Plastic Notched Impact Testing: Principles, Standards, and Test Procedures

Plastic notched impact testing is a fundamental mechanical evaluation method used to measure the impact resistance and notch sensitivity of plastic materials under dynamic loading. Performed according to recognized standards such as ISO 179-1, ISO 180, ASTM D256, and equivalent GB standards, the test provides essential toughness data for material selection, quality control, and product development. This article explains the technical principles, specimen requirements, testing procedures, and typical applications of notched impact testing for plastics.

Technical Overview

Plastic notched impact testing quantifies the energy required to fracture a standardized plastic specimen containing a machined notch, struck by a swinging pendulum at high velocity. The notch functions as a controlled stress concentrator, producing a triaxial stress state at its tip that accelerates brittle crack initiation. Because the test evaluates the material under conditions resembling real-world defects, scratches, and sharp corners, it reveals notch sensitivity that unnotched impact testing cannot detect.

The test is performed in two principal configurations. The Charpy configuration, covered by ISO 179-1 and ASTM D6110, supports the specimen horizontally on two anvils and strikes it at the center of the span, directly behind the notch. The Izod configuration, covered by ISO 180 and ASTM D256, clamps the specimen vertically as a cantilever beam and strikes it above the grip, on the notched side. In both cases, the absorbed impact energy is recorded in joules and normalized by the residual cross-sectional area at the notch to yield notched impact strength, expressed in kJ/m² under ISO standards or in J/m under ASTM standards.

Typical pendulum impact testers operate with energy capacities ranging from 1 J to 50 J and a striking velocity of approximately 3.5 m/s. Test results are classified by break behavior: complete break, hinged break, partial break, or non-break, and the reported value must always be accompanied by this designation to be technically meaningful.

Testing Samples

Izod Test Specimens

For ISO 180 testing, the standard specimen measures 80 mm × 10 mm × 4 mm and is tested edgewise with the notch facing the striking edge. For ASTM D256 testing, the standard specimen measures 63.5 mm × 12.7 mm in length and width, with preferred thicknesses of 12.7 mm, 6.4 mm, or 3.2 mm depending on the material form and available stock. Specimens are produced by injection molding or compression molding, or by machining from sheet, rod, or finished parts.

Charpy Test Specimens

ISO 179-1 uses the same 80 mm × 10 mm × 4 mm geometry as ISO 180, supported on anvils with a span of 62 mm for edgewise impact or 70 mm for flatwise impact. ASTM D6110 specifies a larger specimen of 127 mm × 12.7 mm × 12.7 mm supported on a 102 mm span. The choice between edgewise and flatwise striking, and between instrumented and conventional pendulums, must be documented with the results.

Notch Geometry and Preparation

The notch geometry has a dominant influence on test results and must be machined with precision. ISO standards define three notch types: Type A with a 0.25 mm notch radius, Type B with a 1.00 mm radius, and Type C with a 0.10 mm radius, all machined to a depth of 2 mm. ASTM D256 uses a similar 0.25 mm radius notch with a 2.54 mm depth. Notches must be cut with a sharp, dedicated single-point cutting tool at controlled feed rates, because a dull cutter or improper cutting technique creates residual stresses and plastic deformation at the notch root that artificially alter the measured impact strength. Notch radius should be verified periodically using optical projection or profilometry.

Specimen Conditioning

Prior to testing, specimens must be conditioned to a standard 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 the material specification dictates otherwise. For hygroscopic materials such as polyamides, extended conditioning to reach moisture equilibrium may be required, since moisture content significantly affects impact behavior.

Testing Items

The principal measurable quantities and deliverables of plastic notched impact testing include:

• Charpy notched impact strength, determined per ISO 179-1, ASTM D6110, or GB/T 1043.1, expressed in kJ/m².
• Izod notched impact strength, determined per ISO 180, ASTM D256, or GB/T 1843, expressed in kJ/m² for ISO or J/m for ASTM.
• Absorbed impact energy in joules for each individual specimen.
• Break behavior classification for each specimen: complete break, hinged break, partial break, or non-break.
• Statistical summary including mean value, standard deviation, and coefficient of variation.
• Optional temperature-dependent testing, for example from −40 °C to elevated temperatures, to characterize the brittle-to-ductile transition.

Testing Methods

Equipment and Calibration

Testing is conducted on a pendulum impact machine equipped with interchangeable hammers of suitable energy capacity. The machine must be verified in accordance with ISO 13802 or equivalent procedures, covering pendulum moment of inertia, friction losses, center of percussion, and velocity at impact. The selected pendulum capacity must ensure that the energy absorbed by the specimen falls between approximately 10 % and 80 % of the available energy so that the measurement remains accurate; if a specimen does not break or absorbs excessive energy, a higher-capacity hammer must be used and the result reported accordingly.

