Elongation at break is one of the most critical mechanical parameters for evaluating the ductility and structural performance of modified PEEK (polyetheretherketone) materials. This article provides a comprehensive technical guide to the determination of elongation at break for various modified PEEK grades, covering applicable standards such as ISO 527, ASTM D638 and GB/T 1040, specimen preparation, testing conditions, calculation methods and typical result ranges. Whether you are a materials engineer, quality control specialist or R&D professional, this guide will help you understand how accredited third-party laboratories accurately measure the tensile ductility of glass-filled, carbon-filled and bearing-grade PEEK compounds.
Technical Overview
Polyetheretherketone (PEEK) is a semi-crystalline, high-performance engineering thermoplastic belonging to the polyaryletherketone (PAEK) family. It offers an exceptional combination of mechanical strength, thermal stability (continuous service temperature up to approximately 260 °C), chemical resistance and wear performance. Modified PEEK grades are produced by incorporating reinforcing fibers, mineral fillers or solid lubricants into the neat resin matrix to tailor stiffness, strength, friction behavior and dimensional stability for demanding applications.
Elongation at break is the tensile strain at the moment of specimen rupture, expressed as a percentage of the original gauge length. It is a direct indicator of material ductility and deformation capacity. For modified PEEK, this parameter is particularly informative because the type and loading level of the modifier dramatically alters the failure mode: unfilled PEEK behaves as a ductile thermoplastic with elongation values typically between 30% and 50%, whereas fiber-reinforced grades exhibit brittle failure with elongation values often below 3%.
Determining elongation at break in accordance with internationally recognized standards allows manufacturers, processors and end users to compare material datasheets, verify incoming material quality, support design calculations and satisfy regulatory requirements in industries such as aerospace, medical devices and automotive engineering.
Testing Samples
Unfilled (Neat) PEEK
virgin PEEK resin serves as the reference material. Its relatively high elongation at break reflects the ductile, semi-crystalline nature of the polymer and provides a baseline against which modified grades are evaluated.
Glass Fiber Reinforced PEEK
Grades containing 10% to 40% by weight of short glass fibers show significantly increased stiffness and tensile strength but markedly reduced ductility. Elongation at break values typically fall in the range of 1.5% to 4%, depending on fiber content and processing quality.
Carbon Fiber Reinforced PEEK
Carbon fiber filled PEEK (commonly 30% by weight) provides the highest strength-to-weight ratio and enhanced fatigue performance. Elongation at break generally ranges from 1% to 2.5%, confirming a brittle, fiber-dominated failure mechanism.
Bearing and Wear Grade PEEK
Compounds modified with PTFE, graphite and carbon fiber are optimized for tribological applications. Their elongation at break, commonly between 1.5% and 4%, must be balanced against friction coefficient and wear rate requirements.
Mineral or Particulate Filled PEEK
Grades containing mineral fillers, ceramic particles or other additives for improved dimensional stability or radiopacity (medical grades) are tested to quantify the ductility penalty introduced by the filler phase.
Testing Items
The core measured parameter is elongation at break, but the tensile test simultaneously yields several related properties that are normally reported together:
- Elongation at break (εb, %) — strain at the moment of rupture
- Tensile strength (σM, MPa) — maximum tensile stress sustained
- Tensile strength at break (σB, MPa)
- Tensile stress at yield (σy, MPa), where applicable for ductile unfilled grades
- Tensile modulus (Et, GPa), determined at low strain using an extensometer
Testing Methods
Applicable Standards
The determination is performed under internationally harmonized tensile testing standards:
- ISO 527-1 and ISO 527-2 — Plastics: Determination of tensile properties; test conditions for moulding and extrusion plastics
- ASTM D638 — Standard Test Method for Tensile Properties of Plastics
- GB/T 1040.1 and GB/T 1040.2 — Chinese national standards, technically aligned with ISO 527
- ISO 3167 — Multipurpose test specimens
- ISO 294-1 / ASTM D3641 — Injection moulding of test specimens
- ISO 291 / ASTM D618 — Standard atmospheres for conditioning and testing (23 °C ± 2 °C, 50% ± 5% relative humidity)
Specimen Preparation
Test specimens are preferably produced by injection molding the modified PEEK compound into multipurpose dumbbell-shaped specimens. ISO 3167 Type 1A specimens (gauge length 50 mm, width 10 mm, thickness 4 mm) are used under ISO 527, while ASTM D638 Type I specimens (gauge length 50 mm, width 13 mm, thickness 3.2 mm) are used under ASTM D638. Because PEEK is moisture-sensitive during processing, resin must be thoroughly dried before molding (typically 3 hours or more at 150 °C), and melt temperatures between 360 °C and 400 °C with mold temperatures of 170 °C to 200 °C are applied to ensure proper crystallinity. Alternatively, machined specimens can be prepared from compression-molded or finished parts, provided surface machining marks do not initiate premature failure.
