Crack branching in delayed failure

The condition for crack branching in delayed failure was investigated on the basis of energy consideration, and the effect of diffusion behavior of hydrogen atoms on the crack branching was discussed.

The branching angle predicted from the present theory well agreed with the experimental value. The stress intensity factor at crack branching, K_IB, is dependent on the diffusion velocity of hydrogen atoms, i.e. branching appears to occur when the moving velocity of the position with tri-axial tensile stress reaches the upper limit value of diffusion velocity of hydrogen atoms. The time to crack growth initiation is actually independent of stress intensity factor when the crack branching occurs, probably because the diffusion velocity of hydrogen atom has a certain upper limit value.     

The C-H delayed failure mechanism revisited

The often cited “Charles–Hillig” model of delayed failure assumes that corrosive attack on existing flaws in glass is controlled by interactions involving the moisture in the environment, which leads to time and stress dependent failure. The analysis by Inglis of the stress multiplication at a rounded end of a flaw is combined with stress-dependent chemical kinetics and thermodynamics. Failure occurs when the tip stress reaches the ultimate strength of the glass. The model provides a physical interpretation of the empirical “Universal Fatigue Law.” When sufficient data is available, it also provides an algorithm that allows a precise determination of the minimum stress needed to induce time-dependent failure. This model is compared with the competing LEFM model.     

Enhancement in micro-fatigue resistance of UHMWPE and HDPE processed by SCORIM

The tribological performance of UHMWPE and HDPE polymers, processed by a novel polymer process technology SCORIM (Shear-Controlled ORientation TEChnology) in injection moulding, shear controlled orientation in injection moulding (SCORIM) was evaluated against a through-hardened steel under different test conditions and compared with those processed by conventional injection moulding. Results indicate a significant improvement in the wear resistance using the SCORIM technology as a result of an increase in the mechanical properties. SEM microscopy shows a change of wear mechanisms dominated by micro-fatigue when using conventional injection moulding, compared to mild abrasive wear when using the SCORIM technology under the same test conditions. Examination of microstructure reveals the random aggregation of polymer molecules in the samples processed by a conventional injection moulding. Using the SCORIM technology, the molecules were sheared and orientated and a fibril microstructure formed as an in situ fibre reinforced composite. DSC analysis shows an increase in a second phase shish kebab structure in the samples processed by SCORIM technology. The fibril microstructure with an increase in shish kebab structure results in a significant improvement in wear resistance. Using the surface normal to the direction of the orientated molecular fibril microstructure as a contact surface, the possibility of the initiation and development of micro-cracks was reduced, especially the micro-cracks parallel to the contact surfaces.     

Enhancement of wear and corrosion resistance of metal-matrix composites by laser coatings

A novel technique based on laser-induced chemical reduction of metal salts has been developed to produce surface coatings on metal-matrix composites (MMCs). The substrate is predeposited with a paste, containing concentrated salts of the elements to be coated along with a thickening agent, and then subjected to high power laser radiation. The rise in surface temperature during laser irradiation led to the decomposition of salts to their native metals. The combination of metal and metalloid elements in the reaction zone forms an amorphous layer due to the specific chemical ratio and rapid cooling rate. The thickness of the coatings obtained were of the order of 50–100 m. The coatings exhibited amorphous and microcrystalline structures, possessed hardness in the range of 300–1700 H_v (substrate hardness 80–90 H_v), had superior sliding wear resistance and excellent corrosion resistance. The advantages of this process include the formation of complex coatings on MMCs by a simple, versatile technique which does not require any vacuum or inert atmosphere.     

Enhancement of wear resistance of ductile iron surface alloyed by stellite 6

This paper deals with the improvement of the wear resistance of ductile iron surface alloyed by a hypoeutectic stellite 6 alloy. In this regard, the surface alloyed layer with 3 mm thickness deposited on ductile iron using tungsten inert gas (TIG) surface processing. The microstructure, hardness and wear resistance of surface alloyed layer were investigated using optical microscopy, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction analysis, Vickers hardness (HV0.3) and pin-on-plate tests. The results showed that the microstructure of the surface alloyed layer consisted of carbides dispersed in a Co-based solid solution matrix with dendritic structure. This microstructure was responsible for the improvement of the hardness and wear resistance of the coating. Further investigations showed that the dominant mechanism of the wear in the coated and uncoated samples was delamination wear.     

Enhancement of the corrosion resistance of gray cast iron by ion nitriding

The ion nitriding of gray cast iron at 550°C for 1.5 h increases 1.5–2.1 times the corrosion resistance of its surface. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 6, pp. 116–118, November–December, 2006.     

Statistical approach to delayed failure of brittle materials

Delayed failure tests or static fatigue tests were performed on soda-lime glass in water environment. Testing consisted of four-point bend and three-point bend with constant load. The time-to-failure and fracture location of specimens were measured.The theory of stochastic process is combined with the extreme value distribution in stress and the time dependent probability distribution function is obtained to predict the time-to-failure of specimens as well as the location of fracture.It is shown that the theoretical prediction is in good agreement with the experimental data.

