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Static fatigue of sand particle
Journal: Canadian Geotechnical Journal
Manuscript ID cgj-2017-0543.R2
Manuscript Type: Note
Date Submitted by the Author: 27-Feb-2018
Complete List of Authors: Liu, Su; City University of Hong Kong, Department of Architecture and Civil Engineering Wang, Jianfeng; City University of Hong Kong,
Is the invited manuscript for consideration in a Special
Issue? : N/A
Keyword: Static fatigue, sand particles, strength degradation, uniaxial compressive load
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Static fatigue of sand particle
Su Liu1 and Jianfeng Wang
2
1 Research Associate
Department of Architecture and Civil Engineering,
City University of Hong Kong, Hong Kong
2 Associate Professor
Department of Architecture and Civil Engineering,
City University of Hong Kong, Hong Kong
Corresponding Author
Dr. Jianfeng Wang
Department of Architecture and Civil Engineering
City University of Hong Kong, Hong Kong
Tel: (852) 34426787, Fax (852) 27887612
E-mail: [email protected]
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Abstract
Static fatigue is of particular concern in studying the time-dependent mechanical
behavior of brittle geo-materials. In this regard, the time-dependent strength behavior
of individual particles is essential for understanding the creep behavior of sand. In this
study, short-term strength test and static fatigue test of individual sand particles
subjected to uniaxial compressive load were carried out using a mini-loading
apparatus and modified oedometer frames, respectively. The sand particles in the
static fatigue test were loaded in an incremental manner, and the load at each stress
level was maintained for a fixed period of time. Scatter of the strength of sand
particles was described using the Weibull distribution. Long-term strength from the
static fatigue test of individual sand particles is found to be less than the short-term
strength.
Keywords: Static fatigue; sand particles; uniaxial compressive load; strength
degradation
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Introduction
Static fatigue (or delayed fracture) is the phenomenon of a material failing at
stresses smaller than the short-term strength after a period of constant loading
(Callister 2005). The short-term strength is measured from tests under an
instantaneous loading, whereas the long-term strength is determined from a failure
test of a specimen after a period of sustained loading. For granular materials, the
macro-scale long-term strength of an assembly of particles depends on many factors,
including particle rearrangement, interparticle friction, and the time-dependent
strength of individual particles at the micro-scale (Lade and Karimpour 2010). When
the loading stress is greater than the long-term strength and less than the short-term
strength of an individual particle, cracks become unstable and start propagating due to
the gas and moisture adsorption. A sudden fracture occurs when the crack has grown
to such a size that the applied stress can propagate a new surface (Wiederhorn et al.
2002). The time to delayed failure decreases as the loading stress increases.
The phenomenon of static fatigue has been observed in a few types of
homogenous and continuous materials (e.g., glasses and ceramics). About one third of
the short-term fracture stress is enough to produce delayed fracture in glass if the load
is maintained for a number of weeks (Orowan 1944). Water vapor corrosion is the
most important cause of the delayed fracture in glass (Wiederhorn et al. 2011). Based
on the Griffith's criterion, the time-dependent strength can be related to the
rate-dependent strength for ceramics. Therefore, a strength-probability-time (SPT)
diagram can be generated by simple measurements of the strain rate dependence of
fracture strength (Davidge et al. 1973). This theory has been adopted by Kwok and
Bolton (2013) in DEM simulations of the creep behavior of sand due to progressive
crush. The deformation during the time to delayed failure is negligible in glasses and
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ceramics, both of which are brittle materials.
Static fatigue is also found in natural earth materials (e.g., rocks and sands),
which usually contain multiple types of minerals and discontinuities. These properties
result in a higher variability in strength. For rocks, static fatigue is mainly caused by
the growth of pre-existing internal microcracks to a critical length (Erarslan and
Williams 2012). An exponential function was adopted by Schmidtke and Lajtai (1985)
to fit the time-stress data of Lac du Bonnet Granite in a log-log space. The existence
of a long-term strength given by the exponential function was further confirmed by
Damjanac and Fairhurst (2010) through in situ, laboratory and numerical analyses.
