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Effects of different load schemes on the fatigue crack growth rate
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
2006 (English)In: Journal of Testing and Evaluation, ISSN 0090-3973, E-ISSN 1945-7553, Vol. 34, no 4, 333-341 p.Article in journal (Refereed) Published
Abstract [en]

An experimental study was conducted on a nickel-based super alloy, Inconel 718, at an elevated temperature of +400 degrees C. The basic question was how to conduct the experiments in order to obtain fatigue crack growth data as free from load history effects as possible in the stage II region. This was investigated by use of four different load sequences. An important question was to investigate if the so called constant K-I,K-max-method provides an upper bound of the fatigue crack growth rate in the stage II region. The results indicate that the constant K-I,K-max-method gives an upper bound in fatigue crack growth rate in the current measuring range but also gives a statistically significant lower exponent of the crack growth equation. A more reliable method to establish the fatigue crack growth parameters is to propagate the crack under constant stress-intensity factor range. This should be done at two different stress-intensity factor ranges for a few millimetres at each range. Also, Delta K-reducing procedure should not be used, when performing fatigue crack growth testing in the stage II region, in order to minimize the risk of transient crack closure effects.

Place, publisher, year, edition, pages
West Conshohocken, PA: American Society for Testing and Materials , 2006. Vol. 34, no 4, 333-341 p.
Keyword [en]
fatigue crack growth, crack closure, load scheme, constant KImax-method, experiments
National Category
Applied Mechanics Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-15865DOI: 10.1520/JTE14097ISI: 000239197400010Scopus ID: 2-s2.0-33845245634OAI: oai:DiVA.org:kth-15865DiVA: diva2:333907
Projects
Konsortium Materialteknik för demonstration och utveckling av termiska energiprocesser: KME-101
Funder
Swedish Energy Agency
Note

QC 20100525

Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Fatigue crack growth experiments and analyses - from small scale to large scale yielding at constant and variable amplitude loading
Open this publication in new window or tab >>Fatigue crack growth experiments and analyses - from small scale to large scale yielding at constant and variable amplitude loading
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is on fatigue crack growth experiments and assessments of fatigue crack growth rates. Both constant and variable amplitude loads in two different materials are considered; a nickel based super-alloy Inconel 718 and a stainless steel 316L. The considered load levels extend from small scale yielding (SSY) to large scale yielding (LSY) for both materials.

The effect of different load schemes on the fatigue crack growth rates is investigated on Inconel 718 and compact tension specimens in Paper A. It is concluded that load decreasing schemes give a to high Paris law exponent compared to constant or increasing load amplitude schemes. Inconel 718 is further analyzed in Paper B where growth rates at variable amplitude loading in notched tensile specimens are assessed. The predictions are based on the fatigue crack growth parameters obtained in Paper A. The crack closure levels are taken into consideration and it is concluded that linear elastic fracture mechanics is incapable of predicting the growth rates in notches that experience large plastic cyclic strains. Even if crack closure free fatigue parameters are used and residual stresses due to plasticity are included. It is also concluded that crack closure free and nominal fatigue crack growth data predict the growth rates equally well. However, if the crack closure free parameters are used, then it is possible to make a statement in advance on the prediction in relation to the experimental outcome. This is not possible with nominal fatigue crack growth parameters.

The last three papers consider fatigue crack growth in stainless steel 316L. Here the load is defined as the crack tip opening displacement parameter. Paper C constitutes an investigation on the effect of plastic deformation on the potential drop and consequently the measured crack length. It is concluded that the nominal calibration equation obtained in the undeformed geometry can be used at large plastic deformations. However, two conditions must be met: the reference potential must be taken in the deformed geometry and the reference potential needs to be adjusted at every major change of plastic deformation. The potential drop technique is further used in Paper D and Paper E for crack length measurements at monotonic LSY. Constant amplitude loads are considered in Paper D and two different variable amplitude block loads are investigated in Paper E. The crack tip opening displacement is concluded in Paper D to be an objective parameter able to characterize the load state in two different geometries and at the present load levels. Furthermore, if the crack tip opening displacement is controlled in an experiment and the local load ratio set to zero, then only monotonic LSY will appear due to extensive isotropic hardening, i.e. elastic shake-down. This is also the reason why the linear elastic stress-intensity factor successfully could merge all growth rates, extending from SSY to monotonic LSY along a single line in a Paris law type of diagram, even though the generally accepted criteria for SSY is never fulfilled. For the variable amplitude loads investigated in Paper E, the effect of plastic deformation on measured potential drop is more pronounced. However, also here both the crack tip opening displacement parameter and the linear elastic stress-intensity factor successfully characterized the load state.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 58 p.
Series
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0531
Keyword
Fracture mechanics, fatigue crack growth, finite elements analysis, small scale yielding (SSY), large scale yielding (LSY), low cycle fatigue (LCF), crack tip opening displacement (CTOD), crack closure, potential drop method, constant amplitude load, variable amplitude load, inconel 718, stainless steel 316L
National Category
Applied Mechanics Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-109710 (URN)
Public defence
2013-02-01, F3, Lindstedtsvägen 26, Stockholm, 10:00 (Swedish)
Opponent
Supervisors
Note

QC 20130108

Available from: 2013-01-08 Created: 2013-01-08 Last updated: 2013-01-11Bibliographically approved

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