PART 2: INTRODUCTION TO CONTINUUM MECHANICS

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1 7 PART : INTRODUCTION TO CONTINUUM MECHANICS In the following sections we develop some applications of tensor calculus in the areas of dynamics, elasticity, fluids and electricity and magnetism. We begin by first developing generalized expressions for the vector operations of gradient, divergence, and curl. Also generalized expressions for other vector operators are considered in order that tensor equations can be converted to vector equations. We construct a table to aid in the translating of generalized tensor equations to vector form and vice versa. The basic equations of continuum mechanics are developed in the later sections. These equations are developed in both Cartesian and generalized tensor form and then converted to vector form.. TENSOR NOTATION FOR SCALAR AND VECTOR QUANTITIES We consider the tensor representation of some vector expressions. Our goal is to develop the ability to convert vector equations to tensor form as well as being able to represent tensor equations in vector form. In this section the basic equations of continuum mechanics are represented using both a vector notation and the indicial notation which focuses attention on the tensor components. In order to move back and forth between these notations, the representation of vector quantities in tensor form is now considered. Gradient For Φ = Φ(x,x,...,x N a scalar function of the coordinates x i,i =,...,N, the gradient of Φ is defined as the covariant vector Φ,i = Φ, xi i =,...,N. (.. The contravariant form of the gradient is g im Φ,m. (.. Note, if C i = g im Φ,m, i =,, 3 are the tensor components of the gradient then in an orthogonal coordinate system we will have C = g Φ,, C = g Φ,, C 3 = g 33 Φ,3. We note that in an orthogonal coordinate system that g ii =/h i, (no sum on i, i =,, 3 and hence replacing the tensor components by their equivalent physical components there results the equations C( = Φ h h x, C( = Φ h h x, C(3 = Φ h 3 h 3 x 3. Simplifying, we find the physical components of the gradient are C( = h Φ x, C( = h Φ x, C(3 = h 3 Φ x 3. These results are only valid when the coordinate system is orthogonal and g ij =0fori j and g ii = h i, with i =,, 3, and where i is not summed.

2 7 Divergence The divergence of a contravariant tensor A r is obtained by taking the covariant derivative with respect to x k and then performing a contraction. This produces div A r = A r,r. (..3 Still another form for the divergence is obtained by simplifying the expression (..3. The covariant derivative can be represented { } A r,k = Ar r x k + A m. mk Upon contracting the indices r and k and using the result from Exercise.4, problem 3, we obtain A r,r = Ar x r + ( g g x m Am A r,r = ( g A r g x r + g Ar x r A r,r = g x r ( ga r. (..4 EXAMPLE.-. (Divergence Find the representation of the divergence of a vector A r in spherical coordinates (ρ,θ,φ. Solution: In spherical coordinates we have x = ρ, x = θ, x 3 = φ with g ij =0 for i j and g = h =, g = h = ρ, g 33 = h 3 = ρ sin θ. The determinant of g ij is g = g ij = ρ 4 sin θ and g = ρ sin θ. Employing the relation (..4 we find div A r = g x ( ga + In terms of the physical components this equation becomes By using the notation x ( ga + ] x 3 ( ga 3. div A r = g ρ ( g A( + h θ ( g A( + h φ ( g A(3 ]. h 3 A( = A ρ, A( = A θ, A(3 = A φ for the physical components, the divergence can be expressed in either of the forms: div A r = ρ sin θ div A r = ρ ρ (ρ sin θa ρ + θ (ρ sin θ A θ ρ + ρ (ρ A ρ + ρ sin θ θ (sin θa θ+ ρ sin θ φ (ρ sin θ A ] φ ρ sin θ A φ φ. or

