Comprehensive Analysis of Charpy (Simply Supported Beam) Impact Performance of Plastics

Charpy impact testing, also known as simply supported beam impact testing, is one of the most widely used methods for evaluating the impact toughness, brittleness, and notch sensitivity of plastic materials. This article provides a comprehensive analysis of plastic Charpy impact performance, covering the test principle, specimen preparation, key standards such as ISO 179-1, ASTM D6110 and GB/T 1043.1, testing procedures, data interpretation, and typical industrial applications.

Technical Overview

The Charpy impact test, commonly referred to as the simply supported beam impact test, determines the energy absorbed by a standardized plastic specimen when it is fractured by a single blow from a swinging pendulum. A rectangular bar is laid horizontally across two parallel anvils, forming a simply supported beam, and the pendulum striker impacts the specimen at mid-span on the face opposite the notch. The absorbed energy is derived from the difference between the initial and residual heights of the pendulum, corrected for friction and windage losses, and is used to calculate the impact strength in kJ/m².

This method is governed internationally by ISO 179-1 (Plastics – Determination of Charpy impact properties – Part 1: Non-instrumented impact test), in North America by ASTM D6110, and in China by GB/T 1043.1, which is technically aligned with ISO 179-1. In addition, ISO 179-2 covers instrumented Charpy testing, in which a force transducer in the striker records the full force–deflection response, allowing separation of crack-initiation and crack-propagation energies.

Charpy data are essential for material qualification, quality control, and failure analysis because they reveal how a polymer behaves under high strain-rate loading. Results are strongly influenced by notch geometry, temperature, molecular orientation, filler content, and prior aging, which makes the test highly informative for assessing both ductile and brittle failure tendencies of thermoplastics and thermosets.

Testing Samples

ISO and GB Specimens (ISO 179-1 / GB/T 1043.1)

The preferred specimen geometry is a bar of 80 mm × 10 mm × 4 mm, machined from sheet or plate in accordance with ISO 2818 or directly injection molded. Specimens are normally tested edgewise, with the pendulum striking the 10 mm wide face.

• Unnotched specimens (80 mm × 10 mm × 4 mm) for determining unnotched impact strength.
• Type A notch: 0.25 mm notch radius, 2 mm notch depth, leaving an 8 mm ligament, for assessing standard notch sensitivity.
• Type B notch: 1.0 mm notch radius, 2 mm depth, for materials that are too brittle or too notch-sensitive with a Type A notch.
• Type C notch: 0.1 mm notch radius, 2 mm depth, for evaluating surface effects, for example after weathering or coating.

ASTM Specimens (ASTM D6110)

ASTM D6110 typically uses bars of 63.5 mm × 12.7 mm × 3.2 mm (thicker sections such as 6.4 mm are also permitted), with a notch radius of 0.25 mm and a notch depth of 2.54 mm. Because the geometry, support span, and striker differ from the ISO configuration, ISO and ASTM results are not directly interchangeable.

Notch Machining and Conditioning

Notches must be machined with a single-tooth milling cutter using a controlled feed rate, and cutter sharpness must be verified regularly, since notch root quality has a dominant effect on measured values. All specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 10% relative humidity in accordance with ISO 291 or ASTM D618 before testing. A minimum of ten specimens per material and condition is recommended to obtain statistically meaningful mean values and standard deviations.

Testing Items

Typical test items offered for plastic Charpy impact performance analysis include:

• Charpy unnotched impact strength, acU, expressed in kJ/m².
• Charpy notched impact strength, acA, acB, and acC, expressed in kJ/m².
• Low-temperature Charpy impact strength, for example at -20 °C, -30 °C, or -40 °C, to characterize the ductile-to-brittle transition.
• Charpy impact strength after artificial aging, heat aging, UV exposure, or hydro-aging, to evaluate long-term durability.
• Instrumented Charpy impact testing per ISO 179-2, providing peak force, deflection at break, energy to maximum force, total absorbed energy, and complete force–deflection curves.
• Fracture behavior classification: complete break (C), hinged break (H), partial break (P), and non-break (NB).

For engineering evaluations, the ratio of unnotched to notched impact strength is frequently reported as a notch sensitivity index, and the temperature dependence of impact strength is used to construct ductile–brittle transition curves.

