Standard Determination of Plastic Impact Strength: Test Methods, International Standards, and Applic

Impact strength is one of the most critical mechanical properties for evaluating the toughness and fracture resistance of plastic materials under high-speed loading. This article provides a comprehensive technical guide to the standard determination of plastic impact strength, covering Charpy, Izod, tensile-impact, and instrumented falling-weight methods in accordance with ISO, ASTM, EN, GB, and JIS standards. Whether you are qualifying materials for automotive components, packaging, pipes, or electrical enclosures, understanding these test protocols ensures reliable, accurate, and internationally comparable data.

Impact strength is one of the most critical mechanical properties for evaluating the toughness and fracture resistance of plastic materials under high-speed loading. This article provides a comprehensive technical guide to the standard determination of plastic impact strength, covering Charpy, Izod, tensile-impact, and instrumented falling-weight methods in accordance with ISO, ASTM, EN, GB, and JIS standards. Whether you are qualifying materials for automotive components, packaging, pipes, or electrical enclosures, understanding these test protocols ensures reliable, accurate, and internationally comparable data.

Technical Overview

Impact strength quantifies the ability of a plastic specimen to absorb energy during fracture when subjected to a rapidly applied load, typically from a swinging pendulum or a falling mass. Because thermoplastics are highly strain-rate and temperature sensitive, impact performance often governs product design decisions, material selection, and quality control in industries ranging from automotive engineering to consumer packaging.

The determination of plastic impact strength is standardized internationally through several complementary test protocols. The most widely applied standards include:

• ISO 179-1 and EN ISO 179-1: Determination of Charpy impact properties (unnotched and notched specimens)
• ISO 180 and EN ISO 180: Determination of Izod impact strength
• ASTM D256: Determining the Izod pendulum impact resistance of plastics
• ASTM D6110: Determining the Charpy impact resistance of notched specimens of plastics
• ISO 8256 and ASTM D1822: Tensile-impact strength testing
• ISO 6603-2 and ASTM D3763: Instrumented falling-weight (puncture) impact testing
• ISO 7765-1 and ASTM D1709: Free-falling dart impact resistance of plastic film and sheeting
• ASTM D5420 and ASTM D5628: Gardner and dropped-weight impact resistance of rigid flat specimens
• GB/T 1043 and GB/T 1843: Chinese national standards aligned with ISO 179 and ISO 180 respectively
• JIS K 7110 and JIS K 7111: Japanese industrial standards for Izod and Charpy impact testing

Machine verification is governed by ISO 13802, which specifies the calibration requirements for pendulum impact testing machines, and specimen conditioning is controlled by ISO 291 or ASTM D618.

Testing Samples

Standard Pendulum Impact Specimens

For ISO-based Charpy (ISO 179-1) and Izod (ISO 180) tests, the preferred specimen is a rectangular bar of 80 mm length, 10 mm width, and 4 mm thickness. For ASTM D256 Izod testing, the preferred specimen measures 63.5 mm in length, 12.7 mm in width, and 3.2 mm in thickness, while ASTM D6110 Charpy testing typically uses a larger 127 mm by 12.7 mm by 6.4 mm bar supported on a 101.6 mm span.

Notched Specimens

Notches are machined into the specimen to create a controlled stress concentration, and the notch geometry strongly influences the measured result. ISO 179-1 and ISO 180 define three notch types: Type A with a 0.25 mm notch radius and 2.0 mm depth, Type B with a 1.0 mm radius, and Type C with a sharp 0.1 mm radius for assessing extreme notch sensitivity. ASTM methods specify a 2.54 mm deep notch with a 0.25 mm radius. Notching must be performed with a freshly sharpened cutter, and specimens should be machined in accordance with ISO 2818 to avoid introducing residual stresses.

Specimens for Other Impact Methods

Tensile-impact testing (ISO 8256, ASTM D1822) uses small dumbbell-shaped specimens attached to a pendulum cross-arm. Instrumented puncture testing (ISO 6603-2, ASTM D3763) typically uses 60 mm square or 60 mm diameter disks of 1 mm to 4 mm thickness. Film and thin sheeting are tested as large membrane samples clamped over a circular support for free-falling dart methods (ISO 7765-1, ASTM D1709, GB/T 9639).

Specimen Preparation and Conditioning

Specimens may be directly injection molded or machined from molded plaques, sheets, pipes, or finished parts. All specimens must be conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity, generally for a minimum of 16 hours, unless a specific conditioning protocol is specified by the product standard.

