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The Effect of Large Scale Plastic Deformation on Fatigue Crack Length Measurement with the Potential Drop Method
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
2011 (English)In: Journal of Testing and Evaluation, ISSN 0090-3973, E-ISSN 1945-7553, Vol. 39, no 6, 985-1002 p.Article in journal (Refereed) Published
Abstract [en]

A combined experimental and numerical investigation was conducted on the effects of plastic deformation and material resistance on the relationship between the potential difference and crack size, denoted calibration curve, or equation. The stainless steel 316L was used at room temperature for investigating the limitations of the calibration curve. The nominal calibration equation, obtained from the undeformed geometry, can be used for fatigue crack length measurements at large plastic deformation. However, the reference potential must, for reliable crack length measurements, be measured at the deformed state and later adjusted at every major change of (plastic) deformation. The major part of the change in reference potential was attributed to the geometry change and only a minor part to the resistivity change. The scatter in the potential drop measured crack length, measured on a cycle by cycle basis, was about 30 times smaller here compared to the compliance measured crack length. Also, in situ potential drop sampling was possible, i.e., the test did not need to be stopped for crack length measurements.

Place, publisher, year, edition, pages
West Conshohocken, PA: American Society for Testing and Materials , 2011. Vol. 39, no 6, 985-1002 p.
Keyword [en]
potential drop, crack length measurement, fatigue crack growth, large scale yielding, LCF, experiments
National Category
Applied Mechanics Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-92242DOI: 10.1520/JTE103532Scopus ID: 2-s2.0-84856374861OAI: oai:DiVA.org:kth-92242DiVA: diva2:512908
Projects
Variable amplitude fatigue crack growth in the low cycle regime
Funder
Swedish Radiation Safety Authority
Note

QC 20120628

Available from: 2012-03-29 Created: 2012-03-29 Last updated: 2017-12-07Bibliographically 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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