ASTM D256 Standard Test Methods For Determining The Izod Pendulum Impact Resistance of Plastics

ASTM D256 is a standard test method for determining the impact resistance of plastics using a pendulum impact testing machine. This test method is used to evaluate the ability of plastics to withstand impact or shock loads and to quantify their impact resistance. The test involves preparing a standardized test specimen and positioning it on a test fixture.

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ASTM D256 Standard Test Methods For Determining The Izod Pendulum Impact Resistance of Plastics

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Overview
Scope, Applications, and Benefits
Test Process
Specifications
Instrumentation
Results and Deliverables

Overview

According to ASTM D256, the Izod pendulum test method is used to determine the impact resistance of plastics. This test measures the energy required to break a notched or unnotched plastic specimen when struck by a pendulum hammer, hence describing the material’s toughness and resistance to sudden impact.

It is widely used in the plastics industry to select materials, conduct quality control, and conduct R&D. The technique aids manufacturers and engineers in evaluating material response under high-velocity impact for products in which toughness and mechanical performance are key.

Scope, Applications, and Benefits

Scope

ASTM D256 covers the determination of the impact resistance of rigid and semi-rigid plastics using the Izod pendulum method.
It evaluates:

Resistance to impact breaking under a pendulum strike
Energy absorbed during fracture
Behavior of notched vs. unnotched specimens
Influence of processing variables and material formulations

The method enables comparative material studies, the prediction of performance under service conditions, and the assessment of brittleness or toughness.

Applications

Evaluating the impact resistance of engineering plastics
Comparing polymer grades for product design
Quality control in plastic manufacturing
Assessing effects of additives, fillers, or processing conditions
Research and development of new plastic formulations
Testing plastics used in consumer goods, automotive components, electrical housings, and packaging

Benefits

Provides a standardized measure of material toughness
Helps determine the suitability of plastics for impact-prone applications
Enables consistent quality control across production batches
Supports material optimization and performance validation
Provides data for engineering design and safety compliance

Test Process

Specimen Preparation

Prepare five specimens with a machined notch of defined geometry in accordance with ASTM D256 dimensions to ensure controlled fracture.

Specimen Mounting

Clamp the specimen vertically as a cantilever beam in the Izod impact testing apparatus.

Pendulum Impact Testing

Strike the specimen with a pendulum hammer of appropriate energy based on material toughness.

Energy Measurement and Conditioning

Measure the energy absorbed from the pendulum swing difference; for low-temperature testing (Procedure E), condition specimens in a cold chamber prior to impact.

Technical Specifications

Parameter	Details
Test Principle	Pendulum impact on notched or unnotched specimens
Sample Types	Rigid and semi-rigid plastics
Specimen Dimensions	Length: 2.5 in; Width: 0.5 in; Thickness: 0.125 in (typical)
Number of Specimens	Minimum of 5 for averaging
Notch Type	Precision-machined notch of defined depth and radius
Measured Output	Impact energy (ft-lb/in or J/m)
Analysis Basis	Energy absorbed divided by specimen width or notched area
Special Procedure	Subnormal temperature testing for brittle-temperature evaluation

Instrumentation Used for Testing

Izod pendulum impact tester with calibrated hammer
Pendulums of varying weights for different impact energies
Notching machine for precise notch geometry
Temperature-controlled chamber for low-temperature testing (Procedure E)
Digital or analog energy measurement system
Clamping fixtures for cantilever mounting

Results and Deliverables

Izod impact strength in ft-lb/in or J/m
Average impact energy from five test specimens
Comparison of notched vs. unnotched performance
Fracture behavior assessment (brittle or ductile)
Effect of temperature, processing, or additives on impact resistance
Material suitability evaluation for impact-sensitive applications

Frequently Asked Questions

ASTM D256 is the international standard for determining the impact strength of plastic and insulation materials. It describes impact testing using the Izod test method to assess plastics' impact and notched impact strength.

ASTM D256 – Standard Test Method for Determining Plastics' IZOD Pendulum Impact Resistance. ISO 180: Plastics – Determination of Izod Impact Strength of Rigid Materials

The dimensions of a standard specimen for ASTM D256 are 63.5 × 12.7 × 3.2 mm (2.5 × 0.5 × 0.125 in).

The Charpy impact test, also known as the Charpy V-notch test, is a standardized high strain-rate test that determines the amount of energy absorbed by a material during fracture.

The test provides critical data about the toughness of a material, helping manufacturers, designers, and engineers evaluate material performance for safety, durability, and reliability in end-use applications.

Primarily plastics and polymer-based materials are tested, but composites and certain non-metallic materials can also be evaluated for impact strength.

Izod and Charpy impact tests measure impact resistance but differ in specimen placement and striking method. In Izod testing, the specimen is held vertically, while in Charpy testing, it is placed horizontally.

Results are typically expressed in terms of energy absorbed (Joules or ft-lb/in). This indicates the toughness and ability of the material to resist cracking or breaking under sudden load.

Industries such as automotive, aerospace, packaging, electronics, medical devices, and construction use Izod testing to ensure material durability and compliance with quality standards.

A notched specimen is struck by a pendulum hammer, and the amount of energy absorbed by the specimen before it breaks is recorded.

Yes. Test parameters like specimen size, notch type, and temperature conditions can be customized depending on customer needs and industry requirements.

By comparing impact resistance values, engineers can select the most suitable plastic or polymer for applications requiring durability and toughness.

Infinita Lab USA based Testing laboratory offers accredited testing, precise instrumentation, quick turnaround times, and expert interpretation of results to support product development, quality control, and compliance.