京都大学教育研究振興財団助成事業
成 果 報 告 書
平成21年10月13日
財団法人京都大学教育研究振興財団会 長 辻
井 昭 雄 様所属部局・研究科

エネルギー科学研究科

職 名・学 年

助 教

氏

陳

名

友 晴

事 業 区 分平成２１年度 ・ 国際研究集会派遣助成

研 究 集 会 名

第７回 国際応用地質学会 アジア地域会議

発 表 題 目

Crack growth in granite specimens subjected to cyclic uniaxial loading

開 催 場 所

中華人民共和国 四川省成都市 成都理工大学

渡 航 期 間平成21年９月8日

成 果 の 概 要

タイトルは「成果の概要／報告者名」として、A４版２０００字程度・和文で作成し、添付して下さい。 「成果の概要」以外に添付する資料 □ 無■ 有( 発表論文)〜

平成21年9月15日交付を受けた助成金額

150,000円

使 用 し た 助 成 金 額

150,000円

返納すべき助成金額

0円

会 計 報 告学会参加登録料

25,000円

旅費（航空賃）70,000円

宿泊料

105,000円

助 成 金 の 使 途 内 訳( 使 用 旅 費 の 内 容 )
助成金は上記に充当

成果の概要／陳 友晴
今回、京都大学教育研究振興財団の国際会議派遣助成を得て、2009年9月8日〜15日まで中華人民共和国四川省成都市で開催された国際会議およびシンポジウム The
International Symposium on Geological Engineering Problems in Major Construction
Projects in the 7th Asian Regional Conference of IAEG に参加する機会を得た。本会議は、1997年に日本・東京において開催された第１回を皮切りに、概ね２年ごとにアジア各地で開催されており、アジア地域での応用地質学分野の活動や発展にとって重要な役割を担い、各国から多くの参加者を得て地盤災害、気象災害の防災、減災について
総合的な情報交換、技術討論を行っている。今回は、学会テクニカルツアーとして昨2008
年5月に中国四川省を襲った大地震の震災地現地巡検が予定されていることから、アジ
ア地域のみならず、世界中から参加者が集まった。参加者約250名、30を越える国･地域から参加があったことが報告されている。
シンポジウムは、６つのテーマ（大規模建設プロジェクトにおける地質リスクと地質
環境問題、大規模地すべりのメカニズムと管理、活動域内の山岳都市における地質災害とリスク管理、大規模ダム・構造物における基礎問題、地下深部トンネルにおける地質
工学的問題と災害、複雑な地質環境地域における地下空間の安定性）と２つの特別テーマ（2008年5月12日四川大地震からの教訓、中国三峡ダムプロジェクトにおける地質工
学問題の教訓）に分かれて、講演、討論が行われた。それぞれのテーマでは、キーノー
トレクチャーを含めて、大変熱のこもった発表と活発な討論が行われた。多くの参加者
から、各地域で地質災害から人命と財産を守っているのは自分だという自負、また、それを各地の研究者、技術者に知ってもらい役立てて欲しいという信念、そしてより多くのことを学んでいこうとする熱意のようなものがひしひしと感じられる、和やかな中にも緊張した雰囲気が漂う、大変有意義なシンポジウムであった。
シンポジウムでは、室内試験を用いて基礎岩盤（花崗岩）の疲労劣化プロセスを考察する基礎研究の内容について、発表を行った。当初はポスターでの発表予定であったが、予稿原稿を提出後に口頭発表への変更が認められ、講演、討論と貴重な機会を得ることができたことは、大変嬉しい出来事であった。発表内容は、試験機を用いた花崗岩試験
片の圧縮繰り返し載荷試験により発達する内在欠陥であるマイクロクラックについて、破壊まで至らない多数の試験片を作成・観察することにより、その発達過程を明らかにし、劣化のプロセスを検討したものである。試験片の全体的な挙動が比較的安定した状態のときでも、潜在的な欠陥であるマイクロクラックは、確実に成長していることを示すことができ、クラックの伸長と発生によりクラック密度が増加することが、最終的な
加速度的な劣化進行に寄与することが示唆された。クラック進展の特徴的な方向性についても議論をした。発表後、基礎的知見の蓄積として、さらなる実験の継続を期待するという激励のコメントと、実用、応用について、さらによく検討を行っていくことが望ましいという厳しい内容のコメント両方をいただいた。発表時間では討論が終わらず、
コーヒーブレイク時にも、さらにゆっくりと討論を続けることができ、今後の研究の遂行に多くの指針を得た。
また、シンポジウム期間中、今まで興味をもっていたが研究のステップとして踏み込めないままでいる分野（内容）の先行する研究者の方々と、直接討論できる機会に恵まれた。短い時間ではあるが、簡単なアドバイスももらうことができたことは、大変貴重な情報となっている。今後、あらためて交流を深めていけるよう期待をし、研究を進めていくことを計画している。
シンポジウム後半では、昨2008年5月の四川大地震の震災地域の巡検ツアーが開催された。無残な爪痕は１年半を経た今なお癒えておらず、地震災害の迫力に圧倒されるばかりであった。ツアーでは、学会主催者の尽力と中国政府の配慮から、通常は立ち入り禁止とされている地域にも、討論のため立ち入りを認められた部分があり、間近で観察することが可能であった。いくつかの重点的な現場では、現地研究者、技術者の解説もあり、室内での討論同様、長時間にわたる密度の濃い討論が行われた。地震後すでに１年以上が経過しているが、震災の状況を自分の目に焼き付けることができ、今後もこれ
ら地質災害の防災、減災のために微力ではあるが貢献できればという思いを新たにすることができた。今後も精力的に研究を継続できればと考えております。
最後になりましたが、今回の国際会議派遣に対し助成をいただきました京都大学教育
研究振興財団に厚く御礼申し上げます。会議の参加により、研究者として国際的な人脈を広げられたこと、自己の研究についての多くのコメントをいただけたこと、数多くの有益な情報・知見を得ることができたこと、どれをとっても非常に意義のあるものと感じております。どうもありがとうございました。

