Micro Shear Bands: Precursor for Strain Localization in Sheared Granular Materials

Druckrey et al. (2018) reported the results of RPTG analysis for a very dense F35 Ottawa sand specimen tested at ${\sigma }_3=400\mathrm{kPa}$ (F35-D-400 kPa specimen). Strain hardening took place between ${ϵ}_1=0\%$ and ${ϵ}_1=5.1\%$, and the specimen reached a peak PSR of 3.9 at ${ϵ}_1=5.1\%$ (Fig. 2). MSBs were detected during the hardening phase at ${ϵ}_1=0\%–1\%$ [Fig. 5(a)]. As the test progressed, MSBs continued to develop parallel and perpendicular to the direction of the final shear band. MSBs are shown in Fig. 5(b) after thresholding the color maps with values representing the average translation gradient of all particles within the cross section in Fig. 5(a) (i.e., deleting particles that exhibited relative translation gradients smaller than the average translation gradient of all particles within that slice). More MSBs developed at ${ϵ}_1=1.0\%–2.0\%$ [Fig. 5(c)]; they are shown in Fig. 5(d) through thresholding of color maps. It is evident that these MSBs increased in number, thickness, and length compared with the first loading step [Fig. 5(b)]. The MSBs became more defined, extending through the entire cross section of the specimen as compression progressed, and multiple MSBs merged to form a zone of intensive shearing at the center of the specimen at the end of the hardening phase [${ϵ}_1=3.4\%–4.9\%$ Fig. 5(f)]. The orientation of the intensive shearing zone was the same as that of the final major shear band. Most MSBs merged to form the final major shear band as the specimen passed the peak PSR state [${ϵ}_1=4.9\%–6.9\%$, Fig. 5(g)]. The final major shear band became more defined during the postpeak PSR stage and had a thickness of $9d_{50}–10d_{50}$. Particles within this band exhibited high relative translations and were bound by transition bands composed of particles that manifested smaller relative translations.

The RPTGs for very dense glass beads that were tested at a high confining pressure (GB-D-400 kPa) are depicted in Fig. 6. Strain hardening took place in the range of ${ϵ}_1=0\%–3.7\%$, peak PSR was reached between ${ϵ}_1=3.7\%$ and 4.0%, followed by softening in the range of ${ϵ}_1=4.0\%–7.3\%$, and the CS transpired thereafter (Fig. 2). As in the F35-D-400 kPa experiment, zones of intensive shearing into MSBs were detected during the first ${ϵ}_1$ increment (${ϵ}_1=0\%–1\%$), and MSBs with thicknesses of $3d_{50}-5d_{50}$ began to nucleate during the second ${ϵ}_1$ increment [${ϵ}_1=1\%–2.1\%$, Fig. 6(b)] and continued to grow throughout the hardening phase. MSBs are shown in Fig. 6(c) after the thresholding of the color map using average translation gradient of all particles within the cross section in Fig. 6(b). MSBs had a thickness of $3d_{50}–5d_{50}$. The thicknesses of MSBs in this experiment were larger than those observed in the F35-D-400 kPa experiment, and were not as well defined as those in the F35-D-400 kPa experiment. Most MSBs within the GB-D-400 kPa experiment oriented in the same direction as the final major shear band, whereas a few MSBs developed at the same inclination and the opposite direction as the final major shear band. During the peak PSR phase (${ϵ}_1=3.7\%–5.2\%$), most MSBs within the GB-D-400 kPa specimen merged into a large zone of intensive shearing within the specimen and oriented in the direction of the final major shear band. During the postpeak softening stage of the experiment (${ϵ}_1>5.2\%$), the zone of intensive shearing localized into the final major single shear band that continued to evolve and develop during the CS. The final major shear band was much thicker ($\text{thickness}=13d_{50}–14d_{50}$) than that of the F35 sand that had a higher $I_{\mathrm{sph}}$ (least spherical), and more-angular and less-well-defined particles than the F35-D-400 kPa specimen. There were also some particles with large relative translation outside the final major shear band. Large relative translation of individual (or small groups of) particles outside MSBs or the final major shear bands was attributed to the stick-slip nature of glass beads, as manifested by frequent decreases in the PSR response in Fig. 2. Glass beads have relatively uniform roundness/sphericity and a smoother surface texture, which in turn reduces the interlocking and friction between particles. Once the PSR reaches the peak state; it forces the collapse of unstable force chains within the specimen, in which some of the particles within the force chains slide out of the chain due to low interlocking. The sliding manifests itself as oscillations in the PSR. Alshibli and Roussel (2006) presented a detailed experimental investigation of stick-slip phenomena in glass beads.

