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When using slender beams as compression load-carrying members, one has to consider the risk of buckling failure. Apart from cross-sectional properties, length of the beam, and elastic properties, the initial lateral deflection and the boundary conditions will be important in determining the critical load, or buckling capacity. The presence of cracks may release the end constraints and drastically change the boundary conditions.
A growing fatigue crack may then lead to a gradual change from a rigid joint to a more flexible one with its lower extreme as moment-free. Thus it may be expected that the buckling strength will decrease with increasing fatigue crack size and therefore be time-dependent. Such a behavior would be of great concern in dynamically loaded structural elements containing growing fatigue cracks. This situation is highly realistic for a variety of offshore structures which are constructed as complex welded frames where high local stress levels in the nodal points in combination with dynamic loads due to current, wind, and waves may cause fatigue cracks to grow.
In order to provide some further insight into this problem, experiments and corresponding numerical analyses were performed with a Euler-III column of rectangular cross section and with surface cracks of varying length at the clamped end. The numerical results were combined with possible crack length versus load cycle relations to show the time dependence of the buckling load.
From the experiments it was concluded that the calculated elastic buckling load versus crack length relation (based on linear material behavior) represents an upper bound for the buckling load in the crack length interval considered.
buckling, cracks, fatigue, numerical analysis
Manager, Hydro Aluminum, Holmestrand,
Senior research engineer, The Swedish Plastics and Rubber Institute, Sundsvall,
Manager, Structural Engineering, Saga Petroleum, A.S., Hoevik,