13 results found
Walport F, Zhang R, Meng X, et al., 2023, The softening effect of welding on the mechanical properties of cold-worked stainless steel, Journal of Constructional Steel Research, Vol: 211, ISSN: 0143-974X
Cold-working can occur during production of flat sheet products and during fabrication of structural cross-sections. In both cases, when the material is cold-worked, plastic deformations result in material strength enhancements. These strength enhancements are particularly significant for materials such as stainless steel, which exhibit rounded stress-strain behaviour and pronounced strain hardening, and for hollow structural sections, where the strength increases arise in both the corner and flat regions of the cross-sections. Numerous studies have shown the importance of utilising this strength enhancement for efficient structural design, and predictive models have been derived for harnessing these enhancements. However, if cold-worked material is welded, some of the enhanced strength can be lost due to partial softening in the heat affected zone (HAZ). The extent of this strength loss is investigated experimentally in the present study. The experimental programme comprised ten tensile coupon tests on 1 mm thick austenitic (Grade 1.4301) stainless steel sheet material, with central welds parallel and transverse to the direction of cold-rolling, as well as twelve full cross-section tensile tests on 80x60 mm and 60x60 mm hollow sections with thicknesses varying between 2 and 4 mm. Digital image correlation was utilised to determine the local constitutive properties of the base metal, the heat affected zone, and the weld metal of each specimen. The hardness and microstructure of the welded samples, along with the widths of the weld and heat affected zone, were also characterised.
Zhang R, Gardner L, Amraei M, et al., 2023, Mechanical and microstructural testing of additively manufactured stainless steel with laser welded joints, NOLAMP- Nordic Laser Materials Processing Conference
Zhang R, Gardner L, Amraei M, et al., 2023, Testing and analysis of additively manufactured stainless steel corrugated cylindrical shells in compression, Journal of Engineering Mechanics, Vol: 149, ISSN: 0733-9399
Initial geometric imperfections have been identified as the main cause for the large discrepancies between experimental and theoretical buckling loads of thin-walled circular cylindrical shells under axial compression. The extreme sensitivity to imperfections has been previously addressed and mitigated through the introduction of stiffeners; however, sensitivity still remains. Optimized corrugated cylindrical shells are largely insensitive to imperfections and hence exhibit excellent load-bearing capacities, but their complex geometries make their construction difficult and costly using conventional manufacturing techniques. This was overcome in the present study through additive manufacturing (AM). Nine optimized corrugated shells with different diameter-to-thickness ratios, together with one reference circular cylindrical shell, were additively manufactured by means of powder bed fusion (PBF) from austenitic and martensitic precipitation hardening stainless steel. The structural behavior of the AM shells was then investigated experimentally with the testing program comprising tensile coupon tests, measurements of basic geometric properties, and axial compression tests. Numerical analyses were also conducted following completion of the physical experiments. The experimental and numerical results verified the effectiveness of optimized corrugated cylindrical shells in achieving improved local buckling capacity and reduced imperfection sensitivity. Initial recommendations for the structural design of the studied cross-sections are made.
Zhang R, Meng X, Gardner L, 2022, Shape optimisation of stainless steel corrugated cylindrical shells for additive manufacturing, Engineering Structures, Vol: 270, Pages: 1-14, ISSN: 0141-0296
Axially compressed circular cylindrical shells with large diameter-to-thickness ratios are highly susceptible to local buckling, and their load-carrying capacities are known to be very sensitive to initial geometric imperfections. Hence, severe knock-down factors on their theoretical buckling loads are typically prescribed in design specifications, which greatly impair their structural efficiency. With the aim of enhancing load-bearing resistance and reducing sensitivity to imperfections, the shape optimisation and assessment of compressed free-form wavy cylindrical shells, the realisation of which is now viable through additive manufacturing, are the subject of the present study. The adopted optimisation framework employs the Particle Swarm Optimisation (PSO) algorithm, integrating computer-aided geometric design, nonlinear numerical simulations and imperfection sensitivity analyses. The structural performance of the optimised free-form wavy shells is analysed and compared to that of reference circular shells, as well as other types of non circular shell profiles, including sinusoidally corrugated shells, Aster shells and stringer-stiffenedshells. The optimised free-form wavy shell profiles are shown to exhibit increases in ultimate stress of up to 136% compared with the reference circular shell profiles; in general, greater benefits are achieved for more slender cross-sections. In future work, the proposed optimised shells will be manufactured in stainless steel by means of powder bed fusion (PBF), and their structural performance will be further verified through physical experiments. Keywords: Additive manufacturing; Axial compression; Corrugated cylindrical shells; Imperfection sensitivity; Particle Swarm Optimisation (PSO); Shell buckling; Stainless steel; 3D printing.
