Validation of idealised Charpy impact energy transition curve shape

H G Pisarski, B Hayes, J Olbricht, P Lichter and C S Wiesner 

Paper presented at Charpy Centenary Conference (CCC 2001), Poitiers, France, 2-5 Oct.2001

Abstract

An idealised Charpy transition curve is provided in Annex J of BS 7910:1999 which enables the 27J transition temperature (T _27J) to be estimated from Charpy tests conducted at a single temperature. The validity of this transition curve is examined using data representing a range of ferritic steel grades that includes data from parent plate, weld metal and heat affected zone. It is shown that the idealised Charpy transition curve cannot be relied upon to be accurate or to make consistently conservative predictions of T _27J. The main reason for this is that actual Charpy transition curves have a wide range of shapes that cannot be modelled by one curve. Recommendations are made for estimating T _27J from tests conducted at a fixed temperature. The implications of using T _27J derived from the idealised transition curve to estimate fracture toughness using the Master Curve Charpy-fracture toughness correlation are examined.

Introduction

Situations arise when there is a need to assess the significance of crack-like flaws in structural components and appropriate fracture toughness data, in the form of fracture mechanics test results, are not available. In these circumstances, Charpy data are often used to estimate fracture toughness through appropriate correlations. Although fracture mechanics, as we know it, was unknown to Monsieur Charpy, we are certain that he would have been pleasurably surprised that the test he developed at the beginning of the last century was still being used to help establish the fracture resistance of structural components.

Annex J of BS 7910:1999 ^[1] provides a Charpy-fracture toughness correlation which can be used to estimate fracture toughness in the transition regime of the toughness versus temperature curve for ferritic steels. It is based on the Wallin Master-Curve correlation and requires as input the temperature at which 27J (the 27J transition temperature, T _27J) is achieved in standard sized Charpy specimens. However, for components that have already been built or are in operation, Charpy transition curve data are unlikely to be available, so the identification of T _27J is a problem. Codes often require that Charpy tests are conducted at a fixed temperature with a target minimum absorbed energy.

In order to provide a means of estimating T _27J from Charpy results obtained at a different temperature, Annex J of BS 7910 provides a reference, idealised Charpy transition curve. This curve was derived from a proposal ^[2] for defining the limiting thicknesses in steel bridges for the avoidance of brittle fracture. Since the steel specification for bridge construction requires Charpy testing to be conducted at a fixed temperature, an idealised transition curve, referenced to T _27J, was used to derive limiting thickness for a range of minimum design temperatures. This curve enables T _27J to be estimated from Charpy energies up to 101J. However, the BSI committee preparing Annex J decided that this involved excessive extrapolation and limited the range from 5J to 61J, as shown in Fig.1. This paper reviews experimental data to support the idealised Charpy transition curve in Annex J, and suggest an alternative method for estimating T _27J where the data does not support it.

The implications of this can be assessed for the Grade A ship steel plate for which fracture toughness test results are available in addition to Charpy test results shown in Fig.4. The results of the fracture mechanics tests are shown in Fig.11, where they are compared with the prediction made using the Master Curve correlation as per Annex J. The analyses were made for T _27J = -23°C which is the mean estimate from the best tanh fits, for T _27J = -44°C, which is the lowest estimate obtained from the idealised Charpy transition curve for tests conducted at single temperatures above T _27J, and for T _27J = -6°C, which is the highest T _27J estimated from the idealised Charpy transition curve, see Fig.8.

Fig.11. Comparison between fracture toughness predicted from T _27J and experimental data

It is clear that the predicted fracture toughness (K _J) transition curve is not very sensitive to the choice of T _27J, and that the predictions are very conservative with respect to this particular set of experimental data. Consequently, it can be concluded, at least on the basis of the data available here, that the errors in estimating T _27J using the idealised Charpy transition curve are likely to have minimal impact on making non-conservative fracture toughness estimates. The main reason for this is overall conservatism in the fracture toughness correlation. Nevertheless, it is recommended that the best way forward would be to improve/modify the method of estimating T _27J in Annex J, possibly along the lines suggested here, and relax the conservatism in the Master Curve by selecting a higher value of P _f for making fracture toughness estimates.

Conclusions

Analyses of Charpy data obtained on ferritic steels which included parent plate, weld metal and HAZ show that the idealised Charpy transition curve in Annex J in BS 7910:1999 is not always conservative. Consequently, it is recommended that it should not be used to estimate T _27J from Charpy results obtained at a single temperature. Where energies above 27J are measured at a specific temperature, it is recommended that T _27J is assumed to be that temperature. If less than 27J is measured, but more than 21J, it is recommended that T _27J is estimated by assuming that for each 1J below 27J, (T-T _27J) is -1°C, where T is the test temperature. It is recommended that estimates of T _27J are not made when minimum energies below 21J are measured.

References

1. BS 7910:1999 Incorporating Amendment No. 1 (2000). British standards Institution.
2. Burdekin, F.M. (1981). In: The Design of Steel Bridges, Rockey, K.C and Evans, H.R. (Eds). Granada Publishing.
3. PD 5500:2000 (2000). British Standards Institution.