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Finite element analysis and fracture mechanics: an brief introduction

Finite element analysis and fracture mechanics: an brief introduction . J.Cugnoni , LMAF-EPFL, 2014. Stress based design vs Fracture Mechanics approach. Stress based criteria (like Von Mises ) usually define the onset of “damage” initiation in the material

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Finite element analysis and fracture mechanics: an brief introduction

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  1. Finite element analysis and fracture mechanics: an brief introduction J.Cugnoni, LMAF-EPFL, 2014

  2. Stress based design vs Fracture Mechanics approach • Stress based criteria (like Von Mises) usually define the onset of “damage” initiation in the material • Once critical stress is reached, what happens? • In this case, a defect is now present (ie crack) • The key question is now: will it propagate? If yes, will it stop by itself or grow in an unstable manner. Stress concentrator: Critical stress is reached… A crack is formed… Will it extend further? If yes, will it propagate abruptly until catastrophic failure? Stress analysis Fracture mech.

  3. Fracture mechanics oriented design

  4. Stability of a crack: an introduction Potential energy: P0=U0-V0 • Crack propagation : an “energetic” process • Extend crack length: energy is used to create a new surface (break chemical bonds). • Driving “force”: internal strain energy stored in the system • “Energy release rate”: • Change in potential energy P (strain energy and work of forces) for an infinitesimal crack extension dA. Units: J/m2, Symbol: G • measure the crack “driving force” • “Critical Energy release rate”: • Energy required to create an additionnal crack surface. Is a material characteristic (but depends on the type of loading). Units: J/m2, symbol: Gc • Crack propagation occurs if G > Gc • (see the rest of the course for more explanation on these concepts) Assumed Crack extension dA Potential energy: P1= P0 –Er And Er = G*dA New crack surface dA: Dissipates Ed=Gc*dA

  5. Finite Element evaluation of ERR • Using the “J-Integral” approach (see course), it is possible to calculate the ERR G as G = J in linear elasticity. • If we know the displacement and stress field around the crack tip, we can compute J as a contour integral: • How to get these fields: a Finite Element simulation can be used to evaluate the displacement and stress field in any condition! • J-integral can be calculated in Abaqus. W=strain energy density u = displacement field s = stress field G = contour: ending and starting at crack surface

  6. J-integral in Abaqus Crack tip and extension direction • Can be calculated in elasticity / plasticity in 2D plane stress, plane strain, shell and 3D continuum elements. • Requires a purely quadrangular mesh in 2D and hexahedral mesh in 3D. • J-integral is evaluated on several “rings” of elements: need to check convergence with the # of ring) • Requires the definition of a “crack”: location of crack tip and crack extension direction Quadrangle mesh Crack plane Rings 1 & 2

  7. J-Integral in Abaqus: application notes and demonstration • Create a linear elastic part, define an “independent” instance in Assembly module • Create a sharp crack: use partition tool to create a single edge cut, then in “interaction” module, use “Special->Crack->Assign seam” to define the crack plane (crack will be allowed to open) • In “Interaction”, use “Special->Crack->Create” to define crack tip and extension direction (can define singular elements here, see later for more info) • In “Step”: Define a “static” load step and a new history output for J-Integral. Choose domain = Contour integral, choose number of contours (~5 or more) and type of integral (J-integral). • Define loads and displacements as usual • Mesh the part using Quadrangle or Hexahedral elements, if possible quadratic. If possible use a refined mesh at crack tip (see demo). If singular elements are used, a radial mesh with sweep mesh generation is required. • Extract J-integral for each contour in Visualization, Create XY data -> History output. • !! UNITS: J = G = Energy / area. If using mm, N, MPa units => mJ / mm2 !!! • By default a 2D plane stress / plane strain model as a thickness of 1.

  8. Singular elements & meshing tips • To create a 1/sqrt(r) singular mesh: • In Interaction, edit crack definition and set “midside node” position to 0.25 (=1/4 of edge) & “collapsed element side, single node” • In Mesh: partition the domain to create a radial mesh pattern as show beside. Use any kind of mesh for the outer regions but use the “quad dominated, sweep” method for the inner most circle.Use quadratic elements to benefit from the singularity. • Refine the mesh around crack tip significantly.

  9. Mesh Convergence

  10. Resources & help • Abaqus tutorials: • http://lmafsrv1.epfl.ch/CoursEF2012 • Abaqus Help: • http://lmafsrv1.epfl.ch:2080/v6.8 • See Analysis users manual, section 11.4 for fracture mechanics • Presentation and demo files: • http://lmafsrv1.epfl.ch/jcugnoni/Fracture • Computers with Abaqus 6.8: • 40 PC in CM1.103 and ~15 in CM1.110

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