Charpy absorbed energy

The test method may specify either a notched sample or an unnotched sample. If a notched specimen is specified, the dimensions of the notch are also given. Figure 15.31 is an example of these dimensions for a metal sample. In the Charpy test the notched sample is struck from behind the notch. In the Izod test the notch is facing the hammer. The notch, called a stress raiser, concentrates the stresses applied by the impact load. It makes the material brittle and increases the elastic limit in the notch area. Without the notch, many materials will bend without fracture, and the capacity to absorb energy will not be accurately determined. 

The Izod and Charpy impact tests are mostly used. A defined pendulum strikes the specimen sample, notched or un-notched, clamped with a defined device. The absorbed energy is calculated and expressed ... 

Notched Charpy impact strength acN is calculated from the absorbed energy W, related to the smallest initial cross-section of the specimen at notch base ... 

The Charpy tests are performed according to the RSE-M, with instrumented ISO tups. The transition curves of the absorbed energy, the lateral expansion and the fracture appearance are determined. The RTndt shift is deduced from the transition temperature shifts at an absorbed energy of 56 J and lateral expansion of 0.9 mm. The measured values are compared to the upper-bound predictions calculated according to the RSE-M Code. 

In the Charpy test, this may give erroneous results when the specimen does not break into separate pieces, but folds around the impactor, so adding a frictional component to the absorbed energy, unrelated to the impact strength. In addition, it is rarely necessary in service to know the amount of energy needed to destroy an article, but rather the amount of impact that can be sustained without serious damage. 

The 12 mm plaques were used to manufacture Reversed Charpy specimens, 12 mm wide and 70 mm long. These were razor-notched to leave a ligament. y = 1.5 mm and tested at 0°C, with an impact speed of 1 m/s on a span of 60 mm, and using the procedure developed by Hillmansen [5]. The absorbed energy U is integrated used to determine Wp using the equation ... 

The Charpy test and the Izodtest are both pendulum-type impact tests. The difference between these two tests is essentially the orientation of the sample. In the Charpy test, or simple-beam method, the sample is supported at both ends, but is not held down, as shown on the left in Figure 15.30. In the Izod test, or cantilever beam method, the sample is supported on one end in a vice, as shown on the right in Figure 15.30. In both tests the sample is at the bottom of the pendulum arc and the amount of energy absorbed by the sample is measured. 

Measuring loads in high speed tests is difficult because of the short times involved and the presence of transient effects from stress waves. The more simple method which has been developed in both the Izod and Qiarpy tests is to break the spedmoi with a pendulum and measure the energy absorbed. Some form of scaling parameter from the specimen dimmsions is then applied and an impact number derived. The practical utility of these numbers is beyond question but they have many, weU known, drawbacks. In particular, the nund)ers are geometry dependent and do not agree, for example, between the Izod and Charpy tests. 

Instrumented Charpy impact tests on unnotched samples are conducted to determine the energy absorbing capability and dynamic fracture behaviour of the C/SiC composites. The dynamic fracture toughness (ak) is calculated using the following equation ... 

Some specifications require a minimum Charpy v notch impact requirement of 15 ft lb energy absorbed at the minimum expected service temperature. However, this does not mean that a test specimen exhibiting 60 ft lb is four times tougher than the minimum. The main value of notched bar impact testing is as a criterion for acceptance of materials where reliable correlation with service behavior has been obtained. 

In the Charpy test the specimen is supported as a simple beam and is loaded at the midpoint (Figure 3.36b). In the Izod test it is supported as a cantilever and is loaded at its end (Figure 3.36c). The standard energy absorbed in breaking the specimen is recorded. 

Variation of the energy absorbed in a Charpy impact test with the radius of the notch tip in 3 mm thick bars (from Ogorkiewicz, R. M., Ed, Thermoplastics—Properties and Design, Wiley, 1974). 

To determine the Charpy impact strength of an unnotched specimen acu> the energy absorbed by breaking the specimen is related to the initial cross-section area of the specimen ... 

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