Fracture Integrals

J-Integral

A finite element calculation in which a crack is represented in the mesh geometry can be used to calculate the displacement and stress fields in the vicinity of the crack. One straightforward way to evaluate the stress intensity from a finite element solution is through the JJ-integral (Rice, 1968), which provides the mechanical energy release rate. If the crack is subjected to pure mode-II loading, KIK_I can be calculated from JJ using the following relationship: J={1ν2EKI2plane strain1EKI2plane stressJ= \begin{cases} \frac{1-\nu^2}{E}K_I^2 & \text{plane strain}\\ \frac{1}{E}K_I^2 & \text{plane stress} \end{cases} where EE is the Young's modulus and ν\nu is the Poisson's ratio. The JJ-integral was originally formulated as an integral over a closed curve around the crack tip in 2D. It can alternatively be expressed as an integral over a surface in 2D or a volume in 3D using the method of Li et al. (1985), which is more convenient for implementation within a finite element code.

Following the terminology of Healy et al. (2012), the integrated value of the JJ-integral over a segment of a crack front, represented as Jˉ\bar{J}, can be expressed as a summation of four terms: Jˉ=Jˉ1+Jˉ2+Jˉ3+Jˉ4\bar{J}=\bar{J}_1+\bar{J}_2+\bar{J}_3+\bar{J}_4 where the individual terms are defined as: Jˉ1=V0(PjiuiXkqkXjWqkXk)dV0\bar{J}_1=\int_{V_0}\biggl( P_{ji}\frac{\partial u_i}{\partial X_k}\frac{\partial q_k}{\partial X_j} -W\frac{\partial q_k}{\partial X_k} \biggr) dV_0

Jˉ2=V0(WXkPji2uiXjXk)qkdV0\bar{J}_2=-\int_{V_0}\biggl( \frac{\partial W}{\partial X_k} - P_{ji}\frac{\partial^2u_i}{\partial X_j\partial X_k} \biggr) q_k dV_0

Jˉ3=V0(TqkXkρ2uit2uiXkqk+ρuit2uitXkqk)dV0\bar{J}_3=-\int_{V_0}\biggl( T\frac{\partial q_k}{\partial X_k} - \rho \frac{\partial^2 u_i}{\partial t^2} \frac{\partial u_i}{\partial X_k} q_k + \rho \frac{\partial u_i}{\partial t} \frac{\partial^2 u_i}{\partial t \partial X_k} q_k \biggr) dV_0

Jˉ4=A0tiuiXkqkdA0\bar{J}_4=-\int_{A_0} t_i\frac{\partial u_i}{\partial X_k} q_k dA_0 In these equations, V0V_0 is the undeformed volume, A0A_0 is the combined area of the two crack faces, PjiP_{ji} is the 1st Piola-Kirchhoff stress tensor, uiu_i is the displacement vector, XkX_k is the undeformed coordinate vector, WW is the stress-work density, TT is the kinetic energy density, tt is time, ρ\rho is the material density, and tit_i is the vector of tractions applied to the crack face.

The vector field qkq_k is a vector field that is oriented in the crack normal direction. This field has a magnitude of 1 between the crack tip and the inner radius of the ring over which the integral is to be performed, and drops from 1 to 0 between the inner and outer radius of that ring, and has a value of 0 elsewhere. In 3D, JJ is evaluated by calculating the integral Jˉ\bar{J} over a segment of the crack front. A separate qq field is formed for each segment along the crack front, and for each ring over which the integral is to be evaluated. Each of those qq fields is multiplied by a weighting function that varies from a value of 0 at the ends to a finite value in the middle of the segment. The value of JJ at each point on the curve is evaluated by dividing Jˉ\bar{J} by the integrated value of the weighting function over the segment containing that point.

