Available
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www.science
direct.c om
Journal of the European
Ceramic Society 28
(2008) 803–810
Recoverability of composite materials
Chen,
Kuan-Zong
Fung?
Center for Micro/N ano Science and T echnology, Department of
Materials Science and Engine ering, National Cheng K ung
University, 1 Ta-Hsueh Road, Tainan 701, T aiwan, ROC
Received 25 November 2006; received in revised form 27
July 2007; accepted 5 August 2007
1.I n t r o d u c t i o n
Direct restorative composites experience considerable mechanical challenge during function, especially those indicated for posterior restorations. Thus, in order to withstand the mechanical stress generated by the biting forces, these composites contain a high percentage of inorganic reinforcing filler.
A huge variation in the size, shape and constitution of filler particles can be observed in the different commercial resin composites, even for those of the same category or from the same manufacturer. Improvements in filler technology for composites increased the variety of options available and even classifications of such materials have been suggested based on the morphology of the filler particles.
Studies have shown the influence of the size and shape of the filler particles on the mechanical properties of dental composites. These particle characteristics
determine what Braem et al. called “maximum particle packing fraction”, which is the ratio of true particle volume to the apparent volume occupied by the particles in the composite. According to the authors, important mechanical properties, such as Young's modulus, depend upon this ratio. Also, the presence of small spherical particles has been related to a high percentage of filler in the commercial composites, improving the mechanical properties.
Nanotechnology has become a reality in different areas of engineering with the development, through physical and chemical methods, of materials and functional structures containing particles within a size interval of 0.1–100 nm. It is also one of the most noticeable advances in composite filler technology, involving the incorporation of silica fillers of nanometer size. Nanofillers are found in microfill and some hybrid composites that can be considered predecessors of the ne wer nanoparticulate composites. A study evaluating the mechanical properties of experimental composites with or without nanofillers was carried out by Musanje and Ferracane, who observed a positive effect of the presence of nanofiller particles, expressed by an improvement in flexural strength, surface hardness (H) and fracture toughness (K c).
Nanoparticulate composites bring the perspective of creating another category of universal resin composite that joins the optical properties and the polishability required for anterior restorations with the mechanical properties demanded for posterior restorations. However, relatively little information about these new materials is available in the dental literature.
Strength (σ) is an important property for a restorative material. It is dependent upon the material's microstructure, composition, testing method, environment and failure mechanisms. Strength values are valuable when representing information about the flaw population with potential to cause the failure of a restoration or prosthesis, and thus, must be interpreted within a context that involves the analysis of failure and structural reliability, rather than an isolated result. The presence of structural defects with potential to become critical defects, such as microcracks, grains or internal voids depend upon the volume of the material structure.
Measurement of the strength of composites is often performed through flexural tests. The test indicated by the International Standards Organization to evaluate the strength of polymer-based restorative materials is the 3-point bending test (3PBT). This test employs bar-shaped specimens that bend under compressive loading equally distant from the lower supports, promoting tensile stresses in the lower surfaces that are more likely related to the fracture initiation (Fig. 1A). The test configuration tends to confine the area submitted to the stress between the supporting rollers and the loading rollers. The so-called 4-point bending test (4PBT) uses the same bar-shaped specimens, but a different configuration for load appliance based on two load cylinders over the upper surface of the specimens (Fig. 1B) that tend to expose a higher flaw containing area of the material to the stress when compared to the 3PBT. Therefore, it is expected that higher strengths are measured with the three-point bending test.
Fig. 1
3-point (A) and 4-point (B) bending tests.
Statistical parameters are commonly applied to data from mechanical strength tests to determine the level of structural reliability of the materials. The Weibull modulus (m), or shape parameter, describes the variation in the distribution of strength values from different materials and also establishes a direct relationship with the size and distribution of the defects present in a specific volume of material. In this sense, high Weibull modulus indicates a smaller error range, and potentially greater clinical reliability. In addition, the Weibull analysis can offer more clinically relevant parameters, such as the 5% failure probability (σ0.05).
Data obtained from the flexural tests were submitted to Student's t-test for differences between composites in each flexural test and for the flexural test methods with the same composite (
= 0.05). Weibull statistics were also carried out in order
s
to obtain the shape (m) and scale (σ0) parameters of both composites.
