All About the IZOD Impact Test

The IZOD impact test is an ASTM (American Society for Testing and Materials) standardized method used to determine the impact resistance, or toughness, of a material. When designing a structure or part, it is important for engineers to know the toughness of a material to ensure that their designs not only satisfy the intended application, but are also safe, resilient, and long-lasting. To determine impact resistance (toughness) using an IZOD impact test, a weighted pendulum is dropped from a prescribed height and strikes a notched test specimen. When the specimen breaks, the energy absorbed and the material’s impact resistance can be related back to the potential energy of the pendulum before it is released. This article will discuss the IZOD impact test, show how the resulting metric is calculated, and explain its importance in engineering design.

What Is an IZOD Impact Test?

The IZOD impact test is used to determine the impact resistance (toughness) of a material, or the material’s tendency to resist breaking when subjected to a sudden force or impulse. The IZOD impact test method, also known as the notched IZOD test, is standardized under the ASTM International test procedure. The test is often used in the polymer industry to determine the toughness of plastics but can be used on any material. The materials needed to complete an IZOD impact test are a rectangular test specimen with a machined notch, a pendulum impact test fixture, and a vise or similar holding device to fix the test specimen.

The IZOD impact test is important because it:

1. Allows engineers to determine the toughness of candidate materials, permitting parts and structures to be designed with appropriate materials, dimensions, and safety factors.
2. Enables engineers to observe how materials perform when subjected to impact loading. 

Engineers use the IZOD test to characterize the ability of materials to absorb energy during an impact. This simple, quick test can be used to determine the temperature at which a material changes from ductile to brittle behavior (ductile-to-brittle transition temperature). Some steels experience a ductile-to-brittle transition around 0 °C, well within the service temperature range for many applications with critical safety concerns, like ships and bridges.

The IZOD impact test is commonly used for plastics, while the Charpy test is commonly used for metals, however, both methods can be used for any material. For example, various plastics manufacturers determined the average toughness for polyoxymethylene (POM, acetal) to be 90 J/m2  and the average toughness for Nylon 6,6 to be 100 J/m2 by using the IZOD impact test.

The IZOD impact test method is standardized by ASTM D256 for plastics and ASTM E23 for metals. The IZOD impact test method is also standardized by ISO (International Organization for Standardization) 180. The major difference between the ASTM and ISO standards is the size of the test specimen. The ASTM standard requires the test specimen should have dimensions of 2.5” (L) x 0.5” (W) x 0.125” (T) and the depth beneath the notch is 0.4” while the ISO standard says the test specimen should have dimensions of 3.15” (L) x 0.39” (W) x 0.15“ (T) and 0.31” of material beneath the notch.

How to Calculate the IZOD Impact Test

First, set the height of the pendulum hammer. Then, the potential energy of the pendulum at this height is recorded. Next, the pendulum is dropped and then strikes the test specimen. Fourth, the kinetic energy of the pendulum is determined when the specimen breaks. The law of conservation of energy is used to determine the kinetic energy at breakage. The impact strength of a material is considered the kinetic energy absorbed by the specimen at breakage.

The units for impact strength as determined by the IZOD test are typically expressed as J/m or ft-lb/in. This can be interpreted as the kinetic energy, or amount of work, required (J, ft-lb) to deform the material a certain amount (m, in).

The IZOD impact test is commonly used for plastics, while the Charpy test is commonly used for metals, however, both methods can be used for any material. For example, various plastics manufacturers determined the average toughness for polyoxymethylene (POM, acetal) to be 90 J/m2  and the average toughness for Nylon 6,6 to be 100 J/m2 by using the IZOD impact test.

How To Conduct the Test Procedure for the IZOD Impact Test

To conduct an IZOD impact test, the rectangular test specimen (2.5” (L) x 0.5” (W) x 0.125” (T)) must first be prepared. A notch is then added to the test bar to control the fracture location. The notch is usually a V-shaped groove made by making two cuts into the side of the test bar, at 45 degrees to each other, such that the two cut surfaces meet at a 90-degree angle and the intersection is 0.4" deep. The vertex of the V-shaped notch should be approximately at the midpoint of the length of the specimen. 

Once the test specimen is prepared, it is fixed into one end of the testing apparatus, with the notched side facing the pendulum hammer, effectively creating a cantilever. The notch should be visible above the jaws of the fixture. After the specimen is secured, the standard pendulum hammer is prepared to drop from a height of 24”. The pendulum is released, and the hammer strikes the part above the notch. The hammer is replaced with heavier hammers dropped from the same height until the specimen fractures. Once fractured, the material’s impact strength is calculated by dividing the impact energy by the thickness of the specimen.

To conduct an IZOD test at a reduced temperature, the prepared test specimens are placed in a freezer until the specimen’s temperature reaches equilibrium. The specimens are retrieved one by one and impact tested in accordance with ASTM. The material's impact strength is calculated in the same manner as before. 

Factors That Affect IZOD Impact Energy

The factors that affect IZOD impact strength are described below:

1. Yield Strength

Certain heat-treating processes (such as quenching and precipitation hardening) are used to increase yield strength. However, these processes also decrease ductility, and therefore the energy required to fracture the part. Therefore, higher yield strengths generally correlate with lower impact strengths.

2. Ductility 

Ductility refers to a material’s ability to plastically deform prior to fracture. Therefore, ductile materials are more likely to have higher impact strengths. For more information, see our guide on Ductility.

3. Notches 

The notch in the test specimen localizes stresses in the test specimen during the impact test. The notch sensitivity differs based on the material. Therefore, the notch tip radius and notch depth are highly important.

4. Temperature and Strain Rate 

Both temperature and strain rate affect the impact strength of a material. The temperature and strain rate interact with each other, such that sensitivity to strain rate increases as test temperature decreases. At temperatures below the ductile-to-brittle transition temperature (DBTT), some normally ductile materials (like low-carbon steel and polypropylene) become brittle. Consequently, little to no strain occurs since less strain is required before a fracture occurs. Therefore, a material's tolerance for a high strain rate is significantly reduced at low temperatures.

5. Fracture Mechanism

Fractures are classified as ductile (dominated by microvoid coalescence), brittle (dominated by cleavage), or mixed (having characteristics of both ductile and brittle fracture). Microvoid coalescence occurs when voids form in a material’s crystal structure as a result of applied forces overtaking the strength of interatomic bonds. As strain increases, the voids become larger and coalesce to form larger voids until a fracture occurs. 

Cleavage occurs when cracks form along definite planes of a material’s crystal structure. Cleavage happens almost instantaneously once a crack initiates, and is the common mechanical failure mechanism for brittle materials. Therefore, materials that fracture by cleavage have much lower impact strength, since these materials experience less plastic deformation before fracturing.

The IZOD Impact Test vs. the Charpy Impact Test

Both the IZOD impact test and the Charpy impact test use weighted pendulums to determine the toughness, or impact strength, of a material. However, there are four main differences between the IZOD test and the Charpy impact test. They are compared in Table 1 below:

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