Fatigue

Author: Fatigue in steel structures

Fatigue

Fatigue is the mechanism whereby cracks grow in a structure. Growth only occurs under fluctuating stress. Final failure generally occurs in regions of tensile stress when the reduced cross-section becomes insufficient to carry the peak load without rupture. Whilst the loading on the structure is stationary the crack does not grow under normal service temperatures. Many structures, such as building frames, do not experience sufficient fluctuating stress to give rise to fatigue problems. Others do, such as bridges, cranes, and offshore structures, where the live loading is a higher proportion of the total load.

In welded steel structures, fatigue cracks will almost certainly start to grow from welds, rather than other details, because:

• Most welding processes leave minute metallurgical discontinuities from which cracks may grow. As a result, the initiation period, which is normally needed to start a crack in plain wrought material, is either very short or no-existent. Cracks therefore spend most of their life propagating, i.e. getting longer.
• Most structural welds have a rough profile. Sharp changes of direction generally occur at the toes of butt welds and at the toes and roots of fillet welds, see Figure 1. These points cause local stress concentrations of the type shown in Figure 2. Small discontinuities close to these points will therefore react as though they are in a more highly stressed member and grow faster.

The fatigue strength of a welded component is defined as the stress range which fluctuating at constant amplitude, causes failure of the component after a specified number of cycles. The stress range is the difference between the maximum and minimum points in the cycle. The number of cycles to failure is known as the endurance or fatigue life.

Fatigue and Eurocode 3

The main provisions of Eurocode 3 rely upon a set of fatigue resistance curves, equally spaced, upon which are classified a set of constructional details. The concept for fatigue strength design follows the Recommendations of the European Convention for Constructional Steelwork (ECCS). The recommendations define a set of equally spaced fatigue strength curves with a constant slope of m = 3 (for normal stress), or m = 5 (for shear stress, hollow section joints, and some particular details). In addition to this approach another concept supported mainly by recent developments and research in the field of fatigue for "offshore" structures is referred to in Eurocode 3 as the geometrical stress concentration concept (also called the "hot spot stress" method). To determine the fatigue strength provisions given in Eurocode 3, a compilation of fatigue data of various sources was carried out. This work has provided an opportunity to re-evaluate existing fatigue test data and allowed for a more consistent approach to the classification of detail categories.

Fatigue of steel structural components, especially welded steel details, is a particularly complex problem, and many factors may exert an influence on the fatigue life. In Eurocode 3, the fatigue strength refers to the complete failure of the structural element. This condition corresponds, usually, to the criterion generally adopted by structural laboratories or reported in literature.

Local stress concentrations are taken into account in an implicit manner in the derivation of the S-N curve from fatigue test results. Great care must be taken when assessing fatigue strength from tests on small scale specimens instead of large scale specimens. The scale effect due to weld geometry may have a greater influence on the fatigue strength in small test specimens than in large test specimens.

When assessing the fatigue strength by the so-called geometric stress range method, according to Eurocode 3, the geometric stress concentration as defined by equation  must be properly evaluated. The local geometry of the weld must not be taken into account in the calculation procedure of the design stress range, since the local discontinuity effect is already introduced in the derivation of the S-N curves. However, when determining the design stress, secondary stresses arising from joint eccentricity or due to joint stiffness, stress redistribution due to buckling or shear lag, and effects such as prying action, should be taken into account.

Each individual fatigue strength curve is defined in a conventional way by a slope constant of m = 3 (slope = -1/3). The constant amplitude limit is set at 5 million cycles. The slope constant m = 3 was a best fit for a large number of different structural details tested in fatigue. The figure of 5 million cycles for the constant amplitude fatigue limit is a compromise between 2 million cycles for "good" details and 10 million cycles for details which create a severe notch effect. For any stress range of constant amplitude below this limit, no fatigue damage is expected to occur.

Fatigue depends on the whole service loading sequence (not one extreme load event). Fatigue of welds is not improved by better mechanical properties. Fatigue is very sensitive to the geometry of details. Fatigue requires more accurate prediction of elastic stress. Fatigue makes more demands on workmanship and inspection.