INTRODUCTION TO FINITE ELEMNT METHOD

1. INTRODUCTION:• The finite element method is a numerical method for solving problems of engineering and

mathematical physics.

Typical problem areas of interest:• Structural analysis, heat transfer, fluid flow, mass transport, and Electromagnetic

potential• Useful for problems with complicated geometries, loadings, and material properties where

analytical solutions can not be obtained.• The finite element formulation of the problem results in a system of simultaneous

algebraic equations for solution, rather than requiring the solution of differential equations.• Process of modeling a body by dividing it into an equivalent system of smaller bodies or

units (finite elements) interconnected at points common to two or more elements (nodal points or nodes) and/or boundary lines and/or surfaces is called discretization.

• The solution for structural problems typically refers to determining the displacements at each node and the stresses within each element making up the structure that is subjected to applied loads.

• In nonstructural problems, the nodal unknowns may, for instance, be temperatures or fluid pressures due to thermal or fluid fluxes.

Contd…

Brief History:• Grew out of aerospace industry• Post-WW II jets, missiles, space flight• Need for light weight structures• Required accurate stress analysis• Paralleled growth of computers

Common FEA Applications• Mechanical/Aerospace/Civil/Automotive• Engineering• Structural/Stress Analysis• Static/Dynamic• Linear/Nonlinear• Fluid Flow• Heat Transfer• Electromagnetic Fields• Soil Mechanics• Acoustics• Biomechanics

General Steps In Finite Element method Analysis:

There are two general direct approaches traditionally associated with the finite element method as applied to structural mechanics problems.• One approach, called the force, or flexibility, method, uses internal forces as the

unknowns of the problem.• The second approach, called the displacement, or stiffness, method, assumes the

displacements of the nodes as the unknowns of the problem.

For computational purposes, the displacement (or stiffness) method is more desirable because its formulation is simpler for most structural analysis problems.

1. Discretization: The first step involves dividing the body into an equivalent system of finite elements with associated nodes and choosing the most appropriate element type to model most closely the

actual physical behavior.Model body by dividing it into an equivalent system of many smaller bodies or units (finite elements) interconnected at points common to two or more elements (nodes or nodal points) and/or boundary lines and/or surfaces.

Contd…

Feature• Obtain a set of algebraic equations to solve for unknown (first) nodal quantity

(displacement).• Secondary quantities (stresses and strains) are expressed in terms of nodal values of

primary quantity.

Types of Elements:

• The primary line elements consist of bar (or truss) and beam elements (1D). They have a cross-sectional area but are usually represented by line segments. In general, the cross-sectional area within the element can vary, but it will be considered to be constant for the sake of this section.

• These elements are often used to model trusses and frame structures. The simplest line element (called a linear element) has two nodes, one at each end.

• The basic two-dimensional (or plane) elements are loaded by forces in their own plane (plane stress or plane strain conditions). They are triangular or quadrilateral elements.

• The simplest two-dimensional elements have corner nodes only (linear elements) with straight sides or boundaries The elements can have variable thicknesses throughout or be constant.

• The most common three-dimensional elements are tetrahedral and hexahedral (or brick) elements; they are used when it becomes necessary to perform a three-dimensional stress analysis. The basic three-dimensional elements have corner nodes only and straight sides.

FEM FORMULATION PROCEDURE:

Principles of FEA:

• The finite element method (FEM), or finite element analysis (FEA), is a Computational technique used to obtain approximate solutions of boundary value problems in engineering.

• Boundary value problems are also called field problems. The field is the domain of interest and most often represents a physical structure.

• The field variables are the dependent variables of interest governed by the differential equation.

• The boundary conditions are the specified values of the field variables (or related variables such as derivatives) on the boundaries of the field.

• For simplicity, at this point, we assume a two-dimensional case with a single field variable φ(x, y) to be determined at every point P(x, y) such that a known governing equation (or equations) is satisfied exactly at every such point.

• A finite element is not a differential element of size dx × dy.• A node is a specific point in the finite element at which the value of the field variable is

to be explicitly calculated.

The Mathematical and Physical FEM

Mathematical FEM: • The centerpiece in the process steps of the Mathematical FEM is the

mathematical model which is often an ordinary or partial differential equation in space and time.

• A discrete finite element model is generated from of the mathematical model.• The resulting FEM equations are processed by an equation solver, which

provides a discrete solution.• FEM discretization may be constructed without any reference to physics.• The concept of error arises when the discrete solution is substituted in the

mathematical and discrete models. • This replacement is generically called verification. • The solution error is the amount by which the discrete solution fails to satisfy

the discrete equations. This error is relatively unimportant when using computers. More relevant is the discretization error, which is the amount by which the discrete solution fails to satisfy the mathematical model.

PHYSICAL FEM:• The processes of idealization and discretization are carried out concurrently to

produce the discrete model. • Indeed FEM discretization may be constructed and adjusted without reference

to mathematical models, simply from experimental measurements. • The concept of error arises in the physical FEM in two ways, known as

verification and validation. • Verification is the same as in the Mathematical FEM: the discrete solution is

replaced into the discrete model to get the solution error. As noted above, this error is not generally important.

