CERES :: Browsing by Author "Mehmanparast, Ali"

Browsing by Author "Mehmanparast, Ali"

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Open Access
An accelerated corrosion-fatigue testing methodology for offshore wind applications
(Elsevier, 2021-04-30) Mehmanparast, Ali; Vidament, Azenor
Offshore wind turbines are subjected to cyclic loading conditions during their operational lifespan which is typically between 20 and 25 years. An important issue in fatigue design and integrity assessment of offshore wind turbine foundations is the examination of the long-term fatigue and corrosion-fatigue behaviour of steel structures in the high cycle region. High cycle fatigue tests, particularly at low frequencies in a seawater environment, are time-consuming and costly. Therefore, there is an essential need to perform accelerated tests to predict the long-term behaviour of the structures under realistic operational loading conditions. In this work, the existing fatigue acceleration mechanisms have been reviewed and a novel methodology has been proposed for accelerated testing and analysis of fatigue data in different environments (i.e. air, salt-spray and seawater) at higher temperatures. Two distinct equations have been developed and proposed for the calibration and prediction of S-N fatigue life and crack growth behaviour of steels in different environments. The proposed methodology has been validated through comparison with the existing data in the literature and predictions have been made at operational temperatures using high temperature data. The proposed approach is relatively simple to calibrate for a material of interest and enables accelerating S-N fatigue and crack growth testing of the examined materials by a factor of two and three, respectively. The proposed methodology and the obtained results have been discussed in terms of the need for accelerated testing for fatigue design and integrity assessment of offshore wind monopiles, especially those which are close to the end of initial design life and need a comprehensive engineering analysis for life extension or decommissioning.
Open Access
Analysis of fracture toughness properties of wire + arc additive manufactured high strength low alloy structural steel components
(Elsevier, 2020-08-14) Dirisu, Philip; Ganguly, Supriyo; Mehmanparast, Ali; Martina, Filomeno; Williams, Stewart W.
The uncertainty surrounding the fracture behaviour of CMT-WAAM deposited steel, in terms of crack tip condition (J and CTOD) needed to cause crack tip extension, has made this manufacturing technique unpopular to date. Fracture toughness parameters are crucial in the structural integrity assessment of components and structures in various industries for assessing the suitability of a manufacturing process and material. In the offshore wind industry, the EN-GJS-400-18-LT ductile cast grade for the mainframe and hub has lower fracture toughness resistance for its high strength grade. Its high weight level affects the Eigen frequency of the tower and imposes high installation cost incurred from heavy lifting equipment usage. Poor fracture toughness is currently a challenge for wind turbine manufacturers in the quest for a cleaner and cheaper energy in the form of offshore wind. In this study, CMT-WAAM is used in depositing steel components with an oscillatory and single pass deposition strategy. The effects of microstructural variation, as a result of layer by layer deposition and the layer band spacing, on the fracture resistance in the build and welding direction was shown here. The fracture mechanics and failure mode of the WAAM deposited parts were investigated. The microstructural variation, again as a result of the layer by layer deposition and the layer band spacing, are the key parameters that control the fracture toughness of WAAM steel. Anisotropic behaviour in the values was observed between both fracture orientations. The constructive transformation mechanism of the WAAM oscillatory process made way for intragranular nucleation of acicular ferrite on the Ti containing inclusion, thereby improving the toughness of the ER70S-6 deposit with a unique microstructure and Jq value of 640kJ/m2.
Open Access
Analysis of tightening sequence effects on preload behaviour of offshore wind turbine M72 bolted connections
(MDPI, 2019-11-20) Braithwaite, Jarryd; Mehmanparast, Ali
Offshore wind turbines in shallow waters are predominantly installed using a monopile foundation, onto which a transition piece and wind turbine are attached. Previously, the monopile to transition piece (MP-TP) connection was made using a grouted connection, however, cases of grout failure causing turbine slippage, among other issues, were reported. One solution is to use bolted ring flange connections, which involve using a large number of M72 bolts to provide a firm fixing between the MP-TP. It is in the interest of offshore wind operators to reduce the number of maintenance visits to these wind turbines by maintaining a preload (Fp) level above the minimum requirement for bolted MP-TP connections. The present study focuses on the effect of the tightening sequence on the Fp behaviour of M72 bolted connections. A detailed finite element (FE) model of a seven-bolt, representative segment of a monopile flange was developed with material properties obtained from the available literature. Three analyses were made to examine the effect on Fp after tightening, including the initial Fp level applied to the bolts, the tightening sequence and the effect of an additional tightening pass.
