Browsing by Subject "Shear"
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Item Open Access An experimental study of the jamming phase diagram for two-dimensional granular materials.(2020) Zhao, YiqiuWhat affects the transition of a collection of grains from flowing to a rigid packing? Previous efforts towards answering this important question have led to various versions of ``jamming’’ phase diagrams, which specify conditions under which a granular material behaves like solid, i.e., in a jammed phase. In this dissertation, we report two sets of experiments to study the influence of particle shape and of the form of the applied shear strain on the jamming phase diagram of slowly deformed frictional granular materials. We use 2d photoelastic particles to measure the overall pressure of the system and various physical quantities that characterize the contact network such as the averaged number of contacts per particle.
In the first set of experiments, we systematically compare the mechanical and geometrical properties of uniaxially compressed granular materials consisting of particles with shapes of either regular pentagon or disk. The compression is applied quasi-statically and induces a density-driven jamming transition. We find that pentagons and disks jam at similar packing fraction. At the onset of jamming, disks have contact numbers consistent with predictions from an ideal constraint counting argument. However, this argument fails to predict the right contact number for pentagons. We also find that both jammed pentagons and disks show the Gamma distribution of the Voronoi cell area with the same parameters. Moreover, jammed pentagons have similar translational order for particle centers but slightly less orientational order for contacting pairs compared to jammed disks. Finally, we report observations that for jammed pentagons, the angle between edges at a face-to-vertex contact point shows a uniform distribution and the size of a cluster connected by face-to-face contacts shows a power-law distribution.
In the second set of experiments, we use a novel multi-ring Couette shear apparatus that we developed to eliminate shear banding which unavoidably appears in conventional Couette shear experiments. A shear band is a narrow region where a lot of rearrangements of particles occur. The shear band usually has a much smaller packing fraction than the rest of the system. We map out a jamming phase diagram experimentally, and for the first time perform a systematic direct test of the mechanical responses of the jammed states created by shearing under reverse shear. We find a clear distinction between fragile states and shear-jammed states: the latter do not collapse under reverse shear. The yield stress curve is also mapped out, which marks the stress needed for the shear-jammed states to enter a steady regime where many plastic rearrangements of particles happen and the overall stress fluctuates around a constant. Interestingly, for large packing fraction, a shear band still develops when the system remains strongly jammed in the steady regime. We find that the cooperative motion of particles in this regime is highly heterogeneous and can be quantified by a dynamical susceptibility, which keeps growing as the packing fraction increases.
Our observations not only serve as important data to construct theories to explain the origin of rigidity in density-driven jamming and shear-induced jamming but also are relevant to many other key problems in the physics of granular matter from the stability of a jammed packing to the complex dynamics of dense granular flows.
Item Embargo Injury Risks in Behind Armor Blunt Trauma(2024) Op 't Eynde, JoostBody armor protects law enforcement and military personnel from gunshot wounds to the thorax. However, even when a round is stopped, armor can deform into the thorax at high rate and produce injuries. To evaluate armor protection against this behind armor blunt trauma (BABT), an outdated standard developed in the 1970s is currently used. The applicability of the standard to modern design and its biofidelity are questionable. There is a need for biofidelic models and accurate injury criteria for BABT.
To support numerical modeling of high rate insults, material property characterizations are essential. Pure shear tests at high rate and high shear strain were performed on porcine dorsal skin, ventral skin, liver, and lung tissue post-mortem. Synthetic gelatin was subjected to the same shear tests, to evaluate its validity as a tissue surrogate. Instantaneous elastic shear properties of the tissues were determined, and their stress relaxation over short and long timescales. Dorsal skin tissue was found to have the highest shear stiffness, followed by ventral skin, liver, and lung. Synthetic 10-20% gelatin approximates the instantaneous elastic shear properties of porcine dorsal skin but does not show the same viscoelastic relaxation behavior. Synthetic 10% gelatin behaved similarly to 20% gelatin in stress relaxation, but with significantly reduced shear stiffness. Shear moduli of biological tissues increase with increased shear strain, suggesting a non-linear model is appropriate for computational purposes.