3. Results
There was no significant difference between the two composites evaluated, either for the 3PBT (p = 0.307) or the 4PBT (p = 0.275). However, a significantly higher flexural strength with the 3PBT was observed for both, the microhybrid (p = 0.004) and the nanofill composites (p = 0.005), in comparison with the 4PBT, confirming the hypothesis of the study
4. Discussion
Filler morphology and size are always factors of concern when mechanical properties and fracture behavior of composite resins are evaluated, because they affect the filler volume fraction. In the present study, the flexural strength was not affected by the differences of three orders of magnitude in average filler size between the two composites (p = 0.307). This is in accordance with other studies that showed no significant difference in flexural strength bet ween Filtek Supreme? (nanofill) and some microhybrid composites and. According to Mitra et al. a high filler loading was obtained in Filtek Supreme? due to the wide particle distribution and the spherical shape of the filler particles, equaling physical and mechanical properties of microhybrid composites, including the flexural strength. The filler content of this composite is composed of 20 nm non-aggregated silica particles and nanoclusters of 75 nm agglomerated particles that are reported to reach a 0.6–1.4 μm size range that, in turn, corresponds to the average size of the filler particles of Filtek Z250?. In addition, both composites contain spherical-shaped particles that have been associated with reduced stress concentration as compared with the sharp edges present in irregular-shaped filler particles. These factors in association with the similar filler packing between the composites might have produced their similar mechanical behavior.
The Weibull statistics is considered to be an acceptable approach in engineering to evaluate the reliability of a material or component. It provides a way of accessing the dependability of the material, disclosing the probability of failure at any selected level of stress. As a measure of the variability of strength in a material and its dependence on crack size distribution, a higher m, even in association with slightly lower mean fracture strength, is often preferable to a lower m associated with a higher mean fracture strength. A high m could also be useful as an indicator of a more favorable test design to evaluate and compare the strength of materials.
The overall Weibull modulus results confirmed the variability of strength of both composites with the different tests, expressed by means of the standard deviation a nd the coefficient of variability. A significantly higher m was shown for the microhybrid composite when tested by 3PBT. No significant difference, however, was found between the tests for the nanofill composite. Similar results were found by Jin et al. when testing the flexural strength of ceramics. The authors concluded that it is difficult to determine, based on the Weibull modulus, the most suitable test design for different materials.
Weibull statistics have been developed and used since the 1950s in the engineering community to determine equivalent strengths that have been measured by different test configurations. Weibull size scaling is routinely used to predict strength for a 3PBT when strength is measured by a 4PBT, or vice-versa. This is done by m eans of calculating “effective volumes” or “effective areas” that are under stress, and is fairly straightforward for the configurations of this study. Calculation of Weibull size scaling is done with the following formula: σ3/σ4 = (A4/A3)1/m, where σ3 and σ4 are the stresses measured under 3- and 4-point configurations, A3 and A4 are the effective areas of the associated configurations, and m is the Weibull modulus. The effective area for 3PBT is (S/4)[(m + 2)/(m + 1)2] and for 4PBT is (S/12)[(m + 2)(m + 3)/(m + 1)2], where S is the total specimen surface within the load span and is the same for both configurations, for the span lengths and specimen sizes are the same. Calculations predicted a 15% higher strength for the 3-point
configuration in comparison to the 4-point configuration for either material. This is slightly higher than the measured results (about 10%), but within experimental error.
The fracture behavior of the composites was also expressed by σ0 and σ0.05. Both parameters indicated a lower stress for fracture of the composites when submitted to the 4PBT. This might be explained by the fact that a higher volume (53.4%) is exposed to the stress in the 4PBT comparing to the 3PBT (40%). The macroscopic observation of the composite specimens fractured by 4PBT disclosed fractures occurring in different places along the tensile surface over distances between the upper loading rollers, confirming also the wider volume of material involved in this test. In contrast and as expected, the specimens submitted to 3PBT fractured on the tensile surface midway between the two supports. According to Zeng et al. the 3PBT submits only a very small area to the maximum tensile stress, underestimating the flaws located far from the loading rollers and tending to induce a fracture initiation site. In this sense, although the 4PBT presented lower strength values for both composites, it may represent a more reliable approach as a measure of flexural strength of composites than the 3PBT. In addition, it may provide a more realistic and …safe? lower boundary for the resulting strength.