Open Access
Assesment of crack arrest behaviour in modern structural steels
(2021-03) Taylor, Jessica; Mehmanparast, Ali; Farrahi, Gholam Hossein
It is vital to prevent brittle cracks in large structures. This is particularly important for a number of industry sectors including offshore wind, Oil & Gas, and shipbuilding where structural failure risks loss of human life and loss of expensive assets. Wide-plate test methods allow for direct measurement of the crack arrest toughness but this kind of testing is incredibly expensive. Therefore, there is a need for cheaper and simpler test methods which are able to measure a material’s brittle crack arrest toughness. Some modern steels exhibit high Charpy energy – i.e. high initiation fracture toughness, but poor resistance to crack propagation – i.e. low crack arrest toughness. The correlation between initiation and arrest toughness measured through small-scale testing is investigated in five different steels, which include S355 structural steel (with two different thicknesses), X65 pipeline steel, two high strength reactor pressure vessel steels and EH47 shipbuilding steel. In this work, Compact Crack Arrest (CCA) testing, which is standardised in ASTM E1221, has been successfully used to measure the crack arrest toughness of thick sections of EH47 shipbuilding steel. A wide range of supplementary tests were carried out, including instrumented Charpy, drop weight Pellini, fracture toughness, tensile testing, and microscopy. Small scale mechanical tests were used to characterise the materials’ properties and the results were compared to the materials’ microstructures. The results presented in this study have been discussed in terms of the effectiveness of the CCA test method and small-scale test methods for measurement of brittle crack arrest toughness and integrity assessment of large-scale structures.
Open Access
Assessment of fatigue crack growth resistance of newly developed LTT alloy composition for the repair of high strength steel structures
(Elsevier, 2024-06-11) Igwemezie, Victor; Mehmanparast, Ali; Ganguly, Supriyo
Tensile residual stress (TRS) is a well-known factor that deteriorate the integrity of welded joints. Fatigue failure is accelerated by the existence of TRS introduced during the welding process. There have been efforts in the last two decades to develop filler alloys that can reduce TRS by introducing compressive residual stress (CRS) to oppose the TRS in high strength steel welded joints. These works are based on the theory of austenite (γ) to martensite (α’) transformation and the filler is often called a low transformation-temperature (LTT) alloy. Many studies have reported that the fatigue strength (FS) of weld joint made with LTT alloy is many times better than that of the conventional fillers. It is reported to be particularly useful in the repair of high strength steel structures. However, studies on the fatigue crack growth (FCG) behaviour of these LTT alloys is scarce. In this work, we developed Fe-CrNiMo based LTT weld metal composition, assessed its FCG behaviour and compared the results with that of a conventional welding wire (ER70S-6). It is found that ER70S-6 weld metal obtained under relatively fast cooling is extremely tough, but the associated heat affected zone (HAZ) has poor resistance to FCG which obscured the benefit of the tough weld metal. High heat input or condition that results to slow cooling of the ER70S-6 weldment deteriorates its resistance to FCG. Unfortunately, despite its low martensite start temperature of 231±7 and the anticipated beneficial effect of induced CRS, the LTT alloy studied had the lowest FCG resistance. The LTT alloy appears to have an intrinsic microstructural feature or a ‘fault line’ that reduced its resistance to FCG. While the LTT alloy weld metal has poor resistance to FCG, the associated HAZ resisted FCG more than the HAZ associated with ER70S-6 weld metal. It is observed that aligning the ER70S-6 weld metal perpendicular to the crack front produced the highest resistance to fatigue crack initiation and propagation. In the case of ER70S-6, it is believed that the weld metal induced a CRS at the notch tip which resulted to the high fatigue resistance. In the case of the LTT alloy, perpendicular alignment of the weld metal produced slight improvement.
