Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics - PDF

​

ASTM D256 is a standardized test method used to determine the impact resistance of plastic materials using a pendulum-style Izod impact tester. This method is essential for assessing a plastic’s ability to resist sudden shocks and impacts, and it plays a critical role in materials selection for parts that must withstand mechanical abuse during service.

In this article, we'll explain the ASTM D256 test method, cover the required equipment (including universal impact testers), discuss the procedure, and provide guidance on specimen preparation and interpretation of results.

What Is ASTM D256?

ASTM D256, titled “Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics,” evaluates the toughness of a plastic by measuring the energy absorbed by a standard notched specimen when struck by a swinging pendulum.

This test simulates sudden mechanical shock, such as a dropped object or collision, and helps engineers determine how likely a material is to crack or break under sudden loading.

There are five procedures within ASTM D256, with Procedure A (Notched Izod) being the most commonly used.

Scope and Applications

ASTM D256 is widely used in:

• Material development and quality control for plastic resins

• Comparative toughness testing of plastic formulations

• Component and product design validation

• Failure analysis for brittle or cracked plastic parts

• Specifying minimum impact requirements in procurement standards

Typical materials tested include ABS, polycarbonate, acrylic, nylon, polypropylene, and various composites and filled polymers.

Specimen Preparation

ASTM D256 requires precise sample dimensions and notching geometry to ensure consistent results.

Standard Notched Specimen Dimensions:

• Length: 63.5 mm (2.5 in)

• Width: 12.7 mm (0.5 in)

• Thickness: 3.2 mm (0.125 in)

• Notch Depth: 2.54 mm (0.100 in)

• Notch Radius: 0.25 mm (0.010 in)

Specimens must be cut and notched using approved tools to avoid internal stress or heat effects. Notching is typically done using a motorized notch cutter conforming to ASTM D6110.

Samples should be conditioned at 23°C ± 2°C and 50% ± 5% RH for at least 40 hours prior to testing, per ASTM D618.

Required Test Equipment

1. Pendulum Impact Tester

A pendulum-style Izod impact tester delivers a controlled, repeatable blow to the sample. It must comply with ASTM D256 standards for energy range and strike velocity.

Key Specifications:

• Impact energy: 0.5 to 22 Joules (or more depending on material toughness)

• Swing angle: 160° (typical)

• Strike velocity: ~3.5 m/s

• Adjustable pendulum weights for variable energy levels

The pendulum strikes the notched side of the vertically mounted sample, fracturing it and transferring energy into the break.

2. Sample Support and Clamp

A rigid vertical clamp holds the specimen so the notch faces the pendulum. The fixture must eliminate backlash and ensure consistent orientation.

3. Data Display

The tester reports the energy absorbed in:

• Joules (J) or

• Foot-pounds (ft-lbf)

Some systems use digital encoders and microprocessor control for enhanced accuracy.

ASTM D256 Test Procedure

1. Mount the Specimen

   • Insert the notched specimen vertically with the notch facing the pendulum.

   • Ensure it is firmly clamped and vertically aligned.

2. Release the Pendulum

   • The pendulum is released from a known height and strikes the sample at the bottom of its arc.

   • The sample breaks, and the remaining energy of the pendulum is measured.

3. Record and Calculate Energy

   • The machine measures the energy absorbed by the fracture.

   • A minimum of five samples are tested, and the average is reported.

Izod Impact Strength Calculation

The Izod Impact Strength (IS) is calculated by dividing the energy absorbed by the specimen thickness:

Izod Impact Strength (kJ/m) =
  (Impact Energy in J) ÷ (Specimen Thickness in meters)

Or in imperial units:

Izod Impact Strength (ft-lbf/in) =
  (Impact Energy in ft-lbf) ÷ (Specimen Thickness in inches)

Results should be reported as the average of at least five specimens and indicate the type of break (complete, hinge, partial, or no break).

Types of Fracture Observed

Understanding the break type is essential for interpreting results:

• Complete Break – The specimen breaks into two pieces

• Hinge Break – Partially fractured but held together by a small unbroken ligament

• Partial Break – Cracked but not separated

• No Break – Specimen deflects but doesn’t fracture

Only complete breaks are valid for reporting toughness unless otherwise specified.

Advantages of ASTM D256

• Simple and repeatable for QC and R&D labs

• Requires minimal sample preparation

• Suitable for a wide range of thermoplastics and thermosets

• Provides an industry-standard benchmark for toughness and impact resistance

• Supports material comparison and design validation

Related Standards

If ASTM D256 is not ideal for your material or geometry, consider the following:

• ASTM D4812 – Unnotched Izod impact strength

• ASTM D6110 – Charpy impact strength of plastics

• ISO 180 – International Izod equivalent

• ISO 179 – Charpy impact testing

• ASTM D1822 – Tensile-impact testing for plastics

Conclusion

ASTM D256 provides a straightforward and widely accepted method for determining the impact resistance of plastic materials using the Izod pendulum method. With its well-defined procedures, sample specs, and reliable results, it’s the go-to standard for engineers and material scientists evaluating the toughness of plastics.

For industries ranging from automotive to electronics, consumer goods to aerospace, understanding a material’s ability to absorb impact is critical to ensuring durability and safety.

Need ASTM D256 Testing Equipment?
We offer complete Izod and Charpy impact testers, sample preparation tools, and accessories. Contact us today for turnkey solutions tailored to your lab.

 

Similar Specifications:

ASTM E23 - Metals Charpy Impact Test