In situ scanning electron microscopy study of fatigue crack propagation
The fatigue crack propagation rate is influenced by various mechanisms at the very vicinity of the crack tip, e.g., local plasticity and/or creep, microcracking, crack branching, and crack closure induced by plasticity and roughness. To study these mechanisms and their influence on crack propagation...
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| Опубліковано в: : | Проблемы прочности |
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| Дата: | 2008 |
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| Мова: | Англійська |
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Інститут проблем міцності ім. Г.С. Писаренко НАН України
2008
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| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | In situ scanning electron microscopy study of fatigue crack propagation / L. Jacobsson, C. Persson, S. Melin // Проблемы прочности. — 2008. — № 1. — С. 159-162. — Бібліогр.: 5 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1860068195009822720 |
|---|---|
| author | Jacobsson, L. Persson, C. Melin, S. |
| author_facet | Jacobsson, L. Persson, C. Melin, S. |
| citation_txt | In situ scanning electron microscopy study of fatigue crack propagation / L. Jacobsson, C. Persson, S. Melin // Проблемы прочности. — 2008. — № 1. — С. 159-162. — Бібліогр.: 5 назв. — англ. |
| collection | DSpace DC |
| container_title | Проблемы прочности |
| description | The fatigue crack propagation rate is influenced by various mechanisms at the very vicinity of the crack tip, e.g., local plasticity and/or creep, microcracking, crack branching, and crack closure induced by plasticity and roughness. To study these mechanisms and their influence on crack propagation rate during different loadings, in situ scanning electron microscope studies have been performed. Throughout the load cycles images were taken and analyzed with an image analysis technique to measure the displacements around the crack tip. The obtained data can be used to determine compliance curves at any point along the crack, crack shapes, and the displacementfield in the crack tip vicinity. The technique has been used to analyze which mechanisms of crack propagation are realized during, e.g., fatigue with overloads, and thermomechanical fatigue. The results were compared with resultsfrom measurements using the direct currentpotential drop technique, and it was found that various load conditions promote different mechanisms for crack propagation.
Скорость роста усталостной трещины (РУТ) определяется различными механизмами, реализуемыми в окрестности вершины трещины. Для изучения этих механизмов и их влияния на скорость РУТ для различных режимов нагружения были проведены исследования in situ с помощью электронного сканирующего микроскопа. Микрофотографии, получаемые во время циклического нагружения, анализировались с помощью методики обработки изображений с целью измерения перемещений в окрестности вершины трещины. Полученные результаты использовались для построения кривых податливости для любой точки на линии трещины, определения ее формы и поля перемещений вокруг вершины. По данной методике исследовались механизмы РУТ, реализуемые, в частности, при циклическом нагружении с перегрузками и термомеханической усталости. Полученные результаты сравнивались с данными измерений по методике падения потенциала. При этом было установлено, что разным условиям нагружения соответствуют различные механизмы РУТ.
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| first_indexed | 2025-12-07T17:08:46Z |
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| fulltext |
UDC 539. 4
I n S itu S c a n n in g E le c tr o n M ic r o s c o p y S tu d y o f F a t ig u e C r a c k P r o p a g a t io n
L . J a co b sso n ,1a C . P ersso n ,1b and S. M elin 2,c
1 Division o f Materials Engineering, Lund University, Lund, Sweden
2 Division o f Mechanics, Lund University, Lund, Sweden
a lars.jacobsson@material.lth.se, b christer.persson@material.lth.se, c solveig.melin@mek.lth.se
The fatigue crack propagation rate is influenced by various mechanisms at the very vicinity o f the
crack tip, e.g., local plasticity and/or creep, microcracking, crack branching, and crack closure
induced by plasticity and roughness. To study these mechanisms and their influence on crack
propagation rate during different loadings, in situ scanning electron microscope studies have been
performed. Throughout the load cycles images were taken and analyzed with an image analysis
technique to measure the displacements around the crack tip. The obtained data can be used to
determine compliance curves at any point along the crack, crack shapes, and the displacement field in
the crack tip vicinity. The technique has been used to analyze which mechanisms o f crack propagation
are realized during, e.g., fatigue with overloads, and thermomechanical fatigue. The results were
compared with results from measurements using the direct current potential drop technique, and it
was found that various load conditions promote different mechanisms fo r crack propagation.
