Qing Jie Zhang

Numerical Analysis for Small Punch Creep Tests by Finite-Element Method

A numerical study is presented that simulates small punch creep (SP-C) tests using a finite-element method (FEM). The objective of the present study is to develop a miniaturized testing methodology for high-temperature creep properties. The numerical simulations have been shown to produce deflection versus time curves that are quantitatively similar to the experimental results obtained on tungsten-alloyed 9 % Cr ferritic steels. It is also demonstrated that the numerically predicted curves show the steady state (secondary) creep stage. Furthermore, the numerical simulations reveal that the magnitude of the equivalent stress in the central region of the SP-C specimen shows no significant change with respect to time at the secondary creep stage, supporting the use of the present SP-C testing method to characterize the secondary creep deformation rate. Finally, an approximate equation is proposed for the assessment of the equivalent stress in the SP-C specimen in terms of the load and testing parameters.

Recent Development in Nano and Graded Thermoelectric Materials

In 2003, a joint research project entitled “Nano and graded thermoelectric materials/Photovoltaic-thermoelectric-wind power generation” is established in cooperation among research institutes from Japan and China. The major research institutes include State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, China), Shanghai Institute of Ceramics (Chinese Academy of Sciences, China), State Key Laboratory of New Ceramics and Fine Processing (Tsinghua University, China), Japan Aerospace Exploration Agency (JAXA, Japan) and Foundation for Promotion of Japanese Aerospace Technology (JAST, Japan). The main aim of this project is to develop a photovoltaic-thermoelectric-wind power generation system with high efficiency solar energy conversion. The key works of the project include: (1) fabrication of high efficiency nano thermoelectric materials with a maximum figure of merit Z≥1.3; (2) design and fabrication of nano graded thermoelectric material/component with efficiency conversion larger than 12% for wide temperature range and (3) design and construction of photovoltaic- thermoelectric coupled power generation system. The recent progress about the joint research project is reported in this paper. Emphasis is put on the mechanism, design and fabrication of high efficiency nano graded thermoelectric materials. The future research plan is also mentioned in brief.

Creep Property of Functionally Graded Materials

The present paper investigates the creep phenomenon of the functionally graded materials under high temperature environment by the computational micromechanical method (CMM). Based on the real microstructure of the functionally graded interlayer with different component volume fractions, the emulation experiment is implemented for the creep test numerically and the creep parameters are obtained. A further series of simulation works are carried out to investigate the creep phenomenon of FGM interlayers in more detail. Numerical results show that the creep phenomenon is obvious not only for the metal-rich interlayers but also for the ceramic-rich interlayers. The creep property of ceramic/metal interlayer depends on the material’s properties of the ceramic obviously. It is remarkable that the creep strain rate of the ceramic/metal interlayer is larger than the corresponding one of pure metal under the same load when the modulus of the ceramic component is lower than the one of the metal component.

Characterization of Nanocrystalline CoCrFeNiCuAl High-Entropy Alloy Powder Processed by Mechanical Alloying

The equiatomic multicomponent CoCrFeNiCuAl high-entropy alloy powder was synthesized by mechanical alloying. The effects of milling time and heat treatment on the structure and morphology of the ball milled alloy were investigated. Single BCC solid solution structure appears when the alloy is ball milled more than 30h. The 60h ball milled alloy powder shows a mean particle size of 3 μm, which is actually hard agglomerations of nanosized crystals with crystalline size less than 10nm. The 60h ball milled alloy exhibits good chemical homogeneity. The single BCC solid solution structure transforms to a BCC and a FCC phases when annealled at 600°C for 1h, which can be attributed to the supersaturatable solid solution formation during the mechanical alloying process.

Evaluation on Small Punch Creep Test by Finite Element Method

The Small Punch Creep Tests (SP-C tests) are simulated by a Finite Element Method (FEM). The objective of the present study is to establish a foundation for the SP-C test method by investigating the deformation and stress state of the SP-C test specimen. The emphasis is placed on the dependence relation of the creep strain and the stress on the measurable quantities, such as applied loads and the central deflections. Simulation works are conducted for two different materials, one is the tungsten-alloyed 9% Cr ferritic steel and the other is 12Cr1MoV steel. The numerical results for the central deflection versus time curves are quantitatively similar to the experimental results obtained on tungsten-alloyed 9% Cr ferritic steels. From the numerical results, the relationship between the central deflection and the equivalent creep strain is approximately independent of load, temperature, and material properties. The magnitude of the equivalent stress in the central region of the SP-C specimen shows no significant change with respect to time at the secondary creep stage, an approximate equation is proposed for the assessment of the equivalent stress in the SP-C specimen. As a farther result, the high temperature creep properties of the 12Cr1MoV steel and tungsten-alloyed 9% Cr ferritic steel are appraised by numerical simulation. The results are in good agreement with the results from the standard test method. The results indicate that the small punch test technique is an effective method for the evaluation of the high-temperature creep properties of materials.

Effective Thermal Conductivity of Functionally Graded Composite with Arbitrary Geometry of Particulate

In this paper, a new micromechanical method, the Weighted Residual Self-consistent Scheme (WRSCS), is developed for the prediction of the effective thermal conductivity of particulate composites with arbitrary configurations. The method is based on the concept of the traditional Self-consistent Scheme (SCS). For some special configurations of inclusions, such as spherical or ellipsoidal, the effective conductivity of the composite can be solved without much difficulty using SCS. But for the composite with inclusion of arbitrary geometry, such as polygon or other irregular configurations, it is difficult to get an analytic solution. In the WRSCS, the arbitrary inclusion configuration is modeled by applying collocation points at interface. Based on SCS micromechanical model, the local fields inside the inclusion can be evaluated by using the solution of a single inclusion in an infinite matrix and inclusion interaction is taken into account through the yet unknown average equivalent medium. The solution for calculating the potential field inside the inclusion is obtained by means of Weighted Residual Method (WRM). Using the WRSCS, the effective thermal conductivities for composites with different inclusion’s geometry are calculated. For the case of spherical inclusion, the results from the WRSCS show good agreements with the one from traditional SCS [7, 8]. Examining results corresponding to different inclusion’s geometry, it shows that the effective thermal conductivity depends not only on the volume fractions and the properties of components, but also on the inclusion’s configuration.

