Isoparametric Elements Element Stiffness Matrices Structural Mechanics Displacement-based Formulations

General Approach – Specific Example We will look at manipulation of the mechanics quantities (displacement, strain, stress) using shape functions The approach is quite general, and is used to formulate a number of different elements We will use a specific example to make the development more concrete (Q4) We will start from the nodal displacement representation, work toward strain and stress, and finally element stiffness There is a lot going on here, pay attention to both the overall themes and the detailed steps …

Master Element Mapping Note: we will use a for x and b for h because I can’t remember, pronounce, or legibly write “xi” and “eta” master element actual element

Bilinear Quadrilateral (Q4) Interpolation involves the summation of nodal values multiplied by corresponding shapes functions geometry interpolation field variable interpolation - where - nodal coordinates nodal displacements shape functions

Q4 - Displacements Start with the element displacement field We have to give it some functional form in order to work with it Let it be defined over the element by our interpolation scheme {u} = displacements continuously defined (all components) over an element [N] = the element shape functions in master element coordinates {d} = the nodal (discrete) displacement values

Strain from {u} Now calculate element strains from the displacement field This is just the usual strain-displacement relationship written in compact form with an operator matrix

Q4 – Strain from {d} Let’s now work toward an expression for element strain We have a bit of a difficulty here with direct substitution The shape functions (N1, N2, N3, N4) are defined in terms of the master element coordinates (a,b) But we need to differentiate in terms of the global coordinates (x,y) this operation cannot be done directly - or -

Coordinate Transformation Given any function of the master element coordinates (a,b): We can find derivatives with respect to global (x,y) by using the chain rule: We can combine and rearrange these relationships to get our derivatives:

The Jacobian The Jacobian matrix is an important part of element formulation: For the Q4 element this becomes: note the Jacobian matrix is a function of location within the master element local coordinate derivatives of the shape functions global coordinate locations of the element nodes

Jacobian Interpretation The Jacobian contains information about element size and shape The Jacobian determinant (j) is a scaling factor that relates the differential area of the actual element to the differential area of the master element The Jacobian inverse (G) relates global coordinate system (x,y) function derivatives to master element coordinate system (a,b) function derivatives

Intra-Element Jacobian Variation Here is a single Q4 element (highly-distorted, not recommended) Notice how sub-region size and distortion varies within the element The Jacobian captures local area and distortion differences large area high distortion small area low distortion

Jacobian (determinant) Ratio This is one measure of element quality (which affects element accuracy) Ratio of the highest to lowest quadrature point Jacobian determinant It is 1.0 for any square or rectangular element (same j throughout element) It increases as element distortion increases

Strain/Displacement for Q4 Start with the usual strain-displacement relationship in a slightly different form: Now add the Jacobian approach to master/global coordinate derivative transformation:

Strain/Displacement cont. Now represent the displacement field master element derivatives in terms of the shape functions:

All Together Now … - or - organization shape function derivatives, master coordinates - or - Jacobian inverse terms, master to global coordinate transformation nodal displacements, global coordinates

Stress If we have strain, we can get to stress by bringing in material properties We have to be a little careful here, this simple expression assumes: No initial (residual, assembly) stresses present Linear elastic behavior The general form above does accommodate anisotropic behavior If we further limit ourselves to 2D, isotropic, plane stress, we can write:

Element Stiffness Matrix Recall where the element stiffness matrix fits into the finite element formulation: Take it as a given for the present that the element stiffness matrix [k] is: An integral over the element area in global coordinates (t = thickness) Why is integration required? Think about what [k] does For displacements applied to the element nodes, it determines the required force If one element is larger than another, the force required ought to be greater for the same nodal displacements If an element has a rotated orientation, a coordinate axis displacement can produce forces with multiple coordinate components

Integration in Master Coordinates It is not easy to integrate for the terms in [k] using the global coordinate system (elements are generally distorted and not aligned with global axes) But we can do this instead (matrix dimensions for a Q4 element): Integrate over the master element It is undistorted and aligned with the coordinate system Adjust for the change in coordinates by bringing in the Jacobian determinant j

Quadrature Read “quadrature” as “numerical integration” Why do we want to numerically integrate to establish [k]? To integrate directly is still computationally expensive, even with the change to local coordinates Quadrature involves sampling at discrete points, multiplying by a weighting factor, and summing to get an estimate of the integral this varies point-by-point too … these contain Jacobian inverse terms which vary point-by-point within the element

Gauss Points Gauss quadrature is a method of numerical integration that has optimal characteristics when the underlying functions have polynomial form The figure shows Gauss points for 2nd order and 3rd order quadrature For (a), all four points have a weight of 1.0 (total = 4.0) For (b): 1,3,7,9 weight = .3086; 2,4,6,8 weight = .4938; 5 weight = .7901 (total = 4.0) Note: the quadrature rule is independent of element order (Q4, Q8, Q9)

Element Distortion One of the reasons a distorted element is less than ideal: The integral is estimated by discrete sampling at specific locations within the element If the element is not distorted, the sampled points are highly representative of the un-sampled near by regions of the element If the element is highly distorted, the sampled points are not representative of the un-sampled regions of the element

Element Normal Vectors If you get “inside out element” errors Verify-Element-Normals as a fringe or vector plot (rotate the model to see the vector orientation)

Element Normal Vectors Use Modify-Element-Reverse to get them all going in the same (positive Z, I think, check this) direction