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Silicon carbide fiber and silicon carbide matrix (SiCf/SiCm) tubes produced through the chemical vapor infiltration process have become a candidate cladding material in nuclear applications. The performance of this composite is influenced by many variables such as braiding angle, porosity, material properties, etc., which vary over a range of values due to the inherent fluctuations in the manufacturing process. In this study, the variability in elastic constants of SiCf/SiCm composite has been quantified through multiscale finite element (FE) simulations, variable screening, and high-fidelity surrogate modeling. The key variables dominantly affecting the elastic constants of SiCf/SiCm tubes were identified using global sensitivity analysis. A surrogate to the high-fidelity FE-based model was used in Monte Carlo simulations to generate a hundred thousand samples from which the uncertainty in elastic constants was assessed. It turned out that the coefficient of variation was less than 10%.

Bond resistance, bond strength and electronegativity models have been widely utilized for predicting intrinsic hardness of novel covalent materials. Although these models are generally perceived to be distinct, this manuscript recognizes them to be drawing input parameters from the same basis set comprising of bond density, bond length, electronegativity, and coordination number, while primarily differing in the exponents assigned to them. A numerical bound on the feasible space of these exponents is derived, followed by a search operation of this space to determine optimal values. The resulting model yields hardness estimates in good conformance with experimental data for numerous binary and ternary ceramic compounds. The study also provides a plausible explanation for the diversity of parameter–exponent combinations in the existing intrinsic hardness models, that is derived from the variability exhibited by these exponents in capturing a given experimental dataset at a constant fitting error.

Understanding the structural and mechanical properties of coral skeletons is important to assess their responses to natural and anthropogenic challenges and to predict the long-term viability of hermatypic corals in a changing ocean. Here, we describe the microstructure of the critically endangered staghorn coral (Acropora cervicornis) skeleton and its mechanical properties, spectral and fluidic behavior, including uniaxial compressive strength, resistance to plastic deformation, spectral vibrational response, and flow-field analysis. We evaluated skeletons of A. cervicornis retrieved from a nursery off Broward County, Florida, USA. Optical micrographs and X-ray computed topography revealed a complex system of canals and pores that allow rapid skeletal elongation while retaining sufficient strength to withstand currents, waves, and other physical forces. Compressive loading of the aragonite skeleton resulted in complex stress–strain deformation behavior; the unique pore arrangement resisted catastrophic cracks and prevented instantaneous failure. Vickers microhardness was 3.56 ± 0.31 GPa, which is typical for soft aragonite materials yet sufficient to withstand the hydraulic pressure of ocean waves. Impressions made by the diamond indenter had almost no cracks radiating from their corners, which again demonstrated the ability of the complex skeleton microstructure to suppress crack formation and growth (e.g., from the bites of grazers). Maps of the ν1 mode Raman peak of identation surfaces showed evidence of residual strain. However, the ν1 peak’s position barely changed (from 1083.6 cm−1 outside the impression to 1083.9 cm−1 in the center), indicating weak stress sensitivity. Flow-field analysis revealed small-scale, counter-rotating vortices formed in the skeleton’s wake, which can entrain food particles within range of polyp tentacles and facilitate transport of respiratory gases and wastes. Considered together, our results demonstrate that the perforate skeleton of A. cervicornis is well-adapted to withstand physical forces normally encountered in its shallow-water habitat, but may be susceptible to anthropogenic stressors that alter its architecture.

Coral reefs provide habitat for most oceanic life forms, however, they are declining worldwide due to pollution, increased ocean temperature, diseases and other factors. In this paper, structural a...

Longitudinal elastic stress wave propagation through a 180° bend junction connecting two square bars is analyzed using analytical and numerical approaches and validated against experiments. The aim is to identify conditions under which the one-dimensional stress propagation principles can be applied to this geometry despite complete reversal of the stress wave path and study the mechanism of wave propagation through this geometry. By assuming the junction to move as a rigid body parallel to the input wave direction, the influence of the bend is analyzed for different pulse shapes and durations. For long duration stress pulses, the bend allows the stress wave to “flow” through the junction without distortion, whereas for short duration stress pulses, the wave undergoes significant dispersion. The junction behavior was further analyzed using finite element analysis and the results compared well with those of the analytical model. The wave motion through the junction results in asymmetric deformation of the junction, which generates flexural waves of different amplitudes in both the input and output bars. In general, stress pulses with constant peak amplitude and a smooth transition to the peak value suffer minimal dispersion as they traverse the junction. It is concluded that one-dimensional stress wave theory can be used to successfully model the propagation of long-duration longitudinal stress pulses around a 180° bend junction.

