FRACTURE MECHANICS IN ANSYS R16

Session – 04Fracture Mechanics using Cohesive Zone Material (CZM) Model

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COHESIVE ZONE MATERIAL MODEL (CZM)

• Fracture or delamination along an interface between phases plays a major role in limiting the toughness and the ductility of the multi-phase materials, such as matrix-matrix composites and laminated composite structure. This has motivated considerable research on the failure of the interfaces. Interface delamination can be modeled by using techniques that directly introduce fracture mechanism by adopting softening relationships between tractions and the separations, which in turn introduce a critical fracture energy that is also the energy required to break apart the interface surfaces. This technique is called the cohesive zone material (CZM) model.

• The interface surfaces of the materials can be represented by a special set of interface elements or contact elements, and a CZM model can be used to characterize the constitutive behavior of the interface.

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COHESIVE ZONE MATERIAL MODEL (CZM)

• The concept of CZM was established to study perfectly brittle materials and adopt it to ductile fracture. In CZM, fracture is treated as a gradual phenomenon in which separation resisted by cohesive traction takes place across a cohesive zone. The extension of this cohesive zone ahead of the crack tip is modelled using traction-separation laws (also known as cohesive law) which relates the cohesive stress to the separation in the process zone.

• Fracture simulations using CZM require experience in order to determine input parameters such as:

• Mesh size and• Properties to characterize traction-separation laws

•A sensitivity analysis is recommended prior to utilizing CZM in the simulation of fracture behavior using this approach.

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ELEMENTS

• The interface surfaces of the materials can be represented by a special set of interface elements or contact elements, and a CZM model can be used to characterize the constitutive behavior of the interface.

• Interface Elements: • INTER202: 2D 4-Node Cohesive• INTER203: 2D 6-Node Cohesive• INTER204: 3D 16-Node Cohesive• INTER205: 3D 8-Node Cohesive

• Contact Elements:• CONTA171: 2D 2-Node Surface-to-Surface Contact• CONTA172: 2D 3-Node Surface-to-Surface Contact• CONTA173: 3D 4-Node Surface-to-Surface Contact• CONTA174: 3D 8-Node Surface-to-Surface Contact

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WHAT IS THE DIFFERENCE?

• The difference between using interface elements and contact elements is:

• Interface elements are for modeling interface delamination at the interface of two materials. The elements are capable of representing the cohesive zone between the interface and can account for the separation across the interface.

• Interface delamination with contact elements is referred to as debonding. Debonding is modeled with contact elements which are bonded and have a cohesive zone material model defined.

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DEBONDING vs DELAMINATION

•Debonding occurs when an adhesive stops sticking (adhering) to an adherend or substrate material. The adhesive does not have to be an organic, polymeric material; it could be an inorganic coating, for instance. Debonding occurs if the physical, chemical or mechanical forces that hold the bond together are broken, perhaps by a force or environmental attack.

•Delamination is failure in a laminated material, often a composite, which leads to separation of the layers of reinforcement or plies. Delamination failure can be of several types, such as:

• Fracture within the adhesive or resin• Fracture within the reinforcement• Debonding of the resin from the reinforcement

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DEBONDING vs DELAMINATION

• Therefore, debonding is a special form of delamination…

• There are several advantages to using debonding to model interface delamination. Existing models with contact definitions can be easily modified to include debonding, and standard contact and debonding can be simulated with the same contact definitions.

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DEMO

• Verification Problem – VM248• REF: ALFANO, G. AND CRISFIELD, M. A.,"FINITE ELEMENT INTERFACE MODELS FOR

THE DELAMINATION ANALYSIS OF LAMINATED COMPOSITES: MECHANICAL AND COMPUTATIONAL ISSUES“ INT. J. NUMER. METH. ENGNG 2001, 50:1701-1736.

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DEMO (INTERFACE ELEMENTS)

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DEMO (INTERFACE ELEMENTS)

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DEMO (INTERFACE ELEMENTS)

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DEMO

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DEMO (CONTACT ELEMENTS)

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DEMO (CONTACT ELEMENTS)

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