Degradation Of Fracture Strength Research Articles

Natural Aging Mechanics Characteristics on Fiber Reinforced High Altitude Airship Envelope Composite

For High altitude airship (HAA) with long time affected by space harsh environment, due to the occurrence of natural light radiation aging physical and mechanical properties, fiber-reinforced airship envelope material results in degradation of the mechanical load performance, even causing damage to the balloon airship. Based on the ground aging simulation of the airship envelope material under the natural light, the morphology of airship envelope material in the natural aging process can be observed and the specimen comparison test is carried on for its fracture strength and breaking elongation to qualify its mechanical properties. The specimens with the axial angle of 0°and 90°were respectively tested to get its mechanical properties and the comparative analysis on degradation of fracture strength and breaking elongation before and after its 720h aging is developed too. The test results shows that the mechanical properties of different specimens decrease, wherein the maximum degradation is 0° specimens and that of 90°specimens t is not very significant weakening. Natural aging characteristics of airship envelope material provide support for the design and improvement of anti-aging properties of the airship envelope material.

Effect of grain orientation and heat treatment on mechanical properties of pure W

The effect of grain orientation, heat-treatment temperature and test temperature on the mechanical properties of tungsten (W), which vary depending on plastic working and fabrication process, was investigated by mechanical testing of tensile or bending. Heavily worked W samples (1.5–2.0mm in the final thickness) exhibit degradation of fracture strength due to recrystallization embrittlement after heat-treatment at 1240°C (temperature of diffusion bonding between W and a candidate material of the Fe base support structure). On the other hand, W samples with lower thickness reduction rates do not suffer degradation of fracture strength after heating up to around 1300°C, and show somewhat higher fracture strength by heat-treatment below 1300°C than the samples in the as-received state. The observed behavior is a reflection of recovery of dislocations introduced by plastic working. High temperature tensile testing of ITER grade W with an anisotropic grain structure and S-TUN with an equiaxed grain structure revealed that both W grades exhibit plastic elongation at temperatures higher than 200°C with essentially the same temperature dependence of yield strength, which is relatively insensitive to grain orientation in the structure at 200–1300°C.

Micro-scale damage characterization in porous ceramics by an acoustic emission technique

The fracture mechanism of porous silicon carbide (SiC) was investigated by using the acoustic emission (AE) technique. Four point bending tests were conducted at different loading speeds, and integrated with AE monitoring. The results revealed that the quasi-static fatigue behavior had a critical role on the degradation of fracture strength, and fracture was caused by the microcracks of binder. For the AEs detected during the bending tests, source wave analysis of AE was carried out in order to quantitatively estimate the volume of microcrack. It was found that the crack volume was slightly smaller than that of binder fracture observed by a scanning electron microscope (SEM), thus suggested that the small crack propagates in the binder and produces AEs. It was also found that the longer the testing time, the smaller the microcrack occurred at the lower stress level. Therefore, the fracture mechanism of the porous SiC was found to be controlled by particle binder fracture, which is time dependent. In addition, AE is testified as a useful technique to monitor the small crack of binder, which plays a very important role in material strength and filtering function of porous SiC.

Hygrothermal degradation of two rubber-toughened epoxy adhesives: Application of open-faced fracture tests

The absorption/desorption properties of two commercial, toughened epoxy adhesive systems were evaluated gravimetrically, and by X-ray photoelectron spectroscopy (XPS) and dynamic mechanical thermal analysis (DMTA). Fracture tests on degraded open-faced DCB specimens showed that these two adhesive systems have very different degradation behaviors. The steady-state critical strain energy release rate, G cs , of an adhesive system 1 decreased rapidly with an exposure time in various hot-wet environments, reaching a relatively low value that was stable for over one year, while that of adhesive system 2 remained unchanged for more than one and a half years. A degradation mechanism which accounts for the different characteristics of the two adhesive systems was proposed. A model of fracture toughness degradation, analogous to Fick’s law, was then used to characterize the fracture toughness loss in an adhesive system 1, and the effects of temperature, RH and water concentration were evaluated. The results illustrate the wide variation in water absorption behaviors that can exist among toughened epoxy adhesives, and show how these differences relate to the degradation of fracture strength. The data were also used to assess the applicability of an exposure index (EI), defined as the integral of relative humidity over time, as a means of characterizing an aging history. The fracture strength degradation was measured after aging to achieve a range of EI values, and it was found that the strength loss was independent of the time–humidity path for sufficiently large EI.

