A FINITE ELEMENT MODEL FOR COMPRESSIVE ICE LOADS BASED ON A MOHR-COULOMB MATERIAL AND THE NODE SPLITTING TECHNIQUE

This paper presents a finite element model for the simulation of ice-structure interaction problems, which are dominated by crushing. The failure mode of ice depends significantly on the strain rate. At low strain rates the ice behaves ductile, whereas at high strain rates ice reacts in brittle mode. This paper focuses on the brittle mode, which is the dominating mode for ship-ice interactions. A multitude of numerical approaches for the simulation of ice can be found in the literature. Nevertheless, the literature approaches do not seem suitable for the simulation of continuous ice-structure interaction processes at low and high confinement ratios in brittle mode. Therefore, this paper seeks to simulate the ice-structure interaction with the finite element method (FEM). The objective of the here introduced Mohr-Coulomb Nodal Split (MCNS) model is to represent the essential material behavior of ice in an efficient formulation. To preserve mass and energy as much as possible, the node splitting technique is applied, instead of the frequently used element erosion technique. The intention of the presented model is not to reproduce individual cracks with high accuracy, because this is not possible with a reasonable element size, due to the large number of crack fronts forming during the ice-structure interaction process. To validate the findings of the model, the simulated maximum ice forces and contact pressures are compared with ice-extrusion and double pendulum tests. During validation, the MCNS model shows a very good agreement with these experimental values.

A Finite Element Model for Compressive Ice Loads Based on a Mohr-Coulomb Material and the Node Splitting Technique

This paper presents a finite element model for the simulation of ice-structure interaction problems, which are dominated by crushing at low and medium confinement ratios. The failure mode of ice depends significantly on the strain rate. At very low impact velocities the ice behaves ductile, whereas at high velocities the ice reacts in brittle mode. This paper focuses on the brittle mode, which is the dominating mode for ship-ice interactions. A multitude of numerical approaches for the simulation of ice can be found in the literature. Nevertheless, the literature approaches do not seem suitable for the simulation of continuous ice-structure interaction processes at low and medium confinement ratios in brittle mode. Therefore, this paper seeks to simulate the ice-structure interaction with the FE method. To preserve mass and energy as much as possible, the node splitting technique is applied, instead of the often used element erosion technique. The intention of the presented model is not to reproduce individual cracks with high accuracy, because this is not possible with a reasonable element size, due to the large number of crack fronts forming during the ice-structure interaction process. The objective of the here introduced Mohr-Coulomb Nodal Split (MCNS) model is to represent the essential material behavior of ice in a efficient formulation. To validate the findings of the model, the simulated maximum ice forces and contact pressures are compared with experiments.

Investigation of Crystallographic Aspects of Subsurface Damage Induced by Grinding Using Micro-Raman Tomographic Imaging

The objective of this study is to propose and develop an evaluation method on the process damages of sapphire wafers, regarding crystallographic aspects of the damages by use of tomographic imaging techniques with laser Raman microscopy (called as micro Raman tomographic imaging: mRTI). In this paper, tomographic images with the peak width and the shift of the peak position were observed with mRTI, on c-plane cut sapphire wafers along a-plane, with micro fractures induced by brittle-mode grinding. Fractures along m-plane from the surface to 2–7 microns-depth, and fractures along r-plane from the bottom end of the m-plane fracture to 14 microns-depth, subsequently induced elastic strain field around the fractures, could be visualized without destructions of the wafers.

Experimental Study of the Influence of Deposition of Multilayer CrN/CrCN PVD Coating on Austenitic Steel on Resistance to Cavitation Erosion

The impact of deposition of multilayer CrN/CrCN coating on X6CrNiTi18-10 steel by means of the PVD (physical vapour deposition) method on resistance to cavitation erosion has been investigated. Cavitation tests were performed using a cavitation chamber with a barricade system at the inlet pressure p1 = 600 kPa and the outlet pressure p2 = 123 kPa. Deposition of CrN/CrCN coating allowed increasing duration of the incubation period and decreasing cumulative volume loss until 500 min of exposure. The erosion of the CrN/CrCN–X6CrNiTi18-10 system begins with the removal of microdroplets from the coating surface and surface undulation. The surface undulation increases with the exposure time leading to coating fracture in a brittle mode. Initiation sites of cracks were located inside the PVD coating. Measurements of surface roughness illustrate uneven degradation of the exposed surface and the location of slight and severe erosion zones. The Ra parameters obtained for the CrN/CrCN–X6CrNiTi18-10 system and X6CrNiTi18-10 steel after 180 min of erosion were comparable. An elongation of erosion test up to 600 min resulted in a higher increase in surface roughness of the CrN/CrCN coating–X6CrNiTi18-10 steel system in comparison to that of X6CrNiTi18-10 steel. With increasing exposition time, the rate of increase of the surface roughness decreased due to overlapping damage.

