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Residual Stresses Determination

#0774


Development of Experimental-Numerical Methods and Equipment for Residual Stresses Determination in Welded Pipelines

Tech Area / Field

  • INS-DET/Detection Devices/Instrumentation

Status
8 Project completed

Registration date
25.10.1996

Completion date
20.11.2003

Senior Project Manager
Komashko B A

Leading Institute
MIFI, Russia, Moscow

Supporting institutes

  • Kurchatov Research Center, Russia, Moscow

Collaborators

  • Ecole National Superieure des Arts et Industries de Strasbourg, France, Strasbourg

Project summary

The main objective of the project is a development and testing of the unconventional experimental-numerical techniques of residual stress determination in welded pipelines. These techniques are based, first, on a combined implementation of the hole-drilling and holographic interferometry methods. Then, experimental information thus obtained are used as both input data and verification parameters for numericalmodeling the processes of residual stresses initiation and evolutionunder in-service loading.

The scientific and applied importance of such a research resides from the fact that a high level of residual stresses near weldingseams is often the main reason of fracture of various structures, among them pipelines [1,2].

The metrological and methodological basis of the project is founded upon the methods of holographic interferometry and finite or boundary elements. Different versions and combinations of holographic interferometric techniques and numerical methods are used in order to obtain the input data needed for further determination of both the magnitude and sign of residual stresses in real structures.

Holographic interferometry is currently one of the most advanced optical methods employed in experimental mechanics, which implement laser radiation [3]. A remarkable feature of this method is its capability to perform contactless and high-sensitivity measurements of parameters inherent in solid mechanics on optically rough surfaces of real structures. These parameters are displacement component distributions in the object surface region of interest. The whole-field character of such measurements is of great importance in many mechanical applications.

There are some specific topical mechanical problems in solving of which the remarkable capabilities of combining the holographic interferometry with the modern tools of computer modeling can be revealed with the most efficiency. Among these is the problem related to residual stress evaluation.

Welding is currently the most advanced method of structural components joining and the field of its application is practically unlimited.

However, welding represents one of the most charasteric manufacturing processes, which are accompanied with residual stresses arising. The magnitude of these stresses at the nearest vicinity of the welding seam may reach a very high level that is compared or even over than the material yield limit value. As it is well known, structure's zones of a high residual stress level are the primary sources of corrosion and crack propagation. Thus, a visualization of such areas and reliable quantitative determination of residual stress values should be considered as the essential steps in the course of product maintainability.

One of the most currently and widely used methods of residual stresses determination is the so-called hole-drilling technique [2]. The implementation of holographic interferometry for measurement of local strains in the hole vicinity is capable of ensuring more accurate and reliable results of the residual stress determination compared with traditional techniques such as different types of strain gauge rosette. This is attributed to the fact that the measurement of the displacement components, needed for a calculation of the local strains at the hole vicinity, and further evaluation of the residual stresses can be perfor-med at the nearest vicinity of hole boundary [3]. Thus, the holographic interferometry application results in the highest sensitivity of the hole-drilling technique with respect to the residual strains/stresses determination.

The experimental techniques of residual stress determination are based on the measurement of the displacement component fields at the hole vicinity by means of reflection hologram recording. Special approaches allowing us to define the directions and absolute signs of the principal residual strains is also proposed. This will eventually lead to the wide implementation of holographic interferometry as a tool for integration of modern computer modeling techniques with reliable residual stresses analysis needed for both optimal design and predictive maintenance of different structures.

The technique of residual stresses determination involved are based on the measurement of the displacement component fields at some points of the hole vicinity by means of a reflection hologram recording. An analysis of possible errors made in the residual strain and stress determination can be carried out in order to establish the sensitivity threshold of the technique used. Special numerical models, on which a residual stresses calculation near welding seams have to based, should be established and verified for both thick-walled and thin-walled structures. The accuracy of the approach involved is justified through the use of special test experiments.