• Validation tries to compare the discrete solution against observation by computing the simulation error, which combines modeling and solution errors. Since the latter is typically insignificant, the simulation error in practice can be identified with the modeling error.

• Comparing the discrete solution with the ideal physical system would in principle quantify the modeling errors.

Chapter 2. The Direct Stiffness Method Of FEM

Introduction:• The primary characteristics of a finite element are embodied in the element

stiffness matrix.• For a structural finite element, the stiffness matrix contains the geometric and

material behavior information that indicates the resistance of the element to deformation when subjected to loading.

• For finite elements used in nonstructural analyses, such as fluid flow and heat transfer, the term stiffness matrix is also used, since the matrix represents the resistance of the element to change when subjected to external influences.

Contd…• Basic premise of the finite element method is to describe the continuous

variation of the field variable (physical displacement) in terms of discrete values at the finite element nodes. In the interior of a finite element, as well as along the boundaries (applicable to two- and three-dimensional problems), the field variable is described via interpolation functions that must satisfy prescribed conditions.

• As introduction we present several such principles applicable to finite element analysis. First, and foremost, for spring and bar systems, we utilize the principle of static equilibrium but—and this is essential—we include deformation in the development; that is, we are not dealing with rigid body mechanics.

• For extension of the finite element method to more complicated elastic structural systems, we also state and apply the first theorem of Castigliano’s and the more widely used principle of minimum potential energy .

• Both theorems relate displacements and applied forces to the equilibrium conditions of a mechanical system in terms of mechanical energy.

Steps In Direct Stiffness Method:

Step 1: Select the Element Type

Step 2: Select a Displacement Function

Step 3: Define the Strain/Displacement and Stress/Strain Relationships

Step 4 :Derive the Element Stiffness Matrix and Equations

Step 5: Assemble the Element Equations to Obtain the Global Equations and Introduce Boundary Conditions

Step 6: Solve for the Nodal Displacements

Step 7: Solve for the Element Forces

Direct Stiffness Method For Linear springs and Bars

Introduction:• The direct equilibrium approach, will be used to derive the stiffness matrix for a

one-dimensional linear spring—that is, a spring that obeys Hooke’s law and resists forces only in the direction of the spring.

• Reference points 1 and 2 are located at the ends of the element. These reference points are called the nodes of the spring element.

• The local nodal forces are f1x and f2x for the spring element associated with the local axis x. The local axis acts in the direction of the spring so that we can directly measure displacements and forces along the spring.

• The local nodal displacements are d1x and d2x for the spring element. These nodal displacements are called the degrees of freedom at each node. Positive directions for the forces and displacements at each node are taken in the positive x direction as shown from node 1 to node 2 in the figure.

• The symbol k is called the spring constant or stiffness of the spring.

Contd…

• We know that a prismatic uniaxial bar has a spring constant , where A represents the cross-sectional area of the bar, E is the modulus of elasticity, and L is the bar length.

• Similarly, we know that a prismatic circular cross- section bar in torsion has a spring constant , where J is the polar moment of inertia and G is the shear modulus of the material.

• For one-dimensional heat conduction, , where Kxx is the thermal conductivity of the material.

• And for one- dimensional fluid flow through a porous medium, the stiffness , where Kxx is the permeability coefficient of the material.

• Observe that the stiffness method can be applied to nonstructural problems, such as heat transfer, fluid flow, and electrical networks, as well as structural problems by simply applying the proper constitutive law (such as Hooke’s law for structural problems, Fourier’s law for heat transfer, Darcy’s law for fluid flow and Ohm’s law for electrical networks) and a conservation principle such as nodal equilibrium or conservation of energy.

1. The Linear Spring FEM

1. Select the element type:• Consider the linear spring element (which can be an element in a system of

springs) subjected to resulting nodal tensile forces T (which may result from the action of adjacent springs) directed along the spring axial direction x as shown in Figure below, so as to be in equilibrium.

• The local x axis is directed from node 1 to node 2. • We represent the spring by labeling nodes at each end and by labeling the

element number. The original distance between nodes before deformation is denoted by L.

• The material property (spring constant) of the element is k.

Fig. Linear spring subjected to tensile forces

The Linear Spring FEM contd…

2. Select a Displacement Function:• We must choose in advance the mathematical function to represent the deformed shape of

the spring element under loading. Because it is difficult, if not impossible at times, to obtain a closed form or exact solution,

• We assume a solution shape or distribution of displacement within the element by using an appropriate mathematical function. The most common functions used are polynomials.

• Because the spring element resists axial loading only with the local degrees of freedom for the element being displacements d1x and d2x along the x direction, we choose a

displacement function u to represent the axial displacement throughout the element. • Here a linear displacement variation along the x axis of the spring is assumed, because a

linear function with specified endpoints has a unique path.

Therefore,

• In general, the total number of coefficients a is equal to the total number of degrees of freedom associated with the element. Here the total number of degrees of freedom is two—an axial displacement at each of the two nodes of the element In matrix form, becomes:

Contd…

• We now want to express u as a function of the nodal displacements d1x and d2x.

• This will allow us to apply the physical boundary conditions on nodal displacements directly as indicated in Step 3 and to then relate the nodal displacements to the nodal forces in Step 4. We achieve this by evaluating u at each node and solving for a1 and a2.

