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1.
In recent studies, many groups have investigated the interaction of dislocations and grain boundaries by bi-crystals and micro-specimen experiments. Partially, these experiments were combined with supplementary simulations by discrete dislocation dynamics, but quantitative data for the grain boundary resistance against slip transfer is still missing. In this feasibility study with first results, we use stage-I-fatigue cracks as highly localised sources for dislocations with well-known Burgers vectors to study the interaction between dislocations in the plastic zone in front of the crack tip and selected grain boundaries. The stress concentration at the grain boundary is calculated with the dislocation-free zone model of fracture using the dislocation density distribution in the plastic zone from slip trace height profile measurements by atomic force microscopy. The grain boundary resistance values calculated from common geometric models are compared to the local stress distribution at the grain boundaries. Hence, it is possible to quantify the grain boundary resistance and to combine geometric and stress approach for grain boundary resistance against slip transfer to a self-contained concept. As a result, the prediction of the grain boundary resistance effect based on a critical stress concept is possible with knowledge of the geometric parameters of the grain boundary only, namely the orientations of both participating grains and the orientation of the grain boundary plane. 相似文献
2.
B. Zimanowski;R. Büttner;J. Nestler 《Europhysics letters》1997,38(4):285-290
An experimental study on explosive interaction between transparent melt () and entrapped water () has been performed. Intense explosions occurred, resulting from catastrophic fragmentation of the melt and increasing heat transfer to the water in a cascading process. In earlier experiments a quasi-isochoric brittle reaction of the melt was identified to be the major explosion mechanism. Using a transparent melt, this brittle reaction could directly be observed by high-speed cinematography. The pictures revealed two fragmentation mechanisms: a) formation of leading cracks (mm to cm scale) due to excess water pressure, and b) slower scaled melt fragmentation induced by strain build-up in the melt during rapid cooling.https://doi.org/10.1209/epl/i1997-00239-3 相似文献
3.
Stage-I fatigue cracks are commonly described by the model of Bilby, Cottrell and Swinden (BCS model). However, since several experimental investigations have shown a dislocation-free zone (DFZ) in front of crack-tips, it is necessary to validate the new DFZ model and to examine the deviations to the BCS model. Therefore, the dislocation density distribution is derived from height profiles of slip lines in front of stage-I fatigue cracks in CMSX4® single crystals measured by contact-mode atomic force microscopy. This is possible, because the cracks are initiated at notches milled by focused ion beam technique directly on slip planes with a high Schmid factor. Consequently, the directions of the Burgers vectors are well known; it is possible to calculate the dislocation density distributions from the height profiles. The measured distributions are compared to the calculated distribution function of the DFZ model proposed by Chang et al. The additionally measured microscopic friction stress of the dislocations is then used to calculate the influence of grain boundaries on the dislocation density distribution in front of stage-I cracks. The calculation is done by the extended DFZ model of Shiue et al. and compared with the measured distribution function in polycrystalline specimens. Finally, the crack-tip sliding displacement as a measure for the crack propagation rate is compared for the DFZ model and the BCS model with the experimentally revealed values. The important result: the often used BCS model does not reflect the experimental measurements. On the contrary, the DFZ model reflects the measurements at stage-I cracks qualitatively and quantitatively. 相似文献
4.
Effect of die-upset level on the mechanical properties and microstructure of Nd-Fe-B magnets prepared by spark plasma sintering 总被引:1,自引:0,他引:1
The mechanical properties and microstructure of Nd-Fe-B magnets produced at various die-upset levels have been investigated. The results showed that the Vickers hardness and the fracture toughness of Nd-Fe-B magnets first increased, and then rapidly decreased with the increase in die-upset level. The optimum Vickers hardness and the fracture toughness were obtained at the die-upset level of 40%. The peak intensity of the (1 0 5) peak is the maximum value, the relative intensities of the (2 1 2), (2 1 4) and (3 1 4) peaks decline and the relative intensity of the (0 0 6) peak increases with the increase in die-upset level. The microstructures show that the variation in the size of small spherical grains is not obvious, and the volume fraction of small spherical grains declines gradually with the increase in die-upset level. 相似文献
5.
