Civil Engineering Reference
In-Depth Information
with this in mind, and unnecessary 'stress-raisers' should be avoided. It will
also be advantageous to restrict, where practicable, the locations of joints to low
stress regions such as at points of contraflexure or near the neutral axis. Further
information and guidance on fatigue design are given in [30-33].
1.3.3 Brittle fracture under impact load
Structural steel does not always exhibit a ductile behaviour, and under some cir-
cumstancesasuddenandcatastrophicfracturemayoccur,eventhoughthenominal
tensilestressesarelow.Brittlefractureisinitiatedbytheexistenceorformationof
a small crack in a region of high local stress. Once initiated, the crack may prop-
agateinaductile(orstable)fashionforwhichtheexternalforcesmustsupplythe
energyrequiredtotearthesteel. Moreseriousarecrackswhichpropagateathigh
speedinabrittle(orunstable)fashion,forwhichsomeoftheinternalelasticstrain
energy stored in steel is released and used to fracture the steel. Such a crack is
self-propagating while there is sufficient internal strain energy, and will continue
until arrested by ductile elements in its path which have sufficient deformation
capacity to absorb the internal energy released.
Theresistanceofastructuretobrittlefracturedependsonthemagnitudeoflocal
stress concentrations, on the ductility of the steel, and on the three-dimensional
geometricalconstraints.Highlocalstressesfacilitatecrackinitiation,andsostress
concentrationsduetopoorgeometryandloadingarrangements(includingimpact
loading) are dangerous. Also of great importance are flaws and defects in the
material,whichnotonlyincreasethelocalstresses,butalsoprovidepotentialsites
for crack initiation.
The ductility of a structural steel depends on its composition, heat treatment,
and thickness, and varies with temperature and strain rate. Figure 1.11 shows the
increase with temperature of the capacity of the steel to absorb energy during
Crack initiation
and propagation easy
Crack initiation
more difficult
Crack initiation
difficult
Temperature
Figure 1.11
Effect of temperature on resistance to brittle fracture.
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