How do Charpy values determined using sub-

size specimens relate to results from full-size

specimens? 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/cm

2

(corresponding to 20J to 40J in full-size specimens).

The relations are given below:

ΔT1 = -0.7 (10-t)2 (Ref.1)

ΔT2 = 51.4 ln (2 (t/10)0.25

-1) (Ref.2)

Both give similar results in thickness range between 2 and 10mm, see table below (T2 is

recommended in BS 7910).

Thickness, t

mm ΔT1, °C ΔT2, °C

9 -1 -3

7.5 -4 -8

5 -18 -20

2.5 -39 -45

For Charpy requirements to be equivalent, the measured Charpy energy (normalised by the

specimen net section area in J/cm 2 units) has therefore to be specified at a colder temperature

for sub-size specimens. The table below gives two examples using ΔT 2.

10 x 10mm

specimens

5 x 10mm

specimens

2.5 x 10mm

specimens

Equivalent

Charpy properties

At temperature

27J

(34J/cm2)

-20°C

14J

(34J/cm2)

-40°C

7J

(34J/cm2)

-65°C

Equivalent

Charpy properties

At temperature

40J

(50J/cm2)

-20°C

20J

(50J/cm2)

-40°C

10J

(50J/cm2)

-65°C