Computational Fracture Mechanics

Anderson’s book, third ed. , chap 12

Energy domain integral method:

- Formulated by Shih et al. (1986):

- Generalized definition of the J- integral (nonlinear materials, thermal strain, dynamic effects).

- Relatively simple to implement numerically, very efficient.

Elements of Theory

Finite element (FE) code ABAQUS version 6.5

CF Shih, B. Moran and T. Nakamura, “Energy release rate along a three-dimensional crack front in a thermally stressed body”, International Journal of Fracture 30 (1986), pp. 79-102

ABAQUS: - suite of powerful engineering simulation programs

- based on the finite element method

- for simple linear analyses and most challenging nonlinear simulations

Abaqus 6.5 :

creates input files (.inp) that will be processed by Abaqus standard.

products associated with Abaqus:

For details see the Getting Started Manual of Abaqus 6.5

can be used for producing/ importing the geometry to be analyzed.

is useful to monitor/control the analysis jobs and display the results (Viewer).

Abaqus Standard : general-purpose analysis product that can treat a wide range of problems.

CAE : interactive, graphical environment allowing models to be created quickly.

Optional capabilities (offshore structures, design sensitivity calculations)

Abaqus Explicit : intended for modeling brief, transient dynamic events (impact) uses an explicit dynamic finite element formulation.

In 2D, under quasistatic conditions, J may be expressed by

and,

The contour surrounds the crack tip.

The limit indicates that shrinks onto the crack tip.

For details see the Theory Manual of Abaqus 6.5, section 2.16

n : unit outward normal to

q : unit vector in the virtual crack extension direction.

w : strain energy density

displacement gradient tensor

x1 , x2 Cartesian system

0n H qTJ lim ds

H I u σTw

Energy Domain Integral :

H : Eshelby’s elastic energy-momentum tensor (for a non-linear elastic solid)

: Cauchy stress tensor

With q along x1 and the field quantities expressed in Cartesian components, i.e.

10 1

jij i

uJ lim wn n ds

x

The expression of J (see eq. 6.45) is recovered

1

0q

In indexed form, we obtain

1 1

1 211 12

21 22 2 2

1 2

0

0HT

u u

x xw

w u u

x x

1

2n

n

n

1 1

1 211 121 2 1 2

21 22 2 2

1 2

0 1 1

0 0 0

u u

x xwn n n n

w u u

x x

n H qT

Thus,

1 2n ds dx dy with

The previous equation is not suitable for a numerical analysis of J.

Transformation into a domain integral

Following Shih et al. (1986),

is a sufficiently smooth weighting function in the domain A.

on

on C

qq

0

m = -n on

A includes the crack-tip region as 0

m : outward normal on the closed contour C C C

q qiq xNote that,

m H q t u qT

C C C C C

ds ds

A

: the surface traction on the crack faces.t σ m

0n H qTJ lim ds

1q

1

0i

on

q x on C

otherwise arbitrary

with

(*)

m H q m I u σ qTT T

C C C C

ds w ds

m I σ u qC C

w ds

σ σT since

m q m σ u qC C C C

w ds ds

m σ u qC C

ds

σ m u qT

C C

ds

Derivation of the integral expression

0

t u qC C

ds

t σ msince

(*)

m H q m H q m H q m H q m H qT T T T T

C C C C C

J ds ds ds ds ds

Noting that,

=

0

=

q

→ Line integral along the closed contour enclosing the region A.C C C

Using the divergence theorem, the contour integral is converted into the domain integral

H q t u qT

A C C

J div dA ds

A C C

J dA ds

H : q h q t u q

Tdiv div A a A a + A: a

Under certain circumstances, H is divergence free, i.e. , 0ki iH

indicates the path independence of the J-integral.

In the general case of thermo-mechanical loading and with body forces and crack face tractions:

, 0ki iH

the J-integral is only defined by the limiting contour 0

,k ki ih HorIntroducing then the vector, divh = H in A

Using next the relationship,

Contributions due to crack face tractions.

- This integral is evaluated using ring elements surrounding the crack tip.

- Different contours are created:

In Abaqus:

First contour (1) = elements directly connected to crack-tip nodes.

The second contour (2) are elements sharing nodes with the first, … etc

8-node quadratic plane strain element (CPE8)

12

Refined meshContour (i)

Crack

1q

0q nodes outside

nodes inside

q

0 1 qException: on midside nodes (if they exist) in the outer ring of elements

J-integral in three dimensions

Local orthogonal Cartesian coordinates at the point s on the crack front:

0

n H qTJ s lim d

Point-wise value

J defined in the x1- x2 plane crack front at s

For a virtual crack advance (s) in the plane of a 3D crack,

L : length of the crack front under consideration.

: surface element on a vanishingly small tubular surface enclosing the crack front along the length L.

dA ds d

L

T

Numerical application (bi-material interface):

x

y

Material 1

Material 2

ba2h

and h/b = 1

a = 40 mm

b = 100 mm

h = 100 mm

• SEN specimen geometry (see annex III.1):

Material 1:

a/b = 0.4

MPa.

Remote loading:

Materials properties (Young’s modulus, Poisson’s ratio):

Plane strain conditions.

E1 = 3 GPa

1 = 0.35

Material 2: E2 = 70 GPa

2 = 0.2

• Typical mesh:

Material 1= Material 2

Refined mesh around the crack tip

Number of elements used: 1376Type: CPE8 (plane strain)

Material 1

Material 2

Simulation of the stress evolution (isotropic case)

Simulation of the stress evolution (bi-material)

Isotropic Bi-material

KI KII KI KII

Annex III 0.746 0. / /

Abaqus 0.748 0. 0.752 0.072

Results:Material 1 Material 2 Bi-material

J (N/mm) J (N/mm) J (N/mm)

Abaqus 0.1641 0.0077 0.0837

MPa mSIF given in

(*) same values on the contours 2-8

for the isotropic case (i =1,2).2

21I

i i

KJ

E ( )

• Ones checks that:

(*)

- For an interfacial crack between two dissimilar isotropic materials (plane strain),

where

and 3 4i i 2 1

ii

i

EG

21i

ii

EE

• Relationship between J and the SIF’s for the bi-material configuration:

plane strain, i = 1,2

H. Gao, M.Abbudi and D.M. Barnett, “Interfacial Crack-tip fields in anisotropic elastic solids thermally stressed body”, Journal of the Mechanics and Physics of Solids 40 (1992), pp. 393-416

- Extracted from the Theory Manual of Abaqus 6.5, section 2.16.2.

Disagreement with the results of Smelser et al.

KI and KII are defined here from a complex intensity factor, such that

with