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The sustainable and resource-efficient production of offshore wind turbines requires the use of modern high-strength fine-grained structural steels (Martin and Schroeter, 2005). This applies to wind turbines in terms of increasing turbine sizes as well as to maintenance and installation vessels and equipment (Ummenhofer et al., 2013). Without the demanded high load-bearing capacities and boom lengths, the economic realization of these goals would be inconceivable. During the assembly of high-strength steel structures, unacceptable defects can occasionally be found in the weld area, although the welding process was executed in accordance with the specifications. In most cases, the economical solution would be local thermal gouging of the affected areas and re-welding. Corresponding standards hardly provide any information on adequate repair concepts, and there is no uniform scientific data on which to base such concepts. This applies particularly to the consideration and optimization of welding-induced stresses due to the high shrinkage hindrance of the gouging grooves and degradation of the adjacent microstructures by gouging and re-welding. The result, especially in the case of high-strength steel grades, are frequently recurring imperfections as well as a missing consideration of the additionally induced welding stresses in the design of the structure. In this context, at BAM component-relevant investigations focused on welding residual stress evolution and microstructural degradation during repair of weld joints due to local thermal gouging and re-welding are carried out within the scope of a FOSTA project (P1311, IGF 20162N).
In this study, several relevant findings are discussed based on examples of structural engineering focusing on mechanical-technological properties and residual stresses, for instance found by (Schasse, 2017). Also experimental and numerical work as conducted by (Wongpanya, 2008) and weld tests under defined shrinkage restraint in special weld test-setups for research projects, e. g. FOSTA-P922 (Kannengiesser and Schroepfer, 2015) and P1011 (Kannengiesser and Schroepfer, 2017) have shown that an optimization of the welding-induced stresses of high-strength structural steels is specifically achievable by means of adapted heat control concepts (Schroepfer, 2017). The present research involves systematic investigations of influences of shrinkage restraint, the number of repair cycles and heat control during repair welding of the relatively new developed offshore-relevant high-strength steel S500MLO (EN 10225-1). For the quantification of the shrinkage restraint of weld joints, the concept of restraint intensity established by (Satho et al., 1973) was applied analogous to recent research, e. g. (Schwenk et al., 2008). By means of structural mechanics calculations, geometries of self-strained specimens were identified, that represent different defined rigidity conditions of repair welds of real components, cf. Fig. 1. It could be shown that with increasing weld joint restraint intensity significantly higher residual stresses in the weld metal and heat affected zone up to 80 % of the nominal yield strength occur, cf. Fig. 2. In relation to existing results, it has been shown that a safe repair of such welds can only be achieved by means of appropriate repair concepts and heat control taking the high welding stresses and special microstructures of high-strength steels into account.
Finally, the aim of this research is to derive recommendations for repair concepts appropriate to the stresses and materials involved providing a basis for standards and guidelines, especially for SMEs, in order to avoid damage and, in most cases, expensive reworking and to improve the full utilization of the potential of high-strength steels.
The sustainable and resource-efficient production of wind energy plants requires the use of modern high-strength fine-grain structural steels. This applies to both foundation and erection structures. During the assembly of steel structures, unacceptable defects can occasionally be found in the weld area. In most cases, the economical solution would be local thermal gouging of the affected areas and re-welding. Due to the high shrinkage restraint of the joint groove in the overall structure, the superposition of global and local welding-induced stresses may lead to crack formation and component failure, particularly in interaction with the degradation of the microstructure and mechanical properties of high-strength steels during the repair process [1]. Corresponding standards hardly provide any information on adequate repair concepts, and there is no uniform scientific data on which to base such concepts.