Fig. 7 displays the results of the RPTG for very dense GS#40 Columbia grout sand tested at high confining pressure (GS40-D-400 kPa). The experiment exhibited strain hardening until ${ϵ}_1=4.7\%$. Peak PSR phase for this experiment was broad and endured through Increments 5 and 6 (${ϵ}_1=4.7\%–6.7\%$ and 6.7%–8.6%, respectively), whereas slight softening occurred during Increment 7 (${ϵ}_1=8.6\%–11.4\%$) and thereafter (Fig. 2). The initial ${ϵ}_1$ increment (${ϵ}_1=0\%–1\%$) showed no evidence of MSBs; only two horizontal layers of larger relative displacement were apparent as a result of image processing and should not be considered as strain localization. Through Increments 2 and 3 (${ϵ}_1=1.0\%–1.9\%$ and 1.9%–3.3%, respectively) during the strain hardening phase, a complex network of MSBs emerged similar to those in the F35-D-400 kPa experiment, with thicknesses of $2d_{50}–4d_{50}$. Many MSBs developed in conjugate directions with inclinations similar to that of the final major shear band, whereas some extended through the entire specimen. Near the end of strain hardening (${ϵ}_1=3.3\%–4.7\%$), the MSBs began to merge and show a preferential alignment with that of the final major shear band. During the peak PSR phase, few MSBs remained, and the final major shear band was nearly fully developed. Full development of the final major shear band commenced during the minor postpeak PSR softening and continued through the CS phase. The final major shear band for the GS40-D-400 kPa experiment had a thickness in the range of $10d_{50}–13d_{50}$ and was not as well-defined nor as delineated as the shear band for the F35-D-400 kPa experiment (thickness $9d_{50}–10d_{50}$). However, the final major shear band for the GS40-D-400 kPa experiment was more delineated and better defined than the shear band in the GB-D-400 kPa experiment (thickness $13d_{50}–14d_{50}$), indicating the influence of particle morphology on the growth of shear bands in granular materials.

The relatively high confining pressure is conducive to forcing shearing along a well-defined single shear band for a very dense specimen. Of the three 400 kPa experiments that exhibited a final major shear band, the thickness of the final shear band of F35 sand was the lowest, followed by that of GS40 sand; the final major shear band of glass beads was the thickest. This trend was also apparent for MSBs thicknesses, in which F35 sand had the thinnest MSBs, followed by GS40 sand and glass beads. A careful look at particle morphology values in Table 1 shows that F35 sand had the highest roughness (surface texture), highest nonsphericity, and second-highest roundness, excluding the GB. The GS40 sand particles were more spherical, more angular, and had a surface roughness lower than F35 sand. It appears that particle roundness and surface roughness are the main contributors to particle interlocking, relative particle translation, and specimen tendency to develop a shear band. Many glass beads will be part of the shear band, because they are spherical and have smooth surfaces; therefore, the rotation of particles during shearing is not suppressed due to friction and interlocking between particles. The high ${\sigma }_3$ and very dense packing will suppress the ability of nonspherical particles to rotate, and nonround corners (represented by $I_R$) contribute to a higher interlocking between particles, which will result in a slightly thicker shear band for GS40 sand than for F35 sand.

Pena et al. (2007) and Andò et al. (2012a, b) investigated the effect of particle shape on the rotation of particles and the thickness of the shear band. Pena et al. (2007) used a molecular dynamic approach to study the micromechanical behavior of 2D granular assemblies in a shear cell. A Voronoi tessellation scheme was used to create random 2D polygons (particles) within the periodic shear cell. The shear strain localization was investigated by quantifying the particle rotation along the height of the specimen. The distribution of average particle rotation along the height of the specimen was quantified. The thickness of the shear band was defined as the width of the distribution with a rotation angle higher than 80% of the maximum average rotation. They showed that the width of the localization zone in an isotropic medium is larger than the width of the localization zone in elongated particles (i.e., a ratio of 0.55–0.45 of the cell height).

