High-resolution 3D weld toe stress analysis and ACPD method for weld toe fatigue crack initiation

Weld toe fatigue crack initiation is highly dependent on the local weld toe stress-concentrating geometry including any inherent flaws. These flaws are responsible for premature fatigue crack initiation (FCI) and must be minimised to maximise the fatigue life of a welded joint. In this work, a data-rich methodology has been developed to capture the true weld toe geometry and resulting local weld toe stress-field and relate this to the FCI life of a steel arc-welded joint. To obtain FCI lives, interrupted fatigue test was performed on the welded joint monitored by a novel multi-probe array of alternating current potential drop (ACPD) probes across the weld toe. This setup enabled the FCI sites to be located and the FCI life to be determined and gave an indication of early fatigue crack propagation rates. To understand fully the local weld toe stress-field, high-resolution (5 mu m) 3D linear-elastic finite element (FE) models were generated from X-ray micro-computed tomography (mu-CT) of each weld toe after fatigue testing. From these models, approximately 202 stress concentration factors (SCFs) were computed for every 1 mm of weld toe. These two novel methodologies successfully link to provide an assessment of the weld quality and this is correlated with the fatigue performance

Magnetic properties of silicon steel after plastic deformation

The energy efficiency of electric machines can be improved by optimizing their manufacturing process. During the manufacturing of ferromagnetic cores, silicon steel sheets are cut and stacked. This process introduces large stresses near cutting edges. The steel near cutting edges is in a plastically deformed stress state without external mechanical load. The magnetic properties of the steel in this stress state are investigated using a custom magnetomechanical measurement setup, stress strain measurements, electrical resistance measurements, and transmission electron microscopic (TEM) measurements. Analysis of the core energy losses is done by means of the loss separation technique. The silicon steel used in this paper is non-grain oriented (NGO) steel grade M270-35A. Three differently cut sets of M270-35A are investigated, which differ in the direction they are cut with respect to the rolling direction. The effect of sample deformation was measured—both before and after mechanical load release—on the magnetization curve and total core energy losses. It is known that the magnetic properties dramatically degrade with increasing sample deformation under mechanical load. In this paper, it was found that when the mechanical load is released, the magnetic properties degrade even further. Loss separation analysis has shown that the hysteresis loss is the main contributor to the additional core losses due to sample deformation. Releasing the mechanical load increased the hysteresis loss up to 270% at 10.4% pre-release strain. At this level of strain, the relative magnetic permeability decreased up to 45% after mechanical load release. Manufacturing processes that introduce plastic deformation are detrimental to the local magnetic material properties

Calibration and validation of extended back-face strain compliance for a wide range of crack lengths in SENB-4P specimens

Compliance equations based on back-face strain or crack mouth opening displacement, and potential drop techniques are widely used to calculate fatigue crack growth rate. Standard ASTM E647 for fatigue crack growth rate testing does not include compliance based equations for single edge notched four-point bending (SENB-4P) specimens. Equations developed based on finite element (FE) analysis have been reported in literature; however, they are limited to the long crack propagation regime (i.e., relative crack length ratios a/W > 0.15). No compliance relations for crack growth in the physically short crack regime were found in literature. This work reports on a complementary numerical-experimental study towards the development of compliance equations with an extended application range in terms of relative crack length. FE simulations have been used to calibrate a back-face strain compliance equation for calculation of relative crack length in the range 0.05 ≤ a/W ≤ 0.5 for SENB-4P specimens. Four point bending fatigue tests were performed on SENB specimens extracted from 50 mm thick welded steel plates. Direct current potential drop (DCPD) for crack monitoring was used as benchmark and validation of the crack lengths determined from a back-face strain gage

Front face strain compliance for quantification of short crack growth in fatigue testing

Short fatigue crack growth investigation is of considerable scientific interest as it comprises a significant portion of the total fatigue life of a structure. It is very challenging to accurately quantify this stage of fatigue crack growth experimentally. In this article, a novel front face strain compliance technique for single-edge notched specimens subjected to four-point bending is proposed. Finite element analysis is performed to determine the correlation between crack length and strain change near the crack. This relationship is then validated by experiments in which strains are measured by strain gauges attached near the short crack, and crack length is quantified by examining beachmark lines at the fracture surfaces. Based on the numerical and experimental results, it is concluded that the strain measured near the notch allows quantifying short crack growth for normalised crack lengths in the range 0.01 <= a/W <= 0.06 (a/W being the ratio of crack length over specimen width). A compliance equation based on the front face strain is finally presented

