Charpy Impact Steel Testing: Part One

The Charpy V-notch-CVN impact test also known as the Charpy V- notch test, is a standard test used to measure the impact energy (also referred to as notch toughness) absorbed by a material during fracture. The notch provides a point of stress concentration within the specimen and improves the reproducibility of the results. The absorbed energy is computed by working out the potential energy lost by a pendulum through breaking a specimen. Results from tests performed at different temperatures are used to determine the ductile-to-brittle transition temperature of materials. Although the test is governed by a standard test procedure, several variables influence the test result repeatability. In fact, through convention, the test is performed on three specimens at the same temperature and the results are averaged. However, the test is still susceptible to a number of uncertainties as outlined in Lont (2000) and Splett et al. (2008), giving rise to erratically distributed data. The sources of disturbance can be grouped as follows:

• Specimen (e.g. notch geometry, inhomogeneous distribution of atoms during the early stages of nucleation, duplex grain structures including both coarse and fine grains lead to inconsistent energy distribution, chemical composition).
• System (e.g. machine stiffness and friction, calibration settings).
• Environment (e.g. ambient and specimen temperatures).
• Procedure (e.g. human error). When combined with a highly sparse data distribution, this suggests that modelling Charpy impact test data is a challenging task.

Today, the Charpy impact test is used in a number of ways. It is used in many industries for testing materials, for example the construction of pressure vessels and bridges to determine how storms will affect the material used.

The current British Standard for Charpy testing is BS EN ISO 148-1:2009 and the American Standard is ASTM E23. The standards differ only in the details of the strikers used. The standard Charpy-V specimen, illustrated in Figure 1. is 55 mm long, 10mm square and has a 2 mm deep notch with a tip radius of 0.25 mm machined on one face.

Figure 1: Standard Charpy-V notch specimen

To carry out the test the standard specimen is supported at its two ends on an anvil and struck on the opposite face to the notch by a pendulum as shown in Figure 2. The specimen is fractured and the pendulum swings through, the height of the swing being a measure of the amount of energy absorbed in fracturing the specimen. Conventionally three specimens are tested at any one temperature, see Figure 3, and the results averaged.

Figure 2: The Charpy testing machine

A characteristic of carbon and low alloy steels is that they exhibit a change in fracture behaviour as the temperature falls with the failure mode changing from ductile to brittle.

If impact testing is carried out over a range of temperatures the results of energy absorbed versus temperature can be plotted to give the 'S' curve illustrated in Figure 3.

This shows that the fracture of these types of steels changes from being ductile on the upper shelf to brittle on the lower shelf as the temperature falls, passing through a transition region where the fracture will be mixed.

Figure 3: Schematic Charpy-V energy and % age crystallinity curves