Artificial neural network investigation of hardness and fracture toughness of hydroxylapatite

Hardness and fracture toughness of hydroxylapatite were investigated by artificial neural network (ANN). Hardness and fracture toughness of hydroxylapatite were predicted by using its sintering temperature, sintering time, relative density, and grain size with ANN. It was found that prediction results of its hardness and fracture toughness closely matched with the experimental results.     

喷砂射孔降破技术在川西地区的应用研究

喷砂射孔作为降低地层破裂压力的预处理技术,在川西地区已被广泛地使用。本文对喷砂射孔降低地层破裂压力技术的机理进行了研究,对近几年来川西地区喷砂射孔现场试验的9个主要施工参数（喷嘴直径、喷嘴个数、磨料体积浓度、磨料性质、砂量、施工排量、施工泵压、喷射速度、喷砂时间）进行了研究分析。分析得出：喷射速度和喷砂时间为两个最关键的参数,为了达到降低破裂压力的效果,喷射速度应大于260m/s,喷砂时间应达到20~30m in。研究分析结果经过应用后,施工参数得到了优化,进行现场试验取得了良好的施工效果。     

Two Modes and Criterion of Brittle Fracture

The plastic zone at the tip of the flaw (including acute cark and common notch) was investigated. Forthe notch specimen, a formula of toughness K was proposed, and its physical meaning was emphasized.Twomodes of brittle fracture was identified and the evaluating criterion was established.     

The Effects of Environmental Temperature and Additional Pressure on the Bonding Characteristics of Co-cured Carbon/Epoxy-Aluminum Single Lap Joints

The aim of this study was to investigate the bonding characteristics of a filament wound structure with an aluminum mandrel under different temperature conditions. During the filament winding process, the filament tension generates pressure in the pre-wound plies and mandrel. To simulate pressure in the filament wound structure with a single lap joint specimen, a pressure-generating device has been introduced and applied to the bonding part of the specimen. The single lap joint, composed of a carbon/epoxy prepreg and aluminum beam, was fabricated by an autoclave degassing molding process, at a pressure of 0.7 MPa (absolute including vacuum) and maximum curing temperature of 135°C. The environmental temperature conditions of ?20°C, 25°C, and 50°C were used in the experimental tests, in order to consider the service condition of passenger cars. The static bonding strength and fatigue life of the specimen under different temperature conditions with and without the additional pressures were measured and compared.     

Role of cement content on the properties of self-flowing Al2O3 refractory castables

In this work, Al₂O₃ self-flowing castables (SFCs) were produced based on various cement contents. The SFCs were sintered at 1273 K, 1573 K and 1773 K and the exhibited properties were experimentally determined. Among the properties determined in this work are bulk density (BD), apparent porosity (AP), water absorption (WA), cold crushing strength (CCS), modulus of rupture (MOR) and fracture toughness (K_IC). It is found that additions of 5% cement lead to SFCs with maximum MOR and K_IC values after firing at 1773 K. Firing at 1573 K leads to a reduction in both, MOR and K_IC. In SFC containing 3% cement, maximum K_IC values of 3.53 MPa m^1/2 were achieved after firing at 1573 K. In the low cement SFCs (1 wt%) after firing at 1773 K the exhibited K_IC values were below those obtained in either the SFC-3 or SFC-5, but they were significantly high (3.43 MPa m^1/2).     

On a 3D crack tracking algorithm and its variational nature

The crack propagation problem for linear elastic fracture mechanics has been studied by several authors exploiting its analogy with standard dissipative systems theory (see e.g. Nguyen (2000), Mielke (2005) and Francfort and Marigo (1998)). In recent publications Salvadori and Carini (2011) and Salvadori and Fantoni (2013) minimum theorems were derived in terms of crack tip “quasi static velocity” for two- and three-dimensional fracture mechanics. They were reminiscent of Ceradini's theorem Ceradini (1965, 1966) in plasticity.Such an incremental picture naturally leads to explicit methods for integration in time, with well know drawbacks in terms of accuracy and stability. The present work introduces an implicit Newton–Raphson based crack tracking algorithm which is endowed with a variational formulation.     

Oxidation studies of anodes layer microstructures in metal supported solid oxide fuel cells

The effect of oxidation on the microstructural and mechanical stability of ceramic layers in metal supported solid oxide fuel cells is reported. Half-cells that are produced with a reduced nickel based anode are oxidized for different times and temperatures in order to assess stability limits. Samples are analyzed in terms of the effective cell curvature and microstructure, where further insight is obtained via the observation of microstructures before and after oxidization. The interpretation is aided by a comparison to the behavior of structures without electrolyte layer. Electrolyte cracking and anode delamination are observed after oxidation, where the latter is absent in case of oxidation experiments without electrolyte layer, highlighting the failure relevance of strain induced by electrolyte deposition.     

Influence of surface defects on the biaxial strength of a silicon nitride ceramic - Increase of strength by crack healing

Failure of brittle materials starts in general from defects which exist in the volume or on the surface of the specimens. Surface flaws, which are more dangerous than volume flaws, can be introduced by machining. They decrease the strength of specimens and components.For this investigation silicon nitride specimens were produced using different machining conditions. About half of them were strength tested by use of the biaxial ball-on-three balls (B3B) test. It has been shown that better (more gentle) machining increases the strength but may also cause an increased scatter of strength data.The remaining specimens were heat treated (annealed) at 1000 °C in air and afterwards also strength tested using the B3B test. Compared to the non heat treated specimens a significant increase in strength could be proven, which was - depending on the machining conditions - between almost 300 MPa and more than 500 MPa. The scatter of strength data was largely decreased.The improvement was caused by the formation of a thin (0.5-2 μm) glassy layer which filled surface cracks and surface related pores during annealing.     

Weak interface dominated high temperature fracture strength of carbon fiber reinforced mullite matrix composites

Weak fiber/matrix interface dominates the toughening properties of ceramic matrix composites. This paper reports a novel sol-gel fabricated carbon fiber reinforced mullite matrix composite, in which the fiber/matrix interface was inherently weak in shear properties (∼25 MPa), measured in-situ by fiber push-in tests. The interface microstructure was chemically sharp, characterized by transmission electron microscopy. The outcome of the weak interface was the full trigger of the toughening mechanisms like crack deflection, etc., leading to significant enhancement of the fracture toughness of the composite (∼12 MPa√m), measured by single edged notch beam method. Finally, due to the weak fiber/matrix interface and large thermal expansion mismatch of the fiber and matrix, the high temperature fracture strength was enhanced in the temperature range from 25 to 1200 °C, which is attributed to the enhancement of the interfacial property at elevated temperatures that favors better load transfers between composite constituents.     

Mechanical properties of SPS sintered chalcogenide glass-ceramics with externally doped crystals

The low mechanical properties limit the application of chalcogenide glasses in the infrared lens. This paper proposes a general method for enhancing the mechanical strength of chalcogenide glasses. A series of GaAs crystal-doped Ge₁₀As₂₀Se₁₇Te₅₃ and ZnS crystal-doped As₂S₅ glass-ceramics were prepared by spark plasma sintering (SPS). The effects of crystal doping on the optical and mechanical properties of glass-ceramics were studied utilizing FTIR, XRD, SEM, ultramicroscopy, and Vickers hardness. The results show that adding GaAs crystal and ZnS crystal significantly improves the hardness and fracture toughness of the chalcogenide glasses. The infrared transmittance and mechanical properties can be further improved by reducing the crystal size.