Notched impact sensitivity testing measures how plastics behave under rapid impact loading in the presence of a stress-raising notch, using standardized Charpy and Izod pendulum methods such as ISO 179, ISO 180, ASTM D256, and ASTM D6110. By comparing notched and unnotched impact strength, this testing quantifies a material's notch sensitivity, a critical parameter for material selection, quality control, and failure prevention in automotive, electrical, and consumer product applications.
Technical Overview
Notched impact sensitivity testing is a fundamental mechanical evaluation method for plastic materials that quantifies the reduction in impact resistance caused by a deliberately machined stress concentrator, or notch. Many thermoplastics, including ABS, polycarbonate, polypropylene, nylon, POM, PVC, PBT, and PC/ABS blends, exhibit ductile behavior when tested unnotched but can fail in a brittle manner when a sharp notch is present. This occurs because the notch root creates a triaxial (multiaxial) stress state and elevates the local strain rate, suppressing shear yielding and promoting craze formation and rapid crack propagation.
The principal standardized techniques are the Charpy notched impact test, governed by ISO 179-1, ASTM D6110, and GB/T 1043.1, and the notched Izod test, governed by ISO 180, ASTM D256, and GB/T 1843, with related methods in the JIS K 7110 series. Test specimens are bars into which a V-shaped notch of controlled radius and depth is machined. ISO defines three notch types: Type A with a tip radius of 0.25 mm, Type B with a radius of 1.0 mm, and Type C with a sharp 0.1 mm radius, each cut to a nominal depth of 2.0 mm. A swinging pendulum strikes the specimen and fractures it, and the absorbed energy, corrected for friction and air resistance, is normalized by the cross-sectional area to yield impact strength in kJ/m² for ISO methods or in J/m for ASTM Izod methods.
Notch sensitivity itself is typically expressed as the ratio of unnotched impact strength to notched impact strength. A high ratio indicates a strongly notch-sensitive material whose toughness collapses in the presence of scratches, sharp corners, weld lines, or processing defects, which is essential information for safe part design and material qualification.
Testing Samples
Charpy Specimens (ISO 179 / ASTM D6110 / GB/T 1043.1)
ISO Charpy specimens are normally injection-molded or machined bars measuring 80 mm × 10 mm × 4 mm, tested on a support span of 62 mm with the notch centered and facing away from the striking edge. ASTM D6110 Charpy bars are commonly 127 mm × 12.7 mm × 6.4 mm or 3.2 mm thick, with a 45° V-notch machined to leave a specified remaining ligament depth.
Izod Specimens (ISO 180 / ASTM D256 / GB/T 1843)
Izod specimens are clamped as a vertical cantilever and struck on the notched face above the clamp. ISO Izod bars use the same 80 mm × 10 mm × 4 mm geometry as Charpy. ASTM D256 bars typically measure 63.5 mm × 12.7 mm with thicknesses from 3.2 mm to 12.7 mm, featuring a 45° notch with a 0.25 mm tip radius and a remaining depth of 10.16 mm.
Notch Preparation and Conditioning
Notch quality is decisive for valid results. Notches are cut with a single-tooth milling cutter at controlled feed rate and speed, and the notch radius is verified microscopically or by profilometry, because machining marks, burns, or an oversized radius can shift impact values significantly. Specimens are conditioned in a standard laboratory atmosphere of 23 °C and 50% relative humidity per ISO 291 (or ASTM D618) prior to testing, and at least five, preferably ten, specimens per condition are evaluated.
Testing Items
Typical test items evaluated within a notched impact sensitivity program include:
- Charpy notched impact strength (type A, B, or C notch) in kJ/m²
- Izod notched impact strength in J/m or ft·lb/in
- Unnotched Charpy or unnotched cantilever beam impact strength per ASTM D4812, used as the baseline for the sensitivity ratio
- Notch sensitivity index, calculated as unnotched strength divided by notched strength
- Comparative testing across notch radii (0.1 mm, 0.25 mm, 1.0 mm) to rank radius-dependent embrittlement
- Temperature-dependent impact testing, for example at −40 °C, −20 °C, 0 °C, and 23 °C, to locate the ductile–brittle transition
- Fracture classification per ISO 179-1: complete break (C), hinged break (H), partial break (P), or non-break (NB)
- Instrumented impact data per ISO 179-2, including force–deflection curves, peak force, crack initiation energy, and propagation energy
- Statistical reporting of mean value, standard deviation, and coefficient of variation
Testing Methods
Charpy Pendulum Method
In the Charpy configuration, the notched bar rests horizontally on two anvils separated by a 62 mm span (ISO), and the pendulum strikes the face opposite the notch at mid-span. The pendulum is released from a known height, and the difference between the initial and final swing heights, corrected for windage and bearing friction, equals the absorbed energy. Impact velocities of approximately 2.9 m/s or 3.8 m/s are used depending on pendulum capacity, and the hammer is selected so that the absorbed energy falls within the reliable range of roughly 10% to 80% of its capacity.
