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Path following methods Prof. Dr. Eleni Chatzi, Dr. Konstantinos Agathos Lecture 4 - 10 October, 2019 Institute of Structural Engineering, ETH Z¨ urich October 11, 2019 Institute of Structural Engineering Method of Finite Elements II 1

# Prof. Dr. Eleni Chatzi, Dr. Konstantinos AgathosPath following methods Prof. Dr. Eleni Chatzi, Dr. Konstantinos Agathos Lecture 4 - 10 October, 2019 Institute of Structural Engineering,

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### Text of Prof. Dr. Eleni Chatzi, Dr. Konstantinos AgathosPath following methods Prof. Dr. Eleni Chatzi, Dr....

Path following methods

Prof. Dr. Eleni Chatzi, Dr. Konstantinos Agathos

Lecture 4 - 10 October, 2019

Institute of Structural Engineering, ETH Zurich

October 11, 2019

Institute of Structural Engineering Method of Finite Elements II 1

Outline

1 Introduction

2 The Newton-Raphson method

3 The Newton-Raphson method in structural mechanics

4 Path-following Methods

Institute of Structural Engineering Method of Finite Elements II 2

Learnig goals

Understanding the general solution process for nonlinearproblems

Understanding limitations of the process

Gaining a basic understanding of path following solutionmethods

Institute of Structural Engineering Method of Finite Elements II 3

Significance of the lecture

Applications include any problem described by nonlinear PDEs, suchas:

Structural mechanics problems involving:

Geometrical nonlinearities

Material nonlinearities

Material failure

Fluid mechanics

Institute of Structural Engineering Method of Finite Elements II 4

The Newton-Raphson method in 1-D

ProblemGiven the function:

f (x) : R→ R

find:

x : f (x) = 0

where:f (x) is a general nonlinear function

The derivative of f (x) is available

Institute of Structural Engineering Method of Finite Elements II 5

The Newton-Raphson method in 1-D

Solution:

If an initial estimate x0 is available, the function can be locallyapproximated by a truncated Taylor series:

f (x) ≈ f (x0) + dfdx |x0 (x − x0) = f (x0) + f ′ (x0) ∆x0

Then this equation can be solved to obtain a better approximationto the solution:

∆x0 = −(f ′ (x0)

)−1 f (x0) → x1 = x0 + ∆x0

Institute of Structural Engineering Method of Finite Elements II 6

The Newton-Raphson method in 1-D

The process can be repeated iteratively until a sufficient accuracy isobtained:

∆xi = −(f ′ (xi )

)−1 f (xi ) → xi+1 = xi + ∆xi

The absolute value of f (xi+1) can be used to determine theachieved level of accuracy

Alternatively the difference between two consecutive solutions(|xi+1 − xi |) could be used

Institute of Structural Engineering Method of Finite Elements II 7

The Newton-Raphson method in 1-D

The whole solution can be written as an algorithm:

Data: f (x),f ′ (x), x0, tol , maxitResult: x

for i ≤ maxit do∆xi = −

(dfdx |xi

)−1f (xi )

xi+1 = xi + ∆xiif |f | ≤ tol then

returnend

endreturn

Algorithm 1: Newton-Raphson algorithm

Institute of Structural Engineering Method of Finite Elements II 8

The Newton-Raphson method in 1-D

But how do we know that the method will eventually converge tothe correct value?

It can be shown that: (x − xi+1) ≤ C (x − xi )2

Provided that:

The initial estimate x0 is close enough to the solution x

f is sufficiently smooth

Institute of Structural Engineering Method of Finite Elements II 9

The Newton-Raphson method in n-DThe method can be extended to vector valued functions:

f (x) : Rn → Rn

and systems of nonlinear equations:

f (x) = 0

By considering that:

f (x) ≈ f(xi)

+ ∂f∂x |xi ∆xi

∆xi = −(∂f∂x |xi

)−1f(xi)

xi+1 = xi + ∆xi

For the n-D version of the method, we will use superscripts to indicate theiteration i and we will drop the index for the increment to simplify notation.

Institute of Structural Engineering Method of Finite Elements II 10

The Newton-Raphson method in n-DOr component wise:

f1 (x)f2 (x)

...fn (x)

n×1

f1(xi)

f2(xi)...

fn(xi)

n×1

+

∂f1(xi)∂x1

∂f1(xi)∂x2

· · · ∂f1(xi)∂xn

∂f2(xi)∂x1

∂f2(xi)∂x2

· · · ∂f2(xi)∂xn...

... . . . ...∂fn(xi)

∂x1

∂fn(xi)∂x2

· · · ∂fn(xi)∂xn

n×n

∆x1∆x2

...∆xn

n×1

x i+1

1x i+1

2...

x i+1n

n×1

=

x i

1x i

2...

x in

n×1

∂f1(xi)∂x1

∂f1(xi)∂x2

· · · ∂f1(xi)∂xn

∂f2(xi)∂x1

∂f2(xi)∂x2

· · · ∂f2(xi)∂xn...

