Plastic Unnotched Impact Strength Analysis: Test Methods, Standards, and Applications

Unnotched impact strength analysis is one of the most widely used mechanical tests for evaluating the toughness of plastic materials, measuring the energy a specimen absorbs when fractured by a pendulum striker without a pre-machined notch. This article explains the test principle, specimen requirements, governing standards such as ISO 179-1, ISO 180, ASTM D256, and GB/T 1043.1, as well as typical calculation methods and industrial applications. It also clarifies the practical differences between unnotched and notched impact testing to help engineers and quality managers select the right approach for material qualification.

Technical Overview

Unnotched impact strength quantifies the ability of a plastic material to resist fracture under a high-speed bending load. A rectangular specimen without any stress-concentrating notch is struck by a pendulum hammer, and the energy absorbed during fracture is divided by the original cross-sectional area of the specimen. In the ISO and GB standard systems the result is expressed in kJ/m², while ASTM-based Izod results are expressed in J/m or ft·lb/in.

Because no notch is present to initiate a crack, the measured energy includes both the crack initiation stage and the crack propagation stage. This makes the unnotched test a measure of the overall toughness of the material as processed, whereas notched testing deliberately suppresses initiation energy and emphasizes resistance to unstable crack growth from a sharp stress raiser. As a result, unnotched values are typically several times higher than notched values for the same material.

ISO 179-1 classifies post-test failure behavior into four categories: complete break (C), hinge break (H), partial break (P), and non-break (NB). Very tough or highly ductile materials frequently fail to break completely under the available pendulum energy, in which case the result is reported as "no break" and a higher-energy pendulum, a lower test temperature, or an alternative method such as instrumented impact or drop-weight testing may be required.

Unnotched impact response is strongly influenced by temperature, loading rate, molecular orientation, crystallinity, moisture content, specimen thickness, and injection-molding conditions such as gate location. Careful control of these variables is essential for obtaining reproducible and comparable data.

Testing Samples

Standard Specimen Geometry

For Charpy testing to ISO 179-1 and GB/T 1043.1, the preferred unnotched specimen is the 80 mm × 10 mm × 4 mm bar, usually the type 1B multipurpose test piece defined in ISO 3167, tested flatwise on a 62 mm support span. For Izod testing to ISO 180 and GB/T 1843, the same 80 mm × 10 mm × 4 mm specimen is clamped vertically as a cantilever beam. ASTM D256-based testing uses a 63.5 mm × 12.7 mm bar with a preferred thickness of 3.2 mm or 6.4 mm, and unnotched Izod data are commonly generated using the reverse-notch technique.

Specimen Preparation

Specimens are preferably injection molded directly to final dimensions in accordance with ISO 294 or the equivalent, since molding conditions strongly affect orientation and therefore impact results. Alternatively, specimens may be machined from compression-molded sheets, extruded profiles, or finished parts, in which case the machining direction relative to the flow direction must be recorded and all edges must be smoothed to remove machining marks that could act as artificial stress raisers. For injection-molded bars, the position relative to the gate should be documented because of skin-core and orientation effects.

Conditioning

Specimens are conditioned to ISO 291 (23 °C ± 2 °C, 50 % ± 5 % relative humidity, typically at least 16 hours) or to ASTM D618 Procedure A before testing. Moisture-sensitive materials such as polyamides must be tested in a defined moisture state, either dry-as-molded or equilibrated at 50 % relative humidity, since absorbed water can act as a plasticizer and substantially raise impact strength.

Sample Quantity

A minimum of five valid results is normally required, and ten specimens are commonly tested for statistically robust data. Individual values, the arithmetic mean, and the observed failure category should all be reported.

Testing Items

Typical test items covered under an unnotched impact strength analysis program include:

• Unnotched Charpy impact strength in kJ/m² to ISO 179-1 or GB/T 1043.1
• Unnotched Izod impact strength in kJ/m² to ISO 180 or GB/T 1843, and in J/m or ft·lb/in to ASTM D256-based procedures
• Absorbed impact energy in joules and verification of pendulum capacity utilization
• Failure behavior classification (complete, hinge, partial, or non-break)
• Temperature-dependent impact testing, for example at −40 °C, −20 °C, 0 °C, and 23 °C, to identify the ductile–brittle transition region
• Instrumented impact analysis providing force–deflection curves, peak force, and separation of initiation and propagation energies where required
• Comparative evaluation of material grades, toughener content, filler or reinforcement loading, recycled content, and processing conditions

Testing Methods

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

The specimen is supported horizontally on two anvils spaced 62 mm apart and struck at mid-span by the striking edge of a swinging pendulum. Standard pendulum energies of 1 J, 2.75 J, 5.5 J, 11 J, and 22 J are available, and the absorbed energy should fall between 10 % and 80 % of the nominal pendulum energy for a valid test. The unnotched impact strength is calculated as the friction-corrected absorbed energy divided by the cross-sectional area of the specimen.