A statistical, physical-based, micro-mechanical model of hydrogen-induced intergranular fracture in steel

Intergranular cracking associated with hydrogen embrittlement represents a particularly severe degradation mechanism in metallic structures which can lead to sudden and unexpected catastrophic fractures. As a basis for a strategy for the prognosis of such failures, here we present a comprehensive physical-based statistical micro-mechanical model of such embrittlement which we use to quantitatively predict the degradation in fracture strength of a high-strength steel with increasing hydrogen concentration, with the predictions verified by experiment. The mechanistic role of dissolved hydrogen is identified by the transition to a locally stress-controlled fracture, which is modeled as being initiated by a dislocation pile-up against a grain-boundary carbide which in turn leads to interface decohesion and intergranular fracture. Akin to cleavage fracture in steel, the “strength” of these carbides is modeled using weakest-link statistics. We associate the dominant role of hydrogen with trapping at dislocations; this trapped hydrogen reduces the stress that impedes dislocation motion and also lowers the reversible work of decohesion at the tip of dislocation pile-up at the carbide/matrix interface. Mechanistically, the model advocates the synergistic action of both the hydrogen-enhanced local plasticity and decohesion mechanisms in dictating failure.

Fatigue Strength and Fracture Mechanisms of Porous Ceramics

Static and cyclic fatigue strength in porous SiC and Cordierite ceramics were evaluated using quasi-static and cyclic tests, i.e. applied force, P, increasing and applied force range, ΔP, increasing tests, respectively. The evaluation method used Weibull distribution, in which the fracture parameters are stress at failure, σf, for quasi-static loading and maximum stress at failure, σmax, f, for cyclic loading. The results revealed that the static fatigue behavior had an important role on the degradation of fracture strength and that cyclic component of loading do not influence on the fracture strength. Statistical characteristics of the fatigue strength were expressed by P-S-N curve estimated by the results of ΔP-increasing tests. During the fatigue experiments, crack nucleation was detected using acoustic emission (AE), and the fracture surfaces were observed by SEM. Based on these results, fracture mechanisms of the porous SiC and Cordierite ceramics were controlled by particle binders, and the binder fractures have a great variety of magnitude.

723 Si-基セラミックスの溶融塩 Na_2SO_4 による腐食挙動と強度劣化

Several trials have been conducted for applying Si-based ceramics as the materials used for the high temperature structure such as gas turbine components, because of there superior fracture toughness and thermal shock resistance under high temperature condition. However, various problems concerning oxidation and corrosion under high temperature environment have been reported. From among various high temperature corrosion problems, the molten salt corrosion has been pointed out as the most important one which caused serious fracture strength degradation. Purpose of this study is to evaluate the corrosion behavior and the degradation in the mechanical property of SiC and Si3N4 caused by molten salt. As a result, the influence of molten salt Na2SO4 upon the mechanical property of structural ceramics SiC and Si3N4 was made clear to be caused by the progressive corrosion reaction, i.e. the successive repetition of the generation and removal of the SiO2 membrane under the high temperature condition.

Effect of Milling Conditions on the Strength of Alumina–Silicon Carbide Nanocomposites

Processing effects of wet ball‐milling on the microstructure and fracture strength of Al2O3/5 vol% SiC nanocomposites were investigated. Homogeneous microstructure and a high fracture strength of 1200 MPa could be achieved under the milling condition of relatively low wear contents of Al2O3 grinding media, maintaining the homogeneity of the ball‐milled powders. The fracture strength decreased with increasing wear content. Degradation of fracture strength was caused by abnormal grain growth related to wear particles from the Al2O3 balls.

