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Steel Designers' Manual - 6th Edition (2003)
256 Fracture and fatigue
The BS 7910: 1999 document includes design curves for assessing the acceptability
of a known flaw at a given stress level. There are three levels of sophistication
in the analysis requiring more precise information about the stresses and material
properties as the assessment becomes more advanced. The procedure is very powerful
as it considers the possibility of either fracture or plastic collapse as alternative
failure processes. The first level uses a simple approach with built-in safety
factors and conservative estimates of the material fracture toughness and the
applied and residual stresses in the structure.The standard allows for toughness estimates
from Charpy impact tests to be used at Level 1. Level 2 is the normal assessment
route for steel structures and requires more accurate estimates of the stresses,
material properties and defect sizes and shapes. Level 3 is much more sophisticated
and can accommodate the tearing behaviour of ductile metals.The details of dealing
with multiple flaws, residual stresses and combinations of bending and membrane
stresses are all dealt with in the document. In many practical cases, a Level 1 assessment
is sufficient.
Annex D in BS 7910: 1999 describes the manual procedure for determining the
acceptability of a flaw in structure using the Level 1 procedure. An equivalent flaw
parameter, a,
is defined as the half length of a through-thickness flaw in an infinite
plate subjected to a remote tension loading. An equivalent tolerable flaw parameter,
am,
can then be estimated and used to represent a variety of different defect
shapes and sizes of equivalent severity:
a
m
1 Ê Kmat
ˆ
=
2p
Ë smax ¯
2
(7.6)
Equivalent part-thickness flaw dimensions can then be estimated from graphical
solutions presented in Annex D of the standard. The possibility of plastic collapse
of the cracked sectioned must be checked by calculating the ratio, known as Sr, of
a reference stress to the flow stress of the material. The flow stress is taken to be
the average of the yield and tensile strengths. The reference stress is related to the
applied and residual stresses in the structure and depends on the geometry of the
structure and the defect. Details of its calculation are given in Annex C. Provided
that the Sr parameter is less than 0.8, there is no risk of failure by collapse and any
failure will be as a result of fracture.
The parameter Kmat in Equation (7.6) is a measure of the material fracture toughness,
which may well not be valid plane strain KIc value but is appropriate to the
conditions and section sizes under review. Furthermore, Level 1 allows for the
possibility of estimating a fracture toughness value from Charpy impact data as
described in Annex E of the standard.
Tough structural steels are extremely tolerant of the presence of cracks. If one
considers a typical structural steel, with a yield stress of 275 N.mm-2 and a minimum
fracture toughness of 70 MPa÷m, subjected to a maximum allowable stress of
165 N.mm-2 , then the maximum allowable flaw from Equation (7.6) is about 60 mm
long. (Remember that am
is the half length of a through-thickness crack in an infinite
plate.) The correction for a finite width plate makes less than 10% difference