In this manuscript, we present an improved model for long rod penetration into thick ceramic targets by using a modified version of the Walker–Anderson model along with our dynamic expanding cavity model. This model is simpler to use and captures additional physics such as the dwell-penetration transition phenomena in ceramics as compared to the Walker–Anderson model. The target response is assumed to occur in a hemispherical region containing nested comminuted, cracked and elastic regions of deformation. A dynamic expanding cavity model, recently developed by the authors, along with an exponential pressure-shear response of brittle materials in the form of the extended Mohr-Coulomb model, is used to capture the stress fields in these regions. Incorporating this constitutive model reduces the requirement on (often expensive) experimental data as it uses a single set of parameters to predict the response of many brittle ceramics, which is especially beneficial in understanding and exploring the behavior of new materials. The predictions of the model are in good agreement with experimental results and is also used to quantify the effect of amorphization on boron carbide.

Transient rheological properties of poly(ethylene glycol) diacrylate (PEGDA) hydrogels under Couette flow conditions are determined via molecular dynamics simulations and validated by high strain r...

The mechanical properties and amorphization response of a carbon nanotube (5 wt.%) boron carbide (CNT-B4C) composite with 1 μm grain size are investigated, and compared to those of coarse-grained (10 μm grain size) and ultrafine-grained (0.3 μm grain size) monolithic boron carbides. The quasi-static and dynamic uniaxial compressive strengths for CNT-B4C were statistically the same as those of the ultrafine-grained ceramic and higher than the coarse-grained material, contradicting the expected grain size hierarchy (Hall-Petch-type relationship). Addition of CNTs to B4C resulted in decreased quasi-static hardness compared to the large grain size material; however, dynamic hardness was substantially improved compared to quasi-static values. CNT pullout and crack bridging were observed to be possible toughening mechanisms. Finally, Raman spectroscopy was used to quantify amorphization, and it was concluded that addition of CNTs to boron carbide does not alter the propensity for amorphization, but does improve mechanical properties by enhanced toughening.

Dynamic capacity, defined as the load under which rolling element bearing raceways will survive for 1 million revolutions with 90% probability of survival, is commonly used in bearing life-rating standards. This term was introduced by Lundberg and Palmgren to simplify life prediction equations, and was derived using the elastic Hertzian-theory of contact mechanics for earlier, relatively impure bearing materials. Modern ultra-clean steels with fewer impurities can survive for multi-millions of contact stress cycles, even under elastic-plastic loading conditions. Under such conditions, elastic-plastic stresses are significantly different from the elastic Hertz contact stresses. Current experimental and finite-element study, shows accounting for plastic deformation of the material necessitates significant correction in the material parameters used in the expressions for dynamic capacity calculations.

The simple shear response of soft polymers under large deformation (>50%) and strain rates spanning 10−3 – 103 s−1 is characterized by developing quasi-static and split-Hopkinson pressure bar based single-pulse dynamic simple shear experiments rooted in continuum mechanics fundamentals. Cross-linked polydimethylsiloxane (PDMS) is chosen as a model material. By examining the evolution of stress, strain and strain rate, the latter two parameters measured using two-dimensional digital image correlation (DIC), it is demonstrated that dynamic simple shear deformation consists of four distinct stages: momentum diffusion, inertia effect, steady-state material response, and strain rate decay. By isolating the unsteady and steady-state deformation stages, inertia-free material response is captured under a uniform strain rate. It is shown that the shear response of PDMS is nearly linear with a weakly rate-sensitive shear modulus in the investigated strain rate range. Further, by analyzing the DIC strain-field and comparing the kinematic experimental results with those predicted by classical continuum mechanics, it is demonstrated that the proposed experiments not only achieve a nearly theoretical simple shear state that is uniform across the specimen, but also allow for post-test validation of individual experiments based on these criteria.

A computational and experimental study involving coupled electro-thermo-mechanical field equations was conducted to investigate the evolution of temperature and stress gradient within the uranium dioxide (UO2) powder during spark plasma sintering (SPS) process. The numerical simulation was implemented using ANSYS finite element (FE) software, and the influence of various pellet sizes and heating rates on the temperature and stress distribution of the ceramic fuels was analyzed. The reliability of this model was verified by comparing the simulation results with experimentally measured process parameter variations. Finally, the optimum pellet dimensions were determined to reduce the overall temperature and stress gradient within the powder compact throughout the sintering process. The results of the present study could be used for further optimization of the pellet dimensions and achieve uniform microstructure in the pellets prepared by SPS.

A generalized thermodynamic stability criterion for isotropic finite elastic solids is derived using the fundamental balance laws and field equations of continuum mechanics, which is then used to formulate constitutive inequalities for the polynomial form of hyperelastic constitutive equations. Individual thermodynamic constitutive inequalities (called T-C inequalities) are derived for the neo-Hookean, Mooney Rivlin, and three-parameter generalized Rivlin models under three pure homogeneous deformation modes, namely, uniaxial compression, uniaxial tension and shear (simple and pure), and are compared against two commonly used adscititious inequalities, the Baker-Ericksen (B-E) and E-inequalities. The range of stable model constants as defined by the T-C inequalities is represented by a region in an N-dimensional coordinate space (N is the total number of model constants), which is defined as the Region of Stability (ROS). It is shown that the ROS is a function of material deformation and evolves with the limiting strain, shrinking from an initially large region representing the necessary condition of thermodynamic stability to a converged region under infinite limiting strain that is equivalent to the ROS defined by the E-inequalities. By investigating the evolution of the ROS under different deformation modes, the implication of T-C inequalities on the selection of experimental routines and filtering of erroneous test data and model constants is discussed. It is also demonstrated that while the E-inequalities are over-restrictive for hyperelastic materials with small to moderate limiting strains, an observation supported by recent experimental evidence, the B-E inequalities are inaccurate under moderate to large limiting strain conditions. The applicability of the proposed mathematical framework to other hyperelastic strain energy density forms, such as exponential/logarithmic functions, is demonstrated by investigating the thermodynamic stability of the Fung-Demiray model. It is shown that the commonly assumed restriction that the Fung-Demiray model constants must be positive can be relaxed so that some typical material behaviors under small to moderate limiting strains can also be modeled.

Shockwave propagation in polyethylene glycol diacrylate (PEGDA) hydrogels is simulated for the first time using nonequilibrium molecular dynamics simulations. PEGDA hydrogel models are built using the “perfect network” approach such that each crosslink junction is comprised of six chain connections. The influence of PEGDA concentration (20–70 wt%) on shock behavior is investigated for a range of particle velocities (200–1000 m/s). In agreement with reported experimental results in the literature on gels with similar densities, shock velocity and pressure in PEGDA hydrogels are found to increase with polymer concentration, within a range bounded by pure water and pure polymer behaviors. Nonlinear relationships are observed for shock pressure and shock front thickness as a function of concentration, and a logarithmic equation is proposed to describe this behavior. In addition, the relationship between pressure and shock front thickness is compared with hydrodynamic theory. Deviation from hydrodynamic predictions is observed at high particle velocities and this deviation is found to be related to viscosity changes. A power-law relationship between strain rate and pressure in PEGDA hydrogels is identified, similar to that of metals. However, a power-law exponent of 1.4 is computed for all gel concentrations, whereas an exponent of 4 is typically reported for metals.

