Understanding Common Variable Types in Elasticity Theory

1. Common Variable Types in Elasticity Elasticity theory is a mathematical model of material deformation. Using principles of continuum mechanics, it is formulated in terms of many different types of field variables specified at spatial points in the body under study. Some examples include: Scalars - Single magnitude mass density , temperature T , modulus of elasticity E , . . . Vectors Three components in three dimensions displacement vector Matrices Nine components in three dimensions stress matrix Other Variables with more than nine components , e 1 , e 2 , e 3 are unit basis vectors Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

2. Index/Tensor Notation With the wide variety of variables, elasticity formulation makes use of a tensor formalism using index notation. This enables efficient representation of all variables and governing equations using a single standardized method. Index notation is a shorthand scheme whereby a whole set of numbers or components can be represented by a single symbol with subscripts In general a symbol a ijk with N distinct indices represents 3 N distinct numbers Addition, subtraction, multiplication and equality of index symbols are defined in the normal fashion; e.g. Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

3. Notation Rules and Definitions Summation Convention - if a subscript appears twice in the same term, summation over that subscript from one to three is implied; for example A symbol a ijmnk is said to be symmetric with respect to index pair mn if A symbol a ijmnk is said to be antisymmetric with respect to index pair mn if Useful Identity . . . symmetric . . . antisymmetric Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

4. The matrix a ij and vector b i are specified by Determine the following quantities: I ndicate whether they are a scalar, vector or matrix. Following the standard definitions given in section 1.2, Example 1-1: Index Notation Examples Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

5. Special Index Symbols Kronecker Delta Properties: Alternating or Permutation Symbol 123 = 231 = 312 = 1, 321 = 132 = 213 = -1, 112 = 131 = 222 = . . . = 0 Useful in evaluating determinants and vector cross-products Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

6. Coordinate Transformations v e 1 e 3 e 2 e 3 e 2 e 1 x 3 x 1 x 2 x 1 x 2 x 3 To express elasticity variables in different coordinate systems requires development of transformation rules for scalar, vector, matrix and higher order variables a concept connected with basic definitions of tensor variables. The two Cartesian frames ( x 1 ,x 2 ,x 3 ) and differ only by orientation Using Rotation Matrix transformation laws for Cartesian vector components Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

7. Cartesian Tensors General Transformation Laws Scalars, vectors, matrices, and higher order quantities can be represented by an index notational scheme, and thus all quantities may then be referred to as tensors of different orders. The transformation properties of a vector can be used to establish the general transformation properties of these tensors. Restricting the transformations to those only between Cartesian coordinate systems, the general set of transformation relations for various orders are: Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

8. Example 1-2 Transformation Examples The components of a first and second order tensor in a particular coordinate frame are given by Determine the components of each tensor in a new coordinate system found through a rotation of 60 o ( /6 radians) about the x 3 -axis. Choose a counterclockwise rotation when viewing down the negative x 3 -axis, see Figure 1-2. The original and primed coordinate systems are shown in Figure 1-2. The solution starts by determining the rotation matrix for this case The transformation for the vector quantity follows from equation (1.5.1) 2 and the second order tensor (matrix) transforms according to (1.5.1) 3 x 3 x 1 x 2 x 1 x 2 x 3 60 o Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

9. Principal Values and Directions for Symmetric Second Order Tensors The direction determined by unit vector n is said to be a principal direction or eigenvector of the symmetric second order tensor a ij if there exists a parameter ( principal value or eigenvalue ) such that Relation is a homogeneous system of three linear algebraic equations in the unknowns n 1 , n 2 , n 3 . The system possesses nontrivial solution if and only if determinant of coefficient matrix vanishes scalars I a , II a and III a are called the fundamental invariants of the tensor a ij Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

10. Principal Axes of Second Order Tensors It is always possible to identify a right-handed Cartesian coordinate system such that each axes lie along principal directions of any given symmetric second order tensor. Such axes are called the principal axes of the tensor, and the basis vectors are the principal directions { n (1) , n (2) , n (3) } x 3 x 1 x 2 Principal Axes Original Given Axes Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

11. Example 1-3 Principal Value Problem Determine the invariants, and principal values and directions of First determine the principal invariants The characteristic equation then becomes Thus for this case all principal values are distinct For the 1 = 5 root, equation (1.6.1) gives the system which gives a normalized solution In similar fashion the other two principal directions are found to be It is easily verified that these directions are mutually orthogonal. Note for this case, the transformation matrix Q ij defined by (1.4.1) becomes Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

12. Vector, Matrix and Tensor Algebra Scalar or Dot Product Vector or Cross Product Common Matrix Products Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

13. Calculus of Cartesian Tensors Field concept for tensor components Comma notation for partial differentiation If differentiation index is distinct, order of the tensor will be increased by one; e.g. derivative operation on a vector produces a second order tensor or matrix Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

14. Vector Differential Operations Directional Derivative of Scalar Field Common Differential Operations Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

15. Example 1-4: Scalar/Vector Field Example Scalar and vector field functions are given by Calculate the following expressions, , 2 , u , u , u . Using the basic relations: 2 u u u Contours =constant and vector distributions of vector field is orthogonal to -contours (ture in general ) x y - (satisfies Laplace equation) Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

16. Vector/Tensor Integral Calculus Divergence Theorem Stokes Theorem Greens Theorem in the Plane Zero-Value Theorem Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

17. Orthogonal Curvilinear Coordinate Systems Cylindrical Coordinate System ( r, , z) Spherical Coordinate System ( R , , ) e 3 e 2 e 1 x 3 x 1 x 2 R Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

18. General Curvilinear Coordinate Systems Common Differential Forms Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island

19. Example 1-5: Polar Coordinates From relations (1.9.5) or simply using the geometry shown in Figure The basic vector differential operations then follow to be Elasticity Theory, Applications and Numerics M.H. Sadd , University of Rhode Island