K e y w o rd s : fatigue, crack propagation, scanning electron m icroscope, crack shape, crack
closure, potential drop.
In trod u ction . It is o f interest to study the material in the crack tip v icin ity to
understand the effects o f material p lasticity around the crack tip, roughness o f the crack
faces, microstructure, stresses, and deform ations, on the crack propagation. Elber [1, 2]
introduced the concept o f crack closure where the m axim um stress level at the crack tip is
critical. The m axim um load, is calculated from the closure level, w here the crack tip is
under zero load, and the applied stress intensity factor range. To m easure the crack closure
load level, a number o f different m easurem ent techniques are used. Song and Shieh [3]
used the direct current potential drop technique to find the crack closure level, whereas
the A ST M E 647-99 [4] recom m ends to apply the crack mouth opening displacement.
A lso, there are different techniques using m icroscope im ages to measure the displacements
along the crack.
The aim o f this study w as to determine the deform ations along the crack, and how
the deform ations are affected by the different crack propagation m echanism s. H igh
resolution im ages from an in-situ scanning electron m icroscope w as used to measure
deform ations at the crack tip vicinity, and the potential drop technique w as used to
m easure the contact betw een the crack surfaces.
E xp er im en ta l Setup. The fatigue crack propagation experim ents were performed
w ithin a scanning electron m icroscope (SEM ), using a sm all electrically driven load stage
to perform the load cycles, also described by A ndersson et al. [5]. The test specim ens had
dim ensions o f 7 0 x 1 0 m m and w ere cut from 0.5 m m Inconel 718 foil. The specim ens
w ere prepared w ith a single-edge-notch tension (SE N T) crack, that w as pre-cracked in a
servohydraulic load frame to a length o f 0 .7 -1 .0 mm. The specim ens were etched to
produce a recognizable pattern on the surface o f the specim en for the SEM observations.
The specim ens w ere prepared w ith thin w ires w elded at the crack m outh to measure the
electrical potential drop signal over the crack. A direct current o f 1.0 A w as passed
through the specim en and the potential drop signal w as am plified before saved in the
computer. The crack propagation rate and the crack closure leve l were determined. A
summary o f the performed experim ents is found in Table 1.
© L. JA C O B SSO N , C. PER SSO N , S. M ELIN , 2008
ISSN 0556-171X. Проблемы прочности, 2008, № 1 159
mailto:lars.jacobsson@material.lth.se
mailto:christer.persson@material.lth.se
mailto:solveig.melin@mek.lth.se
L. Jacobsson, C. Persson, and S. Melin
T a b l e 1
Experimental Conditions for the Four Crack Propagation Experiments
Experiment a, /im AK, MPa • m1/2 A K eff, MPa • m1/2 R ^ min / ̂ max
i ■ 740 16.6 3.1 0.01
ii ▼ 945 19.7 10.0 0.01
iii ▲ 1673 43.3 26.3 0.01
iv • 1764 65.0 42.0 0.02
Im age A n a lysis M eth od . A n ew im age analysis m ethod w as developed to measure
displacements in the crack tip vicinity from high resolution SEM im ages. Images were taken
along the crack throughout the loading cycles. The displacements betw een different images
w ere determ ined using a cross-correlation function that recognized a selected area o f one
im age in another im age, taken at a different load. The area w as p laced w ithin 2 -1 0 ,am
from the crack, w here the deform ation w ithin the material w as negligib le. Com pliance
curves and crack shapes were determ ined from the m easured displacem ent field.
