Sequential virtual motion camouflage method for nonlinear constrained optimal trajectory control

Nonlinear constrained optimal trajectory planning is a challenging and fundamental area of research. This paper proposes bio-inspired fast-time approaches for this type of problems based on the inspiration drawn from the natural phenomenon known as the motion camouflage. Two algorithms are proposed: the virtual motion camouflage (VMC) subspace method and the sequential VMC method. As a hybrid approach, the sequential VMC method works through a two-step structure in each iteration. First, the VMC subspace method will solve for an optimal solution over a selected subspace. Second, an algorithm consisting of a linear programming and a line search will vary the subspace so that the next VMC subspace result will be guaranteed not to be worse than that of the current step. The dimension and time complexities of the algorithms will be analyzed, and the optimality of the solution via the sequential VMC approach will be studied. Through the VMC approaches, the state and control variables in the kinematics or dynamics models of vehicles in the selected subspace can be represented by a single degree-of-freedom vector, called the path control parameter vector. The reduction in dimension and no involvement of equality constraints will in practice make the convergence faster and easier, and a much smaller computational cost is expected. Two simulation examples, the Breakwell problem and a minimum time robot obstacle avoidance problem with different numbers of obstacles, are used to demonstrate the capabilities of the algorithms.     

AgTaO3 and AgNbO3 thin films by pulsed laser deposition

Silver tantalate AgTaO₃ (ATO) and silver niobate AgNbO₃ (ANO) films have been grown on to the LaAlO₃ (001) and sapphire Al₂O₃ (0112, r-cut) single crystals by pulsed laser deposition technique from stoichiometric ATO and ANO targets. X-ray diffraction study revealed epitaxial quality of ATO and ANO films on the LaAlO₃ (001) whereas on the sapphire r-cut substrate they are preferential (110) and (001) oriented. To characterize microwave films properties in the range from 1 to 40 GHz, coplanar line interdigital capacitors were fabricated by photolithography and lift-off technique. ANO film capacitors show superior properties: frequency dispersion was as low as 13%, voltage tunability (40 V, 200 kV/cm) was about 4.6% at 20 GHz, loss tangent ∼0.106 at 20 GHz, K-factor = tunability / tanδ from 49% @ 10 GHz to 33% at 40 GHz.     

A study on type II structures. Part I:: a modified one-dimensional mass–spring model

Most impact energy-absorbing structures can be classified into one of two types in terms of the shape of the overall quasi-static load–displacement curve in the early stages of deformation. Type I has a relatively “flat-topped” curve, while type II has an initial peak load followed by a “steeply falling” curve. The previous work showed that the deformation of type II specimens is much more sensitive to the impact velocity than that of type I specimens. That is, when the total kinetic energy remains the same for all specimens, smaller final deformations result from higher impact velocities; and this phenomenon is much more significant for type II specimens than for type I specimens. In order to explain this characteristic of type II structures, a one-dimensional mass–spring model with variable mass is proposed for a typical type II structure (i.e., a pair of pre-bent plates), and used to examine the effects of the lateral inertia of the structure under impact. Unlike conventional mass–spring models, our system contains an equivalent variable mass, which is a function of the rotational angle at plastic hinges and comes into effect during the second phase of the dynamic response. Predictions of this analytical model agree very well with an ABAQUS FE simulation of the dynamic response of the pre-bent plates to impact; and this verifies the validity of the mass–spring model proposed.     

A numerical analysis of intergranular penny-shaped microcrack shrinkage controlled by coupled surface and interface diffusion

An axisymmetric finite-element method is developed and applied to simulate healing evolution of intergranular penny-shaped microcracks. In the finite-element method, grain-boundary diffusion and surface diffusion are coupled by the boundary conditions at the triple circle of the penny-shaped microcrack surface and the grain-boundary plane. Matter is transported to the triple circle by grain-boundary diffusion and is taken away from the microcrack tips and deposited onto the microcrack surfaces by surface diffusion, resulting in shrinkage of the intergranular microcracks. The numerical simulations show that the evolution processes of intergranular microcracks are controlled by equilibrium dihedral angle (defined by surface and grain-boundary tensions), microcrack spacing, ratio of grain-boundary diffusion to surface diffusion, and the applied pressure.     

Cumulative-damage modeling of fatigue crack growth in turbine engine materials

Life predictions of turbine engine structural components utilize fracture mechanics principles to determine fatigue crack growth rates. Fatigue cracks grow under conditions of variable temperature, frequency, hold time, stress ratio and stress level. At elevated temperatures, time-dependent material behavior can play a significant role in the material behavior. Cumulative-damage models must account for all these variables as well as interaction effects. The earliest modeling involved interaction schemes and, primarily, time-independent material behavior. More recent work has focused on time-dependence and creep-fatigue interaction effects. A review of current modeling concepts is presented.     

The micromechanisms of fatigue crack growth in a commercial Al-Zn-Mg-Cu alloy

A detailed fractographic and microstructural study, using combined scanning and transmission electron microscopy has been made of the fracture surfaces produced by fatigue of a commercial aluminium alloy 7010:T76 in moist air, dry argon and dry oxygen. In the dry environments fracture is entirely ductile: the fracture surface is non-crystallographic and essentially striation free. In moist air fracture occurs by cleavage on {110} coupled with plastic deformation during the blunting and closing of the crack: this results in the formation of well defined surface striations. Striation topography is controlled by the local orientation of the cleavage plane to the direction of maximum tensile stress and to the slip systems of highest local shear stress. In consequence a range of striation profiles are developed. It is suggested that the cleavage component of growth is the result of hydrogen embrittlement of the matrix ahead of the crack and that dislocation transport is necessary to introduce this hydrogen.     

Numerical Study on the Morphology of a Re-Ignited Laminar Partially Premixed Flame with a Co-Axial Pilot Flame

Re-ignited partially premixed flame(PPF) is a quite extensive flame type in real applications, which is directly relevant to the local and global extinction and re-ignition phenomenon. The authors designed a model burner to establish laminar re-ignited PPFs. Numerical simulations were carried out to reveal the morphology of laminar re-ignited PPF. Based on the distributions of temperature, heat release and radicals, the morphologies of re-ignited flames were explored. W-shaped flames were formed...     

变厚度复合材料热压罐工艺层板厚度控制的实验研究

针对2种碳纤维织物/环氧预浸料,采用热压罐工艺在不同条件下制备了变厚度层板,并通过层板内部形貌、层板厚度、纤维含量、吸胶量、织物渗透率的测试分析,研究了变厚度层板的密实过程和纤维分布的影响因素。结果表明：密实过程中树脂的二维流动导致2种织物变厚度层板厚板区的纤维含量高于薄板区;G0827单向织物的面内渗透率与厚度方向渗透率比值大于G0803缎纹织物,造成G0827织物变厚度层板的纤维分布不均匀性更大;无吸胶材料的条件下层板内纤维分布均匀,说明吸胶材料内树脂的面内流动对层板的纤维分布有很大影响。