The organization of proteins is an important determinant of functionality in soft tissues. However, such organization is difficult to monitor over time in soft tissue with complex compositions. Here, we establish a method to determine the alignment of proteins in soft tissues of varying composition by polarized Raman spectroscopy (PRS). Unlike most conventional microscopy methods, PRS leverages non-destructive, label-free sample preparation. PRS data from highly aligned muscle layers were utilized to derive a weighting function for aligned proteins via principal component analysis (PCA). This trained weighting function was used as a master loading function to calculate a principal component score (PC1 Score) as a function of polarized angle for tendon, dermis, hypodermis, and fabricated collagen gels. Since the PC1 Score calculated at arbitrary angles was insufficient to determine level of alignment, we developed an Amplitude Alignment Metric by fitting a sine function to PC1 Score with respect to polarized angle. We found that our PRS-based Amplitude Alignment Metric can be used as an indicator of level of protein alignment in soft tissues in a non-destructive manner with label-free preparation and has similar discriminatory capacity among isotropic and anisotropic samples compared to microscopy-based image processing method. This PRS method does not require a priori knowledge of sample orientation nor composition and appears insensitive to changes in protein composition among different tissues. The Amplitude Alignment Metric introduced here could enable convenient and adaptable evaluation of protein alignment in soft tissues of varying protein and cell composition. Statement of significance Polarized Raman spectroscopy (PRS) has been used to characterize the of organization of soft tissues. However, most of the reported applications of PRS have been on collagen-rich tissues and reliant on intensities of collagen-related vibrations. This work describes a PRS method via a multivariate analysis to characterize alignment in soft tissues composed of varying proteins. Of note, the highly aligned muscle layer of mouse skin was used to train a master function then applied to other soft tissue samples, and the degree of anisotropy in the PRS response was evaluated to obtain the level of alignment in tissues. We have demonstrated that this method supports convenient and adaptable evaluation of protein alignment in soft tissues of varying protein and cell composition.

The microstructure and mechanical properties of a 70 wt.% B6O–30 wt.% B4C composite are compared to a monolithic B6O, both prepared by spark plasma sintering (SPS). Optical and scanning electron micrographs showed significant phase segregation in the composite material along with extensive porosity in regions containing larger amounts of B6O. Raman spectroscopy and energy dispersive spectroscopy (EDS) indicated the presence of distinct B4C-dominant and B6O-dominant phases as well as near-homogeneous matrix interphase. The porous structure of the B6O resulted in a lower hardness of the B6O-dominant phase (11.3 GPa) and matrix interphase (16.7 GPa) as compared to the B4C-dominant phase (29.8 GPa) and the monolithic B6O sample (34.3 GPa). Contrary to theoretical predictions made elsewhere in the literature, the current mechanical testing of both samples indicated no improvement in hardness or compressive strength in the composite material as compared to the monolithic B6O material; however, the incomplete sintering and resulting porosity in the B6O phase of the composite had significant deleterious effects on its properties.

Hydrogels exhibit a fluid-like viscous response under high shear strain rates with significant rate- and microstructure-dependent rheological properties. In this study, the transient shear response of poly(ethylene glycol) diacrylate (PEGDA) hydrogels is characterized by application of the power-law fluid model to incompressible Navier–Stokes equation of start-up planar Couette flow. To impart the necessary boundary conditions for model calibration, a split-Hopkinson pressure bar based single-pulse dynamic simple shear experiment is developed, in which unsteady momentum diffusion between two shear plates is measured using two-dimensional digital image correlation (DIC). The measured shear profiles and their characteristic self-similarity under these boundary conditions are utilized to calculate power-law exponent and transient-state viscosity via finite difference simulations.

An analytical model is developed to predict shockwave propagation and attenuation in hydrogels by combining the classical method of shock characteristics and a solution for the shock front structure. To guide the development of the model, molecular dynamics (MD) simulations are performed. Specifically, a one-dimensional shock pulse in poly(ethylene glycol) diacrylate (PEGDA) hydrogels is simulated with the nonequilibrium MD method. The role of polymer concentration on the shock response is evaluated by constructing hydrogels with 20, 35, and 50 wt% PEGDA concentrations in an idealized crosslinked network. Steady-state pressure-density and shock-particle velocity relationships are established using the Murnaghan equation of state. Shock front structure is characterized by a power-law equation that relates the shock front thickness with shock pressure. These results are used as critical input for the shock propagation and attenuation model. The model is then evaluated via comparison with the classical method of characteristics. It shows significant improvement in accuracy and successfully captures salient features of shockwave attenuation, including the shock pressure amplitude, the velocities of the shock and release waves, and the attenuation timeline. Hydrogels with higher polymer concentrations exhibit a shorter attenuation time at all particle velocities studied. This behavior is attributed to differences in bulk properties and shock front structure in hydrogels with different polymer/water concentrations.