Open Access
Assessment of mechanical and fatigue crack growth properties of wire+arc additively manufactured mild steel components
(Wiley, 2022-07-20) Shamir, Muhammad; Igwemezie, Victor C.; Lotfian, Saeid; Jones, Rhys; Asif, Huzaifa; Ganguly, Supriyo; Mehmanparast, Ali
A study has been conducted to evaluate the mechanical and fatigue crack propagation properties of wire + arc additively manufactured ER70S-6 components. A parallel-built deposition strategy was employed to fabricate the additively manufactured wall. The hardness values were slightly higher at the bottom and top of the wall due to the presence of Widmanstätten ferrite and carbides. The characterization of mechanical properties in both orientations; parallel and perpendicular to the deposition direction showed a marginal difference in yield strength and ultimate tensile strength. The crack growth rates were correlated with linear elastic fracture mechanics parameter ΔK and compared with an oscillation-built deposition strategy from the literature. The crack growth rates of both deposition strategies were found to be very similar to each other. Furthermore, it has been demonstrated that the variability in the crack growth histories can be reasonably well captured by using the NASGRO crack growth equation.
Open Access
Compact crack arrest testing and analysis of EH47 shipbuilding steel
(Elsevier, 2021-04-30) Taylor, Jessica; Mehmanparast, Ali; Kulka, Rob; Moore, Philippa; Farrahi, Gholam Hossein; Xu, Li
It is vitally important to measure the brittle crack arrest properties of shipbuilding steels to ensure that accidental damage will not result in total structural failure. Wide-plate test methods allow for direct measurement of the crack arrest toughness but this kind of testing is incredibly expensive. Therefore, there is a need for cheaper and simpler test methods which are able to measure a material’s brittle crack arrest toughness. In this work, Compact Crack Arrest (CCA) testing, which is standardised in ASTM E1221, has been successfully used to measure the crack arrest toughness of thick sections of EH47 shipbuilding steel. The results from this study have been compared to small-scale test methods. It was found that instrumented Charpy testing gives an overprediction of the CCA results, and nil-ductility transition temperature (NDTT) from Pellini tests gives a conservative estimate. The results presented in this study are discussed in terms of the effectiveness of the CCA test method for measurement of brittle crack arrest toughness and integrity assessment of large-scale structures.
Open Access
Comparative study between S-N and fracture mechanics approach on reliability assessment of offshore wind turbine jacket foundations
(Elsevier, 2021-06-14) Shittu, Abdulhakim Adeoye; Mehmanparast, Ali; Hart, Phil; Kolios, Athanasios
This paper investigates from a structural reliability assessment (SRA) perspective the fatigue reliability using the S-N curve approach compared with the fracture mechanics (FM) approach for a typical welded offshore wind turbine (OWT) jacket support structure. A non-intrusive formulation was developed for an OWT jacket support structure in 50 m deep water, consisting of a sequence of steps. First, stochastic parametric 3D (three-dimensional) Finite Element Analysis (FEA) simulations are performed, taking into account stochastic variables such as wind loads, wave loads and soil properties using facilities within the software package ANSYS. Secondly, the FEA results are post-processed using an Artificial Neural Network (ANN) response surface modelling technique deriving the performance functions expressed in terms of stochastic variables. Finally, the First Order Reliability Method (FORM) is applied in calculating the reliability index values of components. The developed framework was applied to elucidate the fatigue damage process, including the small to long crack transition amongst other stages, for structural steels used for OWT jacket applications. The FM formulation investigated includes a crack growth formulation based on the bilinear crack growth law, considering both segments of the crack growth law as non-correlated and correlated in calculating the reliability index (RI). Sensitivity analysis results showed a strong dependence of the structure's reliability levels on the uncertainties of the crack growth law constants measured in terms of coefficient of variation (COV). Also investigated, was the reliability of the structure reassessed and updated in the presence of assumed structural health monitoring/ condition monitoring (SHM/CM) data. The results from the case study revealed that fracture reliability is highly sensitive to the initial crack size. It is recommended to apply the S-N curve method at the design stage while the FM approach applied towards the end of the design life as the structure approaches failure.