Zhang R, Mohsen A, Piili H, et al., 2022, Microstructure, Mechanical Properties and Cross‐sectional Behaviour of Additively Manufactured Stainless Steel Cylindrical Shells, SDSS 2022 - The International Colloquium on Stability and Ductility of Steel Structures
Huang C, Kyvelou P, Zhang R, et al., 2022, Mechanical testing and microstructural analysis of wire arc additively manufactured steels, Materials and Design, Vol: 216, ISSN: 0264-1275
Wire arc additive manufacturing (WAAM) is a metal 3D printing method that allows the cost-effective and efficient production of large-scale elements, and has thus gained great interest from architects and structural engineers. Integration of this novel technology into the construction industry, however, requires the development of a clear understanding of the mechanical behaviour of WAAM materials. To this end, a comprehensive experimental study into the mechanical properties and microstructure of WAAM plates made of normal- and high-strength steels has been undertaken and is reported herein. A total of 137 as-built and machined tensile coupons were tested, extracted in various directions relative to the print layer orientation from WAAM plates of two nominal thicknesses, built using different deposition strategies. Theinfluence of the geometric undulations inherent to the WAAM process and deposition strategy on the resulting mechanical properties was investigated. Microstructural characterisation was also performed by means of optical microscopy (OM) and electron backscatter diffraction (EBSD). The WAAM normal-strength steel plates exhibited a principally ferritic-pearlitic microstructure, while the WAAM high-strength steel plates displayed a mixed microstructure featuring ferrite, bainite and martensite. The EBSD analysis revealed a weak crystallographic texture, which explained the observed mechanical properties being almost isotropic. No significant differences in tensile properties were observed with the different deposition strategies, except for some variation in ductility. The geometric undulations of the as-built coupons resulted in some reduction in effective mechanical properties and a degree of anisotropy. Overall, the examined WAAM material exhibited consistent mechanical properties, a Young’s modulus comparable to conventionally-produced steel plates, marginally lower strength, reflecting the slower cooling conditions than is customary, and good
Zhang R, Buchanan C, Matilainen V-P, et al., 2021, Mechanical properties and microstructure of additively manufactured stainless steel with laser welded joints, Materials and Design, Vol: 208, Pages: 1-20, ISSN: 0264-1275
Powder bed fusion (PBF) is a commonly employed metal additive manufacturing (AM) process in which components are built, layer-by-layer, using metallic powder. The component size is limited by the internal build volume of the employed PBF AM equipment; the fabrication of components larger than this volume therefore requires mechanical joining methods, such as laser welding. There are, however, very limited test data on the mechanical performance of PBF metal with laser welded joints. In this study, the mechanical properties of PBF built 316L stainless steel parts, joined together using laser welding to form larger components, have been investigated; the microstructure of the components has also been examined. 33 PBF 316L stainless steel tensile coupons, with central laser welds, welded using a range of welding parameters, and with coupon half parts built in two different orientations, were tested. The porosity, microhardness and microstructure of the welded coupons, along with the widths of the weld and heat-affected zone (HAZ), were characterised. The PBF base metal exhibited a typical cellular microstructure, while the weld consisted of equiaxed, columnar and cellular dendrite microstructures. Narrow weld regions and HAZs were observed. The PBF base metal was found to have higher proof and ultimate strengths, but a similar fracture strain and a lower Young’s modulus, compared with conventionally manufactured 316L stainless steel. The strengths were dependent on the build direction – the vertically built specimens showed lower proof strengths than the horizontal specimens. The laser welds generally exhibited lower microhardness, proof strengths and fracture strains than the PBF base metal which correlated with the observed structure. This work has demonstrated that PBF built parts can be joined by laser welding to form larger components and provided insight into the resulting strength and ductility.
