Advanced

High Strain Fatigue Crack Growth and Crack Closure

Andersson, Michael LU (2005)
Abstract (Swedish)
Popular Abstract in Swedish

I en komponent som utsätts för upprepande på- och avlastningar, d.v.s. cyklisk belastning, kan defekter i form av sprickor initieras och börja växa. Detta fenomen kallas utmattning och kan leda till haveri med potentiellt katastrofala följder. Det är därför viktigt att förstå fenomenen som ligger bakom utmattning och att ha tillförlitliga modeller för att förutsäga livslängden hos en detalj. Genom god kunskap om utmattningsförloppet kan man dessutom utnyttja materialet bättre och öka både tillförlitlighet och prestanda exempelvis hos en flygmotor.



En mekanism som visat sig ha stor betydelse för hur fort en utmattningsspricka växer är sprickslutning. Sprickslutning innebär att... (More)
Popular Abstract in Swedish

I en komponent som utsätts för upprepande på- och avlastningar, d.v.s. cyklisk belastning, kan defekter i form av sprickor initieras och börja växa. Detta fenomen kallas utmattning och kan leda till haveri med potentiellt katastrofala följder. Det är därför viktigt att förstå fenomenen som ligger bakom utmattning och att ha tillförlitliga modeller för att förutsäga livslängden hos en detalj. Genom god kunskap om utmattningsförloppet kan man dessutom utnyttja materialet bättre och öka både tillförlitlighet och prestanda exempelvis hos en flygmotor.



En mekanism som visat sig ha stor betydelse för hur fort en utmattningsspricka växer är sprickslutning. Sprickslutning innebär att sprickan stänger lokalt nära spetsen, trots att en öppnande last läggs på globalt, vilket gör att den effektiva last som driver sprickan minskar. Slutningen kan orsakas av olika mekanismer, såsom plastisk deformation, ojämnheter i sprickvägen eller oxidation av sprickytorna.



För att förstå hur sprickslutningen påverkar spricktillväxten är det viktigt att kunna mäta när sprickan öppnar. I avhandlingen har det undersökts om elektrisk potential drop (PD) teknik, där elektrisk ström skickas genom provstaven och det elektriska spänningsfallet över sprickmunnen mäts, kan användas för tillförlitliga sprickslutningsmätningar. Genom en kombination av numeriska simuleringar och jämförelse mot slutningsmätningar med hjälp av in situ observationer i ett svepelektronmikroskop konstaterades det att PDtekniken är användbar för att mäta sprickslutning.



I avhandlingen studeras också sprickpropagering vid höga lastamplituder, dels för Ti-6Al-4V vid rumstemperatur, dels för Inconel 718 vid 650°C. Vid höga lastamplituder kan de metoder som används vid nominellt elastiska förhållanden inte användas. Det visade sig att genom att använda töjningsintensitetsfaktorer eller den cykliska J-integralen kunde spricktillväxtdata för en rad förhållanden korreleras, om hänsyn togs till sprickslutning. För Inconel 718 vid 650°C fanns dessutom ett kraftigt beroende av lastfrekvensen, vilket orsakas av en kombination av cykliska och tidsberoende sprickpropageringsmekanismer. Genom en effektiv cyklisk J-integral och en korrektionsfaktor för frekvensen kunde en livslängdslag för högtemperaturspricktillväxt i Inconel 718 formuleras.



Arbetet avslutas med en in situ studie av utmattningssprickpropagering i ett svepelektronmikroskop. Det konstaterades att på en mikroskala är spricktillväxten en oregelbunden process som starkt påverkas av den lokala mikrostrukturen. Detta gör det svårt att förutsäga tillväxt över enstaka cykler, medan över många cykler fungerade det väl att använda spänningsintensitetsfaktorn, kompenserad för sprickslutning. (Less)
Abstract
Understanding of the growth of fatigue cracks is of utmost importance since such growth often has a profound influence on the life of components subjected to cyclic loading.



Thus, reliable fatigue life models enable a more efficient use of materials and improve the performance and efficiency in many applications. This thesis deals with the growth of fatigue cracks subjected to high load amplitudes.



One mechanism that is known to have a strong influence on fatigue crack growth is crack closure, i.e. premature contact between the crack surfaces, caused by for instance residual plastic deformation, crack surface asperities or oxidation of the crack surfaces.



Closure can reduce the... (More)
Understanding of the growth of fatigue cracks is of utmost importance since such growth often has a profound influence on the life of components subjected to cyclic loading.



Thus, reliable fatigue life models enable a more efficient use of materials and improve the performance and efficiency in many applications. This thesis deals with the growth of fatigue cracks subjected to high load amplitudes.



One mechanism that is known to have a strong influence on fatigue crack growth is crack closure, i.e. premature contact between the crack surfaces, caused by for instance residual plastic deformation, crack surface asperities or oxidation of the crack surfaces.