Open Access
Comparative study of structural reliability assessment methods for offshore wind turbine jacket support structures
(MDPI, 2020-01-26) Shittu, Abdulhakim Adeoye; Mehmanparast, Ali; Wang, Lin; Salonitis, Konstantinos; Kolios, Athanasios
Offshore wind turbines (OWTs) are deployed in harsh environments often characterized by highly stochastic loads and resistance properties, thus necessitating the need for structural reliability assessment (SRA) to account for such uncertainties systematically. In this work, the SRA of an OWT jacket-type support structure is conducted, applying two stochastic methods to predict the safety level of the structure considering various design constraints. The first method refers to a commercial finite element analysis (FEA) package (DesignXplorer© from ANSYS) which employs direct simulations and the six sigma analysis function applying Latin hypercube sampling (LHS) to predict the probability of failure. The second method develops a non-intrusive formulation which maps the response of the structure through a finite number of simulations to develop a response surface, and then employs first-order reliability methods (FORM) to evaluate the reliability index and, subsequently, the probability of failure. In this analysis, five design constraints were considered: stress, fatigue, deformation, buckling, and vibration. The two methods were applied to a baseline 10-MW OWT jacket-type support structure to identify critical components. The results revealed that, for the inherent stochastic conditions, the structural components can safely withstand such conditions, as the reliability index values were found acceptable when compared with allowable values from design standards. The reliability assessment results revealed that the fatigue performance is the design-driving criterion for structural components of OWT support structures. While there was good agreement in the safety index values predicted by both methods, a limitation of the direct simulation method is in its requirement for a prohibitively large number of simulations to estimate the very low probabilities of failure in the deformation and buckling constraint cases. This limitation can be overcome through the non-intrusive formulation presented in this work.
Open Access
Corrosion fatigue crack growth mechanisms in offshore monopile steel weldments
(Wiley, 2017-04-20) Adedipe, Oyewole; Brennan, Feargal P.; Mehmanparast, Ali; Kolios, Athanasios; Tavares, Isaac
Offshore structures are generally fabricated of welded joints, which are considered as potential spots for crack initiation owing to the degree of stress concentrations imposed by the weld geometry and the effects of residual stresses introduced by welding processes. There are significant numbers of the current and anticipated offshore installations coupled with the use of newer materials and fabrication techniques. It is therefore important to understand the crack growth mechanisms in these structures accompanied with the effects of mean or residual stresses for a safe estimation of their service lives. In this paper, crack growth results of HAZ and weld materials similar to those used for offshore installations are presented. Tests were conducted in air and in simulated free-corrosion conditions at loading frequencies of 0.3 Hz in seawater, 5 Hz in air and at loading ratios of 0.1, 0.5 and 0.7. Results showed that crack growth rates were influenced by mean stresses, materials microstructure accompanied by welding procedure and environment. Crack growth results showed good agreement when compared with those obtained from other steels used for offshore structures.
Open Access
Corrosion-fatigue crack growth behaviour of wire arc additively manufactured ER100S-1 steel specimens
(Elsevier, 2022-05-19) Ermakova, Anna; Ganguly, Supriyo; Razavi, Javad; Berto, Filippo; Mehmanparast, Ali
The wire arc additive manufacturing (WAAM) technology is a promising fabrication technique which has been proven to have many advantages for producing large structures; however, the fatigue and corrosion-fatigue performance of WAAM steel components for application in the marine environments is still unexplored. In this study, the WAAM technique was employed to fabricate four specimens made of ER100S-1 steel, which were then tested under cyclic loading conditions in seawater to assess the corrosion-fatigue crack growth (CFCG) behaviour and hence suitability of this fabrication technology for offshore renewable energy applications. The test duration, cracking mechanisms and CFCG rate were investigated for each specimen and the material’s behaviour was investigated by considering the microstructural examinations. Furthermore, the obtained results were compared with the BS7910 standard recommended trends and experimental data available in the literature for conventionally built weldments made of different grades of steel which are commonly used for offshore applications.