The analysis of the fractured surface using fractographic principles is a well-established analytic tool to determine the failure behavior of brittle materials. It is based on the principle that the history of the fracture process is encoded on the fractured surface of the material. According to Le May and Begnall, “investigations of structural failure by brittle fracture should take into account, at least, two separate aspects: (1) the point from which the fracture developed is relevant in order to determine whether the fracture initiated from a manufacturing effect, whether it experienced a prior fatigue or stress corrosion that originated the crack that led to failure; (2) it should indicate that, for whatever the defect present, the load applied was enough for unstable fracture to occur”.
Characteristically, brittle materials, such as composites, present a population of flaws of different sizes, geometries and orientations. Fracture occurs when the load exceeds a critical value for the propagation of the largest and most favorably oriented
flaw. These flaws are induced by intrinsic imperfections in the structure of the material, by processing or by mechanical grinding and polishing, and might potentially reduce the strength of the material.
The defects that led to the fracture initiation were identified as non-homogeneous distributions of organic and inorganic phases, inclusions, cracks and voids. Both, the manufacturing of the materials and the handling procedure might generate these defects. In the present study, defects similar presenting a smooth darker area of matrix, were strongly associated with the initiation of fracture.
Surface flaws were identified as the fracture initiation site for 86.6–96.6% of the specimens, independent of the group. The low area exposed to the maximum tensile stress in the bending test makes fractures more likely to develop from the surface. In this sense, bending specimens are considerably sensitive to surface or edge damage during grinding or polishing. Internal critical defects were rare and always associated with moderate to high fracture strength. The internal irregular defects, are more stress inducing than internal spherical-shaped pores, requiring a lower load to propagate the fracture.
The polymerization method employed, that was common to both tests, also might have affected the fracture behavior of the composites. According to Le May and Begnall, fracture initiation and propagation depend on the total local stress, which includes the externally applied load and the residual stress of the specimen. This is of major importance, because the production of 25 mm long bar-shaped specimens requires an overlapped light activation procedure when using an 11 mm diameter light guide, resulting in areas of the specimen that are exposed to twice the light and. Attempting to avoid the effect of the inhomogeneous polymerization of the specimens some authors have suggested the use of oven-LCUs.
Natural and restored teeth are subjected to cyclic loading during normal masticatory function. Therefore, the wear process and failure due to fatigue stress are phenomena of relevance from the clinical standpoint and should instigate future work.
5. Conclusion
Higher flexural strength was produced by the 3-point bending test than by the 4-point bending test, independent of the composite evaluated. The flexural strength and the fracture behavior of both composites were similar, despite the difference of the average filler size of the composites tested, probably due to the microstructural arrangement of the nanofillers in clusters that approximate the average size of the filler of the microhybrid composite and due to the similar filler volume in both composites.
Acknowledgment
This work is financially supported by the Council of Agri- culture, Executive Y uan, Taiwan; Grant 96AS-10.1.1-AD-U1.
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英文论文APA格式 英文论文一些格式要求与国内期刊有所不同。从学术的角度讲，它更加严谨和科学，并且方便电子系统检索和存档。 版面格式
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毕业设计（论文）外文参考文献及译文 英文题目Component-based Safety Computer of Railway Signal Interlocking System 中文题目模块化安全铁路信号计算机联锁系统 学院自动化与电气工程学院 专业自动控制 姓名葛彦宁 学号 200808746 指导教师贺清 2012年5月30日