... . . . ...∂fn(xi)

∂x1

∂fn(xi)∂x2

· · · ∂fn(xi)∂xn

−1

n×n

f1(xi)

f2(xi)...

fn(xi)

n×1

Institute of Structural Engineering Method of Finite Elements II 11

The Newton-Raphson method

We recall the linearized equilibrium equation:

KT∆un = −Ri = fext − finti

Typically external loads are scaled by a factor λ:

KT∆un = −Ri = λfext − finti

Institute of Structural Engineering Method of Finite Elements II 12

The Newton-Raphson method

For a given value of λ, the above corresponds to a Newton-Raphsoniteration where:

R = Ri + ∂Ri

∂un ∆un = finti − λfext + KT∆un = 0

The iteration is repeated until

|R| <= tol

where tol is the tolerance.

Institute of Structural Engineering Method of Finite Elements II 13

The Newton-Raphson method

Typically:

Different (increasing) values are given to the load factor

Displacements are obtained using the Newton-Raphson method

The load-displacement curve of the structure can be obtained

Institute of Structural Engineering Method of Finite Elements II 14

The Newton-Raphson method

The above procedure is called load control and can be illustrated asfollows:

Solve withNewton-Raphson

Repeat for nextincrement

Institute of Structural Engineering Method of Finite Elements II 15

The Newton-Raphson method

The above procedure is called load control and can be illustrated asfollows:

Solve withNewton-Raphson

Repeat for nextincrement

Institute of Structural Engineering Method of Finite Elements II 15

The Newton-Raphson method

The above procedure is called load control and can be illustrated asfollows:

Solve withNewton-Raphson

Repeat for nextincrement

Institute of Structural Engineering Method of Finite Elements II 15

The Newton-Raphson method

The above procedure is called load control and can be illustrated asfollows:

Solve withNewton-Raphson

Repeat for nextincrement

Institute of Structural Engineering Method of Finite Elements II 15

The Newton-Raphson method

The above procedure is called load control and can be illustrated asfollows:

Solve withNewton-Raphson

Repeat for nextincrement

Institute of Structural Engineering Method of Finite Elements II 15

The Newton-Raphson method

The above procedure is called load control and can be illustrated asfollows:

Solve withNewton-Raphson

Repeat for nextincrement

Institute of Structural Engineering Method of Finite Elements II 15

The Newton-Raphson method

In many cases the solution path is not monotonic

Phenomena such as snap-through and snap-back can be present

The Newton-Raphson method will either fail or not provide thefull path in those cases

Such phenomena are often of interest in practice e.g.when theovercritical behavior of a structure needs to be known

Institute of Structural Engineering Method of Finite Elements II 16

Example

Solution path involving snap-through:

Institute of Structural Engineering Method of Finite Elements II 17

Example

Solution path involving snap-through:

Institute of Structural Engineering Method of Finite Elements II 17

Example

Solution path involving snap-through:

Institute of Structural Engineering Method of Finite Elements II 17

Example

Solution path involving snap-through:

Institute of Structural Engineering Method of Finite Elements II 17

Path-following methods

In this category of methods:

It is attempted to follow the full solution path includingsnap-back and snap-through phenomena

The load factor is included as an additional unknown in thenonlinear system of equations

The choice of this constraint leads to different methods

Institute of Structural Engineering Method of Finite Elements II 18

Path-following methods

After adding ∆λ as an unknown and the constraint as an equationthe new system of equations is obtained:[

R (u, λ)g (u, λ)

]=[

00

]where:

u is the vector of nodal unknownsλ is the load factorR is the residual of the Newton-Raphson methodg is the additional constraint

Institute of Structural Engineering Method of Finite Elements II 19

Path-following methods

The above system can be solved by employing the Newton-Raphsonmethod. The linearization of the system yields: R

(ui, λi

)+∂R

(ui, λi

)∂u ∆u + ∂R

(ui , λi)∂λ

∆λ

g(ui , λi)+ ∂g

(ui , λi)∂u ∆u + ∂g

(ui , λi)∂λ

∆λ

=[

00

]

where:∆u is the nodal displacement increment∆λ is the load factor increment

Institute of Structural Engineering Method of Finite Elements II 20

Path-following methods

The above system can be solved by employing the Newton-Raphsonmethod. The linearization of the system yields:[

KT −fexthT s

] [∆u∆λ

]=[−Ri

−g i

]

where:∆u is the nodal displacement increment∆λ is the load factor increment

h Is the gradient of g with respect to u: h = ∂g∂u

s is the derivative of g with respect to λ: s = ∂g∂λ

Institute of Structural Engineering Method of Finite Elements II 20

Path-following methods

The coefficient matrix in this system is not symmetric.

