Plastic Notch Sensitivity Impact Testing: Notched Charpy and Izod Evaluation of Polymers

Plastic notch sensitivity impact testing quantifies how sharply the impact resistance of a polymer drops when a stress-concentrating notch is machined into the specimen, using standardized notched Charpy and notched Izod pendulum methods. Guided by ISO 179-1, ISO 180, ASTM D256, ASTM D6110 and the corresponding GB/T standards, this test is essential for material selection, quality control and failure analysis because real plastic components almost always contain sharp geometric features such as ribs, screw bosses, snap-fits and weld lines that behave like notches under impact loading.

Technical Overview

Notch sensitivity describes the degree to which a plastic material loses its impact toughness when a sharp stress raiser is introduced. Under impact loading, the root of a notch creates a triaxial tensile stress state and constrains plastic deformation, forcing materials that would otherwise fail in a ductile manner to fracture in a brittle, low-energy mode. Ductile polymers such as ABS, polycarbonate and nylon can absorb a large amount of energy in unnotched form yet show dramatically reduced notched impact strength, whereas intrinsically brittle materials such as unfilled polystyrene or PMMA show comparatively little additional loss.

Notch sensitivity impact testing therefore measures notched impact strength and compares it with unnotched performance, often across a series of notch radii, to build a complete picture of how a material will tolerate scratches, sharp corners, weld lines and similar defects in service. The principal standardized methods are notched Charpy impact (ISO 179-1, ASTM D6110, GB/T 1043.1, JIS K 7110) and notched Izod impact (ISO 180, ASTM D256, GB/T 1843, JIS K 7111). Unnotched and reverse-notched reference values are obtained under ISO 179-1/ASTM D4812 and ASTM D256 reverse-notch procedures. Results are expressed in kJ/m² for ISO Charpy/Izod or in J/m (ft-lbf/in) for the ASTM cantilever beam method, and the two systems are not directly interchangeable.

Testing Samples

Injection-Molded Standard Bars

The reference specimen for ISO methods is a milled-notched bar of 80 mm × 10 mm × 4 mm, machined with a single-edge V-notch of 2.0 mm depth. The reference notch radius is 0.25 mm (Type A), with 0.10 mm (Type C) and 1.00 mm (Type B) alternatives used for notch-sensitivity studies. ASTM specimens are typically 63.5 mm × 12.7 mm × 3.2 mm (or 6.4 mm) with a 0.25 mm radius notch cut to leave a nominal 10.2 mm residual width.

Samples Machined from Plates, Pipes and Finished Parts

When molded bars are unavailable, specimens can be cut and notched from compression-molded sheets, extruded profiles, pipe walls, fittings or actual components, provided orientation, thickness and notch geometry are documented. This route is widely used for evaluating PE and PVC pressure-pipe compounds, glass-filled engineering grades and parts suspected of in-service embrittlement.

Conditioned and Environmental-Exposure Samples

Hygroscopic materials such as PA6 and PA66 are tested in both the dry-as-molded and moisture-equilibrated states, since absorbed water plasticizes nylon and can raise impact strength severalfold. Specimens may also be conditioned or tested at low and elevated temperatures to determine the ductile-to-brittle transition behavior relevant to cold-climate service.

Testing Items

Typical test items assessed in a notch sensitivity impact program include:

• Notched Charpy impact strength (ISO 179-1, ASTM D6110, GB/T 1043.1)
• Notched Izod impact strength (ISO 180, ASTM D256, GB/T 1843)
• Unnotched or reverse-notched impact strength as the ductility baseline
• Notch sensitivity index, calculated by comparing unnotched and notched values at one or more notch radii
• Radius-dependence studies at 0.10, 0.25, 0.50 and 1.00 mm radii to map sensitivity to defect sharpness
• Ductile-to-brittle transition temperature from temperature-sweep testing
• Instrumented impact parameters: peak force, energy to crack initiation, propagation energy and total fracture energy
• Fracture mode classification (ductile tearing, brittle cleavage, mixed or hinged break)

Testing Methods

Specimens are machined and notched under controlled conditions, because notch quality dominates the result. The notch is cut with a shaped milling cutter or broach using fresh, sharp tooling at controlled feed rates to avoid heating, rubbing or residual compressive stresses at the notch root. Notch radius and depth are verified by optical projection or profile measurement, since a dull cutter can raise apparent impact strength by 20% or more.

