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Growth of fatigue cracks subjected to non-proportional Mode I and II
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
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 [en]
Fatigue crack growth, Mode II overloading, Crack closure, Sequential mixed mode loading
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-585ISBN: 91-7178-241-9 (print)OAI: oai:DiVA.org:kth-585DiVA: diva2:14506
Public defence
2006-01-24, Sal F3, Lindstedtsvägen 26, Stockholm, 00:00
Opponent
Supervisors
Note

QC 20101004

Available from: 2006-01-16 Created: 2006-01-16 Last updated: 2013-01-14Bibliographically approved
List of papers
1. The effect of plasticity on incipient mixed-mode fatigue crack growth
Open this publication in new window or tab >>The effect of plasticity on incipient mixed-mode fatigue crack growth
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.

Keyword
fatigue crack propagation, incipient crack growth, non-proportional mixed-mode loading, plastic effects
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8980 (URN)10.1046/j.1460-2695.2003.00622.x (DOI)000183686400002 ()
Note
QC 20101004Available from: 2006-01-16 Created: 2006-01-16 Last updated: 2017-12-14Bibliographically approved
2. Reduction of mode I fatigue crack growth rate due to occasional mode II loading
Open this publication in new window or tab >>Reduction of mode I fatigue crack growth rate due to occasional mode II loading
2004 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 26, no 10, 1083-1093 p.Article in journal (Refereed) Published
Abstract [en]

The effect of mode 11 loading on subsequent mode I crack growth is experimentally investigated on steel AISI 01. The results show that the mode I crack growth rate decreases after a single mode 11 load cycle. This effect remains also after the crack has propagated through the mode 11 plastic zone. The crack growth reduction is shown to be caused by crack closure due to tangential displacement of crack-surface irregularities. The durability of this reduction has a decisive influence on the fatigue life when the mode I R-ratio is not as high as to keep the entire load cycle above the closure level.

Keyword
crack closure, fatigue crack growth, mode II overloading
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8981 (URN)10.1016/j.ijfatigue.2004.03.003 (DOI)000223223300006 ()2-s2.0-3042773344 (Scopus ID)
Note
QC 20101004 QC 20110922Available from: 2006-01-16 Created: 2006-01-16 Last updated: 2017-12-14Bibliographically approved
3. Mode I fatigue crack growth reduction mechanisms after a single mode II load cycle
Open this publication in new window or tab >>Mode I fatigue crack growth reduction mechanisms after a single mode II load cycle
2006 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 73, no 13, 1833-1848 p.Article in journal (Refereed) Published
Abstract [en]

A single Mode 11 load cycle, large enough to create residual displacements, decreases the subsequent Mode I crack growth rate. The distance for Mode I crack growth rate to fully recover, i.e., revert to the same da/dN as before Mode 11 load, is much longer than Mode 11 plastic zone size. The higher Mode 11 load, the larger is the reduction in growth rate and the longer the recovery distance. Higher Mode I R-ratio means smaller reduction in growth rate. Above a certain R-ratio, no reduction occurs at all. In the present study it is found that the reduction in growth rate is solely caused by crack closure due to tangential displacement of crack-surface irregularities that induce a surface mismatch between the upper and lower crack faces. The mechanism is called Mode II-induced crack closure. A model based on both analytical and experimental results is developed in order to estimate the degree of Mode II-induced crack closure after a Mode 11 load.

Keyword
crack closure, fatigue crack growth, Mode II overload
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8982 (URN)10.1016/j.engfracmech.2006.02.015 (DOI)000239984900007 ()2-s2.0-33744944545 (Scopus ID)
Note
QC 20101004. Tidigare version publicerad som Report 399, KTH Solid Mechanics.Available from: 2006-01-16 Created: 2006-01-16 Last updated: 2017-12-14Bibliographically approved
4. Fatigue crack growth: mode I cycles with periodic mode II loading
Open this publication in new window or tab >>Fatigue crack growth: mode I cycles with periodic mode II loading
2008 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 30, no 5, 931-941 p.Article in journal (Refereed) Published
Abstract [en]

The influence of periodic Mode II loading on Mode I fatigue crack growth is studied using a new type of loading device. The average rate of crack growth is found to be governed mainly by four parameters; Delta K-I and the Mode I R-ratio, the magnitude of the Mode II load and the Mode II period, M (M = number of Mode I load cycles per Mode II load). The Mode 11 load has to be large enough to create residual tangential displacements of the crack faces otherwise no effect occurs at all. Two mechanisms are found, Mode II-induced crack closure that reduces the crack propagation rate (long range) and a mechanism that increases the growth rate temporarily at every Mode I I load (short-range). For a specific Mode I load and Mode I I magnitude, it is possible to find a minimum crack growth rate for a certain Mode II period, M-opt. At high R-ratios the Mode II-induced closure mechanism disappears, but the temporary increase in growth rate at every Mode II load is still active. A semi-analytical model for the average crack growth rate during sequential Mode I and Mode II loading is presented.

Keyword
sequential mixed mode I, II loading, crack closure, fatigue crack growth
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8983 (URN)10.1016/j.ijfatigue.2007.06.004 (DOI)000254687400015 ()2-s2.0-38949215022 (Scopus ID)
Note
QC 20101004. Tidigare version publicerad som Report 400, KTH Solid Mechanics.Available from: 2006-01-16 Created: 2006-01-16 Last updated: 2017-12-14Bibliographically approved

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