Tangent Stiffness Matrix

very rapidly.

23 It should be noted that each iteration involves three computationally expensive steps:

1. Evaluation of internal forces fmt (or reactions)

2. Evaluation of the global tangent stiffness matrix Kt

3. Solution of a system of linear equations

16.2.1.2 Modified Newton-Raphson

24 This method is essentially the same as the Newton-Raphson however in Eq. 16.23 (K^) is replaced by Kt which is the tangent stiffness matrix of the first iteration of either 1) the first increment Kt = Ky0, Fig. 16.4, or 2) current increment, Fig. 16.3 Kt = Kyn Fig. 16.3

Nonlinear Fem Iteration And Increment
Figure 16.3: Modified Newton-Raphson Method, Initial Tangent in Increment

25 In general the cpu time required for the extra iterations required by this method is less than the one saved by the assembly and decomposition of the stiffness matrix for each iteration.

26 It should be mentioned that the tangent stiffness matrix does not necessarily have to be the true tangent stiffness matrix; an approximation of the true tangent stiffness matrix or even the initial stiffness matrix will generally produce satisfactory results, albeit at the cost of additional iterations.

16.2.1.3 Secant Newton

27 This method is a compromise between the first two. First we seek two displacements by two cycles of modified Newton-Raphson, then a secant to the curve is established between those

Newton Raphson
Figure 16.4: Modified Newton-Raphson Method, Initial Problem Tangent

two points, and a step taken along it, Fig. 16.5.

28 Subsequently, each step will be taken along a secant connecting the previous two points. Hence, starting with

the secant slope can be determined and then

du,2

29 This process can be generalized to (KS)

16.2.2 Acceleration of Convergence, Line Search Method Adapted from (Reich 1993)

30 The line search is an iterative technique for automatically under- or over-relaxing the displacement corrections so as to accelerate the convergence of nonlinear solution algorithms.

Incremental Secant
Figure 16.5: Incremental Secant, Quasi-Newton Method

3i The amount of under- or over-relaxation is determined by enforcing an orthogonality condi-t.ion between the displacement, corrections and the residual loads TZ , which amounts to forcing the iterative change in energy to be zero.

32 The displacement corrections are multiplied by a scalar value sk defining the amount of under- or over-relaxation such that, the total displacements W+l,k are defined as

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Responses

  • flavia
    What is tangent stiffness matrix?
    8 years ago
  • niko
    How to compute the tangent stiffness matrix?
    1 year ago
  • Katrin Seiler
    Why tangent stiffness is required?
    1 year ago

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