Towards the understanding of variable amplitude fatigue

Fatigue life estimation is of high importance during the design stage of a machine or component. Basic fatigue calculations are made based on the use of an S-N curve. As far as constant amplitude loads are applied, this approach works well. However, most part of components in service are subjected to a variable amplitude load spectrum. In this case, linear approaches for fatigue life estimation can lead to over conservative results, which in other words means a heavier and more expensive machine. To further investigate the effect of (complex) service spectra (measured or statistically calculated), simpler load variations must be studied. This paper aims to show the general trend of these events and suggest the underlying physical phenomena behind load and interaction effects. As it will be highlighted, overloads are frequent in a spectrum and they are believed to be responsible for retardation effects. The plasticity induced crack closure mechanism is the most profound explanation for them to occur

Testing methodologies for corrosion fatigue

Offshore constructions are subjected to cyclic loading conditions. This situation is combined with the corrosive nature of the surrounding environment. It is of actual concern whether the combined effect is more damaging or not than the superposition of each effect independently. This literature review first introduces the reader to corrosion fatigue. Thereafter a critical comparison of some typical lab-scale fatigue corrosion test setups is given. Special emphasis is devoted to the instrumentation of the setup. This is followed by a design criteria summary which will be used to design a new corrosion fatigue test set-up for evaluating the fatigue properties of steel components in sea water environment

A comparison of fatigue lifetime prediction models applied to variable amplitude loading

The loads imposed on e.g. offshore structures can vary considerably with time. Lifetime prediction methodologies need to consider possible acceleration and retardation of the crack growth rate due to load sequences. Models based on a linear accumulation of damage will have a limited accuracy and are not considered as a valuable asset in lifetime prediction of structures subjected to variable amplitude loading. This necessitates more complex nonlinear damage evolution models that can be applied in a so-called cycle-by-cycle analysis. In this paper, a comparison is made between three cumulative damage models (Miner, modified Miner and weighted average) and two yield zone models (Wheeler and Willenborg). Experimental data of fatigue crack growth in offshore steel subjected to sequential loading is used as basis of the comparison. The modified Miner model is the most promising of the cumulative damage models but the determination of the parameter α requires laboratory tests. Evaluation of the effects of variation in the model input parameters on estimated lifetime reveals a large influence for the Miner and weighted average approaches

Online fatigue crack growth monitoring with clip gauge and direct current potential drop

Fatigue is a well-known failure phenomenon which has been and still is extensively studied. Often structures are designed according to the safe-life principle so no crack initiation occurs. Nowadays there is a high emphasis on cost-efficiency, and one might rather opt for a fail-safe design. Therefore a certain amount of crack growth can be allowed in structures, but then a good knowledge of stresses and related crack growth rates is needed. To this end, extensive studies are done to obtain a material’s Paris law curve. Within the framework of research for offshore wind turbine constructions, tests were done to determine the crack growth rate of a high strength low alloy (HSLA) steel. A dedicated LabVIEW program was developed to be able to determine an entire Paris law curve with a single specimen, by controlling the stress intensity factor range (ΔK). The program is controlled by the readings of a clip gauge, which make it possible to plan the amount of crack growth per ΔK block and thus plan an entire test in advance. The potential drop technique was also applied in order to obtain the Paris law curve. Clip gauge results were compared with direct current potential drop monitoring. This comparison was done by means of an a/W-N diagram and the resulting Paris law curves. The results show a very good correlation between both methods and with the visual confirmation

Accelerating corrosion in a laboratory set-up for corrosion-fatigue of offshore steels

Corrosion-fatigue is a dangerous failure mechanism that is not yet fully understood. Structures subjected to corrosion-fatigue are over conservative in design, which is economically unfavourable. To counter this, representative laboratory experiments simulating the corrosion-fatigue conditions of an offshore structure should be performed. Lab testing is, for obvious reasons, performed at frequencies much higher than these of wave and wind actions. However, this means that corrosion needs to be accelerated in the same manner. In this work two different ways to accelerate corrosion were selected, namely temperature and oxygen content adaptation. S-N curves were determined in different test conditions in order to evaluate the damage evolution. It has been found that high temperatures and high levels of oxygen content will result in earlier failure. The fracture surfaces are somewhat different than fracture surfaces obtained due to fatigue in air. More crack initiation sites can be observed and the fracture surface is generally rougher due to corrosion

Effects of variable amplitude loading on fatigue life

This paper is a review of published research on variable amplitude loading of steels. The use of service spectra for different industrial sectors and specifically for offshore applications is first considered. Constant amplitude fatigue failure models are not representative for these applications. The JONSWAP spectrum shows potential to be used as service spectrum for offshore structures. Further investigation of variable amplitude fatigue is needed to get insight in the various phenomena linked to the variable amplitude. Observed trends in fatigue crack growth rate in variable amplitude fatigue tests on steels, such as the effects of overloads and underloads (occurring as single events, sequential events or block loadings), are discussed. Furthermore, suggestions of the underlying physical phenomena behind the load interaction effects due to variable amplitude loading are presented. It can be concluded that the plasticity induced crack closure mechanism is the most profound explanation for the acceleration effect in overloads and the retardation effect observed in underloads

Cycle-by-cycle simulation of variable amplitude fatigue crack propagation

In variable amplitude fatigue of high strength low alloy (HSLA) steel components, overloads can severely retard subsequent crack propagation for a number of cycles. In order to be able to predict fatigue crack propagation with a reduced degree of conservatism, retardation has to be taken into account. Of all numerical models that have been developed over time, crack tip plasticity models are selected based on the need for a detailed and fast cycle-by-cycle simulation of high cycle. After introducing the load interaction zone concept, common to all crack tip plasticity models, the Wheeler and Willenborg models are discussed, implemented and compared to experimental data. It is concluded that the Modified Wheeler model provides the most promising results, whereas the main limitation of Willenborg models is the need for extensive experimental data