Determination of Notched Impact Strength of Plastics: Principles, Standards, and Test Methods

Notched impact strength determination is one of the most widely performed mechanical tests for plastics, quantifying a material's resistance to brittle fracture under rapid loading in the presence of a stress-raising notch. This article explains the test principle, specimen preparation, the Charpy and Izod methods defined in ISO 179, ISO 180, ASTM D256, ASTM D6110, and GB/T 1043.1, and how the results are applied in material selection, quality control, and failure analysis.

Technical Overview

Notched impact strength is a measure of the energy required to break a plastic specimen that contains a standardized machined notch, subjected to a blow from a swinging pendulum. The absorbed energy is normalized by the cross-sectional area at the notch root and expressed in kJ/m² (or J/m and ft·lbf/in in ASTM practice). The test is governed by internationally recognized standards including ISO 179-1, ISO 180, ASTM D256, ASTM D6110, GB/T 1043.1, and GB/T 1843.

Because thermoplastics behave very differently under impact loading than under slow tensile loading, notched impact testing is indispensable for evaluating notch sensitivity, assessing the risk of brittle fracture in service, and comparing the toughness of competing grades. The deliberately introduced notch creates a triaxial stress state and a stress concentration at the tip, which accelerates crack initiation and reveals differences in material ductility that a smooth specimen may mask.

Principle of the Pendulum Test

A pendulum hammer of known potential energy is released from a defined height and strikes the mounted specimen. The difference between the initial energy of the pendulum and the residual energy after breaking the specimen, corrected for friction and windage losses, is the absorbed impact energy. Dividing this corrected energy by the area of the ligament at the notch yields the notched impact strength.

Why the Specimen Is Notched

Real plastic parts almost always contain stress raisers such as weld lines, gate marks, sharp corners, scratches, and voids. The machined notch simulates these features in a reproducible way, forcing brittle crack initiation at a known location and making results comparable between laboratories and material batches.

Testing Samples

Injection-Molded Specimens

The preferred ISO specimen is machined or molded from the multipurpose test specimen defined in ISO 3167, with typical notched bar dimensions of 80 mm × 10 mm × 4 mm. Notching is performed after molding using a single-tooth fly-cutter or a dedicated notching machine in accordance with ISO 2818, because the geometry and surface quality of the notch dominate the test result.

Specimens Machined from Sheets and Parts

For plates, compression-molded sheets, extruded profiles, pipes, or finished components, specimens are cut and milled to the required bar dimensions. ASTM D256 typically uses bars of 63.5 mm × 12.7 mm × 3.2 mm or 12.7 mm × 6.4 mm, while ISO practice favors the 10 mm × 4 mm cross-section. The direction of machining relative to processing-induced orientation must be recorded, since it can significantly influence the measured energy.

Notch Geometry

• ISO type A notch: radius 0.25 mm, remaining ligament width 8.0 mm on a 10 mm wide bar — the default notch for most comparative testing.
• ISO type B notch: radius 1.0 mm — used to study notch sensitivity, as it gives higher values than type A for ductile materials.
• ISO type C notch: radius 0.1 mm — used for very tough materials such as certain polyamide grades.
• ASTM D256 notch: depth of 2.54 mm with a radius of 0.25 mm on the 12.7 mm wide face.

Conditioning

Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 16 hours in accordance with ISO 291 or ASTM D618 before testing. Hygroscopic materials such as polyamides should be tested in both the dry-as-molded and moisture-conditioned states, because moisture content dramatically changes impact behavior.

Testing Items

• Charpy notched impact strength per ISO 179-1, GB/T 1043.1, and ASTM D6110.
• Izod notched impact strength per ISO 180, ASTM D256, and GB/T 1843.
• Charpy and Izod unnotched impact strength for baseline ductility comparison.
• Notched impact strength at low temperatures (for example −40 °C or −30 °C) for outdoor, automotive, and cold-chain applications.
• Instrumented impact testing, which records force–deflection curves to separate initiation and propagation energies.
• Tensile-impact strength per ISO 8256 for very flexible or thin materials.

Testing Methods

Charpy Method (ISO 179-1 / GB/T 1043.1 / ASTM D6110)

The specimen is positioned horizontally on two support anvils, typically 62 mm apart, with the notch facing away from the striker. The pendulum strikes the center of the bar opposite the notch, placing the notch root in tension. Both flatwise and edgewise orientations are permitted, and the orientation must be stated in the report. Available hammer energies commonly range from 1 J to 50 J, with 2.75 J, 5.5 J, and 7.5 J hammers used most frequently for notched plastics.