According to Pena et al. (2007), the thinner shear bands in the elongated particles were attributed to the shape of the particles, which increased the interlocking and exhaustion of the movement of the particles, causing a thinner shear band. The 2D polygons used in their study had a smaller degree of freedom compared with particles within a 3D granular assembly. A smaller degree of freedom affects the relative translation between particles. The elongated particles in the medium had a uniform aspect ratio of 2.3, which is considered a high aspect ratio compared with the particles used in this paper. Another important element was the angularity of the isotropic and elongated particles used in Pena et al. (2007). Both isotropic and elongated particles had angular shapes, which increased the interlocking between the particles.

Andò et al. (2012b) used an ID-tracking method to investigate the 3D kinematic behavior of Hostun HN31 (angular grains, $d_{50}=338\mathrm{\mu m}$) and Caicos ooids (rounded grains, $d_{50}=420\mathrm{\mu m}$) sands in triaxial experiments. The tracking scheme is similar to the method used in this paper. The 3D translation and rotation particles were used to identify the shear bands within the specimens. They concluded that specimens with angular particles exhibited a thicker shear band than did specimens composed of rounded particles. They argued that this happens due to the interlocking between the particles, which expands the influence zone of the rotation of the particle. Because the influence zone is larger, the rotation of a particle affects a larger area and causes a thicker shearing zone. The findings of the present paper in terms of the influence of particle morphology agree in principle with Andò’s et al. (2012a, b) explanation, keeping in mind that the present paper offers quantitative measurements of morphology indexes based on 3D images of sand grains.

RPTG for #1 dry glass sand tested at high confining pressure (DG-D-400 kPa experiment); results are displayed in Fig. 8. This experiment experienced strain hardening up to ${ϵ}_1=5\%$, peak PSR at ${ϵ}_1=6.9\%$, and a very minor softening into the CS during Increment 6 (${ϵ}_1=6.9\%–8.9\%$) (Fig. 2). Failure of this specimen was characterized as bulging based on the specimen’s surface deformation, which is an external manifestation of a more complex internal failure mode. Strain localization began to form during the first ${ϵ}_1$ increment (${ϵ}_1=0\%–1\%$) (Fig. 8). In the second ${ϵ}_1$ increment (${ϵ}_1=1\%–2\%$), MSBs formed in a diagonal direction through the specimen, which would seem to be early evidence of a major well-defined single shear band. However, during the third ${ϵ}_1$ increment (${ϵ}_1=2\%–3.5\%$), several MSBs developed throughout the specimen in the opposite direction and began to compete with the previously developed MSBs. MSBs in this third increment had thicknesses of $3d_{50}–4d_{50}$. During the fourth ${ϵ}_1$ increment (${ϵ}_1=3.5\%–5\%$), a complex network of MSBs emerged with a preferential inclination; however, intersecting MSB caused a relatively large zone of intensive shearing within the middle of the specimen. During the loading increment approaching peak PSR (${ϵ}_1=5\%–6.9\%$), MSBs developed in opposite directions, resulting in the development of two conical shearing surfaces near the specimen’s top and bottom and an intensive shearing zone in its middle. Conflicting MSBs in a cross-hatched pattern continued to compete with each other throughout the remainder of the experiment, retaining the conical shearing zones near the top and bottom of the specimen and many MSBs in the central zone. Cross-hatching of MSBs forced other lateral MSBs to manifest and push outward toward the specimen surface and to push groups of particles outward, resulting in specimen bulging at the surface. During the last two ${ϵ}_1$ increments (${ϵ}_1=8.9\%–17.4\%$, Fig. 8), particles within the sheared central part of the specimen experienced large translations which made it difficult to track most particles within that zone, so the translation gradient is not displayed for these particles.

DG sand had the highest angularity of surface corners according the o roundness classification (Table 1), which appears to be the main morphology index affecting strain localization of very dense specimens tested under high confining pressure. In addition, it had the second-highest $I_{\mathrm{sph}}$ and $R_q$ values. As the specimen tried to fail along a single shear band, a high degree of interlocking developed, causing a high resistance to shearing that forced the development of multiple NSBs and a zone of high dilation in the middle part of the specimen. Compared with the other specimens in Fig. 2, DG-D-400 kPa exhibited the smallest softening because it continued to resist shearing with a high degree of interlocking between particles, whereas a single shear band caused a decrease in PSR for F35-D-400 kPa.