• Back substituting the values:• In matrix form:

• Where :

Contd…

• N1 and N2 are called the shape functions because the Ni’s express the shape of the assumed displacement function over the domain (x coordinate) of the element when the ith element degree of freedom has unit value and all other degrees of freedom are zero.

• In this case, N1 and N2 are linear functions that have the properties that N1= 1 at node 1 and N1 = 0 at node 2, whereas N2 = 1 at node 2 and N2 =0 at node 1.

• Also, N1 + N2 =1 for any axial coordinate along the bar.

• In addition, the Ni’s are often called interpolation functions because we are interpolating to find the value of a function between given nodal values.

Fig. (a) Spring element showing plots of (b) displacement function u and

Shape functions (c) N1 and (d) N2 over domain of element

3. Define the Strain/Displacement and Stress/Strain Relationships:

• The tensile forces T produce a total elongation (deformation) d of the spring.

• Here d1x is a negative value because the direction of displacement is opposite to the positive x direction, whereas d2x is a positive value.

• The deformation of the spring is then represented by:

• For a spring element, we can relate the force in the spring directly to the deformation. Therefore, the strain/displacement relationship is not necessary here.

• The stress/strain relationship can be expressed in terms of the force/deformation relationship instead as:

4: Derive the Element Stiffness Matrix and Equations

• We now derive the spring element stiffness matrix. By the sign convention for nodal forces and equilibrium, we have:

• or

• In a single matrix equation yields:

• This relationship holds for the spring along the Y axis. From our basic definition of a stiffness matrix and application.

• Here k is called the local stiffness matrix for the element. We observe from that k is a symmetric (that is, kij = kji square matrix (the number of rows equals the number of columns in k).

• The main diagonal of the stiffness matrix is always positive. Otherwise, a positive nodal force Fi could produce a negative displacement di—a behavior contrary to the physical behavior of any actual structure.

5 : Assemble the Element Equations to Obtain the Global Equations and Introduce Boundary Conditions

• The global stiffness matrix and global force matrix are assembled using nodal force equilibrium equations, force/deformation and compatibility equations, and the direct stiffness method described.

• This step applies for structures composed of more than one element such that

where k and f are now element stiffness and force matrices expressed in a global reference frame. (the ∑ sign used in this context does not imply a simple summation of element matrices but rather denotes that these element matrices must be assembled properly according to the direct stiffness method.

6: Solve for the Nodal Displacements• The displacements are then determined by imposing boundary conditions, such

as support conditions, and solving a system of equations, F = Kd, simultaneously.

7 :Solve for the Element Forces• Finally, the element forces are determined by back-substitution, applied to each

element, into equations

The direct stiffness method for elastic Bar Consider an elastic bar subjected to axial forces only. • This element, which we simply call a bar element, is particularly useful in the

analysis of both two- and three dimensional frame or truss structures. Formulation of the finite element characteristics of an elastic bar element is based on the following assumptions:

1. The bar is geometrically straight.

2. The material obeys Hooke’s law.

3. Forces are applied only at the ends of the bar.

4. The bar supports axial loading only; bending, torsion, and shear are not transmitted to the element via the nature of its connections to other elements.

• The last assumption, while quite restrictive, is not impractical; this condition is satisfied if the bar is connected to other structural members via pins (2-D) or ball and socket joints (3-D).

• Recognize this element as the familiar two-force member of elementary statics, meaning, for equilibrium, the forces exerted on the ends of the element must be collinear, equal in magnitude, and opposite in sense.

Contd…

Fig. A bar (or truss) element with element coordinate system and nodal displacement notation.

• Figure above depicts an elastic bar of length L to which is affixed a uniaxial coordinate system x with its origin arbitrarily placed at the left end. This is the element coordinate system or reference frame. Denoting axial displacement at any position along the length of the bar as u(x), we define nodes 1 and 2 at each end as shown and introduce the nodal displacements;

u1 = u(x = 0) and u2 = u(x = L).

• Thus, we have the continuous field variable u(x), which is to be expressed (approximately) in terms of two nodal variables u1 and u2. To accomplish this discretization, we assume the existence of interpolation functions N1(x ) and N2(x ) (also known as shape or blending functions) such that:

Contd…• To determine the interpolation functions, we require that the boundary values of

u(x ) (the nodal displacements) be identically satisfied by the discretization such that:

• This lead to the following boundary (nodal) conditions:

• As we have two conditions that must be satisfied by each of two one- dimensional functions, the simplest forms for the interpolation functions are polynomial forms:

• Application of conditions represented by the previous Equation above yields a0 = 1, b0 = 0 while Equation above results in a1 = −(1/L) and b1 = x/L. Therefore, the interpolation functions are

• And the continuous displacement function is represented by the discretization

Contd…

• This can be expressed in matrix form as:

• The task remains to determine the relation between the nodal displacements and applied forces to obtain the stiffness matrix for the bar element.

• Recall from elementary strength of materials that the deflection of an elastic bar of length L and uniform cross-sectional area A when subjected to axial load P is given by:

• where E is the modulus of elasticity of the material. Using the above Equation, we obtain the equivalent spring constant of an elastic bar as.