Optimized Design of Multi-Ingredient Radar Absorbing Materials Using Genetic Algorithms

A novel technique for optimized design of radar absorbing coatings(R AC) containing multi-ingredient microwave absorbers and adhesive agent using genetic algorithms is presented in this paper. The objective function involving the maximum bandwidth of more than a desired intensive-absorption virtually bears the advantages of both thin-thickness a d lightweight of RAC by incorporating an idea that variable restrictions in both species and volume fraction of microwave absorber as well as thickness of coating layers are dynamical ly tracked into the genetic algorithms. Given a predefined set of available microwave absorbers with freque ncy-dependent electromagnetic parameters, densities and so on, the proposed technique simultaneously dete rmines the optimal ingredients choice and respective fraction and thickness for each lay er, which greatly simplifies preparation and fabrication for multi-ingredient radar absorbing materials. Introduction This paper focuses on the optimized design of multi-ingredient rada r absorbing materials (RAC), which involves the solution of an N-dimensional optimization problem with m ulti-goal. Usually, broadband, thin-thickness and intensive absorbing together with light weig ht are necessary for the RAC in view of application, i.e., the RAC not only need exhibit a low re flection coefficient over a wide frequency range, but also need to be thin and lightweight. To ac hieve the multi-goal design, genetic algorithms (GA) offer significant advantages for this type of problem owing to their robustness and independence of performance function derivatives [1]. Seve ral previous researches have derived design data for thin-thickness, broadband RAC using GA [ 2-5]. Considering that the problem of designing RAC involves a trade-off between conflicting goal s, namely those of minimizing the total coating thickness while achieving maximum a bsorption, researchers used to introduce multi-goal to a single objective function in the form of penalty function as an accepted technique, which was literally a compromise between conflicting g oals. Recently, the authors [6] described a novel solution, which achieved broadband, thin-layer RAC by tr acing dynamically thickness restriction incorporated in the genetic algorithm. The latt er technique overcome some of the drawbacks of the tradeoff between different goals, and easily l ends itself to multi-goal optimization. This paper proposes a continuation of the previous work [6], incor porating a technique for tracing dynamically restrictions of such variables as fraction of each ingredient and thickness of each layer within the genetic algorithm, by using the concept of m ulti-ingredient. Model of RAC Containing Multi-component Microwave Absorbers Fig.1 shows two-piece of typical sections in different RAC, which indicates that there are different species and volume fractions of absorbers in each layer. According to such RAC, a constructive model on RAC containing multi-component radar absorbers and adhesive agent is proposed and given in Fig.2. Fig.2 shows a schematic of RAC made of n-layer with adhesive and ingr ingredients from a N species of available absorbing-reagents in each layer. Supposed that e ac ingredient is equably Key Engineering Materials Online: 2003-09-15 ISSN: 1662-9795, Vol. 249, pp 367-372 doi:10.4028/www.scientific.net/KEM.249.367

Investigation on the Effect of Test Parameters on Small Punch Creep Tests by Finite Element Method

The small punch creep (SP-C) test technique is a new method which is applied to evaluate the high temperature creep properties of materials by using miniature specimen. In the present paper, the Finite Element Method (FEM) is employed to simulate the SP-C test in order to investigate the effects of test parameters on testing results of the SP-C test. In this attempt, we perform systematic numerical simulations of SP-C tests by changing friction coefficient, specimen thickness, the diameter of punch ball and the inner diameter of lower die, and discuss the effects of the variation of test parameters on test results in detail. The resulting regression equations for assessing the effects of testing parameters on test results are obtained. It is found that the test results are influenced significantly by the specimen thickness, the diameter of punch ball and the inner diameter of lower die. However, the effects of friction coefficient on the results of the SP-C test can be neglected.

Fabrication of (TiB2-Fe)/Fe Functional Gradient Material by SHS/QP

Abstract. Distributions of adiabatic and combustion temperatures were deter mined by the compositions in (TiB2-Fe)/Fe FGM. (TiB2-Fe)/Fe FGM was fabricated by the SHS/QP method. EPMA analysis shows composition of FGM product layers changes gradual ly across the sample without distinct interface. Back-scattered electron image shows that the microstructure of FGM also changes gradually. TiB 2 grains size decreases from TiB 2-rich region to Fe-rich region shown by SEM. The microhardness of TiB 2-Fe /Fe FGM decreases from TiB 2-rich region to Fe-rich region.

Fabrication and Formation Mechanism of Flowerlike Bicephalous TiB2 Nanowhisker Clusters

Flowerlike bicephalous titanium diboride (TiB2) nanowhisker clusters were fabricated by planetary ball-milling. The pure cleavage theory and crack growth mechanism were used to explain the formation process. The high-resolution transmission electron microscopy and nano electronic beam diffraction show that TiB2 superhard material always has a cleavage plane along the direction of [0001] zone axe and [10 0] zone axe because of its big interplane distance, weak bonding force between planes and low index directions. The flowerlike bicephalous TiB2 nanowhisker can be gained after repeated cleavage under high strains. The discovery could open a new path to fabricate C32 type superhard ceramic nanowhiskers.