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Fast multiaxial high cycle fatigue evaluation in the probabilistic fatigue post-processor AROMA-PF
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).ORCID iD: 0000-0003-0275-5557
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).ORCID iD: 0000-0002-2917-5045
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).ORCID iD: 0000-0001-8068-2360
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.

Place, publisher, year, edition, pages
2015.
Keyword [en]
High cycle fatigue, fatigue post-processor, probabilistic HCF models
National Category
Aerospace Engineering
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-177823OAI: oai:DiVA.org:kth-177823DiVA: diva2:874432
Conference
ISABE-2015-20128
Note

QC 20151204

Available from: 2015-11-26 Created: 2015-11-26 Last updated: 2016-02-22Bibliographically approved
In thesis
1. Probabilistic high cycle fatigue models - volumetric approaches
Open this publication in new window or tab >>Probabilistic high cycle fatigue models - volumetric approaches
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:nbn:se:kth:diva-178060 (URN)978-91-7595-780-7 (ISBN)
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
2. Experiments, Computations and Models for Probabilistic HCF Design
Open this publication in new window or tab >>Experiments, Computations and Models for Probabilistic HCF Design
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:nbn:se:kth:diva-182436 (URN)978-91-7595-873-6 (ISBN)
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

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