Open Access
Corrosion-fatigue crack growth behaviour of wire arc additively manufactured ER70S-6 steel parts in marine environments
(Elsevier, 2022-07-17) Ermakova, Anna; Ganguly, Supriyo; Razavi, Javad; Berto, Filippo; Mehmanparast, Ali
A crucial part of the structural integrity assessment of marine structures is the analysis of the fatigue crack growth behaviour of the welded joints in seawater environments, where the cracks often initiate and propagate under corrosion-fatigue loading conditions. In recent years, technological developments have facilitated the fabrication of steel components and structures using additive manufacturing technologies. Among the existing technologies, the Wire Arc Additive Manufacturing (WAAM) technique has proven to offer great potentials for fabrication of large-scale structures. The present study investigates the corrosion-fatigue crack growth (CFCG) behaviour of the WAAM parts fabricated using ER70S-6 low carbon steel wire to assess the suitability of this technology for future marine structures. In this experimental study, the cracking behaviour and test duration in corrosion-fatigue tests were investigated and analysed in conjunction with the microstructural examination of the tested specimens. Moreover, the obtained results were compared with the recommended trends available in BS7910 standard for conventional welded joints and the data available in the literature on widely used offshore structural steel weldments. The CFCG results obtained from this study contribute to the overall knowledge and design requirements for the new optimised functionally graded structures made with WAAM technology for marine applications.
Open Access
Crack growth direction effects on corrosion-fatigue behaviour of offshore wind turbine steel weldments
(Elsevier, 2020-10-24) Jacob, Anaïs; Mehmanparast, Ali
In this study corrosion-fatigue tests have been conducted on fracture mechanics specimens extracted from an S355 G10+M structural steel welded plate. The tests have been performed on compact tension specimens with the crack tip located in the heat affected zone. The corrosion-fatigue test results from this study have been compared with the data available on the base metal as well as air tests on the same material. Moreover, the obtained results have been compared with the corrosion-fatigue data available in the literature on a wide range of steels and also the fatigue trends for welded joints in free-corrosion condition recommended in the BS7910 Standard. The effect of the specimen orientation, with respect to the weld region, is also examined in this study and it has been found that higher corrosion-fatigue crack growth rates are generally observed in the tests with 0° orientation. The results have also shown that the corrosive environment has significant effects on the fatigue crack growth acceleration at the beginning of the tests; however, as the crack propagates, the environmental damage effect on crack growth behaviour becomes less pronounced. The results presented in this study are discussed in terms of improvement in the structural integrity assessment of offshore wind turbine monopiles.
Open Access
Crack growth monitoring in corrosion-fatigue tests using back face strain measurement technique
(Elsevier, 2018-12-31) Mehmanparast, Ali; Albani, Pietro; Igwemezie, Victor C.
Corrosion-fatigue crack growth tests are known to be considerably time consuming, particularly due to low loading frequencies which often result in several months of testing. This study focuses on development of a material and load dependent numerical model which correlates back face strains with crack lengths for standard compact tension, C(T), specimen geometry. To validate numerical predictions, calibration fatigue crack growth tests were conducted in air on C(T) specimens made of S355 steel, which is widely employed in offshore wind industry. The results obtained from these tests at different load levels have been compared with those predicted from the numerical model. Characterization of isotropic-kinematic hardening behaviour for the material adopted was carried out using the data available in the literature. The numerical model presented in this work has proven to generate accurate estimates of crack length in corrosion-fatigue tests. This model can be used in future experimental test program on S355 steel without needing to obtain experimental correlations between crack length and back face strains from calibration tests performed in air.
