Plastic Izod Impact Analysis: Standards, Test Methods, and Applications

Plastic Izod impact analysis is a standardized mechanical test method used to evaluate the impact toughness and notch sensitivity of polymer materials under high-speed loading conditions. Governed by internationally recognized standards such as ISO 180, ASTM D256, and GB/T 1843, this cantilever beam impact technique provides essential data for material selection, quality control, and product compliance across the automotive, electronics, packaging, and construction industries. This article explains the testing principles, specimen requirements, procedures, calculation methods, and practical applications of Izod impact testing for plastics.

Technical Overview

Plastic Izod impact analysis, also known as cantilever beam impact testing, is one of the most widely adopted methods for quantifying the impact resistance of plastic materials. In this test, a rectangular specimen is clamped vertically as a cantilever beam and struck near its free end by a pendulum of known potential energy. The difference between the initial energy of the pendulum and its residual energy after impact, corrected for friction and windage losses, represents the energy absorbed by the specimen during deformation and fracture.

The method is standardized in ISO 180 (Plastics — Determination of Izod impact strength), ASTM D256 (Determining the Izod Pendulum Impact Resistance of Plastics), GB/T 1843, and JIS K 7110. Results are expressed as impact strength in kJ/m² under the ISO system, or in J/m and ft·lb/in of notch under the ASTM system. These values allow engineers to compare the toughness of candidate materials, verify incoming resin quality, and confirm that finished parts meet specification limits.

Izod impact data is particularly important because impact performance cannot be reliably predicted from tensile or flexural properties alone. Polymers exhibit strongly rate- and temperature-dependent behavior, and the presence of a notch—a deliberate stress concentrator—can reduce apparent toughness by an order of magnitude. The test therefore reveals brittleness, quantifies the effectiveness of impact modifiers such as rubber or elastomer phases, and exposes processing-related weaknesses including thermal degradation, moisture absorption, contamination, and poor weld-line fusion.

Testing Samples

Standard Injection-Molded Specimens (ISO 180)

The preferred ISO specimen is Type 1, measuring 80 mm × 10 mm × 4 mm, normally produced by injection molding in accordance with ISO 294 or machined from multipurpose test pieces conforming to ISO 3167. Molding parameters such as melt temperature, mold temperature, and injection speed must be documented and controlled, because they influence crystallinity, molecular orientation, and residual stress, all of which affect impact results.

ASTM D256 Specimens

The standard ASTM specimen measures 63.5 mm × 12.7 mm × 3.2 mm (2.5 in × 0.5 in × 1/8 in), with 6.4 mm thickness also commonly tested for certain applications. Because ISO and ASTM specimens differ in geometry, notch depth, and clamping configuration, results from the two standards are not directly interchangeable and should never be compared without clear labeling.

Notched Specimens

Notched testing evaluates resistance to crack propagation from a sharp stress concentrator. ISO 180 defines three notch geometries: Type A with a 0.25 mm radius, Type B with a 1.0 mm radius, and Type C with a 0.10 mm radius; the notch depth for the Type 1 specimen is 2.0 mm, leaving an 8.0 mm ligament. ASTM D256 specifies a 2.54 mm (0.100 in) notch depth with a 0.25 mm radius. Notches must be machined with a sharp cutter under controlled feed and speed, following ISO 2818 where applicable, because notch radius accuracy and machining quality are among the largest sources of variability in the test.

Unnotched and Reversed-Notched Specimens

Unnotched specimens measure the overall energy absorption of a material free of an artificial stress raiser, which is useful for very brittle or highly filled plastics. For very ductile materials such as polyethylene that do not fracture when unnotched, the reversed-notch configuration—striking the face opposite the notch—provides a meaningful ranking. Both variants are valuable when building a complete picture of material behavior.

Specimens Machined from Finished Products

Specimens may be cut directly from sheets, pipes, profiles, or molded components for verification testing or failure analysis. In such cases, the cutting direction relative to melt flow or machine direction must be recorded, and care must be taken to avoid machining marks, local overheating, and plane-stress effects at thin sections. Results from machined specimens may differ from molded bars due to skin-core structure, orientation, and residual stress in the finished part.

Testing Items

Typical test items covered within a plastic Izod impact analysis program include:

• Notched Izod impact strength (Type A, B, or C notch), reported in kJ/m² or J/m
• Unnotched and reversed-notch Izod impact strength
• Absorbed impact energy in joules (J) for individual specimens
• Fracture classification: complete break, hinge break, partial break, or non-break
• Temperature-dependent testing, including sub-ambient conditions such as -40 °C and -30 °C for automotive and outdoor applications
• Directional comparison of specimens cut parallel versus perpendicular to flow or machine direction
• Conditioning-state comparison, for example dry-as-molded versus moisture-conditioned polyamide samples
• Statistical evaluation of mean value and standard deviation across a minimum of five specimens

Testing Methods

Specimen Conditioning

Prior to testing, specimens are conditioned in a standard atmosphere of 23 °C and 50% relative humidity in accordance with ISO 291 or ASTM D618, typically for a minimum of 16 hours and up to 88 hours depending on the material specification. Hygroscopic materials such as polyamide may require additional drying or defined moisture conditioning, since even small amounts of absorbed water can dramatically plasticize the polymer and raise impact strength.