Component-based Safety Computer of Railway Signal Interlocking System 1 Introduction Signal Interlocking System is the critical equipment which can guarantee traffic safety and enhance operational efficiency in railway transportation. For a long time, the core control computer adopts in interlocking system is the special customized high-grade safety computer, for example, the SIMIS of Siemens, the EI32 of Nippon Signal, and so on. Along with the rapid development of electronic technology, the customized safety computer is facing severe challenges, for instance, the high development costs, poor usability, weak expansibility and slow technology update. To overcome the flaws of the high-grade special customized computer, the U.S. Department of Defense has put forward the concept：we should adopt commercial standards to replace military norms and standards for meeting consumers’demand [1]. In the meantime, there are several explorations and practices about adopting open system architecture in avionics. The United Stated and Europe have do much research about utilizing cost-effective fault-tolerant computer to replace the dedicated computer in aerospace and other safety-critical fields. In recent years, it is gradually becoming a new trend that the utilization of standardized components in aerospace, industry, transportation and other safety-critical fields. 2 Railways signal interlocking system 2.1 Functions of signal interlocking system The basic function of signal interlocking system is to protect train safety by controlling signal equipments, such as switch points, signals and track units in a station, and it handles routes via a certain interlocking regulation. Since the birth of the railway transportation, signal interlocking system has gone through manual signal, mechanical signal, relay-based interlocking, and the modern computer-based Interlocking System. 2.2 Architecture of signal interlocking system Generally, the Interlocking System has a hierarchical structure. According to the function of equipments, the system can be divided to the function of equipments; the system
Analysis of the role of complaint management in the context of relationship marketing Author: Leticia Su′arez ′Alvarez, University of Oviedo, Spain Abstract This research aims to contribute to the relationship-marketing strategy by studying the role of complaint management in long-term relationships. Two factors distinguish it from other studies: it takes into account two types of customers, consumers and firms, and the result variable selected is the probability of ending an ongoing relationship. Two questionnaires were designed for every population. One of them was auto-administrated to a sample of consumers in the north of Spain, and the other one was sent to a representative sample of Spanish firms. The data analyses were conducted using structural equation modeling. The findings confirm the importance that theory accords to the relationship-marketing strategy, and also provide evidence for the importance of complaint management. Thus having a good complaint-handling system and trained and motivated staff who are fully committed to the firm’s objectives are fundamental requisites for firms to be able to build a stable customer portfolio. Keywords complaint management; relationship marketing; relationship termination; trust; satisfaction Introduction Nowadays, the main task for tourism firms is undoubtedly to deliver superior value to customers. One way that these firms can achieve part of this value is by maintaining quality relationships with their customers. In fact, it is well known that managing these relationships is critical for achieving corporate success. Thus the general aim of the present research is to analyze the most important factors that contribute to relationship stabilization between tourism firms and their customers. This research canters on retail travel agencies. We chose this particular type of tourism firm for two reasons. First, competition between retail travel agencies is becoming much more intense, fundamentally due to the advent of the Internet as an alternative distribution channel for tourism services (Wang & Cheung, 2004). The second reason is the current phenomenon of disintermediation, or the tendency of some tourism service providers to contact the end-customer directly. Because of these two developments, retail travel agencies urgently need to develop a strategy that allows them to maintain a stable portfolio of customers over time if they are to remain in the market for the long term. In order to achieve the proposed objective, we set out a causal model that incorporates a number of factors that can condition the future of the relationships between travel agencies and their customers. Specifically, we chose two variables that
规范 5.2－混凝土配合比的选择 5.2.1－混凝土配比的确定有以下规定： （a）和易性和稠度使混凝土在浇筑时，易于成型和易于与钢筋粘结，不会离析或泌水。 （b）按第四章的要求，混凝土具有抵抗侵蚀的性能。 （c）符合5.6节中强度试验要求。 5.2.2－不同材料用在不同部分，起不同作用，要评测每一个组合。 5.2.3－混凝土配比要与5.3节或5.4节相一致，而且要满足适用性。 5.3－以现场试验和（或）试拌配料 注释 R5.2－混凝土配合比的选择 “普通混凝土，重混凝土，大体积混凝土配比选择标准”(ACI 211.1) 5.1给出了选择混凝土配比的细节规则。（提供了两种选择和调整普通混凝土配比的方法：估计重量和绝对体积方法。给出了两种方法的计算实例。按绝对体积方法配制的重混凝土的配比查看附录。）“结构用轻骨料混凝土配比选择标准”(ACI 211.1) 5.2给出了轻质混凝土选料的方法。（提供了选择和调整不同建筑等级轻骨料混凝土的配比方法。） R5.2.1－所用水灰比要足够低，或者轻骨料混凝土的抗压强度足够高,以满足强度标准（见5.3或5.4）和特殊环境（第四章）的要求。该条规范不包括极恶劣暴露环境的要求，如：酸性条件，高温条件，同时也没考虑美学效果，如表面装修。这些方面超出了该规范的范围，应包含在工程技术要求中。混凝土配比要满足该规范的最低要求，以及合同附加的条款。 R5.2.3－本规范强调混合物的现场试验或试验室试拌（见5.3）作为首选的混凝土配比方法。 R5.3－以现场试验和（或）试拌配料 在选择合适的混凝土混合料时，遵循以下三步。第一，确定样品标准偏差。第二，确定要求的混凝土平均抗压强度。第三，根据传统配制试验或合理的经验记录，选择满足该平均强度要求的混凝土配合比。Fig.R5.3是选择配料的流程图。