The failure mode of very dense specimens sheared at a low confining pressure (${\sigma }_3=15\mathrm{kPa}$) occurred through bulging, based on observations of specimen surface. The F35-D-15 kPa experiment (Fig. 9) showed early signs of randomly oriented MSBs with thicknesses in a range between $2d_{50}$ and $3.5d_{50}$ concentrated in the central part of the specimen until approximately ${ϵ}_1=3.5\%–5\%$, when MSBs had mostly merged into a centralized zone except for the conical shearing zones near the top and bottom of the specimen. As shearing progressed, a centralized hourglass-like (X-shaped in 2D slice visualizations) shearing pattern dominated the internal microstructure, forcing particles to move toward the surface of the specimen in the lateral direction, and some of these particles were part of MSBs (Fig. 9, ${ϵ}_1=11.9\%–17.4\%$); thick transparent markers were added to the image to show the hourglass pattern, and the thin lines show structured MSBs being forced outward. The outward thrust of particles and MSBs caused surface bulging of the specimens. Fig. 10 displays cross-sectional slices within the conical shearing zones at the top and bottom of the specimen as well as a cross section near the midheight of the specimen for the ${ϵ}_1=11.9\%–17.4\%$ increment. The circular pattern within the top and bottom cross-sectional slices revealed that the shape of the failure zone was in fact conical. The middle axial cross-section slice shows the high RPTG between the conical shearing zones and multiple MSBs with no preferred orientation other than random RPTGs toward the circumference of the specimen in the lateral direction (specimen’s axisymmetric plane).

The GS40-D-15 kPa experiment (Fig. 11) also exhibited early signs of MSB development (thicknesses between $3.5d_{50}$ and $4d_{50}$) and retained several major MSBs after ${ϵ}_1=5\%$ with inclinations similar to the conical shape observed near the top and bottom of the F35-D-15 kPa experiment, resulting in a centralized cross-hatched shearing pattern. The ends of the MSBs pushed toward the specimen surface, and particles outside those shearing zones were forced to move outward laterally, resulting in bulging of the specimen surface. A similar failure mode was observed in the DG-D-15 kPa experiment at early strains (Fig. 12), with MSB thicknesses between $3d_{50}$ and $3.5d_{50}$. A cross-hatched MSB pattern formed, and conical shearing zones at the top and bottom of the specimen developed and persisted. However, particle relative translation eventually showed a large centralized zone of particle intensive shearing pushing particle groups outward laterally to cause specimen bulging. The GB-D-15 kPa experiment (Fig. 13) exhibited many distributed MSBs and zones of localized shearing throughout the experiment, many of which were sporadically distributed throughout the specimen. Thicknesses of MSBs at early strain increments were between $3d_{50}$ and $5d_{50}$. Some structure was retained in cross-hatched MSBs; however, some of the structured deformation was randomly orientated, and shearing zones near the specimen top and bottom did not have a well-defined conical structure. Specimen bulging was caused by MSB ends pushing out toward the specimen surface as well as pushing groups of particles laterally outward.

The role of confining pressure on internal shearing mechanisms of granular material was significant. A cross-hatched MSB failure pattern developed during strain hardening for both low (${\sigma }_3=15\mathrm{kPa}$) and high (${\sigma }_3=400\mathrm{kPa}$) confining pressures. However, at low confining pressure, MSBs were thicker, and shearing zones were not as well structured as they were at higher confining pressure. This is attributed to a more stable microstructure imposed by higher confining pressure. A high confining pressure typically forced MSBs and shearing zones to merge into a single major shear band during the peak PSR and softening phases, except in the DG-D-400 kPa experiment. In experiments conducted at low confining pressure, the failure manifested in specimen surface bulging with a complex network of internal MSBs. Cross-hatched MSBs were the typical mode of the internal failure (singular hourglass pattern in the F35-D-15 kPa experiment), in which the cross-hatched MSBs forced groups of particles and lateral MSBs outward as well as conical shearing zones at the specimen top and bottom.

