Experimental Study on the Mechanical Properties of Friction, Collision and Compression of Tiger Nut Tubers

The mechanical properties of agricultural materials can provide the basis for the design and optimisation of agricultural machinery. There are currently very few studies on the mechanical properties of tiger nut tubers, which is not conducive to the design and development of machinery for their harvesting and processing. To obtain the mechanical parameters of tiger nut tubers, this study investigated the effects of variety (Zhong Yousha 1 and Zhong Yousha 2), moisture content (8%, 16%, 24%, 32% and 40%), contact material (steel, aluminium, plexiglass and polyurethane), release height (170 mm, 220 mm, 270 mm and 320 mm), loading speed (5 mm/min, 10 mm/min, 15 mm/min and 20 mm/min), compression direction (vertical and horizontal) on the friction, collision and compression mechanical properties of the tubers. The results were as follows: Both moisture content and contact material had a significant effect (p < 0.01) on the sliding friction coefficient (0.405–0.652) of the tubers; both variety and moisture content had a significant effect (p < 0.01) on the angle of repose (27.96–36.09°); contact material, moisture content, release height and variety all had a significant effect (p < 0.01) on the collision recovery coefficient (0.376–0.672) of tubers; variety, loading speed, moisture content and compression direction all had significant effects (p < 0.01) on the damage force (87.54–214.48 N), deformation (1.25–6.12 mm) and damage energy (82.38–351.08 mJ) of the tubers; only moisture content and compression direction had significant effects (p < 0.01) on the apparent elastic modulus (12.17–120.88 MPa) of the tubers. The results of this study can provide a reference for the design and optimisation of machinery for the harvesting and processing of tiger nut tubers.

Double-Level Energy Absorption of 3D Printed TPMS Cellular Structures via Wall Thickness Gradient Design

This paper investigates the deformation mechanism and energy absorption behaviour of 316 L triply periodic minimal surface (TPMS) structures with uniform and graded wall thicknesses fabricated by the selective laser melting technique. The uniform P-surface TPMS structure presents a single-level stress plateau for energy absorption and a localized diagonal shear cell failure. A graded strategy was employed to break such localized geometrical deformation to improve the overall energy absorption and to provide a double-level function. Two segments with different wall thicknesses separated by a barrier layer were designed along the compression direction while keeping the same relative density as the uniform structure. The results show that the crushing of the cells of the graded P-surface TPMS structure occurs first within the thin segment and then propagates to the thick segment. The stress–strain response shows apparent double stress plateaus. The stress level and length of each plateau can be adjusted by changing the wall thickness and position of the barrier layer between the two segments. The total energy absorption of the gradient TPMS structure was also found slightly higher than that of the uniform TPMS counterparts. The gradient design of TPMS structures may find applications where the energy absorption requires a double-level feature or a warning function.

Deformation of NaCoF₃ perovskite and post-perovskite up to 30 GPa and 1013 K: implications for plastic deformation and transformation mechanism

Texture, plastic deformation, and phase transformation mechanisms in perovskite and post-perovskite are of general interest for our understanding of the Earth's mantle. Here, the perovskite analogue NaCoF3 is deformed in a resistive-heated diamond anvil cell (DAC) up to 30 GPa and 1013 K. The in situ state of the sample, including crystal structure, stress, and texture, is monitored using X-ray diffraction. A phase transformation from a perovskite to a post-perovskite structure is observed between 20.1 and 26.1 GPa. Normalized stress drops by a factor of 3 during transformation as a result of transient weakening during the transformation. The perovskite phase initially develops a texture with a maximum at 100 and a strong 010 minimum in the inverse pole figure of the compression direction. Additionally, a secondary weaker 001 maximum is observed later during compression. Texture simulations indicate that the initial deformation of perovskite requires slip along (100) planes with significant contributions of {110} twins. Following the phase transition to post-perovskite, we observe a 010 maximum, which later evolves with compression. The transformation follows orientation relationships previously suggested where the c axis is preserved between phases and hh0 vectors in reciprocal space of post-perovskite are parallel to [010] in perovskite, which indicates a martensitic-like transition mechanism. A comparison between past experiments on bridgmanite and current results indicates that NaCoF3 is a good analogue to understand the development of microstructures within the Earth's mantle.

