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Rock Mechanics and Rock Engineering

, Volume 52, Issue 11, pp 4217–4235 | Cite as

Experimental Study of Cracking Characteristics of Kowloon Granite Based on Three Mode I Fracture Toughness Methods

  • Louis Ngai Yuen Wong
  • Tian Yang GuoEmail author
  • Wing Ki Lam
  • Jay Yu Hin Ng
Original Paper

Abstract

Mode I fracture toughness is an important material parameter of rock. To date, a number of laboratory methods have been developed to measure the mode I fracture toughness. Although the fracture toughness values measured by different methods have been compared in many previous studies, the effect of the specimen configuration and pre-existing notch shape on the fracture toughness value and the associated cracking characteristics have not been comprehensively studied. In the present study, the failure modes and mode I fracture toughness of Kowloon granite, a major type of granite in Hong Kong, are studied experimentally based on the chevron bend (CB), semi-circular bend (SCB) and cracked chevron notched semi-circular bend (CCNSCB) methods. The fracture toughness measured by the SCB test is much lower (56.5% lower) than that measured with the CCNSCB test. The measured CB fracture toughness is between the SCB and CCNSCB results. The failure modes of specimens containing the chevron notch (CB and CCNSCB specimens) are found to differ from those containing the straight-through notch (SCB specimens), especially with respect to the post-failure behavior. The cracking mechanism of the SCB and CCNSCB is studied macroscopically and microscopically. The results show that the straight-through notch and chevron notch, which are the most commonly used notch shapes adopted in various mode I fracture toughness tests, have significant effects on the cracking behavior and crack characteristics. Further investigation on the thin sections prepared from the specimens after the mode I fracture toughness tests provides us with new insights into the microscopic characteristics of tensile cracks in granite under mode I loading condition.

Keywords

Fracture toughness Semi-circular bend specimens Cracking characteristics Microscopic investigation 

List of Symbols

A

Deviation area enclosed by the dominant crack profile and the central pre-existing notch line

ADA

Absolute deviation area of the dominant crack

a

Pre-existing notch length

a0

Minimum chevron notch length

a1

Maximum chevron notch length

ac

Critical crack length in chevron notch ligament

B

Specimen thickness

D

Chervon bend specimen diameter

d

Deviation distance of the dominant crack

dmax

Maximum deviation distance of the dominant crack

KIC

Mode I fracture toughness

L

Chevron bend specimen length

Lv

Vertical length of studied dominant crack

P

Load applied on specimen

Pmax

Peak load recorded in the fracture toughness test

R

Specimen radius

Rs

Saw blade radius

RDA

Relative deviation area of the dominant crack

S

Half span of the two supporting rollers

t

Notch width

W

Short rod specimen length

Y

Minimum (i.e., critical) value of the dimensionless stress intensity factor

β

Normalized straight-through notch length

β0

Normalized minimum chevron notch length

β1

Normalized maximum chevron notch length

βB

Normalized specimen thickness

θ

Chevron angle

μ

Parameter depends on β0 and βB

ν

Parameter depends on β0 and βB

Notes

Acknowledgements

The first and second authors acknowledge the support from the HKU Start-up Fund, the General Research Fund 2017/18 (#17303917) of the Research Grants Council (Hong Kong), and the Hung Hing Ying Physical Sciences Research Fund 2017-18. The second author acknowledges the Postgraduate Scholarship at the University of Hong Kong. The authors would also like to thank Xinyu Xiao for her supportive work in experimental tests. MTR Corporation is acknowledged for providing the rock cores. The Public Works Central Laboratory (PWCL) of the Civil Engineering Development Department (CEDD) is acknowledged. The data for this paper are available by contacting the corresponding author at guoty@hku.hk or the first author at lnywong@hku.hk.

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Copyright information

© Springer-Verlag GmbH Austria, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Earth SciencesThe University of Hong KongHong KongChina

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