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Predictive Joints Design

#2833


New Approaches to Predictive Joints Design Based on Detailed Description of Local Elasto-Plastic Strain History by Combining the Holographic Interferometry Data and Numerical Simulation

Tech Area / Field

  • SAT-AER/Aeronautics/Space, Aircraft and Surface Transportation
  • INS-MEA/Measuring Instruments/Instrumentation
  • PHY-OPL/Optics and Lasers/Physics

Status
3 Approved without Funding

Registration date
21.07.2003

Leading Institute
Central Aerodynamic Institute, Russia, Moscow reg., Zhukovsky

Collaborators

  • Bremer Institute of Aplied Beam Technology (BIAS), Germany, Bremen\nUniversität Stuttgart / Institut fuer Technische Optik, Germany, Stuttgart\nUniversidad Complutense de Madrid, Spain, Madrid\nNorwegian University of Science and Technology / Faculty of Natural Science and Technology, Norway, Trondheim\nAirbus Deutschland GmbH, Germany, Bremen\nUniversity of Electro-Communications / Department of Communication and Systems Engineering, Japan, Tokyo

Project summary

Introduction. This proposal is a description of the Second Phase of the ISTC Project #808. The First Phase of the Project was performed from April 1, 1999 to September 30, 2000. The Funding Party of the Project was the European Union. All essential reports, which are related to the First Project Phase, have been presented to the ISTC as well as to the Funding Party.

The main objective of the project is a development and implementation of new research tools and obtaining on this base a wide set of non-traditional data, which represent both necessary and essential foundation in the course of creating advanced design techniques. These techniques have to be capable of increasing both reliability and maintainability of various demountable and permanent joints proceeding from the low-cyclic-fatigue lifetime standpoint. Permanent joints of main interest represent pin-loaded (pin or rivet) joints of plane thin-walled aircraft structural elements. A set of demountable joints under consideration consists of welded joints of both thin- and thick-walled plane structural elements as well as welded joints of cylindrical both thin-walled shells and thick-walled tubes. Solving the problem of reliable life-time prediction of various high-loaded joints and their optimal design will eventually lead to developing new structural elements having a higher level of reliability and a less cost comparing with traditional types of joints.

The Research Team involved consists of two main groups of highly skilled scientific workers. The expertise field the first of them is mainly related to advanced experimental strain analysis of different structures and material testing by holographic and speckle interferometry. The personnel involved into the project has been very successful during the last ten years in providing new results and approaches, which can be used as powerful tools for a development in various scientific and applied subjects of solid and fracture mechanics. The second part of the Research Team is mainly specialized in the field of both static and fatigue strength investigation of various structures by numerical and hybrid methods. Personnel involved have a wide experience in implementing both finite element and traditional experimental techniques to strain/stress analysis more than 20 years.

Expected Results and Their Application. A set of new research tools and obtained on this base non-traditional data, which represent both necessary and essential foundation of advanced design techniques, will be created as the result of the project performing. These techniques have to be capable of increasing both reliability and maintainability of various demountable and permanent joints proceeding from the low-cyclic-fatigue lifetime standpoint. It is assumed that most of the developed techniques would gain a wide acceptance in different fields of modern civil engineering. Most of inpidual project results, which integrally form the main result, can be recognized as belonging to applied research, which are capable of implementing in both further scientific investigations and design practice. But a set of project results, which are mainly related to residual stresses determination near welded joints, should be considered as very close to a stage of the technology development. It is planned both applying for some patents in this direction and further fast promoting of the techniques developed in various fields of modern engineering.

The first set of inpidual project results is related to pin and rivet joints of plane thin-walled structural elements, which are mainly inherent in aircraft and space engineering.

1. A wide set of unique experimental data obtained by means of sophisticated measurement techniques, which are based on reflection hologram interferometry. This information is presented in a form of the database, which includes information related to quantitative describing of local elasto-plastic behaviour of work-hardening aluminium alloy in low-cyclic deformation range under regular cyclic loading of definite type.

2. Hybrid technique that is capable of reliable description of local elasto-plastic strain evolution near separate joint’s element at a stage of fatigue damage accumulation under regular cyclic loading. Experimental database is the first essential fragment of the technique to be created. A set of interrelated numerical models of different level of sophistication, which are arranged in the framework of the MSC/NASTRAN software, is the second component of the hybrid approach.

3. Non-traditional engineering technique for a description of fatigue damage accumulation process in irregular zones of structures under in-service (random) cyclic loading. This technique has to be capable of refined taking into account the “memory” effect and local elasto-plastic strain history including contact interaction processes of different types. The results obtained will be of direct implementing in design practice especially in the field of aircraft engineering. At the same time the approach to be developed represents a powerful research tool that would be of considerable interest for various fields of modern engineering.