In this study, several relevant findings are discussed based on examples of structural engineering focusing on mechanical-technological properties and residual stresses, e.g. [1]. Further experimental and numerical work as conducted by [2] and weld tests under defined restraint conditions in special weld test-setups [3] show that an optimization of the welding-induced stresses of high-strength structural steels is achievable by means of an adapted heat control. The present research involves systematic investigations of influences of shrinkage restraint, the number of repair cycles and heat control during repair welding of a recently available high-strength offshore steel S500MLO (EN 10225-1). A quantification of the shrinkage restraint of repair weld joints is achievable by means of restraint intensity concept [4], analogous to previous studies [5]. Using structural mechanics calculations, geometries of self-restrained specimens are identified representing different defined rigidity conditions of repair welds considering actual high-strength steel components. Welding experiments with DIC analyses (digital image correlation) of the occurring strains during welding and XRD analyses (X-ray diffraction) of the resulting residual stresses after welding and cooling show increasing transient loads and significantly elevated residual stress profiles in the weld area with increasing restraint intensity. Especially in the heat affected zone, tensile residual stresses of up to 80 % of the nominal yield strength occur when welding under increased restraint conditions. In relation to the presented existing results, this indicates that a safe repair welding is primarily achievable by means of appropriate repair concepts and heat control taking into account the high welding stresses and special microstructures of high-strength steels. Finally, the aim of this research is to derive recommendations for repair concepts appropriate to the stresses and materials involved providing a basis for standards and guidelines, especially for SMEs, in order to avoid damage and, in most cases, expensive reworking and to improve the full utilization of the potential of high-strength steels.
Creep-resistant steels such as the 13CrMoV9-10 used in pressure vessel construction, with wall thicknesses of 250 mm and above, are most commonly submerged arc welded (SAW). The characteristic high heat input during the SAW-process introduces very high, localized residual stresses, which can lead to stress relief cracking (SRC) if the mandatory post weld heat treatment (PWHT) of the welding joint is performed improperly. Current PWHT parameters, such as heating rate and holding time at a specific holding temperature are mostly based on both empirically experience and conventional free shrinking welding experiments to characterize the SRC-susceptibility of the weld. These cannot adequately depict the higher residual stresses caused by the structurally induced stiffness of the surrounding construction and its constraint on the thermal shrinkage of the weld. Nor do they consider the resulting metallurgic effects such as the growth of precipitates during the PWHT and the resulting differences in hardness of the grain boundaries or the premature aging via a dissimilar precipitation behavior. The effect of the heating rate of the PWHT on the SRC susceptibility has to date not been adequately studied either.
This study discusses the development of a repeatable, precise and time efficient methodology to study the effects of different stress levels and heating rates on the SRC susceptibility of the coarse grain heat affected zone (CGHAZ). For that purpose, samples were thermically treated to simulate a CGHAZ and subsequently exposed to representative levels of stress during PWHTs with defined parameters. The recorded stress and heating rate dependent strains were mathematically analyzed via curve tracing/ calculus to identify interdependent effects. This procedure facilitates the measurement of material characteristics such as carbide growth on grain boundaries at the µm-scale via an integrated value over the entire sample volume. The first and second derivate show a slight, precipitate dependent, increase in hardness of the sample, depending on the heating rate and applied stress. Additionally, the formation and growth of special carbides is well known to have a volumetric effect, which together with the material characterization and this new methodology is believed to generate an improved assessment of the SRC susceptibility of SAW microstructures of creep-resistant CrMoV steels. The main advantage of the suggested methodology is the material independence. It thus provides, a comparative value between welds under ideal conditions in the lab and real-world scenarios by including an idealized, simulated component rigidity.
Effect of Inter-Layer Dwell Time on Distortion and Residual Stresses of Laser Metal Deposited Wall
(2019)
The laser metal deposition is an advanced manufacturing technology enabling the production of large-sized parts and partially or completely elimination of machining and welding. The process is characterised by non-uniform local heating of the buildup leading to a stress distribution, which may exceed the yield strength of the material and leads to loss of dimensional accuracy. The interlayer dwell time has a strong influence on the temperature field. The effect of the interlayer dwell time on the distortion and the stress distribution during laser metal deposition of a single-pass wall on the edge of 2 mm thick plate was studied experimentally and numerically. The deposited material was IN625 and the substrate material was AISI 316. A decrease of the residual displacement, due to a uniform shrinkage after the deposition of the last layer and a lower level of the residual compressive longitudinal plastic strain, has been observed in the studies without dwell time. The peak increment of the free edge displacement corresponds to the first layer and hence the subsequent layers will be deposited on the already plastically deformed buildup. The tensile residual longitudinal stress near the top of the buildup and transverse stress near the edges of the buildup is higher than yield strength in the studies with dwell time.