The macro-scale time-dependent behavior of sand samples, which stems mainly from
a repetitive cycle of particle crushing, interparticle contact fatigue, rearrangement of
particles and redistribution of contact stresses (Takei et al. 2001), has been reported
by a number of authors (Kuwano and Jardine 2002; Baxter and Mitchell 2004; Kiyota
and Tatsuoka 2006; Lade et al. 2009; Gao et al. 2012). The time-dependent particle
crushing was identified by comparing the grain size distributions obtained before and
after creep or stress relaxation tests (McDowell and Khan 2003; Lade and Karimpour
2010, 2016; Chen and Zhang 2016). Not until recently, the micro-scale process of
static fatigue at the silica sand contacts was revealed by scanning electron microscope
(SEM) observations (Michalowski and Nadukuru 2012; Wang and Michalowski 2015;
Michalowski et al. 2017). This contact fatigue results in the fracture of asperities and
a time-dependent increase of contact stiffness. However, in their studies, no effort was
made to investigate the fracture and the time-dependent strength behavior of
individual sand particles. In this regard, the objective of this work is to explore the
static fatigue phenomenon of individual sand particles subjected to uniaxial
compressive load. Single-particle crushing and static fatigue tests were carried out on
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Leighton Buzzard sand (LBS), a typical quartzitic sand, to yield understanding of the
time-dependent strength behavior of sand particles. The static fatigue tests on
individual grains were performed in a stable humidity environment, to minimize the
influence of water vapor. The original contribution of this study stems from an
experimental testing program on static fatigue behavior of a quartz sand leading to an
improved understanding of the time effects, especially from a statistical point of view,
of static fatigue on the strength of single sand particles.
Material and Method
Material Tested
The LBS particles used in this study are mainly composed of quartz. The grain
size ranges from 0.9 mm to 1.1 mm (Fig. 1). To minimize any possible shape effects
on the static fatigue behavior, particles with similar shapes were selected by
hand-picking. Nevertheless, a high variability in particle strength may still exist due to
the irregular particle shape and inhomogeneous mass distribution. This high
variability makes the behavior of individual particles very complicated, and results in
difficulties in static fatigue test due to the uncertainty of individual particle strength.
Short-term Strength Tests
A mini-loading apparatus developed by Zhao et al. (2015) was used for single
particle short-term strength test. The sand particle was loaded between two copper
platens under a quasi-static loading condition by moving the lower platen at a
displacement rate of 0.1 mm/min. In order to avoid unstable particle rotation during
loading, sand particles in both short-term strength and static fatigue tests were placed
and measured in the steadiest possible condition, under which the particle is in static
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equilibrium with the minimum gravitational potential. Force and displacement were
measured by a load cell and a linear variable differential transformer (LVDT),
respectively. Loading continued until the particle was fractured entirely.
Fig. 2 shows a typical force-displacement relationship obtained from one of 37
short-term strength tests. It can be seen that the force is roughly proportional to the
displacement with a slowly increasing slope of the curve, till a displacement of 0.043
mm, where a sudden drop of the force occurs as the particle is entirely fractured.
Jaeger (1967) defined the single particle strength as:
ff 2
F
dσ = , (1)
where Ff is the peak force causing the particle fracture (Nakata et al. 1999), d is the
particle diameter, defined as the initial distance between the platens before the start of
the particle crushing test. Fig. 3 shows the curves for the survival probability of a
particle under a given stress σf (and the corresponding force Ff) sufficient to cause
failure. Because the diameter of every testing particle is approximately 1 mm, the
curves almost overlap each other. About 56% of the data points fall within the range
of 30 N to 90 N.
Static Fatigue Tests
Static fatigue tests were carried out using the front-loading oedometer frame.
Particles were loaded between two copper cylinders (Fig. 4). LVDTs were read every
10 seconds to record the displacement. Therefore, the time to failure at the ultimate
failure stress could be determined with a precision of 10 s. A flow chart showing the
procedure of a static fatigue test is shown in Fig. 5.
As shown in Fig. 3, the highest particle strength (i.e., 208.6 MPa) is about ten
times greater than the lowest (i.e., 20.6 MPa) from a typical short-term strength test.
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Therefore, it is impossible to predict the strength of an untested LBS particle, and then
load it at a specific stress level proportional to it. A convenient way to obtain the
demanded data of delayed fracture is to load particles at a low stress level, which is
maintained for a fixed period of time. Those particles surviving are then loaded in an
incremented manner until all have failed. To avoid large times to delayed failure, the
step-loading approach was commonly used in performing static fatigue tests for
ceramics and glass materials with high variability in strength (e.g., Davidge et al.