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

The specimen is clamped as a vertical cantilever and struck near its free end at a defined distance above the clamp. ISO-type and ASTM-type clamping geometries and striker radii differ, and the two systems use different specimen dimensions and result units, so ISO and ASTM unnotched Izod values are not directly convertible. For tough materials that do not break, ASTM D256 offers the reverse-notch configuration in which a notched specimen is struck on the face opposite the notch, effectively removing the stress raiser from the loaded edge.

Calculation and Reporting

The unnotched impact strength is computed as the corrected absorbed energy divided by the product of specimen width and thickness, reported in kJ/m² for ISO and GB methods. A complete test report should identify the standard used, specimen type and dimensions, preparation and conditioning history, pendulum energy, test atmosphere, individual and mean results, and the failure category observed for each specimen.

Equipment Verification and Quality Control

Pendulum impact machines should be verified in accordance with ISO 13802 or equivalent national procedures, covering potential energy, velocity, friction losses, and anvil and striker geometry. A centering jig should be used to guarantee repeatable specimen placement, and when a specimen does not break, the test should be repeated with a higher-energy pendulum or the result reported as non-break rather than an artificially low value.

Applications

Unnotched impact strength data are used across the full plastics value chain, from resin development to end-product qualification. Typical application scenarios include:

• Automotive engineering: validation of interior trim, bumpers, wheel-arch components, and battery housings, including sub-zero impact performance for cold-climate service
• Pipes and fittings: assessment of PVC, polyethylene, and polypropylene systems against transportation, installation, and accidental impact loads
• Packaging: durability evaluation of crates, containers, pallets, closures, and thin-wall injection-molded parts
• Electrical and household appliances: qualification of housings and structural components for drop and impact resistance
• Construction: impact performance of profiles, panels, glazing, and roofing elements
• Material research and development: quantifying the effect of impact modifiers, mineral fillers, glass fibers, and recycled content on toughness
• Quality control: incoming resin inspection, batch release, supplier comparison, and failure analysis of field returns in accredited third-party laboratories or in-house facilities

FAQ

What is the difference between notched and unnotched impact strength?

Unnotched testing measures the total energy needed to initiate and propagate a crack in a smooth specimen, reflecting the overall toughness of the as-molded material. Notched testing introduces a sharp machined notch that acts as a crack initiator, so the result mainly reflects resistance to brittle crack propagation and material notch sensitivity. Unnotched values are therefore substantially higher and the two results answer different engineering questions.

Why does my specimen not break during the test?

Highly ductile materials such as certain polyethylenes, toughened polypropylenes, and ABS grades may bend or hinge without fracturing. In that case, retest with a higher-energy pendulum, reduce the specimen thickness, lower the test temperature, or use instrumented pendulum or drop-weight testing, and report the result as non-break in accordance with the standard.

Should I choose the ISO or the ASTM method?

The choice is driven by the specification, customer requirement, or target market. ISO and GB methods use the 80 mm × 10 mm × 4 mm specimen and report kJ/m², while ASTM-based procedures use different specimen geometry and report J/m or ft·lb/in. Because geometry, striker radius, and clamping differ, results from the two systems cannot be converted by a simple factor and should not be compared directly.

How does temperature affect unnotched impact strength?

Most plastics become significantly more brittle as temperature decreases. Testing at the intended service temperature, for example −30 °C or −40 °C for automotive exterior parts, is essential because room-temperature data can overstate real-world winter performance.

How many specimens are needed for a reliable result?

A minimum of five valid specimens per condition is normally specified, and ten are often tested to reduce scatter. Impact data can show wide distribution, particularly near a ductile–brittle transition, so individual values should always be reported alongside the mean.