Izod Impact (Notched) ASTM D256

20 Aug

Written By MTE

Izod Impact Test (Notched Izod) - ASTM D256 and ISO 180

The Notched Izod Impact Test is a crucial method used to gauge a material's resistance to a swinging pendulum's impact. This singular point test offers a straightforward quality control measure to determine a material's specified impact characteristics or to compare the general toughness of different materials.

Equipment and Procedure for Izod Impact Testing - ASTM D256

The test commences by securing the specimen in the pendulum impact test fixture. Here, the notched side is positioned facing the pendulum's striking edge. Once prepared, the pendulum is let loose, allowing it to impact the specimen. If the specimen doesn't break upon this impact, a heavier hammer is employed until the desired failure is achieved.

Certain materials may show a diminished impact strength at colder temperatures. Therefore, to mimic real-world usage conditions, these materials might need to be tested at reduced temperatures. For such Reduced Temperature Tests, the specimens undergo conditioning at a set temperature in a freezer. Once equilibrium is achieved, each specimen is rapidly removed and impacted. Notably, neither the ASTM nor ISO standards provide explicit guidelines on conditioning duration or the time gap between removal from the freezer and impact. However, commonly followed norms from other standards suggest 6 hours of conditioning and a 5-second gap post removal.

Specimen Specifications for Testing

Dimensions: 64 x 12.7 x 3.2 mm (2½ x ½ x 1/8 inch).
Standard thickness: 3.2 mm (0.125 inch). However, 6.4 mm (0.25 inch) is favored as it lessens the chances of bending or crushing.
Depth below the notch: 10.2 mm (0.4 inches).

For the ISO standard:

Derived from a Type 1A multipurpose specimen after removing end tabs.
Dimensions: 80 x 10 x 4 mm with an 8mm depth below the notch.

Izod Impact Test Method

1. Equipment:

Izod impact testing machine with a pivoting arm.
Standardized specimens as per ASTM D256 or ISO 180.
Freezer (for Reduced Temperature Tests).

2. Test Specimen Preparation:

Prepare the specimens with dimensions as per ASTM D256: 63.5 × 12.7 × 3.2 mm (2.5 × 0.5 × 0.125 in) or according to ISO 180 standards.
Ensure a notch is made in the specimen, typically to determine impact energy and notch sensitivity.

3. Test Procedure:

Clamp the specimen in the pendulum impact test fixture with the notched side facing the striking edge of the pendulum.
If testing at reduced temperatures:
- Condition the specimens at the specified temperature in a freezer until they reach equilibrium.
- Quickly remove each specimen from the freezer and place it in the test fixture for immediate impact. Aim for a gap of 5 seconds from removal to impact.
Raise the pendulum arm to a specific height ensuring constant potential energy.
Release the pendulum arm. It will swing down and hit the notched sample, breaking the specimen.
Observe and record the height to which the pendulum arm swings after impacting and breaking the specimen.

4. Calculations:

Calculate the energy absorbed by the specimen by analyzing the difference in potential energy from the initial height to the height post-impact.
Express the results in energy lost per unit of thickness at the notch (ft·lb/in or J/cm for ASTM, J/m2 for ISO). Alternatively, for ASTM, it may be reported as energy lost per unit cross-sectional area at the notch (J/m2 or ft·lb/in2).

5. Result Interpretation:

Higher values indicate greater material toughness.
The test outcome, typically for ASTM, is based on the average of 5 specimens, whereas ISO results derive from an average of 10 specimens.

6. Conclusion:

This test provides vital insights regarding the material's ability to withstand sudden impacts and is crucial for determining the suitability of a material for specific applications.

Safety Precautions:

Ensure the testing area is clear of any obstructions and bystanders.
Always wear protective eyewear and gloves during the testing procedure.
Handle the pendulum with care to prevent accidental release.

Notes:

Remember, the Izod test uses a cantilevered beam configuration as opposed to Charpy's three-point bending setup.
Factors affecting test results include specimen dimensions, the hammer's initial height, its mass, the notch's sharpness, and the specimen's temperature. Ensure all variables are controlled and consistent for accurate comparisons.

Historical Background and Necessity of Izod Impact Strength Test

Origin of the Izod Impact Test The Izod impact strength test is named in honor of the English engineer Edwin Gilbert Izod. In 1903, Izod presented this test in his address to the British Association, which was later documented in the journal "Engineering".

Comparison with Charpy Impact Test Although the Izod test shares similarities with the Charpy impact test, it distinguishes itself in the arrangement of the specimen during testing. Specifically, while the Charpy test places the specimen in a three-point bending configuration, the Izod test uses a cantilevered beam setup.

Understanding the Importance of Impact Tests Impact, as characterized, implies the application of a substantial force within an exceedingly brief time frame, leading to a swift momentum and energy transfer. This phenomenon's outcomes differ when the equivalent energy is transferred over a longer duration. Many engineering structures undergo such impacts in their lifespan and might manifest cracks. If unaddressed, these cracks could propagate, eventually causing significant failures.

Impact tests are instrumental in evaluating the shear fracture resilience of different materials under identical testing conditions. Moreover, for a singular material, this test can be pivotal in identifying its ductile-to-brittle transition temperature. This is observed as a sharp decrease in impact strength with decreasing temperatures.

Toughness of a material pertains to its capacity to absorb energy during slow plastic deformation. However, the rate of strain is influential. While brittle materials exhibit low toughness due to their minimal plastic deformation, ductile substances can display brittle behavior when subjected to high-energy impacts. This duality underscores the importance of the Izod impact test.

Crucial Variables in the Test The accuracy and relevance of the Izod impact test depend on multiple variables:

Specimen dimensions, particularly the rectangular cross-section below the notch.
The hammer's initial height, which dictates its impact velocity.
The hammer's mass, which, combined with its velocity, determines the kinetic energy upon impact.
The notch's sharpness or curvature.
The specimen's temperature.

ASTM's Role in Standardizing the Test for Plastics ASTM International has defined the standard for the Izod impact test for plastics under ASTM D256. The results can be represented in terms of energy lost per unit of thickness at the notch, such as ft·lb/in or J/cm. Alternatively, it can be expressed based on the energy lost per unit cross-sectional area at the notch, like J/m2 or ft·lb/in2. In contrast, Europe employs the ISO 180 methodologies, where results solely focus on the cross-sectional area at the notch, measured in J/m2. The conventional specimen dimensions for ASTM D256 are 63.5 × 12.7 × 3.2 mm (2.5 × 0.5 × 0.125 in), with the most typical thickness being 3.2 mm (0.13 in). However, the width can range between 3.0 and 12.7 mm (0.12 and 0.50 in).

MTE