Loading rates and the fracture toughness transition curve

The overall effect of loading rate on toughness can be illustrated with reference to the typical fracture toughness transition curves shown in Fig.1 for static and dynamic loading rates. The loading rate can be expressed as the rate of displacement or the strain rate or, for fracture mechanics consideration, as the rate of increase of the stress intensity factor dK/dt or Κ.

At temperatures near the lower shelf of the transition curve (at temperature T₁ in Fig.1) increasing loading rates Κ are likely to lead to a reduction in fracture toughness (but there are material-specific exceptions).

At temperatures in the upper shelf of the transition curve (at temperature T₃ in Fig.1), the initiation toughness increases for most steels with increasing loading rate although there can be exceptions.

In the transition region (at temperature T₂ in Fig.1), increasing loading rate (K₂ > Κ₁) can lead to an increase in toughness if ductile behaviour prevails (for low Κ conditions), but a loading-rate induced change in fracture mode is possible (Κ₃ versus Κ₂ or Κ₁ in Fig.1 at T₂), leading to marked reduction of toughness at a given temperature.

Reference

1. C S Wiesner and H J MacGillivray: ' Loading rate effects on tensile properties and fracture toughness of steel', 1999 TAGSI Seminar, Cambridge, 29 April 1999, IoM Publication, P.149, London, 2000.