3 73 Curl The contravariant components of the vector C =curl A are represented C i = ɛ ijk A k,j. (..5 In expanded form this representation becomes: C = ( A3 g x A x 3 C = ( A g x 3 A 3 x C 3 = ( A g x A x. (..6 EXAMPLE.-. (Curl Find the representation for the components of curl A in spherical coordinates (ρ, θ, φ. Solution: In spherical coordinates we have :x = ρ, x = θ, x 3 = φ with g ij =0fori j and g = h =, g = h = ρ, g 33 = h 3 = ρ sin θ. The determinant of g ij is g = g ij = ρ 4 sin θ with g = ρ sin θ. The relations (..6 are tensor equations representing the components of the vector curl A. To find the components of curl A in spherical components we write the equations (..6 in terms of their physical components. These equations take on the form: C( = h g θ (h 3A(3 ] φ (h A( C( = h g φ (h A( ] ρ (h 3A(3 (..7 C(3 = h 3 g ρ (h A( ] θ (h A(. We employ the notations C( = C ρ, C( = C θ, C(3 = C φ, A( = A ρ, A( = A θ, A(3 = A φ to denote the physical components, and find the components of the vector curl A, in spherical coordinates, are expressible in the form: C ρ = ρ sin θ θ (ρ sin θa φ ] φ (ρa θ C θ = ρ sin θ φ (A ρ ] ρ (ρ sin θa φ (..8 C φ = ρ ρ (ρa θ ] θ (A ρ.

4 74 Laplacian The Laplacian U has the contravariant form U = g ij U,ij =(g ij U,i,j = ( ij U g x i. (..9,j Expanding this expression produces the equations: U = ( ij U x j g x i U = ( ij U x j g x i U = g g x j U = g x j { } im U j + g x i mj g g + ( ij U g x i ( gg ij U x i x x i ij U + g x i. j gij U ] g x j (..0 In orthogonal coordinates we have g ij =0fori j and g = h, g = h, g 33 = h 3 and so (..0 when expanded reduces to the form U = h h h 3 ( h h 3 U x h x + ( h h 3 U x h x + ( ] h h U x 3 h 3 x 3. (.. This representation is only valid in an orthogonal system of coordinates. EXAMPLE.-3. (Laplacian Find the Laplacian in spherical coordinates. Solution: Utilizing the results given in the previous example we find the Laplacian in spherical coordinates has the form ( U = ρ ρ sin θ U + ( sin θ U + ( ] U. (.. sin θ ρ ρ θ θ φ sin θ φ This simplifies to U = U ρ + U ρ ρ + U ρ θ + cot θ U ρ θ + U ρ sin θ φ. (..3 The table gives the vector and tensor representation for various quantities of interest.

5 75 VECTOR GENERAL TENSOR CARTESIAN TENSOR A A i or A i A i A B A i B i = g ij A i B j = A i B i A i B i = g ij A i B j C = A B C i = g e ijk A j B k A i B i C i = e ijk A j B k Φ = grad Φ g im Φ,m Φ,i = Φ x i A =diva g mn A m,n = A r,r = g x r ( ga r A i,i = A i x i A = C =curl A C i = ɛ ijk A k,j C i = e ijk A k x j U g mn U,mn = ( gg ij U g x j x i ( U x i x i C =( A B C i = A m B i,m C i = A m B i x m C = A( B C i = A i B j,j C i = A i B m x m C = A C i = g jm A i,mj or C i = g jm A i,mj C i = ( Ai x m x m ( A φ g im A i φ,m A i φ,i ( A ( A g im ( A r,r,m ɛ ijk g jm ( ɛ kst A t,s,m A j x j x i A r x i x r A i x j x j Table Vector and tensor representations.

