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Materials TestingMaterialprüfung

2/2018 Volume 60 www.Materialstesting.de

Materials – Components – Technology – Application

60 YearsMaterials

Testing

SteBLife – A new short-time procedure for the calculation of S-N curves and failure probabilities p. 121


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60 (2018) 2 © Carl Hanser Verlag, München Materials Testing

urements. Moreover, during the last two decades, the fatigue behavior of metallic (ferromagnetic) materials has been charac-terized by the change in temperature, elec-trical resistance and magnetics which has been getting more attention [4-8].

Generating S-N data for the design pro-cess of structures and components as well as for an optimal material selection needs to be performed in a quick and cost-effective way.

this field. The S-N curve, which is still used today for an optimized material selection and an enhanced component design, is also often named as Wöhler curve providing the relationship between the applied load and the lifetime of the specimen [1-3]. From a classic point of view, the cyclic deforma-tion behavior of metal is characterized by the plastic strain amplitude derived from mechanical stress-strain hysteresis meas-

The failure caused by material fatigue is a central issue for a lot of technical applica-tions. A large number of current and his-torical examples have shown, that such failure could lead to terrible accidents and serious loss, which means in the worst case, even the loss of human life. The first systematic fatigue investigations were per-formed by August Wöhler around 150 years ago, who was one of the pioneers in

A fast calculation of Wöhler or S-N curves is of major importance in the design process of complex components and for selection of optimized materials. In the context of this work, it is shown how the nondestruc-tive testing methods, the digitalization of the measurement techniques as well as signal processing can be combined with a new short-time pro-cedure in order to acquire potentially more information from fatigue processes, while experimental effort and costs are reduced significantly. The results of this combination show not only considerable advantages over conventional methods aimed at determining S-N curves, but also over established short-term procedures, due to the possibility to separate several material mechanisms by means of data analysis and to use this for fatigue life calculations based on the results of a few specimens only. The SteBLife approach is a new short-time calculation method developed at the Chair of Nondestructive Testing and Quality Assurance at Saarland University, Germany, which takes into account that there is no linear re-lation between the elastic, elastic-plastic and plastic portion of the mate-rial response in the deformation process. With respect to the test strategy, the number of fatigue tests can be reduced for SteBLifemtc and SteBLifemsb to three and five, respectively, or in the case of SteBLifestc to one single constant amplitude test with a special step-shaped specimen geometry when compared with approximately 15 required fatigue tests for the con-ventional determination of Wöhler or S-N curves. Moreover, SteBLifemsb offers the possibility to calculate scatter bands for different failure proba-bilities, which is an important feature, even in the case of inhomogeneous materials. Within the scope of the presented work, the change in tempera-ture of SAE 1045 (C45E) specimens was measured during fatigue tests by an infrared camera in order to feed the thermal response back into the new SteBLife approach for a reliable fatigue life calculation.

Peter Starke, Alexander Bäumchen and Haoran Wu, Saarbrücken, Germany

SteBLife – A new short-time pro-cedure for the calculation of S-N curves and failure probabilities

Article Information

Correspondence AddressDr.-Ing. Peter StarkeChair of Non-Destructive Testing and Quality AssuranceSaarland UniversityAm Markt Zeile 4D-66125 SaarbrückenE-mail: [email protected]

KeywordsThermography, magnetics, nondestructive testing, fatigue life evaluation, SteBLife


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The reason why S-N curves are still fa-vored for structural fatigue design is mainly devoted to tradition and the fact that loads are related to stresses and strains. Especially strains have been able to be sensed over the past decades, repre-sented by means of strain gauges, exten-someters or similar sensors. Until today, there is still an intense discussion with respect to the validity of the Palmgren-Miner linear damage accumulation rule and the importance of the nonlinearity in a damage accumulation process. The un-derstanding of the load-damage relation is of major importance for the development of fatigue life calculation methods or eval-uation models, which are aimed at pre-dicting the lifetime of a specimen, a com-ponent or even a whole structure outgoing from the initial state or in any state during service. In this context, the discussion about appropriate damage parameters in fatigue life evaluation and monitoring should be based on a level of broader physical understanding.

