Papers by Keyword: Cleavage

Abstract: The contribution is focused on a new methodology description for determination of threshold stress, as the third parameter in Beremin local approach to cleavage fracture that is using three-parameter Weibull statistics. Nature of the methodology lies in tensile testing of rounded notched specimens at liquid nitrogen temperature and corresponding calculations. Reactor pressure vessel steel was chosen as an example for the illustration.

Abstract: The materials used in the treatment of an adult patient with severe cleft palate need to be choose very carefull, according with the biological aspects. First of all, is mandatory to choose a proper material for endodontic treatment and use a correct technique. Second, the cement used in order to fix a RPD (rapid palatinal disjunctor) has to be glass-ionomer cement, because the oral and nasal cavity are still communicating. On the other hand, the dental ceramic material used in the prosthetic treatment has to be a special one, pink-colored in those regions where cleavage is present. For this reason, the cases with cleavage need a complex treatment, usually during a few years and several steps. They need a good collaboration between doctors of different specialities: maxillo-facial surgery, orthodontics, prosthetics and plastic surgery. The aim of the present study is to underline the assessment of ceramic dental materials during the laborious treatment in the case of an adult with cleft palate, in order to obtain the best results.

Abstract: The complexity of calibration procedure for determination of Weibull distribution parameters as well as necessity to perform FEM calculations is one of the shortcomings of conventional versions of Local Approach (LA). This report presents an attempt to ascertain interrelation between local and global characteristics of fracture of pre-cracked and notched specimens of reactor pressure vessel steels, high-strength steels etc. Criterion of cleavage fracture initiation ahead of a crack (notch) is formulated. Dimensionless parameter characterizing the effect of embrittlement by the crack or notch is proposed on the basis of this criterion. Based on the findings of low-temperature tests of pre-cracked or notched specimens of structural steels, it is proved that this parameter may be determined by both local and global characteristics.

Abstract: A high strength bolt with hot dip galvanizing treatment failed shortly after being installed for two days on a padeye of mooring dolphin. The bolt was installed with increasing stress on the second day in which the value was nearly twice. Investigation results showed the bolt fractured with the nut head separated from the pin. Corroded fracture surface with brittle characteristic and no plastic deformation observed dominating the failed area. Characterization of hardness and chemical composition followed with microstructure and fractography observation on the fracture surface then conducted in order to analyze the reason for this brittle fracture occurrence. Results indicate that, while the bolt conforms to the material specification in term of chemical composition, the hardness value was high. The microstructure observation reveals a transgranular crack propagation and cleavage failure occurred. The cleavage failure was clearly observed under fractography observation using scanning electron microscope. Failed galvanize layer due to mechanical failure becomes preferential site for hydrogen evolution in marine environment, which leads to hydrogen diffusion into the matrix, thus results in hardness increase. The increasing stress during installation become detrimental to the bolt and facilitate the hydrogen induce cracking. Detrimental effect of hot dip galvanize layer is pointed out in the application of high strength material in marine environment.

Abstract: Quantification of characteristics that govern intergranular fracture initiation and propagation of this fracture micromechanism in competition with cleavage one is main aim of the paper. A NiCr steel of commercial quality and the same steel with an increased content of impurity elements, Sn and Sb, were used. Step cooling ageing was applied in order to induce intergranular embrittlement. Standard and pre-cracked Charpy type specimens were both tested in three-point bending to determine fracture toughness characteristics. In order to characterise the quantitative differences in fracture surfaces roughness a fractal analysis was applied. A boundary level of fractal dimension has been determined to be 1.12: fracture surface roughness with a higher value reflects high level of intergranular embrittlement and thus fracture resistance degradation.

Abstract: Flowerlike bicephalous titanium diboride (TiB2) nanowhisker clusters were fabricated by planetary ball-milling. The pure cleavage theory and crack growth mechanism were used to explain the formation process. The high-resolution transmission electron microscopy and nano electronic beam diffraction show that TiB2 superhard material always has a cleavage plane along the direction of [0001] zone axe and [10 0] zone axe because of its big interplane distance, weak bonding force between planes and low index directions. The flowerlike bicephalous TiB2 nanowhisker can be gained after repeated cleavage under high strains. The discovery could open a new path to fabricate C32 type superhard ceramic nanowhiskers.

Abstract: The phenomena occurring during rapid crack propagation in brittle single crystals were studied by cleaving silicon specimens on the low energy cleavage planes under tensile and bending. The experiments revealed new phenomena not previously reported, and new crack path instabilities in particular. The well defined boundary conditions of the tested specimens and crack velocity measurements enabled rationalization of the observed phenomena and the velocity-surface instabilities relationship in particular. In contrast to amorphous materials, the observed instabilities are generated at relatively low velocity, while at high velocity the crack path remains stable. No evidences for mirror, mist, and hackle instabilities, typical in amorphous materials, were found.

Abstract: The elastic finite element analysis (FEA) and the experimental method of testing the cleavage strength of the joint were used to investigate the effect of the recessing as well as its length on the stress distribution in both the mid-bondline and the adherend near the interface along the bondline of adhesively bonded steel cleavage joint. The results from the FEA simulation showed that the peak values of the stresses distributed in the mid-bondline were nearly the same when the length of the arranged recessing was not greater than 10 mm except that the shear stress Sxy was increased a little when the length of the recessing was increased. For the normal stress Sy near the interface of the joint with a 16 mm length recessing, the peak stress in the adherend is about 49% higher than the one in the adhesive at the left edge of the joint. And it is supported with the results from the experiments that the ultimate load of the steel-to-steel cleavage joint decreased a little when the gap length was less than 10 mm.

Abstract: Green sand casting and chill mould casting methods are representing the slow and fast cooling rates of the brass casting, respectively. The compositions of the raw material for this study were about Cu70 and Zn30, which falls under alpha (α) brass. Slow cooling rate casting shows coarse dendritric structures with large spacing between the dendrites. On the other hand, faster cooling rate casting shows finer grains with shorter dendrite spacing. The developed structure during solidification influences the properties of the cast samples. As grain size decreases, the strength of the cast brass increases; micro-porosity in the casting decreases and the tendency for the casting to fracture also decreases. However, the macro-examinations of fracture surfaces of these castings show the differences in the cast samples. Fracture surfaces of the sand cast specimen show larger dimples taking longer time to break indicating higher elongation. However, chill cast specimen shows smaller dimples and cleavage type fracture surface having higher strength and lower elongation.

Abstract: The normal stress distributed in the mid-bondline of the adhesively bonded joint under cleavage loading was investigated using the elastic finite element method (FEM) and the strain gauges method to reveal the real normal stresses distribution in the metal-to-metal joint while the load was increased. The results from the finite element analysis (FEA) showed that there is always a peak stress of the normal stress Sy in the mid-bondline occurred at a point close to the loading pin axis. When the load was increased from 0.5 kN to 3 kN, there was also a point located at about x = 16mm along the length of specimen where there is without any normal stress at all. The result of stress Sy from the FEA is nearly the same as that one obtained from the strain gauges method. It was also found that there was a evidently hardness change in the bonded zone of the adherend made from structural steel or pure copper, which can be used to explain the procedure of the joint and discuss the distribution model of the normal stress Sy in the joint under the cleavage loading.