The first term in Jˉ\bar{J}, Jˉ1\bar{J}_1, represents the effects of strain energy in homogeneous material in the absence of thermal strains or inertial effects. The second term, Jˉ2\bar{J}_2, accounts for the effects of material inhomogeneities and gradients of thermal strain. For small strains, this term can be represented as: Jˉ2=V0σij[αijθˉXk+αijXkθˉ]qkdV0\bar{J}_2 = \int_{V_0} \sigma_{ij} \left [ \alpha_{ij} \frac{\partial \bar{\theta}}{\partial X_k} + \frac{\partial \alpha_{ij}}{\partial X_k} \bar{\theta} \right] q_k dV_0 where σij\sigma_{ij}, αij\alpha_{ij} and θˉ\bar{\theta} are the Cauchy stress, thermal conductivity, and difference between the current temperature and a reference temperature, respectively.

The third term in Jˉ\bar{J}, Jˉ3\bar{J}_3, accounts for inertial effects in a dynamic analysis, and the fourth term, Jˉ4\bar{J}_4 accounts for the effects of surface tractions on the crack face.

The MOOSE implementation of the JJ-integral calculator can be used for arbitrary curved 3D crack fronts, and includes the Jˉ1\bar{J}_1 and Jˉ2\bar{J}_2 terms to account for the effects of quasistatic mechanically and thermally induced strains. The last two terms have not been implemented. For quasistatic loading conditions, there are no inertial effects, so Jˉ3=0\bar{J}_3=0. Jˉ4\bar{J}_4 would be nonzero for pressurized cracks, and should be implemented in the future to consider such cases.

Pointwise values J(s)J(s) are calculated from the JJ-integral over a crack front segment Jˉ\bar{J} using the expression J(s)=Jˉ(s)q(s)dsJ(s) = \frac{\bar{J}(s)}{\int q(s) ds} To use the JJ-integral capability in MOOSE, a user specifies a set of nodes along the crack front, information used to calculate the crack normal directions along the crack front, and the inner and outer radii of the set of rings over which the integral is to be performed. The code automatically defines the full set of qq functions for each point along the crack front, and outputs either the value of JJ or KIK_I at each of those points. In addition, the user can ask for any other variable in the model to be output at points corresponding to those where JJ is evaluated along the crack front.

Interaction Integral

The interaction integral method is based on the JJ-integral and makes it possible to evaluate mixed-mode stress intensity factors KIK_I, KIIK_{II} and KIIIK_{III}, as well as the T-stress, in the vicinity of three-dimensional cracks. The formulation relies on superimposing Williams' solution for stress and displacement around a crack (in this context called 'auxiliary fields') and the computed finite element stress and displacement fields (called 'actual fields'). The total superimposed JJ can be separated into three parts: the JJ of the actual fields, the JJ of the auxiliary fields, and an interaction part containing the terms with both actual and auxiliary field quantities. The last part is called the interaction integral and for a fairly straight crack without body forces, thermal loading or crack face tractions, the interaction integral over a crack front segment can be written (Walters et al., 2005): Iˉ(s)=V[σijuj,1(aux)+σij(aux)uj,1σjkϵjk(aux)δ1i]q,idV+V[ϵij,1σij(aux)biui,1(aux)]q +dV+V[σij,j(aux)ui,1(aux)Cijkl,1ϵklϵij(aux)]qdV,\bar{I}(s) = \int_V \left[ \sigma_{ij} u_{j,1}^{(aux)} + \sigma_{ij}^{(aux)} u_{j,1} - \sigma_{jk} \epsilon_{jk}^{(aux)} \delta_{1i} \right] q_{,i} \, \mathrm{d}V + \int_V \left[ \epsilon^*_{ij,1} \sigma_{ij}^{(aux)} - b_i u_{i,1}^{(aux)} \right] q \ + \mathrm{d}V +\int_V \left[ \sigma_{ij,j}^{(aux)}u_{i,1}^{(aux)} - C_{ijkl,1}\epsilon_{kl} \epsilon_{ij}^{(aux)} \right] q \, \mathrm{d}V, where σ\sigma is the stress, uu is the displacement, and qq is identical to the qq-functions used for JJ-integrals. The second integral in this equation accounts for the effects of the gradient of the eigenstrain, ϵ\epsilon^*, and for the effects of the body force, bb. If the eigenstrain is a thermal strain, its gradient is computed using the gradient of the temperature field and the temperature derivative of the eigenstrain. In that case, the user supplies the temperature and eigenstrain name through the temperature and eigenstrain_name parameters, respectively. If the eigenstrain is from another source, its gradient must be computed by the eigenstrain material model in the form of a RankThreeTensor object, whose name is specified for the fracture integrals using the eigenstrain_gradient parameter. An alternative way to compute the influence of generic eigenstrains (it does not need to depend only on the temperature variable) on the interaction integral is to feed Lagrange auxiliary variables into the domain integral action. These eigenstrain variables can be computed using nodal patch recovery capabilities and their gradient can be coupled into the interaction integral object. Both these approaches yield the same results to a numerical tolerance given by the field recovery method. For two dimensions, the generic eigenstrain components XX, XY, and YY (optionally ZZ) need to be provided. For three dimensions, the components XY and XZ need to be provided as well (see input additional_eigenstrain_{component}). Finally, the last integral accounts for spatially-varying elastic properties in the material (such as for functionally graded materials) using a "non-equilibrium" formulation.