6 76 EXAMPLE.-4. (Maxwell s equations In the study of electrodynamics there arises the following vectors and scalars: E =Electric force vector, E]=Newton/coulomb B =Magnetic force vector, B]=Weber/m D =Displacement vector, D]=coulomb/m H =Auxilary magnetic force vector, H]=ampere/m J =Free current density, J]=ampere/m ϱ =free charge density, ϱ] =coulomb/m 3 The above quantities arise in the representation of the following laws: Faraday s Law This law states the line integral of the electromagnetic force around a loop is proportional to the rate of flux of magnetic induction through the loop. This gives rise to the first electromagnetic field equation: E = B or ɛ ijk E k,j = Bi. (..5 Ampere s Law This law states the line integral of the magnetic force vector around a closed loop is proportional to the sum of the current through the loop and the rate of flux of the displacement vector through the loop. This produces the second electromagnetic field equation: H = J + D Gauss s Law for Electricity or ɛ ijk H k,j = J i + Di. (..6 This law states that the flux of the electric force vector through a closed surface is proportional to the total charge enclosed by the surface. This results in the third electromagnetic field equation: D ( = ϱ or gd i g x i = ϱ. (..7 Gauss s Law for Magnetism This law states the magnetic flux through any closed volume is zero. This produces the fourth electromagnetic field equation: B =0 or ( gb i g x i =0. (..8 The four electromagnetic field equations are referred to as Maxwell s equations. These equations arise in the study of electrodynamics and can be represented in other forms. These other forms will depend upon such things as the material assumptions and units of measurements used. Note that the tensor equations (..5 through (..8 are representations of Maxwell s equations in a form which is independent of the coordinate system chosen. In applications, the tensor quantities must be expressed in terms of their physical components. In a general orthogonal curvilinear coordinate system we will have g = h, g = h, g 33 = h 3, and g ij =0 for i j. This produces the result g = h h h 3. Further, if we represent the physical components of D i,b i,e i,h i by D(i,B(i,E(i, and H(i

7 77 the Maxwell equations can be represented by the equations in table. The tables 3, 4 and 5 are the representation of Maxwell s equations in rectangular, cylindrical, and spherical coordinates. These latter tables are special cases associated with the more general table. h h h 3 h h h 3 h h h 3 x (h 3E(3 ] x 3 (h E( h h h 3 x 3 (h E( ] x (h 3E(3 h h h 3 x (h E( ] x (h E( h h h 3 x (h 3H(3 ] x 3 (h H( h h h 3 x 3 (h H( ] x (h 3H(3 h h h 3 x (h H( ] x (h H( = h B( = h B( = h 3 B(3 = J( h = J( h = J(3 h 3 + D( h + D( h + D(3 h 3 ( D( x h h h 3 + ( D( h x h h h 3 + ( ] D(3 h x 3 h h h 3 = ϱ h 3 ( B( x h h h 3 + ( B( h x h h h 3 + ( ] B(3 h x 3 h h h 3 =0 h 3 Table Maxwell s equations in generalized orthogonal coordinates. Note that all the tensor components have been replaced by their physical components.

8 78 E z y E y z = B x E x z E z x = B y E y x E x y = B z H z y H y z = J x + D x H x z H z x = J y + D y H y x H x y = J z + D z D x x + D y y + D z z = ϱ B x x + B y y + B z z =0 Here we have introduced the notations: D x = D( B x = B( H x = H( J x = J( E x = E( D y = D( B y = B( H y = H( J y = J( E y = E( D z = D(3 B z = B(3 H z = H(3 J z = J(3 E z = E(3 with x = x, x = y, x 3 = z, h = h = h 3 = Table 3 Maxwell s equations Cartesian coordinates r r E z r θ E θ z E r z E z r E r θ r (re θ r r (rd r+ r D θ θ = B r = B θ = B z r H z r θ H r z H θ z H z r r (rh θ H r r θ + D z z = ϱ r r (rb r+ r = J r + D r = J θ + D θ = J z + D z B θ θ + B z z =0 Here we have introduced the notations: D r = D( B r = B( H r = H( J r = J( E r = E( D θ = D( B θ = B( H θ = H( J θ = J( E θ = E( D z = D(3 B z = B(3 H z = H(3 J z = J(3 E z = E(3 with x = r, x = θ, x 3 = z, h =, h = r, h 3 =. Table 4 Maxwell s equations in cylindrical coordinates.

