Plastic impact strength testing evaluates a material's ability to absorb energy under rapid loading conditions, providing critical data for material selection, product design, and quality control across industries such as automotive, electronics, packaging, and construction. This technical protocol outlines internationally recognized testing approaches based on ISO, ASTM, EN, GB, and JIS standards, covering specimen preparation, test procedures, data interpretation, and practical applications. By following a standardized impact testing scheme, manufacturers and accredited third-party laboratories can ensure repeatable, comparable results that support product certification and failure prevention.
Technical Overview
Impact strength is a measure of a plastic material's toughness, defined as the energy absorbed per unit cross-sectional area when a specimen fractures under a high-speed, single-strike load. Unlike tensile or flexural tests conducted at slow strain rates, impact testing simulates real-world events such as drops, collisions, and sudden loading, where plastics frequently exhibit reduced ductility and brittle fracture behavior.
Plastics are inherently strain-rate and temperature sensitive. A polymer that deforms ductilely in a slow tensile test may fail in a brittle manner under impact, particularly at low temperatures, in the presence of stress concentrators such as notches, weld lines, or sharp corners, or after environmental aging. Impact testing therefore serves several essential purposes:
- Ranking and comparing the toughness of candidate materials during design and material selection
- Assessing notch sensitivity and the influence of processing defects on fracture behavior
- Verifying performance after thermal aging, UV exposure, chemical contact, or low-temperature conditioning
- Supporting quality control, incoming material inspection, and certification of finished components
The principal test families include pendulum-based methods (Charpy and Izod), tensile impact, falling-weight and falling-dart impact, and instrumented high-speed puncture testing. These methods are governed by standards published by ISO, ASTM International, EN, GB, JIS, and referenced in UL material evaluation programs such as UL 746A.
Testing Samples
Injection-Molded Specimens
The preferred specimen source is a directly injection-molded test bar, which reflects the material behavior under representative processing conditions. For ISO-based Charpy and Izod tests, the standard specimen measures 80 mm × 10 mm × 4 mm. For ASTM-based Izod testing, the standard bar is 63.5 mm × 12.7 mm × 3.2 mm (with thicknesses up to 12.7 mm permitted for certain materials). Gate location, mold temperature, melt temperature, and holding pressure must be documented because molding conditions significantly influence measured impact values.
Machined Specimens from Sheets, Plates, Pipes, and Finished Parts
When evaluating semi-finished products or actual components, specimens may be machined to the standard geometry in accordance with ISO 2818. Machining must be performed with sharp tools, adequate cooling, and controlled feed rates to avoid introducing heat, residual stress, or surface cracks that would distort the measured impact strength. Orientation effects must also be considered: specimens cut parallel and perpendicular to the processing direction may yield substantially different values for oriented or fiber-reinforced plastics.
Notch Preparation
Notched testing is the most widely used condition because the notch creates a controlled stress concentration and measures resistance to brittle crack propagation. ISO standards define three notch types: Type A with a 0.25 mm root radius, Type B with a 1.00 mm root radius, and Type C with a 0.10 mm root radius. ASTM D256 specifies a 0.25 mm radius notch with a 2.54 mm depth. Notch cutting must use a sharp, freshly prepared cutting tool, as a dull tool or improper feed produces a deformed or overheated root radius and can reduce the reported impact strength by a significant margin.
Conditioning Prior to Testing
Specimens shall be conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 10% relative humidity in accordance with ISO 291 or ASTM D618. A minimum conditioning period of 88 hours (ISO) or 40 hours (ASTM Procedure A) after molding or machining is recommended. For materials with significant moisture dependence, such as polyamides, dry-as-molded and humidity-equilibrated states should be tested and reported separately.
Testing Items
A comprehensive plastic impact testing scheme typically includes the following test items, selected according to the material form, application requirements, and applicable product standards:
- Charpy impact strength, notched and unnotched (ISO 179-1, ASTM D6110, GB/T 1043, JIS K 7111)
- Izod impact strength, notched and unnotched (ISO 180, ASTM D256, ASTM D4812, GB/T 1843, JIS K 7110)
- Tensile impact strength (ISO 8256, ASTM D1822)
- Instrumented puncture impact, multiaxial loading (ISO 6603-2, ASTM D3763)
- Falling-dart impact resistance of plastic films and thin sheets (ISO 7765-1, ASTM D1709)
- Drop-weight impact resistance of pipes, fittings, and molded parts (ASTM D5628, ASTM D2444, ISO 3127)
- Impact strength at reduced temperatures (typically −40 °C, −30 °C, or −20 °C) to determine the brittle–ductile transition
- Impact strength after thermal aging, UV weathering, or chemical exposure as part of durability evaluation programs
Reported results are expressed in kilojoules per square meter (kJ/m²) for ISO methods, and in joules per meter (J/m) or foot-pounds per inch (ft-lb/in) for ASTM methods, together with the observed fracture classification.
Testing Methods
Pendulum Charpy Impact Test
The Charpy method (ISO 179-1, ASTM D6110) uses a rectangular specimen supported horizontally against two anvils and struck at the midspan by a swinging pendulum. The energy absorbed during fracture is calculated from the difference between the initial and final pendulum heights, corrected for windage and friction. The impact strength is obtained by dividing the absorbed energy by the cross-sectional area of the specimen beneath the notch. ISO 179-1 requires that the absorbed energy fall between 10% and 80% of the nominal pendulum capacity; pendulums of 1 J, 2 J, 4 J, 5 J, 7.5 J, 15 J, 25 J, and 50 J are commonly available. Each fracture is classified as complete break (C), hinged break (H), partial break (P), or non-break (N), and this classification must accompany the numerical result.
