Problems for Chapter 10

 

Approximate Theories for Solids with Special Shapes:

Rods, Beams, Membranes, Plates and Shells

 

 

 

 

10.7.    Solutions to Problems Involving Membranes, Plates and Shells

 

 

10.7.1.    A thin circular membrane with radius R is subjected to in-plane boundary loading that induces a uniform biaxial membrane tension with magnitude .   Vertical displacement of the membrane is prevented at .  The membrane is subjected to a uniform out-of-plane pressure with magnitude p on its surface.   Calculate the displacement field in the membrane, assuming small deflections.  This problem can be solved quite easily using Cartesian coordinates.

 

 

 

10.7.2.    The figure shows a thin circular plate with thickness h, Young’s modulus E and Poisson’s ratio .  The edge of the plate is clamped, and its surface is subjected to a uniform out-of-plane pressure with magnitude p.  Calculate the displacement field and internal moment and shear force in the plate, assuming small deflections.  This problem can easily be solved using Cartesian coordinates.

 

 

 

 

 

 

10.7.3.    The figure shows a circular elastic plate with Young’s modulus E, Poissions ratio .  The plate has thickness h and radius R, and is clamped at its edge.  The goal of this problem is to calculate the mode shapes and natural frequencies of vibration of the plate.  The solution to 10.6.2 may be used.

10.7.3.1.           Show that the general solution to the equation of motion for the freely vibrating plate is given by

 

where A,B,C,D,   are arbitrary constants,  are Bessel functions of the first and second kinds, and  are modified Bessel functions of the first and second kinds, with order n, while  and  are a wave number and vibration frequency that are related by

 

10.7.3.2.           Show that most general solution with bounded displacements at r=0 has the form

 

10.7.3.3.           Write down the boundary conditions for  at r=R, and hence show that the wave numbers  are roots of the equation

 

10.7.3.4.           Show that the corresponding mode shapes are given by

 

10.7.3.5.           Calculate  for 0<n<3, 1<m<3 and tabulate your results.   Compare the solution to the corresponding membrane problem solved in Section 10.7.2.

10.7.3.6.           Plot contours showing the mode shapes for the modes with the lowest four frequencies.

 

 

 

10.7.4.    Repeat the preceding problem for a plate with a simply supported edge.   You should find that the wave numbers  are the roots of the equation

 

Calculate the natural frequencies for .

 

 

 

 

10.7.5.    Use Rayleigh’s method, with a suitable guess for the mode shape, to estimate the fundamental frequency of vibration of the circular plate described in Problem 10.7.3. Compare the approximate solution with the exact solution.

 

 

 

 

10.7.6.    Use Rayleigh’s method, with a suitable guess for the mode shape, to estimate the fundamental frequency of vibration of the circular plate described in Problem 10.7.4. Compare the approximate solution with the exact solution.

 

 

 

 

10.7.7.    A thin film with thickness  is deposited on the surface of a circular wafer with radius R and thickness h.  Both film and substrate have Young’s modulus E, and Poisson’s ratio . An inelastic strain  is introduced into the film by some external process (e.g. thermal expansion), which generates stresses in the film, and also causes the substrate to bend.

 

In Section 10.7.3, expressions were derived relating the substrate curvature to the mismatch strain in the film.  These expressions are only valid if the substrate curvature is small.  For mismatch strains, the wafer buckles, as shown in the figure.  The goal of this problem is to estimate the critical value of mismatch strain that will cause the wafer to buckle.

10.7.7.1.           Assume that the displacement of the mid-plane of the wafer-film system is

 

Calculate the distribution of strain in the film and substrate, and hence deduce the total strain energy density of the system.  It is best to do this calculation using a symbolic manipulation program.

10.7.7.2.           Calculate the values of  that minimize the potential energy of the plate, and hence show that the two curvatures satisfy

 

and find a formula for the constant C.

10.7.7.3.           Hence, plot a graph showing the equilibrium values of the normalized curvature  as a function of a suitably normalized measure of mismatch strain, for a Poisson’s ratio .

