index and Sw2d is the degree of sphericity both defined by. Wadell (1933) (1932). In these relations, A is the projection area and P is the perimeter of the 2D ...
Proceedings of the 13th BGA Young Geotechnical Engineers’ Symposium, Manchester, 30 June to 2 July 2014
Study of particle crushing under compression S. Luo Department of Civil Engineering, the University of Bristol, Bristol, UK Supervisors: Dr. E.Ibraim, Dr. A.Diambra
ABSTRACT: When the confining pressure is sufficiently large, grain breakage can occur and this can have a significant influence on the performance of a wide range of geotechnical systems such as shallow foundations, embankments and dams, railway substructures. However, the mechanics of particle crushing still remains one of the most difficult problems in geosciences. This paper analyses chalk particles with approximately 10mm size under uniaxial compression. The crushing behaviour, the distribution of the generated fragments and the data of particle shape description had been presented. It emerges that there is no clear link between these various parameters and further consideration of the internal structure of chalk is necessary.
KEYWORDS: single particle, crushing, compression. 1
INTRODUCTION
Wadell (1933) (1932). In these relations, A is the projection area and P is the perimeter of the 2D particles. Feret diameter is the distance between two parallel tangents to the particle outline while and are respectively the largest and the smallest values of for a given outline; and are respectively the diameters of the maximum inscribed circle and the minimum circumscribed circle: as can be observed in the Figure 1, the areas of the maximum inscribed circle and the minimum circumscribed circle are respectively 107.91 and 75.10 , while and are respectively 11.22 mm and 10.85 mm, A is 90.39 and P is 36.92mm. The results for 11 particles are presented in Table 1. All the data represents the average of at least four particle positions and corresponding photos taken of the by the digital microscope.
The link between the breakage of particles and the mechanical response of the soil is attracting much attention and the main challenge remains the development of a continuum constitutive model that would incorporate the changing of the particle size induced by the crushing process. However, before moving to an agglomerate of individual soil particles, our initial aim is to concentrate on the behaviour of individual particles first. Therefore, this study examines the mechanical response of single particles under uniaxial compression. 2
MATERIAL
While several individual particles of different materials will be considered, so far only chalk particles grouped by the different equivalent area diameters of 10mm, 7mm, 5mm and 2mm have been considered. However, this paper presents only the data obtained for 10mm equivalent diameter particles. 2.1
Particle shape characteristic
The literature is abundant on shape parameters like the form, sphericity, roundness, circularity, regularity, irregular shapes and roughness. Several parameters had therefore been considered to provide a link between the shape of particle and the mechanical behaviour of single particle under uniaxial compression. An optical microscope was used for measuring the pertinent parameters of a large number of particles, while the software ImageJ and Matlab had been used to generate the shape descriptors as defined ISO (2006): (1) Figure 1. Photo from Microscope with parameters
;
(2)
; ;
(4) ;
;
Table 1. The particle shape defines parameters of 10mm group. Shape Description
(3)
da(mm)
AR
IR
C
Cw2D
Sw2D
(5)
Particle Name 10_1
10.53
0.90
0.76
0.79
0.89
0.90
(6)
10_2
9.01
0.80
0.71
0.77
0.88
0.85
10_3
9.18
0.96
0.78
0.76
0.87
0.89
10_4
9.12
0.99
0.82
0.81
0.90
0.90
Where da is the equivalent area diameter, is the aspect ratio, is irregularity, C is the circularity, Cw2d is also a circularity index and Sw2d is the degree of sphericity both defined by
1
Proceedings of the 13th BGA Young Geotechnical Engineers’ Symposium, Manchester, 30 June to 2 July 2014 Force-displacement disgram of group 2
10_5
9.06
0.89
0.75
0.80
0.89
0.88
250
10_6
8.77
0.93
0.79
0.82
0.91
0.89
200
10_7
9.37
0.94
0.76
0.80
0.89
0.88
150
10_8
9.32
0.89
0.76
0.79
0.89
0.89
10_11
8.57
0.91
0.73
0.78
0.88
0.89
10_13
10.20
0.86
0.76
0.81
0.90
0.88
10_14
9.85
0.88
0.74
0.79
0.89
0.90
3
Force(N)
10mm Number6 10mm Number13
100
50
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
Displacement(mm)
Figure 5. Force-displacement diagram of group 2
EXPERIMENTAL DATA AND RESULT
Force-displacement diagram of group 3 250
3.1
Experiment set up
200 10mm Number4
Force(N)
This crushing machine used for the particle uniaxial compression tests is a displacement controlled electromechanical loading frame able to provide various constant displacement rates. The particles are placed between two rigid plates, of which one is fixed to the loading ram that incorporates an LVDT for axial displacement measurements and a 5kN loading cell. The speed of the displacement was set as 0.005mm/min. The photo of particle before and after test can be shown in figure 2 and figure 3. A recording camera for the study of the crushing mechanisms completes the experimental set-up.
3.2
10mm Number14
100
50
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
Displacement(mm)
Figure 6. Force-displacement diagram of group 3
Table 2. The distribution of crushed fragments of 10mm. Particle Name Total Number Number
Group1 Figure 2. Particle before test
10mm Number11
150
Figure 3. Particle after test
Results
The force-displacement diagrams recorded for each particle are plotted in Figures 4-6. Each figure shows the results that appear relatively similar. The critical crushing force ranges are 150200N(group 1), 100N(group 2) and below 50N(group 3). Comparing the shape of the particles and the mechanical behaviour, even the particles have similar shape, the limit forces are still in a quite wide range. After crushing, the particles were recovered and the number and size of the generated fragments assessed. The distribution of the different sizes of the crushed fragments is showed in table 2. No clear correlation between the crushing behaviour and the distribution of the crushed fragments is observed. It is possible that the internal fabric of the particles to be responsible for these differences.
Group2
Group3
4
Fragments Size 10_1
7-6 mm 5-4 mm 3-1 mm 7
1
0
6
10_2
3
0
2
1
10_3
6
0
2
4
10_5
5
0
1
4
10_7
7
1
1
5
10_8
8
1
0
7
10_6
4
0
2
2
10_13
8
1
1
6
10_4
6
1
0
5
10_11
6
1
1
4
10_14
5
1
0
4
CONCLUSION AND FUTURE WORK
Force-displacement diagram of group 1 250
From the data shown above, two points could be drawn: first, the analysis of the internal structure of the particles is important; second, there is no clear links between the crushing behaviour and the crushed fragments, and further investigation will be conducted. In the future, insight into the breakage mechanisms of various particles of different sizes and shapes made of different materials will be explored through some advanced complementary investigative techniques.
Force(N)
200
10mm Number1
150
10mm Number3 10mm Number5 100
10mm Number7 10mm Number8 10mm Number2
50
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
Displacement(mm)
5
Figure 4. Force-displacement diagram of group 1
REFERENCES
ISO (2006). Representation of results of particle size analysis. Part 1-6. Part 6: Descriptive and quantitative representation of particle shape and morphology. Draft International Standard ISO/DIS 9276, Geneva Wadell, H. (1932). Volume, shape, and roundness of rock particles. Journal of Geology 40, 443–451. Wadell, H. (1933). Sphericity and roundness of rock particles. Journal of Geology 41, 310–330.
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