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Welding handbook 8.0 Fatique in welded joints

8.0 Fatigue in welded joints
©SSAB

8.1 Introduction 8.2 Fatigue load

Fatigue is a very important consideration for welded Fig. 8.1 shows one loading cycle with different stress
structures that are subjected to variable loads. Up to definitions needed to analyze the fatigue load.
90% of all failures in welded structures are believed to
be related to fatigue. The importance is even greater
when using high strength steel because the fatigue
strength of a welded joint in “as welded” condition is
the same, independent of the steel grade. This can lead For welded joints use stress range as design criteria
max stress σ
to the misunderstanding that there is no advantage Fatique max
to utilizing high strength steel in structures subjected Stress load stress range σ max
to fatigue. For structures with a low fatigue load, for amplitude σ
example, a low number of cycles and/or a lean load mean stress σ mean
spectrum, or if the structure is post weld treated with 0
Stress
e.g. High frequency Mechanical Impact at highly amplitude σ
min stress σ min
stressed areas, high strength steels are advantageous.
Time
Fatigue is a process whereby a component is subject
to a variable load, which is lower than the static
strength of the material. The material starts to Fig. 8.1: Stress parameters for one loading cycle.
deteriorate and finally a crack is created. The final
stage is a complete failure of the structure. Often,
long before the failure, the component has lost its To characterize a fatigue load, two quantities are
function. needed. For fatigue in welded joints, the stress range
Δσ and the stress ratio, R, are usually used.
The fatigue strength of welded joints is a
multi-discipline subject. It involves design, Normally, a structure is not subjected to constant
dimensioning, production and quality. The general load, called constant amplitude (CA). Instead, the
design is very important for the component’s fatigue amplitude varies with the time, called variable
life, such as, where and which kind of welded joints amplitude (VA). Most of the fatigue strength data is
are used. Getting the appropriate and/or highest tested in CA. To do the dimensioning, the given data
fatigue strength depends on good weld quality, in CA is compared with external load in VA. This is
which in turn, depends on the welding engineer, the a very complex task and experience is often the key
welder and the quality control personnel. It is also to success.
important not to add any additional welded joints,
hooks, etc., after the design has been finalized.

The purpose of this chapter is to give general
information regarding fatigue of welded joints,
not to cover the entire field of fatigue.

This handbook contains general suggestions and information without any expressed or implied warranty of any kind. SSAB hereby expressly disclaims
all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications. 47
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.