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Résumé Des essais de rupture différée ou essais de fatigue statique ont été exécutés sur un verre calcique sodé dans un environnement d'eau. Les essais consistaient en une flexion en quatre points et en une flexion en trois points à charge constante. On a mesuré le temps à la rupture et la localisation de la rupture sur les éprouvettes.En combinant la théorie du processus stochastique avec la distribution aux valeurs extrèmes de la contrainte et de la fonction de distribution de la probabilité de rupture en fonction du temps, on a pu prédire le temps à la rupture des éprouvettes ainsi que la localisation de la rupture.On montre que la prédiction théorique est en bon accord avec les données expérimentales.

     

The method of testing of XLPE cable insulation resistance to partial discharges and electrical treeing

The paper gives a method of model XLPE medium voltage cable insulation testing for partial discharges and electrical treeing with a point to plane test geometry. Based on this method, a comparative estimate of insulation resistance to treeing and partial discharges have been made. XLPE crosslinked by different methods (steam medium and hot nitrogen medium) and with varying contents of inorganic filler kaolin were tested. The characteristics of partial discharges were measured by means of a pulse height analyser characterized with a microscope. The data have been analysed statistically.     

On the unloading compliance method for crack length measurement

Unloading compliance tests have been performed on blunt notched Charpy specimens in order to quantify the extraneous displacements arising during unloading in a three point bend fracture toughness testing fixture. An expression is derived which quantifies the extraneous displacements and enables the unloading compliance to be accurately evaluated. Results are presented from tests on pre-cracked Charpy specimens which demonstrate close agreement between measured crack lengths and those predicted using the corrected unloading compliance.     

Force criterion for delayed failure of metastable austenitic steel

The phenomenon of delayed failure detected in metastable austenitic steels is connected with martensite formation during plastic deformation of austenite. In these steels a zone with a martensitic structure exhibiting a high level of residual internal microstresses develops ahead of a stress concentrator. In model representations a scheme has been suggested for distribution of deformation zones and stresses ahead of a stress concentrator. It is proposed that during delayed fracture a crack generates close to the boundary of plastically and elastically deformed martensite. A force criterion has been established for crack generation during delayed failure of metastable austenitic steel, i.e., the maximum local tensile stress (MLTS). The threshold value of the maximum local tensile stress does not depend on stress concentrator geometry and it is characterized by the tendency of metastable austenitic steel towards delayed failure.Central Scientific-Research Institute of Ferrous Metallurgy, Moscow. Translated from Problemy Prochnosti, No. 11, pp. 69–72, November, 1989.     

Enhancement of the corrosion resistance of titanium alloys of the Ti-Al-Mo-V system by carbonitriding

We study the regularities of formation of coatings on titanium alloys of the Ti-Al-Mo-V system in the process of saturation in carbon-and-nitrogen-containing media (with an oxygen content of at most 0.01–0.0005 vol.%) and their corrosion-electrochemical behavior in an 80% aqueous solution of sulfuric acid. The difference between the phase compositions of the coatings formed in different temperature ranges of saturation is revealed. Thus, nitride coatings are formed on the surface at temperatures below 1100°C and carbonitride coatings are formed above 1100°C. It is shown that the physicochemical characteristics of nitride coatings formed in carbon-and-nitrogen-containing media are better than the corresponding characteristics of nitride coatings obtained by nitriding for the same temperature, time, and gas-dynamic parameters of saturation. As the content of cubic δ-nitride in the coating increases, the corrosion characteristics of the surface after saturation in carbon-and-nitrogen-containing media increase. Despite a significant surface roughness of carbonitride coatings, their corrosion resistance is high. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 4, pp. 78–83, July–August, 2006.     

Finite-element analysis of probability of delayed failure in brittle structures

A formula is derived for predicting the time-dependent probability of fracture of brittle materials under multiaxial stress states. A finite-element analysis is performed to calculate its value for a ceramic beam (silicon nitride—Si₃N₄) subject to bending moment and also for the root of a ceramic gas turbine blade. It is found that the formula can be conveniently used in practical design of components made from brittle materials.     

Enhancement of aging resistance of glass fiber reinforced cement

Long-term weathering tests revealed that glass fiber-reinforced cement (GFRC) might exhibit tensile strength reduction and ductility loss with aging. The main goal of this research was to assess the effectiveness of polyvinyl alcohol (PVA) powder in enhancing the durability characteristics of GFRC sheets. Accelerated aging was achieved by using low pressure steam curing. The strength and ductility of GFRC sheets were measured by the direct tension test. The test results show that the incorporation of PVA powder into GFRC improves the mechanical behavior and changes the failure mode from brittle to ductile. To investigate the mechanism of the enhancement, the fiber-matrix interface was examined by polarizing optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX). It was found that the addition of the PVA powder results in the deposition of a polymeric film on the fiber surface and thus prevented the accumulation of calcium hydroxide in the interfacial zone. The use of the PVA powder led to a more ductile interfacial microstructure and to better bonding between fiber and matrix, which is believed to be responsible for the tensile property enhancement.

     

调制光热法低温接触界面层热阻研究

选择航天器和低温工程中常用金属材料铜、不锈钢,用激光光热法原理,研究低温接触界面层热阻.实验得到了铜在常温下的调制频率与相位差间的实验数据,通过对实验数据的线性拟合得到了铜的热扩散系数,以及铜-不锈钢在300K和20 K低温下的接触界面层热阻.实验表明调制光热法能够测量具有微结构特征的接触界面层热阻,且具有非接触测量的特点.