Inventory management Inventory Control On the so-called "inventory control", many people will interpret it as a "storage management", which is actually a big distortion. The traditional narrow view, mainly for warehouse inventory control of materials for inventory, data processing, storage, distribution, etc., through the implementation of anti-corrosion, temperature and humidity control means, to make the custody of the physical inventory to maintain optimum purposes. This is just a form of inventory control, or can be defined as the physical inventory control. How, then, from a broad perspective to understand inventory control? Inventory control should be related to the company's financial and operational objectives, in particular operating cash flow by optimizing the entire demand and supply chain management processes (DSCM), a reasonable set of ERP control strategy, and supported by appropriate information processing tools, tools to achieved in ensuring the timely delivery of the premise, as far as possible to reduce inventory levels, reducing inventory and obsolescence, the risk of devaluation. In this sense, the physical inventory control to achieve financial goals is just a means to control the entire inventory or just a necessary part; from the perspective of organizational functions, physical inventory control, warehouse management is mainly the responsibility of The broad inventory control is the demand and supply chain management, and the whole company's responsibility. Why until now many people's understanding of inventory control, limited physical inventory control? The following two reasons can not be ignored: First, our enterprises do not attach importance to inventory control. Especially those who benefit relatively good business, as long as there is money on the few people to consider the problem of inventory turnover. Inventory control is simply interpreted as warehouse management, unless the time to spend money, it may have been to see the inventory problem, and see the results are often very simple procurement to buy more, or did not do warehouse departments . Second, ERP misleading. Invoicing software is simple audacity to call it ERP, companies on their so-called ERP can reduce the number of inventory, inventory control, seems to rely on their small software can get. Even as SAP, BAAN ERP world, the field of
A virtual instrument based measurement and control platform for vibration analysis of motor LI Wen1, LI Li2 Abstract:This paper describes the establishing process of motor vibration analyzing system based on virtual instrument. This system consists of data acquisition module, vibration spectrum analysis module, and control module. Real-time collection of data acquisition module can be operated during motor running process. Power spectrum, harmonic analysis and STFT intensity spectrum can be done with vibration spectrum analysis module; and the performance of motor under different control laws can be easily studied by call control algorithms with control module. The test results show that the platform is feasible and effective. Key words: motor vibration; virtual instrument; spectrum analysis module; control module 2.1 data acquisition module Place DAQ Assistant Express VI and Write to Measurement File .vi into While Circulation in program block diagram while set the cycle time as 60s, then set the channel, sensitivity, sample rate and other parameters in DAQ Assistant . Run the program can collect experimental data and the data can be stored by the designated route in the folder. Fig.4 shoes its program Fig.4. program of DAQ Data acquisition
1.Sales Agreement The agreement, (is) made in Beijing this eighth day of August 1993 by ABC Trading Co., Ltd., a Chinese Corporation having its registered office at Beijing, the People’ Repubic of China (hereinafter called “Seller”) and International Trading Co., Ltd., a New York Corporation having its registered office at New York, N.Y., U.S.A. (hereinafter called “Buyer”). 2.WITNESSETH WHEREAS, Seller is engaged in dealing of (product) and desires to sell (product)to Buyer, and WHEREAS, Buyer desires to purchase(product) from Sellers, Now, THEREFORE, it is agreed as follows: 3.Export Contract th This Contract is entered into this 5 day of August 1993 between ABC and Trading Co., Ltd. (hereinafter called “Seller”) who agrees to sell, and XYZ Trading Co., Ltd. (hereinafter called “Buyer”) who agrees to buy the following goods on the following terms and condition. 4.Non-Governmental Trading Agreement No. ＿＿ This Agreement was made on the＿day of＿ 19＿, BETWEEN ＿(hereinafter referred to as the Seller) as the one Side and ＿ (hereinafter referred to as the Buyer) as the one other Side. WHEREAS, the