Test methods for corrosion-fatigue of offshore structures

The increasing demand for renewable energy, necessitates offshore wind turbine structures to be installed in deeper waters and more remote areas. They are subjected to challenging conditions, i.e., the combination of cyclic loads from wind, waves, and currents, and the corrosive nature of the seawater environment. The literature lacks experimental data on the corrosion-fatigue of structural steels, especially concerning the short crack propagation regime. This study involves developing test methods and equipment for corrosion-fatigue testing of welded structural steel and quantifying the propagation rate of short cracks (starting from corrosion pits). The developed models (calibrated by the experiments) allow accurate prediction of the remaining lifetime of steel structures exposed to corrosion damage

High-resolution 3D weld toe stress analysis and ACPD method for weld toe fatigue crack initiation

Weld toe fatigue crack initiation is highly dependent on the local weld toe stress-concentrating geometry including any inherent flaws. These flaws are responsible for premature fatigue crack initiation (FCI) and must be minimised to maximise the fatigue life of a welded joint. In this work, a data-rich methodology has been developed to capture the true weld toe geometry and resulting local weld toe stress-field and relate this to the FCI life of a steel arc-welded joint. To obtain FCI lives, interrupted fatigue test was performed on the welded joint monitored by a novel multi-probe array of alternating-current potential drop (ACPD) probes across the weld toe. This set-up enabled the FCI sites to be located, the FCI life to be determined and gave an indication of early fatigue crack propagation rates. To understand fully the local weld toe stress-field, high-resolution (5 μm) 3D linear-elastic finite-element (FE) models were generated from X-ray Micro-Computed Tomography (μ-CT) of each weld toe after fatigue testing. From these models, approximately 202 stress concentration factors (SCFs) were computed for every 1 mm of weld toe. These two novel methodologies successfully link to provide an assessment of the weld quality and this is correlated with the fatigue performance

Pitting corrosion and its transition to crack in offshore wind turbine supporting structures

Offshore wind turbine support structures undergo pitting corrosion due to the marine environment. Besides, these structures are subject to fatigue loads due to wind and waves. The corrosion-fatigue phenomenon is considered as one of the most dangerous damage mechanisms for offshore structures. Corrosion pits attract stress which is why they are prone to turn into the crack(s). The main objective of this work is to implement computational studies on pitting corrosion and pit-to-crack transition. For pitting corrosion simulations, the phase-field modeling method is chosen. On the other hand, in order to predict the potential location of crack initiation, the stress concentration factor (SCF) concept is used for which a linear elasto-static stress analysis is implemented using the finite element method

A Numerical Investigation on the Pitting Corrosion in Offshore Wind Turbine Substructures