Energy absorption characteristics of circular-celled honeycombs under in-plane quasi-static compressive loadings

The energy absorption characteristics of hexagonally packaged circular-celled honeycombs and quadrilater-ally packaged circular-celled honeycombs are obtained under in-plane quasi-static compressive loadings through finite element analysis. The stress–strain curves, deformation modes, energy absorption efficiency, specific plateau stress, normalized energy absorption and energy absorption diagrams are discussed. The cell arrangement patterns influence the shapes of stress–strain curves and deformation modes. The densification strain is in linear relationship with the relative density and the specific plateau stress is proportional to relative density. The hexagonally packaged circular-celled honeycombs have the largest specific plateau stress in the x2 direction for a given relative density. The normalized energy absorption is nearly proportional to the strain before densification and increases with increasing relative density for a given strain in one compression direction. The envelope line in the energy absorption diagram is approximately a straight line tangent to the shoulder points through the origin. The hexagonally packaged circular-celled honeycombs outperform the quadrilaterally packaged circular-celled honeycombs in in-plane energy absorption.

A correction to van Wyk’s model of the compressibility of fibrous materials

van Wyk put forward a compression model of fibrous materials utilizing a library of analytical approaches, including the continuum mechanics, stereological, geometrical probability, least square method, and excluded area concept. In this letter, we wish to point out a key error noted in van Wyk’s work with the objective of correcting misconceptions that are held by the majority of us. Through this contribution, we question the “inverse cube” pressure-volume relationship of random fibrous materials. The pressure-volume relationship has been revisited by modifying the formulation of the mean length of a fiber element between consecutive contacts projected on the compression direction.

High-speed tensile fractures in granite and gneiss: an experimental study

&lt;p&gt;Tensile fractures are ubiquitous in impact structures formed because of high strain rate deformations of the earth&amp;#8217;s crust. At regions far from the point of meteorite impact, intense rupturing, fragmentation, and pulverisation are an implication of pressure waves limiting at the tensile strength of the host rock with little influence of shock deformation or shear failure. The branching and anastomosing of the fractures are controlled by the local stress state and anisotropy. Thus, a network of infilled fractures or impact breccia dikes is a common feature in the subsurface of impact sites.&lt;/p&gt;&lt;p&gt;We have investigated the failure processes under high strain rates responsible for the formation of Mode-I breccia dikes, at the laboratory scale. The control of planar fabric structures in the development of anastomosing tensile fracture networks was studied through high-strain-rate Brazilian disc tests on gneiss (foliated) and granite (isotropic) samples. A Split Hopkinson Pressure Bar, equipped with high-speed photography (~10&lt;sup&gt;5&lt;/sup&gt; fps), was employed in the study. The gneissic foliation in the gneiss samples were oriented at &amp;#952; = 0, 45 and 90&amp;#176; to the compression direction. The strength of granite lies between 24 and 26 MPa, and the gneisses failed in the range of 29-37MPa at about 70-90 &amp;#956;s. The fracture network formation was seen in the time series images. There is a stark disparity in the nature of failure of granite from gneiss and the geometry of clasts formed in each rock type. While granite samples fail with pulverised clasts localised along a single fracture spanning the diameter of the sample along the compression direction, the gneisses further developed a network of secondary fractures forming large elongate clasts. Preferential orientation of secondary crack growth in relation to the foliation is strongly influenced by &lt;em&gt;&amp;#952;&lt;/em&gt; in gneiss samples. The aspect ratio of the pulverised clasts (size &lt; 10mm) formed in granite was about 1:2, whereas the gneisses produced larger clasts. The clasts in gneisses had an aspect ratio of 1:4 for &lt;em&gt;&amp;#952;&lt;/em&gt; = 45 and 90&amp;#186;, and 1:5 for &lt;em&gt;&amp;#952;&lt;/em&gt; = 0&amp;#186;.&lt;/p&gt;&lt;p&gt;The branching and anastomosing nature of fractures is similar in fracture networks observed from the field and in the experiments, thus providing an insight into the formation of high-speed impact breccia dikes in isotropic and foliated rocks. Our experiments demonstrate that monomict breccia dikes may by formed &lt;em&gt;in situ&lt;/em&gt; inclusive of clasts, rather than by infilling in previously formed tensile fractures.&lt;/p&gt;

Transient effects of a pre-existing lattice preferred orientation on the strength of foliated quartzite