R esu lts and D iscu ssion . Crack shapes and com pliance curves were determined
during a number o f experim ents w ith different loading histories. C om pliance curves from
points at different distances from the crack tip are compared in Fig. 1. The com pliance
curve m easured at the crack mouth show ed neglig ib le influence from the plastic zone, and
had a sm all knee at a higher load than the com pliance curves m easured at the crack tip,
that show ed clear effects from the plastic zone and closure o f the crack surfaces. The knee
on the curves from the potential drop (PD ) m easurem ents show ed in Fig. 2a, indicates the
closure level. A bove this level the crack w as fu lly open and the change in electrical
conductivity w as on ly due to the high stresses in front o f the crack, and b elow this level
the contact betw een the crack surfaces increased with decreasing load. The load level at
the knee on the PD -curves w as higher than that at the knee on the com pliance curves
m easured at the crack mouth.
Fig. 1. Compliance curves measured at three different distances from the crack tip: ( • ) 2 ^m;
(A ) 20 ,Mm; (■ ) at the crack mouth.
The crack shape obtained by linear elastic fracture m echanics (LEFM) considerations
does not alw ays describe the true crack shape w e ll because o f the influence from the
plastic zone and the microstructure. The LEFM solutions, as w ell as crack shapes
com pensated for closure levels from com pliance curves and PD m easurem ents, are show n
in Fig. 3. The LEFM solution w ill be the sam e, independent o f the closure level w hen the
160 ISSN 0556-171X. npoôëeMbi npounocmu, 2008, N 1
In Situ Scanning Electron Microscopy Study
Fig. 2. Compliance curve with closure level at 1200 N (a) and the corresponding PD-curve with
closure level at 1700 N (b) for K max = 65 MPa • m1/2, R = 0.02, overload ratio 1.26, a = 1.8 mm,
specimen thickness 0.5 mm, and displacements measured 15 /im from the crack tip. (Experiment iv.)
x icT1*
о
о
uLi______ c______ c______ c______ c______ i______ a
-3 -2,5 -2 -1.5 -1 -0.5 a
Dist from crack tip (m) x kt4
Fig. 3. Experimental results are compensated for the effective stress intensity factor range measured
from the compliance curve (▼) and from the PD signal (O). (Experiment iv. The opening
displacements divided by the effective stress intensity factor range. The solid line shows the LEFM
solution.)
i и* * 11“
Dist. from crack tip (m) xiC® Dist fromcraDk lip (m) x Kf®
a b
Fig. 4. Crack shapes (a) and ^-deformations (b) divided by effective stress intensity factor range
versus distance from crack tip for the four experiments. (Solid line is the LEFM solution.)
ISSN 0556-171X. Проблемы прочности, 2008, N 1 161
L. Jacobsson, C. Persson, and S. Melin
displacem ent divided by the effective stress intensity factor range is plotted. W hen the
experim ental results were divided by the closure levels m easured from the com pliance
curves and the PD-curves, the results diverged (Fig. 3).
In Fig. 4a, the crack shapes are plotted for four different experim ents (Table 1). This
close to the crack tip, the microstructure and the change in crack direction affects the
crack shape. In Fig. 4b, the deform ation w as divided w ith the effective stress intensity
factor found from com pliance curves. The results show ed sim ilarities betw een the four
experim ents and also the LEFM solution w as c lose to the experim ental results. The crack
tip is show n in Fig. 5.
Fig. 5. SEM image o f the crack tip at maximum load. (Experiment iii.)
C on clusions. Experim ents have been performed w ithin a SEM and the im ages were
used to determine deform ations in the v icin ity o f the crack tip. C om pliance curves were
determ ined and com pared w ith results from PD m easurem ents. The PD measurem ents
gave higher values for the crack closure load than the com pliance m easurem ents. W hen
the experim ental results, com pensated for crack closure w ere com pared w ith the LEFM
solution, the crack shapes at the crack tip v icin ity becam e comparable even w hen the
crack shapes w ere irregular due to the microstructural variations.
Acknowledgments. The authors give their acknowledgements to the Swedish Gas Turbine
Center for the financial support.
1. W. Elber, Eng. Fract. M ech, 2, 37-45 (1970).
2. W. Elber, in: Damage Tolerance in Aircraft Structures, ASTM STP 486 (1971), pp. 230-242.
3. P. S. Song and Y. L. Shieh, Int. J. Fatigue, 26, 429-436 (2004).
4. Standard Test Method fo r Measurement o f Fatigue Crack Growth Rates, ASTM E647-99
(1995).
