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Experiments on Laser powder bed fusion (LPBF) of powdered Ti on Al2O3 substrate were conducted and the interface formation was studied using a multi-material fluid dynamics model. Results show that the melt pool is relatively shallow, with relatively flat interlayer interface under LPBF's conduction mode. In this condition, a thin sheath of molten Al2O3 forms and acts as a lubricating film for the molten Ti, leading to Rayleigh instability due to high flow inertia. Keyhole formation penetrates the Al2O3 substrate, resulting in a wavy interlayer interface. The recoil pressure from the keyhole and overall melt inertia are suppressed by the highly viscous molten Al2O3, thereby improving single-track melt pool stability. However, the thermal expansion coefficient difference between Ti and Al2O3 led to the formation of transverse cracks. Achieving a defect-free metal-on-ceramic single track remains a challenge, despite this study serving as a guide for melt track and interface control. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) offers the advantages of direct near-net-shape production, reduced material waste, and shortened production lead time, showing great potential to revolutionise the manufacturing industry. The flexible movements of the deposition head and the build platform allow directed energy deposition (DED) to conduct the repair process of damaged high-value metallic parts. However, the resulting heterogeneous microstructure and its effect on the mechanical properties of the repaired parts have not been widely realised. In this work, the repair of Monel alloy parts, known for their excellent mechanical properties and high corrosion resistance, was conducted by the laser-DED process with low and high laser power settings, respectively. Different from the fine, equiaxed grains in the original part, the as-deposited Monel alloy consists of large columnar grains. The mechanical performance across the interface between the original and newly deposited material was tested and analysed along horizontal and vertical loading directions. The yield strength and elongation of repaired Monel alloy parts were 409.1 MPa and 35.2% along the horizontal loading direction, which both significantly surpass the corresponding values along the vertical loading direction. This study lays the groundwork for designing a laser-DED process to achieve high performance for repaired metallic parts. [ABSTRACT FROM AUTHOR]

Multiple cracks can be observed in many of engineering structures such as pressure vessels and pipelines. Under continuous loading conditions, these small and closely distanced multiple cracks can grow and coalesce into a large one. Subsequently, it will pose a serious challenge to the integrity and safety of the engineering structures. Although a lot of research works were carried out for predicting fatigue growth of multiple cracks, few literatures focusing on nonlinear elastic-plastic analysis of multiple cracks' fracture behaviors can be referred to. Therefore, to understand the influence of multiple cracks on integrity and safety of offshore pipelines is indeed desirable in engineering practice. In this study the systematic analyses on the fracture behaviors of two collinear 3-D cracks are performed for the pipelines subjected to a series of the loading conditions. A parametric study on the effect of different separation distances of the two interacting collinear cracks is performed. Based on the numerical results, the interaction factor is introduced to quantify the interaction of the two interacting cracks, and the proposed function for interaction factor can be useful for the preliminary fracture assessment of the surface crack affected by the interactions. Moreover, for biaxial loadings, the results indicate that the most severe fracture response can be produced by the tension load combined with high internal pressure. [ABSTRACT FROM AUTHOR]

Abstract: Offshore pipelines installed by reeling method are subjected to large plastic strains. When the steel pipes are joined by girth welding, both surface and embedded cracks are inevitably initiated in welding zone. The pipe lines should have adequate resistance against both crack extension by tearing and unstable fracture during installation as well as during operation. However, common flaw assessment procedures, e.g. BS 7910:2005 , are not explicitly developed for such situations with large plastic strains. The main objective of the current paper is to find a way to assess fracture capability of a practical pipeline subject to large plastic deformation. In our study, the evolution of crack tip opening displacement (CTOD) of the pipeline with semi-eliptical surface crack in weld girth is investigated under biaxial loading conditions (uniaxial tensile and internal pressure) using three dimensional elastic–plastic finite element analysis. The effects of crack depth, ratio of semi-major axis to semi-minor axis and internal pressure are examined. The results show that at moderate levels of global strain, the variation of CTOD with global strain can be well approximated by a simple linear relationship under tensile loading as well as biaxial loading conditions. Comparing the fracture assessment for the welded pipe made by BS 7910:2005 with that made by our current study, it is found that the assessment from BS7910:2005 is over conservative. A CTOD-estimation method for strain -based fracture is suggested for the pipelines when the global strain is 3%. [Copyright &y& Elsevier]

A new framework of non-local model for the strain energy density is proposed in this paper. The global strain energy density of the representative volume element is treated as a non-local variable and can be obtained through a special integral of the local strain energy density. The local strain energy density is assumed to be dependent on both the strain and the rotation-gradient. As a result of the non-local model, a new strain gradient theory is derived directly, in which the first and second strain gradients, as well as the triadic and tetradic stress, are introduced in the context of work conjugate. For power law hardening materials, size effects in thin metallic wire torsion and ultra-thin cantilever beam bend are investigated. It is found that the result predicted by the theoretical model is well consistent with the experimental data for the thin wire torsion. On the other hand, the calculation result for the micro-cantilever beam bend clearly shows the size effect. [ABSTRACT FROM AUTHOR]