Open Access
Creep-fatigue crack growth testing and analysis of pre-strained 316H stainless steel
(Elsevier, 2016-07-21) Mehmanparast, Ali; Davies, C. M.; Nikbin, Kamran
Material pre-straining is known to have significant effects of the mechanical response and crack growth behaviour of steels. In this paper, the influence of material pre-straining on the subsequent creep-fatigue crack growth behaviour of Type 316H stainless steel at 550 °C has been examined by performing tests on compact tension specimens that were extracted from blocks uniformly pre-compressed at room temperature. Creep-fatigue crack growth tests on pre-compressed material were performed at the frequency of 0.01 Hz and R-ratio of 0.1. The crack growth data obtained from these experiments have been correlated with the C* and K fracture mechanics parameters and the results are compared with the existing creep crack growth data on the pre-compressed and as-received material at 550 °C. The results obtained have also been compared with the creep-fatigue data from experiments on weldments where the crack tip was located in the heat affected zone (HAZ). The crack growth behaviour in creep-fatigue tests on pre-compressed material has been found similar to that of HAZ material and are higher than that of the as-received material. Moreover, depending on the loading condition and frequency the crack growth data obtained from creep-fatigue tests on pre-compressed material may be characterized using C* or ΔK fracture mechanics parameters.
Open Access
Current trend in offshore wind energy sector and material requirements for fatigue resistance improvement in large wind turbine support structures – A review
(Elsevier, 2018-11-20) Igwemezie, Victor C.; Mehmanparast, Ali; Kolios, Athanasios
At present, the UK government is driving the survival of the wind energy industry by using interventions that encourage investment in the sector. The use of a Contract for Difference (CfD)/Strike price model by the UK government supports the wind industry and guarantees that wind energy generators have a stable premium over a period of 15–20 years; however, this may not last forever. The growth and stability of the wind industry will depend essentially on continued reductions in wind energy cost, even below that of fossil-fuel based energy sources. Huge cost reduction beyond the present strike price of £ 57.50/MWh for some projects to be delivered in 2022/2023 may be achieved quickly through efficient and optimized turbine support structure. Consequently, the offshore wind industry is currently making enormous efforts to upscale wind turbines (WTs) from 8 MW to 9.5MW,10MW and then 12 MW HAWT (Horizontal Axis Wind Turbine). This level of upscaling no doubt creates tough challenges because the mass of the turbine increases linearly with the cube of the rotor radius. Monopiles having diameters larger than 7 m have been proposed, with a wall thickness section in the range of 70–110 mm. It is generally thought that Thermo-Mechanical Controlled Process (TMCP) steels are well suited for extra-large (XL-WTs). This paper reviews the present status of WTs and critically assesses the material factors in the structural integrity concerns that may confront the use of XL steel plates in the design of XL-WT support structures.
Open Access
Determination of long-term creep properties for 316H steel using short-term tests on pre-strained material
(Springer, 2021-01-07) Zhou, Haoliang; Mehmanparast, Ali; Nikbin, Kamran
Determination of long-term creep rupture properties for 316H steel is both costly and time-consuming and given the level of scatter in the data would need substantial number of tests to be performed. The primary objective of this study is to estimate the long-term creep properties of cross-weld (XW) and as-received (AR) 316H stainless steel by performing accelerated tests on pre-compressed (PC) material. In this work, uniaxial creep rupture tests have been performed on XW specimens and the results have been used to establish a correlation with accelerated test results on the PC material. Moreover, tensile tests have been performed on XW specimens at room temperature and 550 °C to examine the pre-conditioning effects on the mechanical response of the material. Similar power-law creep properties have been found for the creep strain rate and rupture time behaviour of the XW and PC specimens. It also has been found that the creep ductility data points obtained from XW and PC specimens fall upon the estimated trend for the AR material at 550 °C when the data are correlated with the applied stress normalised by 0.2% proof stress. The results show that the long-term creep properties of the XW and AR material can be estimated in much shorter time scales simply by performing tests on the PC material state.