The technique concerned consists of the following main steps:


- Holographic recording the surface area to be investigated at the initial stage (the first exposure);
- Drilling a small hole (about 3 mm diameter) through the entire depth for thin-walled specimen or structure and, for the other cases, a hole with the depth equal to hole diameter;
- Recording the holographic interferogram of the hole vicinity after residual stress energy release (the second exposure);
- Determination of principal strain directions at the point of interest by analyzing the recorded fringe patterns;
- Interpretation of holographic interferograms in terms of absolute changes of hole diameter including determination of physical sign of these changes in the directions of principal strains;
- Introducing some analytical or numerical model describing an elastic or elasto-plastic deformation of the selected part of the object to be investigated with hole. Note that a choice of the correct model of stress/strain concentration factor determination is apparently the most complex step for all destructive methods of residual stress determination. Some special holographic test experiments can be carried out in order to derive or refine above-mentioned model.

Finite and boundary element methods are the main numerical techniques, which are used for modeling the processes of residual stress initiation and evolution in welded structural elements in the context of this project. Two main numerical approaches related to a residual stress simulation should be noted. These approaches are an axisymmetric and three-dimensional formulation of corresponding mechanical problems. A set of the most suitable algorithms and computer codes has to be developed to reach the project objectives. A verification of the software to be created is founded upon experimental data obtained by the above-described techniques.

The main objective of the project is developing currently available techniques and creating new unconventional methods of residual stresses determination based on data obtained by holographic interferometry and corresponding numerical simulation procedures. All these techniques supplemented with corresponding database are capable to be widely implemented in various fields of engineering both for optimal design and evaluation of structure's maintainability and reliability. This fact represents a considerable contribution in applied subjects in the field of solid mechanics.

It is expected that interaction between partners will be substantial and should facilitate as much as possible:


- The combination of various experimental and numerical techniques supported by the partners to produce corroborative data.
- The transfer of technology and expertise between partners and outside Russia.
- The effective cooperation among the Russian and possible foreign partners for obtaining the highest improvement of knowledge in the use of holographic interferometric and numerical techniques for experimental mechanics.

The main results of the project will be presented as a series of scientific papers and presentations in international conferences.

The specialists proposing this project have expertise in development and application of experimental and combined methods in the field of strength investigation of different structures. The experience of holographic interferometry and finite element technique implementation to strain/stress analysis consists of 20 years [3].

The offered methods and equipment can be effectively used not only for determination of residual stresses in welded pipelines, but also and in other welded designs. The majority of unique experimental data was received for the first time by means of various original holographic and speckle interferometry methods.

Most part of experimental work needed for the project performing is planning to be carried out at the specialized holographic laboratory of MEPhl. The equipment of this laboratory allows us to record high-quality holographic interference fringe patterns. The small-sized holographic installation created within the framework of the project will enable to receive holographic interferograms on industrial objects of research, including on pipelines.

The fulfillment of work under project will suppose an active role of the foreign partners, possessing experience development in the field of laser holography and mathematical numerical modeling of deformation processes. Executing of joint seminars and working meetings, exchange of the technological information is planned with partners:

Laboratory of Experimental Mechanics of Materials, Illinois Institute of Technology, Chicago (USA).
Polytechnicum of Strasbourg University (France).
Department of International Scientific Projects of Council of Europe.
The development of the project is the appreciable contribution to federal program "Safety", interdepartmental program "Risk" and international program of the Open Agreement of Council of Europe on prevention of accidents "Formose".


References
1. Birger.l.A. (1963) Residual stresses. Mashgiz, Moscow.
2. Kobayashi.A.S. (ed).(1987) Handbook of Experimental Mechanics Printice Hill, Englewood Cliffs, New Jersey.
3. Shchepinov.V.P., Pisarev.V.S., Novikov.S.A. and others. (1996) Strain and Stress Analysis by Holographic and Speckle Interferometry. John Wiley, Chichester.D


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