.

# What is the effect of elevated ('dynamic') loading rate on tensile properties in steel?

Frequently, it is impossible or impractical to carry out tensile tests at high loading rates on steel used in existing structures. Therefore, efforts have been underway since the 1970s to predict the effect of loading rate on tensile and toughness properties without having to carry out additional tests. Based on an extensive test programme on steels of varying strength, Rolfe and Barsom (1977) developed an empirical predictive relationship for structural steel to calculate the influence of the time period and the temperature of a dynamic event on the yield stress:

[1]

σyd is the yield stress in MPa at the temperature T (in °K) and the time period t (in seconds) of the event; σys is the static, room temperature yield stress in MPa. This formula would give a rise in yield strength of 65MPa for a isothermal test time of 40ms(corresponding to an average strain rate of about 0.05s-1 for a 50mm gauge length steel specimen at room temperature) and a rise of 140MPa for a isothermal test time of 0.4ms (corresponding to an average strain rate of about 5s-1 for conditions as above).

Recent research work at TWI (Wiesner et al, 2004) has provided an alternative equation for the estimation of dynamic yield strength for ferritic steels at a given temperature and strain rate:

[2]

The constants in equation [2] have been estimated by best fitting the equation to test data available in the open literature at the time. The temperature and strain rate ranges covered by the test data are as follows:

Estimation of strain rate is needed in order to use equation [2], while the loading time period is required in equation [1]. Both equations are expected to give similar results, when loading time is converted to strain rate (or viceverse). The choice between the equations for estimation of dynamic yield strength will, in practice, depend which input parameter (loading time or strain rate) can be readily estimated.

## Reference

S T Rolfe and J M Barson: 'Fracture and Fatigue Control in Structures', Prentice-Hall, New Jersey, 1977, p86.

C S Wiesner, W Xu, F M Burdekin, W Zhao and Y Tkach: 'The effects of dynamic loading on structural integrity assessments', Research Report, Health and Safety Executive, UK, 2004. ISBN 0 7176 2831 0.