The treatment for functionally graded materials is currently limited to cases for which the gradient of the Young's modulus is in the direction of crack extension. For example, this could be used to represent a crack that is perpendicular to an interface between two materials, where the transition between the materials is assumed to be approximated by a smooth function such as E(X1)=E0expβX1E(X_{1}) = E_{0} \exp{\beta X_{1}}, where X1X_{1} is the position in the direction of crack extension, E0E_{0} is the stiffness of the material behind the crack tip, and β\beta is a parameter that governs the rate of the material property change. To specify the spatially varying properties, the user must define the functionally_graded_youngs_modulus and functionally_graded_youngs_modulus_crack_dir_gradient parameters, which reference scalar material properties that define the spatially varying Young's modulus and its gradient in the crack direction, respectively. These material properties can be defined arbitrarily by the user, but it is up to the user to ensure that the gradient is indeed in the crack direction. Note that the user must still also specify youngs_modulus to define that modulus at the crack tip. See the "non-equilibrium" formulation in Kim and Paulino (2004) for details.

The integration integral is automatically adapted to axisymmetric solids if an axisymmetric coordinate system is chosen. For example, the interaction integral for homogeneous materials can be defined as follows: Iˉ(s)=1RV[[σijuj,1(aux)+σij(aux)uj,1σjkϵjk(aux)δ1i]q,i+[(u1rσθθ(aux)+u1(aux)rσθθσjkϵjk(aux))qr+1r(σ1j(aux)uj,1σθθ(aux)u1,1+σθθ(u1,1(aux)u1(aux)r))]q]rdV,\bar{I}(s) = \frac{1}{R} \int_V \left[ \left[ \sigma_{ij} u_{j,1}^{(aux)} + \sigma_{ij}^{(aux)} u_{j,1} - \sigma_{jk} \epsilon_{jk}^{(aux)} \delta_{1i} \right] q_{,i} + \left[ (\frac{u_{1}}{r} \sigma_{\theta\theta}^{(aux)} + \frac{u_{1}^{(aux)}}{r} \sigma_{\theta\theta} - \sigma_{jk} \epsilon_{jk}^{(aux)})\frac{q}{r} + \frac{1}{r}(\sigma_{1j}^{(aux)} u_{j,1} - \sigma_{\theta\theta}^{(aux)} u_{1,1} + \sigma_{\theta\theta}(u_{1,1}^{(aux)} - \frac{u_{1}^{(aux)}}{r})) \right] q \right] r \, \mathrm{d}V, where RR is the radial location of the crack, rr is a relative location with respect to the crack tip, u(aux)u^{(aux)} is the auxiliary displacement field, and 11 refers to the local direction of crack advancement, which, in the case of a radial crack, it would coincide with the radial direction. See Nahta and Moran (1993) for details. Note that this form of the interaction integral can be integrated with other features such as its application to functionally graded materials.

