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1.
Usama Umer Jaber Abu Qudeiri Mohammad Ashfaq Abdulrahman AL-Ahmari 《浙江大学学报(A卷英文版)》2016,17(11):873-885
Chip morphology predictions in metal cutting have always been challenging because of the complexity of the various multiphysical phenomena that occur across the tool-chip interface. An accurate prediction of chip morphology is a key factor in the assessment of a particular machining operation with regard to both tool performance and workpiece quality. Although finite element (FE) models are being developed over the last two decades, their capabilities in modeling correct material flow around the tool tip with shear localization are very limited. FE models with an arbitrary Lagrangian Eulerian (ALE) approach are able to simulate correct material flow around the tool tip. However, these models are unable to predict any shear localization based on material flow criteria. On the other hand, FE models with a Lagrangian formulation can simulate shear localization in the chip segments; they need to make use of a mesh-based chip separation criterion that significantly affects material flow around the tool tip. In this study a mesh-free method viz. smoothed particles hydrodynamics (SPH) is implemented to simulate shear localization in the chip while machining hardened steel. Unlike other SPH models developed by some researchers, this model is based on a renormalized formulation that can consider frictional stresses along the tool-chip interface giving a realistic chip shape and material flow. SPH models with different cutting parameters are compared with the traditional FE models and it has been found that the SPH models are good for predicting shear localized chips and do not need any geometric or mesh-based chip separation criteria. 相似文献
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This paper studies the micro-cutting characteristics of aluminum alloy (2A12) based on a series of orthogonal experiments and finite element method (FEM) simulations. An energy-based ductile failure law was proposed in the FEM simulation. The simulated cutting forces and chip morphology were compared with experimental results. The simulation result indicates that there is a close relationship between the cutting force and cutting heat. The micro-cutting force decreases as the heat flux vector increases. Both the cutting heat and the micro-cutting force need a finite time to achieve a steady state. It is observed that with the cutting speed of 169.95 m/min and uncut chip thickness of 6 μm, the heat flux vector in the workpiece increases to a stable value after 0.06 ms; meanwhile, the principal cutting force decreases to a steady state correspondingly, i.e., the micro-cutting process achieves the steady state. It is concluded that the steady state micro-cutting simulation can reflect the cutting process accurately. 相似文献
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To investigate cutting performance in the helical milling of carbon fiber reinforced polymer (CFRP), experiments were conducted with unidirectional laminates. The results show that the influence of cutting parameters is very significant in the helical milling process. The axial force increases with the increase of cutting speed, which is below 95 m/min; otherwise, the axial force decreases with the increase of cutting speed. The resultant force always increases when cutting speed increases; with the increase of tangential and axial feed rates, cutting forces increase gradually. In addition, damage rings can appear in certain regions of the entry edges; therefore, the relationship between machining performance (cutting forces and hole-making quality) and cutting parameters is established using the nonlinear fitting methodology. Thus, three cutting parameters in the helical milling of CFRP, under the steady state, are optimized based on the multi-objective genetic algorithm, including material removal rate and machining performance. Finally, experiments were carried out to prove the validity of optimized cutting parameters. 相似文献
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根据木工单板剪切机工作的特点,研制了一种基于旋转编码器和单片机控制的单板剪切机数控系统,主要由绝对位置测量系统、单片机控制系统、剪切系统和单板夹持系统四部分组成,可以通过设置剪切参数,方便调节剪切长度,最大程度避免浪费,提高工效. 相似文献
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Guoxuan Qin Mengmeng Jin Guoping Tu Yuexing Yan Laichun Yang Yanmeng Xu Jianguo Ma 《天津大学学报(英文版)》2017,23(2):168-175
A 0.18 μm CMOS low noise amplifier(LNA) by utilizing noise-canceling technique was designed and implemented in this paper. Current-reuse and self-bias techniques were used in the first stage to achieve input matching and reduce power consumption. The core size of the proposed CMOS LNA circuit without inductor was only 128 μm 9226 μm. The measured power gain and noise figure of the proposed LNA were 20.6 and 1.9 dB,respectively. The 3-dB bandwidth covers frequency from 0.1 to 1.2 GHz. When the chip was operated at a supply voltage of 1.8 V, it consumed 25.69 mW. The high performance of the proposed LNA makes it suitable for multistandard low-cost receiver front-ends within the above frequency range. 相似文献
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本文根据实验观察,对高速切削条件下连续型带状锯齿形切屑的形成过程进行了分析研究.研究结果表明,在锯齿形切屑的形成过程中,变形材料经历了塑性剪切变形和集中剪切滑移两个变形过程.本文作者分析了锯齿形切屑周期性形成的变形机理,对锯齿形切屑形成的有关变形参数进行了设计计算. 相似文献
