Uniaxial and multiaxial fatigue behaviour of wire arc additively manufactured ER70S-6 low carbon steel components

Wire arc additive manufacturing (WAAM), also known as directed energy deposition (DED) process, is an efficient additive manufacturing technology, offers high potential to rapidly fabricate large-scale parts with complex geometries layer-by-layer. However, the fundamental understanding of the fatigue behaviour of such parts and the material requirements need to be significantly improved at all levels before this unique technology can be implemented for critical applications. This work aims to investigate the fatigue behaviour of WAAM built ER70S-6 steel under uniaxial, torsion and multiaxial loading conditions. Specimens were extracted in two different orientations: vertical and horizontal, to explore if the orientation direction has any effect on the fatigue results. Scanning Electron Microscopy (SEM) was conducted to examine the fracture surface of broken specimens and identify crack initiation regions and fracture mechanisms. The obtained results were compared with the fatigue data available in the literature on common structural steels fabricated using conventional welding and WAAM technique, showing similar fatigue behaviour with wrought S355 specimens. Moreover, the uniaxial data set on ER70S-6 WAAM specimens was evaluated according to the DNV RP-C203 standard for continuous welds, demonstrating advantageous fatigue resistance in the examined material. © 2022 The Author(s

Fatigue crack growth under remote and local compression – a state-of-the-art review

There is an ever increasing need for accurate understanding of the fatigue crack growth behaviour in major engineering materials and components. With the move towards more complex, probabilistic assessments, the traditional ‘safe’ or conservative approach for prediction of fatigue crack growth rate may no longer be attractive. Current codes and standards tend to be ambiguous about the treatment of compressive stress cycles: on the one hand code guidance on fatigue crack initiation may be non-conservative, while assessment of crack propagation may be inconsistently conservative. Where codes are non-conservative they could lead to dangerous assessments. The current paper provides a critical review of state-of-the-art in literature and a study of current code implications

Experimental and numerical investigation of the weld geometry effects on Type IV cracking behaviour in P91 steel

The focus of the present study is on creep crack growth behaviour in Type IV region of P91 steel weldments at 650 °C. In the experimental studies on small- and large-scale single-edge notched specimens in tension, SENT, the effects of weld dimensions and specimen size on the creep crack growth behaviour of the material are investigated. The experimental results demonstrate that the crack starts to propagate at an angle normal to the loading direction, subsequently deviates towards the Type IV region and the specimen eventually ruptures when the crack growth angle becomes parallel to the loading direction. The creep rupture data for SENT specimens compared well with those of the round bar specimens for P91 welded joints. In addition, the data for crack growth rates from the deviating crack path were correlated with the C* fracture mechanics parameter and showed good agreement with standard compact tension test data. To predict the creep crack growth behaviour in the Type IV region, finite element simulations were performed in conjunction with a multiaxial ductility damage criterion at the weld/base metal interface. Given that a lower failure strain along the Type IV region is prominent, it is shown that the cracking, in line with the experiments, followed the HAZ region and led to the final creep rupture in the net sectio

A review of present status and challenges of using additive manufacturing technology for offshore wind applications

Offshore wind is an efficient sustainable source of energy, which is a preferable alternative to burning fossil fuels in Europe and worldwide. About 85% of existing offshore wind turbines are supported using monopile foundations, which are made of large welded plates. The locked in residual stresses in a monopile structure have a great impact on its fatigue life. The new emerged technology of additive manufacturing (AM), which is widely used in other industries such as aerospace and automotive, has the potential to significantly improve a lifespan of the structure by managing the residual stress fields and microstructure in the future monopiles, and moreover reduce the manufacturing cost. In order to achieve this goal, new materials that are used for additive manufacturing parts fabrication and their behaviour in the harsh marine environment and under operational loading conditions need to be understood. Also purely welding fabrication technique employed during AM process is likely to significantly affect crack growth behaviour in air as well as in seawater. This paper presents a review of additive manufacturing technology and suitable techniques for offshore structures. Existing literature that reports current data on fracture toughness and fatigue crack growth tests conducted on AM parts is summarised and analysed, highlighting different steel grades and applications, with the view to illustrating the requirements for the new optimised functionally graded structures in offshore wind structures by means of AM technique

Shot peening effects on residual stresses redistribution of offshore wind monopile multi-pass weldments