 

 

 

10.7.8.    The figure shows an elastic plate with Young’s modulus E, Poisson’s ratio  and thermal expansion coefficient .  The plate is circular, with radius R and its edge is clamped.  The plate is initially stress free and is then heated to raise its temperature by , inducing a uniform internal force   in the plate.  The goal of this problem is to calculate the critical temperature that will cause the plate to buckle, using the governing equations listed in Section 10.6.2.

10.7.8.1.           Assume an axially symmetric buckling mode, so that  with .  Show that w satisfies the governing equation

 

10.7.8.2.           Show that the general solution to this equation is

 

where A,B,C,D, are arbitrary constants,  are Bessel functions of the first and second kind of order zero, and  is the wave number for the nth buckling mode. Find an expression for  in terms of  and relevant geometric and material properties.

10.7.8.3.           Show that the wave number  satisfies , and deduce an expression for the critical temperature change at which the plate can first buckle.

10.7.8.4.           Plot the buckling mode associated with this temperature.

 

 

 

10.7.9.    Repeat the preceding problem for a plate with a simply supported edge.   You should find that the wave numbers  are the roots of the equation

 

Calculate the critical buckling temperature and plot the corresponding buckling mode for .

 

 

 

 

10.7.10. The interface between a thin film and its substrate contains a tunnel crack with length 2a.  The film is initially stress free, then heated to raise its temperature by , inducing a uniform biaxial stress field   in the film.  At a critical temperature, the film buckles as shown in the figure.   For R>>h the buckled film can be modeled as a plate with clamped edge.  The goal of this problem is to calculate the critical temperature required to cause the film to buckle, and to calculate the crack tip energy release rate as the buckled film delaminates from the substrate.  Assume that the displacement of the mid-plane of the film has the form , and use the Von-Karman plate bending theory of Section 10.6.3.

10.7.10.1.       Write down expressions for the mid-plane strain  and the curvature change tensor  for the film in terms of  and , and hence find a formula for the internal force  and moment   in terms of , ,  and material properties.

10.7.10.2.       Hence, show that the equations of equilibrium in terms of  and  reduce to

 

10.7.10.3.       Assume that  at .  Hence, calculate the smallest value of  for which a solution with nonzero w exists.

10.7.10.4.       Deduce an expression for the critical temperature required to cause the film to buckle

10.7.10.5.       Assume that  exceeds the critical value.   Calculate the resulting displacement field

10.7.10.6.       Hence calculate the decrease in energy of the film during the buckling

10.7.10.7.       Use the preceding result to deduce the crack tip energy release rate during delamination.

 

 

 

 

10.7.11. Consider a thin circular plate with thickness h, Young’s modulus E, Poisson’s ratio  and thermal expansion coefficient . Let  be a cylindrical-polar coordinate system and let  be a triad of unit vectors parallel to the natural (covariant) basis vectors for the coordinate system. Assume that the deformation of the plate is axially symmetric, so that the displacement field can be expressed as .   Show that the Von-Karman equations governing the motion of the plate can be reduced to the following form

10.7.11.1.       Mid-plane strain-deflection relation  

10.7.11.2.       Displacement-curvature relation  

10.7.11.3.       Membrane stress-strain relation  

10.7.11.4.       Moment-curvature relations

 

10.7.11.5.       Equations of motion

  

 

 

 

 

 

10.7.12. The Von-Karman equations for a circular plate cannot be solved analytically, even for the case of axially symmetric deformations.  However, it is very straightforward to set up a simple spreadsheet to calculate the potential energy of the plate, and find a numerical solution by using the spreadsheet’s solver function to minimize the potential energy. To this end, consider a circular plate with radius R, thickness h, Young’s modulus E, Poisson’s ratio  and thermal expansion coefficient .  Assume that the plate is initially stress free, and is then heated to increase its temperature uniformly by .  At the same time, the surface of the plate is subjected to a uniform pressure p acting perpendicular to its surface. Assume that the deformation of the plate is axially symmetric, so that the displacement field can be expressed as .

 

10.7.12.1.       Show that the potential energy of the plate can be expressed as

 

where  and  are related to the displacements by the equations listed in the preceding problem.

