Civil Engineering Reference
In-Depth Information
Deformed
test piece
F
IGURE
8.11
'Barrelling' of a mild steel test
piece in compression
and fracture loads since, due to compression, the area of cross section increases as
the load increases producing a 'barrelling' effect as shown in Fig. 8.11. This increase
in cross-sectional area tends to decrease the true stress, thereby increasing the load
resistance. Ultimately a flat disc is produced. For design purposes the ultimate stresses
of mild steel in tension and compression are assumed to be the same.
The ductility of mild steel is often an advantage in that structures fabricated from
mild steel do not generally suffer an immediate and catastrophic collapse if the yield
stress of a member is exceeded. The member will deform in such a way that loads
are redistributed to other adjacent members and at the same time will exhibit signs of
distress thereby giving a warning of a probable impending collapse.
Higher grades of steel have greater strengths thanmild steel but are not as ductile. They
also possess the same Young's modulus so that the higher stresses are accompanied
by higher strains.
Steel structures are very susceptible to rust which forms on surfaces exposed to oxygen
and moisture (air and rain) and this can seriously weaken a member as its cross-
sectional area is eaten away. Generally, exposed surfaces are protected by either
galvanizing
, in which they are given a coating of zinc, or by painting. The latter system
must be properly designed and usually involves shot blasting the steel to remove the
loose steel flakes, or millscale, produced in the hot rolling process, priming, undercoat-
ing and painting. Cold-formed sections do not suffer from millscale so that protective
treatments are more easily applied.
ALUMINIUM
Aluminium and some of its alloys are also ductile materials, although their stress-
strain curves do not have the distinct yield stress of mild steel. A typical stress-strain
curve is shown in Fig. 8.12. The points 'a' and 'b' againmark the limit of proportionality
and elastic limit, respectively, but are difficult to determine experimentally. Instead
a
proof stress
is defined which is the stress required to produce a given permanent
strain on removal of the load. In Fig. 8.12, a line drawn parallel to the linear portion
of the stress-strain curve from a strain of 0.001 (i.e. a strain of 0.1%) intersects the
stress-strain curve at the 0.1% proof stress. For elastic design this, or the 0.2% proof
stress, is taken as the working stress.