Closure can reduce the effective load driving the crack, thus influencing the crack growth rate. In this thesis the use of the electrical potential drop (PD) technique for crack closure measurements has been investigated by a combination of numerical simulations and experiments. It has been shown that crack closure has a strong influence on the variations of the PD-value during a load cycle. Also, crack opening measured with the PDtechnique is consistent with closure measurements made from in situ fatigue crack growth observations using a scanning electron microscope.



Furthermore, the growth of fatigue cracks subjected to high load amplitudes in Ti-6Al-4V at room temperature and Inconel 718 at 650°C has also been studied. It has been shown that crack growth under high strain amplitudes can be analysed using a strain intensity approach or by using the cyclic J-integral. In both cases crack closure plays an important role and must be accounted for by using an effective strain intensity range or an effective cyclic J-integral. For Inconel 718 at 650°C, crack growth occurs by a combination of cyclic and time dependent growth, and thus the load frequency is of



importance for the crack growth rate. A crack growth law based on the product between the effective cyclic J-integral and a function compensating for the frequency was proposed.



Finally an in situ SEM study of crack growth in an aluminium alloy was performed.



It was shown that on a micro scale crack growth is a highly irregular process that is strongly influenced by the local microstructure at the crack tip. Also, there is no correlation between the local crack opening and the crack growth rate. Thus, it is difficult to predict the crack growth during an individual load cycle, but for crack growth rate on a macro scale the stress intensity range, compensated for crack closure, is a proper measure. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Härkegård, Gunnar, Department of Machine Design and Materials Technology, Norwegian University of Science and Technolog
organization
publishing date
type
Thesis
publication status
published
subject
keywords
materialteknik, Materiallära, Material technology, crack closure, high strain fatigue, crack grotwh
pages
106 pages
publisher
Department of Mechanical Engineering, Lund University
defense location
Room M:B, M-building, Ole Römers väg 1, Lund Institute of Technology, Lund University, Lund, Sweden
defense date
2005-09-22 10:15
external identifiers
  • other:ISRN:LUTFD2/TFMT--05/1012--SE(1-106)
ISBN
91-628-6599-4
language
English
LU publication?
yes
id
ada00844-8c56-4c59-be4b-faf2183aadbf (old id 545260)
date added to LUP
2007-09-06 10:56:37
date last changed
2016-09-19 08:45:01
@phdthesis{ada00844-8c56-4c59-be4b-faf2183aadbf,
  abstract     = {Understanding of the growth of fatigue cracks is of utmost importance since such growth often has a profound influence on the life of components subjected to cyclic loading.<br/><br>
<br/><br>
Thus, reliable fatigue life models enable a more efficient use of materials and improve the performance and efficiency in many applications. This thesis deals with the growth of fatigue cracks subjected to high load amplitudes.<br/><br>
<br/><br>
One mechanism that is known to have a strong influence on fatigue crack growth is crack closure, i.e. premature contact between the crack surfaces, caused by for instance residual plastic deformation, crack surface asperities or oxidation of the crack surfaces.<br/><br>
<br/><br>
Closure can reduce the effective load driving the crack, thus influencing the crack growth rate. In this thesis the use of the electrical potential drop (PD) technique for crack closure measurements has been investigated by a combination of numerical simulations and experiments. It has been shown that crack closure has a strong influence on the variations of the PD-value during a load cycle. Also, crack opening measured with the PDtechnique is consistent with closure measurements made from in situ fatigue crack growth observations using a scanning electron microscope.<br/><br>
<br/><br>
Furthermore, the growth of fatigue cracks subjected to high load amplitudes in Ti-6Al-4V at room temperature and Inconel 718 at 650°C has also been studied. It has been shown that crack growth under high strain amplitudes can be analysed using a strain intensity approach or by using the cyclic J-integral. In both cases crack closure plays an important role and must be accounted for by using an effective strain intensity range or an effective cyclic J-integral. For Inconel 718 at 650°C, crack growth occurs by a combination of cyclic and time dependent growth, and thus the load frequency is of<br/><br>
<br/><br>
importance for the crack growth rate. A crack growth law based on the product between the effective cyclic J-integral and a function compensating for the frequency was proposed.<br/><br>
<br/><br>
Finally an in situ SEM study of crack growth in an aluminium alloy was performed.<br/><br>
<br/><br>
It was shown that on a micro scale crack growth is a highly irregular process that is strongly influenced by the local microstructure at the crack tip. Also, there is no correlation between the local crack opening and the crack growth rate. Thus, it is difficult to predict the crack growth during an individual load cycle, but for crack growth rate on a macro scale the stress intensity range, compensated for crack closure, is a proper measure.},
  author       = {Andersson, Michael},
  isbn         = {91-628-6599-4},
  keyword      = {materialteknik,Materiallära,Material technology,crack closure,high strain fatigue,crack grotwh},
  language     = {eng},
  pages        = {106},
  publisher    = {Department of Mechanical Engineering, Lund University},
  school       = {Lund University},
  title        = {High Strain Fatigue Crack Growth and Crack Closure},
  year         = {2005},
}