Instrument and Pendulum Selection

Testing is performed on a calibrated pendulum impact tester verified according to ISO 13802, with interchangeable pendulums spanning common capacities from 1 J to 25 J or higher and an impact velocity of approximately 3.5 m/s. The pendulum capacity is selected so that the corrected absorbed energy falls between 10% and 80% of the nominal capacity, as required by ISO 180. Friction and windage losses are checked by performing free-swing (no-specimen) cycles before each test series, and the total measurement uncertainty of the machine must remain within the tolerance specified by the relevant standard.

Test Procedure

• Measure the width and thickness of the specimen at the notch ligament to an accuracy of 0.02 mm and verify the notch depth and radius.
• Clamp the specimen vertically with the notch facing the pendulum, positioning the notch plane level with the top surface of the clamp.
• Release the pendulum from its latched height so that the striking edge contacts the specimen above the clamp at the specified velocity.
• Record the absorbed energy indicated by the machine, applying the correction for friction and windage.
• Examine the fractured specimen and classify the break type as complete, hinge, partial, or non-break; non-break results must be reported as such rather than as a numeric value.
• Test at least five valid specimens per condition, discard results affected by clamping errors or mis-strikes, and calculate the mean and standard deviation.

Calculation and Reporting

Under ISO 180, the notched impact strength equals the corrected absorbed energy divided by the cross-sectional area of the specimen beneath the notch (width multiplied by ligament thickness), expressed in kJ/m². Under ASTM D256, the result is expressed as energy per unit thickness of the specimen in J/m or ft·lb/in, noting that 1 ft·lb/in equals approximately 53.4 J/m. A complete test report identifies the material and grade, molding method and conditions, specimen and notch type, conditioning state, pendulum capacity, test temperature, individual and mean results, standard deviations, and the designation of each fracture type.

Applications

Izod impact analysis supports a broad range of engineering and commercial objectives, from research and development to routine production control and certification support by accredited third-party laboratories:

• Material development: screening impact modifiers, filler loadings, blend ratios, and new polymer grades during formulation work.
• Material selection and comparison: ranking candidate plastics for housings, brackets, enclosures, and load-bearing components before tooling investment.
• Quality control: verifying incoming resin batches, monitoring compounding consistency, and detecting degradation or contamination in recycled content.
• Automotive engineering: validating bumpers, interior trim, dashboards, wheel arch liners, and battery pack housings against low-temperature impact requirements.
• Electrical and electronic products: confirming the robustness of appliance and consumer electronics enclosures molded from ABS, PC, or PC/ABS blends.
• Pipes, fittings, and construction products: assessing toughness of PVC, PE, and PP components used in pressure and drainage systems.
• Packaging and consumer goods: evaluating drop and shock resistance of containers, closures, and household articles.
• Failure analysis: comparing impact properties of failed parts against reference material to identify processing faults, aging, or environmental stress effects.

Because impact requirements frequently appear in customer specifications, regulatory requirements, and standards published by organizations such as ISO, ASTM, and UL, documented Izod data is often a mandatory element of material data sheets, datasheet declarations, and product compliance files.

FAQ

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

In the Izod configuration, the specimen is clamped as a vertical cantilever beam and struck at its free end. In the Charpy configuration, defined in ISO 179 and ASTM D6110, the specimen lies horizontally on two supports and is struck at midspan, opposite the notch. The two methods use different stress states and produce different values, so results must never be mixed or converted between configurations.

Why are most Izod tests performed on notched specimens?

The notch creates a controlled, sharp stress concentrator that simulates scratches, gate marks, screw bosses, and other real-world defects. Notched results measure the material's resistance to brittle crack propagation, which is usually the property that governs service failures. Comparing notched and unnotched values also quantifies the notch sensitivity of a polymer.

Can ISO 180 results be compared directly with ASTM D256 results?

No. The two standards use different specimen dimensions, notch depths, ligament sizes, clamping arrangements, and units. A material tested by both methods may show noticeably different values even when it fractures in a similar manner. Any comparison must state the standard used, and conversion between kJ/m² and J/m is not valid because the denominators differ.

What does a non-break result mean, and how is it handled?

A non-break result indicates that the specimen bent or partially tore without completing a fracture, which is common for very tough materials such as polycarbonate or ductile polyolefins. The result is reported as NB rather than a number, and if a numeric value is required, a higher-energy pendulum or a sharper notch type may be selected, subject to standard requirements.

What are typical notched Izod impact values for common plastics?

Indicative room-temperature notched values under ISO 180 include approximately 2 to 6 kJ/m² for general-purpose polystyrene, 2 to 4 kJ/m² for PMMA, 4 to 12 kJ/m² for rigid PVC, 12 to 30 kJ/m² for ABS, 3 to 6 kJ/m² for polypropylene homopolymer, and 60 kJ/m² or higher—often non-break—for polycarbonate. Actual values depend strongly on grade, additives, morphology, and processing, so data sheets for the specific material should always be consulted.

How does temperature affect Izod impact strength?

Impact strength generally decreases as temperature drops, and many plastics pass through a ductile-to-brittle transition. For this reason, outdoor and automotive applications are frequently tested at -30 °C or -40 °C using an environmental chamber, and the test temperature must always be stated alongside the result.

How many specimens are required for a valid test?

ISO 180 and ASTM D256 both require a minimum of five valid specimens per condition, with additional specimens recommended when results scatter widely or when anisotropic materials are machined from plates or parts in different directions. Mean values and standard deviations should be reported to reflect material variability honestly.