To exploit the symmetry and sparsity of KT the followingpartitioning procedure is employed:

∆uI = KT−1fext, ∆uII = −KT

−1Ri

∆λ = −g i + hT ∆uII

s + hT ∆uI , ∆u = ∆λ∆uI + ∆uII

Institute of Structural Engineering Method of Finite Elements II 21

Path-following methods

The choice of the additional constraint g is crucial

Different alternatives exist

In some cases problem specific constraints are needed

Some widely used alternatives are demonstrated in the following

Institute of Structural Engineering Method of Finite Elements II 22

Setting g = λ− λ results in load control:

g = λ− λ ⇒ h = ∂g∂u = 0, s = ∂g

∂λ= 1

g i = λi − λ

[KT −fext0 1

] [∆u∆λ

]=[−Ri

λ− λi

]

∆λ = λ− λi , ∆u = KT−1(λfext − fint

i)

Institute of Structural Engineering Method of Finite Elements II 23

Path-following methods: Displacement control

Setting g = T · u− u results in displacement control:

g = T · u− u ⇒ h = ∂g∂u = TT , s = ∂g

∂λ= 0

g i = T · ui − u

[KT −fextT 0

] [∆u∆λ

]=[

−Ri

u − T · ui

]

In the above T =[

0 0 . . . 1 . . . 0 0]

where the entry 1corresponds to a selected dof

Institute of Structural Engineering Method of Finite Elements II 24

Path-following methods: Displacement control

Displacement control in a pathinvolving snap-through

Displacement control in a pathinvolving snap-back

Institute of Structural Engineering Method of Finite Elements II 25

Path-following methods: Displacement control

Displacement control in a pathinvolving snap-through

Displacement control in a pathinvolving snap-back

Institute of Structural Engineering Method of Finite Elements II 25

Path-following methods: Displacement control

Displacement control in a pathinvolving snap-through

Displacement control in a pathinvolving snap-back

Institute of Structural Engineering Method of Finite Elements II 25

Path-following methods: Arc-length

Setting g =√

(u− u0)T · (u− u0) + (λ− λ0)2 −∆s results in arclength control (Crisfield 1981):

g i =√

(ui − u0)T · (ui − u0) + (λi − λ0)2 −∆s

h = ∂g∂u =

(ui − u0)

g , s = ∂g∂λ

=(λi − λ0)

g

KT −fext(ui − u0)T

g

(λi − λ0)

g

[ ∆u∆λ

]=[

Ri

−g i

]

In the above, superscript 0 refers to the beginning of the step

Institute of Structural Engineering Method of Finite Elements II 26

Path-following methods: Arc-length

We observe that for i = 0 the system becomes:

KT −fext(u0 − u0)T

g

(λ0 − λ0)

g

[ ∆u∆λ

]=[−Ri

−g i

]

Institute of Structural Engineering Method of Finite Elements II 27

Path-following methods: Arc-length

We observe that for i = 0 the system becomes:

[KT −fext0 0

] [∆u∆λ

]=[−Ri

−g i

]

Institute of Structural Engineering Method of Finite Elements II 27

Path-following methods: Arc-length

We observe that for i = 0 the system becomes:

[KT −fext0 0

] [∆u∆λ

]=[−Ri

−g i

]

The coefficient matrix is singular!

To obtain the load factor at the first iteration a predictor step isintroduced.

Institute of Structural Engineering Method of Finite Elements II 27

Path-following methods: Arc-length

For the predictor step the system is solved for the external loads:

∆up = KT−1fext

Then the increment of the load factor is computed as:

∆λp = ± ∆s‖∆up‖

Institute of Structural Engineering Method of Finite Elements II 28

Path-following methods: Arc-length

The sign of the increment is determined by the current stiffnessparameter:

κ = fextT ∆u

∆uT ∆u

Institute of Structural Engineering Method of Finite Elements II 29

Path-following methods: Arc-length

Setting g = (∆up)T (u− u1)+ ∆λp (λ− λ1) results in analternative method for arc length control (Riks 1972):

g i = (∆up)T(ui − u1

)+ ∆λp

(λi − λ1

)

h = ∂g∂u = ∆up, s = ∂g

∂λ= ∆λp

[KT −fext

(∆up)T ∆λp

] [∆u∆λ

]=[−Ri

−g i

]

In the above, superscript 1 refers to the predictor step

Institute of Structural Engineering Method of Finite Elements II 30

Path-following methods: Arc-length

Illustration of arc-length methods:

Crisfield: Riks:

Institute of Structural Engineering Method of Finite Elements II 31

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