Density plays a role similar to that of confining pressure on the mechanisms of shearing. Figs. 14 and 15 show the results for the F35-L-15 kPa and DG-L-15 kPa experiments in which evidence of the onset of MSBs was detected at early loading stages of the experiments (${ϵ}_1=2\%$, Fig. 3). Sporadic MSBs were detected during early loading stages of both experiments with thicknesses between $3d_{50}$ to $5d_{50}$ (Figs. 14 and 15). As the experiments progressed, the MSBs evolved into less-defined geometric patterns compared with the patterns observed in dense specimens. This can be caused by the less-stable fabric in medium-dense specimens, in which there is relatively more void space between particles coupled with a low confining pressure. The failure mechanism in both specimens was manifested through a complex network of MSBs, which was very close to the failure mode of the F35-D-15 kPa and DG-D-15 kPa experimental results (X-shaped hourglass, Figs. 10 and 12). MSB orientations were random during the hardening phase of the experiments and transitioned into a more defined zone of intensive shearing composed of multiple MSBs.

In order to investigate the effect of particle kinematic behavior on particle-scale contact and interaction of particles within triaxial specimens, the evolution of average contact numbers of individual particles and their corresponding rotations were investigated. While rotating, particles may initiate new contacts and eliminate others. The dynamics of contacts (i.e., the number of contacts lost and created) is related to the degree of rotation, void space around particles, particle morphology, initial density of the specimen, and applied confining pressure. The effects of CRAs, specimen density, confining pressure, and particle aspect ratio on ACN were studied in detail using 3D measurements. The aspect ratio of particles was divided into three size categories, 1–1.75, 1.75–2.5, and greater than 2.5, based on the range of measurements of tested sands. Most particles had aspect ratios between 1 and 2.5. The CRA color map for particles within a central axial section perpendicular to the final shear bands is depicted in Fig. 16. The glass beads are excluded from Fig. 16 because of the lack of a meaningful trend within the specimens. Particles within shear bands or within zones of intensive shearing exhibited higher rotation angles than particles outside these shear zones. Some particles rotated as much as 40°–50°. The number of particles that exhibited a high rotation increased as the confining pressure decreased for very dense specimens (Fig. 16, compare F35-D-15 kPa, DG-D-15 kPa, and GS40-D-15 kPa with F35-D-400 kPa, DG-D-400 kPa, and GS40-D-400 kPa, respectively).

In order to investigate the effect of CRA on the ACN of particles, the CRA was divided into three subgroups, and the evolution of the ACN of each subgroup was examined as the experiments progressed. F35 specimens (Fig. 17) had different ranges of CRA depending on initial specimen density and confining pressure. The F35-D-400 kPa specimen [Fig. 17(a)], which had a high initial density and was exposed to a high confining pressure (${\sigma }_3=400\mathrm{kPa}$), exhibited smaller CRA, in the range between 0° and 28°, compared with F35-D-15 kPa [Fig. 17(b)], and F35-L-15 kPa [Fig. 17(c)], which had CRA values, ranging between 0° and 58° and 0° and 56°, respectively. The smaller void space and relatively high confining pressure for the F35-D-400 kPa specimen reduced particle rotation, manifesting rotations considerably smaller than those of the F35-L-15 kPa and F35-D-15 kPa specimens. For all three F35 specimens, particles with fewer contacts during the initial state (smaller ACN) had more freedom to rotate and exhibited higher CRA values. In addition, particles with higher CRA lost more contacts with their neighboring particles (highest change in the ACN) than did particles with smaller CRA as the shearing progressed. The high initial ACN during shearing supports the notion that smaller rotation angles are due to higher interaction with neighboring particles. Fig. 17(d) compares the evolution of ACN for F35 specimens with different initial densities. The ACN of particles in both the F35-L-15 kPa and F35-D-15 kPa specimens approached a plateau during the final stages of both experiments when the CS was reached. Rothenburg and Kruyt (2004) investigated the evolution of CN using DEM simulation of 2D granular discs and showed that the CN reached a constant value (i.e., critical contact number) during the final stages of the DEM simulations. The initial CN value for the dense specimens was higher than the CN for the medium-dense specimen, which was expected [Fig. 17(d)].

