Introduction - The Behaviour and Design of Steel Structures to EC3 - page 8

1.3 Material behaviour

1.3.1 Mechanical properties under static load

The important mechanical properties of most structural steels under static load
are indicated in the idealised tensile stress-strain diagram shown in Figure 1.6.
Initiallythesteelhasalinearstress-straincurvewhoseslopeistheYoung'smod-
ulus of elasticity E . The values of E vary in the range 200 000-210 000 N/mm 2 ,
and the approximate value of 205 000 N/mm 2 is often assumed (EC3 uses
210000N/mm 2 ).Thesteelremainselasticwhileinthislinearrange,andrecovers
perfectly on unloading. The limit of the linear elastic behaviour is often closely
approximated by the yield stress f y and the corresponding yield strain ε y = f y / E .
Beyond this limit the steel flows plastically without any increase in stress until
the strain-hardening strain ε st is reached. This plastic range is usually consider-
able, and accounts for the ductility of the steel. The stress increases above the
yield stress f y when the strain-hardening strain ε st is exceeded, and this contin-
ues until the ultimate tensile stress f u is reached.After this, large local reductions
in the cross-section occur, and the load capacity decreases until tensile fracture
takes place.
Theyieldstress f y isperhapsthemostimportantstrengthcharacteristicofastruc-
turalsteel.Thisvariessignificantlywiththechemicalconstituentsofthesteel,the
most important of which are carbon and manganese, both of which increase the
yield stress. The yield stress also varies with the heat treatment used and
withtheamountofworkingwhichoccursduringtherollingprocess.Thusthinner
plateswhicharemoreworkedhavehigheryieldstressesthanthickerplatesofthe
same constituency. The yield stress is also increased by cold working. The rate
of straining affects the yield stress, and high rates of strain increase the upper or
first yield stress (see the broken line in Figure 1.6), as well as the lower yield
stress f y . The strain rates used in tests to determine the yield stress of a particular

Upper yield stress

Stress

f

u

Strain-hardening E

f

st

y

Tensile rupture

Plastic
Elastic E

Not to scale

0

y

Strain

st

Figure 1.6 Idealised stress-strain relationship for structural steel.