Impact Strength Analysis and Testing of Plastic Materials

Impact strength testing is one of the most widely applied methods for evaluating the toughness, ductility, and fracture resistance of plastic materials under rapid loading conditions. This article provides a technical overview of plastic impact strength analysis and testing, covering standard methods such as Izod, Charpy, instrumented puncture, and falling dart impact in accordance with ISO, ASTM, and GB standards. From material selection and quality control to failure analysis, the information below helps manufacturers choose the correct test protocol and interpret results reliably through accredited third-party laboratories.

Technical Overview

Impact strength describes the ability of a plastic material to absorb mechanical energy and resist fracture when subjected to a sudden, high-speed load. Because plastics are viscoelastic, their behavior under impact loading can differ dramatically from their performance in slow tensile or flexural tests. A polymer that shows high elongation at break in a tensile test may still fail in a brittle manner under impact, particularly at low temperatures, in the presence of stress concentrators such as notches, or after environmental degradation.

Impact testing therefore serves several essential purposes in polymer engineering:

• Comparing the toughness of candidate materials during design and material selection.
• Verifying the effect of modifiers, fillers, fibers, and recycled content on toughness.
• Detecting ductile-to-brittle transitions as a function of temperature.
• Controlling manufacturing quality, including molding conditions and part consistency.
• Supporting failure analysis when components fracture in service.

Modes of Impact Loading

Standardized impact tests apply energy through different loading geometries: flexural impact on a notched or unnotched bar (Izod cantilever bending and Charpy three-point bending), tensile impact on a small dumbbell specimen, and multi-axial puncture or drop-weight impact on plates, films, or finished parts. Each mode probes a different stress state, which is why results from different test types are not interchangeable.

Ductile Versus Brittle Failure

In ductile failure, the specimen yields, whitens, or deforms substantially before breaking, absorbing a large amount of energy. In brittle failure, a sharp crack propagates rapidly with little plastic deformation and low energy absorption. Many plastics exhibit a ductile-to-brittle transition temperature below which impact performance drops sharply, making low-temperature impact testing a critical requirement for outdoor, automotive, and cold-chain applications.

Testing Samples

Molded or Machined Impact Bars

The standard flexural impact specimen for ISO methods is a bar of 80 mm × 10 mm × 4 mm, injection molded or machined from sheet, plate, or molded parts. Notched specimens are prepared with an A-type notch (tip radius 0.25 mm), B-type notch (1.0 mm radius), or C-type notch (0.10 mm radius) machined under controlled conditions. ASTM methods typically use bars of 63.5 mm × 12.7 mm with thicknesses from 3.2 mm to 12.7 mm and a milled notch of 0.25 mm radius. Notch quality, cutting tool sharpness, and machining parameters strongly influence results and must be tightly controlled.

Films and Thin Sheets

Flexible films used in packaging are evaluated with circular or square coupons, commonly 100 mm × 100 mm or larger, mounted in a clamping fixture for free-falling dart or instrumented dart impact testing.

Plates, Finished Parts, Pipes, and Fittings

Flat plaques (for example 60 mm × 60 mm or 100 mm × 100 mm panels) are used for instrumented puncture impact. Segments cut from pipes and fittings are tested with drop-weight or falling-mass methods using spherical or tumbling strikers to simulate external blows during handling and installation. Where a finished component cannot be cut into standard bars, specimens machined from defined locations, including flow direction and gate regions, allow assessment of anisotropy.

Specimen Conditioning

Unless otherwise specified, specimens are conditioned at 23 °C and 50% relative humidity in accordance with ISO 291 or ASTM D618 before testing. For hygroscopic materials such as polyamide, dry-as-molded and moisture-equilibrated states should be clearly distinguished, since moisture content has a strong toughening effect.

Testing Items

Typical impact strength test items for plastic materials include:

• Notched Izod impact strength, in J/m (ASTM D256) or kJ/m² (ISO 180).
• Unnotched Izod impact strength.
• Notched and unnotched Charpy impact strength in kJ/m² (ISO 179-1, ASTM D6110).
• Instrumented puncture impact properties: peak force, deflection at peak force, energy to peak force, and total failure energy (ISO 6603-2, ASTM D3763).
• Falling dart impact resistance of films, reported as F50 or mean failure mass/energy (ISO 7765-1, ASTM D1709).
• Drop weight or Gardner impact resistance of rigid sheets and painted parts (ASTM D5420, ASTM D5628).
• Tensile impact strength (ASTM D1822, ISO 8256).
• Impact resistance of thermoplastics pipes to external blows (ISO 3127, ASTM D2444).
• Low-temperature impact strength at -40 °C, -30 °C, -20 °C, or other specified temperatures.
• Impact retention after heat aging, UV weathering, chemical exposure, or sterilization.