• Ultimately, we wish to compute the nodal displacements given some loading condition on the element.

• To obtain the necessary equilibrium equations relating the displacements to applied forces, we proceed from displacement to strain, strain to stress, and stress to loading, as follows. In uniaxial loading, as in the bar element, we need consider only the normal strain component, defined as

Contd….

• which, when applied to Equation

• which shows that the spar element is a constant strain element. This is in accord with strength of materials theory: The element has constant cross-sectional area and is subjected to constant forces at the end points, so the strain does not vary along the length. The axial stress, by Hooke’s law, is then

• And the associated axial force is:

• Lets now use it to relate the applied nodal forces f1 and f2 to the nodal displacements u1 and u2.

Contd…

• The equations are expressed in matrix form as:

• Stiffness matrix for the bar element is given by:

• As is the case with the linear spring, we observe that the element stiffness matrix for the bar element is symmetric, singular, and of order 2 × 2 in correspondence with two nodal displacements or degrees of freedom.

• It must be emphasized that the stiffness matrix given by the Equation above is expressed in the element coordinate system, which in this case is one-dimensional. Application of this element formulation to analysis of two- and three-dimensional structures is considered in the next Topics.

Finite element method for truss

• Vector transformation in 2-dimension • Local coordinates are always chosen to represent the individual element

conveniently. Global coordinates are chosen to be convenient for the whole structure.

• Given the nodal displacement of an element, represented by the vector d in figure below, we want to relate the components of this vector in one coordinate system to components in another. For general purposes, we will assume in this section that d is not coincident with either the local or the global axis. In this case, we want to relate global displacement components to local ones. In doing so, we will develop a transformation matrix that will subsequently be used to

develop the global stiffness matrix for a bar element. We define the angle ɵ to be positive when measured counterclockwise from X to x. We can express vector displacement d in both global and local coordinates by:

Contd….• We will now relate i and j to i’ and j’ through use of the following Figure:

• From vector addition : • Form laws of cosine : • And because i is, by definition, a unit vector, its magnitude is given 1: • There for we obtain

Now a is in the i’ direction and b is in the –j’ direction. Therefore,

Contd…

• This yields :

• Similarly:

• So from

• Combining similar terms:

• In matrix form:

• Where:

• is called the transformation (or rotation) matrix

Direct Stiffness method for Truss

Contd….

• We will now use the transformation relationship to obtain the global stiffness matrix for a bar element in truss. We need the global stiffness matrix of each element to assemble the total global stiffness matrix of the structure.

• We have shown that for a bar element in the local coordinate system,

• We now want to relate the global element nodal forces f to the global nodal displacements d for a bar element arbitrarily oriented with respect to the global axes as was shown in Figure above. This relationship will yield the global stiffness matrix k of the element. That is, we want to find a matrix k such that

Contd…

• Or, in simplified matrix: • We know from vector transformation relationship:

In matrix:

Or as where:

Similarly, because forces transform in the same manner as displacements, we have

or

So relating the force and the displacements:

Substituting the above transformations:

Contd…

• However, to write the final expression relating global nodal forces to global nodal displacements for an element, we must invert T. This is not immediately possible because T is not a square matrix. Therefore, we must expand d’, f’ , and k’ to the order that is consistent with the use of global coordinates even though f’1y and f’2y are zero. Using vector transformation for each nodal displacement, we thus obtain

Or

where:

Similarly we can write

Contd…• Also, k’ must be expanded to a 4 x 4 matrix. Therefore, in expanded form

becomes:

• Because f’1y and f’2y are zero, rows of zeros corresponding to the row numbers f’1y and f’2y appear in k’.

• Finally we obtain:

• Solving for f:

• where T-1 is the inverse of T. but it can be shown that T-1 is equal to TT . • where TT is the transpose of T. (Property of orthogonal Matrix)

Contd…• Equating equations and

• We obtain the transformed elemental matrix as:

The obtained matrix would be:

• The stiffness matrix and force matrix for each element can be summed by using the direct stiffness method to obtain

• K now relates the global nodal forces F to the global nodal displacements d for the whole structure by:

Computing Stress in bars (Truss) • For a bar, the local forces are related to the local displacements by the following

equation. This equation is repeated here for convenience.

• The usual definition of axial tensile stress is axial force divided by cross-sectional area. Therefore, axial stress is:

• where f’2x is used because it is the axial force that pulls on the bar.

• Substituting the axial stress in the above equation:

• Now by using the transformation concept:

• In simpler form where:

Finite Element Method for Beam

A. ELEMENTARY BEAM THEORY (Euler- Bernoulli beam theory ):• Figure below (a) depicts a simply supported beam subjected to a general,

distributed, transverse load q(x ) assumed to be expressed in terms of force per unit length. The coordinate system is as shown with x representing the axial coordinate and y the transverse coordinate. The usual assumptions of elementary beam theory are applicable here:

1. The beam is loaded only in the y direction.

2. Deflections of the beam are small in comparison to the characteristic dimensions of the beam.

3. The material of the beam is linearly elastic, isotropic, and homogeneous.

4. The beam is prismatic and the cross section has an axis of symmetry in the plane of bending

Contd…

• Considering a differential length dx of a beam after bending as in Figure above b (with the curvature greatly exaggerated), it is intuitive that the top surface has decreased in length while the bottom surface has increased in length. Hence, there is a “layer” that must be un deformed during bending. Assuming that this layer is located distance from the center of curvature O and choosing this layer (which, recall, is known as the neutral surface) to correspond to y = 0, the length after bending at any position y is expressed as:

And the bending strain is then:

• From basic calculus, the radius of curvature of a planar curve is given by:

where v = v(x) represents the deflection curve of the neutral surface.• In keeping with small deflection theory, slopes are also small, so above is

approximated by:

Contd…

• such that the normal strain in the direction of the longitudinal axis as a result of bending is:

• And the corresponding normal stress is:

• where E is the modulus of elasticity of the beam material. Equation above shows that, at a given cross section, the normal stress varies linearly with distance from the neutral surface.

• As no net axial force is acting on the beam cross section, the resultant force of the stress distribution given by Equation above must be zero. Therefore, at any axial position x along the length, we have:

• Noting that at an arbitrary cross section the curvature is constant, Equation above gives:

Contd…

• The above equation is satisfied if the xz plane (y = 0) passes through the centroid of the area. Thus, we obtain the well-known result that the neutral surface is perpendicular to the plane of bending and passes through the centroid of the cross-sectional area.

• Similarly, the internal bending moment at a cross section must be equivalent to the resultant moment of the normal stress distribution, so

• The integral term in the above equation represents the moment of inertia (Iz ) of the cross sectional area about the z axis, so the bending moment expression becomes:

• Combining Equations of normal stress and bending moment, we obtain the normal stress equation for:

• In the above equation as per the sign convention compressive (negative) and tensile (positive) stress values are obtained correctly depending on the sign of the y location value.

B. Beam stiffness• A beam is a long, slender structural member generally subjected to transverse

loading that produces significant bending effects as opposed to twisting or axial effects.

• This bending deformation is measured as a transverse displacement and a rotation. Hence, the degrees of freedom considered per node are a transverse displacement and a rotation.

• Consider the beam element shown in Figure below. The beam is of length L with axial local coordinate and transverse local coordinate . The local transverse nodal displacements are given by ’s and the rotations by ’s. The local nodal forces are given by ’s and the bending moments by ’s as shown. We initially neglect all axial effects.

At all nodes, the following sign conventions are used:

1. Moments are positive in the counterclockwise direction.

2. Rotations are positive in the counterclockwise direction.

3. Forces are positive in the positive y direction.

4. Displacements are positive in the positive y direction.• Figure below indicates the sign conventions used in simple beam theory for positive

shear forces and bending moments .

The differential equation is derived as follows. Consider the beam shown in Figure below subjected to a distributed loading (force/length). From force and moment equilibrium of a differential element of the beam, shown in the next Figure (c),we have:

Beam Stiffness Matrix Based on Euler-Bernoulli Beam Theory

• (Considering Bending Deformations Only)• The differential equation is derived as follows. Consider the beam shown in

Figure (a) above subjected to a distributed loading (force/length). From force and moment equilibrium of a differential element of the beam, shown in Figure above (c), we have:

• Or, simplifying the above equation, we obtain

• The final form of the above equation relating the shear force to the bending moment, is obtained by dividing the left equation by and then taking the limit of the equation as approaches 0. The term then disappears.

Contd…• Also, the curvature k of the beam is related to the moment by

• where r is the radius of the deflected curve shown in Figure above (b), is the transverse displacement function in the direction, E is the modulus of elasticity, and I is the principal moment of inertia about theaxis.

• The curvature for small slopes: is given by:

Combining the above two equations: • Solving Eq. above for M and substituting this result into the shear force and the

distributed load equations found earlier we obtain.

• For constant EI and only nodal forces and moments, the above equation becomes:

• Now we will use the above equation to develop a stiffness matrix in direct method just like we did for trusses and bars.

1. Select the element type• Represent the beam by labeling nodes at each end and in general by labeling the

element number (figure).

2. Select a displacement function:Assume the transverse displacement variation through the element length to be:

• The complete cubic displacement function Eq. (above) is appropriate because there are four total degrees of freedom (a transverse displacement and a small rotation at each node). The cubic function also satisfies the basic beam differential equation—further justifying its selection. In addition, the cubic function also satisfies the conditions of displacement and slope continuity at nodes shared by two elements.

Contd..• Using the same procedure as described in FEM of bars, we express as a function

of the nodal degrees of freedom ,, , and as follows:

• where for the assumed small rotation Φ. Solving the above equations for a1 through a4 in terms of the nodal degrees of freedom and substituting into the transverse displacement equation we have:

Contd…

In matrix form:

• N1, N2, N3, and N4 are called the shape functions for a beam element. These cubic shape (or interpolation) functions are known as Hermite cubic interpolation (or cubic spline) functions.

