Tensile Testing of Polymers

Tensile Testing of Polymers

Tensile testing measures how a polymer material behaves when it's pulled apart in a straight line — how much force it takes to stretch, how far it stretches before breaking, and whether it fails suddenly or gives way gradually. It's one of the most widely used mechanical tests in the plastics industry, and for good reason — a huge range of real-world failures come down to something being pulled, stretched or put under tension, whether that's a cable sheath under strain, a packaging film being stretched during sealing, or a molded part under sustained load. 

Unlike flexural testing, which bends a sample, or impact testing, which strikes it suddenly, tensile testing applies a steady, controlled pull until the material yields or breaks. METS Laboratories carries out tensile testing on polymers to confirm whether a material meets its mechanical specification before it's used in a load-bearing, structural or consumer-facing application.

What is Polymer Tensile Testing?

Tensile testing applies a steadily increasing pulling force to a polymer sample and measures its mechanical response, generating several key properties in the process:

• Tensile strength: the maximum stress the material withstands before breaking
• Yield strength: the stress at which the material starts to deform permanently rather than springing back
• Elongation at break: how far the material stretches, as a percentage of its original length, before fracturing
• Modulus of elasticity: a measure of the material's stiffness in the early, straight-line part of the stress-strain curve

Polymer Products Subjected to Tensile Testing

Tensile testing applies wherever a polymer material or part needs to withstand pulling or stretching forces in service:

• Injection-molded and extruded plastic components
• Plastic films, sheets and packaging materials
• Cables, wire insulation and tubing
• Fibers and textile-grade polymers
• Adhesives and bonded polymer joints
• Raw polymer resin and compounded material prior to processing

Importance of Tensile Testing

Confirming tensile performance matters for a few practical reasons:

• Confirms a material meets the strength and elongation values its specification claims
• Predicts how a part will behave under sustained pulling or stretching loads
• Detects batch-to-batch variation in polymer formulation or processing
• Supports material selection decisions during product design
• Required for quality control and regulatory compliance across many industries
• Flags brittleness or excessive stretch before a part fails in service

Tensile Testing Method

Tensile testing follows a standard pull-to-failure procedure, run on a universal testing machine:

• A specimen is prepared to a standard shape, usually a "dog-bone" profile with a narrower gauge section in the middle
• The specimen is clamped at both ends in the testing machine's grips
• The machine pulls the specimen apart at a controlled, constant rate until it yields, necks, or fractures
• Force and displacement (or strain, via an extensometer) are recorded continuously throughout the test
• Stress and strain values are calculated from this data, based on the specimen's original cross-sectional area and gauge length

Standards That Govern Tensile Testing of Polymers

Tensile testing follows internationally recognized methods, selected based on the material type and the market the product is destined for:

• ASTM D638: the standard method for tensile properties of plastics, widely used in North America
• ISO 527: the equivalent international standard, commonly specified for products sold into European and other international markets
• ASTM D882: a dedicated method for tensile testing of thin plastic sheeting and film
• ISO 37: covers tensile testing of vulcanized rubber and thermoplastic elastomers specifically

The standard applied should always match what the product's specification or destination market requires, since specimen shape and test speed differ between methods and aren't directly comparable.

How are Tensile Test Results Evaluated?

Results are reported as tensile strength (MPa), elongation at break (%), and tensile modulus (MPa or GPa), then compared against:

• The material's approved specification or datasheet values
• The relevant standard's reporting requirements
• Batch-to-batch consistency data, to flag a shift in raw material or processing

A material's tensile properties should always be read alongside its intended application — high tensile strength with low elongation suits a rigid structural part, while high elongation matters more for a flexible film or elastomer.

Factors Affecting Tensile Test Results

Tensile performance isn't a fixed property of a polymer type — it shifts with formulation, processing and test conditions:

• Polymer grade and any fillers, reinforcements or additives present
• Specimen thickness, shape and gauge length
• Test speed, since polymers often behave differently under fast versus slow pulling
• Temperature and humidity at the time of testing
• Processing history, including molding or extrusion conditions and any orientation effects

Advantages of Regular Tensile Testing

Running tensile tests as a routine part of quality control, rather than only during initial material qualification, gives manufacturers:

• Confirmation that each production batch meets its mechanical specification
• Earlier detection of a raw material or processing change affecting strength or elongation
• Data to support material selection and design validation
• Documentation to back up compliance and customer specification claims

Tensile Testing at METS Laboratories

• Testing carried out on the client's own material, film or finished part sample
• Results reported against ASTM, ISO or the specific standard the product is specified to
• Testing available across rigid plastics, films, fibers and elastomeric materials
• Reports structured to support material qualification and compliance documentation
• Part of a wider Middle East, UK, China and India laboratory network for manufacturers working across multiple markets

METS Lab's ISO/IEC 17025-accredited Ajman facility carries out mechanical testing on polymers and composite materials, supporting compliance with UAE's construction and industrial standards alongside international methods such as ASTM and ISO.

Conclusion

A polymer's tensile properties determine whether it holds together under pulling and stretching loads, or fails well before its intended service life, and that's not something that can be judged by handling the material. METS Laboratories carries out tensile testing to measure strength, elongation and modulus against ASTM, ISO and other applicable standards. If a polymer material or finished part needs its tensile performance confirmed, get it tested first.

Frequently Asked Questions

1. What's the difference between tensile testing and flexural testing?

Ans: Tensile testing pulls a sample apart in a straight line to see how it handles stretching. Flexural testing bends it instead. Both matter — a material can perform well in one and poorly in the other, depending on how it's actually used.

2. What does "elongation at break" actually mean?

Ans: It's how much a material stretches before it snaps, shown as a percentage of its original length. A low number means the material is stiff and breaks with little stretch; a high number means it stretches a lot before failing.

3. Can tensile testing tell if a plastic part will snap under normal use?

Ans: It gives a strong indication. If the expected load on a part is close to or above its tested yield or breaking strength, that's a sign the material or design needs rethinking before it's used.