Zhang R, Gardner L, Buchanan C, 2021, Testing of additively manufactured stainless steel material and cross-sections, The 10th International Conference on Advances in Steel Structures
Zhang R, Gardner L, Meng X, et al., 2021, Optimisation and compressive testing of additively manufactured stainless steel corrugated shells, Eurosteel 2021 – 9th European Conference on Steel and Composite Structures
Zhang R, Gardner L, Buchanan C, et al., 2021, Testing and analysis of additively manufactured stainless steel CHS in compression, Thin Walled Structures, Vol: 159, ISSN: 0263-8231
Additive manufacturing, also referred to as 3D printing, has the potential to revolutionise the construction industry, offering opportunities for enhanced design freedom and reduced material use. There is currently, however, very limited data concerning the performance of additively manufactured metallic structural elements. To address this, an experimental and numerical investigation into the cross-sectional behaviour of circular hollow sections (CHS),produced by powder bed fusion (PBF) from Grade 316L stainless steel powder, is presented. The experimental programme comprised tensile coupon tests, initial geometric imperfection measurements and five axially loaded stub column tests on specimens with a range of diameter to-thickness (D/t) ratios. Similar cross-sectional behaviour to that of conventionally produced stainless steel CHS was observed, with the more slender cross-sections displaying increased susceptible to local buckling. In parallel with the experimental study, numerical simulations were carried out initially to replicate the experimental results and then to conduct parametric studies to extend the cross-sectional capacity data over a wider range of D/t ratios. The generated experimental and numerical results, together with other available test data on stainless steel CHS from the literature, were used to evaluate the applicability of existing design approaches for conventionally formed sections to those produced by additive manufacturing. Keywords: Additive manufacturing; Circular hollow sections; Current design approaches; Digital image correlation (DIC); Powder bed fusion (PBF); Stainless steel; Stub column testing; Tensile coupon tests; 3D printing.
Zhang R, Liu J, Wang W, et al., 2020, Fire behaviour of thin-walled steel tube confined reinforced concrete stub columns under axial compression, JOURNAL OF CONSTRUCTIONAL STEEL RESEARCH, Vol: 172, ISSN: 0143-974X
Li J, Zhang R, Liu J, et al., 2018, Determination of the natural frequencies of a prestressed cable RC truss floor system, MEASUREMENT, Vol: 122, Pages: 582-590, ISSN: 0263-2241
Cao L, Liu J, Li J, et al., 2018, Experimental and analytical studies on the vibration serviceability of long-span prestressed concrete floor, Earthquake Engineering and Engineering Vibration, Vol: 17, Pages: 417-428, ISSN: 1993-503X
An extensive experimental and theoretical research study was undertaken to study the vibration serviceability of a long-span prestressed concrete floor system to be used in the lounge of a major airport. Specifically, jumping impact tests were carried out to obtain the floor’s modal parameters, followed by an analysis of the distribution of peak accelerations. Running tests were also performed to capture the acceleration responses. The prestressed concrete floor was found to have a low fundamental natural frequency (≈ 8.86 Hz) corresponding to the average modal damping ratio of ≈ 2.17%. A coefficients βrp is proposed for convenient calculation of the maximum root-mean-square acceleration for running. In the theoretical analysis, the prestressed concrete floor under running excitation is treated as a two-span continuous anisotropic rectangular plate with simply-supported edges. The calculated analytical results (natural frequencies and root-mean-square acceleration) agree well with the experimental ones. The analytical approach is thus validated.
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