The plastic unnotched impact test measures the energy required to fracture a plastic specimen that contains no machined notch, providing a direct assessment of the material's inherent impact toughness under rapid loading. Performed with a pendulum impact tester in accordance with standards such as ISO 179-1, ASTM D4812, and GB/T 1043.1, this test is widely used for material qualification, quality control, and product development across the automotive, packaging, pipe, and consumer goods industries. This guide explains the test principle, specimen requirements, testing procedures, result calculation, and typical applications of unnotched impact testing for plastics.
Technical Overview
The plastic unnotched impact test is a standardized pendulum-based method used to determine the impact resistance of plastic materials by measuring the energy absorbed when a freely swinging pendulum strikes and fractures a smooth, rectangular specimen. Unlike notched impact testing, the unnotched configuration has no stress-concentrating notch machined into the specimen, so the measured result reflects the bulk ductility and toughness of the polymer itself rather than its notch sensitivity.
During the test, a pendulum hammer of a calibrated energy capacity is released from a defined height, strikes the specimen at a controlled velocity, and breaks it. The difference between the striking energy and the residual energy of the pendulum, corrected for friction and windage losses, represents the energy absorbed by the specimen. This absorbed energy is divided by the original cross-sectional area of the specimen to obtain the unnotched impact strength, expressed in kJ/m² (or ft·lb/in² in some ASTM procedures).
Unnotched impact values are typically several times higher than notched values for the same material, because the fracture must initiate from a naturally flawless surface. The test is therefore particularly valuable for evaluating highly ductile materials such as polyethylene, polypropylene, and toughened compounds, as well as for comparing overall material toughness in quality control and material selection programs.
Testing Samples
Standard Specimen Geometry
The most common specimen for unnotched Charpy testing is the ISO 3167 multipurpose test specimen Type A, machined or molded to the preferred Type 1 impact bar with dimensions of 80 mm × 10 mm × 4 mm. Alternative dimensions may be used when agreed between interested parties, provided the actual specimen dimensions are reported with the results. For Izod-type unnotched testing under ASTM D4812, specimens of 63.5 mm × 12.7 mm × 3.2 mm (or other agreed thicknesses) are typically used.
Specimen Preparation and Orientation
Specimens may be directly injection molded or machined from compression-molded sheets, finished parts, or semi-finished products. Machining must be performed carefully to avoid introducing scratches, stress, or heat damage on the specimen surface, since surface quality strongly influences unnotched results. Testing orientations include edgewise (eU), flatwise (fU), and vertically supported (pU) configurations, and the orientation must be clearly recorded in the test report.
Conditioning and Quantity
Prior to testing, specimens shall be conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity for a minimum of 16 hours in accordance with ISO 291 or ASTM D618, unless otherwise specified. A minimum of five valid specimens per condition is recommended; ten or more specimens may be required when results show high scatter, which is common for ductile or fiber-reinforced materials.
Testing Items
Typical testing items covered under the plastic unnotched impact test include:
- Unnotched Charpy impact strength (ISO 179-1, GB/T 1043.1), reported in kJ/m²
- Unnotched cantilever beam (Izod) impact strength (ASTM D4812, GB/T 1843), reported in kJ/m² or ft·lb/in²
- Absorbed impact energy of individual specimens, reported in joules (J)
- Classification of failure behavior: complete break (C), hinge break (H), partial break (P), or non-break (N)
- Temperature-dependent impact strength, including sub-ambient testing at −20 °C, −30 °C, or −40 °C for cold-weather performance evaluation
- Comparative unnotched impact testing of different batches, grades, or formulations for quality control purposes
Testing Methods
Charpy Unnotched Method (ISO 179-1 / GB/T 1043.1)
In the Charpy configuration, the unnotched specimen is supported horizontally against two anvils separated by a defined span, and the pendulum strikes the specimen at the midpoint of the unsupported span with a striking edge velocity of approximately 2.9 m/s or 3.5 m/s. The absorbed energy is read from the pendulum scale or electronic display, corrected for friction and air resistance in accordance with ISO 13802, and divided by the cross-sectional area (width × thickness) of the specimen to calculate the unnotched Charpy impact strength in kJ/m².