《Physics letters. A》2014,378(28-29):1961-1964
6.
A fiber-reaction zone-matrix three-phase model is developed to evaluate the interfacial fracture toughness of titanium alloys reinforced by SiC monofilaments. Based on fracture mechanics, theoretical equations of GIIc are presented, and the effects of several key factors such as crack length and the interface reaction zone thickness on the critical applied stress necessary for crack growth and interfacial fracture toughness are discussed. Finally, the interfacial fracture toughness of typical composites including Sigma1240/Ti-6Al-4V, SCS-6/Ti-6Al-4V, SCS-6/Timetal 834, SCS-6/Timetal 21s, SCS-6/Ti-24Al-11Nb and SCS-6/Ti-15V-3Cr are predicted by the model. The results show that the model can reliably predict the interfacial fracture toughness of the titanium matrix composites. 相似文献
7.
The elastic properties of double-stranded DNA (ds-DNA) molecules are believed to play an important role in their biological functions. By using a mesoscale model, we construct a simple cylinder-DNA-surface system to explore the radial elastic property of a ds-DNA molecule through a Langevin-dynamics-based computer simulation. The numerical predictions of the radial elastic property are favorable with the recent experimental results. The analysis of the hydrogen bonds and base stacking interaction shows that local conformation transition occurs through the breaking of local hydrogen bonds, and this transition minimizes the inner strain aggregated during compression. This behavior provides an alternative method for studying the local property of ds-DNA, which is expected to be helpful in better understanding the local interaction between ds-DNA and protein, and the mechanics of the short-segment DNA molecule. https://doi.org/10.1209/0295-5075/89/48003 相似文献
8.
Hao Qu;Giovanni Zocchi 《Europhysics letters》2011,94(1)
We derive an analytic expression for the bending elastic energy of short DNA molecules, valid in the entire range from low to high energies. The elastic energy depends on three parameters: the length of the molecule (2L ), the bending modulus B , and a critical torque τc at which the molecule develops a kink. In the kinked state, the elastic energy is linear in the kink angle, i.e. the torque at the kink is constant (= τc ). τc depends (weakly) on the sequence around the nick, but is about 27 pN×nm. We measure it for a specific sequence, through experiments where the elastic energy of constrained DNA molecules is directly measured.https://doi.org/10.1209/0295-5075/94/18003 相似文献
10.
金属玻璃的断裂机理与其断裂韧度的关系 总被引:1,自引:0,他引:1
本文选取了三种不同断裂韧度值的金属玻璃Zr41.25Ti13.75Ni10Cu12.5Be22.5,Ce68Al10Cu20Co2和Fe41Co7Cr15Mo14Y2C15B6,通过压缩实验测量了它们的应力-应变关系;样品断裂以后观察了其断口形貌,发现这三种金属玻璃具有不同的断裂模式.经过对这三种金属玻璃做缺口三点弯曲实验,利用数字散斑技术研究了缺口前端应变集中方向弹性应变场的演化过程.根据金属玻璃的屈服准则,阐述了不同断裂韧度值的金属玻璃的断裂机理. 相似文献
11.
12.
Application of ultrasound technology in modulating the hydration process during paddy germination was analyzed in this study. The effect of hydropriming (24 h) and sono-hydro priming (ultrasound priming, 12 h) on the hydration behaviour of paddies was determined at different temperatures (25–40 °C). Ultrasound pulse was applied for 10 min after every one hour for sono-hydro priming. Germination potential and microstructure analysis of treated paddies were also performed. Downward concave curve observed in hydration process of paddies indicates initial high-water absorption through diffusion process. Sono-hydro priming process showed higher hydration rate compared to hydropriming. The changes in moisture content during hydration processes fitted to theoretical (Fick's model) and empirical model (Peleg model) exhibited high regression coefficient (R2 > 0.95) indicating suitability for predicting hydration behaviour in both paddies for germination. The Peleg model adequately predicted saturation moisture content and sono-hydro priming efficiently increased the water absorption rate. Effective moisture diffusivity determined from Fick's diffusion model increased for sono-hydro priming. Activation energy estimated from effective moisture diffusivity required in sono-hydro priming (Ea = 20.32 and 19.19 KJ/mol respectively) for pigmented rice and non-pigmented rice was lower than hydropriming (Ea = 27.11 and 32.15 KJ/mol respectively). Both hydration processes were endothermic and non-spontaneous inferred from thermodynamic properties. Sono-hydro priming exhibited < 95% germination potential with shorter soaking time (12 h) owing to the high mass transfer rate. SEM micrograph revealed water absorption through various micro-cavities during sono-hydro priming. Thus, sono-hydro priming potentially reduced the soaking process (approximately 50%) with higher germination rate in paddies beneficial for commercial malting of grains. 相似文献
13.
