength: typically exhibit a distinct yield point followed by plastic deformation and necking, which increases with higher crystallinity. Tensile strength for thermoplastics could be in a range of 10-120 MPa.
• Elastic Modulus: generally lower than thermosets but varies widely (e.g., LDPE is flexible, while PEEK is rigid) and increases with the degree of crystallinity. Modulus drops progressively with a sharp drop at glass transition (Tg) or melting (Tm) temperatures.
• Elongation at Break: ductile polymers (e.g., LDPE, plasticized PVC) can stretch 100-1000% before breaking. Semi-crystalline polymers (e.g., HDPE, PP) show moderate elongation (10–600%) depending on crystallinity. Brittle thermoplastics (e.g., PS, PMMA) fracture at low strains (<10%).
• Common thermoplastics: PE, PP, PET, Nylon, PS, PMMA

Thermosets

• Tensile Strength: often brittle failure occurs, with minimal plastic deformation before fracture. Some toughened epoxies may show limited yielding. Tensile strength for thermosets could be in a range of 30-100 MPa.
• Elastic Modulus: higher than most thermoplastics due to rigid 3D networks, providing better dimensional stability. Modulus values are retained up to the glass transition temperature (Tg) with a sharp drop near it.
• Elongation at Break: highly crosslinked thermosets (e.g., standard epoxy) are brittle, elongating only 1-5%. Toughened thermosets (e.g., rubber-modified epoxies) may reach 10–50% elongation due to energy-dissipating mechanisms.
• Common thermosets: Epoxy, Phenolic, Polyurethane

References

A class of polymer, some common examples include polypropylene and polyethylene.

They soften and melt upon heating (i.e. potentially recyclable), high viscosity when melted, therefore difficult to saturate fibres. Usually needs a lot of pressure and heat to process.

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

The glass transition temperature (T_g) is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. It is one of the most important properties of any amorphous polymer.

Periodic 3-D, repeating array of molecules.