1973; Davis and Mould 1984; Bermejo et al. 2008). It should be noted that by using
this step-loading approach, the load is not steady throughout the loading process.
In the 1st stage of the test, the load on the particle is continuously increased from
0 N to 29.43 N (i.e., corresponding to 300 g of weight on the hanger of oedometer
frame) at an interval of 4.905 N. The duration between load increments within this
stage is negligible. According to Fig. 3, about 86.1% of particles would survive at a
load of 29.43 N. Then, the survived particles are stressed at 29.43 N for 12 hours
overnight in the 2nd stage. In the 3
rd stage, the load placed on the survived particles is
increased at an interval of 2.4525 N or 4.905 N for up to 24 times. The load is
sustained for half an hour at each stress level, leading to a total duration of 12 hours in
this stage. In the last stage, in order to save testing time, the load on the particle is
continuously increased at an interval of 0.981 N until the particle is entirely fractured.
This rapid loading would cause an instantaneous failure of the particle.
Results and Discussion
A typical load-time and displacement-time relationship from a static fatigue test
are shown in Fig. 6. For this test, the load was increased with an interval of 4.905 N in
the 3rd stage. The particle was fractured at an ultimate load of 68.67 N after sustaining
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the load for about 370 s.
Two groups of static fatigue tests were conducted simultaneously using two
modified oedometer frames at a temperature of 22±1 oC and a relative humidity of
70±2 %. In the 3rd stage, the load interval used for one group was 2.4525 N (SF-25),
and 4.905 N for the other group (SF-50). Assuming two identical sand particles are
loaded simultaneously, the different interval loads would lead to different ultimate
failure loads and different amount of time to the delayed failure in the 3rd stage.
Detailed test results from both groups are listed in Table 1. A failure time of zero
means the sample undergoes an instantaneous failure at the ultimate failure stress. The
probability of survival under a given stress σf for particles in both groups are then
compared with that from short-term strength tests in Fig. 7. Note, that the data from
tests with instantaneous failure mode are also included, because excluding these data
from the 4th stage (i.e., data from particles with higher strength), would result in an
unrealistic distribution of survival probability. The data for the short-term strength
tests in Fig. 7 is the same as that in Fig. 3. The obvious reduced strength for the two
groups of static fatigue test clearly suggests the dependence of the particle strength on
the loading time. The loading rate of SF-25 can be regarded as lower than that of
SF-50. This lower rate of loading leads to an overall lower distribution of the static
fatigue strength (σf) for SF-25.
The Weibull distribution, which is widely used to describe the scatter of the
strength in brittle materials, can be written as:
fs
f 0
exp
m
Pσσ
= −
, (2)
where Ps is the survival probability of a particle under a stress σf, σf0 is the
characteristic stress where 37% of the particles survive, and m is the Weibull modulus,
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which is used to describe the scatter of the strength in brittle material and increases
with the decreasing variability in particle strength. σf0 is 77.4 MPa, 74.7 MPa and 63.5
MPa for short-term strength, SF-50 and SF-25 tests, respectively. Equation (2) can be
rewritten as
f
s f 0
1ln ln lnmP
σσ
=
. (3)
The data from both short-term strength and static fatigue tests are plotted using
equation (3) in Fig. 8. The Weibull modulus m for the short-term strength test is 2.16,
which is lower than the value of about 3 reported by Wang and Coop (2016) for the
same type of LBS but with a larger particle size. The higher value of m for the static
fatigue test (i.e., m=2.34 for SF-50 and m=2.87 for SF-25) indicates a decreasing
variability in strength due to the time effects.
The current study does not allow us to explore the micro-scale mechanisms of
static fatigue of sand particles. From the observations of X-ray CT scanning (Zhao et
al. 2015), there is no obvious initial internal microcrack in LBS particles. However, a
rich surface texture, which indicates a distribution of initial surface cracks, is
observed in the scanning election microscope (SEM) images (Wang and Michalowski
2015) of quartz sand. The time-dependent micro-fracturing of asperities has been
observed under constant oblique force, leading to a decrease in roughness and an
increase in stiffness (Michalowski et al. 2017). The phenomenon of delayed fracture,
therefore, could be the result of combined surface and internal crack propagation, at
different load levels.
