TWI Industrial Member Report Summary 219/1983
By O L Towers
Background
The main objectives of this work were:
a ) to examine the influence of different striker geometries,
b) to study the influence of specimen thickness on the energy absorbed in a Charpy V-notch impact test,
c) to assess alternative allowances for the effect of thickness in sub-size specimens.
To achieve the above the literature was reviewed and tests were performed on a ferritic steel submerged arc weld metal and the following plain materials: five ferritic steels, three austenitic stainless steels, two aluminium bronzes and an aluminium alloy. Tests were carried out, using both the striking edges specified in the American Code ASTM E23-81 and that specified in other standards, in order to establish whether this variable has a significant effect on test results. It is found that the blunter striking edge to the American standard tends to result in higher absorbed energies for Levels above 0.75J/mm2 (60J in a full-size specimen), but below this there is no apparent effect. For ferritic steels displaying ductileibrittle behaviour with temperature, the transition temperature (at a constant absorbed energy per unit ligament area) is reduced with smaller specimen thicknesses. A shift in transition temperature of 0.7 (10-t)2°C, where t = specimen thickness in mm, fits the data reasonably well, although there is considerable scatter. For fully ductile behaviour, or 'upper shelf' behaviour for ferritic steels, the energy absorbed per unit ligament area is generally less for thinner specimens, although the effect is, in some cases, either minimal or reversed, (particularly for relatively low absorbed energies). These observations appear to be consistent with a model which had already been used to explain the effect of specimen thickness and of splitting on absorbed energy. Finally, the requirements of national product and fabrication codes for sub-size Charpy specimens are collated and comments are made on the approaches used. The principnl recommendations for ferritic structural and pressure vessel steels are that the effect of specimen thickness on the ductilelbrittle transition temperature should, where possible, be used to model the improved resistance to brittle fracture of the thinner sections, without attempting to correlate the results to those which would have been obtained in full-size specimens. Alterations to the existing pressure vessel standards ASME section Vlll and BS 5500, in particular, are recommended.