In the same way as for the JJ-integral, the pointwise value I(s)I(s) at location ss is obtained from Iˉ(s)\bar{I}(s) using: I(s)=Iˉ(s)q(s)dsI(s) = \frac{\bar{I}(s)}{\int q(s) \mathrm{d}s} Next, we relate the interaction integral I(s)I(s) to stress intensity factors. By writing JSJ^S, with actual and auxiliary fields superimposed, in terms of the mixed-mode stress intensity factors JS(s)=1ν2E[(KI+KI(aux))2+(KII+KII(aux))2]+1+νE(KIII+KIII(aux))2=J(s)+J(aux)(s)+I(s)\begin{aligned} J^S(s) &= \frac{1-\nu^2}{E} \left[ \left( K_I+ K_I^{(aux)} \right)^2 + \left( K_{II} + K_{II}^{(aux)} \right)^2 \right] \\ &+ \frac{1+\nu}{E} \left( K_{III}+ K_{III}^{(aux)} \right)^2 \\ & = J(s) + J^{(aux)}(s) + I(s) \end{aligned} the interaction integral part evaluates to I(s)=1ν2E(2KIKI(aux)+2KIIKII(aux))+1+νE(KIIIKIII(aux))\begin{aligned} I(s) &= \frac{1-\nu^2}{E} \left( 2 K_I K_I^{(aux)} + 2 K_{II} K_{II}^{(aux)} \right) \\ & + \frac{1+\nu}{E} \left( K_{III} K_{III}^{(aux)} \right) \end{aligned} To obtain individual stress intensity factors, the interaction integral is evaluated with different auxiliary fields. For instance, by choosing KI(aux)=1.0K_I^{(aux)} = 1.0 and KII(aux)=KIII(aux)=0K_{II}^{(aux)} = K_{III}^{(aux)} = 0 and computing the volume integral I(s)I(s) over the actual and auxiliary fields, KIK_I can be solved for.

If all three interaction integral stress intensity factors are computed, there is an option to output an equivalent stress intensity factor computed by: Keq=KI2+KII2+KIII21ν.K_{eq} = \sqrt{ K_I^2 + K_{II}^2 + \frac{K_{III}^2}{1-\nu}}. The TT-stress is the first second-order parameter in Williams' expansion of stress at a crack tip and is a constant stress parallel to the crack (Larsson and Carlsson, 1973) (Rice, 1974). Contrary to JJ, TT-stress depends on geometry and size and can give a more accurate description of the stresses and strains around a crack tip than JJ alone. The TT-stress characterizes the crack-tip constraint and a negative TT-stress is associated with loss of constraint and a higher fracture toughness than would be predicted from a one-parameter JJ description of the load on the crack. TT-stresses can be calculated with the interaction integral methodology using appropriate auxiliary fields (Nakamura and Parks, 1992). The current implementation of the TT-stress computation is valid for two-dimensional and three-dimensional cracks under Mode I loading.

Usage

The MOOSE implementation of the capability to compute these fracture integrals is provided using a variety of MOOSE objects, which are quite complex to define manually, especially for 3D simulations. For this reason, the to compute JJ-integrals or interaction integrals, the DomainIntegral Action should be used to set up all of the required objects.

[DomainIntegral]
  integrals = JIntegral
  boundary = 800
  crack_direction_method = CrackDirectionVector
  crack_direction_vector = '1 0 0'
  radius_inner = '4.0 5.5'
  radius_outer = '5.5 7.0'
  output_variable = 'disp_x'
  output_q = false
  incremental = true
  symmetry_plane = 1
[]
(contrib/moose/modules/solid_mechanics/test/tests/j_integral/j_integral_3d.i)

C-Integral

The C(t) integral is a fracture integral that is obtained in essentially the same way as the JJ-integral described above, but by replacing displacements and strains with displacement rates and strain rates:

C(t)=Γ(Pjiu˙iXkdsW˙dy)dΓ{C}(t)=\int_{\Gamma}\biggl( P_{ji}\frac{\partial \dot{u}_i}{\partial X_k}ds -\dot{W}dy \biggr) d\Gamma

This integral is computed in an analogous way to the J-integral: A domain integral is solved on the crack front geometry defined in the mesh. This integral becomes the CC^{*} integral as creep behavior becomes steady state. Then, creep crack growth behavior during secondary creep can be obtained from this integral. A number of approximate expressions for CC^{*} are available in the literature (Bong Yoon et al., 2003).

