Fracture Mechanics Analyses for Interface Crack Problems - A Review

Recent developments in fracture mechanics analyses of the interfacial crack problem are reviewed. The intent of the review is to renew the awareness of the oscillatory singularity at the crack tip of a bimaterial interface and the problems that occur when calculating mode mixity using numerical methods such as the finite element method in conjunction with the virtual crack closure technique. Established approaches to overcome the nonconvergence issue of the individual mode strain energy release rates are reviewed. In the recent literature many attempts to overcome the nonconvergence issue have been developed. Among the many approaches found only a few methods hold the promise of providing practical solutions. These are the resin interlayer method, the method that chooses the crack tip element size greater than the oscillation zone, the crack tip element method that is based on plate theory and the crack surface displacement extrapolation method. Each of the methods is validated on a very limited set of simple interface crack problems. However, their utility for a wide range of interfacial crack problems is yet to be established

Development of a software for the design of adhesive joints

Tese de mestrado integrado. Engenharia Mecânica. Faculdade de Engenharia. Universidade do Porto. 200

Rotordynamic Computation of a Permanent-Magnetic excited Synchronous Machine due to Electromagnetic Force Excitation

For the acoustical computation of electromagnetic noise, it is very important to consider both, the rotor and stator vibrations of the electrical machine. Rotor vibrations can be transmitted as structure-borne sound to connected systems which might be excited at their resonances and radiate airborne sound. In order to predict the dynamical behaviour of complex electrical machine rotors (such like rotors of permanent-magnetic excited synchronous machines) in frequency domain, finite element (FE) computations can be efficiently applied using rotating coordinates. Hereby, it has to be taken into account that rotor vibrations are mainly influenced by stiffness and damping of the built-in laminated stacks and mechanical joints. Therefore, a FE model of the rotor is required which takes these parameters into account. In order to obtain the material properties, two experimental set-ups are considered. On the one hand, a generic lap joint is considered to determine the stiffness and damping of mechanical joints. On the other hand, a test rig for laminated stacks is presented which allows for the determination of direction-dependent stiffness and damping of laminated stacks by a shear and dilatation test. All identified parameters are included into the FE model. Thereby, local stiffness and damping of mechanical joints are modelled by so-called thin-layer elements. In order to prove the quality of the rotor FE model, a numerical modal analysis without considering rotor spin speed is carried out and compared to experimental results. Electromagnetic force densities are computed in the air gap of the electrical machine using an electromagnetic FE model. To cover different FE meshes of the mechanical and electromagnetic model, a method is presented which allows for converting force densities into equivalent nodal forces on the rotor surface. These excitation forces are used to compute electromagnetically caused rotor vibrations dependent on rotor spin speed by a frequency domain rotor dynamic analysis

Bonded Repair of Composite Structures; A Finite Element Approach

This thesis addresses the issues surrounding the application of the finite element method to analyse composite structure repairs with an emphasis on aircraft applications. A comprehensive literature survey has been carried out for this purpose and the results are presented. A preliminary study and a comparative study of different modelling approaches have been completed. These studies aim to explore and identify the problems in modelling repairso n simplec ompositep anelsw ith speciala ttention given to adhesivem odelling. Three modelling approaches have been considered: Siener's model which is an extension of the traditional plane strain 2D model used for adhesively bonded joints, Bait's model which is a promising new approach and a full 3D model. These studies have shown that these methods are complementary providing a different insight into bonded repairs. They have also highlighted the need for a new modelling approach which will provide an overall view of bonded repairs. Improved modelling approachesh ave been developedf or externallyb onded patch and flush repairs. These models enable the study of adhesive failure as well as composite adherendf ailures.T hesea pproachesh aveb eena ppliedt o real repairs and the predicted results compared to experimental data. Four case studies have been conducted: external bonded patch repairs to composite plates, a scarf joint for bonded repairs, a flat panel repaired with a scarfed patch and a repaired curved panel. These case studies have shown that bonded repairs to composite structures can be analyseds uccessfullyu sing PC-basedc ommercialf inite elementc odes

Analysis of fiber reinforced plastic bolted flange joints

Fiber Reinforced Plastic (FRP) composites are extensively used in the areas of pressure vessels and piping and FRP bolted flange joints have experienced a spectacular development to provide continuity for the flow of fluid through piping systems. In spite of the increased use of FRP composites in bolted flange joints and the good knowledge of these structures and their material behavior the procedure used for their design is still that of metallic flanges. There is a major concern to appropriately address the anisotropic behavior of composite materials in a flange design. Therefore, it is necessary to make a precise evaluation of the bolt and gasket loads in order to be able to predict the integrity of FRP bolted flange joints. This thesis presents two analytical model cases; one with the flange hub and the other one without the flange hub. These models are supported by numerical finite element modeling and experimental test data. The study treats FRP bolted flange joints integrity and leak tightness based on the anisotropy and flexibility analysis of all joint elements including the gasket, bolts, and flanges. In the analytical models for the flange with and without the hub, the composite flanges are subdivided into three major categories, namely: ring flange, hub, and shell. The experimental study was carried out on a well-equipped test bench, used for Hot Blow out test of PTFE gaskets. The rig was modified to accommodate an NPS 3 FRP bolted flange joint designed according to ASME BPV Code Section X. Furthermore, three different numerical models based on 3D anisotropic layered shell and solid element models were conducted to compare and verify the results obtained from analytical and experimental approaches. In spite of the rigorous mathematical analysis and complexity of the laminate composite flange, comparing the results proved that the proposed analytical models for FRP flanges with and without the hub, are efficient, accurate and reliable in predicting the longitudinal and tangential stress distributions on the flange surface and radial displacement of the flange. Moreover, the results demonstrated that the FE model which is developed for FRP flanges with and without the hub can depict the true behavior of FRP bolted flange joints