Corresponding Chinese national standards, such as GB/T 1843 for Izod and GB/T 1043.1 for Charpy impact, are technically aligned with the ISO methods and are frequently specified for domestic market access.

Testing Methods

Izod Impact Test (ASTM D256 / ISO 180)

In the Izod configuration, the notched specimen is clamped as a vertical cantilever beam and struck by a swinging pendulum on the notched side just above the clamp. The pendulum, available in energy capacities typically from 0.5 J to 25 J, fractures the specimen, and the energy absorbed is calculated from the difference between the initial and final pendulum heights. ASTM D256 reports energy per unit specimen width (J/m), while ISO 180 reports energy divided by the cross-sectional area at the notch (kJ/m²). Because the geometries and calculations differ, ASTM and ISO notched Izod values are not directly convertible. ASTM D256 also classifies failure modes as complete break, hinged break, partial break, or non-break; valid notched Izod values normally require complete breaks, and non-breaks must be retested with a higher-capacity pendulum or reported as such.

Charpy Impact Test (ISO 179-1 / ASTM D6110)

In the Charpy configuration, the specimen rests horizontally against two supports and is struck by the pendulum at the center, opposite the notch. ISO 179-1 defines edgewise and flatwise strikes with several notch types, and also permits testing unnotched specimens. For quality control of very brittle or fiber-reinforced materials, the Charpy unnotched method often provides a clearer ranking than notched testing. The absorbed energy must generally fall between 10% and 80% of the pendulum capacity for a valid measurement, and at least five specimens are tested with the mean and standard deviation reported. Instrumented pendulum systems, which record force versus time during the strike, further separate crack initiation energy from propagation energy.

Instrumented Puncture Impact (ISO 6603-2 / ASTM D3763)

Instrumented impact testing drives a hemispherical striker, commonly 20 mm in diameter, through a clamped flat specimen at a controlled high velocity while a force transducer in the tup records the complete force-deflection response. The resulting curve distinguishes brittle failure (a sharp force peak followed by sudden load drop) from ductile failure (yielding, stable crack growth, and energy absorption through drawing). Key reported parameters include peak force, deflection and energy at peak force, and total energy to failure. This method is particularly valuable for engineering plastics, painted automotive components, and multi-layer structures, because it captures the full failure process rather than a single energy number.

Falling Dart and Drop Weight Impact (ISO 7765 / ASTM D1709 / ASTM D5420)

For plastic films, the free-falling dart method (ASTM D1709, ISO 7765-1) drops a rounded dart with an incremental mass onto a clamped film; the failure mass is determined statistically, typically using a staircase sequence with twenty or more specimens. Rigid sheet, laminates, and finished parts are evaluated with Gardner or drop-weight tests (ASTM D5420, ASTM D5628), which report impact energy at which 50% of specimens fail. These pass-fail statistical methods are well suited to thin or anisotropic products where pendulum bar testing is impractical.

Tensile Impact Test (ASTM D1822 / ISO 8256)

The tensile impact test fractures a small dumbbell-shaped specimen attached between a pendulum crosshead and a fixed grip, measuring the energy absorbed under high-speed tensile loading. It is useful for materials that are too flexible for reliable flexural impact results and for evaluating very brittle or highly filled compounds.

Temperature-Controlled and Environmental Testing

Impact tests can be performed inside temperature chambers covering sub-ambient to elevated conditions. A temperature sweep reveals the ductile-to-brittle transition, a critical design parameter for automotive exterior parts, cold-climate infrastructure, and refrigeration components. Testing after thermal aging, artificial weathering, or chemical exposure quantifies impact retention and long-term durability.