In some industrial applications, welding is the only alternative to join different parts. However, the major problem in welded structures is the tensile residual stresses that are inevitably produced during welding process. Surface tensile stresses can threaten the performance of the weldments as they act as an accelerant in fatigue crack initiation and failure. Shot peening is a well-known method which can enhance weldments' fatigue performance and longevity by inducing surface compressive residual stresses in order to eliminate or limit tensile residual stresses. In this study, the effect of shot peening on redistribution of multi-pass welding residual stresses was numerically and experimentally investigated on very large components typical of welded joints used in offshore wind turbine monopiles. In experimental part of the study, the residual stresses were measured using the Incremental Centre Hole Drilling (ICHD) method before and after shot peening. Finite element studies were carried out using 3D welding models and random shot peening analyses. Moreover, extensive finite element analyses were conducted to study the effect of model dimensions and the number of passes on prediction of welding residual stresses. Interesting set of results obtained from both the numerical studies and the ICHD measurements were in good agreements and showed that shot peening can be advantageous even for large components with multi-pass welded joints. Additionally, reducing the number of weld passes in finite element models could considerably lower the computational time without affecting the accuracy of results at surface regions of the models

Shot peening effects on residual stresses redistribution of offshore wind monopile multi-pass weldments

In some industrial applications, welding is the only alternative to join different parts. However, the major problem in welded structures is the tensile residual stresses that are inevitably produced during welding process. Surface tensile stresses can threaten the performance of the weldments as they act as an accelerant in fatigue crack initiation and failure. Shot peening is a well-known method which can enhance weldments' fatigue performance and longevity by inducing surface compressive residual stresses in order to eliminate or limit tensile residual stresses.In this study, the effect of shot peening on redistribution of multi-pass welding residual stresses was numerically and experimentally investigated on very large components typical of welded joints used in offshore wind turbine monopiles. In experimental part of the study, the residual stresses were measured using the Incremental Centre Hole Drilling (ICHD) method before and after shot peening. Finite element studies were carried out using 3D welding models and random shot peening analyses. Moreover, extensive finite element analyses were conducted to study the effect of model dimensions and the number of passes on prediction of welding residual stresses. Interesting set of results obtained from both the numerical studies and the ICHD measurements were in good agreements and showed that shot peening can be advantageous even for large components with multi-pass welded joints. Additionally, reducing the number of weld passes in finite element models could considerably lower the computational time without affecting the accuracy of results at surface regions of the models

Damage Analysis of Ship Collisions with Offshore Wind Turbine Foundations

Nowadays, a large number of wind turbines are being installed offshore due to more stable and steady flow of wind at sea and also less noise and visual impact compared to onshore wind farms. With the growing number of offshore wind installations, particular attention should be paid to the safe operation of assets. Offshore wind assets are subject to extreme environmental conditions and high dynamic stresses caused by wind, waves and currents. More importantly, they are largely exposed to hazards associated with collision with either commercial ships or infield support vessels passing closely at high speeds. To date, the damage analysis of collisions between infield support vessels and offshore wind turbine foundations has received very limited attention. In this study, a numerical nonlinear finite element analysis (NLFEA) approach is developed to evaluate the damage to wind turbine foundations when stricken by an offshore support vessel. The model is applied to a case study where 4000 tons class vessels collide with two common types of fixed-bottom foundations, namely monopile and jacket structure in shallow and deep waters respectively. Various accident scenarios are identified and the resulting damage to wind turbine foundations are analyzed. The number, location and the extent of damage to the members in each scenario are determined and the effects of reinforcement on the structure response are evaluated. The results of this research provide a good understanding of the factors that affect magnitude of damage caused by ship-wind turbine collision accidents and give an insight on how the next generation of wind turbine foundations can be designed in a more “collision-friendly” way

Probabilistic Assessment of Creep-Fatigue Crack Propagation in Austenitic Stainless Steel Cracked Plates

This study investigates the effects of uncertainties in the prediction of creep-fatigue crack propagation in 316L(N) austenitic stainless steel plates containing a semi-elliptical surface defect. Different parameters in geometry, material behavior and test condition are considered as random variables in probabilistic assessments. Monte-Carlo sampling method is employed to estimate the probability distribution of desired outputs, i.e.such as propagated crack sizes, stress intensity factors and creep rupture life. It is shown that, generally, the uncertainty in prediction of crack sizes in both trough-wall direction and along the surface of the plate will be increased by increasing the time (crack size). Furthermore, probabilistic evaluations are performed using different reliability methods to calculate the probabilities of exceedance of available experimental results. These evaluations clarified the importance of consideration of uncertainties in creep-fatigue crack growth prediction. Sensitivity analysis, are carried out to provide useful information about the order of importance of random variables. It is found that, initial crack size may be an important random variable in such probabilistic assessments