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Experiments, Computations and Models for Probabilistic HCF Design
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).ORCID iD: 0000-0003-0275-5557
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High Cycle Fatigue (HCF) failure is a common failure type for many mechanical components. Traditional HCF design is based on the deterministic safety factor approach, typically used in conjunction with the point stress method. A current development is to explicitly model the uncertainty of the design set-up, and compute the probability of failure, pf. If pf can be computed in an appropriate way, the contributions to fatigue can be identified and managed. Probabilistic design gives improved control over safety, which helps to avoid overly conservative design.

One of the applications dealt with in this work is gas turbine compressor blades. For this type of component requirements on safety coincide with requirements on high efficiency, low weight, etc. In such case, methods for accurate fatigue assessment become extra important.

In order to perform an appropriate fatigue design, certain requirements must be fulfilled. For example, the fatigue model that is used must be accurate and the relevant material parameters must be accurately determined. Other requirements are that the mesh used in the FE-computations for the stress field is fine enough, a HCF post-processor that enables application of fatigue models to real components must be available and a method for computation of pf including all uncertainties should also be available.

In Paper A, it is shown that for a gas turbine compressor blade, it is the number of elements through the blade’s thickness that is the most important mesh property for convergence in .

In Paper B, it is investigated which test strategy that should be used in order to perform accurate estimations of material parameters in multiaxial HCF criteria by use of as few laboratory tests as possible when different types of scatter are present, and when the cost to perform the fatigue tests is taken into consideration. It is shown that performing tests on few stress ratios located far away from each other is the best strategy, and that for tests performed in a high quality laboratory, scatter in specimen misalignment has an insignificant influence on the parameter estimation.

In Papers C and D, the prediction accuracy for the probabilistic volume based Weakest Link (WL) model and the Volume method for the Probability of Fatigue (VPF) is investigated. A novel specimen design is suggested for investigation of the volume effect. Based on the results, the newly developed VPF is favoured for design purpose. In Paper D, the HCF post-processor AROMA-PF is also presented, and used for computation of pf for a real gas turbine compressor blade geometry. The behavior of the predicted fatigue probability curves is very different between WL and the VPF for low pf-values.

In Paper E, a new method for fatigue probability assessment is presented. The classification of aleatory uncertainty type 1 and type 2 is also introduced. The suggested method is applied to a bladed disk in a gas turbine for computation of pf. The results show that the epistemic uncertainty in the modeling of the aero-forcing gives the major contribution to uncertainty in pf.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 45 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0588
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-182436ISBN: 978-91-7595-873-6 (print)OAI: oai:DiVA.org:kth-182436DiVA: diva2:904467
Public defence
2016-03-11, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (Swedish)
Opponent
Supervisors
Projects
COMP10
Note

QC 20160222

Available from: 2016-02-22 Created: 2016-02-18 Last updated: 2016-02-22Bibliographically approved
List of papers
1. FE-mesh effect of the volume based weakest-link fatigue probability applied to a compressor blade
Open this publication in new window or tab >>FE-mesh effect of the volume based weakest-link fatigue probability applied to a compressor blade
2012 (English)In: Proceedings of the ASME Turbo Expo: Volume 7, Issue PARTS A AND B, 2012, ASME Press, 2012, no PARTS A AND B, 427-438 p.Conference paper, Published paper (Refereed)
Abstract [en]

When dealing with design process of compressor blades, predominantly deterministic models are used for High Cycle Fatigue (HCF) investigations. The existing scatter in factors such as material inhomogeneity of the blade material and loading condition is accounted for by safety factors that often end up in conservative designs. An alternative way to account for these uncertainties is the application of probabilistic models. More information about the scatter in different sources together with probabilistic models can lead to a more robust design process. In order to compute the stresses acting in a compressor blade, the Finite Element (FE) method is widely used as standard tool. This method may show mesh dependence. Therefore, mesh requirements always exist in FE computations. In this work, a probabilistic HCF investigation is carried out for a transonic compressor rotor blade. The sensitivity of the volume based weakest-link probabilistic model (WL) due to different mesh properties of the blade is investigated. The goal is to provide advice for better finite element meshing of the blades based on linear type solid elements for the computation of stress history. The mesh types of the blade are the input parameters for the probabilistic HCF investigation. A stress invariant based HCF local criterion, Sines, and a critical plane criterion, Findley, are used in weakest-link to describe the failure probability for the 12% Cr-steel material used for the compressor blade. The estimation of the weakestlink and the local HCF criteria material parameters are performed using HCF experimental data based on 2 million load cycles obtained for smooth and notched specimens. The study shows that the choice of the mesh property through the thickness of the compressor blade has much more effect on the failure probability predictions compared to the in-plane mesh property of the blade.