Step 3 : Define the Strain/Displacement and Stress/Strain Relationships

• Assume the following axial strain/displacement relationship to be valid:

• we can relate the axial displacement to the transverse displacement by:

• where we should recall from elementary beam theory the basic assumption that cross sections of the beam (such as cross section ABCD) that are planar before bending deformation remain planar after deformation. and, in general, rotate through a small angle

•

Contd…

• From elementary beam theory, the bending moment and shear force are related to the transverse displacement function. Because we will use these relationships in the derivation of the beam element stiffness matrix, we now present them as

Step 4 Derive the Element Stiffness Matrix and Equations:• First, derive the element stiffness matrix and equations using a direct

equilibrium approach. We now relate the nodal and beam theory sign conventions for shear forces and bending moments.

Contd…

• Where the minus signs in the second and third of (the above equations) are the result of opposite nodal and beam theory positive bending moment conventions. In matrix form, the above equation becomes:

• Where the stiffness matrix is then:

Note :This method doesn’t include the effect of transverse axial shear deflections. • Castiglione's method for finding beam and frame deflections is a convenient

way to include the effects of the transverse shear term. E.g (Timoshenko beam)

Distributed Loading in Beams: • Beam members can support distributed loading as well as concentrated nodal

loading. Therefore, we must be able to account for distributed loading. Consider the fixed-fixed beam subjected to a uniformly distributed loading w shown in Figure below.

• Therefore, guided by the results from structural analysis for the case of a uniformly distributed load, we replace the load by concentrated nodal forces and moments tending to have the same effect on the beam as the actual distributed load.

Example Determining Equivalent Loads

• Goal is to ensure equivalent loads produce same strain energy• By making use of the work equivalence method:• This method is based on the concept that the work of the distributed load w() in

going through the displacement field v() is equal to the work done by nodal loads and in going through nodal displacements and for arbitrary nodal displacements.

24231211 )()()()()( xNvxNxNvxNxv

2324

22

333

2322

22

331

11)(

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)(

xL

xL

xN

xL

xL

xN

xxL

xL

xN

xL

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xN

2211

0

42

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0

21

0

11

0

24

0

23

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122

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)()()()(

)()()()(

)()()()(

)()(

Lv

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dxxNdxxNvdxxNdxxNvqW

dxxNqdxvxNqdxxNqdxvxNqW

dxxNvxNxNvxNqW

dxxvqqdxxvW

LLLL

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L

LL

Equivalent Loads (continued)

• In the same way one can use the work equivalence method to analyze other type of loadings.

2121 MvvFW

22112121 122

1

122

1 Lv

LvqLMvvFW

12

22qL

M

qLF

M

FF

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General Formulation:• In general, we can account for distributed loads or concentrated loads acting on

beam elements by starting with the following formulation application for a general structure:

• where F are the concentrated nodal forces and Fo are called the equivalent nodal forces,

• Assume that concentrated nodal forces are not present , as we are solving beam problems with distributed loading only.

• For a uniformly distributed load w acting over a one-element beam, we have:

Finite Element method for Frames: • Frames are used in Structures like buildings and bridges.• Frame is a combination of beam element in plane and trusses. • Consider a two dimensional arbitrarily oriented beam element:• Local displacements transformed in to global

From the figure we can see that only a displacement in the direction of local coordinate ŷ exists so we take only the second term in equation of the transformation matrix and expand it to a three degree of freedom equation at each node.

Contd…

• Where : is the transformation matrix for beam elements

Note:- Here the axial effect is not yet included. And that rotation is invariant with respect to either coordinate system. For example, rotation , and moment

Recall a stiffness matrix for beam element in elemental coordinate system was found earlier to be:

Contd…

• By using the general formula of stiffness matrix transformation in to global stiff ness;

• The general global stiffness matrix is:

• The above stiffness matrix is for beam element that includes shear and bending resistance. Local axial effects are not yet included.

FRAME FEM contd…

• To include the axial effect consider the following figure:

• The element now has three degrees of freedom per each node.• Recall for axial loading of on bars:

• Recall also the shear and bending Effect of beams:

Contd…

• Combining the axial loading, the shear and bending moment in local coordinate we have.

• The matrix in Eq. above now has three degrees of freedom per node and it includes axial effects (in the direction), as well as shear force effects (in the direction) and principal bending moment effects (about the axis).

Contd..

• Next step is to relate the local displacement to the global by the transformation matrix with 3 degrees of freedom at each node.

Contd…

• By using the general formula of stiffness matrix transformation in to global stiff ness; which is:

• we obtain the general transformed global stiffness matrix for a beam element that includes axial force, shear force, and bending moment effects as follows:

CHAPTER THREE:

Introduction To FEM for Two Dimensional Finite Elements

• 2D planar elements are thin-plate elements.• 2D coordinates define a position on the 2D element surface.

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• Once the nodal displacements are determined these stress can be evaluated directly

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Contd..3.2 Rectangular plane stress Element:

Two advantages of the rectangular element over the triangular element are :• Ease of data input and • simpler interpretation of output stresses.

A disadvantage of the rectangular element is that the • simple linear-displacement rectangle with its associated straight sides poorly

approximates the real boundary condition edges.

Step 1: Select Element Type• Consider the rectangular element shown in Figure below (all interior angles are

90o)with corner nodes 1–4 (again labeled counterclockwise) and base and height dimensions2b and 2h, respectively.

