Predictive Design of Joints
New Approaches to Predictive Joints Design Based on a Detailed Description of Local Elasto-Plastic Strain History by Combining the Holographic Interferometry Data and Numerical Simulation
Tech Area / Field
- MAN-MAT/Engineering Materials/Manufacturing Technology
8 Project completed
Senior Project Manager
Kulikov G G
Central Aerodynamic Institute, Russia, Moscow reg., Zhukovsky
- Bremer Institute of Aplied Beam Technology (BIAS), Germany, Bremen\nAeronautical Research Institute of Sweden, Sweden, Bromma
Project summaryIn many cases joints are the primary sources of weakness in a structure, both from a static strength point of view and from a fatigue standpoint. Both a process of the low-cyclic fatigue and an existence of residual stresses are the main reason for the crack occurrence and the rapid growth of such cracks thus resulting in the fracture of various joints.
The main objective of the project is a development and implementation of new design techniques, which have to be capable of increasing a reliability and maintainability of various demountable and permanent joints proceeding from the low-cyclic-fatigue lifetime standpoint. Proposed techniques, first, are founded upon unique information being obtained by the holographic interferometry method. This information describes a local elasto-plastic strain and crack history in contact interaction zones of bolted and rivets joints.
The second essential component of the techniques employed is based on modern powerful computational tools and sophisticated algorithms which are capable of a detailed numerical reproducing both a local and a general deformation process in real joints of various design for each loading cycle to be investigated, including the case of random loading.
Moreover, the approach involved allows us to take into account an influence of residual stresses on joints' lifetime. The methods of residual stresses determination being used for this purpose are based on the holographic interferometric measurements of the tangential displacement components at the vicinity of small holes or slits made on the object surface area of interest between exposures when hologram is recorded.
A reliable design of pin-loaded joints based on life-time criteria requires an analysis of local elasto-plastic strain history near each separate joint element taking into account the actual character of contact interaction and a change of local material mechanical properties under cyclic in-service loading. Solving the same problem in the case of welded joints has to be founded upon a quantitative determination of an initial residual stresses level and a faithful description of evolution of these stresses during structure's operating.
In order to obtain with a required accuracy both the deformation parameters describing the strain or crack history and local material mechanical properties in joints' fragments characterized by a high displacement and strain gradients level, whole-field displacement measurements have to be implemented. Such measurements can be effectively carried out through the use of various holographic interferometric techniques.
Note those almost all-available criteria of both a static strength analysis and a fatigue life prediction, which are currently used for both a structure's design and for a maintainability evaluation, have a force character. Historically, the most powerful and having an evident mechanical interpretation deformation criteria did not find a widespread application. This situation resides in a deficiency of quantitative data on a local displacement fields evolution before and after a fatigue crack initiation under cyclic loading. Obtaining the above-mentioned information by means of holographic interferometry implementing is one of the main objectives of the project proposed.
An availability of deformation criteria of fatigue crack initiation and propagation up to fracture is an essential condition of joints' lifetime prediction proceeding from a local strain analysis. Developing such criteria in the framework of this project will be founded upon the unique database. The main part of this database will include a quantitative description of local deformation processes, processes of crack initiation and growth under cyclic loading in terms of corresponding displacement and strain fields. The corresponding measurements will be carried out by means of holographic interferomety.
The following step in the course of using a local strain-stress analysis for an optimal joint design should be directed toward taking into account an actual spectrum of in-service cyclic loading. The currently available methods of fatigue lifetime estimations based on the local strain-stress analysis under random cyclic loading are of limited usefulness now. The main reason of this resides in the fact that these techniques are capable to take roughly into account an influence of a "perfect notch" only without detailed describing the strain or crack history and considering the actual contact interaction character. Thus, the above-mentioned methods need a body of new specific experimental data for both development and reliable practical implementation.
Holographic interferometry, being used for obtaining information needed for the project performing, is currently the most advanced optical method employed in experimental mechanics which implement laser radiation. A remarkable feature of this method is its capability to perform contactless and high-sensitivity measurements of displacement component fields on optically rough surfaces of real structures both in elastic and a plastic deformation range. The whole-field character of such measurements, performing when the object investigated is subjected to cyclic loading, is of great importance in mechanical applications consisting of the project subject.