Equipment and Laboratory Requirements

Pendulum testers must be verified for energy scale, friction and windage losses, anvil and support geometry, and striker dimensions in accordance with ISO 13802 or equivalent calibration procedures. Accredited third-party laboratories operating under ISO/IEC 17025 maintain calibrated instrumentation, controlled notching equipment, documented conditioning, and validated test procedures, ensuring that reported values are traceable and reproducible.

Applications

Automotive

Notched Izod and instrumented puncture impact data at room and low temperatures are essential for polypropylene bumpers, TPO fascias, PC/ABS interior trims, and under-hood components. Testing after painting, plating, or heat aging verifies that processing and finishing have not embrittled the part.

Electronics and Electrical Equipment

Housings, enclosures, connectors, and structural brackets made of ABS, PC, PC/ABS, and modified PPO are qualified with Izod or Charpy impact data, often as part of polymeric material evaluation programs referenced in UL and IEC safety schemes.

Packaging and Consumer Goods

Falling dart and drop impact tests validate the toughness of PE and PP films, containers, closures, and household products, correlating directly with drop, abrasion, and transport performance.

Pipes, Fittings, and Construction Products

Drop-weight impact testing of PVC, PE, and PP pipes per ISO 3127 or ASTM D2444 demonstrates resistance to installation damage and external blows, a core requirement for pressure pipe qualification and standards compliance.

Material Development and Failure Analysis

Impact testing quantifies the effectiveness of rubber toughening, compatibilization, fiber reinforcement, and recycled content addition. When a component fails in service, comparison of impact data from failed and reference material, combined with fracture surface examination, helps identify root causes such as degradation, contamination, excessive regrind, or molding defects.

FAQ

What is the difference between Izod and Charpy impact tests?

The Izod test strikes a vertically clamped cantilever specimen on the notched side, while the Charpy test strikes a horizontally supported beam opposite the notch. Both use a pendulum, but specimen clamping, striking direction, and standard reporting units differ, so results are not directly comparable between the two configurations.

Why are specimens notched?

The notch acts as a controlled stress concentrator that accelerates crack initiation under the triaxial stress state at the notch tip. Notched results primarily measure notch sensitivity and crack propagation resistance, which is highly relevant because real parts almost always contain sharp features, weld lines, or defects.

Are notched and unnotched results interchangeable?

No. Unnotched impact strength reflects the total energy the material can absorb, including yielding and plastic deformation, whereas notched values emphasize resistance to brittle crack growth. Tough, notch-insensitive materials show small differences between the two, while notch-brittle materials can show differences of an order of magnitude or more.

Can ASTM J/m values be converted to ISO kJ/m² values?

Not reliably. The two systems use different specimen dimensions, notch geometries, and calculation bases (energy per unit width versus energy per cross-sectional area). Comparisons should always be made within a single standard and specimen type.

Why do impact results sometimes show large scatter or "non-break" outcomes?

Impact fracture is inherently statistical and sensitive to notch quality, internal voids, orientation, and temperature. When a specimen does not break, the recorded value represents only a lower bound; the correct procedure is to retest with a higher-energy pendulum or report the result as non-break. Multiple specimens and statistical reporting are standard practice.

How does temperature affect impact strength?

As temperature decreases, thermoplastics lose segmental mobility and tend to shift from ductile to brittle failure, causing a steep drop in absorbed energy. Determining the ductile-to-brittle transition through temperature-controlled testing is essential for parts exposed to cold environments.

How many specimens are required?

Flexural pendulum methods typically require a minimum of five to ten specimens per condition, while statistical drop-weight and falling dart methods generally require twenty or more specimens to establish the 50% failure energy with confidence.

Can finished products be tested instead of molded bars?

Yes, whenever possible. Specimens can be machined from designated locations of a finished part, or complete parts can be subjected to drop-weight, instrumented puncture, or product-specific impact tests, which better reflect real geometry, orientation, and processing effects.

What information should be included in an impact test report?

A complete report should identify the material, specimen preparation and notch details, conditioning, the test standard and configuration, pendulum capacity, test temperature, individual and mean results with standard deviation, break classification, and any deviations or non-break occurrences.

Why outsource impact testing to accredited third-party laboratories?

Accredited laboratories provide calibrated and verified impact testers, controlled notching equipment, traceable temperature conditioning, and experienced interpretation of failure modes. Independent results are widely accepted for supplier qualification, regulatory submissions, and dispute resolution.