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The effect of plasticity on incipient mixed-mode fatigue crack growth
KTH, Superseded Departments, Solid Mechanics.
KTH, Superseded Departments, Solid Mechanics.ORCID iD: 0000-0001-8068-2360
2003 (English)In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 26, no 7, 577-588 p.Article in journal (Refereed) Published
Abstract [en]

Several criteria for the prediction of incipient crack path direction of non-proportional mixed-mode fatigue cracks, immediately after a change of load from steady mode I conditions are investigated. The analysis is based on two-dimensional plane strain FE-simulations in which the actual elasto-plastic stress distribution is used for the MTS, MTSR and MEPSR criteria. The purpose of the analysis is to compare the numerical results with experimental results as well as with previous predictions based on solely elastic stress analyses, taken from the literature. It is shown that the influence from elasto-plastic deformation on crack branching direction is of utmost importance. It is found that the incipient crack growth of metals falls into two categories: high strength metals follow the MTSp criterion whereas more ductile metals follow the MTSRp criterion. The subscript p indicates that the elasto-plastic evaluation of the respective criterion should be used.

Place, publisher, year, edition, pages
2003. Vol. 26, no 7, 577-588 p.
Keyword [en]
fatigue crack propagation, incipient crack growth, non-proportional mixed-mode loading, plastic effects
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-8980DOI: 10.1046/j.1460-2695.2003.00622.xISI: 000183686400002OAI: oai:DiVA.org:kth-8980DiVA: diva2:14502
Note
QC 20101004Available from: 2006-01-16 Created: 2006-01-16 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Growth of fatigue cracks subjected to non-proportional Mode I and II
Open this publication in new window or tab >>Growth of fatigue cracks subjected to non-proportional Mode I and II
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis deals with some aspects of crack growth in the presence of cyclic loading, i.e. fatigue. The cyclic load cases studied here are primary of non-proportional mixed mode type. Under non-proportional loading the principal stress directions rotate and, generally, the ratio between the principal stresses vary. A new criterion has been presented for prediction of incipient crack path direction after changes in load from steady Mode I to non-proportional loading.

The criterion is based on FE-simulations which are used to compute the actual elasto-plastic stress state in the vicinity of the crack tip. The predictions of the criterion capture several phenomena observed in the literature, which indicates that plasticity effects have to be included in a criterion for crack path predictions under non-proportional loading. The effects of Mode II overloads on subsequent Mode I crack growth have been studied relatively little in the literature. Also, the results deviates substantially. In the present thesis, this load case has been investigated in detail, both experimentally and analytically. The results show that the Mode I crack growth rate decreases after a single Mode II load, if the R-ratio is not as high as to keep the entire Mode I load cycle above the closure level. This is based on the fact, shown in this thesis, that the reduction is caused by crack closure due to tangential displacement of crack-surface irregularities.

A new loading device is presented. With this device, it is possible to apply sequential loading in Mode I and Mode II in an automated way, without having to dismount the specimens. This loading device is used to study the influence of periodic Mode II loading on Mode I crack growth. The main parameters concerning the influence of periodic Mode II loading on Mode I crack growth are; (i) the Mode I R-ratio, (ii) the Mode II magnitude and (iii) the Mode II periodicity, M (number of Mode I loads for every Mode II load). The mechanisms involved are mainly RICC (Roughness-Induced Crack Closure) and a Mode II mechanism that increases the growth rate temporary at every Mode II load. Hence, the latter becomes more significant for low M-values. The higher the Mode I R-ratio the smaller is the reduction.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. 34 p.
Series
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0398Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472
Keyword
Fatigue crack growth, Mode II overloading, Crack closure, Sequential mixed mode loading
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-585 (URN)91-7178-241-9 (ISBN)
Public defence
2006-01-24, Sal F3, Lindstedtsvägen 26, Stockholm, 00:00
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Supervisors
Note

QC 20101004

Available from: 2006-01-16 Created: 2006-01-16 Last updated: 2013-01-14Bibliographically approved

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