Jianxun Zhang

Effects of welding residual stresses on fatigue crack growth behaviour in butt welds of a pipeline steel

Abstract In the present test the fatigue crack growth rate in the parent plate, weld and cross-bond regions was measured and the results were correlated with the stress intensity range ΔK and the effective stress intensity range ΔK eff . It is indicated that the welding residual stresses strongly affect the crack growth rate. For the weld metal and cross-bond compact tension specimens in which crack growth is along the weld line the fatigue crack growth rate increases as the crack grows. However, for the T compact tension specimen in which crack growth is perpendicular to the weld line at a constant value of applied ΔK the crack growth rate initially decreases as the crack grows. Particularly, at a low constant value of applied ΔK the crack growth rate obviously decreases and the crack fails to grow after short crack growth. When the crack grows to intersect the welded zone, the fatigue crack growth rate gradually increases as the crack grows further. It is clear that the effect of welding residual stresses on the crack growth rate is related to the position of the crack and its orientation with respect to the weld line. Finally, the models of welding residual stress redistribution in the compact tension specimens with the growing crack and its influence on the fatigue crack closure are discussed. It appears that for a butt-welded joint one of the crack closure mechanisms may be considered by the bend or rotation deformation of crack faces due to the welding residual stress redistribution as the fatigue crack grows in the welded joint.

An investigation on the effects of side assisting gas flow and metallic vapour jet on the stability of keyhole and molten pool during laser full-penetration welding

This paper reports on a study aiming at separating the effects of side assisting gas flow from a metallic vapour jet on the transient behaviour of a molten pool and a keyhole during laser full-penetration welding. To achieve the research purpose, laser welding process was simulated under three different conditions: in the presence of both side gas flow and metallic vapour jet, in the presence of side gas flow alone and in the presence of metallic vapour jet alone. It was found that the side gas flow not only pushed the molten melt to flow towards the rear part of the molten pool but also formed, on the molten pool surface, a proper pressure distribution which helped maintain both the humping in the rear part of the molten pool and the concave around the upper exit of the keyhole. Under the condition with side gas flow, the swelling formed around the keyhole blocked the side gas flow and on the other hand the side gas flow pushed them to flow backwards to the rear part of the molten pool more effectively, thereby enlarging and stabilizing the keyhole exit. Furthermore, the peak value of the average pressure in the region composed of the molten pool and keyhole decreased step by step with the growth of the concave. Finally, the interaction between the metallic vapour and molten melt was well controlled using the side gas flow, which led to an improvement in the stability of the molten pool and a reduction in spatters and pores.

Studies on the fracture mechanics parameters of weldment with mechanical heterogeneity

Abstract The fully plastic solutions of welded centre-cracked strip for plane stress problem were carefully investigated with the fully plastic finite element method. It was introduced for assessing the fracture mechanics parameters of weldment with mechanical heterogeneity that there existed an equivalent yielding stress and equivalent strain hardening exponent in the vicinity of crack tip keeping the assessment of fracture mechanics parameters of weldment in the same way as the homogeneous material. The equivalent yielding stress and equivalent strain hardening exponent of various matched weldment were computed and the effect of weld metal width were calculated and discussed on equivalent yielding stress and equivalent strain hardening exponent near crack tip. The engineering approach was given for estimating the fracture mechanics parameters of weldment with mechanical heterogeneity in elastic-plastic range.

Large Deflection of Geometrically Asymmetric Metal Foam Core Sandwich Beam Transversely Loaded by a Flat Punch

The objective of this work is to study the large deflection of geometrically asymmetric metal foam core sandwich beam under transverse loading by a fiat punch. A yield criterion is proposed for geometrically asymmetric metal foam core sandwich structures, and then analytical solution for the large deflection of a fully clamped slender sandwich beam is obtained, in which the interaction of bending and stretching induced by large deflection is considered. Also, finite element (FE) analysis is carried out. Comparisons of the analytical solution with numerical results are presented and good agreements are found. The effects of asymmetric factor, core strength and loading punch size on the structural response of asymmetric sandwich beam are discussed in detail. It is shown that the axial stretching induced by large deflection has significant effect on the load-carrying and energy absorption capacities of geometrically asymmetric sandwich structure, but the effect of asymmetry is negligible as the deflection is larger than the depth of the sandwich beam.

A YIELD CRITERION AND PLASTIC ANALYSIS FOR PHYSICALLY ASYMMETRIC SANDWICH BEAM WITH METAL FOAM CORE

A yield criterion for physically asymmetric sandwich cross-sections is proposed in this paper. Using the yield criterion, analytical solutions for the large deflections of fully clamped asymmetric slender sandwich beams transversely loaded by a flat punch at the midspan are derived considering the core strength effect and interaction of bending and axial stretching. Finite element (FE) method is employed to predict the large deflection behavior of the sandwich beams. Good agreement is achieved between the analytical predictions and FE results. Effects of asymmetric factor, core strength and loading punch size are also discussed. It is demonstrated that core strength and loading punch size have significant influences on the load-carrying and energy absorption capacities of physically asymmetric metal sandwich beams while the asymmetry effect could be neglected when the deflection exceeds sandwich beam depth.

