Impingement Density Analysis on Heat Transfer and the Appearance of Edge Cracks in Conventional Slab Using Hydraulic Nozzles

In this work, the fluid dynamics and heat transfer of two hydraulic nozzles used in the secondary cooling of the conventional slab continuous casting machine were analyzed. Impingement density maps, the jet opening angle and heat flux associated with different operating conditions (impingement distance, pressure) were experimentally determined. The opening angle and impingement density footprint were found to vary considerably in shape and magnitude with varying operating pressure and distances. Finally, it was found that when short operating distances are used, a greater heat extraction gradient occurs in the major axis of the impingement footprint, which promotes edge-cracks in the slab in plant.

Effect of Excess Atomic Volume on Crack Evolution in a Deformed Iron Single Crystal

This paper presents a molecular dynamics study of how the localization and transfer of excess atomic volume by structural defects affects the evolution and self-healing of nanosized cracks in bcc iron single crystals under different mechanical loading conditions at room temperature. It is shown that deformation is initially accompanied by a local growth of the atomic volume at the crack tips. The crack growth behavior depends on whether the excess atomic volume can be transferred by structural defects from the crack tips to the free surface or other interfaces. If an edge crack is oriented with respect to the loading direction so that dislocations are not emitted from its tip or only twins are emitted, then the sample undergoes a brittle-ductile fracture. The transfer of the excess atomic volume by dislocations from the crack tips prevents the opening of edge cracks and is an effective healing mechanism for nanocracks in a mechanically loaded material.

Initiation and Suppression of Crack Propagation during Magnesium Alloy Rolling

The conventional rolling of magnesium alloy with a single pass and large reduction will cause severe edge cracking. The sheet without cracks can be achieved by limited width rolling. The microstructure evolution of the sheet with cracks after conventional rolling and the sheet without cracks after limited width rolling is explored, and an effective mechanism for solving edge cracks is proposed. Conventional rolling can fully develop twin evolution due to high deformation, and three stages of twinning evolution can be observed and the secondary twins easily become the nucleation points of micro cracks, resulting in a large number of cracks propagating along the twin lamellae. Cracks terminate at dislocation accumulation because the accumulation of a large number of dislocations can hinder propagation. Dislocation shearing of twins to eliminate the high localization caused by twins and induce the tensile twins to weaken the basal surface texture provides an effective plastic deformation mechanism of crack inhibition, which is useful for expanding the engineering application of magnesium alloy rolled sheets.

Thermal fracture of functionally graded thermal barrier coatings with pre-existing edge cracks and multiple internal cracks imitating a curved interface

This work is devoted to the problem of thermal fracture of a functionally graded coating on a homogeneous substrate (FGC/H) with an emphasis on the analysis of a special system of cracks that simulates a curved interface. The FGC/H structure contains the pre-existing crack system in the FGC, both edge cracks (which are often seen in FGC/H structures) and internal cracks. The stress intensity factors are calculated. (Generally, both Mode I and Mode II are nonzero.) Then, using the appropriate fracture criterion for mixed-mode fracture conditions, the crack propagation direction (so-called fracture angles) and critical loads, when this propagation is initiated, are determined. The application of fracture criteria requires knowledge of the fracture toughness near the crack tips. Thus, it is assumed that the fracture toughness of an FGC, as well as other material properties, continuously varies through the thickness of the coating. For multiple cracks, it is also important to know the weakest crack that starts to propagate first, and the initial direction of this growth. Therefore, the main attention is paid to the evaluation of the fracture angles for the cracks for different parameters of the FGC/H structure. Both cases of a homogeneous semi-infinite medium with a system of cracks imitating a curved interface and FGC/H structures with identical crack systems are studied.

Perbandingan Penilaian Kondisi Perkerasan Jalan Menggunakan Metode Pavement Condition Index dan Metode Bina Marga) (Studi Kasus Ruas Jalan Laksamana R.E. Martadinata – Bandar Lampung)

Jalan merupakan prasarana dalam mendukung laju perekonomian serta berperan sangat besar dalam kemajuan dan perkembangan suatu daerah. Dengan pertumbuhan lalu lintas yang semakin cepat, maka harus diimbangi pula dengan peningkatan sarana transportasi yang memadai sehingga ruas jalan tidak menimbulkan hambatan dan kemacetan. Penelitian ini bertujuan untuk mengetahui nilai kondisi perkerasan jalan berdasarkan Metode Pavement Condition Index dan Metode Bina Marga serta membandingkan kedua metode tersebut. Berdasarkan hasil analisis kondisi perkerasan jalan dengan Metode Pavement Condition Index menunjukkan bahwa kerusakan yang terjadi yaitu retak buaya (alligator cracks), retak tepi (edge cracks), pengausan agregat (polished aggregate) dan Lubang (potholes) dengan nilai PCI keseluruhan adalah 94,25 sempurna (excellent). Sedangkan hasil analisis kondisi perkerasan jalan dengan Metode Bina Marga menunjukkan bahwa kerusakan yang terjadi yaitu lubang dan retak dengan nilai Bina Marga keseluruhan adalah 0,36 baik (good). Pada metode PCI mendapatkan nilai sempurna (excellent) yang beraarti belum perlu adanya perbaikan. Sedangkan pada metode Bina Marga mendapatkan nilai baik (good) dan masuk dalam program pemeliharaan rutin.

Stress Corrosion Cracking of Graphene

We use molecular dynamics (MD) simulations to study the stress corrosion cracking (SCC) of monolayer graphene sheets with an initial edge cracks. Two types of edge cracks are considered in the simulations; one with armchair edges and another one with zigzag edges. All the simulations are conducted at 300 K and the corrosive environment is O2 molecules. Tensile stresses are induced in the graphene sheet by applying mode–I loading. To understand the mechanism of the sub–critical crack growth during SCC, we expose the graphene sheets to O2 molecules at strains of 0.047 and 0.076. Our MD simulations capture the chemisorption process between the O2 molecules and pre–stressed graphene sheet. Oxygen molecules react with carbon radicals at the edges of the crack tip and gets adsorbed to the graphene surface. The atomic stresses in the vicinity of crack tip relaxes due to the adsorption of O2 molecule. Our results show that the reaction of O2 molecules with the carbon radicals at the crack tip can cause the failure of C–C bonds which leads to the sub critical cracking.