Advanced Evaluation of Fatigue Phenomena Using Non-Destructive Testing Methods

Abstract

The comprehensive characterization of the change in metallic materials’ microstructure due to an applied load is of prime importance for the understanding of basic fatigue mechanisms or more general damage evolution processes. If those mechanisms and processes are to be understood to a much greater extent, advanced fatigue life calculation methods being far away from linear damage accumulation models, have to be realized providing more than “classic fatigue data” only. Among others the PHYBAL (physically based fatigue life calculation) method including current enhancements and a thereon-based development named SteBLife (step-bar fatigue life approach) have been developed over the last 10 years. These methods allow the efforts in experimentation to be reduced by more than 90 % and therefore offer the possibility to take further fatigue relevant parameters into account. This therefore allows a variety of S,N-curves dependent on those fatigue relevant parameters to be generated with those methods easily establishing a multidimensional dataset. To just name a few examples of those parameters such as the influence of temperature, loading conditions, geometry as well as thermal and mechanical ageing processes on the fatigue behavior can now be calculated in accordance to a process being straightforward leading to an important step with regard to improving the efficiency of assessing structural components. Consequently, safety factors can be defined more in accordance to structural needs, being of highest interest with respect to the increasing number of ageing infrastructure such as highways, bridges or others. A lot of this ageing infrastructure has a strong need to be managed with respect to its structural integrity and the engineering community therefore tries the residual life of this infrastructure to be determined as appropriate as possible. In that context non-destructive testing parameters are increasingly considered to characterize a metallic material’s microstructure allowing more precise information to be obtained regarding the actual damage condition and the integrity of a component. The paper will address the high capability of non-destructive testing techniques for the evaluation of damage evolution processes also with respect to mechanism based fatigue as well as residual life calculations according to PHYBAL and SteBLife.

Characterization of the fatigue behaviour and lifetime evaluation of metallic materials based on thermographic NDT-methods

“…Test data published in [18][19][20] have been used along with new test results, which were obtained from the same batch of specimens. These data were used to examine the former work of Risitano et.…”

“…The change in temperature, commonly used in the PHYBALLSV related experiments [18] and the SteBLife-method [19,20], which is calculated by the formula = 1 − ( 2 + 3 )/2, are further corrected by subtracting the very first value from every test, in order to make sure, that all the tests start with = 0. [18,19] When not only taking the change in temperature into account, but also considering thermodynamics, it is meaningful to calculate the area of the region enclosing the -curve, which represents the dissipated energy during the fatigue process [17,29].…”

“…[18,19] When not only taking the change in temperature into account, but also considering thermodynamics, it is meaningful to calculate the area of the region enclosing the -curve, which represents the dissipated energy during the fatigue process [17,29]. It is emphasized, that this calculation is linked to the time , load frequency and the IR-camera recording frequency .…”

“…The fatigue tests were performed until failure or to a maximum number of cycles = 2 · 10 6 . During the tests, temperature measurement areas on the specimen are defined with the same method as described in [18][19][20] …”

“…Since Reifsnider and Williams applied an IR-camera in this field for the first time [7], there is an increasing number of publications dealing with similar methods [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Among these different activities having been published, the parameters commonly used to describe the change in temperature of defined measurement areas are [3,4,6], [5] or [12], which are given different physical meanings by different authors while the symbols being similar.…”