Place, publisher, year, edition, pages
ASME Press, 2012
Series
Proceedings of the ASME Turbo Expo, 7
Keyword
Conservative designs, Deterministic models, Experimental datum, Failure Probability, Finite element meshing, Material inhomogeneity, Probabilistic models, Transonic compressor rotor, Compressors, Exhibitions, Gas turbines, Loading, Product design, Safety factor, Finite element method
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-128976 (URN)10.1115/GT2012-69852 (DOI)000335868800048 ()2-s2.0-84881183212 (Scopus ID)978-079184473-1 (ISBN)
Conference
ASME Turbo Expo 2012: Turbine Technical Conference and Exposition, GT 2012; Copenhagen; Denmark; 11 June 2012 through 15 June 2012
Note

QC 20130918

Available from: 2013-09-18 Created: 2013-09-17 Last updated: 2016-02-22Bibliographically approved
2. On the optimal choice of experiments for determination of parameters in multiaxial HCF-criteria
Open this publication in new window or tab >>On the optimal choice of experiments for determination of parameters in multiaxial HCF-criteria
2014 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 61, 315-324 p.Article in journal (Refereed) Published
Abstract [en]

Material parameters in multiaxial HCF-criteria must be estimated from experiments, which always are affected by scatter from several sources. A higher number of tests provides a more reliable parameter estimation, but the cost for the testing increases. In this paper, by starting from a known fatigue limit distribution for the titanium alloy Ti-6-4, test series are simulated according to different predefined test strategies. In Monte Carlo simulations, scatter in the material properties and in specimen alignment in the testing machine is taken into account. Based on the results obtained from the simulations, it is determined which one of the defined strategies that is most beneficial for HCF-criteria material parameter estimation, and also how many tests that should be performed in order to maximize the benefit of the information obtained from the fatigue testing.

Keyword
Experiments, Fatigue criteria, HCF, Monte Carlo
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-142321 (URN)10.1016/j.ijfatigue.2013.08.024 (DOI)000331919900032 ()2-s2.0-84893736361 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20140305

Available from: 2014-03-05 Created: 2014-02-28 Last updated: 2017-12-05Bibliographically approved
3. An investigation of the prediction accuracy for volume based HCF models using scaled geometries and scaled loading
Open this publication in new window or tab >>An investigation of the prediction accuracy for volume based HCF models using scaled geometries and scaled loading
2015 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 82, no 2, 317-324 p.Article in journal (Refereed) Published
Abstract [en]

he prediction accuracy for the volume based Weibull- and V* high cycle fatigue models is investigated and compared to the point method. A high number of fatigue tests are performed in rotating bending for single notched cylindrical specimens manufactured in a 12% Cr-steel, a high quality structural steel. The specimens are designed with different highly stressed volume and stress gradient by scaling the geometry, but with the same stresses at the corresponding scaled points. Thus, the maximum stress in the notch is the same. Experiments are performed for three specimen sizes at several stress levels. The volume based Weibull- and V*-models, as well as the point stress method are fitted to the experimental results. Based on the results, the V*-model is favored for design purposes.

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
HCF, Size effect, Scaled geometries, Weakest-link, Failure probability
National Category
Materials Engineering
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-160353 (URN)10.1016/j.ijfatigue.2015.04.024 (DOI)000365365900021 ()2-s2.0-84959557264 (Scopus ID)
Note

QC 20150219

Available from: 2015-02-19 Created: 2015-02-19 Last updated: 2017-12-04Bibliographically approved
4. Fast multiaxial high cycle fatigue evaluation in the probabilistic fatigue post-processor AROMA-PF
Open this publication in new window or tab >>Fast multiaxial high cycle fatigue evaluation in the probabilistic fatigue post-processor AROMA-PF
2015 (English)In: conference proceedings from the 22nd International Symposium on Air Breathing Engines, 2015Conference paper, Published paper (Other academic)
Abstract [en]

The probabilistic high cycle fatigue (HCF) post-processor AROMA-PF, developed mainly for HCF design of compressor blades, is presented. Several local multiaxial HCF criteria have been implemented for computation of the fatigue effective stress, and two volume based probabilistic HCF models have been implemented for computation of the fatigue probability: Weakest-link (WL) and the Volume method for the Probability of Fatigue (VPF). It is shown that for the type of stress history that acts in a compressor blade, for stress invariant based criteria, the effective stress can be expressed in closed form. This enables fast HCF evaluations. By comparing WL and VPF to test data obtained for Ti-6-4 specimens, it is seen that the highest transferability is obtained for WL used in combination with Crossland´s HCF criterion. The results also indicate on that the true fatigue behavior at low failure probabilities is between the predictions obtained by use of WL and VPF.

Keyword
High cycle fatigue, fatigue post-processor, probabilistic HCF models
National Category
Aerospace Engineering
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-177823 (URN)
Conference
ISABE-2015-20128
Note

QC 20151204

Available from: 2015-11-26 Created: 2015-11-26 Last updated: 2016-02-22Bibliographically approved
5. Probabilistic fatigue design of gas turbine compressor blades under aleatory and epistemic uncertainty
Open this publication in new window or tab >>Probabilistic fatigue design of gas turbine compressor blades under aleatory and epistemic uncertainty
2016 (English)Report (Other academic)
Abstract [en]

In this work, a new method for fatigue probability assessment is introduced. The method is applied to a bladed disk in a gas turbine for computation of the fatigue probability for a specific load case. Both epistemic and aleatory uncertainties are modeled. The aleatory uncertainty is of two types: Type 1 aleatory uncertainty is modeled by use of stochastic variables that influence the problem, including both design variables and stochastic parameters. Type 2 aleatory uncertainty is modeled by use of a probability that remains even if all stochastic variables are replaced by deterministic values. The fatigue behavior of a material exhibits type 2 aleatory uncertainty. The results show that the epistemic uncertainty in the modeling of the aero-forcing and the damping gives a large uncertainty in the computed failure probability. The new method is also used to study the influence on the probability of fatigue that comes from the stochastic variables.

Publisher
20 p.
Keyword
failure probability, fatigue assessment, high cycle fatigue, uncertainty, compressor blade
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-182430 (URN)
Projects
COMP10
Note

QC 20160219

Available from: 2016-02-18 Created: 2016-02-18 Last updated: 2016-02-22Bibliographically approved

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