What is the Charpy V-Notch (CVN) Impact Test?

Before we can answer this question, we must provide the answer to what the Charpy Test measures in a material. It is a test that is designed to give information on a material property we know as toughness. 

Toughness itself is often confused with a materials strength but, the differences are quite significant. Generally speaking, strength is related to how much force the metal can support, while, in the science of metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing. Good toughness, therefore, is necessary to resist the initiation and retard the growth of cracks.

During your time as practitioners and supervisors it’s possible that you may have to produce a welding procedure that requires toughness testing and perform controlled welding in the shop/field. 

The Charpy V Notch (CVN) test is a basic method for defining toughness and, in this process, toughness is defined in “joules or ft lbs” at the “temperature of test”. There are other, more complex methods but the CVN test is cheap, is a good Quality Control (QC) method, and is globally accepted. 

In the test itself, a notched specimen is hit by a swinging pendulum and the amount of energy required to break the specimen is recorded. The energy can be measured simply by allowing the pendulum to drop from a fixed height and measuring how high it swings up after fracturing the specimen. If the specimen is brittle and absorbs little energy, the pendulum may swing up almost to its original height. If the specimen is tough, the pendulum will only swing up a small distance. The value of energy absorbed is read directly from a dial on the machine.  A typical impact testing machine is shown in Figure. 1.

Figure 1. Schematic of a Typical Charpy Impact Testing Machine

In carrying out a test, the specimen is loaded into the anvil with a pair of special tongs that facilitate location of the specimen in the machine. The specimen is supported at each end as shown in Figure 2 and the pendulum strikes the back of the specimen behind the notch. This puts the side with the notch into tension, causing an initiation of fracture from the notch

Figure 2 Charpy Test Specimen under Load

The most commonly used specimen, the Charpy V-notch (CVN), is 10 × 10 × 55 mm and has a V-notch 2 mm deep with a 0.25 mm tip radius. Subsize specimens are also sometimes used when testing thinner material such as welds in small diameter, thin walled pipes.. 

The temperature of test is important because once a materials toughness is established at a certain temperature, then that toughness may lessen as the temperature of operation lessens. This is particularly true of steels so that a steel that is tough at room temperature may fracture in a brittle mode under the right loading conditions at low temperatures. It’s not difficult to imagine that low temperatures of service will be experienced in some areas of Canada and, certainly, in the Far North.

As an example, Figure 3 shows the results of testing at different temperatures on a particular steel. The graph shows low energy at low temperatures, then an increase in energy through a temperature interval known as the transition range. At higher temperatures, the curve flattens out to give an upper shelf. The temperature, at which a certain energy (for example 20 Joules) is reached, is often used as a method of comparing the toughness of materials/welds.

Figure 3. Charpy Energy in Joules vs Temperature of Test.

When conducting impact tests on welds, it is most important that the specimens are machined from the right location, and that the notch is oriented correctly. A totally misleading result may be obtained if the tip of the notch is in the wrong position. 

It is important to note that the width of the specimen probably encompasses more than one weld pass, and the proportion of each pass sampled is likely to affect the results. A slight shift in the location of the specimen, or a small change in the sequence of weld passes may well have a significant effect on the result of the impact test. When testing to a specification, strict adherence to the given procedures is essential. 

For example, when impact tests are required as part of the classification of an electrode, they are machined from the same test weld as the tensile specimens. The test plate is 20 mm thick and the impact specimens are located mid-thickness as shown in Figure 4 below. The notch is placed perpendicular to the plate surface on the centreline of the weld.

Figure 4. Location of Impact Specimens in the Weld Metal as per CSA W 48 “Electrode and Filler Metal Classification.

The testing of the heat affected zone (HAZ) can be more troublesome because of its narrow width and the fact that it is usually sloping. In most cases, the notch samples only part of the HAZ. To locate the position, the specimens should be machined as over-length blanks, then etched to reveal the weld and HAZ. The desired position of the notch can then be marked and the specimens cut to final length. Figure 5 shows the location of a typical HAZ impact test.

Figure 5 Typical location of a CVN test piece sampling the HAZ

When impact tests are required in the qualification of welding procedures, specimens from several positions within the joint may be required, particularly for thick plates. For example, a test may require the sampling of 50% of the weld metal and 50% of the HAZ. In the case of thick joints welded from both sides and where the root of side one is back gouged, tests in the back gouged region may be called for as this area will have seen additional heat cycling.

If CVN tests for weld metal and/or HAZ toughness are required by design then the locations of the test pieces and the number of tests required will be detailed in the qualification and/or the construction code and the results tabulated in a Procedure Qualification Record (PQR)