Testing Items

The principal impact-related test items determined for plastic materials include:

• Charpy impact strength, unnotched and notched, expressed in kJ/m² (ISO 179-1, ASTM D6110, GB/T 1043)
• Izod impact strength, unnotched and notched, expressed in kJ/m² under ISO 180 or in J/m (ft·lb/in) under ASTM D256
• Tensile-impact strength in kJ/m² (ISO 8256, ASTM D1822)
• Instrumented puncture impact properties, including peak force, deflection at peak force, energy to peak force, and total puncture energy in joules (ISO 6603-2, ASTM D3763)
• Free-falling dart impact resistance of films, reported as dart mass at 50 % failure probability (ISO 7765-1, ASTM D1709)
• Low-temperature and elevated-temperature impact strength for assessing the ductile-to-brittle transition
• Failure mode classification: complete break, hinge break, partial break, or non-break, distinguishing brittle from ductile behavior

Testing Methods

Charpy Impact Test (ISO 179-1, ASTM D6110, GB/T 1043)

In the Charpy configuration, the specimen is placed horizontally on two supports as a simply supported beam and struck at mid-span by the pendulum, on the face opposite the notch. The absorbed energy is corrected for friction and windage losses, and the impact strength is calculated by dividing the corrected energy by the cross-sectional area under the notch. The pendulum capacity must be selected so that the absorbed energy falls between 10 % and 80 % of the nominal capacity. The Charpy method is particularly suitable for brittle or notch-sensitive materials and is the preferred configuration in European and ISO-based testing programs.

Izod Impact Test (ISO 180, ASTM D256, GB/T 1843)

In the Izod configuration, the specimen is clamped vertically as a cantilever beam and struck by the pendulum at a point 22 mm above the clamping plane, with the notch facing the striker. ASTM D256 Method A is the classical notched Izod procedure, while Methods B and C address unnotched and reversed-notch configurations. Because the clamped end of the specimen experiences bending stresses, Izod results are generally not directly interchangeable with Charpy data.

Tensile-Impact Test (ISO 8256, ASTM D1822)

The tensile-impact method fractures a dumbbell specimen in uniaxial tension at high strain rate and is used for materials that are too flexible or too tough to fail in flexural pendulum tests. The absorbed energy divided by the original cross-sectional area yields the tensile-impact strength.

Instrumented Falling-Weight and Puncture Impact (ISO 6603-2, ASTM D3763)

Instrumented testing drives a hemispherical striker, typically 20 mm in diameter at approximately 4.4 m/s, into a clamped disk while a load cell and displacement sensors record the complete force-deflection curve. This yields not only the total failure energy but also peak force, stiffness, and the distinction between ductile and brittle failure modes, making it the method of choice for automotive interior safety assessments and multi-axial impact qualification.

Free-Falling Dart Test for Films (ISO 7765-1, ASTM D1709)

For flexible films and thin sheeting, a hemispherical-headed dart is dropped onto a clamped membrane using the staircase method to determine the mass causing 50 % specimen failure, which characterizes the practical impact toughness of packaging films.

Applications

Standardized impact strength determination supports a broad range of engineering and commercial objectives:

• Material qualification and datasheet generation for new resins, compounds, and recycled plastics
• Automotive engineering: verifying low-temperature ductility of bumpers, interior trim, and trim materials at temperatures down to -30 °C or -40 °C
• Pipes and fittings: confirming toughness of PVC, PE, and PP piping systems for water and gas distribution
• Packaging: rating drop and shock resistance of films, bottles, crates, and transport containers
• Electrical and electronic equipment: validating enclosures and housings in line with requirements such as those referenced in UL 746A for polymeric materials
• Construction products: impact verification of window profiles, door panels, sheets, and glazing elements
• Quality control in production, incoming material inspection, and root-cause failure analysis
• Comparative studies of additives, fillers, glass-fiber reinforcement, and impact modifiers

Accredited third-party laboratories routinely perform these determinations under ISO/IEC 17025 management systems, providing internationally recognized test reports for regulatory submission and customer acceptance.

FAQ

What is the difference between the Charpy and Izod impact tests?

The Charpy test strikes a horizontally supported beam at mid-span, whereas the Izod test strikes a vertically clamped cantilever specimen near its free end. The stress states differ, and clamping stresses in the Izod setup can affect results, so data from the two methods must never be compared directly.

Why are notched impact values much lower than unnotched values?

The notch creates a triaxial stress concentration and accelerates crack initiation, which suppresses plastic deformation. The ratio of notched to unnotched strength is therefore a useful measure of the notch sensitivity of a material.

Which standard system should I choose, ISO or ASTM?

The choice depends on the market and the customer specification. ISO methods with the 80 mm by 10 mm by 4 mm specimen dominate European and Asian specifications, while ASTM methods with the 63.5 mm by 12.7 mm by 3.2 mm specimen are prevalent in North America. Results are not numerically interchangeable.

How does temperature affect plastic impact strength?

Most thermoplastics become significantly more brittle below their glass transition temperature. Testing at low temperatures, such as -20 °C or -40 °C, is essential for outdoor, automotive, and cold-chain applications to identify the ductile-to-brittle transition.

How many specimens are required for a valid determination?

ISO 179-1 generally requires ten specimens for a statistically valid determination, while ASTM D256 allows a minimum of five, with additional specimens recommended when scatter is high. Brittle failure modes typically require more specimens than ductile ones.

What do complete break and non-break results mean?

ISO 179-1 classifies outcomes as complete break, hinge break, partial break, or non-break. A non-break or partial break usually indicates ductile, tough behavior, while a complete brittle break signals notch sensitivity or embrittlement and should be noted in the test report.