Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
Chapter 7
Fracture and fatigue
by JOHN YATES
Structural steelwork is susceptible to several failure processes. The principal
amongst these are wet corrosion, plastic collapse, fatigue cracking and rapid fracture,
often termed brittle fracture. In this chapter, failure by rapid fracture and
fatigue cracking will be discussed.
7.1 Fracture
7.1.1 Introduction
The term brittle fracture is used to describe the fast, unstable fractures that occur
with very little energy absorption. In contrast, ductile fracture is a relatively slow
process that absorbs a considerable amount of energy, usually through plastic deformation.
Some metals, such as copper and aluminium, have a crystalline structure
that enables them to resist fast fracture under all loading conditions and at all temperatures.
This is not the case for many ferrous alloys, particularly structural steels,
which can exhibit brittle behaviour at low temperatures and ductile behaviour at
higher temperatures. The consequence of a brittle fracture in a structure may be an
unexpected, catastrophic failure. An understanding of the fundamentals of this
subject is therefore important for all structural engineers.
The introduction and development of fracture mechanics technology allows the
engineer to examine the susceptibility of steel structures, especially their welded
joints, to failure assuming that a defect of a given size is present and knowing the
operating conditions.
7.1.2 Ductile and brittle behaviour
Ductile fracture is normally preceded by extensive plastic deformation. Ductile
fracture is slow, and generally results from the formation and coalescence of voids.
These voids are often formed at inclusions due to the large tensile stresses set up
at the inclusion/metal interface, as seen in Fig. 7.1(a). Ductile fracture usually goes
through the grains but, if the density of inclusions or of pre-existing holes is higher
248