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戴必胜 《黄冈师范学院学报》2006,26(6):63-68
用三唑酮(6.25%) 乙蒜素(93.75%)混合配制成160 m g/L至1 600 m g/L不同浓度的药剂(10种),对九重葛插穗浸泡处理3 h后扦插。实验结果,各种处理中最佳的插穗生根数、根长、根径的平均值分别为23.2根、20.5 mm、1.1 mm,最高成活率达到86.7%,所测生长指标与对照组的差异极显著(P<0.01)。在160 m g/L至400 m g/L范围,药剂的处理浓度与插穗的生根数、根长、根径以及成活率的关系均符合二次曲线模型(R2>0.98),而与枝叶的生长呈负相关(R2>0.95),但与对照的差异不显著。统计分析结果,三唑酮.乙蒜素混剂对九重葛插穗的最佳处理剂量为266 m g/L。 相似文献
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In developed countries, healthy retirees can fulfill their life, but may fear growing old. Yet, there is little empirical data on the relationship between this fear and life satisfaction. This cross-sectional, correlational survey study tested whether a new, summated measure of Fears About Growing Old (FAGO)—derived from exemplifications of Laslett, who posited the theory of the Third Age—significantly predicted life satisfaction and retirement satisfaction after adjusting for significant social participation covariates. A total of 190 Canadian retirees at three senior centers in Ontario, Canada, completed surveys. A pilot study established the reliability and validity of the scales, including the FAGO, used to assess the independent variable. In a regression analysis, fear (R 2 change = .06) was found to be a statistically significant predictor of life satisfaction when controlling for five covariates (current activity, circumstance and pursuing own interest as two reasons for retirement, postretirement work, and perceived social support); overall R 2 = .26. For retirement satisfaction, fear significantly explained variance in the outcome (R 2 change = .04) while controlling for two significant covariates (current activity and perceived social support); overall R 2 = .14. A work by gender interaction on satisfaction was not found. Other than fear about loss of mobility, men rated loss of partner very high; women rated mortal disease very high. The lowest fear was loss of retirement income for men and loss of earning-power for women. Canada's poverty preventive programs successfully supported senior postretirement life. The FAGO was useful to find senior needs. 相似文献
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为了提高大坝变形分析模型的预测精度并检验模型的泛化能力,研究了大坝变形分析的BP神经网络模型,并基于神经网络BP算法和传统的统计模型建立了大坝变形分析的融合模型．结合陈村大坝多年的变形观测数据,对上述3种模型进行了试算及分析．分析结果表明,统计模型的平均预测精度为±0．477mm．BP神经网络模型的平均预测精度为±0．390mm,融合模型的平均预测精度为±0．318mm,相比统计模型和BP神经网络模型分别提高了33％和18％,且泛化能力较强,具有广泛的适用性． 相似文献
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董海东 《荆门职业技术学院学报》2007,22(3):6-9
细长轴的外形并不复杂,但由于其本身的刚度低,车削时又受切削力、重力、切削热等因素的影响,容易产生弯曲变形以及振动、锥度、腰鼓形、竹节形等缺陷,但只要抓住中心架和跟刀架的使用、解决工件热变形伸长以及合理选择车刀的几何参数三个关键技术,问题就迎刃而解。 相似文献
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Michael J. Kieffer Yaacov Petscher C. Patrick Proctor Rebecca D. Silverman 《Scientific Studies of Reading》2016,20(6):436-454
Language comprehension is crucial to reading. However, theoretical models and recent research raise questions about what constitutes this multifaceted domain. We present two related studies examining the dimensionality of language comprehension and relations to reading comprehension in the upper elementary grades. Studies 1 (Grade 6; N = 148) and 2 (Grade 3–5; N = 311) contrasted factor models of language comprehension using item level indicators of morphological awareness and vocabulary (Studies 1 and 2) and syntactic awareness (Study 2). In both studies, a bifactor model—including general language comprehension and specific factors for each language component—best fit the data, and general language comprehension was the strongest predictor of reading comprehension. In Study 2, the morphology-specific factor also uniquely predicted reading comprehension above and beyond general language comprehension. Results suggest the value of modeling the common proficiency underlying performance on tasks designed to tap theoretically distinct language comprehension skills. 相似文献
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KHAN M.M.A DHAR N.R. 《浙江大学学报(A卷英文版)》2006,7(11):1790-1799
In all machining processes, tool wear is a natural phenomenon and it leads to tool failure. The growing demands for high productivity of machining need use of high cutting velocity and feed rate. Such machining inherently produces high cutting temperature, which not only reduces tool life but also impairs the product quality. Metal cutting fluid changes the performance of machining operations because of their lubrication, cooling and chip flushing functions, but the use of cutting fluid has become more problematic in terms of both employee health and environmental pollution. The minimization of cutting fluid also leads to economical benefits by way of saving lubricant costs and workpiece/tool/machine cleaning cycle time. The concept of minimum quantity lubrication （MQL） has been suggested since a decade ago as a means of addressing the issues of environmental intrusiveness and occupational hazards associated with the airborne cutting fluid particles on factory shop floors. This paper deals with experimental investigation on the role of MQL by vegetable oil on cutting temperature, tool wear, surface roughness and dimen- sional deviation in turning AISI-1060 steel at industrial speed-feed combinations by uncoated carbide insert. The encouraging results include significant reduction in tool wear rate, dimensional inaccuracy and surface roughness by MQL mainly through reduction in the cutting zone temperature and favorable change in the chip-tool and work-tool interaction. 相似文献