Yingchun Ding 《Physica B: Condensed Matter》2012,407(12):2190-2200
The structural stability, mechanical properties and thermodynamic parameters such as Debye temperature, minimum thermal conductivities of orthorhombic-A2N2O (A=C, Si and Ge) are calculated by first principles calculations based on density functional theory. The calculated lattice parameters, elastic constants of Si2N2O and Ge2N2O using PBEsol function are consisted with the experimental data and other calculated values. The full set elastic constants of the orthorhombic-A2N2O (A=C, Si and Ge) are calculated by stress–strain method. The mechanical moduli (bulk modulus, shear modulus and Young's modulus) are evaluated by the Voigt–Reuss–Hill approach. The orthorhombic-C2N2O exhibits larger mechanical moduli than the other two structures. The hardness of orthorhombic-A2N2O (A=C, Si and Ge) is evaluated according to the intrinsic hardness calculation theory of covalent crystal relying on Mulliken overlap population. The results indicate that the orthorhombic-C2N2O is a super hard material. Furthermore, the mechanical anisotropy, Debye temperature and minimum thermal conductivity of the orthorhombic-A2N2O (A=C, Si and Ge) have been estimated by empirical methods. The orthorhombic-Ge2N2O shows the lowest thermal conductivity, which may have useful applications as gas turbine engines and diesel engines. 相似文献
14.
Mechanical properties of DNA, in particular its stretch-dependent extension and loop formation properties, have been recognized as effective probes for understanding possible biochemical roles played by them in a living cell. Single stranded DNA (ssDNA), which till recently was presumed to be a simple flexible polymer continues to spring surprises. Synthetic ssDNA, like polydA (polydeoxyadenosines) have revealed an intriguing force-extension (FX) behavior exhibiting two plateaus, which is absent in polydT (polydeoxythymidines). Loop closing time in polydA had also been found to scale exponentially with inverse temperature, unexpected from generic models of homopolymers. Here we present a new model for polydA which incorporates both a helix-coil transition and an overstretching transition, accounting for the two plateaus. Using transfer matrix calculation and Monte Carlo simulation we show that the model reproduces different sets of experimental observations, namely FX characteristics and looping behavior, quantitatively. It also predicts a weak, nonmonotonic behavior in the temperature-extension characteristic of polydA.https://doi.org/10.1209/0295-5075/100/68004 相似文献
15.
Very recently, experimental evidence showed that the hydrogen is retained in dithiol-terminated single-molecule junction under the widely adopted preparation conditions, which is in contrast to the accepted view[Nat. Chem. 11 351 (2019)]. However, the hydrogen is generally assumed to be lost in the previous physical models of single-molecule junctions. Whether the retention of the hydrogen at the gold-sulfur interface exerts a significant effect on the theoretical prediction of spin transport properties is an open question. Therefore, here in this paper we carry out a comparative study of spin transport in M-tetraphenylporphyrin-based (M=V, Cr, Mn, Fe, and Co; M-TPP) single-molecule junction through Au-SR and Au-S(H)R bondings. The results show that the hydrogen at the gold-sulfur interface may dramatically affect the spin-filtering efficiency of M-TPP-based single-molecule junction, depending on the type of transition metal ions embedded into porphyrin ring. Moreover, we find that for the Co-TPP-based molecular junction, the hydrogen at the gold-sulfur interface has no obvious effect on transmission at the Fermi level, but it has a significant effect on the spin-dependent transmission dip induced by the quantum interference on the occupied side. Thus the fate of hydrogen should be concerned in the physical model according to the actual preparation condition, which is important for our fundamental understanding of spin transport in the single-molecule junctions. Our work also provides guidance in how to experimentally identify the nature of gold-sulfur interface in the single-molecule junction with spin-polarized transport. 相似文献
16.
