Charpy (Simply Supported Beam) Impact Testing of PE Pipes: Principles, Standards, and Procedures

Charpy impact testing, known as the simply supported beam impact test, is the principal method for evaluating the impact toughness, notch sensitivity, and low-temperature brittleness resistance of PE (polyethylene) pipes. By striking a specimen resting on two supports with a calibrated pendulum, the test quantifies the energy absorbed during fracture and confirms whether a pipe grade will behave in a ductile manner under sudden impact loading. This guide covers the testing principles, specimen preparation, procedures, calculation methods, and practical applications of simply supported beam impact testing for PE80 and PE100 piping systems.

Technical Overview

The simply supported beam impact test, internationally known as the Charpy impact test, is a standardized method for measuring the impact resistance of plastic materials. A rectangular specimen is laid horizontally across two support anvils, forming a simply supported beam, and a swinging pendulum hammer strikes the specimen at mid-span. The energy absorbed by the specimen during fracture is derived from the difference between the initial and residual heights of the pendulum and is expressed as impact strength in kJ/m².

For polyethylene (PE) pipes used in water supply, gas distribution, mining, and drainage systems, impact toughness is a critical performance property. PE materials exhibit a ductile-to-brittle transition at low temperatures and are inherently notch sensitive, so a pipe that performs well at room temperature may become brittle during cold-climate installation or when damaged by scratching or gouging. The Charpy test therefore serves as a fast, low-cost screening method for verifying material quality, detecting embrittlement, and comparing the low-temperature toughness of different PE compounds.

The test is performed according to recognized standards, including ISO 179-1 (determination of Charpy impact properties of plastics), ISO 9854-1 (thermoplastics pipes, pendulum impact strength by the Charpy method), ASTM D6110 (Charpy impact resistance of notched specimens of plastics), and the Chinese national standard GB/T 18743, which specifies the simply supported beam method for thermoplastic pipes for fluid transport. Pipe product specifications and purchaser contracts define the applicable standard, notch geometry, and test temperature.

Testing Samples

Specimens Cut Directly from the Pipe Wall

For finished pipe evaluation, specimens are machined from the pipe wall with their length aligned parallel to the pipe axis. The arc-shaped pieces retain a portion of the original outer surface, and the pendulum normally strikes the outer surface so that the test reflects the stress state experienced in service. If curvature would destabilize the specimen on the supports, the inner surface may be machined flat, provided the remaining wall thickness is documented and the specimen still meets the minimum dimensional requirements of the governing standard.

Standard Machined Specimens

For material qualification and compound comparison, specimens are compression-molded or injection-molded from PE resin and machined to ISO 179 dimensions:

• Type 1: 80 mm × 10 mm × 4 mm, the preferred specimen for pipe-grade PE.
• Type 2: 60 mm × 10 mm × 3 mm, used when material availability is limited.
• Type 3: 120 mm × 15 mm × 10 mm, used for materials showing very high impact energy absorption.

Notch Preparation

Because unnotched PE rarely breaks in a brittle manner, notched specimens are used to assess notch sensitivity. The notch concentrates stress and accelerates the ductile-to-brittle transition. Standard notch geometries per ISO 179 include:

• Notch A: 0.25 mm notch-root radius, the default for pipe-grade PE.
• Notch B: 1.00 mm notch-root radius, a milder, less severe notch.
• Notch C: 0.10 mm notch-root radius, a very sharp notch for research purposes.

Notches are machined with a profiled cutter or dedicated notch-cutting apparatus. The notch must be smooth, symmetrical, and free of machining marks, and the remaining ligament thickness must be measured on each specimen.

Conditioning

Specimens are conditioned in a standard atmosphere of 23 °C ± 2 °C and 50% ± 10% relative humidity (ISO 291) before testing. For low-temperature evaluation, specimens are conditioned in a temperature-controlled chamber at the test temperature (for example 0 °C, −20 °C, or −30 °C) long enough to reach thermal equilibrium, and each specimen is tested within a few seconds of removal so that it does not warm up before impact.

Testing Items

A typical PE pipe Charpy impact test program determines the following results:

• Notched Charpy impact strength (acN, kJ/m²): the energy absorbed per unit cross-sectional area of a notched specimen, the primary acceptance parameter.
• Unnotched Charpy impact strength (acU, kJ/m²): measured when the specification requires it; tough PE grades often show "no break".
• Break classification: complete break (C), hinge break (H), partial break (P), or non-break (NB), recorded for every specimen.
• Low-temperature impact performance: impact strength at 0 °C, −20 °C, or lower to verify cold-climate suitability.
• Temperature dependence: a series of tests across a temperature range to map the ductile-to-brittle transition region.
• Notch sensitivity: comparison of results obtained with different notch radii or with unnotched specimens.
• Batch-to-batch consistency: statistical comparison of mean values and standard deviations for quality control charting.

