Last time, we introduced factors that affect the Charpy impact energy of a specimen. These factors are discussed here.
Yield Strength and Ductility
For a given material, the impact energy will be seen to decrease if the yield strength is increased (i.e., if the material undergoes some process that makes it more brittle and less able to resist plastic deformation). Examples include cold working or precipitation hardening.
Temperature and Strain Rate
Most of the impact energy is absorbed by means of plastic deformation during the yielding of the specimen. Therefore, factors that affect the yield behavior and ductility of the material (such as temperature and strain rate) will influence the impact energy. This type of behavior is more prominent in materials with a body-centered-cubic structure, where lowering the temperature reduces ductility more markedly than face-centered-cubic materials.
The notch-bar impact test over a range of temperatures is more meaningful than at a single temperature so that the temperature at which the ductile-to-brittle transition can be determined (Fig. 2). Steel A shows higher notch toughness at room temperature, but its transition temperature is higher than that of steel B. The material with the lowest transition temperature is preferred.
Metals tend to fail by one of two mechanisms: microvoid coalescence or cleavage.
Cleavage can occur in body-centered-cubic materials, where cleavage takes place along the specific crystal planes. Microvoid coalescence is the more common fracture mechanism, where voids form as strain increases and eventually join together before failure occurs. Of the two fracture mechanisms, cleavage involves far less plastic deformation and hence absorbs far less fracture energy (see references 4 and 5 for more detail).
Next Time: A discussion about the variables that affect the transition temperature and a look toward the future.