Browsing by Author "Igwemezie, Victor C."

Now showing 1 - 8 of 8
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen 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.
  • Thumbnail Image
    ItemOpen Access
    The influence of microstructure on the fatigue crack growth rate in marine steels in the Paris Region
    (Wiley, 2020-07-28) Igwemezie, Victor C.; Mehmanparast, Ali; Brennan, Feargal
    This paper presents a study on the effect of microstructure on the fatigue crack growth (FCG) rate in advanced S355 marine steels in the Paris Region of the da/dN versus ΔK log–log plot. The environments of study were air and seawater (SW), under constant amplitude sinewave fatigue loading. Fundamentally, three phenomena (crack tip diversion, crack front bifurcation and metal crumb formation) were observed to influence the rate of FCG. These phenomena appear to be a function of the material microstructure, environment and crack tip loading conditions. The three factors retarded the crack growth by reducing or redistributing the effective driving force at the main active crack tip. A crack path containing extensively the three phenomena was observed to offer strong resistance to FCG. In SW, the degree of the electrochemical dissolution of the microplastic zone appears to be an additional primary factor influencing FCG in the steels
  • Thumbnail Image
    ItemOpen Access
    Materials selection for XL wind turbine support structures: A corrosion-fatigue perspective
    (Elsevier, 2018-06-23) Igwemezie, Victor C.; Mehmanparast, Ali; Kolios, Athanasios
    The continued growth of the offshore wind industry will depend essentially on reductions in wind energy production cost. Large cost reductions can be achieved through efficient, economic and optimised wind turbine support structures. To achieve maximum offshore wind adoption beyond 2020, significant industrial and research efforts are being made in optimised material selection and application. Fatigue and corrosion damage are the greatest challenges today in design and life estimation of wind turbine support structures. S355 steel is currently used in fabrication of most wind turbine monopile support structures. Clear understanding of their corrosion-fatigue properties and accurate steel selection will support the optimisation and economic design of extra-large wind turbines. This paper presents the fatigue crack growth test results of advanced S355 TMCP steel in air and seawater, and compares the results with studies on commonly available S355 steel. The results show that S355 TMCP steels generally offer higher fatigue damage tolerance than normalised S355 steels in air and the factor decreases and tends towards a common value with increase in stress intensity factor range. However, in seawater there is no significant difference in fatigue crack growth rates for all the S355 ferritic steels considered in this study.
  • Thumbnail Image
    ItemOpen Access
    Physical metallurgy of modern creep-resistant steel for steam power plants: microstructure and phase transformations
    (Hindawi, 2016-11-30) Igwemezie, Victor C.; Ugwuegbu, Chima C.; Mark, U.
    The fact that the microstructure of steel depends on its composition and the heat treatment given to it has been heavily exploited in the design of steel for power plant applications. To obtain a steel that can function at the higher temperature where power plants operate without failure for extended life, heat treatment is needed to produce fine and highly stable dispersion of carbides, nitrides, and intermetallic compounds in the microstructure of the material. A significant contribution also comes from solid solution strengthening by substitutional solutes. We review here various types of phases, microstructures, functions, and interacting effects of the various alloying elements in the design of steel for modern power plant application.
  • Thumbnail Image
    ItemOpen Access
    A review of LTT welding alloys for structural steels: design, application and results
    (Elsevier, 2022-04-06) Igwemezie, Victor C.; Shamir, Muhammad; Mehmanparast, Ali; Ganguly, Supriyo
    A great deal of effort goes into production of modern steel for structural applications. The structural integrity of the steel becomes compromised when it is welded to form engineering components. The structural capacity of the steel joints is further reduced if the joint is to serve in a fluctuating stress environment. This is because, the fatigue strength (FS) of the steel structure is now shifted to the welded joints. One of the major factors that deteriorate the FS of welded joints is tensile residual stress (TRS). There have been efforts in the last two decades to develop welding alloys capable of mitigating TRS in welded joints based on the phase-transformation of austenite (γ) to martensite (ά). This paper reviews the design, application and results of these alloys often referred to as Low Transformation Temperature (LTT) welding alloys. It also presented the factors affecting them and areas where performance data are lacking.
  • Thumbnail Image
    ItemOpen Access
    Waveform and frequency effects on corrosion-fatigue crack growth behaviour in modern marine steels
    (Elsevier, 2020-01-25) Igwemezie, Victor C.; Mehmanparast, Ali
    The primary focus of this work is to investigate the sensitivity of cyclic waveform, frequency (f), load level and microstructure on the corrosion-fatigue crack growth rate (CFCGR) in modern normalised-rolled (NR) and thermomechanical control process (TMCP) ferrite-pearlite steels in the Paris Region of the da/dN vs. ΔK log-log plot. Constant amplitude sinewave (si) and trapezoid waveform (generally referred to here as hold-time (h-t)) were used under frequencies of 0.2 Hz, 0.3 Hz and 0.5 Hz and stress ratio of 0.1. Comparison is also made between the crack path in the S355 TMCP steel under si and h-t in seawater (SW). The role of microstructure in retarding or accelerating fatigue crack growth in SW is also discussed. Experimental results showed that the CFCGR corresponding to the si is higher than that of the h-t for all the load levels and frequencies examined. It was observed that reduction in the f and fatigue load level increased the CFCGR for the h-t but had little effect on the si. Generally, f in the range 0.2–0.5 Hz had little effect; and for a given f an increase in load led to a reduction in the CFCGR, in the Paris Region (PR) for both si and h-t in SW. Under both si and h-t, the CFCGR in the TMCP steels (e.g. S355G8 + M, S355G10 + M) is lower than that of the normalised steels (e.g. S355J2 + N). Metallurgical analyses on the fractured surface of corrosion-fatigue specimens show that the main active crack tip blunting process is the primary factor controlling the CFCGR of steel at high stress intensity factor range (SIFR) and low f in SW. The results obtained from this study have been discussed in terms of the potential impact on the structural design and integrity of offshore wind turbine foundations.