The structural, electronic, elastic, mechanical and thermal properties of Ti3Au, Ti3Pt and Ti3Ir intermetallic compounds crystallizing in A15 structure have been studied using density functional theory within generalized gradient approximation (GGA) for the exchange correlation potential. Elastic properties such as Young's modulus (E), rigidity modulus (G), bulk modulus (B), Poisson's ratio (σ) and elastic anisotropic factor (A) have been calculated. From the present study it is noted that Ti3Ir is the hardest compound among the three materials studied due to its larger bulk modulus. Also, it is more ductile in nature. 相似文献
17.
Blends of cis-polyisoprene (CPI) and chloroprene rubber (CR) have been prepared in different blend compositions by solution casting. Structural characterization of these blends has been done using X-ray diffraction and scanning electron microscopy. The experimental values of thermo-mechanical properties, mechanical properties and thermal conductivity of so-prepared blends determined using dynamic mechanical analyzer and thermal constant analyzer have been presented. Crosslink density has been determined using different models. Experimental results from thermo-mechanical properties show that all the blends are immiscible. Tensile strength, toughness, Young's modulus and thermal conductivity of these blends were found to be higher than that of pure CPI and pure CR. However, mechanical properties of 25/75(V/V) of CPI/CR blend and thermal conductivity of 75/25(V/V) of CPI/CR blend have been found to be highest. 相似文献
18.
电磁脉冲武器能够通过\"前、后门\"耦合效应对箱体内部电子元器件及电路板造成损伤,从而对电气电子设备的安全性构成严重威胁,因此,开展箱体电磁屏蔽效能的分析研究具有重要意义.推导了任意入射波条件下电大开孔箱体屏蔽系数的解析解,并在此基础上对箱体屏蔽效能进行了分析研究.首先通过矢量分解,得出任意入射平面波的坐标分量;再基于Cohn模型,获得了电大开孔的等效电偶、磁偶极子;然后通过镜像原理,计算出总的赫兹电矢量位、磁矢量位;最终求得电大开孔箱体内部任意观测点的电场解析解,用于箱体屏蔽系数计算.设计了5组验证性实验,仿真结果表明:该解析算法相对CST的均方误差为11.565 d B,绝对误差为8.015 d B,相关系数为0.921,从而验证了该算法的准确性;解析算法仿真的平均耗时为0.183 s,仅占CST耗时的1/7530,从而验证了该算法的高效性. 相似文献
19.
Vipul Srivastava Sanjay Bhajanker Sankar P. Sanyal 《Physica B: Condensed Matter》2011,406(11):2158-2162
The structural and mechanical properties of LnO (Ln=Sm, Eu, Yb) compounds have been investigated using a modified interionic potential theory, which includes the effect of Coulomb screening. We predicted a structural phase transition from NaCl (B1)- to CsCl (B2)-type structure and elastic properties in LnO compounds at very high pressure. The anomalous properties of these compounds have been correlated in terms of the hybridisation of f-electrons of the rare earth ion with conduction band and strong mixing of f-states of lanthanides with the p-orbital of neighbouring chalcogen ion. For EuO, the calculated transition pressure, bulk modulus and lattice parameter are close to the experimental data. The nature of bonds between the ions is predicted by simulating the ion-ion (Ln-Ln and Ln-O) distances at high pressure. The second order elastic constants along with shear modulus and Young's modulus, elastic anisotropy and Poisson's ratio are also presented for these oxides. 相似文献
20.
《Europhysics letters》1997,38(6):435-440
Simultaneous propagation of cracks in glass plates isstudied in a directional crack growth experiment.The main result of this study is the existence of a periodic crack patternpropagating straight in a finite domain of crack spacings.Its stability range decreases with the system size, and leads toa single intrinsic selected crack spacing for large systems.https://doi.org/10.1209/epl/i1997-00264-2 相似文献