Frequently Asked Questions
Full-size Charpy specimens are 10mm thick by 10mm wide. Sub-size Charpy specimens referred to in the following are defined as those of identical width but smaller thickness.
When relating sub-size Charpy results to full-size values, there are two issues of concern. One is the value of impact energy and the generally accepted method adopts a simple net section area-scaling rule to calculate impact energy values for thinner specimens. The second issue is the inherent shift (for identical material) in brittle-to-ductile transition for thinner ferritic steel.
Two relations have been developed to address this issue, based on the measured transition temperature shift for normalised Charpy energies between 25J/cm2 and 50J/cm2 (corresponding to 20J to 40J in full-size specimens).
The relations are given below:
|ΔT1 = -0.7 (10-t)2
|ΔT2 = 51.4 ln (2 (t/10)0.25 -1)
Both give similar results in thickness range between 2 and 10mm, see table below (T2 is recommended in BS 7910).
|ΔT1, °C||ΔT2, °C|
Although it is possible to simply normalise the impact energy by the cross section area of the respective specimens and apply the test temperature shift, Wallin (Ref 3) proposes a method to adjust Charpy values which is applicable to the full ductile to brittle transition curve, based on knowing the upper-shelf energy of a 10m thick Charpy specimen (KV10-US). His method is illustrated in Figure 1, and the equation to calculate the relative Charpy energy between subsize and 10mm thick specimens is given below: