Phillip Jannotti

Raman spectroscopic characterization of the core-rim structure in reaction bonded boron carbide ceramics

Raman spectroscopy was used to characterize the microstructure of reaction bonded boron carbide ceramics. Compositional and structural gradation in the silicon-doped boron carbide phase (rim), which develops around the parent boron carbide region (core) due to the reaction between silicon and boron carbide, was evaluated using changes in Raman peak position and intensity. Peak shifting and intensity variation from the core to the rim region was attributed to changes in the boron carbide crystal structure based on experimental Raman observations and ab initio calculations reported in literature. The results were consistent with compositional analysis determined by energy dispersive spectroscopy. The Raman analysis revealed the substitution of silicon atoms first into the linear 3-atom chain, and then into icosahedral units of the boron carbide structure. Thus, micro-Raman spectroscopy provided a non-destructive means of identifying the preferential positions of Si atoms in the boron carbide lattice.

Instrumented Penetration of Metal Alloys During High-Velocity Impacts

A methodology is presented for characterizing the failure behavior of metallic targets due to high-velocity and hypervelocity impacts. Time-resolved sub-scale terminal ballistic experiments were performed at approximately 1.2 km/s to assess the feasibility of using high-speed optical imaging, photon Doppler velocimetry, and high-speed 3D digital image correlation for measuring back face deformation. Spherical copper impactors were fired into aluminum alloy targets with thickness equal to one half the impactor diameter. The approach has implications for determining the susceptibility of metallic targets to different failure modes including bulk plastic deformation resulting in tensile failure, cratering, plugging, spallation and adiabatic shear band formation. Results will be used to assist in validation of large-scale computational models used to model ballistic impact.

Measurement of Residual Stresses in B4C-SiC-Si Ceramics Using Raman Spectroscopy

Processing-induced residual stresses in reaction bonded B4C-SiC-Si ceramic composites were investigated using Raman peak shift measurements. The measured stresses in the residual silicon phase were compared with classical formulation which predicts stress development due to thermal mismatch between a particle and the surrounding matrix. It was found that the residual stress does not remain uniform in a given particle as predicted in the classical formulation. The two methods matched at the boundary between the particle and the matrix, but varied drastically both in magnitude and nature in the interior of the particle. This variation became more dramatic when the particle was of irregular shape with high aspect ratio.

Micro-Raman Spectroscopic Evaluation of Residual Microstresses in Reaction Bonded Boron Carbide Ceramics

Raman spectroscopic mapping was verified as a viable method to assess the spatial distribution of residual microstresses originating from thermal mismatch between various phases in reaction bonded boron carbide ceramics. Beneficial residual compressive stresses in the residual silicon phase have been suggested to lead to enhanced mechanical performance. However, no literature exists to definitively confirm the presence of such residual stresses. Using Raman scans of the residual silicon phase, the 520 cm−1 Raman peak, characteristic of stress-free crystalline silicon, was tracked in order to detect peak shifts indicative of residual microstresses. Based on the observed peak shifts for individual point scans, the presence of residual stresses of a detectable level have been confirmed. Additionally, based on peak broadening and peak intensity reduction, which is indicative of disorder, it appears likely that the residual silicon becomes highly disordered as a result of the processing. The results of these Raman studies have major implications as they can used for developing microstructure-property-performance relationships for reaction bonded ceramics.