Within the new fatigue life calculation method SteBLife (stepped bar fatigue life evaluation), which is a new development

at the Chair of Nondestructive Testing and Quality Assurance at Saarland Uni-versity, Germany, a stepped specimen geometry with several gauge lengths is designed in order to evaluate the local material responses during fatigue tests through the application of different NDT techniques [9]. This short-time procedure offers an enormous reduction in experi-mental time, costs and offers therefore the possibility to take more fatigue rele-vant parameters into account. Conse-quently, safety factors can be defined more individually and replacement strat-egies can get a flexible characteristic, which is of highest interest with a closer look at large infrastructures and complex loaded components or structures.

SteBLife combines the potential of NDT methods, the digitalization of the measure-ment techniques as well as signal process-ing to acquire comprehensive information concerning the fatigue behavior with a si-multaneous reduction of experimental ef-forts and costs, which means in the end, fewer experiments but more information.

Instead of conventional stress-strain hysteresis, temperature measurements [5, 6] are used, whereas the current measure-ments in the framework of this research are mostly focused on infrared camera techniques. For further investigations, also electromagnetic measurements based on a self-developed modular hall probe array will be used. In general, the changes of these physical quantities are directly linked by cross ef-fects to microstructural changes in the bulk material obtained during quasi-static or even fatigue loading.

Material

The SAE 1045 (C45E) steel is in accordance with DIN EN 10083-1. The material was de-livered in form of round bars with a length of 5 m, a diameter of 20 mm and a scaly surface due to the heat treatment. The heat treatment by the manufacturer consists of austenitization at 850 °C, followed by a controlled slow cooling process to ambient temperature to receive a mostly ferritic-pearlitic microstructure which is shown in a light micrograph in Figure 1.

In the micrograph, ferrite is displayed whitish and the pearlite is characterized by the cementite lamellas (black) and the fer-rite (white) regions in between.

The SAE 1045 (C45E) steel in the nor-malization condition has a Brinell hard-ness of 210 and a mostly ductile material behavior due to applied loads. The chemi-cal composition from the analysis in com-parison to DIN EN 10083-1 as well as the mechanical and quasistatic properties are summarized in Tables 1 and 2.

Experimental setup

Stress-controlled constant amplitude tests (CATs) on SteBLife- and hourglass speci-mens were carried out at ambient tempera-ture with a frequency of 5 Hz on a servohy-draulic testing system by Shimadzu type EHF-L with a maximum cyclic/quasistatic load of 20/25 kN using a sinusoidal load-time function at a load ratio of R = -1.

The fatigue tests were performed to a maximum number of cycles Nmax of 2 × 106 or until specimen failure. To characterize the microstructure-based fatigue behavior in detail, the change in temperature ΔT was calculated from the surface tempera-ture of the specimen, which was measured along the gauge length by an infrared cam-era continuously during the fatigue tests. This quantitative change is directly related to deformation-induced changes of the mi-crostructure in the bulk material and is considered to represent the current fatigue state. Since metallic materials are very good heat conductors, the microstructure related dissipated energy is superimposed by heat transfer processes, which requires a temperature stabilized grip system of the servohydraulic test setup.

For these investigations, an infrared camera by Micro-Epsilon type thermoIM-AGER TIM 450 has been used. This system provides a spectral range of 7.5-13 μm, an optical resolution of 382 × 288 pixels and a thermal sensitivity of 40 mK. The data ac-

Parameters DIN Manufacturer’s certificate

Yield strength Rp0.2 (MPa) min. 305 413

Tensile strength Rm (MPa) min. 580 710

Ultimate strain A (%) min. 16 23.5

Notch impact strength KCU (J × cm-2) – 56

Hardness (HB) – 210

Table 2: Mechanical and quasistatic properties of normalized SAE 1045 (C45E)

Figure 1: Pearlitic-ferritic microstructure of normalized SAE 1045 (C45E)

Table 1: Chemical composition of normalized SAE 1045 (C45E)

Alloying elements (wt.-%)

C Si Mn P S Cr Mo Ni

DINmin. 0.42 - 0.50 - - - - -

max. 0.50 0.40 0.80 0.030 0.035 0.40 0.10 0.40

Manufacturer’s certificate 0.47 0.23 0.72 0.012 0.013 0.06 0.014 0.07


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quisition software was programmed ac-cording to the requirements of the fatigue tests on National Instruments LabView 2015. The software allows to extract ther-mographs and to measure point/line/field temperatures on the specimen surface.