Usage

To compute CC-integrals, the DomainIntegral Action should be used to set up all of the required objects. In particular, two additional inputs are required integrals = CIntegral and inelastic_models. The former refers to the type of integral requested ('C') and the latter refers to the name of the power law creep model –only supported material model at present. The power law exponent in the material model is used to compute the strain energy rate density in the fracture integral under the assumption of a creep strain rate field subject to steady-state (secondary) crack growth.

[DomainIntegral]
  integrals = CIntegral
  boundary = 1001
  crack_direction_method = CurvedCrackFront
  crack_end_direction_method = CrackDirectionVector
  crack_direction_vector_end_1 = '0.0 1.0 0.0'
  crack_direction_vector_end_2 = '1.0 0.0 0.0'
  radius_inner = '12.5 25.0 37.5'
  radius_outer = '25.0 37.5 50.0'
  intersecting_boundary = '1 2'
  symmetry_plane = 2
  incremental = true
  inelastic_models = 'powerlawcrp'
[]
(contrib/moose/modules/solid_mechanics/test/tests/j_integral_vtest/c_int_surfbreak_ellip_crack_sym_mm.i)

References

  1. Kee Bong Yoon, Tae Gyu Park, and Ashok Saxena. Creep crack growth analysis of elliptic surface cracks in pressure vessels. International Journal of Pressure Vessels and Piping, 80(7):465 – 479, 2003.[BibTeX]
  2. Brian Healy, Arne Gullerud, Kyle Koppenhoefer, Arun Roy, Sushovan RoyChowdhury, Matt Walters, Barron Bichon, Kristine Cochran, Adam Carlyle, James Sobotka, Mark Messner, and Robert Dodds. Warp3d-release 17.3.1. Technical Report UILU-ENG-95-2012, University of Illinois at Urbana-Champaign, 2012.[BibTeX]
  3. Jeong-Ho Kim and Glaucio H. Paulino. Consistent formulations of the interaction integral method for fracture of functionally graded materials. Journal of Applied Mechanics, 72(3):351–364, 07 2004.[BibTeX]
  4. S.G. Larsson and A.J. Carlsson. Influence of non-singular stress terms and specimen geometry on small-scale yielding at crack tips in elastic-plastic materials. Journal of the Mechanics and Physics of Solids, 21(4):263–277, July 1973.[BibTeX]
  5. F. Z. Li, C. F. Shih, and A. Needleman. A comparison of methods for calculating energy release rates. Engineering Fracture Mechanics, 21(2):405–421, 1985. doi:10.1016/0013-7944(85)90029-3.[BibTeX]
  6. R. Nahta and B. Moran. Domain integrals for axisymmetric interface crack problems. International Journal of Solids and Structures, 30(15):2027–2040, 1993. URL: https://www.sciencedirect.com/science/article/pii/002076839390049D.[BibTeX]
  7. Toshio Nakamura and David M. Parks. Determination of elastic t-stress along three-dimensional crack fronts using an interaction integral. International Journal of Solids and Structures, 29(13):1597–1611, January 1992. doi:10.1016/0020-7683(92)90011-H.[BibTeX]
  8. J. R. Rice. A path independent integral and the approximate analysis of strain concentration by notches and cracks. Journal of Applied Mechanics, 35(2):379, 1968. doi:10.1115/1.3601206.[BibTeX]
  9. J.R. Rice. Limitations to the small scale yielding approximation for crack tip plasticity. Journal of the Mechanics and Physics of Solids, 22(1):17–26, January 1974.[BibTeX]
  10. Matthew C. Walters, Glaucio H. Paulino, and Robert H. Dodds. Interaction integral procedures for 3-D curved cracks including surface tractions. Engineering Fracture Mechanics, 72(11):1635–1663, July 2005.[BibTeX]