The standard for full-size Charpy specimens is 10mm wide and 10mm thick. The sub-sized Charpy specimens discussed in the figures below are defined as those that are lesser in thickness, but identical in width.

Two issues arise when relating the results of sub-sized Charpy to full-sized values. First, the value of the impact energy, along with the method TWI uses, adopts a plain net section area-scaling principle in calculating the values of impact energy for thin specimens. Second, an inherent shift (in identical materials) in the brittle-to-ductile transition occurs for thin ferritic steel.

To address these, two relations were developed. These were based on the measured temperature shift for normalized Charpy energies between 25 J/cm2 and 50 J/cm2, which correspond to 20 J ? 40 J in full-sized specimens.