Modified PEEK impact strength testing is an essential qualification procedure for evaluating the toughness and fracture resistance of glass-fiber, carbon-fiber, mineral-filled, and impact-toughened polyetheretherketone grades used in aerospace, medical, energy, and industrial applications. This technical guide explains Charpy and Izod test methods under ISO 179, ISO 180, ASTM D256, ASTM D6110, and corresponding GB/T standards, covering specimen preparation, conditioning, instrumentation, data interpretation, and practical factors that influence test results for reinforced and toughened PEEK materials.
Technical Overview
Polyetheretherketone (PEEK) is a semi-crystalline, high-performance engineering thermoplastic known for its glass transition temperature of approximately 143 °C, melting point of approximately 343 °C, outstanding chemical resistance, and excellent mechanical properties across a wide temperature range. Modified PEEK grades, including glass-fiber reinforced, carbon-fiber reinforced, mineral-filled, PTFE- and graphite-lubricated, and impact-toughened formulations, are engineered to enhance stiffness, dimensional stability, wear performance, or fracture resistance for demanding applications.
Impact strength testing quantifies the energy a material absorbs when subjected to rapid, high-strain-rate loading. For modified PEEK, this measurement is critical because components such as gears, brackets, surgical instruments, downhole tools, and connectors frequently experience shock loads during assembly, handling, operation, or accidental drop events. Because PEEK is a notch-sensitive polymer, and because fibrous reinforcements introduce significant anisotropy, standardized impact testing under ISO 179 (Charpy), ISO 180 (Izod), ASTM D256, ASTM D6110, and the corresponding GB/T standards provides comparable toughness data for material selection, batch release, quality control, and design verification.
Testing Samples
Injection-Molded Test Bars
The preferred specimen geometry for impact testing is the injection-molded bar produced under ISO 294-1 (typically 80 mm × 10 mm × 4 mm for ISO methods) or ASTM D3641 (typically 63.5 mm × 12.7 mm with thickness between 3.2 mm and 12.7 mm for ASTM methods). Processing conditions for PEEK, including melt temperature, mold temperature, holding pressure, and cooling rate, strongly influence crystallinity and residual stress, both of which affect impact response. The gate location and melt flow direction must be documented, since fiber orientation in reinforced grades can cause impact strength to differ by a factor of two or more between flow-parallel and flow-perpendicular directions.
Notched Specimens
Notched specimens are machined after molding in accordance with ISO 2818 or the notching requirements of ASTM D256. For ISO testing, the Type A notch with a tip radius of 0.25 mm and a depth of 2 mm is the standard geometry, while ASTM notches typically have a 0.25 mm radius and a 2.54 mm depth. Notch machining quality, including tip radius accuracy, surface finish, and the absence of machining-induced microcracks or thermal damage, is one of the largest sources of data scatter. Sharp, well-machined notches produce a controlled triaxial stress state that measures resistance to crack propagation rather than crack initiation.
Machined Specimens from Plates and Finished Parts
When component-level verification is required, specimens may be machined from compression-molded plates, extruded stock shapes, or actual finished parts. Machining parameters must be controlled to avoid overheating, smearing, or introducing surface cracks, and the machining direction relative to the notch should be reported. Machined specimens are particularly useful for evaluating weld lines, flow-induced fiber orientation, and thickness effects in real components.
Conditioning Requirements
All specimens must be conditioned before testing to ISO 291 (23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 16 hours, with longer periods preferred for semi-crystalline polymers) or ASTM D618 Procedure A. Prior to molding, PEEK granules must be dried according to the material manufacturer's recommendations, typically several hours at approximately 150 °C, since residual moisture can cause voids, hydrolytic degradation, and artificially low impact values. A minimum of five specimens per condition is required by the standards, although ten specimens are recommended for reinforced grades to properly characterize scatter.
Testing Items
A comprehensive impact evaluation program for modified PEEK typically includes the following test items:
- Charpy impact strength, notched and unnotched, expressed in kJ/m²
- Izod impact strength, notched and unnotched, expressed in kJ/m²
- Tensile impact strength determined in accordance with ISO 8256
- Instrumented falling-dart or puncture impact testing of plate specimens in accordance with ISO 6603-2
- Temperature-dependent impact profiling from subambient to elevated temperatures
- Break classification and ductile-to-brittle transition assessment
- Fiber orientation dependence studies for reinforced grades
Testing Methods
Charpy Impact Test (ISO 179-1 / ASTM D6110 / GB/T 1043)
In the Charpy configuration, the specimen is supported horizontally on two anvils with a support span of 62 mm and struck at the midpoint by a swinging pendulum. Specimens are normally tested edgewise, with the notch facing away from the striker, using the standard designation 1eA for an edgewise specimen with a Type A notch. The striking edge radius is 2 mm and the nominal impact velocity is 2.9 m/s or 3.8 m/s. The pendulum capacity must be selected so that the absorbed energy falls between 10 % and 80 % of the available energy, with friction and windage corrections applied as specified in the standard. Impact strength is calculated by dividing the corrected absorbed energy by the cross-sectional area of the specimen at the notch and is reported in kJ/m², together with the break classification: complete break, hinge break, partial break, or non-break.