5. M. Andersson, C. Persson, and S. Melin, Int. J. Fatigue, 28, 1059-1068 (2006).
Received 28. 06. 2007
162 ISSN 0556-171X. npo6neMbi npouHocmu, 2008, № 1
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| id | nasplib_isofts_kiev_ua-123456789-48418 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0556-171X |
| language | English |
| last_indexed | 2025-12-07T17:08:46Z |
| publishDate | 2008 |
| publisher | Інститут проблем міцності ім. Г.С. Писаренко НАН України |
| record_format | dspace |
| spelling | Jacobsson, L. Persson, C. Melin, S. 2013-08-19T13:19:55Z 2013-08-19T13:19:55Z 2008 In situ scanning electron microscopy study of fatigue crack propagation / L. Jacobsson, C. Persson, S. Melin // Проблемы прочности. — 2008. — № 1. — С. 159-162. — Бібліогр.: 5 назв. — англ. 0556-171X https://nasplib.isofts.kiev.ua/handle/123456789/48418 539.4 The fatigue crack propagation rate is influenced by various mechanisms at the very vicinity of the crack tip, e.g., local plasticity and/or creep, microcracking, crack branching, and crack closure induced by plasticity and roughness. To study these mechanisms and their influence on crack propagation rate during different loadings, in situ scanning electron microscope studies have been performed. Throughout the load cycles images were taken and analyzed with an image analysis technique to measure the displacements around the crack tip. The obtained data can be used to determine compliance curves at any point along the crack, crack shapes, and the displacementfield in the crack tip vicinity. The technique has been used to analyze which mechanisms of crack propagation are realized during, e.g., fatigue with overloads, and thermomechanical fatigue. The results were compared with resultsfrom measurements using the direct currentpotential drop technique, and it was found that various load conditions promote different mechanisms for crack propagation. Скорость роста усталостной трещины (РУТ) определяется различными механизмами, реализуемыми в окрестности вершины трещины. Для изучения этих механизмов и их влияния на скорость РУТ для различных режимов нагружения были проведены исследования in situ с помощью электронного сканирующего микроскопа. Микрофотографии, получаемые во время циклического нагружения, анализировались с помощью методики обработки изображений с целью измерения перемещений в окрестности вершины трещины. Полученные результаты использовались для построения кривых податливости для любой точки на линии трещины, определения ее формы и поля перемещений вокруг вершины. По данной методике исследовались механизмы РУТ, реализуемые, в частности, при циклическом нагружении с перегрузками и термомеханической усталости. Полученные результаты сравнивались с данными измерений по методике падения потенциала. При этом было установлено, что разным условиям нагружения соответствуют различные механизмы РУТ. The authors give their acknowledgements to the Swedish Gas Turbine Center for the financial support. en Інститут проблем міцності ім. Г.С. Писаренко НАН України Проблемы прочности Научно-технический раздел In situ scanning electron microscopy study of fatigue crack propagation Исследование in situ роста усталостной трещины с помощью электронного сканирующего микроскопа Article published earlier |
| spellingShingle | In situ scanning electron microscopy study of fatigue crack propagation Jacobsson, L. Persson, C. Melin, S. Научно-технический раздел |
| title | In situ scanning electron microscopy study of fatigue crack propagation |
| title_alt | Исследование in situ роста усталостной трещины с помощью электронного сканирующего микроскопа |
| title_full | In situ scanning electron microscopy study of fatigue crack propagation |
| title_fullStr | In situ scanning electron microscopy study of fatigue crack propagation |
| title_full_unstemmed | In situ scanning electron microscopy study of fatigue crack propagation |
| title_short | In situ scanning electron microscopy study of fatigue crack propagation |
| title_sort | in situ scanning electron microscopy study of fatigue crack propagation |
| topic | Научно-технический раздел |
| topic_facet | Научно-технический раздел |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/48418 |
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