Key Engineering Materials Vols. 297-300

Abstract: Synchrotron X-ray microtomography has been utilized for the 3D characterisation of microstructure in the cell materials of aluminium foams. Tomographs, consisting of about 109 isotropic voxels with a maximum of 1.0µm edge, were collected at the SPring-8 in Japan. A combination of high-resolution phase contrast imaging technique and several state-of–the-art application techniques has enabled the quantitative image analyses of micro-pore, intermetallic particles and grain boundary as well as the assessment of their effects on compressive deformation and fracture behaviours in two kinds of aluminium foams.

Abstract: This paper introduces a newly developed three-dimensional Material Failure Process Analysis code, MFPA3D to model the failure processes of brittle materials, such as concrete, ceramics, fibrous materials, and rocks. This numerical code, based on a stress analysis method (finite element method) and a material failure constitutive law, can be taken as a tool in numerical modeling analysis to enhance our understanding of the failure mechanisms of brittle materials. Properties of material heterogeneity are taken into account. The material is discretized into numerous small elements with fixed size. Fracture behavior can be modeled by reducing the material stiffness and strength after the peak strength of the material has been reached. The evolution of the cracking process down to full fracture implies strain softening, which describes the post-peak gradual decline of stress at increasing strain. In the present study, a Mohr-Coulomb criterion envelop with a tension cut-off is used so that the element may fail either in shear or in tension. Simulated fracture or crack patterns of two examples are found quite realistic, and the results strongly depend on the heterogeneity level.

Abstract: The application of fracture in three-points bending to blanking of tool steel was studied. Several mechanical models of stress blanking were discussed at first. And then the experiments were made in special equipment of three points bending designed by us. Several problems, such as the effect of wedge on fracture, the effect of support condition on the quality of blanking, the effect of the geometrical parameters of notch on blanking were studied with the help of experiment. The suitable parameters of three-point bending for blanking, depth and tip radius of notch, the ratio of blanking length to the diameter of the bar and suitable support in asymmetrical geometry have been proposed.

Abstract: Limit load test results from fourteen forged piping branch junctions are reported. Eight were uncracked. Three had outside, non-through wall cracks running parallel to the run pipe centerline at the flank and the others had crotch corner cracks among the selected specimens. The collapse behaviors such as bulging deformation at the side flanks of junctions with and without crack were depicted. Based on the experimental data, the relationships of limit load with structural dimension and crack size were then summarized and the existing solutions were evaluated in use of the test results.

Abstract: Today, stress measurement methods by thermography and by photoelasticity are widely used to make stress distribution visible. However, it is difficult to separate principal stresses using only one of these methods because only the difference of principal stresses is measured in photoelasticity, and only the sum of the principal stresses is measured in thermograpy. Therefore, the inverse analysis problem must be solved to separate the principal stress in the thermoelastic method and the shear difference integration method must be used for the photoelastic method. Although there are some reports separation of the principal stresses under uniaxial stress by combining the two methods, little research under the biaxial stress has been reported due to the difficulty of experimentation. In this research, the principal stresses under biaxial stress are separated by a combined method. Moreover, it is verified that the thermoelastic stress measurement method is effective to evaluate the stress concentration factor.

Abstract: The authors experimentally investigated the change of mechanical properties of Inconel X-750 alloy under various heat treatments. For the selected specimens, solid solution treatment under different temperatures was carried out, followed air cooling or furnace cooling. Results show that suitable solid solution treatment and air cooling enhances the strength, plasticity, impact toughness at room temperature of the alloy and lowers the hardness of the alloy at room temperature.

Abstract: In order to simplify the production routine and to save cost of hot-rolled TRIP steels, the intercritical rolling process was investigated using thermo-mechanical simulation technology. Both 0.2C-1.5Mn-1.5Si and 0.2C-1.5Mn-1.5Si-0.5Cu TRIP steels were chosen so as to study the effect of 0.5 pct Cu addition on microstructure and retained austenite volume fraction of 0.2C-1.5Mn-1.5Si TRIP steel. It is found that grain size refinement happened during intercritical deformation, and multi-phase microstructure could be achieved after isothermal treatment, so intercritical annealing can be eliminated to avoid grain size coarse and to keep the refine grain size in intercritical-deformed TRIP steels.

Abstract: The composition, shape, size and distribution of non-metallic inclusions in a kind of high strength shaft steel enriched CoNi have been investigated. In situ tensile tests in special designed SEM have been conducted to trace the whole process of crack initiation and propagation till to fracture at inclusion. The experimental results show that the non-metallic inclusions in the high strength shaft steel are primarily AlN. Their average size is about 3.6µm. Sometimes, a string of inclusions distributes along the rolling direction. Non-metallic inclusion can induce crack to be initiated by inclusion debonding. When the inclusion size is larger than the critical size, the crack can propagate as the main crack that induces the specimen to fracture.

Abstract: The effects of three-dimensional crack configurations and delaminations on fracture mechanism and fracture toughness Jc of pipeline steel were investigated experimentally by use of tensile specimens having surface cracks of different depth to length ratio. Comparison with test results of through-thickness cracks and mechanism analyses are made as well. When 3D stress constraint is larger than the strength in the thickness direction the delamination forms. As no delamination occurs in the interior of a surface crack, the constraint is higher and the fracture toughness is lower than that of the through-thickness cracks. Therefore, the nominal fracture toughness obtained from through-thickness cracked specimens is not a real material constancy, and not suitable for safety assessment of pipelines.

Abstract: This paper examined the fundamental problem of an interaction between a soil medium that experiences frost heave in a closed system and an underground pipeline. The coupled thermal transfer and structural analysis, considering the material nonlinearity of freezing soil, for an underground pipeline subjected to the low cyclic load was focused on the development of computational scheme by introducing the effective heat capacity concept and the effective thermal expansion coefficient into the study. The effective heat capacity model in the thermal transfer analysis took into consideration the phase-change effect in the frozen fringe of a soil medium. The comparative analyses between the theory and the actual performances were valuable in establishing a level of confidence in the application of introduced theory to the field. The numerical results in the paper illustrated the influence for the frost heave of a soil medium on the temperature-dependent development of stress fields on metallic underground pipe walls in South Korea.