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Probabilistic high cycle fatigue models - volumetric approaches
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).ORCID iD: 0000-0002-2917-5045
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fatigue is the most frequent failure mode and must be considered in a mechanical designof actual operating components. The fact that mechanical design is most often linkedwith existence of stress raisers and multiaxial time-varying stresses has in the last decadesincreased the research effort worldwide. The goal is to put forward methods and ideasto explain the fatigue phenomenon so that costs can be decreased and reliability can beincreased. The ultimate goal is reliable performance of mechanical components.Most of the available models for High Cycle Fatigue (HCF) assessment are deterministicand are applied to experimental fatigue limits for a failure probability of 50%.These models are not intended to describe the statistical nature of HCF even with theknowledge that HCF has a degree of randomness (stochastic), often showing considerablescatter even in well controlled environments. In traditional product design, safetyfactors, or design factors, are usually assigned in order to assure reliability since thefatigue process is influenced by many different factors, i.e. size effect, gradient effect andload effect, which inherently exhibit scatter.Probabilistic approaches in fatigue design are practical due to the uncertainties associatedwith service loads, material properties, geometrical attributes, and mathematicaldesign models. This approach allows a quantification of risk that is not possible withdeterministic design approaches.In HCF assessment, both the deterministic and probabilistic models share a commoncritical point. The critical point is the transferability of the models, i.e. transferringfatigue data in between different geometries. In order to address the problem of transferability,and hence the prediction capability of the fatigue models in new situations,many engineers and researchers have contributed.The stress gradient and the structural size are known to be important factors affectingthe fatigue life of components. The volumetric approaches based on threshold stresslevels have indicated on good predictive capabilities. In these approaches, it is assumedthat only in some highly stressed material volume, fatigue processes take place.For describing the scatter around the fatigue limit, the weakest link (WL) model iswidely used. In theWL-model, the spatial stress field acting in a component is integratedover either the component material volume or surface and thus the failure probability isobtained. The model is considered to be the state of the art approach in HCF field.In this work, new probabilistic HCF models based on ideas originating from thehighly loaded region concept and the Theory of Critical Distances (TCD) are presented.The new HCF models stem from the hypothesis that fatigue damage can initiate at anyspatial point that is stressed higher than a material specific threshold stress value. Allpoints that fulfill this condition form the highly loaded regions. The new models arefound to have good transferability and improved predictive capability compared to theWL-model when validated with fatigue test data obtained from conducted experimentsusing cylindrical specimens loaded by uniaxial and rotating bending loading modes.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 32 p.
Series
TRITA-HFL, ISSN 1104-6813 ; 0585
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-178060ISBN: 978-91-7595-780-7 (print)OAI: oai:DiVA.org:kth-178060DiVA: diva2:876722
Public defence
2015-12-11, B2, Brinellvägen 23 (02 tr), KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Projects
Turbopower
Note

QC 20151204

Available from: 2015-12-04 Created: 2015-12-04 Last updated: 2015-12-04Bibliographically 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. New models for prediction of high cycle fatigue failure based on highly loaded regions
Open this publication in new window or tab >>New models for prediction of high cycle fatigue failure based on highly loaded regions
2014 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 66, 101-110 p.Article in journal (Refereed) Published
Abstract [en]

The stress gradient and the structural size are known to be very important factors affecting the fatigue life of metallic mechanical components. For multiaxial stress conditions, a relevant criterion must be used to account for both these effects as well as the history of a non-homogeneous stress state. The new probabilistic HCF models in this work fulfill these requirements. A stress field resulting from rotating bending is analyzed. Also, the well-known weakest link model has be compared to the new proposed HCF models based on the concept of the highly loaded regions. The new models take the highly loaded regions (volume, area) and the stress gradient effect into account. These models show significant improvement of failure probability prediction capability compared to the weakest link integral.

Keyword
Experiments, Probabilistic model, Highly loaded region, Gradient effect, High cycle fatigue
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-148599 (URN)10.1016/j.ijfatigue.2014.03.014 (DOI)000338622000011 ()2-s2.0-84901984180 (Scopus ID)
Note

QC 20140812

Available from: 2014-08-12 Created: 2014-08-11 Last updated: 2017-12-05Bibliographically approved
3. A Probabilistic Method for Multiaxial HCF based on highly loaded regions below the threshold depth
Open this publication in new window or tab >>A Probabilistic Method for Multiaxial HCF based on highly loaded regions below the threshold depth
2016 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 87, 91-101 p.Article in journal, Editorial material (Refereed) Published
Abstract [en]

For High Cycle Fatigue (HCF) assessment of structural components, a variety of evaluation methods that take important factors into account are available. The application of probabilistic evaluation methods is not always straightforward. Investigations show that the methods have limited predictive capability and most often poor transferability of fatigue test results for more complex geometries. Here, a new probabilistic HCF model based on the concept of highly loaded region is proposed. A depth parameter is taking on an important role in the proposed model. Different local stress criteria in combination with the new model and two other evaluation methods are studied. The purpose is to determine the combination of criteria and evaluation method that gives the best transferability. The results show that the new model has superior predictive capability than other models regardless the choice of fatigue stress criteria.

Place, publisher, year, edition, pages
Elsevier, 2016
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-177854 (URN)10.1016/j.ijfatigue.2016.01.002 (DOI)000374615900011 ()2-s2.0-84956873859 (Scopus ID)
Note

QC 20151204 QC 20160520

Available from: 2015-11-27 Created: 2015-11-27 Last updated: 2017-12-01Bibliographically 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. Surface and sub-surface probabilistic modelsfor HCF
Open this publication in new window or tab >>Surface and sub-surface probabilistic modelsfor HCF
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this paper, new probabilistic HCF evaluation models based on the surface andthe sub-surface region is presented. The sub-surface model stem from the hypoth-esis that the highest fatigue risk depends on the loading in a region slightly belowthe free surface area. There, the materials microstructural barriers must be over-come by fatigue crack propagation. The size of the region with high load below thesurface affects the fatigue risk. The predictive capability of the new proposed HCFmodel is studied along with comparisons to other state of the art HCF probabilisticmodels. Fatigue data originating from different geometries and materials conductedat different loading modes, rotating bending and uniaxial loading is used for theanalysis. The investigation performed in this work indicate that the new proposedmodels increase the prediction accuracy and hence predictive capability.

Keyword
Experiments, probabilistic model, highly loaded regions, high cycle fatigue
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-178054 (URN)
Note

QC 20151211

Available from: 2015-12-03 Created: 2015-12-03 Last updated: 2015-12-11Bibliographically approved

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