Conclusions
This study endeavors to explore the static fatigue behavior of sand particles. A
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mini-loading apparatus and two modified oedometer frames were used to conduct the
single particle short-term strength and static fatigue tests, respectively. The
phenomenon of static fatigue does exist in the constant loaded individual LBS particle.
A clear tendency for strength degradation is found by comparing the data from static
fatigue tests with those from short-term strength tests. Moreover, static fatigue tends
to have a larger influence on lowering the strength when the particle has a higher
short-term strength. This results in a decreasing variability in strength due to the time
effects.
Acknowledgements
The study presented in this article was supported by the General Research Fund
CityU122813 from the Research Grant Council of the Hong Kong SAR, National
Science Foundation of China (NSFC) grant No. 51779213 and Shenzhen Basic
Research Grant No. JCYJ20150601102053063.
References
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Journal of Geotechnical and Geoenvironmental Engineering, 130(10):
1051-1062.
Bermejo, R., Torres, Y., Anglada, M., and Llanes, L. 2008. Fatigue behavior of
alumina-zirconia multilayered ceramics. Journal of the American Ceramic
Society, 91(5): 1618-1625.
Callister, W. D. 2005. Fundamentals of materials science and engineering: an
integrated approach, 2nd edn. Hoboken: Wiley.
Chen, X., and Zhang, J. 2016. Effect of load duration on particle breakage and dilative
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behavior of residual soil. Journal of Geotechnical and Geoenvironmental
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crystalline rock. Rock Mechanics and Rock Engineering, 43(5): 513-531.
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relationships in ceramics. Journal of Materials Science, 8(12): 1699-1705.
Davis, M. W., and Mould, R. E. 1984. Effect of step size in incremental loading tests
on glass specimens. Journal of the American Ceramic Society, 67(1): 43-48.
Erarslan, N., and Williams, D. J. 2012. The damage mechanism of rock fatigue and its
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Mechanics and Mining Sciences, 56: 15-26.
Gao, Y., Wang, Y. H., and Su, J. C. P. 2012. Mechanisms of aging-induced modulus
changes in sand under isotropic and anisotropic loading. Journal of Geotechnical
and Geoenvironmental Engineering, 139(3): 470-482.
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219-227.
Kiyota, T., and Tatsuoka, F. 2006. Viscous property of loose sand in triaxial
compression, extension and cyclic loading. Soils and Foundations, 46(5):
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Kwok, C. Y., and Bolton, M. D. 2013. DEM simulations of soil creep due to particle
crushing. Géotechnique, 63(16): 1365-1376.
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Lade, P. V., and Karimpour, H. 2010. Static fatigue controls particle crushing and
time effects in granular materials. Soils and Foundations, 50(50): 573-583.
Lade, P. V., and Karimpour, H. 2016. Stress drop effects in time dependent behavior
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delayed increase in penetration resistance after dynamic compaction of sands.
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sand particles using a high-speed microscope camera. Géotechnique, 66(12):
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List of Tables
1 Static fatigue data for LBS.
List of Figures
1 Photograph of LBS particles.
2 A typical load-displacement relationship from a short-term strength test.
3 Probability of survival considering peak stress and peak force.
4 A modified oedometer frame for static fatigue test.
5 Flow chart showing the procedure of a static fatigue test.
6 (a) Load-time and (b) displacement-time relationship of a static fatigue test.
7 Probability of survival for static fatigue and short-term strength tests.
8 Comparison of the data with Weibull function.
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Fig. 1 Photograph of LBS particles
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Fig. 2 A typical load-displacement relationship from a short-term strength test
0
20
40
60
80
100
0 0.02 0.04 0.06 0.08
Force: N
Displacement: mm
Ff
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Fig. 3 Probability of survival considering peak stress and peak force
0 50 100 150 200 250
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
0 50 100 150 200 250
Ff: N
Survival probability
σf: MPa
Stress
Force
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Fig. 4 A modified oedometer frame for static fatigue test
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Fig. 5 Flow chart showing the procedure of a static fatigue test
Yes No
4th stage,
quickly increase
the load until the
particle is
fractured.