9 79 ρ sin θ θ (sin θe φ E ] θ φ E ρ ρ sin θ φ ρ ρ ρ (ρe θ E ρ ρ θ = B ρ ρ (ρe φ= B θ = B φ ρ sin θ θ (sin θh φ H ] θ φ H ρ ρ sin θ φ ρ ρ ρ (ρh θ H ρ ρ θ = J ρ + D ρ ρ (ρh φ=j θ + D θ = J φ + D φ ρ ρ ρ (ρ D ρ + ρ sin θ θ (sin θd θ+ D φ ρ sin θ φ =ϱ ρ (ρ B ρ + ρ sin θ θ (sin θb θ+ B φ ρ sin θ φ =0 Here we have introduced the notations: D ρ = D( B ρ = B( H ρ = H( J ρ = J( E ρ = E( D θ = D( B θ = B( H θ = H( J θ = J( E θ = E( D φ = D(3 B φ = B(3 H φ = H(3 J φ = J(3 E φ = E(3 with x = ρ, x = θ, x 3 = φ, h =, h = ρ, h 3 = ρ sin θ Table 5 Maxwell s equations spherical coordinates. Eigenvalues and Eigenvectors of Symmetric Tensors Consider the equation T ij A j = λa i, i,j =,, 3, (..9 where T ij = T ji is symmetric, A i are the components of a vector and λ is a scalar. Any nonzero solution A i of equation (..9 is called an eigenvector of the tensor T ij and the associated scalar λ is called an eigenvalue. When expanded these equations have the form (T λa + T A + T 3 A 3 =0 T A +(T λa + T 3 A 3 =0 T 3 A + T 3 A +(T 33 λa 3 =0. The condition for equation (..9 to have a nonzero solution A i is that the characteristic equation should be zero. This equation is found from the determinant equation T λ T T 3 f(λ = T T λ T 3 =0, (..0 T 3 T 3 T 33 λ

10 80 which when expanded is a cubic equation of the form f(λ = λ 3 + I λ I λ + I 3 =0, (.. where I,I and I 3 are invariants defined by the relations I = T ii I = T iit jj T ijt ij (.. I 3 = e ijk T i T j T k3. When T ij is subjected to an orthogonal transformation, where T mn = T ij l im l jn,then l im l jn (T mn λδ mn = T ij λδ ij and det (T mn λδ mn =det ( Tij λδ ij. Hence, the eigenvalues of a second order tensor remain invariant under an orthogonal transformation. If T ij is real and symmetric then the eigenvalues of T ij will be real, and the eigenvectors corresponding to distinct eigenvalues will be orthogonal. Proof: To show a quantity is real we show that the conjugate of the quantity equals the given quantity. If (..9 is satisfied, we multiply by the conjugate A i and obtain A i T ij A j = λa i A i. (..5 The right hand side of this equation has the inner product A i A i which is real. It remains to show the left hand side of equation (..5 is also real. Consider the conjugate of this left hand side and write A i T ij A j = A i T ij A j = A i T ji A j = A i T ij A j. Consequently, the left hand side of equation (..5 is real and the eigenvalue λ can be represented as the ratio of two real quantities. Assume that λ ( and λ ( are two distinct eigenvalues which produce the unit eigenvectors ˆL and ˆL with components l i and l i,i=,, 3 respectively. We then have T ij l j = λ ( l i and T ij l j = λ ( l i. (..6 Consider the products λ ( l i l i = T ij l j l i, λ ( l i l i = l i T ij l j = l j T ji l i. and subtract these equations. We find that (..7 λ ( λ ( ]l i l i =0. (..8 By hypothesis, λ ( is different from λ ( and consequently the inner product l i l i must be zero. Therefore, the eigenvectors corresponding to distinct eigenvalues are orthogonal.

11 8 Therefore, associated with distinct eigenvalues λ (i,i=,, 3 there are unit eigenvectors ˆL (i = l i ê + l i ê + l i3 ê 3 with components l im,m=,, 3 which are direction cosines and satisfy l in l im = δ mn and l ij l jm = δ im. (..3 The unit eigenvectors satisfy the relations T ij l j = λ ( l i T ij l j = λ ( l i T ij l j3 = λ (3 l i3 and can be written as the single equation T ij l jm = λ (m l im, m =,, or3 m not summed. Consider the transformation x i = l ij x j or x m = l mj x j which represents a rotation of axes, where l ij are the direction cosines from the eigenvectors of T ij. This is a linear transformation where the l ij satisfy equation (..3. Such a transformation is called an orthogonal transformation. In the new x coordinate system, called principal axes, we have T mn = T ij x i x m x j x n = T ijl im l jn = λ (n l in l im = λ (n δ mn (no sum on n. (..4 This equation shows that in the barred coordinate system there are the components ( T mn = λ ( λ ( λ (3 That is, along the principal axes the tensor components T ij are transformed to the components T ij where T ij =0fori j. The elements T (i(i, i not summed, represent the eigenvalues of the transformation (..9.

