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How do Charpy values determined using sub-size specimens relate to results from full-size specimens?

TWI 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 (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

References

  1. Towers O L: 'Testing of sub-size Charpy specimens: Part 1 - the influence of thickness on the ductile/brittle transition'. Metal Construction, March 1986, 18, (3), No. 171R-176R.

  2. Wallin K: 'Methodology for selecting Charpy toughness criteria for thin high strength steels: Part 1 - determining the fracture toughness'. Jernkontorets Forskning, Report from Working Group 4013/89, 28 December 1994.

  3. BS 7910:2005 'Guidance on methods for assessing the acceptability of flaws in metallic structures'. London, British Standards Institution, 2005 (including Amendment No.1).