Experimental results and discussion
The uniaxial compressive strength
results of the specimen is shown in Table 1. The elastic modulus and Poisson’s
ratio of the rock in the table are calculated by selecting 50% of the peak
strength value (M.H.B, Rao, & Ramamurthy, 2002).
For the uniaxial strength and
elastic modulus, we can see that the variation of uniaxial strength varies
greatly, with the maximum reaching 206.168MPa and the minimum of only
99.841MPa. The average elastic modulus of the rock is 86.835GPa, shows that the
elastic modulus of the rock is large which will cause more elastic deformation
during the load process. The average Poisson’s ratio (0.274) of all samples as
seen in table 1.
At the later stage of the loading
process, the rock samples were peeled off and accompanied by splashing and
destruction. The rock samples emitted a loud sound and accompanied with the
splashing of the rock, which indicated that the rock samples showed strong
brittleness. During the loading process, large elastic energy is stored, and
the energy is quickly released at the moment of failure of rock specimen.
Based on this study, the failure mode
of slate caused by
uniaxial compression can be summed up in 3 categories
which are Tensile Failure (TF), X shear failure (XSF) and Shear Failure (SF) as
illustrated in figure 7. Tensile failure is dominant at angle ?=90° and it is
parallel to the direction of laminations and the
samples had multiple splitting failures along the axis of surface tension as
shown in figure 9a. Under the influence of the axial compressive stress,
the Poisson’s effect will cause the transverse stress to be greater than the
rock’s compressive strength which will cause the destruction of the rock. The
cracks from the sample are similar to that obtained by (Khanlari, Rafiei, & Abdilor, Evaluation of strength anisotropy and
failure modes of laminated sandstones, 2015) but the cracks were
observed at angle ? =0 °. When ? =30°£ ?£ 60°, Shear Failure (SF) occurs.
In this type, the shear failure occurs along the axial to the laminations and
gives rise to two separate shear planes in the middle. The shape of the sample
failure is a ?-shaped which resembles the one of (Khanlari et al, 2015) but in
their experiment it occurs at ? =900 as
shown in fig. 9b.
In X Shear Failure (XSF), the failure cuts
across the laminations perpendicularly. The transverse stress produced by the
Poisson effect is larger than that of the tensile Strength which will cause the
destruction of the rock. This falls
under ? =00as shown in fig. 9c.
Q from my explanations, does these diagrams
correspond to the failure modes?
The rock samples in uniaxial failure forms are complex
and changeable. It is generally assumed that most of the final rock
destructions are parallel to the axis of fracture (failure
along foliation plane). Similar studies were conducted by (MingTien, Chuan Kuo, & HseinJuang, 2006) and (Saroglou & Tsiambaos , 2007), to determine the uniaxial compressive
strength variations of rocks. In this study, the type I and type II maximum
compressive strength occurs when ? =0°, while in type III the maximum
compressive strength occurs when ? =90° as shown in figure 6.