It should be noted that well-known traditional methods of experimental mechanics, such as photoelasticity, strain gages, moire' and others are capable of deriving a limited body of information on material mechanical properties and structure's deformation parameters to be investigated. This results from particular features inherent in these methods, such as the use of modeling materials, a presence of reference grating on the surface of interest, a pointwise and, sometimes, non-direct character of a measurement procedure, etc. The restrictions of implementing traditional experimental methods to strain/stress analysis reveal themselves most widely when information on an elasto-plastic deformation process resulting in fatigue crack initiation and propagation in contact interaction zone is of main interest. It is precisely this information that can be effectively obtained by various holographic intereferometric techniques . The same experimental data can be used as a physical foundation for a development of some deformation criteria related to different stages of a low-cyclic- fatigue process.
The remarkable measurement potentialities of holographic interferometry implementing in experimental mechanics are strengthened with the unique experimental setup located in the laboratories of the Project Proposer. This setup is capable of reliable recording holograms of different types and reconstruction of high-quality interference fringe patterns. Note that specimens and structural elements can be stressed under various loading and boundary conditions with the maximum loading force value about 200000 N in the deformation range from microelastic (10-5 - 10-4) to developed plastic (10-2) strains.
The available equipment is capable of holographic interferograms recording when the object to be studied is deformed in the elasto-plastic range under cyclic loading. In order to avoid undesirable rigid-body displacements of the elements of holographic interferometer optical system between exposures, special mounting and support units are used. In addition, an effective solution of most both scientific and applied problems in the field of solid mechanics by holographic interferometric techniques demands an implementation of various available mechanical and opto-mechanical components.
Two main streamlines can be recognized in the framework of the project proposed. The first of them is related to experimental investigations of local deformation processes by various holographic interferometric techniques. The Project Proposer has developed most of these techniques for the first time. Experimental information that will be obtained through the use of the holographic interferometry method will have a form of the database describing a material mechanical behavior under regular cyclic loading in terms of local displacement and strain fields. This database will also include information on initial and residual stress redistributions near the most typical welding seams under repeated loading.
The second streamline of the project can be specified as the development of experimental-numerical methods of local strain/stress analysis, and cracks initiation and propagation. These methods will be founded upon some new fundamentals obtained as both the results of investigations by holographic interferometry techniques and data resulting from the use of sophisticated approaches to implementing displacement and strain gages in fatigue testing materials and structural elements. An accurate and detailed numerical simulation of deformation processes in problems, which must be solved for reaching the project objectives, is ensured by the available effective software and powerful computational tools. Among them the program products such as the MSC/NASTRAN, the MSC/PATRAN and the MSC/FATIGUE supplemented with specifically developed algorithms which are installed at the workstation IBM RISK 6000/ 3CT should be pointed out. The combined methods to be created are directed toward finding the new ways of predictive joints' design and joints' maintainability evaluation proceeding from deformation criteria of damage accumulation, fatigue crack initiation and propagation up to fracture. The following approaches to be developed should be noted as the essential conditions of the advantageous project performing:
A The combined method of low-cyclic fatigue life-time prediction of pin or rivet joints based on detailed quantitative description of the local strain history, processes of crack initiation and propagation up to fracture in contact interaction zone under regular cyclic loading.
B. The combined method of static strength analysis and fatigue lifetime estimation of welded joints founded upon the data of an experimental residual stress determination.
C. Engineering techniques for an accurate considering damage accumulation processes in joints under in-service irregular cyclic loading resulted from a detailed local elasto-plastic strain/stress analysis in contact interaction zones.
D. Engineering techniques for a determination of joints' damage tolerance based on both traditional force criteria using in fracture mechanics and some new deformation criteria of fatigue fracture.
Most of both experimental and numerical techniques and approaches on which the project performing are 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.
The meeting of the project objectives related to experimental investigations by holographic interferometry can be considered as a major contribution to the development of experimental mechanics for a reliable quantitative description of various local deformation processes including a crack initiation and propagation up to fracture. As a result, the level of practical availability of holographic interferometric techniques for a local strain analysis will be considerably increased. Note that information capacity of holographic interferometry is greatly superior to that of traditional experimental methods.