12 8 EXERCISE.. In cylindrical coordinates (r, θ, z withf = f(r, θ, z find the gradient of f.. In cylindrical coordinates (r, θ, z with A = A(r, θ, z find div A. 3. In cylindrical coordinates (r, θ, z for A = A(r, θ, z find curl A. 4. In cylindrical coordinates (r, θ, z forf = f(r, θ, z find f. 5. In spherical coordinates (ρ, θ, φ withf = f(ρ, θ, φ find the gradient of f. 6. In spherical coordinates (ρ, θ, φ with A = A(ρ, θ, φ find div A. 7. In spherical coordinates (ρ, θ, φ for A = A(ρ, θ, φ find curl A. 8. In spherical coordinates (ρ, θ, φ forf = f(ρ, θ, φ find f. 9. Let r = x ê +y ê +z ê 3 denote the position vector of a variable point (x, y, z in Cartesian coordinates. Let r = r denote the distance of this point from the origin. Find in terms of r and r: (a grad(r (b grad(r m (c grad( (d grad(lnr (e grad(φ r where φ = φ(r is an arbitrary function of r. 0. Let r = x ê +y ê +z ê 3 denote the position vector of a variable point (x, y, z in Cartesian coordinates. Let r = r denote the distance of this point from the origin. Find: (a div ( r (b div (r m r (c div (r 3 r (d div (φ r where φ = φ(r is an arbitrary function or r.. Let r = x ê + y ê + z ê 3 denote the position vector of a variable point (x, y, z in Cartesian coordinates. Let r = r denote the distance of this point from the origin. Find: (a curl r (b curl (φ r where φ = φ(r is an arbitrary function of r.. 3. Expand and simplify the representation for curl (curl A. Show that the curl of the gradient is zero in generalized coordinates. 4. Write out the physical components associated with the gradient of φ = φ(x,x,x Show that g im A i,m = g x i gg im A m ] = A i,i = g x i ga i ].

13 83 6. Let r =( r r / = x + y + z and calculate (a (r (b (/r (c (r (d (/r 7. Given the tensor equations D ij = (v i,j + v j,i, i,j =,, 3. Let v(,v(,v(3 denote the physical components of v,v,v 3 and let D(ij denote the physical components associated with D ij. Assume the coordinate system (x,x,x 3 is orthogonal with metric coefficients g (i(i = h i,i=,, 3andg ij =0 for i j. (a Find expressions for the physical components D(,D( and D(33 in terms of the physical components v(i,i=,, 3. Answer: D(ii = V (i h i x i + V (j h i h i h j x j no sum on i. j i (b Find expressions for the physical components D(,D(3 and D(3 in terms of the physical components v(i,i=,, 3. Answer: D(ij = ( hi V (i h j x j + h ( ] j V (j h i h i x i h j 8. Write out the tensor equations in problem 7 in Cartesian coordinates Write out the tensor equations in problem 7 in cylindrical coordinates. Write out the tensor equations in problem 7 in spherical coordinates. Express the vector equation (λ +µ Φ µ ω + F = 0 intensorform.. Write out the equations in problem for a generalized orthogonal coordinate system in terms of physical components Write out the equations in problem for cylindrical coordinates. Write out the equations in problem for spherical coordinates. Use equation (..4 to represent the divergence in parabolic cylindrical coordinates (ξ,η,z. Use equation (..4 to represent the divergence in parabolic coordinates (ξ,η,φ. Use equation (..4 to represent the divergence in elliptic cylindrical coordinates (ξ,η,z. Change the given equations from a vector notation to a tensor notation B = v A +( v A d dt A ( B C] = d A dt ( B C+ A ( d B dt C+ A ( B d C dt d v dt = v +( v v H c = curl E db dt ( B v + B( v = 0