Microstructure design of HIPed TiO2 ceramics for improved corrosion resistance

HIPed TiO2 ceramics of excellent properties have been reported recently [1–3]. Their potential applications include structural components in production equipment of semiconductors. In this application, the material must withstand severe attack from various chemicals which are commonly used in the production of semiconductors. For designing a material with optimum properties, it is necessary to examine the effect of microstructure on the corrosion resistance. Specimens in this study were prepared with the hot isotatic pressing (HIP) technique, which enables us to change only the grain size while keeping the density high (relative density (RD) ˆ 100%). Details of the powder and pre-sintering conditions have been reported elsewhere [1]. In brief, TiO2 powder was synthesized by a gas phase process and contained 200 and 140 p.p.m. of silica and alumina as impurities, respectively. After compaction, the powder was pre-sintered at 1200 8C for 2 h in air and then HIPed in argon. The HIPing temperature was varied for 1000–1200 8C with the same holding time (2 h) and pressure (150 MPa). Specimens (3 mm 3 4 mm 3 20y40 mm) were cut from each HIPed body and were placed in 100 ml of aqueous solutions of various chemicals. The corrosion was followed by measuring the weight loss and the fracture strength degradation in boiling conditions except for the hydrofluoric acid. The corrosion in the last acid was examined at 25 8C by measuring the weight loss, the concentration of ions dissolved in the solution and the fracture strength. Inductively coupled plasma emission spectrometry (ICP) was applied to measure the concentration of ions in the solution. The fracture strength was measured with the three point bending method with the outer span 30 mm and the cross-head speed of 0.5 mm miny1. The average grain size was determined by the metallographic method on scanning electron microscopy (SEM) micrographs of polished surfaces. Transmission electron microscopy (TEM: JEM-2010, JEOL, Japan) was also used to examine the grain boundaries of the material. Fig. 1 shows the microstructures of the specimen HIPed at 1000 8C. In this observation, no coating or etching was applied. The moderate electrical conductivity of the material caused no charging. The orientation dependence of the escape efficiency of secondary electrons may contribute to generating the contrast for the as-polished surface without etching. The specimen HIPed at 1000 8C has a very uniform structure consisting of fine grains. No pores or cracks were observed. Examination of other specimens showed that the grain size increased markedly

ファインセラミックスの高温衝撃３点曲げ強度について

Though problems on a precise and concise evaluation method of strength behaviors of ceramics still remain open, extensive studies by many investigators have revealed the extremely low fracture toughness of engineering ceramics such as silicon nitride and silicon carbide. However, on the other hand, these engineering ceramics have several excellent properties; high compression strength, high strength at elevated temperatures and high wear resistance. Such opposite aspects in their mechanical properties must be taken into account when we intend to apply these creamics as a structural material. Furthermore, in order to establish the design concept for the application of engineering ceramics, the whole strength characteristics must be systematized by compiling the strength data covering mechano chemical reactions in aggressive environments and further the dynamic response to transientally varying load such as impact load.The aim of the present study was to clarify the impact strength characteristics of representative engineering ceramics at high temperatures. For this purpose, the impact fracture strengths of silicon nitride and silicon carbide at high temperatures upto 1300°C were investigated by using an impact 3-point bending load device. The results indicate that the sintered silicon nitride has excellent high temperature impact strength, i.e., the degradation of fracture strength at high temperatures is less under impact load than under static load. The impact strengths of two types silicon carbides were also described.

Influence of high temperature exposure on the properties of alumina short fibre reinforced AA6061 alloy

Influences of high temperature exposure on the mechanical properties and microstructures of AA6061 alloy matrix composites containing 15 vol % alumina short fibre (FRM) have been examined. When the exposure temperature exceeded 853 K, the degradation in fracture strength of the FRM became remarkable. During the heat treatment, magnesium, which is one of the strengthening elements of the matrix, segregated on the surface of alumina fibres to form a spinel and vaporized from the specimen surface in the vacuum. Therefore, degradation in the strength of the FRM was caused by weakening of the matrix due to the dissipation of magnesium from the matrix. Appropriate conditions for joining the FRM were also discussed.