Open Access
The effect of surface treatment and orientation on fatigue crack growth rate and residual stress distribution of wire arc additively manufactured low carbon steel components
(Elsevier, 2023-04-10) Ermakova, Anna; Razavi, Javad; Cabeza, Sandra; Gadalinska, Elzbieta; Reid, Mark; Paradowska, Anna; Ganguly, Supriyo; Berto, Filippo; Mehmanparast, Ali
The directed energy deposition (DED) processes, such as laser metal deposition or Wire Arc Additive Manufacturing (WAAM), are gradually becoming the preferred method for fabrication of large-scale components using metal additive manufacturing (AM) technology. In this work, the possibility of fatigue life enhancement in WAAM built low carbon steel components, by means of rolling and laser shock peening surface treatment techniques, was investigated. A series of fatigue crack propagation tests were performed on surface treated ER70S-6 and ER100S-1 WAAM built specimens, and the results were analysed and compared with the untreated materials tested under the same loading conditions. The obtained results were interpreted in terms of the sensitivity of the cracking behaviour to the specimen orientation and extraction location. Furthermore, the residual stress profiles were measured, before and after applying the surface treatment techniques, and the effects of locked-in residual stresses on the fatigue performance of WAAM built components were discussed. Finally, a detailed texture analysis was performed on the surface treated and untreated regions of both WAAM built materials considered in this work. The obtained results from this study provide an insight into the advantages and disadvantages of various surface treatment techniques for fatigue life enhancement of WAAM built components with the view to extend the application of this advanced manufacturing technology to a wider range of industrial applications.
Open Access
Effects of plastic pre-straining level on the creep deformation, crack initiation and growth behaviour of 316H stainless steel
(Elsevier, 2016-03-19) Mehmanparast, Ali; Davies, C. M.; Dean, David W.; Nikbin, Kamran
The effects of the material pre-straining level, in the form of plastic pre-compression at room temperature, on the tensile, creep deformation, creep crack initiation and growth behaviour of 316H stainless steel have been examined at 550 °C. Experiments have been performed on the 4%, 8% and 12% pre-compressed specimens and the results are compared with existing data on the pre-compressed material to investigate the change in mechanical response, creep failure, creep crack initiation and growth behaviour of 316H over a range of plastic pre-straining levels. Comparisons are also made to short term and long term test data on the as-received material. It has been found that creep ductility and rupture times decreased with an increase in pre-strain levels considered. The test results obtained from different material states are discussed in terms of the influence of material pre-straining level on the microstructural deformation, mechanical response, creep deformation and crack growth behaviour of the material.
Open Access
Effects of structural steels microstructure and waveform on corrosion-fatigue behaviour of offshore wind turbine foundations.
(2019-10) Igwemezi, Victor Chinedu; Mehmanparast, Ali; Brennan, Feargal
Marine structures in the offshore environment are subjected to constant cyclic wave and wind forces. Also, the foundations are in direct contact with seawater (SW), hence introducing corrosion damage in the structure. Due to the cyclic loading cracks could nucleate and grow, or existing cracks could propagate at loads far less than the maximum design load, and the problem is worsened in marine environment. Offshore Wind Turbines (WTs) are relatively new structures and their long-term corrosion-fatigue performance data are very scarce. In fact, presently there are little or no public data on corrosion-fatigue performance of these sub-grades of S355 steel used in the design of extra-large (XL) Wind turbine support structures (WTSSs). This experimental research primarily seeks to understand the rate at which a crack grows in the modern normalised-rolled (NR) and thermomechanical control process (TMCP) S355 steel subgrades designated as S355G10+M, S355G8+M and S355J2+N under the influence of: fatigue waveforms, change in fatigue load level and material microstructure in seawater (SW) at frequencies within the range of commercial Wind Turbine (WT) operating condition. The test programme employed a soft-stiff frequency range of commercial WTs in the North Sea. The waveforms considered in this study are constant amplitude sinewave and trapezoid waveform (generally referred to here as hold-time) and the experiments were conducted under frequencies of 0.2Hz, 0.3Hz, 0.5Hz. The stress ratio in all tests was 0.1 and the maximum applied loads were 9kN and 10kN. All