ASTM D 256 impact strength testing for plastics

Key facts and scope of ASTM D 256 for impact strength testing

Overview of ASTM D 256

ASTM D 256 is a standard test method developed by ASTM International for determining the impact resistance (toughness) of plastics using a pendulum-type impact tester. This standard, commonly known as the “Izod impact test,” measures the amount of energy required to break a test specimen with a single, sudden blow. The test helps assess a material’s suitability for specific applications by establishing its ability to withstand sudden mechanical impacts. ASTM D 256 is widely used in quality control, material comparison, research, and development within the plastics industry.

The test specifies the size, shape, and conditioning requirements for plastic specimens.
It outlines procedures for both notched and unnotched specimens, although the notched test is most commonly used.
Data obtained are reported as energy absorbed per unit width of the specimen (usually in J/m or ft-lbf/in).

Purpose and application

The primary purpose of ASTM D 256 is to determine the ability of rigid plastics to resist impact-induced fracture. It is applicable to a wide range of thermoplastic and thermoset materials, aiding in material selection and product design. The results are not intended for direct prediction of real-life performance, but rather as comparative values to rank materials or assess manufacturing consistency.

Testing parameters

The standard defines critical parameters, including impact speed, conditioning environment, and specimen dimensions, to ensure repeatable and comparable results. The test can be adapted for different types of impactors and variations in test configuration as specified within the standard, depending on the purpose of the assessment. Precision is influenced by factors such as notch geometry, specimen thickness, and testing temperature.

Takeaway

ASTM D 256 provides a consistent method for evaluating the impact strength of plastic materials, supporting quality control and comparative studies.

Used to measure the toughness of plastics using standardized procedures
Mostly applies to notched specimens for material ranking
Supports material development and production control
Ensures industry-wide comparability of impact strength data

Performing ASTM D 256 impact strength tests step by step

Specimen preparation and conditioning

Begin by preparing specimens according to the precise dimensions specified in ASTM D 256, typically 63.5 mm x 12.7 mm x 3.2 mm for most plastic materials. The specimens may be notched using a specific cutting tool as defined by the standard. Once prepared, specimens are conditioned at a standard temperature and humidity—generally 23°C and 50% relative humidity—for at least 40 hours prior to testing, ensuring reproducibility and material stabilization.

Equipment setup and calibration

Set up the pendulum impact tester as outlined in ASTM D 256. Verify that the machine uses the specified energy range and impact speed appropriate for the material tested. The test fixture should securely hold the specimen in a vertical position, with the notched side facing the impactor. Calibrate the machine as needed to ensure accuracy.

Confirm correct energy range for pendulum hammer
Check alignment of specimen and impactor
Inspect notch geometry and dimensions

Conducting the impact test

Place the conditioned specimen in the fixture, ensuring the notch faces the striking edge of the pendulum. Release the pendulum so it strikes the specimen at the specified speed. The device records the energy absorbed in breaking the specimen. Repeat for the required number of specimens (usually five or more for statistical significance) and record all results, noting any complete or partial breaks as defined in the standard.

Calculating and reporting results

Calculate the energy absorbed per unit width of the specimen, typically reported in joules per meter (J/m) or foot-pounds per inch (ft-lbf/in). Compile results, highlighting any deviation or unusual fractures. Report findings according to ASTM D 256 requirements, including specimen dimensions, notch type, number of samples, and conditioning details.

Determine absorbed energy for each specimen
Report average and individual values
Document test temperature, humidity, and notch details
Note any anomalies or specimen irregularities
Include all required details as per ASTM D 256

Quality control and data interpretation

Use average impact strength values to compare different plastic materials or monitor production consistency. Interpret results with caution as this test measures a specific type of impact behavior, not all aspects of material toughness. ASTM D 256 results are most useful for material ranking, process monitoring, and quality assurance applications in plastics manufacturing.

Summary

ASTM D 256 is a standardized method used for assessing the impact resistance of plastics. The process involves careful specimen preparation, accurate testing, and comprehensive data reporting, allowing for reliable material comparison and quality assurance in production environments.