Contd…

Step 2 Select Displacement FunctionsWe then select the linear displacement functions as:

We can proceed in the usual manner to eliminate the ai’s

Contd…

In terms of the shape functions and unknown nodal displacements:

And the Ni’s are again such that N1 = 1 at node 1 and N1 = 0 at all the other nodes, with similar requirements for the other shape functions. In expanded form,

Contd…

Step 3: Define the Strain/Displacement and Stress/Strain Relationships

Again the element strains for the two-dimensional stress state are given by

• Using the interpolation function Eq. (in the above equation) and taking the derivatives of u and v as indicated, we can express the strains in terms of the unknown nodal displacements as

Contd…

• From the above two equations we observe that εx is a function of y, εy is a function of x, and γxy is a function of both x and y. The stresses are again given by the formulas in chapter 3, where [B] is now that of the above equation and {dg} is that of Eq..

Step 4 : Derive the Element Stiffness Matrix and Equations

• with [D]again given by the usual plane stress or plane strain conditions, of the equation in chapter. Because the [B] matrix is a function of x and y, integration must be performed. The [k] matrix for the rectangular element is now of order 8x 8. The element force matrix is determined by:

Contd…

where [N] is the rectangular matrix of the shape functions N1 through N4. The element equations are then given by:

Steps 5–7• Steps 5–7, which involve assembling the global stiffness matrix and equations,

determining the unknown nodal displacements, and calculating the stress, are identical to those in chapter 3 for the Constant strain triangular element.

Chapter Four

Introduction to FEM for Three- Dimensional Elements

4.1 Three-Dimensional Stress and StrainWe begin by considering the three-dimensional infinitesimal element in Cartesian coordinates with dimensions dx; dy, and dz and normal and shear stresses as shown in Figure below.

From moment equilibrium of the element,• Hence, there are only three independent shear stresses, along with the three normal

stresses

• The element strain/displacement relationships are:

• where u; v, and w are the displacements associated with the x; y, and z directions. The shear strains γ are now given by

• We again represent the stresses and strains by column matrices as

Contd…

• The stress/strain relationships for an isotropic material are again given by

• the constitutive matrix [D]

4.2 : Tetrahedral Element

Step 1 Select Element Type• Consider the tetrahedral element shown in Figure below with corner nodes 1–4.

This element is a four-noded solid.• The nodes of the element must be numbered such that when viewed from the

last node (say, node 4), the first three nodes are numbered in a counterclockwise manner, such as 1, 2, 3, 4 or 2, 3, 1, 4. This ordering of nodes avoids the calculation of negative volumes and is consistent with the counterclockwise node numbering associated with the CST element in the previous chapter.

Contd…

• The unknown nodal displacements are now given by:• Hence, there are 3 degrees of freedom per node, • or 12 total degrees of freedom per element.

Step 2: Select Displacement Functions• The element displacement functions u; v, and w must be linear along each edge

• In the same manner as in 2D, we can express the ai’s in terms of the known nodal coordinates (x1, y1, z1,. . . , z4) and the unknown nodal displacements (u1, v1, w1,. . . w4) of the element. Skipping the straight forward but tedious details, we obtain

Contd..

• where 6V is obtained by evaluating the determinant

and V represents the volume of the tetrahedron. The coefficients α1,β1, γ1, and

δi (i=1, 2, 3,4) are given by:

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Expressions for v and w are obtained by simply substituting vi’s for all ui’s and then

wi’s for all ui’s.

The displacement expression for u with similar expressions for v and w, can be written equivalently in expanded form in terms of the shape functions and unknown nodal displacements as

Contd…

Step 3 : Define the Strain / Displacement and Stress / Strain Relationships

The element strains for the three-dimensional stress state are given by:

Contd…

where, again, the comma after the subscript indicates differentiation with respect to

the variable that follows. Sub matrices B2; B3, and B4 are defined by simply indexing the subscript in the above equation from 1 to 2, 3, and then 4, respectively. Substituting the shape functions, B1 is expressed as:

with similar expressions for B2; B3, and B4.

The element stresses are related to the element strains by

Contd…

Step 4: Derive the Element Stiffness Matrix and Equations

Because both matrices [B] and [D] are constant for the simple tetrahedral element

• where, again, V is the volume of the element. The element stiffness matrix is now of order 12 x 12.

Body Forces:The element body force matrix is given by

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For constant body forces, the nodal components of the total resultant body forces can be shown to be distributed to the nodes in four equal parts. That is,

The element body force is then a 12 x 1 matrix

Surface Forces:Again, the surface forces are given by:

• For example, consider the case of uniform pressure p acting on the face with nodes 1–3; The resulting nodal forces become:

Contd….

• Simplifying and integrating:

4.3: Rectangular Brick element

Isoparametric Formulation:(Linear Hexahedral Element)The basic (linear) hexahedral element [Figure below] now has eight corner nodes with Isoparametric natural coordinates given by s; t, and z’ as shown in the Figure. The element faces are now defined by s; t; z’= ±1. (We use s; t, and z’ for the coordinate axes because they are probably simpler to use than Greek letters ξ, η and ζ).

Contd…

The function use to describe the element geometry for x in terms of the generalized degrees of freedom ai’s is:

• The same form as the above equation is used for y and z as well. Just start with a9 through a16 for y and a17 through a24 for z.

