关键词： Anatomy   Arms   Biology and life sciences   Body limbs   Classical mechanics   Elbow   Kinematics   Legs   Mechanical energy   Medicine and health sciences   Motion   Musculoskeletal system   Physical sciences   Physics   Research Article   Shoulders   Skeletal joints   Velocity  

摘要： Objectives The purpose of this study was to investigate the effect of different incline-speed combinations, at equal external power outputs, on the mechanics and energetics of the double-poling (DP) technique in cross-country skiing. Methods Fourteen elite male cross-country skiers performed treadmill DP on roller-skis at low, moderate, and high mean external power outputs (P-mean) up a shallow incline (5%, INC5), at which DP is preferred, and up a steep incline (12%, INC12), at which DP is not preferred. Speed was set to produce equal Pmean at both inclines. From recorded kinematics and dynamics, arm power (P-arm) and trunk+leg power (PT+L) were derived, as were pole propulsion power (P-pole) and body mechanical energy perpendicular to the treadmill surface (E-body perpendicular to). Results Over a locomotion cycle, the arms contributed 63% to P-mean at INC5 but surprisingly only 54% at INC12 (P<0.001), with no effect of Pmean (P = 0.312). Thus, the trunk and legs contributed substantially to Pmean both at INC5 (37%) and INC12 (46%). At both inclines, PT+L generation during the swing phase increased approximately linearly with Pmean, which increased E-body perpendicular to. Within the poling phase, similar to 30-35% of the body energy which was developed during the preceding swing phase was transferred into propulsive pole power on both inclines. At INC5, the amount of negative PT+L during the poling phase was larger than at INC12, and this difference increased with P-mean. Conclusions The considerable larger amount of negative PT+L during poling at INC5 than at INC12 indicate that the legs and trunk generate more power than 'necessary' during the swing phase and thus must absorb more energy during the poling phase. This larger surplus of PT+L at INC5 seems necessary for positioning the body and poles so that high P-arm generation can occur in a short time. At INC12, less P-arm is generated, probably due to less advantageous working conditions for the arms, related

关键词： 大众7挡   干式双离合自动变速器   机械结构   动力传递路线  

摘要： 技术革新使驾驶员从换档的烦锁过程中解脱出来。今天,装有自动变速箱的汽车占汽车市场的一半。但是,传统的自动变速器在功率损耗较大的情况下,相当一部分发动机在功率传输过程中被功率消耗殆尽。与手动变速箱相比,自动变速箱在失去动力的同时会增加油耗。本文主要对大众7挡干式双离合器自动变速器的机械结构和传动路线进行了分析和探讨。

关键词： rubbing action   ductility   Tailoring   Material properties   Mechanical   Mechanical properties   Stress relaxation   dynamics   friction   Sheet metal  

摘要： Crumpled sheets show slow mechanical relaxation and long lasting memory of previous mechanical states. By using uniaxial compression tests, the role of friction and ductility on the stress relaxation dynamics of crumpled systems is investigated. We find a material dependent relaxation constant that can be tuned by changing ductility and adhesive properties of the sheet. After a two-step compression protocol, nonmonotonic aging is reported for polymeric, elastomeric and metal sheets, with relaxation dynamics that are dependent on the material's properties. These findings can contribute to tailoring and programming of crumpled materials to get desirable mechanical properties.

关键词： Molecular dynamics simulation   Surface crack   Fracture   Mechanical properties  

摘要： In this paper, fracture mechanism and the effects of surface crack on the mechanical properties (modulus and strength) of silica glass are studied through reactive all-atom molecular dynamics (MD) simulations. Tapered surface cracks of different length ranging from 0.25 to 10.0 nm are created by deleting atoms to study the crack length effects. Interatomic interactions are modeled with state-of-the-art reactive force field ReaxFF and fracture energy release rate is determined from discretized atomistic J-integral approach. Simulation results indicate that surface cracks have no effect on fiber modulus, however, fiber strength is significantly affected by surface crack. With the increase in crack length, strength decreases and MD predicted strength-crack length response is in good agreement with theoretical prediction. MD simulations project that about 35 nm size crack could reduce glass fiber strength to 3.5 GPa, which is experimentally observed fiber mean strength. MD simulations show that there is no inelastic process zone with cavities in front of the crack tip. Fracture mode is brittle type where crack growth initiates through Si-O bond breakage and it propagates through sequential bond ruptures.