Increase the
load by 0.981N
Fractured? Instantaneous
failure
2nd stage,
load=29.43N
3rd stage,
29.43N<load≤88.29N for SF-25
29.43N<load≤147.15N for SF-50
Yes
No
No
Yes
No
Yes
Sustain the
load for 0.5 h
Static
fatigue
Fractured?
Fractured during 0.5 h?
Increase the load by
2.4525N or 4.905N
Instantaneous
failure
Sustain the load for 12 h, fractured
during 12 h?
Static
fatigue
Yes
No
1st stage
load<29.43N
Increase the load by
4.905N till load=29.43N
Start
Load the sand particle
with 4.905N of load
FracturedInstantaneous
failure
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(a)
(b)
Fig. 6 (a) Load-time and (b) displacement-time relationship of a static fatigue test
0
20
40
60
80
0 4 8 12 16
Load: N
Time: hour1st stage
2nd stage3rd stage
0
0.01
0.02
0.03
0.04
0 4 8 12 16
Displacement: mm
Time: hour1st stage
2nd stage
3rd stage
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Fig. 7 Probability of survival for static fatigue and short-term strength tests
0.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300
Survival probability
σf: MPa
Short-term
SF-25
SF-50
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Fig. 8 Comparison of the data with Weibull function
-5
-4
-3
-2
-1
0
1
2
3
-2 -1 0 1
ln[ln(1/P
s)]
ln(σf/σf0)
Short-term
SF-25
SF-50
Linear fit of short-term, m=2.16
Linear fit of SF-25, m=2.87
Linear fit of SF-50, m=2.34
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Table 1 Static fatigue data for LBS
No.
Load increased by 2.4525 N Load increased by 4.905 N
d (mm) Load (N) Failure
time (s) d (mm) Load (N)
Failure
time (s)
1 0.94 107.9100 20 1.10 98.1000 180
2 1.02 36.7875 590 0.92 29.4300 0
3 0.96 134.8875 0 0.95 68.6700 370
4 1.06 41.6925 300 1.00 68.6700 610
5 0.93 31.8825 880 1.09 103.0050 690
6 1.05 101.0430 0 1.03 122.6250 10
7 1.01 66.2175 700 0.97 39.2400 180
8 0.96 29.4300 0 1.00 68.6700 620
9 1.00 107.9100 0 0.98 34.3350 80
10 0.98 29.4300 21560 0.97 53.9550 160
11 1.10 88.2900 130 1.06 117.7200 400
12 1.09 73.5750 1060 1.08 29.4300 21620
13 0.96 41.6925 10 1.08 53.9550 1780
14 1.09 56.4075 460 0.96 83.3850 840
15 1.03 44.1450 640 1.04 73.5750 910
16 1.03 56.4075 550 1.02 44.1450 1720
17 1.00 29.4300 40 0.95 157.9410 0
18 1.06 66.2175 1150 0.96 53.9550 10
19 1.00 63.7650 820 0.96 98.1000 300
20 0.97 46.5975 760 0.95 78.4800 1660
21 1.01 112.8150 0 0.93 34.3350 170
22 1.05 85.8375 970 1.02 39.2400 30
23 0.93 61.3125 860 1.02 49.0500 860
24 1.01 39.2400 40 1.00 73.5750 1340
25 0.99 46.5975 220 0.9 53.9550 10
26 0.95 73.5750 1000 1.02 44.1450 10
27 0.96 19.6200 0 0.90 882.9000 30
28 0.93 36.7875 30 0.98 93.1950 540
29 1.04 83.3850 500 1.02 29.4300 10
30 1.02 63.7650 60 0.99 68.6700 10
31 0.99 29.4300 530 0.90 49.0500 130
32 0.92 58.8600 220 1.05 122.6250 110
33 0.99 76.0275 1090 1.00 98.1000 230
34 1.01 46.5975 1070 1.04 29.4300 0
35 1.06 80.9325 10 0.97 88.2900 80
36 0.97 46.5975 1780 1.00 137.3400 30
37 1.00 58.8600 20 1.01 63.7650 1450
38 1.05 34.3350 110 0.94 63.7650 10
39 1.01 41.6925 960 0.99 14.7150 0
40 1.06 51.5025 370 0.99 29.4300 940
41 0.94 76.0275 280 1.06 49.0500 390
42 0.97 73.5750 30
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