For the sake of the thermal stability as well as the accuracy during long test inter-vals, the infrared camera has been modi-fied by an active cooling system. The ex-perimental setup is given in Figure 2.

To get the thermal stability of the speci-men grips realized, a cooling device based on a Peltier element cooled water circuit was developed, which reduces the temper-ature difference between upper and lower grip during the test to less than 1 K as well as the temperature drift during the test to less than 2 K.

Compared to conventional techniques in destructive testing, the application of ther-mography results in a gain of information for the characterization of the cyclic defor-mation behavior of metallic materials.

Theory

The SteBLife approach gives comprehen-sive information regarding the load-func-tion related deformation behavior and thus the load-lifetime relation by means of an S-N curve of a material with only a few or even down to one single specimen. The specimen geometry is characterized by a stepped shape with different gauge length diameters, which consequently result in different local stresses along the axial di-rection of the specimens during loading. The fatigue tests are performed under con-stant amplitude loading and the material response is measured by NDT-related methods, for example, in this case using an infrared camera. Based on this idea and due to the symmetry of the specimen, it is possible to plot one cyclic deformation curve for the gauge length with the small-est diameter and for each further one two cyclic deformation curves, as shown in Fig-ure 3 in a schematically manner.

These cyclic deformation curves provide the database for cyclic stress-material re-sponse curves. In contrast to other short-time procedures like PhyBaL [10, 11], no damage due to the previous steps and the increased load or hardening effects from load increase tests has to be taken into con-sideration and the stress-material respon-serelationship can be used as measured, which is a significant advantage in the case of SteBLife method. In the following, Mor-row and Basquin equations are combined to

calculate the fatigue life and in the end the S-N-curve of the material.

Following this strategy, three different modules were developed up to this date. The first one is SteBLifestc (stc: single test, trend curve) which allows calculating a trend S-N curve based on one single test with a duration of 2 to 3 hours. SteBLifemtc (mtc: multiple tests, trend curve) is based on 2 to 5 StebLife tests at different stress amplitudes to improve the statistics of the provided dataset and to generate a repre-sentative mean S-N curve, which also takes the inhomogeneity of materials into ac-count. The third module SteBLifemsb (msb: multiple tests, scatter bands) offers the possibility to calculate scatter bands for different failure probabilities, which is an important feature even in the case of more inhomogeneous materials like, e. g., cast irons. Therefore, 4 or 5 SteBLife tests are performed at the same stress amplitude and the received lifetimes are transferred into a Gaussian distribution, which allows determining the lifetimes for different fail-

ure probabilities, e. g., 5, 50 and 95 %. Scat-ter bands for different failure probabilities show slight variations in the slopes and, due to this, also slight differences in the fatigue strength coefficients σf’. From the SteBLife calculation of each test, the Nf-σf’ relationship can be drawn, so that it is also possible to calculate individual σf’ values for the respective scatter bands. Within the scope of this paper, examples for all three SteBLife approaches are presented.

Results

For the following temperature measurements, seven measurement fields (5 × 5 pixels) within the software of the infrared camera were defined along the specimen, one in each gauge length (T1, T2o/u, T3o/u, o: upper, u: lower), and two at each shaft (T4o/u). The change in temperature (ΔT) was calculated in accordance with Equations (1) to (3) be-low:

ΔT1 = T1 – 0.5 · (T4o + T4u) (1)

Figure 3: Procedure of SteBLife approach

Figure 2: Experimental setup


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ΔT2o/u = T2o/u – T4u/o (2)

ΔT3o/u = T3o/u – T4o/u (3)

The diameter of both shafts is bigger than the diameter of any other gauge length, so that T4o/u could be considered as the elastic portion of the change in temperature as well as the ambient influences are there-fore subtracted from T1, T2o/u, T3o/u, which leads to the change in temperature (ΔT1, ΔT2o/u, ΔT3o/u). ΔT1, ΔT2o/u, ΔT3o/u are pro-portional to the plastic strain amplitude εa,p and are dedicated to the plastic deforma-tions in the specimen gauge length.

