Plastic Tensile Notch Sensitivity Testing: Principles, Standards, and Evaluation Methods

Plastic tensile notch sensitivity testing measures how sharply a plastic material loses its load-bearing capacity and energy absorption when stress-concentrating notches are introduced into a tensile specimen. Conducted in accordance with international standards such as ISO 8256, ASTM D1822, ISO 527, and ASTM D638, the test compares notched and unnotched specimens to quantify the material's notch sensitivity factor. This article explains the technical principle, sample requirements, test items, methodology, and typical industrial applications of tensile notch sensitivity evaluation for plastics.

Technical Overview

Notch sensitivity is a fundamental mechanical property that describes the degree to which a plastic material's strength and toughness are degraded by the presence of a notch, sharp corner, hole, or other stress concentrator. In a smooth uniaxial tensile specimen, stress is distributed uniformly across the cross-section. When a notch is machined or molded into the specimen, the local stress at the notch root is amplified by the theoretical stress concentration factor, and a triaxial (plane-strain) stress state develops beneath the notch. This triaxial constraint suppresses plastic deformation and promotes brittle, low-energy fracture, which is the essential mechanism behind notch-induced embrittlement.

Ductile plastics, such as many polyolefins and toughened amorphous resins, blunt the notch through local yielding and retain a large fraction of their unnotched strength; they are classified as notch-insensitive. Highly brittle or glass-fiber-reinforced plastics, by contrast, may fracture at loads well below the net-section strength predicted from the unnotched tensile strength, indicating strong notch sensitivity. Quantifying this behavior is critical because real plastic components invariably contain ribs, snap-fits, gate marks, weld lines, and sharp corners that act as built-in notches.

Tensile notch sensitivity evaluation is typically performed by two complementary approaches:

• High-speed tensile-impact testing on notched and unnotched specimens per ISO 8256 and ASTM D1822, which measures the energy absorbed per unit cross-sectional area at impact velocities.
• Quasi-static notched tension testing per ISO 527-1/527-2 and ASTM D638, in which notched specimens are pulled to failure at controlled crosshead speeds and compared with smooth specimens.

Testing Samples

ISO 8256 Tensile-Impact Specimens

ISO 8256 specifies Type 1 and Type 2 dumbbell-shaped specimens designed for pendulum tensile-impact testing. Type 1 specimens are preferred for rigid materials, while Type 2 specimens with a longer narrow section are used for materials requiring more deformation room. Notched variants contain a machined V-notch or U-notch at the center of the narrow section.

ASTM D1822 Specimens

ASTM D1822 defines Type S (short) and Type L (long) specimens for tensile-impact energy measurement. Notched configurations are produced by machining a circumferential or double-sided notch in the gauge section, allowing direct comparison with unnotched controls.

Quasi-Static Notched Tension Specimens

For slow-rate evaluation, multipurpose specimens per ISO 527-2 (1A or 1B) or ASTM D638 Type I are used. Single-edge-notched or double-edge-notched geometries are machined into the gauge section, with the notch depth, angle, and root radius tightly controlled.

Notch Geometry and Preparation

Common notch geometries include the 45-degree V-notch with a root radius of 0.1 mm, 0.25 mm, or 1.0 mm, and the U-notch with a larger radius. Notches may be machined with a dedicated notching cutter after molding, or molded-in when process-related notches are being simulated. Machined surfaces must be free of tool marks, thermal degradation, and residual chips, as artificial defects can distort results. Both single-notched and double-notched (symmetrical) configurations are accepted.

Conditioning and Quantity

Specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity per ISO 291 or ASTM D618 for a minimum of 40 hours, unless a specific service condition applies. A minimum of five valid specimens per condition is recommended for each notched and unnotched group; ten or more are advised for materials with high scatter.

Testing Items

The core measurable outputs of the tensile notch sensitivity test include:

• Tensile-impact strength of unnotched specimens, in kJ/m², per ISO 8256 or ASTM D1822.
• Tensile-impact strength of notched specimens, calculated on the remaining cross-section at the notch.
• Notch sensitivity factor or notch sensitivity ratio, defined as the ratio of notched to unnotched tensile-impact strength; lower ratios indicate stronger notch sensitivity.
• Net-section notched tensile strength versus unnotched tensile strength from quasi-static tests per ISO 527 and ASTM D638.
• Energy to break, peak force, and displacement at break for both geometries.
• Effect of notch root radius, notch depth, temperature, and strain rate on the measured properties.
• Fracture surface classification: ductile tearing, mixed-mode, or brittle flat fracture.