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YANG Ying TONG Mingwei WU Zhijuan College of Power Engineering Chongqing University Chongqing P. R. China 《重庆大学学报(英文版)》2004,3(1):6-10
1.IntroductionTherequirementsforenvironmentalprotectionaregettingstricterworldwide.Notonlythenaturalecologyandresourcesshouldbeconsideredinconservation,butalsotheenvironmentbeprotectedfrombeingsacrificedastheexpenseofproductivityboosts.Aconventionalmachiningprocessusedtotakeoilasthelubricantandcoolantforthesurfacesofmachinedtoolsandworkpieces,whichbroughtforthenvironmentpollution,andincreasedcostsonpost-treatingoftools,workpiecesandcuttingscrap.Asaresult,greenmachiningtechnologyfeaturingcleanp… 相似文献
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Michelle Hoda Wilkerson Rebecca Shareff Vasiliki Laina Brian Gravel 《Instructional Science》2018,46(1):35-60
In computational modeling activities, learners are expected to discover the inner workings of scientific and mathematical systems: First elaborating their understandings of a given system through constructing a computer model, then “debugging” that knowledge by testing and refining the model. While such activities have been shown to support science learning, difficulties building and using computational models are common and reduce learning benefits. Drawing from Collins and Ferguson (Educ Psychol 28(1):25–42, 1993), we conjecture that a major cause for such difficulties is a misalignment between the epistemic games (modeling strategies) learners play, and the epistemic forms (model types) a given modeling environment is designed to support. To investigate, we analyzed data from a study in which ten groups of U. S. fifth graders (n = 28) worked to create stop motion animations and agent-based computational models (ABMs) to discover the particulate nature of matter. Content analyses revealed that (1) groups that made progress—that is, that developed increasingly mechanistic, explanatory models—focused on elements, movement, and interactions when developing their models, a strategy well-aligned with both animation and ABM; (2) groups that did not make progress focused on sequences of phases, a strategy well-aligned with animation but not with ABM; and (3) struggling groups progressed when they received guidance about modeling strategies, but not when they received guidance about model content. We present summary analyses and three vignettes to illustrate these findings, and share implications for research and curricular design. 相似文献
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In this study, we performed a fine grained analysis of writing by children with a specific language impairment (SLI) and examined the contribution of oral language, phonological short-term memory (STM), nonverbal ability, and word reading to three writing constructs (productivity, complexity and accuracy). Forty-six children with SLI were compared with 42 children matched for chronological age, receptive vocabulary (N = 46) and reading decoding (N = 46) on a measure of narrative writing. The SLI group performed worse on all measures compared to children of a similar chronological age. The SLI group produced a greater proportion of orthographic spelling errors than children with similar receptive vocabularies, but were comparable to children matched for reading decoding. The children with SLI showed specific difficulties in the omission of whole words (e.g. auxiliary verbs and subject nouns) and omissions of grammatical morphology (e.g. past tense—ed) reflecting the difficulties shown in their oral language. Receptive grammar made a significant contribution to writing complexity and accuracy. Phonological fluency contributed to writing productivity, such as the production of diverse vocabulary, ideas and content and writing fluency. Phonological STM and word reading explained additional variance in writing accuracy over and above the SLI group’s oral language skills. 相似文献
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With the development of human society,the social hub enlarges beyond one community to the extent that the world is deemed as a community as a whole.Communication,therefore,plays an increasingly important role in our daily life.As a consequence,communication model or the definition of which is not so much a definition as a guide in communication.However,some existed communication models are not as practical as it was.This paper tries to make an overall contrast among three communication models——Coded Model,Gable's Communication Model and Ostensive Inferential Model,to see how they assist people to comprehend verbal and non-verbal communication. 相似文献
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梁晓丹 《北京教育学院学报》2011,(4):29-33
中国共产党建党的特色文化是在马克思主义中国化理论精髓———实事求是、解放思想、与时俱进的指导下发展成熟的。在经济全球化时期发展中国特色文化,要回顾中国共产党革命、建设、改革三个历史阶段的文化建设是如何在马克思主义中国化理论精髓指导下获得成功的,根据历史经验和现实需求提出传承中国式文化、继续发展马克思主义中国化理论、推动马克思主义中国化文化大众化,推动文化产业的繁荣,促进中国特色文化建设的措施。 相似文献