Plastic room-temperature impact strength testing measures the ability of plastic materials to absorb energy and resist fracture under a sudden impact load at a standard ambient temperature of 23 °C. It is one of the most widely performed mechanical evaluations for thermoplastics and thermosets, supporting material selection, quality control, and product design validation. This article explains the test principles, specimen requirements, test items, internationally recognized methods, and typical applications of room-temperature impact testing for plastics.
Technical Overview
Impact strength describes the energy a material absorbs per unit area when it is fractured by a single, high-speed blow. Because plastics are viscoelastic, their toughness depends strongly on temperature and loading rate, so tests are performed under tightly controlled ambient conditions—typically 23 °C ± 2 °C and 50 % ± 10 % relative humidity as defined in ISO 291 and ASTM D618. At room temperature many plastics behave in a ductile manner, but stress concentrations such as notches, weld lines, or surface scratches can trigger brittle fracture, which is exactly the failure mode this test is designed to reveal.
Room-temperature impact testing is carried out on a pendulum-type impact machine. A swinging hammer of defined energy strikes a supported or clamped specimen, and the difference between the initial and final pendulum heights indicates the energy absorbed during fracture. The result quantifies notch sensitivity, brittleness, and overall toughness, and is routinely used for material datasheets, incoming resin inspection, and compliance with product standards. The principal methods and their governing standards include:
- ISO 179-1 – Determination of Charpy impact properties
- ISO 180 – Determination of Izod impact strength
- ASTM D256 – Determining the Izod pendulum impact resistance of plastics
- ASTM D6110 – Charpy impact of plastics
- GB/T 1043.1 and GB/T 1843 – Chinese national standards aligned with ISO 179-1 and ISO 180
- ISO 8256 and ASTM D1822 – Tensile-impact strength of plastics
Testing Samples
Molded Specimens
Preferred specimens are injection-molded bars produced under the conditions specified by the relevant material standard. For ISO methods the standard bar measures 80 mm × 10 mm × 4 mm, while ASTM methods typically use bars of 63.5 mm × 12.7 mm × 3.2 mm, with thicknesses up to 12.7 mm where required. Molding parameters directly influence crystallinity, molecular orientation, and internal stress, so documented and reproducible processing is essential for comparable results.
Machined Specimens
When finished parts, sheets, extruded profiles, or semi-finished stock must be evaluated, specimens can be machined to the standard geometry. Machined samples reveal the actual toughness of the delivered product but introduce additional variables such as machining marks, anisotropy, and skin-core effects, which must be recorded in the test report.
Notched and Unnotched Specimens
Notches act as controlled stress raisers and are machined with a specified cutter radius: ISO defines notch A (0.25 mm radius), notch B (1.00 mm radius), and the sharper notch C (0.10 mm radius). Testing both notched and unnotched specimens allows engineers to separate inherent material toughness from notch sensitivity, a critical distinction for parts containing sharp corners, ribs, holes, or snap-fits.
Specimen Conditioning
Unless otherwise specified, specimens are conditioned to equilibrium at 23 °C ± 2 °C and 50 % ± 10 % relative humidity, commonly for at least 88 hours according to ISO 291 or 40 hours according to ASTM D618 Procedure A. Testing is performed in the same atmosphere so that the entire evaluation truly represents room-temperature behavior.
Testing Items
Typical room-temperature impact test items for plastics include:
- Charpy impact strength – notched and unnotched, edgewise and flatwise impact
- Izod impact strength – notched and unnotched cantilever impact
- Tensile-impact strength – energy absorbed under high-speed tensile loading
- Notch sensitivity assessment – comparison of A, B, and C notch radii
- Fracture behavior classification – complete, hinge, partial, or non-break
- Comparative toughness evaluation of resin grades, production batches, recyclate contents, or processing conditions
Results are expressed as impact strength in kJ/m² under ISO methods, calculated as absorbed energy divided by the cross-sectional area beneath the notch, or as energy per unit width in J/m under ASTM methods. The reported value must always be accompanied by the break type designation to be fully interpretable.