Testing Methods

Apparatus and Calibration

The test is performed on a pendulum impact machine equipped with interchangeable hammers with nominal energies of 1 J, 2.75 J, 5.5 J, 7.5 J, 15 J, 25 J, or 50 J, an impact velocity of approximately 2.9 m/s or 3.8 m/s, and a fixed support span of 62 mm for Type 1 and Type 2 specimens. Before testing, the machine is verified for frame rigidity, center-of-strike alignment, pendulum energy, and friction and windage losses. The hammer energy must be selected so that the corrected absorbed energy remains within the valid working range (roughly 10% to 80% of the nominal pendulum energy); otherwise a different hammer must be used.

Test Procedure

• Measure the width and thickness (or remaining ligament thickness at the notch) of each specimen to the required precision, typically 0.02 mm.
• Set and verify the support span, and confirm that the pendulum strikes the specimen within the tolerance zone at mid-span.
• Perform an empty-swing (blank) test to determine friction and air-resistance corrections.
• Place the specimen on the anvils with the notch facing away from the pendulum, so the hammer strikes the face opposite the notch and the ligament is loaded in bending.
• Release the pendulum from its locked position and allow it to swing freely through the specimen.
• Record the absorbed energy from the scale or transducer, apply the friction correction, and note the break type.
• Test at least ten specimens per condition, or more when the material shows mixed ductile and brittle behavior, and inspect the fracture surfaces for evidence of brittle fracture, contamination, or poor consolidation.

Calculation of Results

The Charpy impact strength is calculated by dividing the corrected fracture energy by the cross-sectional area of the specimen at the notch: acN = Ec × 10³ / (b × hN), where Ec is the corrected absorbed energy in joules, b is the specimen width in millimeters, and hN is the remaining thickness under the notch in millimeters. The result is reported in kJ/m² together with the specimen type, notch designation (for example 1eA for a Type 1 specimen tested edgewise with an A-type notch), test temperature, number of specimens, individual and mean values, standard deviation, and the break classification of every specimen.

Interpretation for PE Materials

Pipe-grade PE100 and PE80 materials are intentionally tough, and their notched Charpy impact strength typically falls in the range of several to a few tens of kJ/m², decreasing as the test temperature drops and as the notch becomes sharper. Unnotched specimens frequently absorb nearly the full pendulum energy and are reported as "no break", which is itself a meaningful indicator of ductility. A batch showing a shift toward lower impact strength, complete brittle breaks, or increased scatter compared with established reference data indicates possible contamination, excessive use of regrind or fillers, inadequate stabilization, or degradation during processing, and should trigger further investigation such as OIT, melt flow rate, or density testing.

Applications

• Incoming inspection of PE resin and compound deliveries at pipe extrusion plants.
• In-process and outgoing quality control of PE80 and PE100 water supply, gas distribution, sewage, mining slurry, and subsea pipes.
• Qualification of new pipe grades, bimodal resins, and crack-resistant materials, complemented by rapid crack propagation studies on larger pipes.
• Verification of low-temperature performance for pipes installed in cold regions or installed by trenchless methods that impose scratches and gouges.
• Evaluation of recycled content, compounding formulations, and long-term aging or UV exposure effects on toughness.
• Failure analysis of field fractures, where Charpy testing of material taken adjacent to the failure helps distinguish material embrittlement from installation or overload causes.
• Third-party certification and compliance testing for regulatory acceptance of piping systems.

FAQ

Why is the test called a simply supported beam impact test?

The specimen rests freely on two anvils without clamping, exactly like a beam on simple supports, and the pendulum applies a sudden bending load at mid-span. This support configuration is the defining feature that distinguishes the Charpy configuration from the Izod test, in which the specimen is clamped as a cantilever.

Which hammer energy should be selected for PE pipe specimens?

Select the lightest hammer that can break the specimen while keeping the absorbed energy within the valid range of roughly 10% to 80% of the nominal energy. For notched pipe-grade PE, 7.5 J or 15 J hammers are common; if the specimen does not break and absorbs nearly the full energy, the result is reported as "no break" rather than a numeric value.

Why do PE specimens often show "no break"?

Polyethylene is a tough, ductile thermoplastic that deforms and draws rather than fracturing catastrophically, particularly without a stress-concentrating notch. Reporting "no break" is technically meaningful because it demonstrates that the material remained ductile under the test conditions; adding a sharp notch or lowering the test temperature converts the behavior to brittle fracture, which is exactly why notched, low-temperature testing is specified for pipe applications.

What test temperature is specified for PE pipes?

Room-temperature testing at 23 °C provides a reference value, but many specifications require testing at 0 °C, −20 °C, or −30 °C to verify low-temperature performance. The exact temperature, notch type, and minimum impact strength are defined by the applicable product standard or the purchaser's technical specification.

What are typical acceptance values for pipe-grade PE?

Acceptance limits are contract- or standard-specific, because values depend strongly on temperature, notch radius, and specimen orientation. As a practical indication, notched Charpy impact strength for modern PE100 pipe compounds commonly lies between about 5 and 30 kJ/m² depending on grade and test temperature, and any significant downward shift from the manufacturer's reference data should be investigated.

How does the Charpy test relate to real pipe performance?

The Charpy test is a rapid screening method for material toughness, not a direct simulation of service loading. For large-diameter gas and water pipes, it is complemented by rapid crack propagation tests (such as the S4 test) and slow crack growth tests (such as the notched pipe test or FNCT), which together characterize the complete fracture behavior of the piping system.