Izod Method (ISO 180 / ASTM D256 / GB/T 1843)

The specimen is clamped as a vertical cantilever with the notch level with the top of the vise and aligned on the tension side. The pendulum strikes the free end of the bar at a defined distance, normally 22 mm, above the clamping plane. Consistent clamping torque is essential because over- or under-clamping shifts the measured energy. ISO results are expressed in kJ/m², while ASTM D256 results are classically reported in J/m or ft·lbf/in, so direct comparison requires geometric conversion.

Calculation and Expression of Results

The notched impact strength is calculated as the corrected absorbed energy E divided by the product of the specimen thickness h and the remaining width b_N at the notch base, multiplied by 10³ to obtain kJ/m². The total energy absorbed by the apparatus, verified with unbroken calibration specimens, must be subtracted, and the absorbed energy should ideally be between 10% and 80% of the hammer capacity for valid measurement.

Fracture Classification

• C — complete break: the specimen separates into two or more pieces.
• H — hinge break: an incomplete break with low residual stiffness at the ligament.
• P — partial break: incomplete break exceeding a defined strained zone.
• N — non-break: the specimen does not reach the defined strained zone.

Per ISO, values from complete and hinge breaks are averaged, while partial breaks and non-breaks are recorded separately, because averaging them with complete breaks would distort the material characterization.

Influencing Factors

• Notch radius, depth, and cut quality: a dull cutter or overheated notch produces artificially low values.
• Temperature and strain rate: many thermoplastics pass through a ductile-to-brittle transition as temperature drops.
• Molecular orientation from molding: impact energy measured along versus across the flow direction can differ substantially.
• Specimen thickness, moisture content, hammer capacity selection, and clamping conditions.

Applications

• Incoming inspection and quality control of resins, compounds, and masterbatches against datasheet limits.
• Material selection and grade comparison for housings of electronics and appliances, where drop and knock resistance matter.
• Automotive interior and exterior components, including bumpers, trims, and under-hood parts verified at low temperature.
• Pipes, fittings, and valves for water and gas systems where resistance to rapid crack propagation is critical.
• Packaging, pallets, and crates subjected to repeated handling shocks.
• Failure analysis, aging studies after heat or UV exposure, and validation of recycled-content compounds.
• Compliance evidence for buyer specifications, EN and UL product standards, and regulatory submissions.

Typical indicative notched Izod values at 23 °C include approximately 1.5–3 kJ/m² for general-purpose polystyrene and PMMA, 3–6 kJ/m² for PP homopolymer, 10–25 kJ/m² for ABS, and 60–90 kJ/m² for polycarbonate; actual values always depend on grade, processing, and notch type, so comparison must reference the exact test conditions.

FAQ

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

In the Charpy test the notched bar lies horizontally on two supports and is struck at mid-span opposite the notch, while in the Izod test the notched bar is clamped vertically as a cantilever and struck above the notch. Charpy is preferred in ISO and European practice; Izod dominates in North American practice. The two methods do not produce interchangeable numbers.

Why is the notch radius so critical?

The stress concentration factor depends strongly on the notch root radius. A sharper radius produces a more severe triaxial stress state and a lower measured strength. Standards therefore specify tight radius tolerances, typically ±0.05 mm for the 0.25 mm type A notch, and require periodic verification of the cutting tool.

How many specimens should be tested?

ISO recommends at least ten specimens when fracture behavior is expected to vary, and never fewer than five for routine comparison; ASTM practice likewise tests a minimum of five valid breaks. If partial breaks or non-breaks occur, additional specimens are tested and the break categories are reported individually.

What do very low or very high results mean?

Values below about 10% of the pendulum capacity or above about 80% carry excessive uncertainty, so a more suitable hammer must be selected. Results that fall into the partial-break or non-break category indicate an extremely tough material relative to the chosen notch and should be documented rather than averaged with complete breaks.

Can results from different standards be compared directly?

No. Differences in specimen dimensions, notch geometry, support or clamping arrangement, and units mean that only tests run under the same standard, notch type, temperature, and conditioning state are directly comparable. Accredited third-party laboratories report the full test conditions alongside the numerical result to guarantee comparability.