Conditioning
After molding, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 88 hours (per ISO 291) prior to testing. This ensures comparable moisture content and temperature equilibrium across all tested grades.
Test Procedure
Testing is conducted on a calibrated universal tensile testing machine fitted with appropriate grips and, for modulus determination, a contact or video extensometer attached to the gauge length. The typical procedure is as follows:
- Measure and record the width and thickness of the narrow section of each specimen at several points; calculate the cross-sectional area
- Mark or fix the gauge length (50 mm for 1A / Type I specimens) and mount the specimen in the grips, ensuring axial alignment
- Apply a preload of approximately 1 MPa to remove slack and zero the strain measurement
- For modulus measurement, run at 1 mm/min (ISO) or the equivalent strain rate defined in ASTM D638, using the extensometer between strain levels of 0.05% and 0.25% (ISO 527)
- Increase to the testing speed specified for the material — typically 5 mm/min or 50 mm/min for 1A specimens under ISO 527-2; brittle fiber-filled grades are commonly tested at 5 mm/min while ductile unfilled grades may use 50 mm/min
- Continue loading until the specimen ruptures; record the increase in gauge length (or crosshead-travel-derived strain, if permitted) at the moment of break
- Test at least five valid specimens and report the arithmetic mean, standard deviation and, if required, the coefficient of variation
Calculation
Elongation at break is calculated as the increase in gauge length at break divided by the original gauge length, multiplied by 100%. Under ISO 527, the preferred method uses direct strain measurement from an extensometer; under ASTM D638, elongation values may be derived from crosshead displacement when the specimen does not fail within the gauge section, though this is noted as giving nominal results. Since modified PEEK specimens fail well before yield, accurate strain capture requires an appropriately responsive extensometer or video strain measurement system.
Typical Results
- Unfilled PEEK: approximately 30% to 50% (ductile failure with necking)
- 30% glass fiber reinforced PEEK: approximately 2% to 3%
- 30% carbon fiber reinforced PEEK: approximately 1.5% to 2.5%
- Bearing-grade PEEK (PTFE/graphite/CF): approximately 1.5% to 4%
Factors Influencing Results
- Molding conditions — poor melt temperature or mold temperature control leads to low crystallinity, weld lines or voids that distort results
- Fiber orientation — flow-induced fiber alignment causes anisotropy; testing direction relative to flow must be documented
- Specimen quality — notches, gate marks or machining damage artificially reduce elongation
- Test speed — higher crosshead speeds generally raise apparent strength and lower measured elongation
- Moisture and conditioning state of the specimens
Applications
Elongation at break data for modified PEEK supports material selection and qualification across many advanced industries:
- Aerospace: structural brackets, fasteners and interior components where weight reduction and predictable ductility are essential
- Automotive: transmission seals, thrust washers, gears and under-hood components exposed to sustained high temperature
- Oil and gas: downhole seals, bearings and electrical connectors requiring chemical and hydrolysis resistance
- Medical: surgical instruments, spinal implants and dental frameworks where biocompatible grades must meet strict mechanical lot-release criteria
- Electronics and semiconductor: wafer carriers, test sockets and insulating parts demanding dimensional precision and cleanliness
- Industrial machinery: pump and valve components, bearing cages and wear strips
FAQ
What is elongation at break and why does it matter for modified PEEK?
Elongation at break is the percentage strain a material sustains before rupturing in tension. For modified PEEK, it quantifies how much ductility is sacrificed when fibers or fillers are added, and it directly informs snap-fit feasibility, impact tolerance, crack resistance and overall part reliability.
Which standards are used for the determination?
The most widely applied standards are ISO 527-1/ISO 527-2, ASTM D638 and the equivalent Chinese national standards GB/T 1040.1/GB/T 1040.2. Accredited third-party laboratories typically offer testing under all three frameworks for seamless datasheet comparability.
Why do filled PEEK grades show much lower elongation than unfilled PEEK?
Glass or carbon fibers carry most of the load and create stress concentrations at the fiber-matrix interface. Once interfacial debonding or fiber breakage begins, cracks propagate rapidly, producing brittle failure at strains of only 1% to 4%.
Is an extensometer always required?
For modulus measurement, an extensometer is mandatory under both ISO 527 and ASTM D638. For elongation at break of brittle fiber-filled grades, extensometer or video strain measurement is strongly recommended because crosshead-displacement-derived strain includes system compliance and can significantly overstate the true value.
How many specimens should be tested?
A minimum of five valid specimens per sample is standard. Additional specimens may be required when results show high scatter, which is common for fiber-filled compounds due to fiber orientation and processing variability.
Can specimens be machined from finished parts?
Yes, small-scale or machined specimens can be used when molded dumbbells are unavailable, but the geometry, machining quality and fiber orientation must be documented, and results may not be directly comparable to datasheet values obtained on injection-molded specimens.