Pitting corrosion is a common cause of concern for steel structures in an offshore environment. As geometric stress concentrating features, corrosion pits can potentially act as fatigue crack initiation sites. The current study is a part of the MAXWind project which, amongst others, aims to develop numerical tools for a more accurate estimate of the remaining lifetime of in-service wind turbines. UGent is responsible for developing an advanced corrosion-fatigue model which will be used to build “smart S-N curves”. The smart S-N curve is a novel concept that takes the level of corrosion into account. To this end, the entire evolution of corrosion fatigue is divided into three major phases including pitting corrosion, short fatigue crack propagation, and long fatigue crack propagation, see Figure 1. The main focus of this work is on pitting corrosion and its transition to short fatigue crack propagation. A phase-field modelling approach [1],[2] is used to simulate the autonomous growth of a corrosion pit. The corrosion phenomenon - pitting corrosion in particular – is a complex electrochemical process that is influenced by various environmental factors such as temperature, dissolved oxygen, pH, salinity, etc. [3]. Phase-field modelling is a robust technique that is capable to incorporate a vast range of influential parameters. In essence, in a phase-field model, each phase (here, metal and electrolyte) possesses a constant value in the bulk (0 for Pitting corrosion model transition transition Integrated model Experimental validation Short fatigue crack model Long fatigue crack model Phase-field modeling approach NR model X-FEM 18 th EAWE PhD Seminar on Wind Energy 2 – 4 November 2022 Bruges, Belgium electrolyte and 1 for steel), with a continuous interpolation between the bulk values across the interface between phases. The evolution of the system is a result of constrained minimization of free energy for which the advective Cahn-Hilliard equation is used. The Nernst-Planck equation is used to describe the diffusion of ions within the electrolyte, and the Butler-Volmer-type kinetic expression is used to calculate the reaction current density throughout the process. For more information and formulations see [1]. First, an electrochemical characterization was performed for structural steel grade S355 in an environment that is representative of the North Sea. This study is crucial to evaluate the electrochemical behaviour of this steel grade and will support further studies towards predicting pit dimensions in offshore wind turbine support structures in the North Sea. To this end, potentiodynamic polarization tests were implemented for S355 steel. An Ag/AgCl electrode was used as reference electrode in the tests and potential values are obtained against this electrode. The corresponding corrosion potential and current density were obtained as -711 mV vs. Ag/AgCl and 0.1534 A/m2 , respectively. For a metal, the more negative the corrosion potential is, the more susceptible it will be to corrosion [4]. In practice, corrosion protection systems and coatings will be applied to the metal structure, which will increase the value of corrosion potential [5]. Using the output of the experiments as input to the phase-field model, a parametric study was performed to assess the effect of the applied potential on geometrical parameters (pit width and depth) and electrochemical parameters associated with pit growth rate (metal cation concentration and reaction current density), see Figure 2. It was found that for an applied potential of -600 mV vs. Ag/AgCl, the corrosion process stays in the activation-controlled regime throughout the simulation time (150 seconds). Applied potentials of -550 to -500 mV vs. Ag/AgCl take the system to the current-resistance-controlled regime where the metal cation concentration does not reach the saturation. The higher the applied potential is, the more pitting corrosion is accelerated until it reaches to the point where any additional increase in applied potential will not have any 18 th EAWE PhD Seminar on Wind Energy 2 – 4 November 2022 Bruges, Belgium additional influence on pit growth rate. As it is easier for the metal ions to diffuse into the bulk electrolyte near the pit mouth in bare steel, pit width increases with a higher velocity in comparison to the pit depth. All numerical results will be validated with dedicated experiments. Any changes in temperature will cause changes in electrochemical parameters such as ionic diffusivity, corrosion potential and corrosion current density. As future work, a parametric study will be conducted on the effect of temperature on the corrosion pit growth rate. Ultimately, pit dimensions extracted at every time step will serve as input to a short fatigue crack propagation model [7]. Once a corrosion pit nucleates, the local stress in the material increases at the discontinuity. Therefore, in parallel to the pitting corrosion study, a finite element analysis was performed to assess the stress concentration factor (SCF) of corrosion pits and to identify the effect of different normalized geometrical parameters on SCF, see Figure 3. According to [8], a crack will initiate from a pit under the following conditions: (a) the pit size must pass a critical value, and (b) the crack growth rate must be higher than the pit growth rate. The concept of SCF can assist to address the first criterion. A pit becomes susceptible to transit to a crack as soon as it causes a considerable increase in the local stress. Results show that an increase in a/2c or b/c leads to enhanced SCF when the loading is oriented parallel to the pit’s major axis (i.e., = 0), see Figure 4 (a) and (b)

Fatigue strength degradation of structural steel in sea environment due to pitting corrosion

Steel support structures of offshore wind turbines (jackets and monopiles) undergo both fatigue and corrosion damage, impacting their lifetime. Due to the time‐variant uncertainties associated with environmental and mechanical loads, having reliable models that allow prediction of the degradation due to corrosion and fatigue is necessary to accurately assess the structural integrity and to support decision-making. This work investigates how pitting corrosion, caused by being exposed to the marine environment, affects the fatigue strength of structural steel. A short crack model is used to estimate the minimum required applied load amplitude which causes a growing crack emanating from the bottom of a semi-elliptical pit. The modeling results show the fatigue strength degradation as a function of the exposure time to the corrosive environment. As exposure time increases, it is observed that degradation happens more quickly in the early years followed by a convergence of the fatigue strength to a minimum value. ALso, a parametric study is done to see the effect of the pit size and sharpness of the degradation of the fatigue strength. It illustrates that for a specific pit sharpness by increasing the pit size the fatgiue strength decreases sharply at first and then tends to converge to a specific value which depends on the sharpness of the pit