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51.
A.N. Guz V.M. Nazarenko V.L. Bogdanov 《Theoretical and Applied Fracture Mechanics》2007,48(3):285-303
The problems of fracture under initial stresses acting along cracks in [A.N. Guz’, Mechanics of Brittle Fracture of Materials with Initial Stresses, Naukova Dumka, Kiev, 1983 (in Russian)] is studied. Other approaches and concepts are also briefly discussed. Results for isolated and near-the-surface cracks are given. 相似文献
52.
53.
The effect of long-range forces on the dynamics of a bar 总被引:2,自引:0,他引:2
Olaf Weckner 《Journal of the mechanics and physics of solids》2005,53(3):705-728
The one-dimensional dynamic response of an infinite bar composed of a linear “microelastic material” is examined. The principal physical characteristic of this constitutive model is that it accounts for the effects of long-range forces. The general theory that describes our setting, including the accompanying equation of motion, was developed independently by Kunin (Elastic Media with Microstructure I, 1982), Rogula (Nonlocal Theory of Material Media, 1982) and Silling (J. Mech. Phys. Solids 48 (2000) 175), and is called the peridynamic theory. The general initial-value problem is solved and the motion is found to be dispersive as a consequence of the long-range forces. The result converges, in the limit of short-range forces, to the classical result for a linearly elastic medium. Explicit solutions in elementary form are given in a broad class of special cases. The most striking observations arise in the Riemann-like problem corresponding to a constant initial displacement field and a piecewise constant initial velocity field. Even though, initially, the displacement field is continuous, it involves a jump discontinuity for all later times, the Lagrangian location of which remains stationary. For some materials the magnitude of the discontinuity-jump oscillates about an average value, while for others it grows monotonically, presumably fracturing the material when it exceeds some critical level. 相似文献
54.
The effect of transverse shear on delamination in layered, isotropic, linear-elastic materials has been determined. In contrast to the effects of an axial load or a bending moment on the energy-release rate for delamination, the effects of shear depend on the details of the deformation in the crack-tip region. It therefore does not appear to be possible to deduce rigorous expressions for the shear component of the energy-release rate based on steady-state energy arguments or on any type of modified beam theory. The expressions for the shear component of the energy-release rate presented in this work have been obtained using finite-element approaches. By combining these results with earlier expressions for the bending-moment and axial-force components of the energy-release rates, the framework for analyzing delamination in this type of geometry has been extended to the completely general case of any arbitrary loading. The relationship between the effects of shear and other fracture phenomena such as crack-tip rotations, elastic foundations and cohesive zones are discussed in the final sections of this paper. 相似文献
55.
For many motion control applications spatial constraints are often more important than temporal constraints. In recent work,
we have developed a spatial control strategy called the ε-controller for mobile robot applications. The control strategy is
based solely on static path geometry with position (in space) feedback. Motivated by this idea, in this paper, we consider
the notion of spatial-based iterative learning control (ILC). Specifically, we consider repetitive operation problems where
corrections are made to the control signal from trial to trial. Unlike traditional ILC, however, which updates control signals
based on the time elapsed along a trajectory, we instead make updates based on path errors and progress along the path. The
idea is demonstrated via simulation for a system with bang–bang velocity control. Experimental results using a high-precision,
two-axis gimbal mechanism are presented to show the effectiveness of the strategy. 相似文献
56.
57.
Elastic and Damage Longitudinal Shear Behavior of Highly Concentrated Long Fiber Composites 总被引:1,自引:0,他引:1
The elastic and damage longitudinal shear behavior of highly concentrated long fiber composites is analyzed by means of a simplified model where it is supposed that the fibers are rigid and touch each other in a regular hexagonal array. In the microscopic unit cell the problem is reduced to six similar problems of antiplane deformation on an equilateral circular triangle (see forthcoming Figure 2). These problems are solved in closed form by the complex variable method, and the solution is used to determine the longitudinal shear moduli, and to study their dependence on the microscopic damage caused by the circumferential debonding at the fiber–matrix interface. Subsequently, the damage evolution is investigated under the hypothesis that the microcracks propagate according to the Griffiths energy criterion. The elastic domain, where there is no damage propagation, is determined and it is shown that it is a polygonal convex set symmetric with respect to the origin. The overall damage evolution is discussed in detail and illustrated with some examples which highlight the very rich nature of the proposed model. 相似文献
58.