To recreate BABT in an experimental setting, a 3D-printed acrylic indenter was developed. This indenter replicates the backface deformation of the body armor into the chest, matching velocity and aerial density of hard body armor. The performance of the indenter was evaluated using the current clay testing standard (n = 52). The obtained deformations in clay match those from previous hard armor experiments. The limitations of using clay as a surrogate for behind armor blunt trauma are discussed in relation to the indenter performance: clay is inconsistent and produces and unpredictable elastic rebound obfuscating the final deformation measurement used in the standard. Equivalent exposures comparing indenter velocity to rifle round velocity are used to translate indenter impacts to in-field scenarios.
Indenter BABT impacts (n = 117) were performed on porcine (n = 16) and human (n = 18) cadavers to establish injury scaling from pig to human. Impactor dynamics were determined using an onboard accelerometer and high-speed video, and rib fractures were assessed using post-test micro-CT imaging and necropsy. Regional injury risk curves were developed for different impact locations on the human cadaver (n = 6) thorax and different injury severity levels, indicating the risk might not be uniform. The injury threshold for anterior ribcage injuries is lower than for the posterior ribcage. The kinetic energy of the impact was scaled according to body mass based on equal velocity scaling, widely used in injury biomechanics. Confidence intervals of injury risk curves substantially overlap for the human and swine cadavers, suggesting that this scaling is appropriate for transferring risk across these species. Residual energy differences of 20 to 30% for similar injury risk between the human and swine cadavers suggest an additional bone quality scaling is desirable since the swine cadavers are generally at an earlier developmental age than available human cadavers. The structural scaling relationships between the human and swine cadavers are valuable in interpreting injury results from live animal BABT tests.
In vivo swine (n = 18) were subjected to BABT impacts to the ribcage. Chest wall and lung injuries were assessed using necropsy and histology, and injury risk curves were developed for different severity injuries based on the kinetic energy of the impact. The resulting injury risks are compared to those obtained for human cadavers. Chest wall injury risk corresponds closely with lung injury risk severity. Injury risks for lateral ribcage impacts in the live swine are close to posterior ribcage impact injury risks in the human cadaver, but injury risks are lower than for frontal impacts in human cadavers. Acoustic emissions of rib fractures were non-invasively detected during BABT impact with the use of hydrophones. Obtained injury risks and fracture detection may guide future armor design and injury monitoring.
A novel modality of lung injury was observed in the live swine impacts. Advancement of the chest wall into the lung tissue at high velocity produces a local compressive shock that can damage alveolar walls and cause bleeding within the lung tissue. A theoretical basis for shock development, experimental shock pressure measurements, and characteristic injuries are presented.
Item Open Access Response of Granular Materials to Shear: Origins of Shear Jamming, Particle Dynamics, and Effects of Particle Properties(2018) Wang, DongGranular materials under shear are common in nature and industry. Previous results show changes of system behaviors when friction is added and particle shapes are varied, e.g. shear jamming for frictional grains. Understanding these changes depends on characterization of deformation induced by shear. However, previous studies mainly focus on yielding processes and are locally symmetric, e.g. shear transformation zones (STZ's). Besides, the grain scale explanation is lacking. In this thesis, I study the shear response of granular materials with various particle properties in two dimension, utilizing a novel setup that suppresses shear banding. Particles made of photoelastic materials can reveal inter-particle contact forces and be customized to have different friction and shapes. I propose novel minimum structures, trimers and branches, that account for shear jamming. These structures are locally asymmetric, which is contrary to STZ's. Systems with three different friction coefficients $\mu$ are studied: $0.15, 0.7$ and one higher than $1.7$. Shear jamming is still observed for the lowest $\mu$ studied, with the lowest value of packing fraction $\phi$ for shear jamming, $\phi_S$, increasing as $\mu$ decreases. Furthermore, these systems for all $\mu$ show abnormal diffusion under cyclic shear. The diffusion exponents show transitions as $\phi$ increases, with a $\mu$-dependent onset $\phi$. This behavior is consistent with the non-affine displacements under linear shear. In addition, systems composed of ellipses exhibit novel structural and mechanical responses different from disks, e.g., nematic ordering and local density variability under shear.