Improved Erosion Resistance by HVOF Sprayed 10%Al2O3-COCrAlTaY Coating on Ti-31

In this present research work the solid particle erosion test carried on uncoated samples (Ti-31), and HVOF sprayed 10%Al2O3 -CoCrAlTaY on Ti-31 are made. Erosion test are done with impact angles of 30º, 60º and 90º. Solid particle erosion studies were carried out using air jet erosion test rig as per ASTM G76-02 standard.All the three angles of uncoated alloys exhibit erosion damage under ductile mode and less amount of erosive loss compared HVOF coated samples. The HVOF sprayed coated Ti-31 at various impact angles is brittle mode. The mechanism of material removal during erosion of brittle materials is explained by using SEM micrographs.

Effect of shear span-to-depth ratio on posttensioned concrete crane beams shear capacity

The shear span-to-depth ratio has the most significant influence on the shear capacity of beams and determines their failure mode. The subject of the current project is the shear capacity of precast posttensioned concrete crane beams disassembled after more than fifty years of being used in an industrial plant. The paper gives the theoretical basis for the shear capacity of such elements as well as standard design models. The conducted tests showed that despite the low shear reinforcement ratio, the elements do not fail in a brittle mode but show a clear indication of prospective destruction. It was also confirmed that in the case of poorly shear-reinforced PC elements, a clear arch action can be distinguished with a low shear span-to-depth ratio, whereas in the case of a higher ratio there is a classical beam action.

Experimental and numerical study of reinforced concrete interior wide beam-column joints subjected to lateral load

Past researches showed that the energy dissipation capacity of the wide beam-column joints was not sufficient. So, two full-scale reinforced concrete interior wide beam-column joints were tested under quasi-static cyclic loads and the performance of the specimens was studied experimentally and numerically. Effect of using wide beams in two directions was investigated and it became clear that the longitudinal reinforcement of transverse wide beam had significant effect on the seismic behavior of the joints. Flexural hinging mechanism in the wide beams occurred instead of torsion brittle mode of failure. Effect of eccentric beams on joints is one of the areas needing research in ACI 352R-02. In the numerical study, it was seen that damage of the joints was concentrated to one side of the joints that the beams shifted to. Besides, concrete grade did not have much effect on behavior of the joint.

Grinding Characteristics, Material Removal, and Damage Formation Mechanisms of Zirconia Ceramics in Hybrid Laser/Grinding

Zirconia ceramics which are sometimes called “ceramic steel” have gained significant interest because of their excellent properties. However, it is desired to maintain the surface quality while increasing the economics of ceramics grinding process. A hybrid laser/grinding (HLG) process was utilized to grind zirconia ceramics which was irradiated with continuous wave laser before grinding in the hybrid process. The feasibility of hybrid laser/grinding of zirconia ceramics was investigated in terms of grinding force and energy, material removal, and damage formation mechanisms. The results show that laser irradiation can induce lateral cracks, which can help material removal and prevent further crack propagating into the base. The results of grinding tests indicate that grinding force and energy decrease significantly as compared with conventional grinding of ceramics. The combinations of the fractured area, the plowing striations, and seldom debris on the ground surfaces in this work indicate the combined material removal mechanism of both brittle mode and ductile mode.

Effects of mean strain and tensile pre-strain on torsional fatigue behaviours of duplex stainless steel SAF2205

Effects of mean strain and tensile pre-strain were investigated on the torsional fatigue behaviours of duplex stainless steel SAF2205. Two equivalent strain amplitudes (0.5%,0.7 %), three strain ratios (-1, - 0.5, -0.25) and 5% tensile pre-strain were chosen. Results indicated that the mean strain had no distinct influences on the torsional fatigue behaviours in terms of cyclic stress reponse and fatigue life while tensile pre-straining made a significant increase in cyclic stress response which was mainly attributed to the cross hardening derived from the loading sequence of monotonic tension preceding to cyclic torsion and led to a reduction in fatigue life. The failure mechanisms were revealed by scanning electron microscope characterized by microcracks initiation at the extrusions in ferrite and phase boundary inhibited further propagation. Additionally, the fractography of all fatigued specimens revealed a quasi-cleavage brittle mode with features of distinct tearing ridges and cleavage facets.

Ductilizing of cast hypereutectic Al–17%Si alloy by friction stir processing

In present research work, multi-pass friction stir processing was utilized for the as-cast Al–17%Si alloy. The multi-pass friction stir processing reproduced the aluminum (Al) matrix with a uniform distribution of the ultra-fine silicon (Si) particles. The multi-pass friction stir processing also resulted in the formation of refining cast microstructure with negligible porosity. The significant reduction in silicon particle size was measured and it reduced from 204 µm to 0.86 µm after the first pass and up to 0.30 µm after two pass friction stir processing. The frequency of fine Si particles was increased after two pass friction stir processing as compared to single pass friction stir processing. The engineering stress–strain curves revealed a significant enhancement in ductility and strength after one and two passes of friction stir processing as compared to the as-cast alloy. After one and two passes of friction stir processing, the ultimate tensile strength of as-cast alloy was enhanced by 24% and 31% and ductility was increased by 300% and 500%, respectively. The secondary electron micrograph of fracture surfaces of tensile specimens was taken before and after friction stir processing. The fractographs revealed the transformation from brittle mode to ductile dimples fracture after the multi-pass friction stir processing process.