Unnotched Izod Method (ASTM D4812 / GB/T 1843)
In the unnotched cantilever beam configuration, the specimen is clamped vertically as a cantilever beam, and the pendulum strikes the free end at a fixed distance above the clamp. The absorbed energy is divided by the width of the specimen to express the result in J/m or converted to kJ/m². This method is widely specified in North American material data sheets and automotive specifications.
Equipment and Calibration Requirements
Testing shall be performed on a calibrated pendulum impact machine with the following key requirements:
- The pendulum energy capacity shall be selected so that the absorbed energy falls between approximately 10 % and 80 % of the nominal capacity; otherwise, a different hammer must be used
- The machine shall be leveled, and friction and windage losses shall be verified and corrected at regular intervals
- Support spans, striker radii, and clamping forces shall conform to the geometry specified in the applicable standard
- For instrumented impact testers, force–deflection curves may be recorded to separate initiation and propagation energies
- Environmental chambers may be fitted for testing at elevated or sub-ambient temperatures, with sufficient dwell time for the specimen to reach thermal equilibrium
Result Evaluation
The mean, individual values, and standard deviation of the unnotched impact strength are reported together with the specimen designation (for example, 1eU), test temperature, hammer capacity, and the observed break type. For very tough materials, specimens may bend without fracturing; in such cases the result is reported as "no break (N)" together with the hammer energy used, which itself provides useful comparative information about ductility.
Applications
Unnotched impact testing is applied across a broad range of industries and material development activities:
- Automotive engineering: evaluation of bumpers, interior trim, dashboard components, wheel arch liners, and battery housing materials for impact durability
- Pipes and fittings: assessment of PVC, PE, and PP pipe compounds where unnotched toughness correlates with resistance to handling and installation damage
- Packaging: qualification of crates, pallets, containers, and caps that must withstand drops and transport shock
- Electrical and household appliances: verification of housing materials made from ABS, PC/ABS, and flame-retardant compounds
- Building products: impact evaluation of profiles, sheets, roofing panels, and glazing materials
- Polymer research and development: comparison of new formulations, impact modifiers, recycled content levels, and processing conditions
- Incoming inspection and quality control: batch release testing against material data sheet specifications at accredited third-party laboratories or in-house facilities
- Failure analysis: establishing whether a failed part was made from a material with adequate or degraded impact toughness
FAQ
What is the difference between notched and unnotched impact tests? A notched test uses a machined notch that acts as a stress concentrator and measures the material's resistance to brittle crack propagation, while an unnotched test measures the energy needed to initiate and propagate fracture in a smooth specimen, reflecting the overall toughness of the material. Unnotched values are usually much higher and are more sensitive to surface defects and processing quality.
Which standard should I choose for unnotched impact testing? ISO 179-1 and GB/T 1043.1 describe the unnotched Charpy method, which is common in Europe and Asia, whereas ASTM D4812 describes the unnotched cantilever beam (Izod) method common in North America. The choice depends on customer specifications, regional market requirements, and the existing material database being compared.
What does it mean if the specimen does not break? If a specimen bends or deforms without fracturing, the result is recorded as "no break," which indicates very high ductility at the test temperature. Testing at a lower temperature or with a higher-energy hammer can help define the ductile-to-brittle transition behavior of the material.
How does temperature affect unnotched impact strength? Plastics become markedly more brittle as temperature decreases. Specifying test temperatures such as −30 °C or −40 °C is essential for outdoor, automotive cold-crank, and cold-chain applications, and the complete temperature profile is often more informative than a single room-temperature value.
Why do unnotched results show high scatter? Because fracture initiation depends on random surface imperfections and internal defects, unnotched values naturally scatter more than notched values, especially in the ductile-to-brittle transition region. Testing a larger number of specimens and reporting statistical measures improves reliability.