The series solutions of unsteady flows of a viscous incompressible electrically conducting fluid caused by an impulsively
rotating infinite disk are given by means of an analytic technique, namely the homotopy analysis method. Using a set of new
similarity transformations, we transfer the Navier–Stokes equations into a pair of nonlinear partial differential equations.
The convergent series solutions are obtained, which are uniformly valid for all dimensionless time 0 ≤ τ < ∞ in the whole
spatial region 0 ≤ η < ∞. To the best of our knowledge, such kind of series solutions have never been reported. The effect
of magnetic number on the velocity is investigated. 相似文献
59.
Vladimir Kobelev 《Meccanica》2006,41(6):653-660
Failure of a composite is a complex process accompanied by irreversible changes in the microstructure of the material. Microscopic
mechanisms are known of the accumulation of damage and failure of the type of localized and multiple ruptures of the fibers
delamination along interphase boundaries, and also mechanisms associated with fracture of fibers. In this work, we propose
a mathematical model of the local mechanism of failure of a composite material randomly reinforced with a system of short
fibers. We implement the Cosserat moment model of crack tip for filament material, reinforced with whiskers or in fiber- reinforced
polycrystalline materials. It is assumed that the angular distribution of the fibers is isotropic and the elastic characteristics
of the fibers are considerably higher than the elastic constants of the matrix. We implement the homogenization procedure
for the effective Cosserat constants similarly to the effective elastic constants. The singular solution in the vicinity of
the crack tip in the Cosserat moment model is found. Using this solution, we examine the bending stresses in the filaments
due to effective moment stresses in the material. The constructed model describes the phenomenon of fracture of the fibers
occurring during crack propagation in those composites. The following assumptions are used as the main hypotheses for the
micromechanical model. The matrix contains a nucleation crack. When the load is increased the crack grows and its boundary
comes into contact with the reinforcing fibers. A further increase of the stress causes bending of the fiber. When~the fiber
curvature reaches a specific critical value, the fiber ruptures. If the stress at infinity is given, the fibers no longer
delay the development of failure during crack propagation The degree of bending distortion of the fiber in the vicinity of
the boundary of the crack is determined by the moment model of the material. The necessity to take into account the moment
stresses in the failure theory of the reinforced material was stressed in [Muki and Sternberg (1965) Zeitschrift f angew Math
und Phys 16:611–615; Garajeu and Soos (2003) Math Mech Solids 8(2):189–218; Ostoja-Starzewski et al (1999) Mech Res Commun
26:387–396]. The moment Cosserat stresses were accounted also for inhomogeneous biomechanical materials by Buechner and Lakes
(2003) Bio Mech Model Mechanobiol 1: 295–301. We should also mention the important methodological studies [Sternberg and Muki
(1967) J Solids Struct 1:69–95; Atkinson and Leppington (1977) Int J Solids Struct 13: 1103–1122] concerned with the moment
stresses in homogeneous fracture mechanics. 相似文献
60.
High velocity penetration of a rigid conical impactor into a ductile target with air gaps between the plates is studied using the cylindrical cavity expansion approximation describing impactor–target interaction. It is showed that the latter model predicts improvement of the ballistic performance of the target with the increase of air gaps. It is found analytically that the ballistic limit velocity of the target consisting of N plates with a fixed total thickness with large air gaps increases with the increase of N. The conditions are discussed when the predicted effects can be most pronounced. 相似文献