The SteBLifestc (stc: single test, trend curve) approach allows to give comprehen-sive information regarding the cyclic defor-mation behavior and also the load-lifetime relation through a S-N curve of the materials with one single specimen only. The specimen geometry is characterized by a stepped shape (see Figure 3) with different gauge length di-ameters, which consequently results in dif-ferent local stresses along the axial direction of the specimens during axial fatigue loading.

Each gauge length is cylindrical and has the same volume. The transition from one gauge length to another is realized by a ra-dius of 4 mm, which results in a mild notch and has to be taken into account (notch fac-tor of 1.11). The fatigue tests are performed under constant amplitude loading and the material response is measured by an infra-red camera. The specimen was loaded by a sinusoidal alternating axial force with an amplitude of Fa = 9.170 kN at a load ratio of R = -1. Based on the SteBLife idea and due to the symmetry of the specimen geometry, one cyclic deformation curve for the diam-eter of 6 mm and each two for the diame-ters of 6.2 mm and 6.4 mm resulting in lo-cal stress amplitudes of 360 (ΔT1), 337 (ΔT2,o/u o: upper, u: lower) and 318 MPa (ΔT3,o/u), respectively. The change in tem-perature is also calculated with respect to the pure elastic loaded shaft of the speci-men in accordance with Equations (1) to (3). The cyclic deformation curves are shown in Figure 4a.

Specimen failure occurred during the test (see Figure 4a) at Nf = 52,320, which

corresponds quite well with the conven-tionally determined lifetimes under consid-eration of the notch factor of 1.11.

The cyclic deformation curves are the da-tabase for cyclic stress-material response curves. For the SteBLifestc calculation, the change in temperature values are extracted at Nf/2 and plotted as a cyclic stress ampli-tude-temperature (CST) curve, which is shown in Figure 4b.

The same procedure is also performed for a SteBLife test with σa = 380 MPa (re-lated to 6 mm). As it is known from fatigue tests with hourglass specimens, higher stress amplitudes result in a higher mate-rial response and consequently a lower number of cycles to failure with 18 231 cy-cles. The cyclic deformation curves and CST curve are shown in Figure 5. The slope of CST curve in Figure 5b is absolutely comparable with that in Figure 4b.

Figure 6 shows cyclic deformation curves for different stress amplitudes from σa = 300 to 400 MPa and underlines the ca-pability of temperature measurements by an infrared camera for the characterization of the cyclic deformation behavior of metal-lic materials. Comparable with stress-strain hysteresis measurements, the change in temperature indicates the cyclic softening behavior as well as the incubation intervals of the normalized SAE 1045 (C45E) steel.

With respect to the cyclic deformation behavior, higher stress amplitudes conse-quently lead to larger values of the mate-rial response in terms of the change in tem-perature.

For the stress amplitudes of σa = 340 and 400 MPa, incubation and cyclic soften-ing is followed by cyclic hardening, whereas for lower stress amplitudes, the first two stages are followed by saturation. Until maximum cyclic softening at 10-13 % of the lifetime of the specimens (σa = 320 to 400 MPa), the cyclic deformation behav-

Figure 4: a) Temperature-based cyclic deformation curves for different diameters of the step-curved specimen, b) stress amplitude-temperature curve in a constant amplitude test with σa = 360 MPa (related to 6 mm) in accordance with the SteBLifestc approach for normalized SAE 1045 (C45E)

Figure 5: a) Temperature-based cyclic deformation curves for different diameters of the step-curved specimen, b) stress amplitude-temperature curve in a constant amplitude test with σa = 380 MPa (related to 6 mm) in accordance with the SteBLifestc approach for normalized SAE 1045 (C45E)