&lt;p&gt;Much of our understanding of the strength of the continental crust is based on flow laws derived from homogeneous mono-mineralic aggregates (quartzites).&amp;#160; However, crystal plastic deformation of rocks in the middle to lower continental crust during orogenic events forms foliations, lineations and lattice preferred orientations (LPOs) which produce physical and viscous anisotropies in rocks.&amp;#160; In some of these orogenic events, such as in the Appalachian mountains, multiple deformation events form different, cross-cutting foliations and overprint existing LPOs.&amp;#160; In order to determine the effects foliation/lineation and preexisting LPO have on the strength of rocks in the middle crust, we deformed a natural quartzite with a cross-girdle LPO from the Moine Thrust in Scotland with the compressive stress at six different primary orientations relative to the foliation and lineation. This quartzite has aligned but distributed fine-grained muscovite which defines a foliation and lineation. &amp;#160;The cores were deformed at the same temperature (800&amp;#176;C), pressure (1500 MPa) and strain rate (1.6*10&lt;sup&gt;-6&lt;/sup&gt;/s) to similar strains (50-58%), leaving the foliation/lineation orientation as the only difference between experiments. &amp;#160;Peak stresses occur at strains of 10-20% and are lowest for the sample with foliation at 45&lt;sup&gt;o&lt;/sup&gt; to the compression direction (400 MPa, the weak orientation).&amp;#160; All other cores (hard orientations) have peak strengths of 600 to 1100 MPa and highest for the cores with lineation perpendicular to the compression direction (1100 MPa). These cores in hard orientations all strain weaken to a similar stress (~500 MPa), but are still ~100 MPa stronger than the core with both foliation and lineation initially oriented at 45 degrees to the compression direction.&amp;#160; Optical microstructures include undulatory extinction, deformation lamellae, and at high strain (58%), the quartzite is more than 50% recrystallized. Scanning electron microscope electron backscatter diffraction analyses indicate that recrystallized grains in all cores reflect the deformation conditions of the experiment and original grains retain their initial LPO. &amp;#160;Strength anisotropy at low strains is due to placing the foliation and lineation at non-ideal (hard) orientations relative to the compression direction and is greatest in cores with the lineation perpendicular to the compression direction. &amp;#160;The evolution to a similar strength at high strains indicates that dynamic recrystallization creates new grains oriented for easy slip in the second (experimental) deformation event. These results suggest that differences in lineation and foliation orientations and a pre-existing LPO may cause strength anisotropy in rocks in the mid to lower continental crust, but this anisotropy may be transient and unlikely to exist to high strains.&lt;/p&gt;

The effect of structure and texture on pure magnesium properties

Deformation modes and twin hardening of pure magnesium under compression in respect of the initial structure and texture were studied in the present work. In general, samples had two types of texture with different alignment of c-axis in respect to a compression direction. In the first case, most of the grains have the c-axis parallel to the compression direction and in the second case, the c-axis was perpendicular with the compression direction. It was found that coarse grained material deformed by slip despite the type of the texture, while the fine grained samples, with c-axis perpendicular to the compression direction, deformed by twinning. The samples which deform by twinning exhibited the highest yield point. It was concluded that combination of the fine grained structure and hard type texture components may introduce twinning as the main deformation mode and may increase the mechanical properties of magnesium and its alloys. The model for twin induced hardening is proposed where twins act as the grain refinement factor.

Highly thermally conductive UHMWPE/NG composites with the segregated structure and their application for heat spreader

In this work, ultra-high molecular weight polyethylene (UHMWPE)/natural flake graphite (NG) polymer composites with the extraordinary high thermal conductivity were prepared by a facile mixed-heating powder method. Morphology observation and X-ray diffraction (XRD) tests revealed that the NG flakes could be more tightly coated on the surface of UHMWPE granules by mixed-heating process and align horizontally (perpendicular to the hot compression direction of composites). Laser flash thermal analyzer (LFA) demonstrated that the thermal conductivity (TC) of composites with 21.6 vol% of NG reached 19.87 W/(m·K) and 10.67 W/(m·K) in the in-plane and through-plane direction, respectively. Application experiment further demonstrated that UHMWPE/NG composites had strong capability to dissipate the heat as heat spreader. The obtained results provided a valuable basis for fabricating high thermal conductive composites which can act as advanced thermal management materials.

Neotectonics of Brazzaville and Kinshasa: linking Congo Basin seismicity and in situ stress in the Inkisi Group

The Congo Basin has been affected by several earthquakes for which the in-situ stress has not yet been reported. This study aims to determine the in-situ stress related to earthquakes in the Congo Basin, particularly those located in the north portion of the Republic of Congo (RC) and in the northwest portion of the Democratic Republic of Congo (DRC). The combined analysis of seismic history of the Congo Basin and of in-situ paleo-stress in the Inkisi Group allowed us to distinguish onshore earthquakes that are linked with preexisting zones of fractures on the continent and offshore earthquakes that are directly linked with transform faults. The Inkisi Group has been affected by two phases of strike-slip tectonics. The first phase, with a direction of compression N142°, is a result of the Gondwanide orogenesis in the Paleozoic. The second phase, with a compression direction of N078°, is related to the present-day stress of earthquakes in the Congo Basin. This phase is still active and is likely attributable to ridge push from the opening of the Atlantic Ocean. It is therefore appropriate that infrastructure construction in Brazzaville and Kinshasa considers seismic risk in the Inkisi bedrock of this area. As an example, we note that several masonry fences along the Congo river have developed fractures.