Crack growth in granite specimens subjected to cyclic uniaxial loading
Y. Chen, K. Watanabe, M. Kobayashi, H. Kusuda, E. Kusaka & M. Mabuchi
Graduate School of Energy Science, Kyoto University, Kyoto, Japan

ABSTRACT: In order to examine the fatigue process of granite, a series of Westerly granite specimens were
subjected to a cyclic loading test under the uniaxial compression with maximum 140 and 160MPa at room
temperature. The development of microcracks within the specimens stressed to different characteristic stages
was observed microscopically and their growth patterns were analyzed using image analysis technique. As aresult, with increasing loading cycles, the number and the population of microcracks within the specimen
increased. At the first stage of loading, many intragranular cracks generated. During the quasi-stable middle
stage, many microcracks preferentially elongated parallel to the loading direction and grew into transgranular
cracks. With further loading, cracks gradually widened, and some of them grew into intergranular cracks. It
was estimated that the generation and growth of these intergranular cracks induced the rock fatigue failure.

Holcomb 1981). However, the mechanical behavior
during the fatigue process has not been clarified.
In many geological engineering problems, granite is one of the most important materials to investigate. In granite, the initiation and the elongationof microcracks play an essential role in the failure
process. It is therefore important to analyze thegrowth of cracks during the fatigue process. In
recent years, for example, Åkesson et al. (2004)
reported the characteristic microcrack growth pattern in every constituent mineral by the microscopic observation of granite specimens after cyclic loading. However, much remains unknown
about the fatigue process of granite, although manystudies have been conducted.
In this study, to obtain further insights into thecrack development pattern in granite during thefatigue process, microcracks in a series of specimens stressed to different stages by cyclic loadingwere observed in detail applying the fluorescentapproach proposed by Nishiyama & Kusuda (1994).
For further examination, microcrack growth patterns were analyzed by the digital image analysis
technique.

1 INRODUCTION
The demand for the long-term safety utilization ofrock structures has increased further in recent years.
It is known that degradation of rocks or rockmasses used in traditional rock structures, i.e.,buildings, bridges, traffic tunnels, and mining galleries, occurs by repeated stress changes in theiruse and operation. For the stability evaluation ofmany rock structures, it is important to reveal thedeterioration characteristics of rocks under repeated stress alternation over the long term.
It is well-known that materials deteriorate byrepeated stress change over a prolonged period andthen finally reach failure even if the changes arelower than its static breaking strength. This phenomenon is generally termed ‘fatigue’, and happens in many kinds of materials (Suresh 1998).
Needless to say, rock is no exception. Many studies
on the fatigue characteristics of rocks have been
carried out since the 1960's. Most of them stressedrock test specimens cyclically by a loading machine. These primary studies reported that the fatigue life was significantly influenced by the magnitude of the applied maximum stresses (e.g., Burdine 1963, Hardy & Chugh 1970). The strain behavior of rocks during the fatigue process wasnearly the same as for other structural materials,
such as metals and concrete (e.g., Haimson & Kim
1971). Attempts to estimate the fatigue life werealso conducted by many researchers (e.g., Costin &2 EXPERIMENTAL
2.1 Sample
Fine-grained Westerly granite from the USA waschosen for the examination. Many rock tests have528

been conducted using this rock, and its fundamental characteristics have been accumulated by many
researchers (e.g., Tapponnier & Brace 1976, Chen
&Wang 1980). The modal composition of thetested samples was about 70% feldspars (including
potassium and plagioclase feldspar), 25% quartz,
and 5% micas (including biotite and muscovite)
and the accessories. The mean grain size was about
0.7mm. The effective porosity of the specimens
ranged around 1%.

increase at first (Stage 1) and then quasi-stable with
gradual increase (Stage 2), and finally the values
suddenly increased (Stage 3) and then the sample
fractured.
Fracture

Strain

2.2 Test condition

CyclesFigure 2. Outline of behavior on the strain of materials
during the fatigue failure process.

2.4 Collecting prefailure specimens for observation

2.3 Monitoring sample behavior

In order to observe the sequence of the changes
occurring within specimens throughout the fatigue
failure process, many specimens were tested in thesame conditions and retrieved before failure.
In a series of the specimens stressed by 140
MPa, the stage in the fatigue process of the retrieved specimen was decided in consideration of
the strain behavior (Fig.3). While in 160 MPa
specimen, a total of six specimens, including theintact (i.e., no-loaded) specimen and the fractured
specimen, were regarded as a series of the specimens to observe the fatigue failure process (Fig.4).
The behavior of four prefailure specimens wasnearly the same as those of the fractured specimen.

To monitor the behavior of the specimens during
the test, axial and lateral strains, and the load were
simultaneously measured every 0.1 second intervals, that is, 100 measurement data for one cycle.
The strains were detected by strain gages directly
attached to the side surfaces of specimen (Fig. 1).

10mm
Strain gages

20mm

20mm

Thin
section
Grain
plane

F

Maximum lateral strain, 10-3

G

in
ra

Rift plane

10mm

Hardway plane

Figure 1.

Stage 2
Stage 1

Cylindrical specimens 10 mm in diameter and 20
mm in length, cored orthogonal to the hardway
plane (Fig. 1), were subjected to the cyclic loading
test under uniaxial conditions at room temperature.
The specified maximum stress was 140 and
160MPa (i.e., about 70 and 80% of the estimateduniaxial compressive strength 200MPa), and theminimum one was 0.5MPa. The loading-unloading
cycle was 0.1Hz, and the applied load was controlled to change linearly.

e
an
pl

Stage 3Size and direction of tested cylindrical specimens.

It is a well-known fact that the number of theloading times to reach failure by the cyclic loading
test shows some variation (Suresh 1998), and ourtest was no exception. However, the strain behaviorof specimens until failure was nearly the same toall samples. Figure 2 shows the general behavior
on the strain of materials during the fatigue failure
process. In our test, the lateral strain showed asame pattern; the strains were stable with liner

5.0

3

4.0

F
3

3.0
2.0

1

3

2

30,00040,000

2

1.0
0

1

0

10,00020,000
CyclesFigure 3. Behavior of maximum lateral strain of selected
140 MPa specimens. Numbers in the figure show the decided
stages.

529

10.0

tended to distribute near the lateral sides of thespecimen.

8.0

3.3 Long cracks parallel to the loading direction

6.0

Long cracks which could not be identified in the
intact specimen were observed in the tested specimens. Most of these cracks ran parallel to theloading direction. In the later stage, many suchcracks grew into transgranular cracks and somecracks elongated to more than about 2mm, cf.
mean grain size about 0.7mm, and cut throughseveral grains (Fig. 5). They developed aroundmica grain in many cases. Long cracks in quartz
parts were comparatively straight, while those infeldspar parts sometimes elongated stepwise.

Maximum lateral strain, 10-3

12.0

4.0
2.0
0

0

10,00020,00030,00040,000

CyclesFigure 4. Behavior of maximum lateral strain of tested
specimens, and outline of six observed 160 MPa specimens.
Real line shows the behavior of fractured specimen.

