Section 2.2 Solid Mechanics Part II Kelly 232.2 One-dimensional Elastodynamics In rigid body dynamics, it is assumed that when a force is applied to one point of an object, every other point in the object is set in motion simultaneously. On the other hand, in static elasticity, it is assumed that the object is at rest and is in equilibrium under the action of the applied forces; the material may well have undergone considerable changes in deformation when first struck, but one is only concerned with the final static equilibrium state of the object. Elastostatics and rigid body dynamics are sufficiently accurate for many problems but when one is considering the effects of forces which are applied rapidly, or for very short periods of time, the effects must be considered in terms of the propagation of stress waves. 2.2.1 The Wave Equation Consider now the dynamic problem. In this case one considers the equation of motion: abdxdρσ=+Equation of Motion (2.2.1a) dxdu=εStrain-Displacement Relation (2.2.1b) εσE=Constitutive Equation (2.2.1c) where ais the acceleration. Expressing the acceleration in terms of the displacement, one then obtains the dynamic version of Navier’s equation, 2222tubxuE∂∂=+∂∂ρ1-D Navier’s Equation (2.2.2)In most situations, the body forces will be negligible, and so consider the partial differential equation 222221tucxu∂∂=∂∂1-D Wave Equation (2.2.3)where ρEc=(2.2.4) Equation 2.2.3 is the standard one-dimensional wave equationwith wave speed c; note from 2.2.4 that chas dimensions of velocity. The solution to 2.2.3 (see below) shows that a stress wavetravels at speed cthrough the material from the point of disturbance, e.g. applied load. When the stress wave reaches a
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