Browsing by Author "Shamir, Muhammad"

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    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.
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    Fatigue life assessment of offshore wind support structures in the presence of corrosion pits
    (Elsevier, 2023-08-14) Shamir, Muhammad; Braithwaite, Jarryd; Mehmanparast, Ali
    Offshore wind turbine support structures, which connect the wind turbine transition piece and/or tower to the seabed, are located below the sea level and are in direct contact with seawater during the entire lifespan; therefore, they are highly susceptible to corrosion damage and cracking. In particular, the pitting corrosion is very crucial in these support structures, as it leads to local stress concentrations and thus affects the fatigue life. Although corrosion protection mechanisms are commonly implemented in offshore wind turbines, they have a finite life and therefore corrosion damage cannot be completely avoided during the entire life cycle, and this can lead to pitting corrosion on the steel surface. This paper aims to investigate the impact of pitting corrosion on fatigue durability of steel structures by performing tests on lab-scale coupons, made of S355 structural steel which is widely employed in fabrication of offshore wind support structures. For this purpose, cross-weld uniaxial samples were initially exposed to seawater for different time durations and then tested under cyclic loading condition. Furthermore, the durability analysis of corroded samples was carried out using a modified NASGRO equation. The results show that the pitting corrosion significantly reduces the fatigue life, and its level of life reduction is strongly dependent on the seawater exposure time. Moreover, the results show that employment of a linear trend for lower stress ranges would result in significant underprediction of the fatigue life, hence over-conservatism in the design life, at longer seawater exposure times.
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    Influence of deposition strategies on tensile and fatigue properties in a wire + arc additive manufactured Ti-6Al-4V
    (Elsevier, 2021-04-15) Syed, Abdul Khadar; Zhang, Xiang; Caballero, Armando; Shamir, Muhammad; Williams, Stewart
    This paper investigates the influence of two different deposition strategies, oscillation and parallel pass, on the tensile and high cycle fatigue properties of a wire + arc additive manufactured Ti-6Al-4V alloy in the as-built condition. In the oscillation build, the plasma torch and the wire feeder continuously oscillated across the wall thickness direction. In contrast, four single layers were deposited consecutively in the same direction along the wall length in the parallel pass build. Test specimens were manufactured in horizontal and vertical orientation with respect to the deposited layers. Compared with the parallel pass build, the oscillation build had lower static strength due to its coarser transformation microstructure. However, the elongation values were similar. The presence of columnar primary β grains has resulted in anisotropic elongation values. The vertical samples with loading axis parallel to the primary β grains showed 40% higher elongation than the horizontal samples. The fatigue strength was comparable with its wrought counterpart and greater than typical material by casting. At 107 cycles, fatigue strength of 600 MPa was achieved for the oscillation build vertical samples and the parallel pass build in both orientations. Only the oscillation build horizontal samples had lower fatigue strength of 500 MPa. Fractography analysis showed that most of the samples (about 70%) had crack initiation from pores, about 20% samples had crack initiated from microstructural features and the rest did not failed (runouts at 107 cycles).
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    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.
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    The role of microstructure and local crystallographic orientation near porosity defects on the high cycle fatigue life of an additive manufactured Ti-6Al-4V
    (Elsevier, 2020-08-21) Shamir, Muhammad; Syed, Abdul Khadar; Janik, Vit; Biswal, Romali; Zhang, Xiang
    Titanium alloys such as Ti-6Al-4V built by most of the additive manufacturing processes are known to contain process induced defects, non-conventional microstructure and strong crystallographic texture; all of which can affect the fatigue strength. In this study we evaluated the effect of crystallographic orientation of α and α lath width around gas pore defects on the high cycle fatigue life of Wire + Arc Additive Manufactured Ti-6Al-4V by means of Electron Back Scattered Diffraction. Here we show that variations in crystallographic orientation of α lath and its width in the vicinity of the crack initiating defect were the main reasons for the considerable scatter in fatigue life. Pyramidal slip systems with high Schmid factor active around the defects resulted in longer fatigue life compared to pyramidal slip with lower Schmid factor. In the absence of pyramidal slip, cracks initiated from active prismatic slip systems. When considering the influence of the microstructure, a higher number of smaller α laths around the defect resulted in longer fatigue life, and vice versa. Overall, the fatigue crack initiation stage was controlled collectively by the complex interaction of porosity characteristics, α lath width and its crystallographic orientation at the crack initiation location