Tensile Impact Test for Plastics

Introduction

The Tensile Impact Test measures the impact resistance and strength of plastic materials. A notched specimen is held between two grips, and a sudden force is applied to measure the energy required to break the material. This test helps evaluate plastic materials for applications that may experience sudden and repeated impacts, such as automotive or aerospace components.

Scope

The Tensile Impact test determines whether plastic materials suit a particular application. The test provides the impact resistance and strength of the material, which is required to select the appropriate material for an application. It also enables the optimization of the design of plastic parts concerning impact resistance in the product development process. It can be applied in quality control to ascertain whether the plastic material meets the minimum standards and requirements.

Understanding Tensile Impact Test for Plastics

The test involves preparing a notched specimen of the plastic material, clamping it between two grips, and striking it with a pendulum at a predetermined velocity. The energy required to break the specimen is calculated from the height of the pendulum before and after the impact. The test is conducted at room temperature but can also be performed at other temperatures to evaluate the material’s behavior under different conditions. The test measures the material’s strength and flexibility by measuring the energy required to cause it to break and the deformation of the material during the test.

Types of Impact Testing

The Charpy and Izod impact tests measure the toughness of a material by quantifying the amount of energy it can absorb before fracturing. This value is correlated to the material’s brittleness—the brittleness of the material determines the capacity of absorption. Glass absorbs less energy due to its brittleness, but materials such as aluminum, on the other hand, could absorb higher amounts. Its knowledge enables predictions of deformation tolerance up to failure. It is essential to consider the differences and similarities between the two tests when determining which one to use for impact strength testing of a material.

Charpy Impact Test

The Charpy impact test, first introduced by Charpy in 1900, is one of the most widely used methods for determining the impact strength of materials. This is attributed to its simplicity and ease of use. In this test, a pendulum is dropped from a certain height to strike a notched specimen, which absorbs energy upon impact. The energy absorbed is determined by measuring the pendulum’s height before and after the fracture.

The specimen, often a 55 mm x 10 mm x 10 mm bar, is placed horizontally with a U-shaped or V-shaped notch facing away from the pendulum to focus stress and encourage fracture. The test can be conducted at ambient or low temperatures below -45°F and follows standards like EN 10045-1, ISO 148, ASTM E23, and ASTM A370. Although this is mainly used on metals, it applies to plastics with ISO 179 and ASTM D6110 standards.

Izod Impact Test

Edwin Gilbert Izod developed the Izod impact test in 1903. This test apparatus and specimen are similar to Charpy’s impact test, though different. In the Izod test, the specimen is clamped vertically with the notch oriented to face the pendulum; the pendulum strikes the specimen above the notch.

Despite their similarities, the Charpy and Izod impact tests have several differences. One is that the Izod test can be carried out on metallic and plastic materials.

Plastic specimens are typically 64 mm x 12.7 mm x 3.2 mm bars with a V-shaped notch, whereas metallic specimens are generally round bars measuring 127 mm x 11.42 mm with one or three V-shaped notches. Common standards for the Izod impact test include ISO 180, ASTM D256, and ASTM E23.

Differences between Charpy and Izod impact test

The differences between the Charpy and Izod impact tests are as follows.

Conclusion

In conclusion, the Tensile Impact Test is the most important method for testing plastic materials’ impact resistance and strength, especially in applications that might be subjected to sudden or repeated impacts, such as automotive and aerospace parts. It helps ensure the materials meet the required durability and performance standards by providing the energy absorbed by breaking a notched specimen. Although both Charpy and Izod impact tests measure material toughness, specimen orientation, notch placement, and testing procedures vary. Hence, application type and material will determine which test to be selected. The two tests can provide abundant knowledge about the material’s behavior when producing products requiring high-impact resistance.

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