calculations where done under Linear Elastic Fracture Mechanics (LEFM) and all tests and investigations were limited to the Paris Region or the Stage II of the da/dN vs. ΔK plot, where LEFM applies. All the tests have been performed in accordance with BS 7910:2013+A1:2015 and ASTM E647-15. This study found that frequency has no obvious effect on the FCGR of ferrite-pearlite steels in air. There was enhancement of the FCGR in seawater by a factor that depended on the waveform and load level. The CFCGR of the sine waveform was found to be higher than that of the holdtime for all the load levels and frequencies used. Using the mean curve, SW enhanced the FCGR by an average factor of 1.48 under hold-time and 2.17 under sinewave throughout theParis Region in the range of 20 - 35 MPa√m with reference to the air mean curve. Similar trend was obtained if mean +2SD is used, giving an average factor of increase of 1.48 under hold-time and 2.30 under sinewave. Comparison of the experimental results on S355G8+M, S355G10+M and S355J2+N has shown that under both sinewave and hold-time waveforms, the CFCG trend in normalised steels (e.g. S355J2+N) is consistently higher than that of the TMCP steels (e.g. S355G8+M, S355G10+M) in both air and sea environments. Change in load level and frequency did not affect the CFCGR of the TMCP steels in SW under sinewave, but slight change was observed with decrease in the load level for the holdtime. The Pmax is found to have a profound influence on CFCGR than the cyclic frequency (in the range of 0.2Hz to 0.4Hz). Decreasing the load level reduces the effect of frequency in SW and the difference in CFCGR for waveforms diminishes with increasing frequency and ∆K. In SW, increase in the fatigue load and decrease in the frequency, especially for holdtime has the highest retarding effect under corrosion dominated regime. This is a consequence of what we referred to here as the microplastic zone size. High load level increases the microplastic zone size closest and ahead of the propagating crack tip and opens up the crack tip region to allow the entry of more damaging chemical species into the plastically deformed regions causing fast attack by the time-dependent corrosion process resulting to rapid crack tip dissolution and blunting. When the main active crack tip is blunted the crack propagation process is considerably reduced or even arrested for a very long time as observed especially for the hold time test. This appears to suggest that for same ∆K value, combination of low stress range and long crack length, a will be more damaging than combination of high stress and short a in SW. Corrosion products are also found to build up in and around the blunted crack tip leading to further retardation of the crack growth. Extensive blunting of the crack front due to availability of time explains why the CFCGR is generally lower in holdtime as compared to sinewave. Fractographic and metallographic analyses were carried out to understand the disparity in the CFCGR between the waveforms under the test conditions. Fractographic examination of the fractured surface of the corrosion-fatigue specimens showed that the CFCG mechanism is by ductile striations both in air and SW. The crack path was always non-planar with complex crack front. Generally, three phenomena were identified that primarily retarded crack growth in the ferrite-pearlite steels in air. These are crack diversion, crack bifurcation and metal crumbs. The three factors retarded the crack growth by reducing or re-distributing the effective driving stress at the main propagating active crack tips. It was found that the main crack tip blunting process is the primary factor controlling the CFCGR of steel at high ∆K and low frequency in a ferrite-pearlite steel in SW. Other fundamental factors are crack angle diversion, branching of crack front and formation of metal crumbs along the crack path. The extent of formation of the aforementioned phenomena is a strong function of the steel microstructure. This implies that microstructure has a strong effect on the FCGR of ferrite-pearlite steels in the Paris Region, both in air and SW. This conclusion is in contrary to current theory that microstructure has little or no effect in the Paris Region of the da/dN vs ∆K sigmoidal curve. Transgranular and quasi-intergranular modes of propagation in both environments were observed. The quasi-intergranular mode is a situation where the fatigue-crack propagated through a thin layer of high solute ferrite ribbon, 𝛼𝐻𝐴 adjacent to the low relief, αLR or the pearlite, P phases. The morphology and chemistry of the phases local to the main crack front and the load level appeared to determine which mode the crack growth would adopt. The angle the crack front made with the least resistant path ahead of it seemed to determine if it would propagate by transgranular or quasi-intergranular mode.