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Damage state evaluation of X6CrNiNb18-10alloys during cyclic deformation by exploiting the fractal analysis of ultrasonic sampling phased array backscatter signals

 
: Schreiber, J.

:
Volltext (PDF; )

South-African Insitute for Non-Destructive Testing -SAINT-:
18th World Conference on Non-Destructive Testing 2012. Proceedings. CD-ROM : WCNDT 2012, 16 - 20 April 2012, Durban, South Africa
Lynnwood Ridge: SAINT, 2012
ISBN: 978-0-620-52872-6
9 S.
World Conference on Non-Destructive Testing (WCNDT) <18, 2012, Durban>
Englisch
Konferenzbeitrag, Elektronische Publikation
Fraunhofer IZFP, Institutsteil Dresden ( IKTS-MD) ()
Fraunhofer IZFP ()

Abstract
In light of today's problems concerning the life time of industrial components, scientists are searching for a commercially viable component control system to define the residual service. The understanding of the mechanisms leading to fracture is necessary to forecast the residual service time of metal constructions. A basic understanding of structural changes on different scaling levels is given by physical Mesomechanics. The plastic deformation and fracture of cyclically loaded materials is associated with the loss of stability, and is considered a relaxation process at different scales. The conventional approach of continuum mechanics describes this process on two levels, micro and macro. Mesomechanics fits between these two scales: the meso level. At this level and due to the high degree of freedom mainly in the near-surface areas, a new deformation structure, the so called mesostructure, develops. The hierarchical behavior of this structure leads to the hypothesis that the deformation structure is of fractal nature and the fractal dimension DF of the deformation structures should be a suitable parameter to characterize the material at different load phases. This approach offers a new concept for monitoring the material state by suitable nondestructive testing methods. The deformation structures of ferromagnetic steel were analyzed using surface topography and Barkhausen Noise signals [1]. In dependence of increasing loads, the derived parameter DF ensures a universal step-like behavior at three plateaus (levels). This behavior allows the assessment of the actual damage state of the material and estimation of the residual time before fracture appears macroscopically.
However, the mesostructure develops not only at the surface or in near-surface areas, but also in the volume of the material. The volume deformation structure can be visualized as a 3-D structure. In order to detect this volume's mesostructure, a nondestructive testing technique with deep penetration behavior such as ultrasonic testing, in particular, the Sampling Phased Array technique, has higher resolutions in comparison to conventional UT techniques, and the option of 3-D imaging of the measurement results was successfully used. Pronounced changes of UT scans detected during on-line measurements of the X6CrNiNb18-10 alloy during multi-axial fatigue tests was fractal analyzed by two different evaluation methods, Gaussian Smoothing and Autocorrelation Function. Thereby, the determined parameter DF increases in a step-like dependency from the cyclic number.
This concept was developed in the framework of reactor safety research, and was later applied in the industrial field for testing of feed-water piping systems in nuclear plants. Results of these investigations will be presented in this contribution.

: http://publica.fraunhofer.de/dokumente/N-243573.html