Measuring absorbed energy with an impact test

Resilience is defined as the ability of a material to store elastic and plastic deformation energy until failure, i.e. the ability to withstand shocks and dynamic loads. In general, a very important index is given by the resilience module, or by the area subtended by the graph of a curve in an effort-deformation diagram.
Resilience, like other mechanical properties that characterize the materials, is very affected by some conditions, in particular the temperature. In fact, materials tend to pass, by effect from temperature, from a ductile behavior to a fragile behavior and vice versa. Important from this point of view, is the ductile-fragile transition temperature, at which there is a sudden jump from one to the other. In general, the effect is:
- At low temperatures, the material tends to fragility by storing less energy.
- At high temperatures, the material tends to behave ductile, managing to absorb higher quantities of energy.
Although this is what is found for most of the cases, there are always exceptions that characterize the behavior of the materials: for example, materials with CFC (cubic centered faces) lattice do not have the transition temperature and the performance leap that follows.

In the laboratory, the resilience test is usually carried out by means of the Charpy test and, sometimes, by means of the Izod test.
The pendulum of Charpy, from the name of its creator, is an instrument made up of a club tied to the base by a hinge. The other side is free to move and is positioned at a height of H. Once the club is left free, it goes down and impacts a suitably carved specimen (a V or U notch may be present), causing it to break, and goes back to a height h, different from the starting one. The measure consists in calculating the difference of potential energy between the initial and the final situation:

where m is the mass of the pendulum; g is the acceleration of gravity; H is the initial height of the pendulum; h is the final height of the pendulum. This value of K, expressed in Joules (being an energy value), represents the resilience of the material. The previously written report can also be reformulated on a trigonometric basis:

where l is the length of the pendulum rod; β is the angle of ascent with respect to the vertical; α is the angle of the initial position with respect to the vertical.
A fundamental aspect when approaching the result of a resilience test is to consider that this value does not have absolute but relative validity. The simple energy data extracted has no meaning if taken individually (by changing the size of the notch or other parameters the value changes), but must be used to make comparisons that allow you to make a correct choice of material avoiding the use of fragile materials in applications in to which a certain degree of ductility is required.