Variable amplitude loading

Harbour, R.J., Fatemi, A., and Mars, W.V., Fatigue crack growth of filled rubber under constant and variable amplitude loading conditions. Fat Fract. Eng. Mat. Struct, 30, 640, 2007. 

To investigate the control factors of welded component s fatigue behaviour, and use the analytical methods for estimating the total fatigue life of welds subjected to variable-amplitude loading histories and surface treatments, in order to find some possible methods to improve the fatigue strength. 

Stresses. The main mechanical properties to consider are maximum stress or stress intensity factor, <7m ix or Kmax, cyclic stress or stress-intensity range, Act or AK, stress ratio R, cyclic loading frequency, cyclic load waveform (constant-amplitude loading), load interactions in variable-amplitude loading, state of stress, residual stress, and crack size and shape, and their relation to component size geometry.31... 

ALTERNATIVE FATIGUE FORMULATIONS FOR VARIABLE AMPLITUDE LOADING OF FIBRE COMPOSITES FOR WIND TURBINE ROTOR BLADES... 

Fatigue, variable amplitude loading, composites, lifetime prediction. 

However, there are indications that the commonly used lifetime prediction methods can lead to inaccurate results for the composite blade s lifetime under the strongly varying loads to which they are subjected. The present paper presents the preliminaries to an extensive study into the fatigue behaviour of wind turbine rotor blade materials under variable amplitude loading. This paper will discuss some of the problems and drawbacks of the currently prescribed lifetime prediction methods, and describes possible ways of tackling these. 

First, the most important loads on wind turbine rotor blades are described and a brief review of fatigue research in composites is presented to place the following approach in a broader perspective. The approach described in this paper concerns modification of the existing formulations. Possible modifications are illustrated with predictions for an existing data set, which was generated by subjecting composite specimens to different variable amplitude load sequences. 

As has been indicated in the previous chapter, quite sophisticated lifetime prediction algorithms are continuously being developed. Unfortunately, no appropriate validation has been performed for any universally applicable theory. Therefore, current guidelines for lifetime prediction methods for variable amplitude loading of composites lag the intricate modeling efforts described in the literature, and resort to the classical and well-known method. This is referred to as the baseline method. 

Alternative Fatigue Formulations for Variable Amplitude Loading... 

Another fatigue testing related consideration is that in an ideal test dK/da = 0, in other words a constant amplitude loading is maintained. A real structure, however, experiences a spectrum of stresses in service, which can result in overloads. A detailed analysis of the subject of variable amplitude loading and the associated concepts are covered by Anderson [16]. 

H Nowak, K Schulte and G Liitjering, Microscopical and mechanical contributions to the cycle by cycle crack growth under variable amplitude loading . Fatigue Crack Growth Under Variable Amplitude Loading, London, Elsevier Applied Science, 1988, pp 109-133. 

Khoramishad H, Crocombe A D, Katnam K B, Ashcroft 1 A (2010), Fatigue damage modelling of adhesively bonded joints imder variable amplitude loading using a cohesive zone model, Engineering Fracture Mechanics, 78, 3212-3225. 

Fatigue (Structural) The progressive localized permanent structural change that occurs in a material subjected to constant or variable amplitude loads at stresses having a maximum value less than the ultimate strength of the material. 

Fatigue assessment with variable amplitude loading... 

Variable amplitude loadings are generally divided in two types ... 

Naumann, E.C., Hardrath, H.R., Guthrie, E.C. Axial load fatigue tests of 2024-T3 and 7075-T6 aluminum alloy sheet specimens under constant and variable-amplitude-loads. In Report TN D-212, NASA (1959)... 

Zhang, S., Marissen, R., Shulte, K., Trautmann, K.K., Nowak, H., Schijve, J. Crack propagation studies on Al 7475 on the basis of constant amplitude and selective variable amplitude loading histories. Fatigue Fract. Eng. Mater. Struct. 10(4), 315-332 (1987)... 

Barsom, J.M. Fatigue crack growth under variable amplitude loading in ASTM A514 grade B steel. ASTM STP 536, 147-167 (1973)... 

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