The right choice of steel – according to the Eurocode - Economical Bridge Solutions based on Innovative Composite Dowels and Integrated Abutments - page 17

In the Hong Kong Code of Practice for the Structural Use of Steel (2005) [5], a ratio f u /
f y = 1.2 is required and therefore 9% more conservative compared to the Eurocode and there-
fore does not allow for high strength, high toughness steels. Further, the elongation at failure
to be not less than 15% is required which is in line with the Eurocode requirement. The ration
1.2 used in the Hong Kong Steel Code is reasonable for conventional steels.

2.3 Toughness

2.3.1 Introduction

There are two ways of material failure: ductile failure and brittle fracture (Tab. 2).

Table 2: Failure mechanisms of materials

Failure mode Deformation of crystal lattice Fractography

Ductile failure
▪ shear
▪ slipping
▪ toughness
▪ dull

Brittle fracture
▪ cleavage
▪ decohesion
▪ brittleness
▪ shiny

Toughness is the resistance of a material to brittle fracture when stressed. Toughness is
defined as the amount of energy per volume that a material can absorb before rupturing. The
material toughness depends on:

Temperature
Materials lose their crack resistance capacity with decreasing temperature (“Fig. 10”).
This relation can be displayed in an impact energy A v - temperature T curve with an
upper shelf region (3: ductile failure), lower shelf region (1: brittle fracture) and a transi-
tion region (2: crack shows shares of cleavage and shear area).
Influence of loading speed
The higher the loading speed, the lower the toughness (“Fig. 11”).
Grain size
The orientation of the crystal lattice varies in the adjacent grains (“Fig. 12”). When-
ever the crack tip reaches the grain boundary, the crack would subsequently change his