Raman spectroscopy has been used extensively to characterize the influence of mechanical deformation on microstructure changes in biomaterials. While traditional piezo-spectroscopy has been successful in assessing internal stresses of hard biomaterials by tracking prominent peak shifts, peak shifts due to applied loads are near or below the resolution limit of the spectrometer for soft biomaterials with moduli in the kilo- to mega-Pascal range. In this Review, in addition to peak shifts, other spectral features (e.g., polarized intensity and intensity ratio) that provide quantitative assessments of microstructural orientation and secondary structure in soft biomaterials and their strain dependence are discussed. We provide specific examples for each method and classify sensitive Raman characteristic bands common across natural (e.g., soft tissue) and synthetic (e.g., polymeric scaffolds) soft biomaterials upon mechanical deformation. This Review can provide guidance for researchers aiming to analyze micromechanics of soft tissues and engineered tissue constructs by Raman spectroscopy.

Macroscale experiments for structural response of materials often do not capture deformation features at the atomistic scale. This limitation becomes more pronounced in materials subjected to shock loading, especially in covalently bonded ceramics such as boron carbide, which exhibit a deleterious mechanism called pressure-induced amorphization. Perhaps because ceramics have high thermal resistance, temperature was never considered a prime factor influencing deformation mechanisms, leaving such phenomena unexplained for more than two decades. In this article, we probe fundamental material behavior using molecular dynamics (MD) to link nanoscale thermodynamics to amorphization during shock loading. We have generated Rayleigh lines via MD shock simulations, subsequently constructing the shock Hugoniot, which aligns well with experimental data. We estimate shock-induced temperature in ceramics to be well above their melting point, resolving a plethora of interconnected scientific issues pertaining to anomalous behavior of boron carbide.

The high strain rate behavior of nonideal poly(ethylene glycol) diacrylate hydrogels under uniaxial tension and transient-state shear deformations is investigated using molecular dynamics (MD) simulations. This work specifically focuses on the influence of first-order loop defects, including their effect on topological evolutions. Two approaches are proposed to systematically introduce first-order loops, allowing separate and controllable investigations of effective cross-link functionality and cross-link density. MD simulations confirm that first-order loop defects are elastically inactive, but the topological disruptions caused by the presence of loop defects influence mechanical behavior. For decreasing effective cross-link functionality but constant cross-link density, a weaker tensile stress-strain response and decreasing shear-thickening behavior are observed. This is due to nonaffine translation of cross-link junction positions during deformation. Hydrogels with lower cross-link density but constant effective functionality show a stronger stress-strain response and an earlier transition between entropic and enthalpic deformation regimes. This behavior is correlated to changes in mesh size caused by the introduction of loops within an elastically active network. However, the resulting range of cross-link densities is not sufficient to cause measurable changes in shear-thickening behavior. To conclude, reductions in effective cross-link functionality are more important to high strain rate behavior than reductions in cross-link density.

Mechanical properties and microstructure of 3D-printed alumina processed using pressurized spray deposition have been compared to a commercial sintered alumina. The 3D-printed alumina microstructure was found to be bimodal in nature, with alumina particles agglomerated into large spheres, which resulted in 6.1% porosity. Compared to the sintered alumina, the 3D-printed material exhibited lower quasi-static and dynamic compressive strength, negligible differences in quasi-static and dynamic Vickers hardness, and negligible differences in quasi-static and dynamic fracture toughness. However, while the dynamic fracture surfaces of 3D-printed alumina were smooth and planar, large undulations were observed under quasi-static loading. It is concluded that the pressurized spray deposition 3D-printing technique is a promising method for processing alumina with properties comparable to that produced by traditional techniques, and further improvements may be gained by eliminating porosity.