Application of 3D Digital Image Correlation In Ballistic Testing

Digital image correlation has been identified as a promising technique for use in terminal ballistic research; however, the limited resolution of ultra-high-speed cameras requires coarse speckle patterns not consistently achievable by conventional methods like spray painting. Thus, a robust and automated speckling technique that can apply a speckle pattern with appropriate speckle size and density is desired. The current study outlines an efficient, repeatable means of applying a high-quality black-on-white speckle pattern to test specimens. The speckle patterns were designed digitally and reproduced on the samples using a 3-axis mill coupled to a permanent marker with a custom-made, spring-actuated marker holder. By automating the process, the resulting speckle pattern is significantly more consistent compared to traditional specking techniques like spray painting or using a permanent marker manually. It also allows for precise control of the speckle size, density, and speckle pattern randomness. The method outlined in this work is capable of easily achieving 0.2–4 mm speckles and can generate speckles at a rate of 1–2 speckles per second. Example patterns ranging in size from 10 × 10 mm to 200 × 200 mm are shown which required as little as 5–10 min to complete (each containing ~600 speckles). High-velocity impact experiments were then performed on 6.35 mm thick aluminum plates that were speckled using the described technique to demonstrate the high-rate, localized deformation that takes place during an impact event. It emphasizes the extreme spatial and temporal difficulties of using digital image correlation for ballistic research and the need for a tailored speckle pattern.

Parametric Study of the Formation of Cone Cracks in Brittle Materials

Brittle materials such as ceramics and oxide glasses are widely employed because they possess a variety of useful properties including hardness, strength, wear resistance and/or transparency. Normal and oblique impacts of spherical projectiles on brittle materials are similar to the classical Hertzian sphere indentation problem, yet different in significant ways. In the case of oblique impacts, these differences can result in the formation of unique damage patterns, including cone cracks, which are not axisymmetric, and they are distinct from partial cone cracks formed by sliding indentation. Some selected results of oblique sphere impacts on brittle targets are shown and discussed, identifying unique features. A parametric study of the factors contributing to the differences between normal and oblique impacts is reported. The effects of normal and tangential velocity, friction, and softening are investigated as factors influencing the peak principal stresses produced by oblique impacts. Friction and projectile softening were found to have a significant impact on peak maximum principal stress values. An obliquely impacting projectile’s lateral motion and its interaction with the sides of the cone crack were also found to have a significant effect on the stress field and the shape of the resulting cone crack.

Impact Response of Coquina

The Castillo de San Marcos Fort in St. Augustine, FL was built over 330 years ago and has endured numerous wars between the Spanish and the British. During these wars, cannonballs were fired at the fort walls and became embedded in the walls. The walls did not shatter, nor did large cracks form. The fort was constructed from a native rock called coquina, found on the east coast of Florida and the west coast of Australia. Coquina is a highly porous sedimentary rock, consisting of crushed shells, fossils, limestone, sand, minerals, and clay. There are no scientific studies illustrating coquina’s ability to withstand cannonball impacts. This research focused on testing coquina and a similar material (a commercial cellular foam) in uniaxial compression. The compression experiments revealed that coquina had two times the specific energy compared to a structural foam. The research revealed that the microstructure of coquina allows impact to be absorbed by progressive failure and hence possesses a high energy absorption capability.

Damage Mechanisms of Chemically Strengthened Glass Bars Due to High-Velocity Ball Impact

Ball impact experiments were conducted on unstrengthened and strengthened glass bars at 261 and 345m/s, respectively. Damage propagation was recorded using a high-speed camera at frame rates of 281,000 frames per second. Immediately after the ball impact on the unstrengthened glass, the damage front reached a maximum velocity of 1,967m/s before falling to zero within a short distance. However, the longitudinal wave created due to the impact continued down the bar towards the rear-end. Upon reflection from the rear-end of the bar, a secondary damage front was initiated at 3,192m/s, which eventually arrested. On the other hand, the damage front in the strengthened glass reached a maximum of 2,275m/s immediately after impact, and then stabilized at 1,921m/s until the bar was consumed. It was determined that the stored elastic energy in the strengthened glass fueled the self-sustained damage and allowed it to propagate at a near constant rate. For both glasses, high-speed imaging allowed for observation of energy dissipation modes such as fracture propagation (fracture surface area), radial bar dilation, and high velocity jetting of fine glass particles at the impact site. In addition to the triangular dilation observed in the unstrengthened glass at the impact site, the strengthening process also led to uniform dilation of the entire rod.