Electrical properties of ZnO/alumina nano composites for high voltage transmission line insulator

The main aim of the present study is to evaluate the impact of the nano-ZnO on the dielectric properties of alumina bodies used in high voltage insulators. In this work, Zinc oxide/alumina nanocomposites were prepared by sol–gel method. The ratios of ZnO were 5, 6 and 7 mass %. The effect of ZnO on the densification, microstructure, mechanical and electrical properties of the prepared bodies were evaluated after sintering at a temperature ranging from 1550 to 1750 °C. Results revealed that incorporation of 7 mass % ZnO enhanced the breakdown voltage, electric resistivity and the dielectric loss, which are the most important factors to evaluate high-voltage insulators. In addition, incorporation of 7 mass % ZnO enhanced the densification and mechanical properties of the alumina nano composites .     

Growth mechanism of stress corrosion cracking in high strength steel

Stress corrosion crack growth rates are measured at sveral stress intensity levels for low-tempered 4340 steel in 0.1N H₂SO₄ solution. The characteristics of the growth rates are divided into three regions of stress intensity factors: Region I near K_1SCC; Region III near unstable fracture toughness, K_1SC; and Region II, which lies between the two. K_1SCC is the value of K at which no crack growth can be detected after 240 hr.

In order to explain these experimental results, the crack initiation analysis reported in a previous paper is extended to the growth rates. A detached crack initiates and grows at the tip of an already existing crack. When the detached crack reaches the tip of the main crack, the process repeats as a new existing crack.

A relationship between crack growth rate, v, and stress intensity factor, K, is obtained as a function of b/a and a = b + d, where b is the distance from the tip of the main crack to the detached crack, and d is the ydrogen atom saturated domain.

The experimental data are in good agreement with the theoretical values in Region II when a = 0.02 mm, b/a = 0.8, c₁/c₀ = 2.8 for 200°C tempered specimens and a = 0.015 mm, b/a = 0.7, c₁/c₀ = 3.0, ρ_b = 0.055 mm for 400°C tempered specimens, where ρ_b is a fictitious notch radius. The plateau part in Region II for 400°C tempered specimens is also successfully explained by the present theory. For Region III, the value of b/a will be almost equal to 1 because v → ∞ for b/a → 1. On the other hand, for Region I, b/a will be zero, since the value of v becomes negligibly small and no crack growth is observable.     

Extruded PZT/polymer composites for electromechanical transducer applications

PZT/polymer composites having a 1–3 parallel connectivity were fabricated by impregnating a sintered, extruded honeycomb configuration of PZT with various polymers. The resultant composites were found to have densities less than 2900 kg/m³, a dielectric constant of ～500 and a piezoelectric d₃₃ of ～300×10^?12 C/N. The 1–3 connectivity increases the piezoelectric voltage coefficient (g₃₃) from 22×10^?3 Vm/N (solid PZT) to ～70×10^?3 Vm/N.The composites have thickness mode electromechanical coupling coefficients (k_t) which are ～25% greater than that of homogeneous PZT, and are readily adaptable for broad bandwidth operation. This combination of electromechanical properties makes these composites ideal for low voltage displacement and pulse echo applications.     

Effect of loading history on stress corrosion cracking in high strength steel

The effect of preloading on crack nucleation time was examined with compact tension specimens having various notch radius in 0.1N-H²SO⁴ aqueous solution for 200°C tempered AISI 4340 steel. Crack nucleation time t_n increases by preloading for a given apparent stress intensity factor K_p2. The curve K_?2 vs. t_n deviates upward from the curve for the non preloading case. A linear relationship between the crack nucleation time and parameter is seen in semi-log diagram, where is taken as the value at t_n=α due to preloading. The apparent threshold stress intensity factor increases with K_?2 which is the apparent stress intensity factor of preloading. A detached crack is nucleated at some distance from the notch root and extends in a form of circle. This distance increases with increasing K_?2. The effect of load reduction during crack growth was examined. When the K-value was reduced from K₁ to K₂, an incubation time was observed before the crack started growing under the K₂-value. The incubation time t_m tends to increase with increasing ΔK = K₁-K₂. The threshold stress intensity factor was also found to increase for high load reduction.In order to explain these experimental results, a new dislocation model is proposed on the basis of stress induced diffusion of hydrogen in high stress region ahead of the notch root or a crack. This model suggests that the change in the crack nucleation time and the increase of the incubation time due to preloading or load reduction are caused by reducing the hydrostatic pressure and by spreading the hydrogen saturated region which requires more time for the hydrogen accumulation due to preloading or load reduction. The theory predicts the experimentally observed relations between and t_n and between log t_in and ΔK.     

Resistance of a zirconium alloy to corrosion cracking under stresses

We describe a procedure for ultrahigh-frequency thermal treatment of é110 alloy and present the results of its textural and structural analysis. We also study the influence of the modes of ultrahigh-frequency treatment of the zirconium alloy on its long-term strength at a temperature of 653 K in corrosive media. The results of fractographic investigation of the fracture surfaces allow us to determine the mechanisms of initiation and propagation of stress-corrosion cracks in shells made of the alloy under consideration in the intact state and after ultrahigh-frequency treatment. Translated from Problemy Prochnosti, No. 2, pp. 122 – 130, March – April, 1998.     

Device for long-term strength testing of unidirectional polymer composite materials

An experimental device is described for testing the long-term strength of polymer composite materials. The model used is a specimen of microplastic, loaded to failure by means of a hydraulic cylinder. The case of simple tensile static loading is considered.     