Four pressure-sensitive phenomenological constitutive models for brittle ceramics are compared to available experimental data over a wide range of materials and confinement pressures. It is shown that the linear Mohr–Coulomb and Drucker Prager models are only applicable at low pressures. While the nonlinear strain-rate dependent Johnson–Holmquist (JH-2) model provides a reasonable fit to experimental data up to HEL for most materials, it completely fails to capture the experimentally observed pressure and strain-rate independent strength saturation of ceramics at pressures beyond HEL. It is demonstrated that the extended Mohr–Coulomb model is the only model that can not only capture the linear response at low pressures, but also the experimentally observed nonlinear response at higher pressures up to HEL and the pressure-independent response beyond HEL with a single curve. Further, the constants used in this model are shown to be universal and applicable to most brittle materials, allowing the model to be used in the absence of additional experimental data. The potential to extend the extended Mohr–Coulomb model to damaged ceramics and its implementation into computational codes is also discussed.

When exposed to harsh environmental conditions, the adhesive bond between epoxy and concrete was found to depend almost entirely on the effectiveness of the mechanical interlock which is contingent upon the ability of the adhesive to properly wet and penetrate the substrate. The current study is an investigation of the existence of a distinct interphase region within cement paste through the combined use of mercury intrusion porosimetry and Raman spectroscopy. A positive correlation between median pore size and depth of epoxy permeation into the cement paste substrate was observed. The Lucas-Washburn equation was found to accurately represent the driving mechanisms behind permeation of curing epoxy into the porous cement paste matrix. Interconnectivity of the capillary and gel pore structures was determined to play an important role in the permeation mechanisms. Analysis of relative conversion of epoxy via Raman spectroscopy in the interfacial region showed evidence of epoxy-cement paste interactions.

Recently, researchers discovered that in contrast to isolated twins, periodic twins with nanoscale spacing can dramatically improve mechanical properties. Ceramics engineers now seek to incorporate this “nanotwinning” into icosahedral solids because of their high strength, high stability, and low mass density. In this manuscript, we assert that boron suboxide, while far less studied than boron carbide (i.e., the most popular icosahedral solid), possesses higher propensity for nanotwinning and higher theoretical promise. For boron suboxide, the influence of processing on twin spacing is explored through mechanical testing and transmission electron microscopy. Quantum-mechanical simulations are then performed to suggest a critical twin spacing that would maximize performance and to show how to track experimental nanotwinning with x-ray diffraction. Finally, transmission electron microscopy and Raman spectroscopy show that amorphization, the localized loss of crystallinity, drives mechanical failure in ways unique to boron suboxide.

This paper depicts efforts for fabricating GaN on diamond microstructure through direct bonding between Ga and C, while excluding the use of adhesive interlayer during spark plasma sintering (SPS) process. The resulting GaN on diamond architecture is seemingly successful, as suggested by macroscopic morphological observations. The microscopic inspection using high-resolution transmission electron microscopy (HRTEM), however, shows a unique, off-the-chart interlayer configuration, wherein the components are migrated, etched, or fused to tentatively form multiple crystal phases. These phases can be constructed based on their utmost stabilities among all possible phases thermodynamically driven under or near the SPS conditions.

Silicon carbide (SiC) fiber-reinforced ceramic matrix (SiCf/SiCm) composites have been identified as potential materials for nuclear fuel cladding. These composites are fabricated by braiding the SiC fibers around a mandrel into a cylindrical tube and then densifying this preform with SiC matrix using chemical vapor infiltration (CVI). However, during this fabrication process residual stresses develop in the composite, either due to the mechanical braiding process or due to high temperature CVI deposition of the SiC matrix phase. Micro-Raman spectroscopy has been employed to measure the residual stress in the SiC fibers at various stages of the fabrication process. Samples of the composite were analyzed after mechanical braiding and at 33%, 66% and 100% CVI densification of the composites. Raman spectra obtained at the above stages revealed significant band shifts of the SiC peaks and these peak-shift corresponded to -716 MPa of residual stress following the braiding process and -1075 MPa after the densification process. Interestingly, the Raman spectra also revealed carbon bands whose origin was traced to the presence of nanoscale turbostratic graphite throughout the nanograined SiC microstructure.