14 84 Change the given equations from a tensor notation to a vector notation. 33. ɛ ijk B k,j + F i =0 34. g ij ɛ jkl B l,k + F i =0 35. ϱ +(ϱv i,i=0 36. ϱ( v i + v v i P m = xm x i + µ v i x m x m + F i 37. The moment of inertia of an area or second moment of area is defined by I ij = (y m y m δ ij y i y j da A where da is an element of area. Calculate ( the moment of inertia I ij,i,j=, for the triangle illustrated in the figure.- and show that I ij = bh3 4 b h 4 b h. b3 h Figure.- Moments of inertia for a triangle 38. Use the results from problem 37 and rotate the axes in figure.- through an angle θ to a barred system of coordinates. (a Show that in the barred system of coordinates ( ( I + I I I I = + cos θ + I sin θ ( I I I = I = sin θ + I cos θ ( ( I + I I I I = cos θ I sin θ (b For what value of θ will I have a maximum value? (c Show that when I is a maximum, we will have I a minimum and I = I =0.

15 85 Figure.- Mohr s circle 39. Otto Mohr gave the following physical interpretation to the results obtained in problem 38: Plot the points A(I,I andb(i, I as illustrated in the figure.- Draw the line AB and calculate the point C where this line intersects the I axes. Show the point C has the coordinates ( I + I, 0 Calculate the radius of the circle with center at the point C and with diagonal AB and show this radius is (I I r = + I Show the maximum and minimum values of I occur where the constructed circle intersects the I axes. Show that I max = I = I + I + r I min = I = I + I r. ( I I 40. Show directly that the eigenvalues of the symmetric matrix I ij = are λ I I = I max and λ = I min where I max and I min are given in problem Find the principal axes and moments of inertia for the triangle given in problem 37 and summarize your results from problems 37,38,39, and or 43. Verify for orthogonal coordinates the relations A ] ê (i = 3 k= Verify for orthogonal coordinates the relation ( A ] ê (i = e (ijk (h (k A(k h (i h h h 3 x j A h ê h ê h 3 ê 3 = h h h 3 x x x 3 h A( h A( h 3 A(3. 3 h (i e (ijr e rsm h h h 3 x j m= ] h (r (h (m A(m h h h 3 x s Christian Otto Mohr ( German civil engineer.

16 Verify for orthogonal coordinates the relation ( A ] ê (i = { (h h 3 A( h (i x (i h h h 3 x Verify the relation ( A B ] ê (i = 3 k= A(k B(i + h (k x k k i B(k h k h (i + (h h 3 A( x + (h h A(3 x 3 ( A(i h (i A(k h k x k x (i ]} 46. The Gauss divergence theorem is written ( F V x + F y + F3 ( dτ = n F + n F + n 3 F 3 dσ z S where V is the volume within a simple closed surface S. Here it is assumed that F i = F i (x, y, z are continuous functions with continuous first order derivatives throughout V and n i are the direction cosines of the outward normal to S, dτ is an element of volume and dσ is an element of surface area. (a Show that in a Cartesian coordinate system F,i i x + F y + F3 z and that the tensor form of this theorem is F,i i dτ = F i n i dσ. V S (b Write the vector form of this theorem. (c Show that if we define u r = u x r, v r = v x r and F r = g rm F m = uv r then F,i i F i,m = g im (uv i,m + u m v i (d Show that another form of the Gauss divergence theorem is g im u m v i dτ = uv m n m dσ ug im v i,m dτ V Write out the above equation in Cartesian coordinates. 47. Find the eigenvalues and eigenvectors associated with the matrix A =. Show that the eigenvectors are orthogonal. 48. Find the eigenvalues and eigenvectors associated with the matrix A = 0. 0 Show that the eigenvectors are orthogonal. 49. Find the eigenvalues and eigenvectors associated with the matrix A = 0. 0 Show that the eigenvectors are orthogonal. S 50. The harmonic and biharmonic functions or potential functions occur in the mathematical modeling of many physical problems. Any solution of Laplace s equation Φ = 0 is called a harmonic function and any solution of the biharmonic equation 4 Φ = 0 is called a biharmonic function. (a Expand the Laplace equation in Cartesian, cylindrical and spherical coordinates. (b Expand the biharmonic equation in two dimensional Cartesian and polar coordinates. Hint: Consider 4 Φ= ( Φ. In Cartesian coordinates Φ=Φ,ii and 4 Φ=Φ,iijj. V