The increased application of higher-strength steels and filler materials necessitates a more profound understanding of the interactions between the welding process, the heat input, the cooling conditions and the resulting metallurgical processes in the weid and its surroundings. Strategies, which help to improve the strength properties and life-time of welded stmctures to such an extent that the utilisation of higher-strength materials can be justified and their lightweight construction potential can thus be exploited to the füll, can only be derived from the interaction between all the influencing factors. Examples of a few investigations on welded joints between higher-strength fine-grained structural steels with regard to the interactions between the main variables influencing the cold cracking are presented in this article.
The increased application of higher-strength steels and filler materials necessitates a more profound understanding of the interactions between the welding process, the heat input, the cooling conditions and the resulting metallurgical processes in the weid and its surroundings. Strategies, which help to improve the strength properties and life-time of welded stmctures to such an extent that the utilisation of higher-strength materials can be justified and their lightweight construction potential can thus be exploited to the füll, can only be derived from the interaction between all the influencing factors. Examples of a few investigations on welded joints between higher-strength fine-grained structural steels with regard to the interactions between the main variables influencing the cold cracking are presented in this article.
Cyclic fatigue behavior of glass fiber reinforced epoxy resin at ambient and elevated temperatures
(2018)
The fatigue behavior of ±45° glass fiber reinforced epoxy resin under cyclic mechanical and constant thermal loading is investigated in this study. Tests at three different temperature levels in the range 296 K to 343 K have been performed in order to create S-N curves for each temperature level. The specimen damage is measured in-situ using optical grayscale analysis. The characteristic damage state (CDS) is evaluated for each specimen. It is shown that the point of CDS is suitable as a failure criterion to compare the resulting S-N curves. With micromechanical formulations, the temperature-dependent matrix effort is calculated for each stress-temperature level. In terms of matrix effort, the longest fatigue life is reached at high temperatures, while, in terms of stress, the lowest fatigue life is reached at the highest temperatures.
Cyclic fatigue behavior of glass fiber reinforced epoxy resin at ambient and elevated temperatures
(2018)
The fatigue behavior of ±45° glass fiber reinforced epoxy resin under cyclic mechanical and constant thermal loading is investigated in this study. Tests at three different temperature levels in the range 296 K to 343 K have been performed in order to create S-N curves for each temperature level. The specimen damage is measured in-situ using optical grayscale analysis. The characteristic damage state (CDS) is evaluated for each specimen. It is shown that the point of CDS is suitable as a failure criterion to compare the resulting S-N curves. With micromechanical formulations, the temperature-dependent matrix effort is calculated for each stress-temperature level. In terms of matrix effort, the longest fatigue life is reached at high temperatures, while, in terms of stress, the lowest fatigue life is reached at the highest temperatures.
Additive manufacturing (AM) by selective laser melting (SLM) offers ample scope for producing geometrically complex parts as compared to the traditional subtractive manufacturing strategies. However, the residual stresses which develop during the process can limit the application of SLM parts because they can reduce the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing. This study aims at the characterization of residual stresses in SLM parts by using different measurement techniques. The material used is the nickel based super Alloy 718. Microstructure as well as surface and bulk residual stresses were characterised. For residual stress analysis X-ray, synchrotron and neutron diffraction were applied. The results show different residual stress states dependent on the penetration depth in the sample offered by the different measurement techniques. Samples of Alloy 718 manufactured by SLM process can show high tensile residual stresses in the surface as high as the yield strength of the wrought alloy. Residual stresses in the bulk show considerably lower stress values.
Welded plate girders are used in heavy steel construction, industrial buildings and bride construction. Residual stresses are present in all plate structures. They are mainly caused by welding. In addition, they influence the load bearing capacity of these welded components. However, Eurocode (EC) does not provide any specific residual stress patterns for consideration of residual stress impact on load capacity. Hence, the decision for a particular problem has to be made by the designer. Many codes, including EC 3, permit the use of non-linear finite element analysis (FEA) for the design of structures. For that purpose, the contribution gives an overview on recent results derived from welded I-girders made from mild steel S355 and high-strength steel S690QL. Weld residual stresses have been investigated by X-ray diffraction (XRD) on small-scale specimen. In addition, residual stresses of welded component-like I-girders have been studied using the sectioning technique. The results confirmed the beneficial effect of certain edge preparation of the plates by flame cutting prior to welding. High-strength steels like the S690 can contribute to both: increased load capacity and decreased influence of weld residual stresses.