The variation of the ACN within the medium-dense specimen was much less than the change in the ACN of the very dense specimen. This difference was caused by extra void space around sand particles within the medium-dense specimen. The CN of particles is influenced by the void space available around them. Particles surrounded by more void space have a higher chance of forming new contacts compared with densely packed particles. The rate of contacts loss was much higher than the rate of forming new contacts in very dense specimens; thus the contact number varied noticeably. However, these two rates balanced each other in a medium-dense specimen, reducing the rate of change of ACN compared with that of a very dense specimen. Rothenburg and Kruyt (2004) investigated the effects of available void space around particles and formulated an analytical solution for the variation of CN that showed that the rate of formation of new contacts depends on the available void space around particles. In the present study, the F35-D-400 kPa specimen exhibited an initial increase in the can, followed by a significant decrease in the ACN as the test progressed. The F35-D-15 kPa specimen exhibited a gradual decrease in ACN throughout the test. Both dense specimens exhibited relatively similar ACNs in the initial stage and the final stage. Mirghasemi et al. (1997) studied the effects of confining pressure on the evolution of the CN of 2D polygon-shaped particles using DEM. They found that the confining pressure has similar effects as initial density of the specimens on the ACN of particles.

Fig. 18 shows the evolution of ACN of DG specimens, in which the range of CRA for the DG specimens and their dependence on the confining pressure and initial density was similar to that of the F35 specimens, except in the DG-D-400 kPa experiment, in which the range of rotation angle was slightly larger than that of the DG-D-15 kPa experiment even though ${\sigma }_3$ was much smaller for the DG-D-15 kPa experiment. The number of particles in DG-D-15 kPa with a CRA between 30° and 41° was very low (38 particles). Similar to the F35 specimens, particles with high CRA in the DG specimens had a smaller initial CN, and the CN continued to decrease as compression progressed, indicating the dependence of CRA on the initial specimen density and the evolution of ACN through the test. Figs. 18(a and b) show the ACN for the DG-D-400 kPa and DG-D-15 kPa experiments, respectively; ${\sigma }_3$ had little effect on the ACN of the DG specimens, which is similar to the results of F35 specimens. Fig. 18(c) summarizes the evolution ACN for the DG-L-15 kPa experiment, in which the rate of change of ACN was much smaller than the ACN for very dense specimens, supporting the dependence of the rate of change of ACN on the free void space available around particles. Fig. 18(d) compares the ACN for DG-D-15 kPa and DG-L-15 kPa specimens, showing that both specimens reached a critical ACN during the final stages of the experiments.

Fig. 19 shows the evolution of the ACN for the GS40 and GB specimens, only very dense specimens of which were scanned. The results are similar to those of the F35 and DG specimens, with the exception of the GB-D-400 kPa specimen, for which the rate of change of the ACN values was much less than that of other very dense specimens that were tested at high confining pressures. The GB specimens exhibited a higher CRA than the other specimen (as high as 300°). The GB particles were nearly spherical with no corners and had a relatively smoother surface, resulting in minimal interlocking between them. The absence of interlocking caused the particles to have more freedom to rotate compared with the other specimens. Particle morphology is another important property that affects particle ACN. Particle aspect ratio and its effect on the evolution of particle ACN was also studied. Fig. 20 summarizes the evolution of the ACN for all 10 triaxial specimens, in which particles that had a higher aspect ratio had a higher initial ACN and a higher ACN as the compression progressed. The rate of change of ACN was similar for all particles, and it seems that the aspect ratio of the particles did not change the rate of change of ACN. The dense specimens tested at high confining pressure (${\sigma }_3=400\mathrm{kPa}$) had an initial rate of change of 0.04–0.2, followed by a sudden decrease at a rate of 0.1–0.3. The decreasing rate diminished and approached 0 as the test progressed. The behavior of dense specimens at low confining pressure (${\sigma }_3=15\mathrm{kPa}$) had a similar trend; however, the initial rate of change was smaller than that of the dense specimens at high confining pressure. The initial increasing rate was between 0.012 and 0.025, followed by a sudden decrease at a rate between 0.17 and 0.25. The ACN approached a constant value during the final stage of the experiment. The loose specimens at low confining pressure (${\sigma }_3=15\mathrm{kPa}$) exhibited a general decrease in the ACN during the test. The ACN initialy decreased at a rate of 0.05–0.08, and as the test progressed this rate approached values close to zero. Ting et al. (1995) and Ng (2004) investigated the effects of the aspect ratio of 2D and 3D particles using DEM and showed that particles with higher aspect ratios have a higher CN and less rotation. The elongation of a particle is expected to affect its tendency to rotate. The GB specimens did not have as many particles with aspect ratios higher than 2.5 due to their spherical shape.