Izod Pendulum Method
In the Izod configuration, the specimen is clamped vertically as a cantilever with the notch aligned with the top of the clamp, and the pendulum, typically at 3.5 m/s, strikes the notched side above the grip. This clamped geometry introduces different bending and shear distributions than Charpy, which is one reason ISO and ASTM impact values are not directly interchangeable.
Notch Sensitivity Evaluation Protocol
A complete sensitivity study tests parallel batches of notched and unnotched specimens from the same molded lot under identical conditions. The notch sensitivity ratio is then computed, and, where required, specimens with different notch radii are compared to characterize how sharply the material degrades as stress concentration increases. Ductile engineering resins may show ratios of 10 or higher, while intrinsically brittle or highly filled materials show ratios closer to 1. Sub-ambient testing further reveals whether notch tip plasticity is suppressed at service temperatures.
Instrumented and Data Analysis Methods
Instrumented pendulum systems per ISO 179-2 record force–time and force–deflection histories, separating the energy consumed in crack initiation from that consumed in propagation. This distinction is valuable for impact-modified compounds and for computing ductility indices. Results are always reported with the notch type, specimen orientation, temperature, break classification, and statistical spread, and accredited third-party laboratories follow strict calibration and verification procedures for pendulum energy scales.
Applications
Notched impact sensitivity testing supports a wide range of engineering and commercial objectives:
- Material screening and ranking of resin grades, impact modifiers, fillers, and glass- or mineral-reinforced compounds
- Automotive qualification of interior trims, bumpers, body panels, and under-hood components exposed to low-temperature impact and stone chipping
- Electrical and electronic enclosures, connectors, and switch housings where UL 746A mechanical performance and long-term property retention programs reference impact behavior
- Pipes, fittings, and valves for water and gas distribution where notch-like defects from handling can initiate brittle fracture
- Appliance and power tool housings, packaging, sports equipment, and consumer goods requiring drop and abuse resistance
- Quality control of incoming resins, verification of recyclate content effects, and batch-to-batch consistency monitoring
- Failure analysis, comparing impact performance of fractured field parts against reference material
- Support of regulatory submissions and customer specifications requiring ISO 179, ISO 180, ASTM D256, ASTM D6110, or GB/T test evidence
FAQ
What is notch sensitivity in plastics?
Notch sensitivity describes how sharply a plastic's impact toughness decreases when a notch or sharp geometrical feature concentrates stress. It is commonly quantified as the ratio of unnotched to notched impact strength; a higher ratio signals a material that is prone to brittle fracture from scratches, sharp corners, or weld lines.
Why are specimens notched before impact testing?
The notch guarantees a controlled crack initiation site, concentrates stress into a small ligament, and raises the local strain rate, producing reproducible and discriminating measurements. Without a notch, many ductile plastics simply bend and do not break, giving little comparative information.
Why do ISO and ASTM impact results differ for the same material?
Specimen dimensions, notch geometry, support span, clamping method, and result units all differ between ISO 179/ISO 180 and ASTM D6110/ASTM D256. ISO reports impact strength in kJ/m², while ASTM reports absorbed energy per unit width in J/m, so values are not numerically convertible and must be compared only within the same standard system.
Which notch radius should be selected?
The Type A notch with a 0.25 mm radius is the default for ISO classification. The Type B notch with a 1.0 mm radius is milder and useful for very tough or rubber-modified grades, while the Type C notch with a 0.1 mm radius is sharper and used to discriminate notch sensitivity in materials that appear insensitive with Type A notches.
What does a "non-break" (NB) result mean?
An NB classification means the specimen did not separate or hinge after being struck, typically indicating impact energy exceeding the material's toughness under those conditions. The absorbed energy is still reported, but specimen geometry, notch type, and temperature should be adjusted, or an instrumented method applied, to obtain a fully valid comparison.
Can notched impact data be used directly for design?
Notched impact values are comparative screening data, not design allowables. They rank materials and lot quality and reveal embrittlement trends, but actual component performance must be validated with application-specific impact, drop, and crash simulation testing.