Investigation into manufacturing Fe–Cu–C alloy parts through indirect selective laser sintering

Abstract The properties of alloy parts can be adjusted conveniently if alloy element powders are used for manufacturing alloy parts by indirect selective laser sintering, but no research has been reported on this so far. In this paper several composite powders have been obtained by blending pure Fe, Cu and graphite powders, which have been used to produce green parts by indirect selective laser sintering. Cu infiltrated Fe–Cu–C alloy was manufactured after green parts had been degreased, high temperature sintered and infiltrated. Contents of composite powders, post-processing of green parts, microstructures and mechanical property of alloys were investigated. The results indicate that: Cu and C can diffuse into γ-Fe when green parts are being sintered at high temperature; the microstructures of alloy are composed of a large quantity of eutectoid structures mixtures of Fe–C and Fe–Cu at room temperature besides a coarse structure; the eutectoid structure varies regionally owing to the inhomogeneous concentration of Cu and C; the yield strength of Fe–Cu–C sample alloys is >300 MPa and the elongation is &lt3%.

The Failure Behavior of Geometrically Asymmetric Metal Foam Core Sandwich Beams Under Three-Point Bending

The failure behavior of geometrically asymmetric sandwich beams with a metal foam core is analytically and experimentally investigated. New initial failure modes of the asymmetric sandwich beams are observed under three-point bending, i.e., face yield, face wrinkling, core shear A, core shear AB, core shear A-AB, and indentation. It is shown that the initial failure modes of sandwich beams depend on the span of the beam, the thicknesses of top and bottom face sheets, core height and material properties. We derived the analytical formulae for the initial failure loads and then constructed the initial failure mechanism maps for the geometrically asymmetric sandwich beams. It is shown that the analytically predicted initial failure mechanism maps are in good agreement with the experimental results, which are clearly different from the symmetric sandwich beams. As a preliminary application, the minimum weight designs are presented for asymmetric metal sandwich beams.

Indentation of sandwich beams with metal foam core

Abstract The quasi-static indentation behavior of sandwich beams with a metal foam core was investigated. An analytical model was developed to predict the large deflections of indention of the sandwich beams with a metal foam core subjected to a concentrated loading. The interaction of plastic bending and stretching in the local deformation regions of the face sheet was considered in the analytical model. Moreover, the effects of the shear strength of the foam core on the indentation behavior were discussed in detail. The finite element simulations were preformed to validate the theoretical model. Comparisons between the analytical predictions and finite element results were conducted and good agreement was achieved. The results show that the membrane force dominates indentation behavior of the sandwich beams when the maximum deflection exceeds the thickness of the face sheet.

Synthesis, evaluation and thermodynamics of a 1H-benzo-imidazole phenanthroline derivative as a novel inhibitor for mild steel against acidic corrosion

A novel 1H-benzo-imidazole phenanthroline derivative (1) was developed as a corrosion inhibitor for mild steel in 1.0 M HCl solution with temperature ranging from 25 °C to 90 °C. Potentiodynamic polarization, electrochemical impedance spectroscopy and weight loss indicates that inhibitor 1 can efficiently protect mild steel from corrosion in acidic medium with better performance under moderately higher concentration and temperature. Based on thermodynamic and kinetic analysis, it is observed that 1 functioned as a mixed-type inhibitor and obeyed Langmuir adsorption isotherm via chemisorption. Further assisted by synergistic effect of iodide ions, inhibition efficiency can be dramatically enhanced up to 99.2%.

An investigation of ultrasonic nanocrystal surface modification machining process by numerical simulation

A simplified model of UNSM was developed by equivalent static loading method.A 3-D model was built to investigate the machining process of UNSM.Small and stable energy dissipation of UNSM process can be found.Lower machining linear velocity can produce severe surface work hardening.Static loading has a greater impact to UNSM result comparing others. As a method for surface severe plastic deformation (S2PD), ultrasonic nanocrystal surface modification (UNSM) enhances metal surface properties through striker peening, a metal dimpling process driven by ultrasonic vibration energy. UNSM treatment introduces residual stress, surface hardening, and nano-crystalline structures into metal surfaces which are beneficial for reducing wear, fatigue, and corrosion properties. In this paper, the process of UNSM is described and a simplified physical model created using the equivalent static loading method is presented. Along with the simplified physical model, a finite elements simulation model was developed. Effective plastic strain was considered as a parameter for evaluating the level of work hardening produced in the simulation. The dynamic processes and energy dissipation were also examined, and it was found that different kinds of energy dissipation occur during UNSM treatment. Comparisons between the processing parameters (processing velocity, static load, and feed rate) were performed using a simulated example of UNSM linear processing. The results show that the linear processing produces a uniform region containing identical distributions of residual stress and effective plastic strain. The effects of the parameters on the processing results (residual stress, plastic deformation and work hardening) were likewise studied using UNSM linear processing. Compared to processing velocity, a high static load produced more work hardening and higher compressive residual stress. Surface deformation and residual stress results were also more sensitive to static load than processing velocity. Feed rate was found to be an important parameter as well, greatly influencing both surface deformation and work hardening.