Test Procedure

• Verify specimen dimensions and notch depth with calibrated instruments; reject specimens with visible molding defects, sink marks, or flash at the notch.
• Condition the specimens in the standard atmosphere and test them promptly after removal from the conditioning chamber.
• Select the appropriate pendulum, confirm the correct support span and striker alignment for the chosen configuration, and perform a blank swing to record windage and friction losses.
• Position the specimen with the notch precisely aligned against the anvil or clamp stop, with the notch facing the direction of strike.
• Release the pendulum from its latched height, record the absorbed energy from the scale or transducer, and inspect the fracture surfaces.
• Classify the break type, calculate the notched impact strength, and repeat on the required number of specimens, discarding and replacing any result affected by a clamping or alignment error.

Calculation of Results

Under ISO standards, the notched impact strength is calculated by dividing the corrected absorbed energy by the cross-sectional area of the specimen at the notch root, that is, the specimen thickness multiplied by the residual width beneath the notch, and is reported in kJ/m². Under ASTM D256, the Izod impact resistance is calculated by dividing the absorbed energy by the specimen width and is reported in J/m or ft-lb/in. Because the two normalization methods differ, ISO and ASTM numerical values are not directly convertible, and the governing standard must always be cited alongside the data.

Applications

• Material comparison and selection: ranking the toughness of competing thermoplastics, engineering resins, and polymer blends during design and procurement decisions.
• Quality control: monitoring incoming resin batches, molding consistency, and the effectiveness of impact modifiers, reinforcements, or annealing processes in production environments.
• Automotive engineering: validating bumpers, interior trim, clips, and under-hood components that must withstand low-temperature impact and stone chipping.
• Pipes, fittings, and infrastructure: assessing impact performance of PVC, PE, and PP piping systems covered by EN, ISO, and GB product standards.
• Electrical and appliance housings: confirming compliance with material toughness requirements in UL, IEC, and related safety specifications.
• Cold-environment performance: determining ductile-to-brittle transition temperatures for outdoor equipment, packaging, and consumer goods.
• Failure analysis and research: evaluating notch sensitivity, embrittlement caused by weathering, thermal aging, or chemical exposure, and supporting formulation development in polymer laboratories.

FAQ

What is the difference between the Izod and Charpy notched impact tests?

The Charpy test supports the specimen horizontally on two anvils and strikes it at midspan, while the Izod test clamps the specimen vertically as a cantilever and strikes it above the clamp. The two methods also use different specimen sizes under ASTM standards and different energy normalization conventions, so results from the two configurations are not interchangeable.

Why is a notch machined into the specimen?

The notch concentrates stress at a known location and imposes a triaxial stress state that suppresses ductile yielding and promotes brittle fracture. This allows the test to reveal notch sensitivity, meaning the degree to which a material loses toughness in the presence of scratches, sharp corners, weld lines, or other stress raisers inherent in real products.

Why do ISO and ASTM results for the same plastic often differ?

Differences in specimen thickness, notch depth, radius, and the method of normalizing absorbed energy produce systematically different numbers between ISO and ASTM procedures. A typical ISO specimen is 4 mm thick whereas ASTM specimens are commonly 3.2 mm or thicker, and orientation-dependent materials respond differently to these geometries. Comparisons must therefore always be made within the same standard and specimen type.

How many specimens should be tested?

ISO standards recommend testing at least five specimens, and preferably ten, unless the material specification requires otherwise, because impact data are inherently scattered. Reporting the mean together with the standard deviation and break-type classification is essential for correct interpretation.

What does a low notched impact strength indicate?

A low value indicates brittle behavior in the presence of stress concentrators, which may result from low temperature, high notch acuteness, excessive mold-in stress, poor fusion of weld lines, degradation during processing, or an intrinsically brittle polymer. It signals that the part may crack or shatter when scratched or impacted in service and that design or material changes may be needed.

What are the limitations of notched impact testing?

The test measures total energy to fracture rather than a true material property, and the value depends on specimen geometry, notch quality, and test conditions. Results can be influenced by machine friction, clamping force, and specimen orientation in anisotropic materials such as fiber-reinforced plastics. For critical applications, notched impact data should be complemented by fracture mechanics testing, instrumented impact analysis, or dynamic mechanical analysis conducted by accredited third-party laboratories or qualified in-house facilities.