4. A set of advanced approaches to acquiring and processing data resulted from local strain measurements by means of sophisticated strain gages set-up. An essence of the technique to be created resides in substituting high accurate but quite time-consuming whole-field measurements with more simple point-wise measurement procedure.

5. Efficient and reliable engineering approaches to a damage tolerance evaluation of cracked joints' fragments under irregular cyclic in-service loading.

The following set of the project results (points 6-8) represents further development, modification and expanding the unique sophisticated technique of residual stresses determination that has been created at the First Project Phase. The approach developed, which has been mainly justified for plane both thin- and thick-walled structures, is based on combining the hole drilling method and reflection hologram interferometry. The main distinctive feature of the technique developed comparing with other destructive methods, including advanced methods based on whole-field optical techniques, resides in a capability of deriving membrane or total residual stress components at the object surface within 5 per cent accuracy for most cases of practical interest.

6. Universal technique for deriving membrane and bending residual stress components in thin-walled structures by means of inverse problem solution. Established approach is of both metrological and applied importance, especially for investigations of residual stress fields in advanced aircraft structures of redundant type. The technique involved is capable of wide practical implementing in various fields of modern engineering.

7. Reference set of main residual stress distributions, which are inherent in both typical welding technologies used for aluminium alloys and typical welded joints of redundant aluminium thin-walled structural elements. These data are of great importance for a process of substituting routine pin and rivet joints traditionally used in aircraft structural elements with advanced welded joints. There are good chances for a wide promotion of main results obtained in both research and industrial applications in the field of aircraft and aerospace engineering.

8. Advanced high-accurate technique for residual stresses determination in curved both thin-walled shells and thick-walled tubes with accuracy and reliability parameters, which can not be achieved by any other destructive methods. The technique to be developed and possible results of its implementing are of great both scientific and applied importance. From the copyright standpoint the technique represents at the moment a set of sophisticated “know-how” and has good chances to be converted into a patent. Information related to residual stress distributions near welds in curved shells and tubes is of considerable promise from the standpoint of wide practical promotion in many fields of modern engineering.

Meeting ISTC goals and objectives. Project realization would have gone a long way toward extending the unique results and achievements in careful design and fatigue testing high-loaded joints of aircraft structural elements into different fields of civil engineering. A set of both techniques to be developed and the results to be obtained are of considerable scientific interest especially in applied sense. 59 persons, whose field of expertise is connected with experimental and numerical solving various topical mechanical problems, will be involved for the project performing. Among them 39 persons have a long-standing experience of developing, testing and manufacturing structural elements and equipment implemented in various military aircrafts and missiles, which are capable of delivering of nuclear weapons. The main results of the project performing will be presented as a series of scientific papers in foreign journals and presentations on international conferences.

Scope of activities. Three scientific streamlines can be recognized in the framework of the planned project. The first of them is related to experimental investigations of local deformation processes and residual stress fields by various holographic and speckle- interferometric techniques (Lines A and B). The second streamline of the project performing can be specified as a development of experimental-numerical (combined or even hybrid) methods of local strain/stress analysis (Lines A and C). The main fundamentals of joints' design techniques are supplemented with data on fatigue crack initiation propagation in real joints (Line D). Most of both experimental and numerical techniques, on which the project is founded upon, have been developed and have to be developed by the Project Proposer. All these techniques correspond, as minimum, to the current state of the art. Sophisticated technique of residual stresses determination, which has been created at the First Project Stage, is of unique metrological properties comparing with other destructive methods traditionally used in this field for inspecting plane structural elements.

Role of Foreign Collaborators. Project performing will be accompanied by a close collaboration with foreign scientists. The main attention will be paid to questions related to an application of advanced coherent optics techniques, which are capable of whole-field displacement measurements, to sophisticated local strain/stress analysis in the course of reaching the project objectives. All persons, who have confirmed their collaboration with TsAGI in the framework of the project, are word-famous and highly skilled experts in the field of developing and implementing holographic and speckle interferometric techniques as well as computer-aided fringe patterns evaluation. The latter point is of decisive importance for promised promotion of main project results. It is planned a series of direct visits to collaborators as well as meetings during international conferences. A set of available materials in common with results of further investigations will be presented as a series of joint publications.

Technical Approach and Methodology. The methodological and metrological basis of the project are founded upon experimental investigations and computer simulation of various stages of local deformation processes, including a fatigue crack initiation and propagation up to fracture as well as a determination of residual stress level. Some of required approaches were available before the project starting point but a set of unique research tools, which are essential for further project development, has been developed during the First Project Phase. Main volume of essential experimental data will be obtained through the use of sophisticated holographic and speckle interferometric techniques, which are capable of contactless and whole-field measurements of required mechanical parameters. Essential numerical procedures are based on the MSC/NASTRAN program products and specialized software developed in TsAGI.


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