IZOD Impact Test: Definition, Importance, and Calculation
A helpful way to find out a material’s toughness
Engineers who work with us here at Xometry use many different impact tests to learn a material’s characteristics, one of which is the IZOD impact test. There are various methods of determining how a material will respond or react; for the IZOD, you’ll need a pendulum and vises.
Here’s more about this test, its important factors, and how it compares to other impact tests (like the Charpy impact test).
What Is an IZOD Impact Test?
The IZOD test is a way of determining the impact resistance of a material, specifically how much load it can handle before it breaks. This will help you choose the right material for your needs. Also called the notched IZOD test, it’s been accepted as a standardized method of doing so by the American Society for Testing and Materials (ASTM) and the International Organization for Standardization (IOS)—although the latter requires a slightly longer and wider piece of material for testing.
You can use it to test almost any material, but it’s most popularly relied on for plastics and polymers. You need a few different supplies and tools for conducting the test, including a rectangular sample of your material with a notch in it, a pendulum impact test tool, and a vise. Below is a diagram from the Xometry team of how the test works.
In addition to impact, the IZOD test also provides an idea of the amount of energy the material can absorb. All of this is imperative to know before these materials are then turned into products or structures that people use every day, such as bridges. When you’re using the IZOD to test, the method is ASTM D256 and for metals it’s the ASTM E23.
What Is the Importance of the IZOD Impact Test?
The IZOD impact test is important because it:
- Allows engineers to determine the toughness of candidate materials, permitting parts and structures to be designed with appropriate materials, dimensions, and safety factors.
- Enables engineers to observe how materials perform when subjected to impact loading.
Why Do We Need IZOD Impact Tests?
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.
How Is the Result of the IZOD Impact Test Calculated?
First, the height of the pendulum hammer is set. The potential energy of the pendulum at this height is recorded. Second, the pendulum is dropped and then strikes the test specimen. Third, 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.
What Are the Units of IZOD Impact Test Strength?
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).
What Is an Example of IZOD Impact Test?
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.
Why Do We Use a Notched Specimen for the IZOD Impact Test?
A notched specimen is used for IZOD tests because the notch produces a stress concentration at the root of the notch and forces the fracture to occur at a repeatable location. By localizing stresses, the notch promotes crack initiation and propagation and minimizes plastic deformation. If the first hammer weight selected for the test does not break the sample, successively heavier hammers are used until the sample does break.
How is the IZOD Impact Test Procedure Performed?
The IZOD test is relatively simple to do, but it does require specific conditions and measurements. Here are the general steps you can expect:
- First, prepare the test specimen, cutting it to the dimensions that the ASTM or IOS calls for and notching a 45-degree V-shaped section. The pendulum will hit right above this to break the material. You’ll want to have a vertex that sits at the midpoint of the length of the test material.
- Then, add the test material to the pendulum testing apparatus, at the bottom where the pendulum will swing and make contact. Secure it with the vise.
- You’ll set the height of the pendulum’s hammer (the standard is 24 inches) and note the potential energy the pendulum will have at this height. Then, release the pendulum.
- After the first test, you can increase the weight of the pendulum’s hammer until it breaks the material, which would then signify that the impact strength value can be calculated.
- If you need to do an IZOD test in cold temperatures, the test material will be placed in a freezer until it reaches equilibrium, and then taken out and tested.
- To get the impact value, you’ll divide the impact energy by the thickness of the specimen. This number will have a unit of J/m or ft-lb/in, which translates to the amount of kinetic energy that’s needed to deform and break the material.
What Machine Is Used for IZOD Impact Testing?
A pendulum testing machine is used for IZOD impact testing. This type of machine typically has a pendulum that can be fixed at varying angles, a hammer at the end of the rod of the pendulum, and a fixture into which test specimens can be positioned at the bottom of the apparatus. There is also typically a scale on the point of rotation of the pendulum that allows users to specify different angles from which to drop the pendulum.
What Is the IZOD Impact Test Standard?
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.
What Are the Factors Affecting IZOD Impact Strength?
There are a few details that make a difference in the impact strength reading you get from a material:
1. Yield Strength
Although a material may be heat-treated to have higher yield strength, heat treating processes can also decrease the matertial’s ductility. This can lead to a lower impact strength overall.
2. Ductility
Materials with higher ductility tend to deform before they’re permanently broken or damaged. Since this is the case, you’ll find these materials tend to have higher impact strengths.
3. Notches
It’s important to get the notch tip radius and notch depth right in order to get an accurate reading of impact strength. This is because the notch localizes the stress and the sensitivity will vary depending on what material you’re using.
4. Temperature and Strain Rate
A lower temperature will influence how brittle a material is and how much strain happens during the test. The cooler it gets, the more likely the material’s strain rate decreases, which is important to be aware of.
5. Fracture Mechanism
You’ll want to understand what kind of fracture is at play, which will come down to the makeup and structure of the material you’re testing. It’ll either be a ductile or brittle fracture, or feature characteristics of both these types. Different kinds of cracks will result, including cleavage and microvoid coalescence.
What Is the Difference Between the IZOD Impact Test and the Charpy Impact Test?
Both tests use the same type of swinging pendulum device, but the IZOD is normally used for testing plastics, whereas the Charpy is popular for testing metals. We’ve covered a few other differences between these tests in the table below.
| Feature | IZOD | Charpy |
|---|---|---|
Feature Specimen Orientation | IZOD The material sits vertically and fixed at one end forming a cantilever | Charpy The material sits horizontally is supported on both ends |
Feature Point of Contact | IZOD The weight strikes the test material above the notch | Charpy The weight strikes directly behind the test material’s notch |
Feature Notch Direction | IZOD The notch faces the weighted hammer | Charpy The notch faces away from the weighted hammer |
Feature Specimen Dimensions | IZOD 2.5” (L) x 0.5” (W) x 0.125” (T) | Charpy 2.16” (L) x 0.39” (W) x 0.39” (T) |
IZOD Test vs. The Charpy Test
How Xometry Can Help
Whether you’re testing plastics or metals, we have various services here at Xometry that cater to both these types of materials. You can get instant quotes for plastic 3D printing, sheet metal fabrication, plastic injection molding, and metal extrusion on our website.
Check out our Technical Datasheet Glossary to learn more about different material properties.
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