Izod Impact Test (ISO 180 / ASTM D256 / GB/T 1843)
In the Izod configuration, the specimen is clamped as a vertical cantilever beam and struck by the pendulum at a defined distance above the notch. ISO 180 uses the same 80 mm × 10 mm × 4 mm bar as the Charpy method with a defined free clamped length of 22 mm, while ASTM D256 uses the larger cross-section North American specimen and reports results in ft-lb/in or converted kJ/m². Unnotched Izod testing may be performed under ASTM D4812 when the unnotched behavior of the material is of interest. Because the stress states and stress concentrations differ between the two cantilever configurations, Charpy and Izod results are not directly convertible and must not be compared interchangeably.
Instrumented and Temperature-Dependent Testing
Instrumented pendulum or falling-dart systems equipped with force transducers record the full force-deflection history of the impact event, allowing separation of crack initiation energy from crack propagation energy. This distinction is particularly valuable for brittle fiber-reinforced PEEK grades, where total absorbed energy alone does not reveal whether the material fails by catastrophic crack growth or by controlled, progressive fracture. Temperature chambers cooled with liquid nitrogen or dry ice, or heated electrically, enable impact testing from approximately −40 °C up to 150 °C or beyond, which is essential for aerospace, automotive, and oil and gas applications that experience wide service temperature ranges.
Data Interpretation and Reporting
A compliant test report must include the complete material designation, filler content, conditioning history, specimen geometry and notch type, test configuration, pendulum energy, impact velocity, break classification, individual values, and the mean with standard deviation. Typical published notched Charpy values for unreinforced PEEK fall in the approximate range of 4 to 8 kJ/m², while glass-fiber reinforced grades often show similar or moderately higher notched values with reduced unnotched ductility, and toughened grades can achieve substantially higher notched toughness. These values vary with grade, crystallinity, notch geometry, and fiber orientation, so verification by an accredited third-party laboratory using standardized procedures is strongly recommended for qualification programs.
Applications
Modified PEEK impact strength data directly support material qualification and design assurance across high-value industries:
- Aerospace: thermoplastic brackets, clips, standoffs, and insulation components requiring validated impact toughness alongside flame, smoke, and toxicity compliance
- Automotive and electrified mobility: transmission gears, bushings, thrust washers, and battery module components exposed to crash, vibration, and road-shock loading
- Medical devices: surgical instrument housings, sterilization trays, and structural implant components where drop impact durability and repeatable sterilization are mandatory
- Oil and gas: downhole tooling components, seals, backup rings, and electrical connector systems subject to handling shocks and impact at elevated pressure and temperature
- Semiconductor and electronics: wafer carriers, test sockets, and precision connectors requiring impact resistance during automated handling
- Industrial machinery: pump and valve components, bearings, wear plates, and compressor parts exposed to impact and erosive service conditions
FAQ
What is the difference between Charpy and Izod impact tests for modified PEEK?
Charpy testing supports the specimen horizontally at both ends and strikes the center, while Izod testing clamps the specimen as a vertical cantilever and strikes the free end above the notch. The stress distributions differ, and the results are not interchangeable. ISO-based supply chains typically specify Charpy, whereas North American specifications more commonly call for Izod.
Why should both notched and unnotched specimens be tested?
Unnotched impact strength reflects the inherent ductility and energy absorption of the bulk material, while notched impact strength reveals sensitivity to stress concentrators and resistance to crack propagation. Because real components contain holes, ribs, sharp fillets, and machining marks, notched data are usually the more conservative and design-relevant value.
How do glass and carbon fibers affect PEEK impact strength?
Fiber reinforcement substantially increases stiffness, strength, and heat deflection temperature but changes impact behavior in a complex way. Notched impact strength of glass-fiber grades is often maintained or modestly improved, while unnotched values decrease because the composite fails in a more brittle manner. Carbon-fiber grades generally show lower notched toughness than glass-fiber grades at equal loading. Both systems exhibit strong anisotropy, so testing in multiple orientations relative to the flow direction is essential.
Which standard should be selected: ISO or ASTM?
The choice is driven by customer specifications, regional regulatory expectations, and existing qualification databases. ISO 179 and ISO 180 use the 80 mm × 10 mm × 4 mm specimen, while ASTM D256 and ASTM D6110 use different geometries, so results from the two families cannot be compared directly. Consistency of method within a program is critical for valid trend analysis.
Does temperature influence the impact strength of modified PEEK?
Yes, significantly. Below room temperature, PEEK and its composites become progressively more brittle, reducing impact energy absorption. Above the glass transition temperature, the unreinforced matrix becomes noticeably tougher, while highly filled grades remain relatively brittle. Temperature-dependent testing at realistic service extremes is recommended for any safety-relevant application.
How many specimens are required and how should scatter be handled?
The standards require a minimum of five specimens per condition, but ten specimens are strongly recommended for fiber-reinforced PEEK because orientation and fiber distribution increase variability. Individual values that deviate due to documented specimen defects may be flagged, but data must not be discarded without justified technical cause. Reporting the mean, standard deviation, and break classification provides the statistical basis for acceptance decisions.
Where can modified PEEK impact strength testing be performed?
Testing should be performed by accredited third-party laboratories with validated pendulum impact equipment, temperature-controlled chambers, and documented compliance with ISO 179, ISO 180, ASTM D256, ASTM D6110, and the relevant GB/T standards, ensuring traceable calibration and internationally recognized test reports for material qualification and regulatory submission.