ASTM D256 Testing Determining the Izod Impact Strength of Plastic Materials
What is ASTM D256 Testing?
Impact resistance in plastics, as measured according to ASTM D256, is determined by the Izod pendulum impact test. Under controlled conditions, a pendulum strikes the material, absorbing a certain amount of energy, and yielding a quantitative value for material toughness and the ability of the material to resist sudden impact failure.
The method is widely used in assessing the resistance of plastic materials to sudden mechanical stresses or shocks, which also forms a vital component in understanding their behavior in field applications, such as packaging, construction, and consumer products.
Why is ASTM D256 Testing Important?
Impact resistance is a key mechanical property that determines how a plastic will perform under conditions involving sudden force or impact.
ASTM D256 testing helps:
- Assess material toughness and brittleness to inform product design and material selection.
- Compare the impact resistance of different plastic grades or formulations.
- Predict performance under real-world conditions where accidental impacts or drops may occur.
- Ensure compliance with product specifications, safety standards, and regulatory requirements.
Manufacturers, designers, and quality control engineers rely on this test to ensure that materials meet durability and reliability expectations, especially in automotive parts, packaging, electronics housings, and consumer goods.
Which Materials Can Be Tested Under ASTM D256?
ASTM D256 applies to a wide range of rigid and semi-rigid plastics, including but not limited to:
- Polypropylene (PP)
- Polyethylene (PE)
- Polystyrene (PS)
- Polyvinyl Chloride (PVC)
- Acrylonitrile Butadiene Styrene (ABS)
- Polycarbonate (PC)
- Nylon (PA)
- Acetal (POM)
Specimens are typically tested in the form of standardized bars, either notched or unnotched, depending on the desired test condition and material behavior.
What Information Do ASTM D256 Test Results Provide?
The test provides data that quantifies a material’s resistance to impact failure.
| Parameter | Unit | Description |
| Impact Strength (Izod Value) | J/m or ft·lb/in | Energy absorbed per unit of specimen width under impact |
| Type of Break | – | Describes fracture mode: complete, partial, hinge, or non-break |
| Notch Sensitivity | – | Indicates how the presence of a notch affects material toughness |
| Brittle or Ductile Behavior | – | Characterizes how the material fails under impact |
These results help classify plastics as brittle, semi-tough, or rigid, aiding in design optimization and material selection for impact-critical applications.
What Industries Use ASTM D256 Testing?
ASTM D256 impact testing is widely applied across industries that demand durable and resilient plastic components:
- Automotive: Bumpers, interior panels, and protective housings
- Consumer Goods: Appliance casings, helmets, toys, and electronics
- Packaging: Containers, bottles, and protective films
- Construction: Pipes, fittings, and panels
- Medical Devices: Casings and protective housings for instruments
- Aerospace: Lightweight components requiring a balance between toughness and weight
What Equipment Is Used for ASTM D256 Testing?
The primary equipment used in this test includes:
- Izod Pendulum Impact Tester: Measures the energy absorbed by the specimen.
- Specimen Notching Cutter: Creates a precise notch to act as a stress concentrator.
- Temperature Conditioning Chamber: Maintains consistent test conditions.
- Specimen Support and Clamping System: Holds the specimen vertically during impact.
Accessories may include different pendulum weights and scales for varying energy ranges, depending on the material’s toughness being tested.
How Is the ASTM D256 Test Performed?
Step-by-Step Procedure:
- Specimen Preparation:
Standard test bars (typically 64 × 12.7 × 3.2 mm) are machined or molded, and a precise notch is cut using a notching cutter. - Conditioning:
Specimens are conditioned at 23°C ± 2°C and 50% ± 5% relative humidity for a minimum of 40 hours before testing. - Mounting the Specimen:
The specimen is clamped vertically in the impact testing machine, with the notch facing the striking edge of the pendulum. - Pendulum Release:
The pendulum is released from a known height, striking the specimen at a fixed velocity. - Data Measurement:
The instrument measures the energy absorbed during fracture, automatically calculating the Izod impact strength. - Fracture Classification:
The type of break (complete, partial, or hinge) is recorded to interpret the fracture mode.
What Are the Standard Test Conditions?
| Parameter | Typical Condition |
| Temperature | 23°C ± 2°C |
| Relative Humidity | 50% ± 5% |
| Pendulum Energy Range | 2.7 – 21.7 J (varies by machine capacity) |
| Specimen Dimensions | 64 × 12.7 × 3.2 mm (standard) |
| Notch Depth | 2.54 mm ± 0.05 mm |
| Striking Velocity | 3.5 ± 0.2 m/s |
Maintaining uniform environmental and geometric conditions ensures the accuracy and reproducibility of results.
Key Properties Measured
ASTM D256 measures the Izod impact resistance—the material’s ability to absorb energy and resist fracture during sudden loading. This reflects both the toughness and brittleness of the plastic.
The test also provides insight into notch sensitivity, helping to predict performance in applications with stress concentrations or sharp edges.
Related Testing Methods
Other impact resistance test methods include:
- ASTM D6110: Charpy Impact Strength of Notched Specimens of Plastics
- ISO 180: Plastics — Determination of Izod Impact Strength
- ASTM D1822: Tensile-Impact Energy of Plastic by Pendulum Method
- ASTM D3763: High-Speed Puncture Properties of Plastics
These methods complement ASTM D256 by providing alternative impact geometries or higher strain-rate evaluations.
Conclusion
ASTM D256 is the leading standard for assessing the impact toughness and fracture behavior of plastics. Since this method quantifies the absorbed energy in an immediate manner, it enables both manufacturers and designers of plastic materials to assess the reliability, durability, and safety of the materials for specific applications.
From material optimization for protection in automotive to consumer durability and industrial safety, ASTM D256 provides the data needed to ensure that material performance is consistent and compliant with quality standards.
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FAQs on ASTM D256 – Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics
What does ASTM D256 measure? 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.
What is the objective of the Izod impact test? 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.
Why is the notch important in ASTM D256 testing? 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.