Testing is performed on a calibrated pendulum impact machine. For Charpy, the bar is laid horizontally on supports and struck edgewise (or flatwise) opposite the notch; for Izod, the bar is clamped as a vertical cantilever and struck on the notched face above the grip. The hammer capacity is selected so the absorbed energy falls within the valid working range of the machine, and corrections for friction and windage losses are applied. Specimens are conditioned to equilibrium at 23 °C and 50% relative humidity in accordance with ISO 291 or ASTM D618 before fracture.

Notch sensitivity itself is evaluated by testing a matrix of specimens: unnotched bars and bars notched at two to four different radii are broken at the same temperature, and the impact strength is plotted against notch root radius. A steep decline with decreasing radius indicates high notch sensitivity and warns that scratches and sharp corners will severely degrade toughness. For deeper insight, instrumented pendulums (ISO 179-2) record the force–deflection history, separating crack-initiation energy from propagation energy and clearly distinguishing ductile from brittle fracture signatures. All work is typically carried out in accredited third-party laboratories or in-house laboratories with calibrated equipment and documented measurement traceability.

Applications

Notch sensitivity impact data directly influence design and material decisions across every sector that uses molded or extruded plastics:

• Automotive: bumper fascias, interior trim, clips and brackets that must survive low-temperature impacts and stone strikes
• Pipes and fittings: PE, PP and PVC pressure systems where scratches and gouges act as service notches
• Electronics and appliances: housings, bezels and connectors subject to drop impact and snap-fit assembly stresses
• Packaging: containers, closures and crates evaluated for crack resistance at corners and gates
• Medical devices: components requiring validated toughness after sterilization and aging
• Material qualification: datasheet comparison of resin grades, recycled-content effects and glass-fiber loading
• Quality control: batch-to-batch release testing and incoming resin verification
• Failure analysis: confirming whether field fractures originated from notch-root stress concentration or material degradation

FAQ

What is notch sensitivity in plastics?

Notch sensitivity is the extent to which a plastic's impact strength decreases in the presence of a sharp notch or defect. It is commonly expressed as the ratio or percentage difference between unnotched and notched impact strength; the larger the difference, the more notch-sensitive the material.

Why is notched impact strength so much lower than unnotched strength?

The notch concentrates stress at its root and suppresses the ability of surrounding material to yield, promoting brittle crack initiation. Materials that rely on shear yielding—such as PC, nylon and many impact-modified grades—are affected most strongly, while craze-forming brittle plastics show a smaller relative drop.

Can Izod and Charpy results be compared directly?

No. The two methods use different specimen geometries, clamping arrangements and stress states, so values differ systematically and cannot be converted. Within each method, ISO and ASTM results also differ because of specimen size and notch depth differences; comparisons must always reference the same standard.

Which notch radius should be specified?

The standard reference radius is 0.25 mm. Sharper radii (down to 0.10 mm) probe worst-case sensitivity to fine scratches, while blunter radii (0.50 or 1.00 mm) better represent generous design fillets. A multi-radius study gives the most complete assessment.

How does temperature affect the results?

Raising temperature generally increases impact strength and promotes ductile fracture; lowering it drives materials toward brittle failure. Many notch-sensitive plastics pass room-temperature tests yet fail brittlely a few tens of degrees colder, so cold-temperature testing is essential for outdoor and automotive applications.

What does a low notched impact value mean for a real part?

It signals that the part is vulnerable to sudden, low-energy brittle fracture originating at sharp internal features, scratches or weld lines. Designers should respond by increasing fillet radii, relocating gates away from high-stress areas, selecting a less notch-sensitive grade, or adding impact modification.