Page 386 - Forensic Structural Engineering Handbook

STEEL STRUCTURES 11.17

and design stresses below this level are assumed to preclude fatigue failure. Axial fatigue
tests of this type are covered by ASTM Specification E466. For complex structures, how-
ever, these tests are not particularly helpful because (1) structures are rarely flaw-free since
fabrication operations often create small but significant flaws, and (2) if welding processes
are used, welding creates both high local residual stress levels and microscopic flaws at the
welds. The effect of these conditions is to eliminate the crack initiation portion of this clas-
sical S/N curve so that only the crack growth portion of the curve applies. This renders the
classical S/N curve a poor predictor of the fatigue behavior of structures. An alternate pro-
cedure for predicting fatigue life in structures using fracture mechanics characterizations
provides a more effective approach to both failure analysis and failure avoidance. This pro-
cedure relates the service cyclic stress range in the structure to the stress intensity range,
ΔK, in the vicinity of the flaws or discontinuities and allows prediction of the rate of flaw
growth at any point in the life of the structure. With this information and knowledge of the
material K , the time at which the growing flaw becomes or will become critical to the
C
integrity of the structure can be predicted. Knowledge of the fracture mechanics crack
growth properties of the material, as well as its K can be a powerful tool in both failure
C
analysis and failure prevention for live loaded structures. The methodology used in these
calculations is also found in Ref. 8 and is based on the following relationship:
da m
AK)
= ( Δ (11.3)
dN
where a = the crack length
N = the number of cycles
ΔK = the stress intensity factor range
A = the material constant
m = the environmental constant obtained from test data.
The units of a and K are as indicated above.
An early application of the fracture mechanics approach to bridges is found in the
AASHTO bridge code, where the stress ranges applied to typical connection details are cor-
related to fatigue lives, assuming the structure is welded or bolted and the fabrication and
inspection procedures have eliminated all macroscopic flaws. This approach has since found
wide application to the design and analysis of other large welded steel structures. Most U.S.
codes and standards, as well as many international codes and standards, have adopted simi-
lar procedures wherein the cyclic stress range is correlated to the fatigue life of typical
welded design details. Examples of this can be found in British Standards BS 5400, BS 7608,
and BS 7910, API-579/ASME FFS-1, AWS D1.1, Eurocodes, etc.

Corrosion and Deterioration
Of all the material properties that relate to service behavior and failure, perhaps the most
difficult to characterize and predict is corrosion resistance. This is primarily due to the fact
that corrosion processes are complex chemical reactions that are sensitive to small changes
in the service environment, the chemical composition of the material, stress, electric cur-
rents, the combinations of materials present, material strength, geometric configuration,
and other factors. Thus, it is not surprising that there are myriad ASTM specifications cov-
ering general corrosion, stress corrosion cracking, electrochemical corrosion measure-
ments, seawater corrosion, and intergranular corrosion, to name but a few. Failure analysis
of corrosion controlled or assisted failures is a complex engineering science, which usually
necessitates familiarity with, or experience in, corrosion engineering. A good general ref-
erence in this field is Ref. 16.