关键词： Accelerated failure time model   Coarse woody debris   Created snags   Douglas-fir   Intensive forest management   Oregon   Structural enrichment   Survival modeling  

摘要： Structural enrichment in commercial tree plantations is a potential tool to increase snag numbers but relatively little information is available about how species, size, and spatial distribution of created snags are associated with longevity of these structures. We created 1197 snags in 31 harvest units from 1997 to 1999 in the Cascade and Coast Ranges, Oregon, USA, by topping live trees with harvesting equipment. We used an experimental design to distribute created snags at three densities and as either single or clumped created snags. We fit Weibull and log-logistic Accelerated Failure Time models and found that the median failure time was insensitive to the choice of distribution. We found a small positive effect of diameter at breast height (DBH) and a slight negative effect of increasing distance between created snags on survival. Assuming Weibull and log-logistic distributions at mean observed values of DBH and distance between snags, median survival times for Douglas-fir (Pseudotsuga menziesii) were 21.0 (95% confidence interval: 19.3, 22.8) years and 21.2 (19.7, 22.8) years, respectively. For western hemlock (Tsuga heterophylla), median survival times were 13.0 (11.9, 14.2) years and 12.5 (11.4, 13.7) years, respectively. Although the two failure distributions had similar median failure times, the log-logistic implies a higher survival probability over time for snags that remained standing at the end of the study period. Created snags can be a useful supplement for harvest units rotated at similar to 45 years and Douglas-fir will be available for longer as standing structures. For example, under the log-logistic model, a predicted 5% of Douglas-fir snags are retained to rotation age, so that 40 snags per hectare would be required at harvest to maintain 2 snags per hectare through stand rotation. Snags created from western hemlock will provide an early rotation pulse but are unlikely to last longer than 20 years. Our results suggest that longevity can be incr

关键词： Stress   Deformation   Crystals   Crystallinity   Crystallization  

摘要： It has previously been shown that polyethylene (PE) with a bimodal molar mass distribution has a high fracture toughness. Our approach has been to use coarse-grained (CG) molecular dynamics (MD) simulations to investigate the effects of including short-chain branches in the high molar mass fraction of bimodal PE on topological features and mechanical behavior of the material. The CG potentials were derived, validated, and utilized to simulate melt equilibration, cooling, crystallization, and mechanical deformation. Crystallinity, tie chain, and entanglement concentrations were continuously monitored. During crystallization, the branched bimodal systems disentangled to a lesser degree and ended up with a higher entanglement density than the linear bimodal systems simulated in our previous study. The increase in entanglement concentration was proportional to the content of the branched high molar mass fraction. A significantly higher tie chain concentration was obtained in the short-chain branched bimodal systems than in the linear systems. The increase in the number of ties was more pronounced than the increase in the number of entanglements. The tie chain concentration was not proportional to the content of the high molar mass fraction. Despite a lower crystal thickness and content, the elastic modulus and yield stress values were higher in the branched bimodal systems. A more pronounced strain hardening region was observed in the branched systems. It was suggested that the higher tie chain and entanglement concentration prior to the deformation, the more extensive disentanglement during the deformation, and the disappearance of formed voids prior to failure point were the reasons for the observed higher toughness of the short-chain branched bimodal PE compared with that of the linear bimodal systems. The toughest system, which contained respectively 25 and 75 wt % low molar mass and branched high molar mass fractions, had the highest tie chain concentration and the

关键词： 机械设计   非线性动力学   动态设计   振动  

摘要： 通过合理有效地对工业产品进行机械设计,可以大大提高机械加工制造的质量。概述机械设计中动力学问题和非线性动力学特点,分析机械设计中的非线性动力学和动态设计问题,阐述机械设计中的非线性动力学和动态设计研究进展。

关键词： Superoxide radical ion   Dissolved organic matters   Kinetic studies   Reaction mechanism  

摘要： Superoxide ion (O-2(center dot-)) is one of the short lived reactive oxygen species (ROS) formed in aquatic environments. The reactions of O-2(center dot-) with the model dissolved organic matter (DOM) were studied using a chemiluminescent analysis method under relevant environmental conditions. The reaction of O-2(center dot-) with DOM produced reduced DOM (DOM center dot-) by fast one-electron-transfer in the initial stage. This process resulted an initial "loss" in the Or decay kinetics. DOM is unstable which will continue react with O-2(center dot-) generating H2O2 to complete a catalytic dismutation cycle. Based on analyzing the observed pseudo-first order O-2(center dot-) decay rates (kpseudo), the quasi-steady-state concentration of DOM is found to be equal to the initial loss of O-2(center dot-). Thus, the rate constant for DOM center dot- with HO2 center dot/O-2(center dot-) is derived to be (1.1 -1.9) x 10(6) M-1 s(-1) in the temperature range of 7.8-41.4 degrees C. Meanwhile, the apparent rate constant for DOM with Or in a flow cell during a short time (2.25 s) is measured as (1.5-3.3) x 10(3) MC-1 S-1 in the temperature range of 8.2-38.6 degrees C. These temperature dependent Or reaction rate constants present an apparent activation energy of (19.6 +/- 2.9) kJ mol(C)(-1) for DOM, while that of DOM center dot- (12.5 +/- 3.5 kJ mol(-1)) is lower. For the pseudo-first order decay rate of Or, the catalyzed-dismutation by metal components ranges from 13 to 23%;the contribution by aromatic ketones of DOM is estimated to be 10-13% by using NaBH4 reduction method. The residual contribution might mainly occur at the quinone-like groups, which contributed 64%-77% to the total dismutation. The pH effects on the apparent catalytic rate constants dominate the reaction of O-2(center dot-) with DOM. The present work suggests that DOM is an important sink for O-2(center dot-) in aquatic environments. Furthermore, we proposed that the reaction of O-2(center dot-) with DO