Testing Methods

Specimen Preparation

Test pieces are injection molded or machined from plaques, pipes, or finished parts. Notches are cut with calibrated cutters, and the notch dimensions (depth, angle, root radius) are verified under an optical measuring projector or microscope. Only specimens within tolerance are accepted, and damaged or degraded specimens are rejected.

Tensile-Impact Procedure (ISO 8256 / ASTM D1822)

The specimen is clamped between the pendulum crosshead and the stationary grip of a tensile-impact tester. The pendulum is released from a defined height, and the specimen is fractured in tension at high velocity. The energy absorbed is corrected for friction and windage, then divided by the original minimum cross-sectional area to obtain the tensile-impact strength. Notched and unnotched groups are tested under identical conditions so that the notch sensitivity ratio is directly comparable.

Quasi-Static Notched Tension Procedure (ISO 527 / ASTM D638)

Notched and unnotched specimens are mounted in a universal testing machine with an extensometer or video strain measurement on the notch region. Testing is conducted at standard speeds, commonly 1 to 50 mm/min, and optionally at low temperatures using an environmental chamber to assess cold brittleness. Load-displacement curves are recorded to capture peak load, energy to break, and post-yield behavior.

Data Reduction

The tensile-impact strength is calculated as the corrected absorbed energy divided by the minimum original cross-sectional area. The notch sensitivity factor is derived by dividing the notched result by the unnotched result. In quasi-static testing, the net-section stress at fracture of the notched specimen is compared with the unnotched tensile strength: if the net-section strength approaches the unnotched strength, the material blunts the notch and is notch-insensitive; if it falls significantly below, the material is notch-sensitive.

Test Report

A complete report includes the material identification, specimen type and preparation method, notch geometry with root radius and depth, conditioning details, test temperature and speed, individual and mean results with standard deviation, the calculated notch sensitivity factor, and a description of the fracture surfaces.

Applications

Tensile notch sensitivity data are used across the plastics value chain to ensure that materials selected for notched or stress-concentrated geometries will not fail in a catastrophic brittle manner. Typical applications include:

• Material selection and ranking for snap-fits, living hinges, clips, ribs, and screw bosses where sharp internal corners are unavoidable.
• Qualification of glass-fiber, mineral-filled, and flame-retardant compounds, where fillers frequently raise stiffness but intensify notch sensitivity.
• Assessment of embrittlement caused by thermal aging, UV exposure, chemical contact, or moisture conditioning.
• Validation of processing quality, including the effect of gate locations, weld lines, and molded-in stresses on local notch performance.
• Automotive interior and exterior components, electrical enclosures and connectors, appliance housings, pipe fittings, packaging structures, and medical devices subject to impact or sharp loading.
• Failure analysis and root-cause investigation of cracked or fractured plastic parts in service.
• Input data for finite element analysis and part design, where notch-sensitive behavior must be accounted for in structural simulations.

FAQ

What is notch sensitivity in plastics?

Notch sensitivity is the extent to which a plastic's strength and toughness decrease when a notch or stress concentrator is present. It results from stress concentration and triaxial constraint at the notch root, which suppress yielding and promote brittle fracture.

Which standards govern tensile notch sensitivity testing?

The primary standards are ISO 8256 and ASTM D1822 for tensile-impact energy, complemented by ISO 527-1/527-2 and ASTM D638 for quasi-static notched tension testing. ISO 291 and ASTM D618 govern specimen conditioning.

How is the notch sensitivity factor calculated?

The factor is typically the ratio of the notched result to the unnotched result, obtained under identical test conditions. A ratio close to unity indicates notch insensitivity, while a substantially lower ratio indicates strong notch sensitivity and a risk of brittle failure at stress raisers.

How does this test differ from Izod or Charpy impact testing?

Izod (ASTM D256, ISO 180) and Charpy (ISO 179) tests load a notched bending specimen, whereas tensile notch sensitivity testing loads the notched specimen in tension, either at impact speed or quasi-statically. Tensile loading better represents many real-world failure modes such as snap-fit insertion, overpressurization, and pulling loads.

What factors most influence the results?

Notch root radius, notch depth, temperature, strain rate, material ductility, filler content, molecular orientation, and specimen conditioning all significantly affect the measured notch sensitivity. Testing must therefore control these parameters rigorously.

What sample quantity and lead time are typical?

Most accredited third-party laboratories require at least five to ten specimens per condition for each notched and unnotched group. Standard testing, including machining, conditioning, and reporting, is commonly completed within five to ten working days, depending on material and conditioning requirements.