Pendulum Izod Impact Test
The Izod method (ISO 180, ASTM D256) uses a specimen clamped vertically as a cantilever beam and struck by the pendulum above the clamp, on the side of the notch. The calculation principle is identical to the Charpy method. Izod testing is the reference method in many North American specifications and in UL material recognition programs. Because specimen geometry, notch dimensions, and support conditions differ between the ISO and ASTM variants, Izod values obtained under the two systems are not directly interchangeable, and the governing standard must always be stated with the result.
Tensile Impact Test
Tensile impact testing (ISO 8256, ASTM D1822) is applied to materials that are too flexible, too thin, or too ductile to fracture completely in flexural pendulum tests. A dumbbell-shaped specimen is attached between a pendulum cross-clutch and a fixed head, and the energy absorbed in tensile fracture is recorded. This method is particularly useful for thin films, elastomers, and highly ductile polyolefins.
Instrumented Puncture and Drop-Weight Impact
Instrumented impact testing (ISO 6603-2, ASTM D3763) drives a striker through a clamped disk or plate at speeds typically between 1 m/s and 4.4 m/s while a load cell records the full force–deflection response. Key outputs include peak force, deflection at peak force, energy to peak force, and total failure energy, along with a clear distinction between ductile and brittle failure modes. This technique is the preferred approach for evaluating automotive body panels, bumpers, helmets, and safety-critical components because it reproduces multiaxial stress states that better represent in-service impact events.
Falling-Dart Test for Films
For flexible films and thin sheets up to approximately 1 mm, the free-falling dart method (ASTM D1709) or the staircase method (ISO 7765-1) determines the dart mass at which 50% of specimens fail. The result is reported as the failing mass in grams and is a primary quality parameter for packaging films.
General Procedure and Data Reporting
For every method, the test protocol should include: verification of the pendulum or drop tower in accordance with ISO 13802 or the relevant ASTM calibration requirements; measurement of specimen dimensions at the fracture cross-section; testing of at least five to ten specimens per condition depending on the governing standard; recording of test temperature and conditioning history; and reporting of individual and mean impact strength values with the fracture classification. Any specimen that does not break is reported as "no break" rather than being averaged with broken specimens.
Applications
Plastic impact strength testing supports a wide range of industrial and regulatory applications:
- Automotive engineering: qualification of bumpers, body panels, interior trim, fuel system components, and battery housings, including verification of low-temperature impact performance at −30 °C or −40 °C
- Electrical and electronic products: evaluation of enclosures, switch housings, connectors, and appliance casings, frequently within the framework of UL 746A polymeric material short-term property evaluations
- Packaging: quality control of films, bottles, crates, and containers using falling-dart and drop-impact methods
- Pipes and fittings: assessment of thermoplastic piping systems for water, gas, and drainage applications using drop-weight methods such as ISO 3127 and ASTM D2444
- Construction products: impact verification of profiles, panels, glazing elements, and safety components under EN-based product standards
- Research and development: comparison of base resins, evaluation of impact modifier formulations, studies of recycling content effects, and root-cause analysis of field failures
- Quality assurance: incoming resin inspection, periodic production audits, and validation of process changes such as tool modifications or molding parameter adjustments
Manufacturers exporting to regulated markets frequently engage accredited third-party laboratories to perform these tests under ISO/IEC 17025 management systems, ensuring that results are accepted by certification bodies and customers worldwide.
FAQ
What is the difference between Izod and Charpy impact tests?
In the Izod test the specimen is clamped vertically as a cantilever and struck above the clamp, while in the Charpy test the specimen lies horizontally on two supports and is struck at midspan. The two methods also use different standard specimen dimensions, so their numerical results are not directly comparable even when testing the same material.
Why are notched specimens so widely used?
The notch introduces a sharp, controlled stress concentration that promotes brittle fracture and amplifies differences in notch sensitivity between materials. Notched impact strength is one of the most discriminating toughness indicators for plastics and is the default condition in ISO 180, ASTM D256, and most product specifications.
How many specimens should be tested?
ISO pendulum methods generally require a minimum of ten specimens per condition unless otherwise agreed, while ASTM methods typically specify at least five. Additional specimens are recommended for highly variable materials or whenever the failure mode is inconsistent.
Can ISO and ASTM impact results be converted to each other?
No direct conversion exists. The two systems differ in specimen geometry, notch depth and radius, support conditions, and unit conventions (kJ/m² versus J/m or ft-lb/in). Any comparison must identify the exact standard used, and procurement specifications should state the required method explicitly.
What does it mean when a specimen does not break?
A "no break" result indicates that the specimen absorbed the full energy of the pendulum without complete fracture. The result is reported as greater than the pendulum energy divided by the cross-sectional area, and testing should be repeated with a higher-capacity pendulum or, preferably, with an instrumented impact method.
How does temperature affect impact strength?
Most plastics exhibit a ductile-to-brittle transition as temperature decreases. Impact strength measured at room temperature may not predict performance in cold climates or freezer applications, so critical parts are routinely tested at their minimum service temperature.
What are typical notched Izod values for common plastics?
As an indicative guide, general-purpose polystyrene shows very low notched Izod strength in the range of 10–20 J/m, ABS typically falls between 150 and 400 J/m, while polycarbonate and certain impact-modified blends can exceed 600 J/m and often fail to break completely under standard pendulum energies. Actual values depend strongly on grade, processing, and conditioning.