Figure 6: Cyclic deformation curves based on the change in temperature for normalized SAE 1045 (C45E)


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ior is mostly related to dislocation reac-tions. From this point, ongoing in addition to dislocation reactions, micro cracks are formed and propagate from the surface. In the last stage, the fracture is indicated by a secondary softening, which is mostly re-lated to the macro crack formation as well as propagation processes in the last 30-40 % of the lifetime. The use of temperature measurements leads to an advantage in comparison to conventional measurement techniques, because it is applicable even in the case of complex geometries without a defined gauge length, but also on materials in higher or high strength conditions, where material reactions can be only hardly detected by mechanical stress-strain hysteresis measurements.

In the following, a combination of Mor-row [12] and Basquin [13] equations is used, as it is already explained in Figure 3, to calculate the fatigue life and at least the S-N curve of the material.

Figure 7 gives an impression regarding the capability of the SteBLifestc approach, which enables the calculation of the fatigue life of the materials based on one single fa-tigue test. Figure 7 shows the calculated result of the SteBLifestc test shown in Fig-ures 4 and 5, which is in very good accord-ance with the conventionally determined lifetimes from constant amplitude tests.

The slopes of the S-N curves are slightly different, which depends on the scatter and inhomogeneity of individual tests, yet both regression calculations show a similar trend in both S-N curves.

In order to improve the result of the fa-tigue life calculation and to set it on a more statistic database, 3 to 5 SteBLifestc tests can be performed. The overall CST curve of 4 SteBLife tests is shown in Figure 8a, which is the basis for the SteBLifemtc (mtc: multiple test, trend curve) calculation. The calculated results lie in between the deter-mined lifetimes from conventional CATs. The values of fatigue strength coefficients are nearly the same, which are around 750 MPa (751 MPa for conventional CATs and 755 MPa for SteBLifemtc). Moreover, the fatigue strength exponents are for both -0.07. This calculation highlights the ad-vantage of this fatigue life calculation method with respect to the reduction of the experimental time by more than 90 %, which leads to enormous financial and sci-entific benefits compared to the conven-tional way to provide S-N data.

In the case of more inhomogeneous ma-terials, scatter bands for different failure probabilities (Pf) may become of interest as

additional information in an S-N dataset. Therefore, the SteBLifemsb (msb: multiple tests, scatter bands) module was designed, which combines 4 to 5 SteBLife tests per-formed at the same stress amplitude. The deviation of lifetimes (number of cycles to failure, Nf) from different SteBLife tests are plotted according to the model of a Gauss-ian distribution [14] (see Figure 9a), which is calculated using Equation (4).

see Equation 4 (4)

From the Gaussian distribution, the life-times of different failure probabilities can be calculated. In this example, we have chosen 5, 50 and 95 % for plotting the scat-ter bands. Other probabilities can also be calculated, e. g., 0.01 %, which is an impor-tant value for the design of components in

the automotive industry. Different fatigue strength exponents can be received from the 5 SteBLife tests and can be plotted as an exponent-lifetime relation. By using the calculated lifetimes, e. g., at a failure prob-ability of 5, 50 and 95 % (see Figure 9a) in the exponent-lifetime relation, also fatigue strength exponents for a failure probability of 5, 50 and 95 % can be derived.

Based on the calculations presented in Figure 9, S-N curves for failure probabili-ties of 5, 50 and 95 % can be determined. The results are shown in Figure 10 to-gether with the lifetimes of 11 CATs with conventional hourglass specimens, which are used as reference values. Within the next months, additional tests on hourglass specimens of normalized SAE 1045 (C45E) will be performed in order to provide a sta-tistically verified dataset.

