Browsing by Subject "Shock"

Body 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 To evaluate armor protection against this behind armor blunt trauma (BABT), an outdated standard developed in the 1970s is currently used.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.

The successful, efficient, and safe turbine design requires a thorough understanding of the underlying physical phenomena. This research investigates the physical understanding and parameters highly correlated to flutter, an aeroelastic instability prevalent among low pressure turbine (LPT) blades in both aircraft engines and power turbines. The modern way of determining whether a certain cascade of LPT blades is susceptible to flutter is through time-expensive computational fluid dynamics (CFD) codes. These codes converge to solution satisfying the Eulerian conservation equations subject to the boundary conditions of a nodal domain consisting fluid and solid wall particles. Most detailed CFD codes are accompanied by cryptic turbulence models, meticulous grid constructions, and elegant boundary condition enforcements all with one goal in mind: determine the sign (and therefore stability) of the aerodynamic damping. The main question being asked by the aeroelastician, ``is it positive or negative?'' This type of thought-process eventually gives rise to a black-box effect, leaving physical understanding behind. Therefore, the first part of this research aims to understand and reveal the physics behind LPT flutter in addition to several related topics including acoustic resonance effects. A percentage of this initial numerical investigation is completed using an influence coefficient approach to study the variation the work-per-cycle contributions of neighboring cascade blades to a reference airfoil. The second part of this research introduces new discoveries regarding the relationship between steady aerodynamic loading and negative aerodynamic damping. Using validated CFD codes as computational wind tunnels, a multitude of low-pressure turbine flutter parameters, such as reduced frequency, mode shape, and interblade phase angle, will be scrutinized across various airfoil geometries and steady operating conditions to reach new design guidelines regarding the influence of steady aerodynamic loading and LPT flutter. Many pressing topics influencing LPT flutter including shocks, their nonlinearity, and three-dimensionality are also addressed along the way. The work is concluded by introducing a useful preliminary design tool that can estimate within seconds the entire aerodynamic damping versus nodal diameter curve for a given three-dimensional cascade.

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