In the micromechanical analysis of braided composite tubes, the effects of curvature are often ignored. This approach is acceptable for thin-walled tubes with a large radius. However, in case of small diameter thick-walled tubes, such an approach may lead to inaccurate results. Therefore, it is of interest to determine when the curvature effects of tubular geometry become important in micromechanics. In this study, the extensional stiffness matrix of 2-D braided composite tubes was calculated using a curved unit-cell. The periodicity was imposed in circumferential and longitudinal directions of the tube. The extensional stiffness coefficients were calculated by simulating an internal pressure test, uniaxial tensile test, and torsion test. The obtained stiffness coefficients were compared to those obtained using a flat unit-cell. The results of curved unit-cell were different from that of flat unit-cell. The differences in stiffness coefficients, calculated using curved and flat unit cells were as high as 13%.

An extended Mohr–Coulomb (M–C) model, capable of capturing the high pressure-dependent shear deformation of ceramics, is incorporated into a spherical dynamic expanding cavity model (d-ECM) to capture the dynamic response of semi-infinite brittle ceramics under steady-state penetration. It is shown that this improved d-ECM can better predict the target resistance of structural ceramics reported in penetration experiments. The brittle ceramic is assumed to undergo cracking when the hoop stress reaches its tensile strength and is subsequently comminuted when the radial stress reaches its compressive strength. The constitutive behavior of the comminuted ceramic is predicted using a modified form of the extended M–C model. The extended M–C model captures the strain rate-independent exponential pressure-shear response of intact ceramics in a normalized universal strength model. A single set of three parameters can be used to describe the response of all intact ceramics. This constitutive model has been suitably modified into a two-parameter exponential model, applicable to comminuted ceramics. These two parameters have been calibrated using experimental penetration data along with analytical estimations, and it has been shown that a single set of universal parameters can be used to describe the response of most comminuted ceramics. By incorporating these models, the improved d-ECM can be used to estimate the target resistance of a ceramic when relevant experimental data is not readily available. The results of the analysis further reveal that, in the case of steady-state penetration into semi-infinite monolithic targets, the properties of the comminuted material have a greater influence on the target resistance of ceramics than those of its intact state. The model proposed is applicable for thick monolithic ceramic targets under steady-state penetration conditions, and not for layered targets.

A methodology is described for characterizing the spatial distribution of thermal mismatch stresses at grain level in B4C-SiC-Si ceramic composites using Raman spectroscopy. Unlike traditional methods to detect residual stresses (e.g., X-ray diffraction) which provide average values of stress over the entire specimen surface, Raman peak-shift analysis provides residual stress distributions within the microstructure at high spatial resolution. While classical formulation predicts uniform compressive stress within a Si-phase surrounded by the ceramic matrix, the Raman measurements revealed non-uniform residual stress distributions in Si when the particle size was larger than 5 microns. For large irregular shaped particles, the two methods coincide only along the interface between the particle and matrix, but vary drastically both in magnitude and nature in the interior of the particle where large tensile stresses have been measured. The presence of anomalous tensile stress in the interior of the minor Si-phase results in defect generation and structural disorder which has been confirmed by TEM analysis. Raman spectroscopic mapping was also used to compute an average macroscale residual stress value for a given material composition allowing links to be drawn between processing, microstructure and properties. The average residual stress within the microstructure was found to correlate well with the estimates based on volume fraction of the constituents and material properties.

Three-dimensional models of amorphized zones beneath quasistatic and dynamic Vickers indentations on boron carbide were constructed using micro-Raman spectroscopy. The square of amorphized zone depth varied linearly with load and the maximum amorphized area occurred beneath the indentation imprint in accord with the maximum shear stress under Hertzian contact. Reduced measurements of amorphization intensity at loads above 10 N may be due to a loss of subsurface amorphized material through lateral cracks. Utilizing an expanding cavity model with power-law (n = 0.79–0.80) and linear (Ep = 0.39–0.45) strain hardening responses, finite element simulations were conducted to determine the critical values of stress and strain required to cause amorphization. These simulations suggest that amorphization may initiate at von Mises stresses and equivalent plastic strains above 6.6 GPa and 0.026, respectively. These results may be useful for validating computational models of boron carbide under complex loading scenarios (e.g., ballistic impact).