Pf= 1

(2⋅ π ⋅4.2523056512⋅107)⋅e

−(Nf −40455.55)2

2⋅4.2523056512⋅107 Equation 4

Figure 8: a) Stress-temperature relation and b) experimental S-N data and SteBLifemtc curve calculated on the basis of the change in temperature and 4 tests in accordance with the SteBLifemtc approach for normalized SAE 1045 (C45E)

Figure 7: Experimental S-N data and SteBLifestc curve calculated on the basis of the change in temperature for a) a SteBLife test with σa = 360 MPa (see Figure 4) and b) σa = 380 MPa (see Figure 5) for normalized SAE 1045 (C45E)


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Figure 10 underlines that the conven-tional determined lifetimes can be reliably described by scatter bands calculated on the basis of SteBLifemsb. With a look to the mathematical description according to Bas-quin [13], the fatigue strength coefficients and exponents of calculated and conven-tional determined S-N data are in the same range. As the authors have expected, the S-N curve calculated for a failure probabil-ity of 50 % shows the best fit.

In a next step, this procedure will be validated on cast iron specimens, which show a much higher scatter behavior than the investigated SAE 1045 steel.

The results of Figures 7, 8 and 10 show that all SteBLife modules have a high po-tential to reduce the experimental efforts up to 90 %, which leads to enormous practi-cal and financial advantages. Depending on the material as well as the application, for which the evaluated datasets are going to be used, only a few or even down to one single constant amplitude test has to be performed, which means an enormous sim-plification in the provision process of fa-tigue data.

Conclusions

Besides extensometers and strain gauges, additional measurement techniques have been introduced in the last decades. Re-cently, NDT methods based on thermal and magnetic sensing technologies have drawn increasing attention for determining of fa-tigue relevant parameters, as these ap-proaches are closely associated with the physical features of materials, it could be of benefit especially to short-time procedures or fatigue life calculation methods.

The above-mentioned measuring tech-niques are highly sensitive to microstruc-tural changes due to dislocation reactions and could therefore offer significant advan-tages in comparison with those conven-tional methods based on techniques, such as stress or strain measurements. Further-more, material properties determined by these techniques are independent of a de-fined gauge length and also applicable to complex geometries like notched speci-mens or even components.

Within the new fatigue life calculation method SteBLife, developed at the Chair of

Nondestructive Testing and Quality Assur-ance at Saarland University, Germany, a stepped specimen geometry with 5 gauge lengths has been designed, in order to eval-uate the local material responses during fatigue processes by combining the appli-cation of different NDT techniques. This short-time procedure allows to reduce the efforts in experimentation by more than 90 % and offers therefore the possibility to take more fatigue relevant parameters into account, which improves the S-N curve to a multidimensional dataset.

With respect to the test strategy, the number of fatigue tests could be reduced to only 3 to 5 by utilizing SteBLifemtc (mtc: multiple tests, trend curve) and SteBLifemsb (msb: multiple tests, scatter bands), re-spectively. Using SteBLifestc (stc: single test, trend curve), this effort could be fur-ther lessened down to only one single CAT with a special step-shaped specimen geom-etry in contrast to approximately 15 fa-tigue tests for the conventional determina-tion of Wöhler or S-N curves.

Moreover, SteBLifemsb offers the possibil-ity to calculate scatter bands for different failure probabilities, which brings an im-portant advantage even in the case of inho-mogeneous materials.

Acknowledgements

The authors would like to thank the German Research Foundation (Deutsche Forschun-gsgemeinschaft DFG) for the financial sup-port of this research (STA 1133/6-1).

The authors would also like to thank Shi-madzu Europe and Micro-Epsilon for their support in technical equipment provision.

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Figure 10: Experimental S-N data and SteBLifemsb

curve with calculated scatter bands for failure

probabilities of 5, 50 and 95 %, based on the

change in temperature (ΔT) from 5 SteBLife tests

with σa = 360 MPa for normalized SAE 1045

(C45E)

Figure 9: a) Gaussian distribution, b) fatigue strength exponent-lifetime relation for 5 tests in accord-ance with the SteBLifemsb approach for normalized SAE 1045 (C45E)


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Bibliography

DOI 10.3139/120.111139Materials Testing60 (2018) 2, pages 121-127© Carl Hanser Verlag GmbH & Co. KGISSN 0025-5300