• Now , we expand equation used in 2D to include the z coordinate as follows:

Contd…

when, s1 = 1, t1 = 1, and z’1 = +1 are used in the above equation we obtain:

Explicit forms of the other shape functions follow similarly:

• For instance, when we let i = 8 and substitute s8 = 1, t8 = 1, z’8= 1 into the equation for N8, we obtain:

Then evaluating all shape functions at node 8, we obtain N8 = 1, and all other shape functions equal zero at node 8. we see that N1 = 0 when s = 1or when t = 1

CHAPTER 5: FEM for Heat Transfer Problems

5.1: One-Dimensional Heat Conduction (without Convection): By conservation of energy, we have:

Where:

Ein : is the energy entering the control volume, in units of joules (J)

∆U: is the change in stored energy, in units of kW h (kWh)

qx: is the heat conducted (heat flux) into the control volume at surface

edge x, in units of kW/m2

qx+dx : is the heat conducted out of the control volume at the surface edge x+dx.

t is time, in h or s.

Q : is the internal heat source (heat generated per unit time per unit volume is positive), in kW/m3; (a heat sink, heat drawn out of the volume, is negative).A: is the cross-sectional area perpendicular to heat flow q, in m2

By Fourier’s law of heat conduction,

where

Kxx: is the thermal conductivity in the x direction, in kW/(m0C).

T: is the temperature, in 0 C.

• By Taylor series expansion, for any general function f (x), we have

• Therefore, using a two-term Taylor series:

• The change in stored energy can be expressed by

Contd…

where c is the specific heat in kW h/(kg 0 C)), and ρ is the mass density in kg/m3.

• simplifying, we have the one-dimensional heat conduction equation as:

• For steady state, any differentiation with respect to time is equal to zero:

• For constant thermal conductivity and steady state:

• The boundary conditions are of the form

Heat Transfer with Convection

• for conservation of energy, we have:

• All terms have the same meaning as in previous, except the heat flow by convective heat transfer is given by Newton’s law of cooling:

• h is the heat-transfer or convection coefficient, in kW/(m2 0C) • T is the temperature of the solid surface at the solid/fluid interface.• T is the temperature of the fluid (here the free-stream fluid temperature).• P: denotes the perimeter around the constant cross-sectional area A.

Contd…

• simplifying, we obtain the equation for one-dimensional heat conduction with convection as:

• we have:

As a boundary condition for the problem of heat conduction with convection.

One dimensional FEM formulation of heat transfer using variational method

• Step 1 Select Element Type:

• Step 2 Choose a Temperature Function:

• are again the same shape functions as used for the bar element. The [N] matrix is then given by

Contd…

• In matrix form, we express:

Step 3 : Define the Temperature Gradient/Temperature and Heat Flux/Temperature Gradient Relationships

The temperature gradient matrix {g}, analogous to the strain matrix {ε}, is given by

The heat flux/temperature gradient relationship is given by:

where the material property matrix is now given by:

Step 4 Derive the Element Conduction Matrix and Equations:

• Substituting the equation derived earlier

• using the fact that the nodal temperatures {t} are independent of the general coordinates x and y and can therefore be taken outside the integrals, we have

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• The minimization is most easily accomplished by explicitly writing the surface integral S3 with {t}left inside the integral as shown:

• where the last term (hT)is a constant that drops out while minimizing . Simplifying:

Contd…

• The first term {fQ}(heat source positive, sink negative) is of the same form as the body-force term, and the second term {fq}(heat flux, positive into the surface) and third term {fh}(heat transfer or convection) are similar to surface

tractions (distributed loading) in the stress analysis problem.• we have the element conduction matrix* for the heat-transfer problem:

• where the first and second integrals are the contributions of conduction and convection, respectively. for each element, we have:

• The conduction part of the [k] matrix for the one-dimensional element becomes:

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• P is the perimeter of the element (assumed to be constant)• we find that the [k] matrix is:

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• When h is zero on the boundary of an element, the second term on the right side (convection portion of [k]) is zero. This corresponds, for instance, to an insulated boundary.

• The force matrix terms, on simplifying and assuming Q, q, and product hT to be constant are:

• Therefore, adding all terms, we obtain:

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• The equation above indicates that one-half of the assumed uniform heat source Q goes to each node, one-half of the prescribed uniform heat flux q (positive q enters the body) goes to each node, and one-half of the convection from the perimeter surface hT goes to each node of an element.

• Finally, we must consider the convection from the free end of an element. For simplicity’s sake, we will assume convection occurs only from the right end of the element, as shown:

• The additional convection term contribution to the stiffness matrix is given by:

• Now N1 = 0 and N2 =1 at the right end of the element. Substituting the N’s

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• The convection force from the free end of the element is evaluated with the shape functions now evaluated at the right end (where convection occurs) and with S3 (the surface over which convection occurs) now equal to the cross-sectional area A of the rod. Hence,

• represents the convective force from the right end of an element where N1(x=L) represents N1 evaluated at x=L, and so on.

Step 5 : Assemble the Element Equations to Obtain the Global Equations and Introduce Boundary Conditions

Contd…

Step 6 :Solve for the Nodal Temperatures

Step 7 : Solve for the Element Temperature Gradients and Heat Fluxes