Testing Methods

Testing is performed on a calibrated pendulum impact tester verified in accordance with ISO 13802. Standard pendulum capacities range from 1 J to 50 J (for example 1, 2, 4, 5, 7.5, 15, 25, and 50 J), with nominal impact velocities of 2.9 m/s for small-capacity pendulums and 3.8 m/s for pendulums of 7.5 J and above.

The general test procedure is as follows:

• Select a pendulum so that the energy absorbed by the specimen falls between 10% and 80% of the pendulum capacity; otherwise the measurement uncertainty becomes unacceptably large.
• Set the anvil support span to 62 mm for ISO Type 1 specimens and confirm the striker contact geometry (2 mm radius for ISO 179-1).
• Place the specimen edgewise on the supports with the notch facing away from the striker, centered with a positioning gauge.
• Release the pendulum from a defined angle and record the absorbed energy from the encoder or dial, applying corrections for friction and air resistance.
• Classify the fracture appearance (C, H, P, or NB) for each specimen.
• Calculate unnotched impact strength as acU = E × 1000 / (b × h) and notched impact strength as acN = E × 1000 / (bN × h), where E is the corrected absorbed energy in joules, b is the specimen width, bN is the residual ligament width under the notch, and h is the thickness in millimeters, yielding results in kJ/m².
• Report the arithmetic mean, standard deviation, and individual values together with the fracture classification for at least ten valid specimens.

For specimens that do not break, ISO 179-1 permits reporting a corrected impact strength calculated from the energy consumed up to a specified deflection using the path function defined in the standard. In instrumented testing, the force–deflection curve is analyzed to separate the energy consumed during crack initiation from that consumed during crack propagation, which is particularly valuable for distinguishing ductile and brittle failure mechanisms in toughened or reinforced compounds.

Applications

Charpy impact performance analysis supports design, material selection, and regulatory compliance across many industries:

• Automotive: interior trim, instrument panels, bumpers, underbody shields, and battery housings, including cold-temperature impact requirements.
• Pipes and fittings: PVC, PE, and PP pressure and drainage systems, where product standards specify minimum Charpy impact energy for qualification.
• Building and construction: PVC window and door profiles, siding, roofing sheets, and flooring, including assessment after weathering.
• Packaging and logistics: returnable crates, pallets, and containers subjected to repeated handling shocks.
• Electrical and electronic equipment: housings, enclosures, and switchgear components where impact resistance at ambient and low temperatures is required.
• Material development: screening of resin formulations, impact modifiers, fillers, glass-fiber reinforcement levels, and recycled content, as well as incoming inspection and lot-release testing by accredited third-party laboratories.

Comparative Charpy data are also widely used in failure analysis, since a shift from hinged or partial breaks to complete brittle breaks often indicates degradation, contamination, excessive regrind content, or improper processing conditions.

FAQ

What is the difference between Charpy and Izod impact testing?

In the Charpy test (ISO 179-1, ASTM D6110, GB/T 1043.1), the specimen rests horizontally on two supports as a simply supported beam and is struck at mid-span, with the notch facing away from the striker. In the Izod test (ISO 180, ASTM D256), the specimen is clamped as a vertical cantilever and struck above the clamp, with the notch facing the striker. The two methods use different geometries and stress states, so their results are not directly comparable.

Why are specimens notched?

The notch acts as a controlled stress concentrator that promotes crack initiation under triaxial stress. This accelerates brittle fracture, shortens testing time, and makes the measurement highly sensitive to differences in material toughness, allowing notch-sensitive materials to be differentiated reproducibly.

How many specimens should be tested?

At least ten specimens per material and test condition are recommended by ISO 179-1 to obtain a representative mean and standard deviation, because impact data of plastics show considerable scatter. Additional specimens are advisable when results span a ductile–brittle transition.

What does a non-break (NB) result mean?

A non-break result means the specimen absorbed the impact without meeting the break criteria defined by the standard, which is typical of very tough materials tested with an undersized pendulum. The result can be reported as a corrected impact strength based on the energy consumed up to a specified deflection, or the test can be repeated with a higher-capacity pendulum so that the absorbed energy lies within the valid 10% to 80% range.

Can results obtained under different conditions be compared?

Comparison is only valid when the specimen geometry, notch type, support span, pendulum velocity, temperature, and conditioning history are identical. In particular, ISO 179-1 and ASTM D6110 values must not be merged, and results at low temperature or after aging should always be compared against an untested reference set tested in the same campaign.