A conception of using the deformation parameters and criteria for description of a local elasto-plastic strain evolution before and after fatigue crack initiation up to fracture is an essential condition of developing the combined techniques of strain/stress analysis. All these techniques supplemented with corresponding database are capable to be widely implemented in various fields of engineering both for a predictive design and for an evaluation of joints' reliability and maintainability. Promoting of the project results can lead to a notable increase in an efficiency of computer modeling that, at the moment, is the main tool of structure's design and maintainability analysis.
Concluding we can say that reaching all project's objectives will mean a development of some new approaches eventually directed toward increasing the capabilities of predictive joints' design and maintainability evaluation from the fatigue strength standpoint. The results of the advantageous project performing may contain some aspects, which can be considered as a major contribution both in fundamentals and applied subjects of solid and fracture mechanics.
The Research Team proposing this project consists of two groups of highly trained scientific workers. The expertise field of one of them is mainly dealt with an experimental strain analysis of different structures and material testing by holographic and speckles interferometry. The personnel involved into the project has been very successful in the last ten years in providing new results and approaches which can be used as an effective means for development in various scientific and applied subjects of solid and fracture mechanics. Sufficiently detailed information about available achievements and capabilities in this line of investigations is contained in the book .
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 combined methods. The personnel involved have the experience of finite element and traditional experimental technique implementation to strain/stress analysis more then 20 years. Some information describing the scientific level of the Research Team in the field of numerical simulation of deformation processes can be found in the works [2-5].
The project is of importance from the standpoint of conversion of activity of Russian researchers formerly connected with a developing and manufacturing various military aircraft to civil research programs. In particular, the project realization will 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.
It should be also noted that bolted and welded joints are critical parts of numerous pipelines using in both nuclear power stations and oil or gas conveying. In this sense, the results of the project performing will facilitate increasing the ecological safety of complicated industrial installations.
We assume that the project performing will be accompanied by a close collaboration with foreign partners having a great experience of developing the techniques and equipment in the fields of holographic interferometry application to strain/stress analysis, computer-aided fringe patterns evaluation, and computer simulation of deformation processes. It is expected that this cooperation will be substantial and should facilitate as much as possible:
- The combination of various experimental techniques supported by the partners to produce corroborative data.
- The transfer of technology and expertise between partners. + Discussions between partners for obtaining the highest improvement of knowledge in the use of both holographic interferometric and computer simulation techniques for a predictive design and maintainability analysis of various structures.
One of possible lines of the above-mentioned collaboration will be directed toward confirming the results obtained and techniques developed to international standards.
It is expected that a series of techniques and results of investigations obtained as the results of the project performing and international cooperation will be capable of applying for the patents. If the patents will be in force, a series of sophisticated techniques both experimental, including low-size setup and numerical can find a market.
In any case, the main fundamental and applied results of the project performing will be presented as a series of collaborative scientific papers and presentations on international conferences.
1. Shchepinov,V.P., Pisarev,V.S. (1996) Strain and Stress Analysis by Holographic and Speckle Interferometry. John Wiley, Chichester.
2. Dzuba,A.S., Ionov,A.A., Kutyinov,V.F. Application of the Finite Element Method to the Structural Analysis of Composite Structures. Composite Materials in Aerospace Design, Ed.by G.I. Zagainov and G.E. Lozino-Lozinski (1996), p.372-388. Chapman & Hall, London.
3. Dzuba,A.S., Grigoriev,V.D., Pisarev,V.S. Comparison of the MSC/NASTRAN and Holographic Interferometry Data on a Local Strain/Stress Evaluation in the Elasto-Plastic Range. Proceedings of 1996 MSC World Users' Conference, Los-Angeles, 1996.
4. Dzuba,A.S., Grigoriev,V.D., Pisarev,V.S. The Use of Holographic Interferometry Data for a Refinement of Local Elasto-Plastic Strain Calculations by FEM. - Simulation and Experiment in Laser Metrology. Proceedings of the International Symposium on Laser Applications in Precision Measurements held in Balatonfured/ Hungary, June 3-6, 1996. Akademie Verlag Series in Optical Metrology, Vol.2, Ed. by W.Juptner and W.Osten, Berlin:Akad. Verl., 1996, p.176-183.
5. Pisarev,V.S., Grigoriev,A.D., Dzuba,A.S. Numerical Simulation of a Transition from Holographically Measured Displacements near Drilled Hole to Residual Strains and Stresses. Ibid., p. 159-169.