Papers by Keyword: Brittleness

Abstract: The paper presents a quantitative assessment of the brittleness of highly hardened silicon carbide ceramics. This characteristic is important for predicting the strength characteristics of the material. The analysis of existing methods for determining the brittleness of the material is carried out. During the research, it was assumed that the production of composite materials in the SiC-Al2O3 and SiC-Al2O3-AlN systems would combine high chemical resistance, wear resistance, and low coefficient of friction. These properties inherent in aluminum oxide and nitride, along with high thermal conductivity and strength, are characteristic of silicon carbide. Materials in the SiC-Al₂O₃ system containing 20, 50 and 80 percent of aluminum oxide – Al₂O₃ were selected as objects of research. Their advantages and disadvantages are shown. The results of an experimental study of samples made of highly hardened silicon carbide ceramics are presented. Based on experimental studies, analytical and graphical dependences are proposed that allow determining the crack resistance of ceramics from the crack length at the corners of the Vickers pyramid. It is shown that these dependences change exponentially, which makes it possible to predict the appearance of cracks at various stages of mechanical processing of products made of highly hardened ceramic materials.

Abstract: The development of methods for predicting the reliability of structural elements based on brittle fracture criteria is a rather complex mathematical task. This is due to the fact that these criteria are usually obtained in the framework of the mathematical theory of cracks, the boundary problems of which allow a limited number of exact analytical solutions. To this we must add that the brittle fracture of materials with disc-shaped circular fractures has been studied in thermomechanics and in the kinetic theory of strength, from our point of view, is not enough and research in this area seems to be relevant to us. In this regard, in this work, within the framework of the linear theory of elasticity, two cases of external impact on a material containing a circular disk-shaped fracture are considered: mechanical, in the form of a uniaxial tensile stress, and temperature, in the form of a temperature gradient in the region of a material containing a circular disk-shaped crack destruction. From the extremum condition, brittle fracture criteria such as the Griffith criterion are obtained both for the case of only mechanical loading of the material with uniaxial tensile stress, and for the case of only temperature exposure of the material in the form of a local temperature gradient at the crack surface.

Abstract: Years ago, especially in Sweden, a new family of ductile iron materials was propagated: Solid solution strengthened ferritic ductile iron. Since the 2011 edition, three grades are integrated into the European Standard EN 1563: EN-GJS-450-18, EN-GJS-500-14 and EN-GJS-600-10. The introduction of these materials into the European standard generated a large interest of many engineers, which came to a real hype meanwhile. The reason is clear: While GJS-450-18 is not very different from the standard GJS-400-15, especially the grade EN-GJS-500-14 promises to have big advantages compared to the standard grade EN-GJS-500-7. Same tensile strength, from 320 MPA to 400 MPa raised 0.2%- proof strength and from 7% to 14 % doubled elongation after fracture are very interesting properties of course and the pure ferritic structure promises a better machinability furthermore. With the higher strength grade EN-GJS-600-10 very early sceptic comments raised, because the Silicon content to reach the required strength is such high, that the risk of an embrittlement of the ferrite even at room temperature and on a tensile test bar is high. But up to now, the grade EN-GJS-500-14 has the reputation to be a high strength and very high ductile material. So, a customer of us also substituted a part from steel to EN-GJS-500-14. All calculations showed a very good performance of the parts, but as soon as in serial production field damages occurred in a manner that the parts completely broke without advance warning. The investigation of the damages showed, that the parts are not only statically and cyclically loaded, but also by strikes. Deeper investigations about strike loads confirmed, that EN-GJS-500-14 is not really ductile under these conditions. The reflection to the philosophy we had with our SiboDur-700 concept, namely to combine a moderate solid solution strengthening with a moderate strengthening by pearlite with Copper, brought us to a new high strength high ductile material we call SiboDur-500. Same strength as EN-GJS-500-14, 10 % lower in 0.2%- proof stress, slightly lower in elongation after fracture, but double to more than four times energy consumption at strike loads depending om temperature!

Abstract: We have performed First-principles density functional calculation by using full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA) of B₂- AlGd (Aluminum compound). The ground state properties along with electronic and elastic properties are studied. The energy ranges are given for bands which are crossing the Fermi level and explained whether the Fermi surface is formed by hole pocket or electron pocket. Bonding properties are analyzed by charge density plot. By B/G_H ratio the brittleness of the material is determined.

