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Daniel Guggenheim School of Aerospace Engineering

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Now showing 1 - 4 of 4
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Closed/Semi-Closed Form Solutions for Face/Core Debonds in Sandwich Beams

2022-05-03 , Niranjan Babu, Siddarth

Sandwich beams are highly susceptible to debonding at the interface between face and core. These debonds can grow and eventually lead to complete failure of the structure. To understand and study such debonds analytically, an Elastic Foundation Analysis (EFA) can be used to incorporate the effects of crack tip deformation in beam theory. In this model, EFA is extended further to better capture the effects of transverse shear. Unlike most models, this approach can be applied for both isotropic and orthotropic face & core materials. The approach uses both normal and rotational springs in the elastic foundation in the bonded region of the beam to capture transverse shear effects. Timoshenko beam theory introduces a rotational degree of freedom to the beam element and the rotational springs are used to capture it. The model is comprehensive and include both the deformation of the debonded part and the substrate. Double Cantilever Beam (DCB) and Single Cantilever Beam (SCB) specimens are chosen to demonstrate the procedure to obtain Mode-I fracture parameters. In the case of Mode-II fracture, the effects of crack face contact can affect the fracture parameters and are usually neglected in analytical approaches. The proposed model extends EFA by introducing a tensionless spring foundation in the cracked region. Tensionless springs are used to capture the compressive stresses across the interface between the debonded face sheet and the substrate. The absence of tensile stresses in the foundation is because when there is tension the debonded face sheet lifts away from the substrate. Apart from compressive stresses, there will also be frictional forces acting between the crack faces. So, the governing equations are modified to capture the friction tractions in the crack faces. An End Notched Flexure (ENF) specimen is chosen to demonstrate Mode-II fracture. Expressions for energy release rates are obtained using J-Integral approach and it is modified to capture the energy lost due to the friction tractions. Solutions for mode partitioning are obtained using the axial and transverse displacements near the crack tip. Results obtained from these expressions are compared with results from finite element models. The model is comprehensive, efficient and would provide accurate results when compared with the other models (from literature).

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Compressive behavior of thick composite shells : benchmark solutions for loss of stabilty and hygroscopic effects

1993-12 , Chung, Chang-Bum

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MOLTEN-BASED 3-D DIRECTWRITE FABRICATION OF MICRO STRUCTURES

2021-12-07 , Chen, Yining

The micro fabrication is the backbone of information age. From phones people use daily to the satellite in the orbit, they all rely on the control unit, sensors that made by microfabrication techniques. With decades of rapid development, the micro fabrication technology has reached a dimension that no one can image in the past. The next state of art micro electric fabrication dimensions has reached 2nm which is 1/6000 that in 1970s. However, these fabrication techniques are mostly planar and become more and more expensive and make it only economically feasible for mass productions. Thus, it has difficulties in making 3D or very high aspect ratio structures due to it is mainly a 2D process. Meanwhile, the demand for 3D micro-structures or high aspect ratio microstructures is increasing in various fields, such as micro needles, micro antenna etc. In this dissertation, we develop a direct-write method to fabricate 3D micro-structures and high aspect ratio microstructures at low cost that can fill this gap. Our method is based on molten materials extruding from a nozzle by pressure to form microstructures on the substrates. These critical dimensions of the structures can be as small as hundred nanometer level while maintaining hundreds of aspect ratio. We explore the mechanism in the fabrication and realize precise control of the process. The bonding and mechanical strength of the fabricated structures are also strong enough to serve as senor or mold for sequential processes. It provides a unique method for micro device that has application in biomedical and microelectronics. The process can be stacked up together to fabricate more complex or more dense structures. Very high dense and aspect ratio structure arrays are readily fabricated. Moreover, we also develop a method to fabricate microneedle arrays. These micro needles arrays can serve as mold to get polymer replica which has many applications in biomedical field, such as medical drug/vaccine delivery, bio-signal monitoring, etc.

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Growth-arrest behavior of small fatigue cracks

1997-05 , Steadman, David Lawrence

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