ASTM D256 Testing Determining the Izod Impact Strength of Plastic Materials

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

Overview

ASTM D256 describes standardised methods for determining the resistance of plastic materials to a sudden, sharp impact, using the Izod pendulum impact test. Impact resistance is a critical mechanical property that reflects a material’s ability to absorb energy during fracture — an essential characteristic for plastic components subject to sudden loads, drops, or collisions in service.

The test involves clamping a notched or unnotched specimen vertically in a vice and striking it with a weighted pendulum at a fixed velocity. The energy absorbed in breaking the specimen is measured from the difference in the pendulum’s swing before and after impact. Results are reported as energy absorbed per unit of specimen width (J/m or ft·lb/in), providing a standardised basis for comparing materials. ASTM D256 includes multiple methods: Method A (notched Izod), Method B (reversed notched Izod), Method C (unnotched Izod), and Method D (tensile impact), offering flexibility for different materials and applications.

Scope, Applications, and Benefits

Scope

ASTM D256 evaluates:

Notched Izod impact resistance of plastic materials
Unnotched Izod impact resistance
Tensile impact resistance (Method D)
Energy absorbed per unit of specimen width
Effect of notch geometry on impact behavior
Influence of temperature on impact resistance
Comparison of brittle vs. ductile fracture behavior

Applications

Engineering thermoplastics qualification
Consumer product plastic components
Automotive plastic parts (bumpers, panels)
Electrical housing and enclosures
Appliance and tooling components
Packaging materials
Research and development in polymer formulation

Benefits

Provides standardized impact performance data
Enables ranking of materials by toughness
Supports material selection for impact-critical designs
Detects brittle-to-ductile transition behavior
Ensures quality consistency in production compounding
Reduces product failures from impact-induced fracture

Test Process

Specimen Preparation

Specimens are molded or machined to standard dimensions; a notch is machined to the specified geometry for Method A.

Conditioning

Specimens are conditioned at 23°C/50% RH or specified test temperature.

Clamping & Striking

Specimen is clamped vertically; the pendulum is released to strike the specimen at the notch.

Energy Recording

Energy absorbed at fracture is read from the pendulum scale and reported as impact strength.

Technical Specifications

Parameter	Details
Applicable Materials	Plastic materials (thermoplastics and thermosets)
Specimen Types	Notched and unnotched Izod specimens
Formula Used	Impact Strength = Absorbed Energy / Specimen Thickness
Impact Energy	Energy absorbed during fracture
Specimen Thickness	Standard machined thickness specimen
Test Environment	Controlled ambient conditions

Instrumentation Used for Testing

Izod pendulum impact tester
Notching tool / milling machine
Specimen conditioning chamber
Micrometer and calipers
Notch depth and radius verification gauge
Data recording system

Results and Deliverables

Notched and/or unnotched Izod impact strength values
Statistical summary (mean, standard deviation)
Failure mode classification (complete break, hinge break, partial break, no break)
Temperature-dependent impact data (if applicable)
Quality assurance reports

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Frequently Asked Questions

ASTM D256 measures the impact resistance or toughness of a plastic material when subjected to a sudden blow. Specifically, it determines how much energy a material absorbs before fracturing, using a pendulum impact tester in an Izod configuration.

The objective of the test is to assess a material’s ability to withstand sudden mechanical impact without breaking. It helps identify whether a plastic is brittle or ductile, guiding material selection for applications prone to impact.

The notch acts as a stress concentrator, ensuring the fracture initiates consistently at a specific point. It helps reveal the material’s notch sensitivity, which is crucial for evaluating its performance in real-world applications involving sharp edges or sudden stress.

Results are commonly reported in J/m, kJ/m², or ft-lbf/in.

The test is mainly used for thermoplastics, thermosetting plastics, and molded polymer components.