Abstract: In this study, an attempt is made to characterize the brittleness of the material by crack formation with the Vickers indenter. To assess the critical load, the sensor of AE signal was used. After preliminary tests on a silica glass, the methodology was applied to HVOF sprayed CrC-NiCr coating in as-sprayed and annealed state. The critical value of indentation force obtained from AE sensing was correlated with the standard method of crack length measurement with good agreement

Abstract: It is known that the torsional extrusion process being used to create the object is complex in cross sectional profile. When compared to conventional extrusion, it is evident that the strength of the titanium alloy could be increased in torsional extrusion. This torsional extrusion process could also be applied the materials having the property of brittleness. Hence, the titanium alloys have huge application in aerospace industries in the area of jet engine components subjected to operating at extreme temperature. Besides, it is also used in critical airframe applications where high strength and fracture toughness are mandatory requirement. Further, it is also used as fasteners and tubing throughout the application of aircraft structures, since it involves a lot of complications with a huge investment for machining. This paper investigates the challenges and a difficulty in the torsional extrusion process ^[2].The material titanium is well known for its high temperature resistance and also possesses poor machining rate. Therefore, the study on Titanium machining is chosen to enrich the role of titanium alloys in the field of engineering science and materials.

Abstract: Ultra-high performance concrete (UHPC) incorporating coarse aggregate was prepared with common raw materials. Fresh concrete had excellent good workability with slump of 265 mm and slump spread of 673 mm. Compressive strength of UHPC at 56 d reached 150 MPa. However, UHPC exhibited high brittleness in terms of spalling failure which occurred during compression loading.The ratio of splitting tensile strength to compressive strength of about 1/18 and the ratio of flexural strength to compressive strength of about 1/14 at 56 d were also associated with the brittleness of UHPC in this research. Mineral admixtures and fluidity of fresh concrete influenced compressive strength of UHPC significantly. Moreover, UHPC had excellent permeation-related durability but considerable shrinkage. Autogenous shrinkage of UHPC was less than half of free shrinkage, for which the reason is unknown and needs further research.

Abstract: It used conventional techniques and materials prepared high strength fiber reinforced concrete whose strength class is above C100 and it studied the effect of fiber content on the mechanical properties and elastic modulus. It also studied the fire resistance of fiber reinforced concrete. Results suggest that the strength of 28d concrete is above 100MPa and the highest strength is 126.4MPa. Under the same ratio conditions, the greater the volume content of steel fiber concrete flexural strength, the splitting tensile strength is higher. The steel fiber volume only affect elastic modulus of concrete little. When it heats to 300 °C, the no fiber concrete comminuted burst while the fiber concrete does not damaged at elevated temperatures up to 300 °C and continue to heat up, the crushing damage occurs at about 460 °C. Has not been damaged concrete specimens at 300 °C, the quality have emerged about 3% decline, while the compressive strength increased by 35%-52%, the highest strength reached 180.3MPa.

Abstract: In order to improve the mechanical properties of high strength concrete, HSC, several studies have been conducted using fly ash, FA. Researchers have made it possible to achieve 100-150MPa high strength concrete. Despite the popularity of this FAHSC, there is a major shortcoming in that it becomes more brittle, resulting in less than 0.1% tensile strain. The main objective of this work was to evaluate the fresh and hardened properties of FAHSC utilizing chopped basalt fiber stands, CBFS, as an internal strengthening addition material. This was achieved through a series of experimental works using a 20% replacement of cement by FA together with various contents of CBFS. Test results of concrete mixes in the fresh state showed no segregation, homogeneousness during the mixing period and workability ranging from 60 to 110 mm. Early and long terms of compressive strength did not show any improvement by using CBFS; in fact, it decreased. This was partially substituted by the effect of FA. Whereas, the split and flexural strengths of FASHC were significantly improved with increasing the content of CBFS as well as the percentage of the split and flexural tensile strength to the compressive strength. Also, test results showed a progressive increase in the areas under the stress-strain curves of the FAHSC strains after the CBFS addition. Therefore, the brittleness and toughness of the FAHSC were enhanced and the pattern of failure moved from brittle failure to ductile collapse using CBFS. It can be considered that the CBFS is a suitable strengthening material to produce ductile FAHSC.

Abstract: In this work, the mechanical properties, such as Vickers microhardness H_v, fracture toughness K_c, yield strength σ_v and brittleness index B_i, of <0001> oriented 6H-SiC crystal are systematically evaluated using a microindentation technique under 0.1-2 kg applied load. It is found the H_v is decreased as the applied load is increased which is mainly attributed to the effect of indenter penetration. The H_v value can be effectively presumed by Kicks law and the Meyers index n is determined to be 1.73. However, the K_c value is measured nearly a constant (~0.148 MPa.m^1/2) which reveals the toughness of 6H-SiC₍₀₀₀₁₎ crystal is much weaker than those of Si₍₁₀₀₎ and GaAs₍₁₀₀₎ crystals. The variation of σ_v to the load is similar to that of H_v. The brittleness index B_i also exhibits deceasing tendency as the applied load is added.