05-01-2013, 02:40 PM
Design of Steel Structures
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Fatigue of steel structures
A component or structure, which is designed to carry a single monotonically
increasing application of static load, may fracture and fail if the same load or even
smaller load is, applied cyclically a large number of times. For example a thin rod bent
back and forth beyond yielding fails after a few cycles of such repeated bending. This is
termed as the ‘fatigue failure’. Examples of structures, prone to fatigue failure, are
bridges, cranes, offshore structures and slender towers, etc., which are subjected to
cyclic loading. The fatigue failure is due to progressive propagation of flaws in steel
under cyclic loading. This is partially enhanced by the stress concentration at the tip of
such flaw or crack. As we can see from Fig. 1.22, the presence of a hole in a plate or
simply the presence of a notch in the plate has created stress concentrations at the
points ‘m’ and ‘n’. The stress at these points could be three or more times the average
applied stress. These stress concentrations may occur in the material due to some
discontinuities in the material itself. These stress concentrations are not serious when a
ductile material like steel is subjected to a static load, as the stresses redistribute
themselves to other adjacent elements within the structure.
At the time of static failure, the average stress across the entire cross section
would be the yield stress as shown in Fig.1.23. However when the load is repeatedly
applied or the load fluctuates between tension and compression, the points m, n
experience a higher range of stress reversal than the applied average stress. These
fluctuations involving higher stress ranges, cause minute cracks at these points, which
open up progressively and spread with each application of the cyclic load and ultimately
lead to rupture.
Design of Steel Structures Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar
Indian Institute of Technology Madras
Fig 1.22 Stress concentrations in the presence of notches and holes
Fig 1.23 Stress pattern at the point of static failure
Design of Steel Structures Prof. S.R.Satish Kumar
Indian Institute of Technology Madras
The fatigue failure occurs after four different stages, namely:
1. Crack initiation at points of stress concentration
2. Crack growth
3. Crack propagation
4. Final rupture
The development of fatigue crack growth and the various stages mentioned above are
symbolically represented in Fig. 1.24. Fatigue failure can be defined as the number of
cycles and hence time taken to reach a pre-defined or a threshold failure criterion.
Fatigue failures are classified into two categories namely the high cycle and low cycle
fatigue failures, depending upon the number of cycles necessary to create rupture. Low
cycle fatigue could be classified as the failures occurring in few cycles to a few tens of
thousands of cycles, normally under high stress/ strain ranges. High cycle fatigue
requires about several millions of cycles to initiate a failure. The type of cyclic stresses
applied on structural systems and the terminologies used in fatigue resistant design are
illustrated