3 OBSERVATION
3.1 Microcrack detecting method
The fluorescent method proposed by Nishiyama &Kusuda (1994) was applied to identify microcracks
within the specimens. Microcracks are detectedbased on a marked difference in brightness underultraviolet light irradiation, because they are fully
filled with acrylic resin mixed with a fluorescent
substance in advance. The method provides quick
and accurate identification of microcracks with an
optical microscope. Thin sections including theaxis of the specimen were prepared for the detailed
observation (Fig. 1).
3.2 Overview of the entire section
First, the crack growth on each entire section was
overviewed. As a result, many pre-existing microcracks were already identified in the intact specimen. In the specimen at the first stage of loading
cycle, extraordinary marked growth of microcracks
was not observed compared with the intact specimen. However, grain boundary cracks widened
slightly in several parts and the number of intragranular cracks increased.
In the specimen during the quasi-stable stage(Stage 2), the number of cracks increased significantly compared with the less stressed specimens.
Most of these developed cracks were observed inquartz and feldspar grains, and some grew parallelto the loading direction. Cracks further increased
and elongated in the specimen at the third stage
near the rock failure (Stage 3). In this stage, cracks
running parallel to the loading direction, whichwere estimated to develop around this stage, were
dominant, and some of these long cracks cut
through more than two grains. These long cracks

Figure 5. Long cracks observed in the specimen stressed toStage 3, 140 MPa. White parts are corresponded to cracks. Qz:
quartz, Fd: feldspar, Bi: biotite.

3.4 Microcracks in major constituent minerals
For further examination, the development of microcracks in each constituent mineral was observedin more detail. In the specimens at Stage 1, slightly
larger numbers of short cracks were observed inquartz grains compared with the intact specimen.

530

Many of these cracks existed within a grain, andthe detailed observation found that most of thesecracks tended to appear around the boundaries ofneighboring mica grains. In the specimens during
Stage 2, cracks parallel to the loading direction
were observed in quartz grains. These cracks werelonger than those observed in the less loaded
specimens, and many of them grew into transgranular cracks, and some grew into the intergranular cracks by passing through the grainboundaries.
In feldspar grains, a great number of fine microcracks were observed along the cleavages but they
existed only inside the grains, and no clear evidence could be found of marked elongation ofthese cleavage cracks with increasing loading cycles. In the later stage of loading, two types ofcrack running parallel to the loading direction wereobserved: cracks with a relatively long lengthgrown from the grain boundary, and relatively
short cracks related to the cleavages.
On the other hand, cracks within biotite or muscovite grains developed little throughout the test,
even in the fractured specimen.

4.2 Analytical results and discussion
4.2.1 Crack direction
The crack distribution both in quartz and feldspar
grains was analyzed separately. Fine microcracks
concerning the cleavages within feldspar grains
were excluded from this analysis, because it wasestimated that they were rarely influenced to therock failure. Crack direction was summarized
every 20 degrees (Fig. 6). As a result, it was foundthat the amount of cracks increased gradually withincreasing loading cycles and increased drastically
immediately before failure. Furthermore, after thelater half of Stage 2, crack growth in feldspar was
dominant compared with in quartz grains, and especially this tendency was remarkable for thecracks running parallel to the loading direction.
4.2.2 Crack in quartz grain
Next, cracks in quartz around mica and cracks inother quartz were analyzed separately. It is becausethat a different crack growth pattern was observed
between quartz grains around mica grains and otherquartz grains. As a result, the amount of cracks inquartz around mica increased drastically at the firststage and then it stayed nearly constant until after
failure. The average crack length showed no significant change until Stage 2 and elongated distinctly during the later stage. In contrast, in otherquartz grains, the tendency was similar to total
cracks, of which amount increased gradually withincreasing loading cycles and increased drasticallyat final stage. This indicates that in quartz aroundmica short cracks firstly appeared and then elongated and conjugated later.

4 CRACK ANALYSIS
4.1 Image analysis
The features of crack growth pattern were analyzed
by applying digital image analysis technique inmore detail. Digitalized image files of the reference
area at a suitable resolution were captured with a
CCD camera attached to an optical microscope,
applying the fluorescent approach. Microcracks in
the images were automatically extracted following
the typical image analysis procedure, i.e. enhancement, smoothing, thresholding, and thinning operations (Kusuda & Nishiyama 1994, Chen et al.
2001). The data on crack width could not be discussed following this procedure because the characteristics of cracks were extracted by the thinning
operation. Furthermore, grain boundaries and mineral species of individual grains were identified bytypical observations of thin sections.
The number of extracted cracks and the length ofeach crack were measured. If a crack had somebranches, the number was counted as one and its
length included the length of branches. To analyze
crack growth patterns, two parameters, crack
population by length (the sum of crack length perunit area) and average length of crack, were calculated based on the length and the number of cracks.