Testing Methods
Charpy Impact Method (ISO 179-1, ASTM D6110, GB/T 1043.1)
The specimen rests horizontally on two supports spaced 62 mm apart for the standard ISO bar, and the pendulum strikes the center of the beam at approximately 2.9 to 3.8 m/s. Testing may be performed edgewise or flatwise, on the narrow or broad face, and with the notch facing toward or away from the striker. Charpy testing is well suited to rigid thermoplastics, thermosets, and fiber-reinforced compounds, and it tolerates slight specimen misalignment better than the Izod configuration.
Izod Impact Method (ISO 180, ASTM D256, GB/T 1843)
The specimen is clamped as a vertical cantilever, with the notch positioned on the impacting side and facing the striker, which hits the bar at a defined point above the clamp. The Izod method historically dominates North American specifications and is widely applied to rigid and semi-rigid plastics, offering excellent discrimination between notch-sensitive materials.
Instrumented and Complementary Techniques
Instrumented pendulum or falling-weight systems record force–displacement curves, separating crack initiation energy from propagation energy and detecting ductile-to-brittle transitions. Where multi-axial loading is relevant, puncture and drop-dart methods such as ISO 6603-2, ASTM D5420, and ASTM D3763 can complement beam impact data for a more comprehensive toughness characterization.
Test Procedure and Data Evaluation
The machine is verified and zeroed, the pendulum capacity is selected so that the absorbed energy falls between roughly 10 % and 80 % of the available energy, and friction and windage losses are corrected. Each specimen is measured, fractured, and classified as complete break, hinge break, partial break, or non-break. ISO methods recommend testing at least ten specimens, while ASTM practice commonly uses five, with mean values and standard deviations reported. Accredited third-party laboratories issue complete reports covering specimen details, conditioning history, standard edition, notch geometry, and break classification so that results remain auditable and comparable between laboratories.
Applications
Room-temperature impact strength data are used across the full life cycle of plastic products:
- Automotive engineering – validating interior trims, bumpers, fasteners, and housings against impact loads during assembly and service
- Electrical and electronics – verifying the drop resistance and mechanical robustness of enclosures, connectors, and switch components under IEC and UL product standards
- Consumer goods and appliances – confirming that casings and structural parts withstand knocks, drops, and handling abuse
- Pipes, fittings, and construction products – demonstrating adequate toughness of PVC, PE, and composite profiles used in pressure and drainage systems
- Packaging – assessing crates, containers, and closures subjected to transport shocks
- Research and quality control – ranking resin grades, optimizing additives and fillers, monitoring batch consistency, evaluating recycled content, and supporting failure analysis
Because many service failures involve a combination of impact, stress concentration, and reduced temperature, room-temperature results are frequently compared with sub-zero data to map the ductile-to-brittle transition of a material across its full operating range.
FAQ
What temperature counts as room temperature for impact testing?
Room temperature is defined by ISO 291 and ASTM D618 as 23 °C ± 2 °C, usually combined with 50 % ± 10 % relative humidity. Specimens are conditioned to this atmosphere before testing, and the test itself is performed under the same conditions.
Why test both notched and unnotched specimens?
Unnotched results reflect the inherent toughness of the material, while notched results reveal how sharply toughness collapses in the presence of a stress concentration. The comparison quantifies notch sensitivity, which is essential when designing parts with corners, holes, ribs, or weld lines.
How do ISO and ASTM results differ?
ISO methods report impact strength in kJ/m² based on the fracture cross-section, whereas ASTM D256 reports absorbed energy per unit notch width in J/m. Specimen geometry, notch depth, and clamping also differ, so ISO and ASTM values are not directly convertible; comparisons should always reference the same standard.
How many specimens are required?
ISO methods call for a minimum of ten specimens when a mean value is required, and ASTM practice typically uses at least five. Additional specimens may be needed for highly scattered results or for anisotropic materials tested in several orientations.
Can finished products be tested instead of molded bars?
Yes. Specimens can be machined from finished parts, sheets, or extruded profiles, and the measured values then characterize the actual processed article. Machined specimens must be free of grinding damage, and the sampling location and orientation should be documented because processing-induced orientation can strongly influence impact behavior.
What does a hinge or partial break result mean?
A hinge break means the two specimen halves remain connected by a thin outer skin, and a partial break means the crack did not traverse the full cross-section. These classifications indicate ductile, tough behavior and are reported as letter codes together with the numeric value because they influence how the data should be interpreted.