Browsing by Author "Li, L."
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Item Open Access Fatigue strengthening of damaged steel members using wire arc additive manufacturing(Elsevier, 2023-03-17) Ghafoori, E.; Dahaghin, H.; Diao, Chenglei; Pichler, N.; Li, L.; Mohri, M.; Ding, Jialuo; Ganguly, Supriyo; Williams, StewartIn this study, a directed energy deposition (DED) process called wire arc additive manufacturing (WAAM) is employed for the fatigue strengthening of damaged steel members. Three steel specimens with central cracks were tested under a high-cycle fatigue loading (HCF) regime: (1) the reference specimen; (2) the WAAM-repaired specimen with an as-deposited profile, and (3) the WAAM-repaired specimen machined to reduce stress concentration factors (SCF). The corresponding finite element (FE) simulation of the WAAM process was calibrated using static experimental results, which revealed the main mechanism. The process was found to introduce compressive residual stresses at the crack tip owing to the thermal contraction of the repair. The FE results also revealed that stress concentration exists at the root of the as-deposited WAAM; this stress concentration can be mitigated by machining the WAAM to a pyramid-like shape. The fractography analysis indicated that the cracks were initiated at the WAAM-steel interface, and microscopic observations revealed that the microcracks were arrested by the porosities in the melted interface. The results of this pioneering study suggest that WAAM repair is a promising technique for combating fatigue damage in steel structures.Item Open Access Measurement and modelling of the residual stresses in autogenous and narrow gap laser welded AISI grade 316L stainless steel plates(Elsevier, 2016-09-22) Elmesalamy, A. S.; Abdolvand, H.; Walsh, J. N.; Francis, J. A.; Suder, Wojciech; Williams, Stewart W.; Li, L.Thick-section austenitic stainless steels have widespread industrial applications, where stress-corrosion cracking is often of major concern. Problems tend to arise in the vicinity of welds, where substantial residual stresses often reside. This paper describes an investigation into the residual stresses in autogenous high power laser welds and narrow gap laser welds (NGLW) in 10 mm thick AISI grade 316L steel plates, using both neutron diffraction and the contour method. The influences of laser power, welding speed and the time interval between weld passes on residual stress were analysed. For the NGLW process, finite element modelling was employed to understand the influence of thermal history on residual stress. The results for the NGLW technique show that the laser power has a significant effect on the peak value of residual stress, while the welding speed has a more significant influence on the width of the region sustaining tensile stresses.Item Open Access Measurement of turbulent flame speed of natural gas/air mixtures at elevated pressure(2010-01-31T00:00:00Z) Li, L.; Witton, John J.Turbulent flame speeds were measured over a range of pressures to 0.8 MPa using a jet flow apparatus fired with a synthetic mixture representing a mid-European natural gas. The equivalence ratio empty set was 0.9. The gas contained significant proportions of ballast gases and higher hydrocarbons. The method adopted was the 'flame angle' technique, using schlieren imaging to obtain the flame vertex angle from the peak density gradient. Image analysis techniques were developed to reduce interpretation errors and give an unbiased result. The data show higher flame speeds than those obtained with pure methane at elevated pressures, using similar methodology, and has an application in numerical modelling of combustors.Item Open Access Schlieren methods for high pressure turbulent jet flames(2009-06-01T00:00:00Z) Li, L.; Fu, Shan; Witton, John J.Turbulent flame speeds of a premixed synthetic natural gas mixture were measured using a jet flow apparatus over a range of pressures to 0·8 MPa at an equivalence ratio of 0·9. The method adopted was the 'flame angle' technique, using schlieren imaging to obtain the flame vertex angle from the peak density gradient. Image analysis techniques were developed to reduce interpretation errors and to give information on the velocity and combined gradient distribution local to the flame fron