Crack population (mm/mm2)

0.6

a

0.4
0.2
0
1.2

b

Cracks in feldspar
Cracks in quartz

1.0
0.8
0.6
0.4
0.2
0

0
~20

20
~40

40
~60

60
~80

80
~100

100
~120

120
~140

140
~160

160
~180

Direction

Figure 6. Crack direction in feldspar and quartz grains, 140
MPa specimens. Loading direction corresponds to 90 degree.
(a) Intact specimen, (b) specimen stressed to Stage 3.

531

For more detailed analysis, cracks in quartz
grains, which stretched from the boundary ofneighboring mica grains, were manually extracted
and divided into several ranks by their length. The
length 0.3mm, which corresponded to the averagegrain size of a quartz grain, was chosen for the
standard of classification. As a result, the numberof cracks less than 0.3mm increased at first but
decreased prominently during Stage 2. The numberof cracks 0.3-0.6mm long increased distinctly afterStage 2. The number of cracks 0.6-0.9mm longgradually increased until immediately before failure and drastically increased after failure. Cracks
over 0.9mm long were not identified until immediately before failure (Fig. 7). It was inferred that thedevelopment of cracks over the average grain size
led to specimen failure.

microscopically and their growth patterns were
investigated using the image analysis technique.
With increasing loading cycles, the number andthe population of microcracks within the specimen
increased. At the first stage of loading, many intragranular cracks generated. During the quasi-stablemiddle stage, many microcracks preferentiallyelongated parallel to the loading direction and grew
into transgranular cracks. With further loading,cracks gradually widened, and some of them elongated and grew into intergranular cracks. It wasestimated that the generation and growth of theseintergranular cracks induced the rock fatigue failure. The decrease in strength by fatigue was causedby the increase in crack population and preferential
orientation of microcracks within the specimen.
REFERENCES

50
40

Number o f crack sÅkesson U., Hansson, J. & Stigh, J. 2004. Characterisation of
microcracks in the Bohus granite, western Sweden, caused
by uniaxial cyclic loading. Engineering Geology 72:
131-142.
Burdine, N.T. 1963. Rock failure under dynamic loading
conditions. Society of Petroleum Engineers Journal March
1963: 1-8.
Chen, Y. & Wang, C.-Y. 1980. Thermally induced acoustic
emission in Westerly granite. Geophys. Res. Lett. 7(12):1089-1092.
Chen, Y., Nishiyama, T. & Ito, T. 2001. Application of image
analysis to observe microstructure in sandstone and granite.
Resource Geology 51(3): 249-258.
Costin, L.S. & Holcomb, D.J. 1981. Time-dependent failureof rock under cyclic loading. Tectonophysics 79: 279-296.
Haimson B.C. & Kim C.M. 1971. Mechanical behavior ofrock under cyclic fatigue. In Proc. thirteenth Symposium
on Rock Mechanics: 845-863. Illinois, USA.
Hardy, H.R. Jr. & Chugh, Y.P. 1970. Failure of geologic
materials under low-cycle fatigue. In Proc. 6th Canadian
Symposium on Rock Mechanics: 33-47. Montreal, Canada.
Kusuda, H. & Nishiyama, T. 1994. Extraction and evaluationof microcracks in rocks by methods of image processing.
Journal of Japan Society of Engineering Geology 34(6):292-299.
Nishiyama, T. & Kusuda, H. 1994. Identification of porespaces and microcracks using fluorescent resins. Int. J.
Rock Mech. Min. Sci. & Geomech. Abstr. 31: 369-375.
Suresh, S. 1998. Fatigue of materials: 1-34. Cambridge:
Cambridge UP.
Tapponnier, P. & Brace, W.F. 1976. Development ofStress-Induced Microcracks in Westerly Granite. Int. J.
Rock Mech. Min. Sci. & Geomech. Abstr. 13: 103-112.

less than 0.3mm
0.3-0.6mm
0.6-0.9mm
more than 0.9mm

30
20

10
0
1

10

100

1,000

10,000100,000

CyclesFigure 7. Correlation between cycles and the elongation of
cracks in quartz grains, 160 MPa specimens. Reference area
is about 25mm2.

5 CONCLUSIONSIn order to examine the fatigue process of granite,
Westerly granite specimens were subjected to acyclic loading test under the uniaxial compression
with maximum 140 and 160MPa at room temperature, and a series of specimens stressed to characteristic stages were retrieved. The development of
microcracks in the tested specimens was observed532