 

10.7.12.2.       To obtain a numerical solution, let  , with i=0,1,2…N and , denote the position and displacements of a set of  N+1 equally spaced points along a radius of the disk.   The derivatives of u and w can be approximated using simple difference formulas.  For example, define i=1,2…N to be the derivatives of u and w at ; similarly, approximate the second derivative of w as  at each i=1,2…N-1.  The value of  at r=R can be estimated by extrapolation as    The integrals in the expression for the potential energy can be evaluated using a simple piecewise-constant approximation (or if you prefer piecewise linear) to the integrand. Use this approach to set up an EXCEL spreadsheet to calculate the total potential energy of the plate.  You should find N=40 sufficient for most practical purposes.

 

10.7.12.3.       Check your spreadsheet by calculating the potential energy of a heated flat plate with , and compare the prediction with the exact solution.

 

10.7.12.4.       The deflection of the plate under loading can be calculated by using the solver feature of EXCEL to determine the values of  and  that minimize the potential energy.   To test your spreadsheet, use it to calculate the deflection of a circular plate with thickness h/R=0.02, with a clamped boundary, which is subjected to a pressure .  You will need to enforce the clamped boundary condition using a constraint.   You can set  but you will find that this makes the plate slightly too stiff.   A better result is obtained by extrapolating the slope of the plate to r=R based on the slope of the last two segments, and enforcing the constraint .    Show that, for this pressure, the numerical solution agrees with the predictions of the exact small-deflection solution.

 

10.7.12.5.       Show that if the deflection of the center of the plate is comparable to the plate thickness, the small deflection solution predicts deflections that are substantially greater than the numerical solution of the Von-Karman equations.   This is because the in-plane strains begin to significantly stiffen the plate.   Plot a graph showing the deflection of the center of the plate (normalized by plate thickness) as a function of normalized pressure .

 

 

 

 

10.7.13.  The interface between a thin film and its substrate contains a circular crack with radius R.  The film has Young’s modulus E, Poisson’s ratio  and thermal expansion coefficient .  The substrate can be idealized as rigid. The film is initially stress free, then heated to raise its temperature by , inducing a uniform biaxial stress field   in the film.  At a critical temperature, the film buckles as shown in the figure.   For R>>h the buckled film can be modeled as a plate with clamped edge, so that the critical buckling temperature is given by the solution to problem 10.7.8.  When the film buckles, some of the strain energy in the film is relaxed.  This relaxation in energy can cause the film to delaminate from the substrate. 

10.7.13.1.       Show that the change in strain energy of the system during the formation of the buckle can be expressed in dimensionless form by defining dimensionless measures of displacement, position and strain as

 

            

so that

 

where  and  is the total strain energy of the circular portion of the film before buckling.

10.7.13.2.       The implication of 10.5.13.1 is that the change in strain energy (and hence the normalized displacement field which minimizes the potential energy) is a function of material and geometric parameters only through Poisson’s ratio  and the dimensionless parameter .  Use the spreadsheet developed in problem 10.7.12 to plot a graph showing  as a function of  for a film with .  Verify that the critical value of  corresponding to  is consistent with the solution to problem 10.7.8.

10.7.13.3.       Find an expression for the crack tip energy release rate  and the dimensionless function .  Hence, use the results of 10.7.13.2 to plot a graph showing the crack tip energy release rate (suitably normalized) as a function of , for a film with .

 

 

 

10.7.14. The figure shows a thin-walled, spherical dome with radius R, thickness h and mass density .  The dome is open at its top, so that the shell is bounded by spherical polar angles .  Calculate the internal forces induced by gravitational loading of the structure, using the membrane theory of shells in Section 10.7.7.

 

 

 

 

 

 

 

10.7.15. The figure shows a thin-walled conical shell with thickness h and mass density .  Calculate the internal forces induced by gravitational loading of the structure, using the membrane theory of shells in Section 10.7.7.   Use the cylindrical-polar coordinates  as the coordinate system.

 

 

 

 

(c) A.F. Bower, 2008
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