The authors of this contribution

Dr.-Ing. Peter Starke, born in 1977, studied Mechanical Engineering at University of Kaisers lautern, Germany. Since 2002, he has been a research assistant at the Chair of Materi-als Science and Engineering, University of Kai-serslautern. He received his engineering doc-toral degree in 2007 working on “The fatigue life calculation of metallic materials under constant

mplitude loading and service loading”. From 2007 to 2013, he worked as the Head of “Fatigue life calculation” group at University of Kaiser-slautern. Since 2013, he is a senior research as-sociate at the Chair of Non-Destructive Testing and Quality Assurance at Saarland University, Germany. His research is mainly focused on the use of nondestructive measurement techniques for the characterization of the fatigue behavior and the fatigue life calculation of metallic and nonmetallic materials in the LCF, HCF and VHCF regime as well as for the evaluation of de-fects and inhomogeneities in material micro-structures.

Alexander Bäumchen, born in 1994, is study-ing Materials Science and Engineering at Saar-land University in Saarbrücken, Germany. Haoran Wu, born in 1988, studied Materials Science and Engineering at Tianjin University, China and Saarland University in Saarbrücken, Germany. After that, he received his master de-gree from Saarland University in 2016, he works at the Chair of Nondestructive Testing and Qual-ity Assurance (LZfPQ), where he finished his by DGZfP (The German Society for Non-Destructive Testing) rewarded master thesis. He is currently working as a research associate with focus on NDT methods and material fatigue.

Abstract

SteBLife – Ein neues Kurzzeitverfahren zur Berechnung von Wöhler-Kurven und Ausfallwahrscheinlichkeiten. Eine zeiteffiziente Ermittlung von Wöhler- oder S-N Kurven ist wesentlich für die Auslegung komplexer Bauteile sowie für eine optimierte Materialauswahl. Im Rahmen dieser Arbeit wird gezeigt, wie zerstörungsfreie Prüfverfahren, Digitalisierung der Messtechnik sowie die Signalverarbeitung mit einem neuen Kurzzeit-verfahren kombiniert werden können, um potenziell mehr Informationen aus dem Ermüdungsverhalten zu gewinnen, während gleichzeitig der ex-perimentelle Aufwand und die Kosten erheblich reduziert werden. Die Ergebnisse dieser zuvor beschriebenen Kombination zeigen nicht nur er-hebliche Vorteile gegenüber herkömmlichen Methoden zur Ermittlung von Wöhler-Kurven, sondern auch gegenüber etablierten Kurzzeitverfah-ren, da hierbei unterschiedliche Werkstoffmechanismen mittels Daten-analyse separiert werden können. Durch diesen Ansatz können auf der Basis weniger Versuche Aussagen zur Lebensdauer eines Werkstoffes getroffen werden. Der SteBLife-Ansatz ist eine neue Kurzzeitberechnungs-methode, die am Lehrstuhl für Zerstörungsfreie Prüfung und Qualitätssi-cherung der Universität des Saarlandes entwickelt wurde. Dabei wird be-rücksichtigt, dass im Verformungsverhalten eines metallischen Werkstof-fes zwischen dem elastischen, elastisch-plastischen und plastischen Anteil kein linearer Zusammenhang besteht. Im Bezug auf die vorgestell-ten Versuchsstrategien kann die Anzahl der Ermüdungsversuche für SteBLifemtc und SteBLifemsb auf drei bis fünf bzw. für SteBLifestc auf einen einzigen Ermüdungsversuch mit einer speziellen stufenförmigen Probe reduziert werden, wohingegen die konventionelle Ermittlung von Wöhler oder S-N Kurven ca. 15 Ermüdungsversuche erfordert. Darüber hinaus bietet SteBLifemsb die Möglichkeit, Streubänder für unterschiedliche Aus-fallwahrscheinlichkeiten zu berechnen, was gerade bei inhomogenen Werkstoffen eine wichtige Funktion darstellt. Im Rahmen der vorgestell-ten Arbeit wird die Temperaturänderung von Proben aus C45E in Ermü-dungsversuchen mittels einer Infrarotkamera gemessen und diese Werk-stoffantwort in der neuen SteBLife-Methode für eine zuverlässige Lebens-dauerberechnung genutzt.

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Materials Testing