A novel statistical model for predicting matrix cracking in high temperature polymer composite laminates

This paper studied the progressive matrix cracking in high temperature polymer composite laminates that could be used for next generation high speed transport airframe structures and aircraft engine components exposed to elevated temperatures. Damage mechanisms of matrix cracking were identified by X-ray radiography at room temperature and in-test photography technique at high temperature. It was found that the non-deterministic scenario is always involved in the procedure of transverse matrix cracking. Monte Carlo simulations using experimentally obtained materials properties were applied to simulate the multiple transverse cracking and compared with the experiment data. Finally, a novel statistical model combining Weibull theory with shear lag model was proposed to predict the matrix cracking based upon the previously obtained probability density function of crack spacing. It is shown that the predictions of this statistical model agree well with the experimental results and can be used to have an in-depth understanding of the random matrix cracking problem in composite laminates.     

Damage due to hydrogen embrittlement and stress corrosion cracking

Damage of metals due to the influence of hydrogen and to stress corrosion cracking is quite frequent and leads to dangerous failures as well as to loss of property and large compensational payments by insurance companies. One reason for this, is that some designers and engineers seem to lack sufficient knowledge of the basic mechanisms of these phenomena and accordingly often have only vague ideas how to prevent such failure causes. Although the basic concepts can be found in a number of good text books it seems worthwile to recall them in a short comprehensive paper.     

单向纤维增强聚合物复合材料压缩渐进破坏单向纤维增强聚合物复合材料压缩渐进破坏

复合材料结构在承压时破坏如何演化,是其强度破坏分析的基础和核心任务。本文提出了基于连续介质损伤力学（CDM）的单向纤维增强聚合物复合材料压缩破坏渐进损伤分析（PDA）模型。建模中考虑了材料非线性行为、失效判断及损伤演化中材料性能退化等基本问题,分别对应于拉压不对称弹塑性本构关系、Puck准则、LaRC05准则及考虑破坏面方向的刚度退化方法。该模型通过用户材料子程序接口VUMAT引入到有限元软件ABAQUS中实现了有限元求解。对文献中提供的纵向、横向及偏轴压缩案例进行了数值计算并与试验数据对比。数值分析结果与试验数据吻合较好,证明了该方法的合理性和有效性,对开展多向层合板压缩破坏分析富有参考价值。      

Elastic field equations for blunt cracks with reference to stress corrosion cracking

The elastic stress field equations for blunt cracks are derived and presented in a form equivalent to the usual sharp crack tip stress fields. These stress field equations are employed in analyzing a dissolution model for the arrest of stress corrosion cracking by crack tip blunting, which is often observed with the arrest of stress corrosion cracks.

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Zusammenfassung Es wird die Spannungsverteilung vor einem Riss mit einem Kruemmungsradius groesser Null fuer den elastischen Fall abgeleitet and in einer den ueblichen Gleichungen fuer den scharfen Riss aehnlichen Form dargestellt. Diese Spannungsgleichungen werden angewandt, um ein Modell fuer das Anhalten eines Bruchs durch die Abstumpfung der Riss-spitze zu untersuchen. Bei der Spannungskorrosion ist dieses Anhalten eines Bruches haeufig von einer Abstumpfung begleitet.

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Résumé Les équations de champ de tension élastique pour des fissures épointées sont dérivées et présentées de la même façon que celles qui décrivent le champ de tension des pointes de fissures aigues. Ces équations de champ de tension sont employées dans l'analyse d'un modéle de dissolution décrivant l'arrêt du craquement corrosif sous tension obtenu par l'émoussement des pointes de fissures, qu'on observe souvent dans l'arrêt du craquement corrosif sous tension.

     

Nanodiamond-filled high-temperature vulcanized silicon rubber composite for high-voltage insulator applications

Journal of Materials Science: Materials in Electronics - Nanodiamond particles with various filling ratios were added into the commercial high-temperature vulcanized silicon rubber composites,...     

单向复合材料横向裂纹黏弹性损伤演化模型

建立一个考虑基体黏弹性的纤维增强聚合物单向复合材料在产生横向裂纹时的损伤演化模型,有效地预测了单向复合材料横向拉伸行为。假设呈现威布尔分布的缺陷会在变形的驱动下演化为损伤,并以此为基础建立了单向复合材料横向损伤演化模型。通过此模型,时间-温度叠加原理（TTSP）得到了更具有物理基础的解释。最后,通过具体例子阐述了此模型的应用,并通过试验对模型预测结果进行了验证。本模型有效地预测了单向复合材料横向拉伸行为。由于单向复合材料横向性能存在脆性,此模型还无法准确预测失效和强度。      

Acoustic properties of particle/polymer composites for ultrasonic transducer backing applications

The acoustic impedance and attenuation in composites made of particle fillers loaded in polymer matrices for transducer backing applications is investigated. The acoustic impedance of tungsten/vinyl composites was modeled, and an experimental matrix identifying variables that contribute to composite attenuation was established. The variable included the particle type, the particle size and volume fraction of a filler, the physical characteristics of the polymer matrix, and the processing route that determined the composite connectivity. Experimental results showed that with an increase in filler particle size or a decrease in volume fraction of filler, there is an increase in composite attenuation. Overall, the various types of filler, the polymer matrix, and the interface between the two contribute to attenuation in the composite, as confirmed by the acoustic properties and the microstructural analysis.