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NIA Seminar by Rakesh Kapania |
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Date: March 7, 2006
Time: 10:30am
Location: NIA, Rm 137
Dynamic Fracture of Adhesively Bonded Composite Structures
Using Cohesive Zone Models
Rakesh Kapania, Virginia Tech
Adhesives are increasingly being used in aerospace and automobiles to reduce the weight. The seminar will provide some of our recent work in simulating the behavior of adhesively bonded joints. Using data obtained from standard fracture test configurations, theoretical and numerical tools are developed to mathematically describe progression of cracks without specifying an initial crack. A cohesive-decohesive zone model, similar to the cohesive zone model known in the fracture mechanics literature as the Dugdale-Barenblatt model, is adopted to represent the degradation of the material ahead of the crack tip. This model unifies strength-based crack initiation and fracture-mechanics based crack progression. The cohesive-decohesive zone model is implemented with an interfacial surface material that consists of an upper and a lower surface that are connected by a continuous distribution of normal and tangential nonlinear elastic springs that act to resist either Mode I opening, Mode II sliding, Mode III sliding, or a mixed mode. The initiation of fracture is determined by the interfacial strength while the progression of the crack by the critical energy release rate. To predict fracture, the adhesive is idealized with an interfacial surface material that is positioned within the bulk material to predict discrete cohesive cracks. The interfacial surface material is implemented through an interface element, which is incorporated in ABAQUS using the user defined element (UEL) option. A rate dependent model, based on experiments carried out on compact tension test specimens, is incorporated into the interface element approach to study the unstable crack growth observed in experiments under quasi-static loading. The compact tension test gives the variation of the fracture toughness with the rate of loading and a relationship between the fracture toughness and the rate of the opening displacement is established. The cohesive-decohesive zone model is implemented through a material model to be used in an explicit code (LS-DYNA). Dynamic simulations of the standard test configurations for Mode I (Double Cantilever Beam) and Mode II (End Load Split) are carried out using the explicit code. Verification of these coupon tests leads to the crash analysis of realistic structures like the square composite tube, bonded and unbonded. These tubes show a very uncharacteristic failure mode: the composite material disintegrates on impact, and this has been captured in the analysis. To overcome the disadvantages of the interface element approach, an alternative method, known as the Extended Finite Element Method (XFEM), is implemented here through an eight-noded quadrilateral plane strain element. The method is used to study simple test configuration like the three-point bend problem and a double cantilever beam.
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