Papers by Keyword: Brittle-Ductile Transition

Abstract: Failure analysis of 4 swirls in the fuel nozzle of a gas turbine was made by macroscopic and microscopic examination, and verified test were conducted. The result showed that the swirls in the fuel nozzle subjected thermal fatigue and failed in different manner for different reasons.

Abstract: The mechanism of brittle-ductile transition (BDT) in machined Si single crystal is investigated by simulating dislocations emission from crack tip along (111) and (111) slip plane under mixed-mode loading. One kind of compression-shear crack is taken into account and the law of strain-energy-density-factor is applied as fracture criteria. The total number of the emitted dislocations and the number of dislocations in each slip plane at the onset of cleavage are calculated. It is found that the ratio of stress intensity factor kII to kI that the crack tip is subjected has significant effect on the BDT in machined Si single crystal. Then the results are applied to study the action of negative rake angle and edge radius of diamond tool in the ultra-precision turning.

Abstract: A series of industry-scale and laboratory-scale W-Cu composites containing copper of 19-51 vol% were prepared by infiltrating a tungsten skeleton with liquid copper. These composites were subjected to a three-point bend test at temperatures between 77 and 363 K, and yield and maximum strengths were evaluated as a function of both the temperature and the composition. Fracture surface was observed using SEM and its fracture mode was examined. Results are summarized as follows. (1) Critical Cu content for the transition from a brittle to a ductile deformation behavior at room temperature was about 20 vol%. (2) Critical Cu content for the transition from a bcc-type to a fcc-type deformation behavior was 38-45 vol%.

Abstract: Grinding of single crystal silicon may be achieved by two modes of material removal: ductile mode and brittle mode. Knowing of the brittle to ductile transition point at which the grinding process changes from the brittle mode to ductile mode is critically important for the realization of ductile mode grinding. This paper uses a new single grain diamond grinding method developed recently by the authors to investigate the brittle to ductile transition during grinding of single crystal silicon in all around. The results indicate that there exist four stages of brittle to ductile transition as the depth of cut is reduced: firstly, the surface cracks outside the grinding groove disappeared, secondly
cracks on the bottom of the groove disappeared, then the lateral cracks ceased in the subsurface region, and finally the median crack is suppressed beneath the grooves. It is not until the depth of cut reaches the last transition point that a crack-free groove can be produced, therefore, the last transition stage is decisive. The critical depth of cut delineating the brittle to ductile transition point derived based on this criterion is 40 nanometers, which is much lower than that based on surface cracks.

Abstract: In this paper, mechanical characteristics of KDP crystal anisotropy are analysed theoretically. Vickers indentation experiments are adopted to validate the variation rule of hardness and fracture toughness in different orientation of KDP crystal plane (100), and a model to calculate critical cutting thickness of brittle-ductile transition is proposed for the KDP crystals. The result shows that, on the crystal plane (100), the minimum value of critical cutting thickness of KDP crystal in brittle-ductile transition appears in the direction [110], but the maximum appears in the direction [010]. Finally, the ultra-precision machining of KDP crystal is performed, and the results agree well with the theoretical conclusions. Super-smooth surface with a roughness RMS of 6.6nm is reached as machined in the crystal direction [010], and 11.2nm to the direction [110].

Abstract: In order to machine high accuracy Potassium Dihydrogen Phosphate (KDP) crystal part, the indentation experiments are carried out with various loads and various orientation angles. The experimental results show that the critical condition of brittle-ductile transition of KDP has strong anisotropy. Therefore, the influence factors on the surface quality of crystal KDP was discussed, it is shown that influences of the tool's geometry parameter, feed rate and Nominal depth of cut etc on the surface quality of KDP are main. Afterwards the cutting experimental study on crystal KDP material is carried out. The experimental results show that the super-smooth surface quality only can be obtained while KDP is ultra-precision machined in ductile mode.

Abstract: A brittle-ductile transition lapping mechanism is proposed for the mechanical lapping of ultra-precision diamond cutting tools, and then the critical depths of cut for brittle-ductile transition in different orientations and on different planes are deduced in theory. Combined the critical lapping depth with the contact accuracy between rotating scaife and lapped tool surface, the influences of processing factors on cutting edge radius are studied. Both the theoretical analyses and experimental results indicate that the vibration of lapping machine tool and surface quality of scaife have enormous influences on the sharpened cutting edge. And lapping compression force has an optimal value. Lapping rate should be considered when lapping velocity is selected. But the smaller the lapping velocity is, the littler the cutting edge radius sharpened. Finally, the optimal selections are performed for each influencing factor and a perfect diamond tool is lapped in ductile mode with a cutting edge radius of 30~40nm and a surface roughness Ra of 0.7nm.