Cylindrical Voids Research Articles

An investigation of the damage tolerance of carbon/Benzoxazine composites with a thermoplastic toughening interlayer

The results of an experimental analysis of the damage tolerance of carbon/Benzoxazine laminates subjected to a 30J impact, compression after impact (CAI) and open-hole compression (OHC) are presented herein. Four configurations of quasi-isotropic laminate with interlaminar polyamide (PA) fibre veils are studied. Damage areas were examined using X-ray imaging and micro-computed tomography (μCT). It was found that interlaminar veils decreased damage area by up to 36% for a 30J impact. The veils, however, caused the formation of long, cylindrical voids due to retention of the resin within the veils which did not permit percolation into the fibre tows. Despite this, the veils increased the CAI strength from 45% of the undamaged compressive strength to 64%. The OHC response of three types of laminates was found to compare well with commercially available carbon/epoxy systems qualified for use in aircraft structures, but was not significantly affected by the interlaminar veils.

2D full-waveform modeling of seismic waves in layered karstic media

We have developed a new propagator-matrix scheme to simulate seismic-wave propagation and scattering in a multilayered medium containing karstic voids. The propagator matrices can be found using the boundary element method. The model can have irregular boundaries, including arbitrary free-surface topography. Any number of karsts can be included in the model, and each karst can be of arbitrary geometric shape. We have used the Burton-Miller formulation to tackle the numerical instability caused by the fictitious resonance due to the finite size of a karstic void. Our method was implemented in the frequency-space domain, so frequency-dependent [Formula: see text] can be readily incorporated. We have validated our calculation by comparing it with the analytical solution for a cylindrical void and to the spectral element method for a more complex model. This new modeling capability is useful in many important applications in seismic inverse theory, such as imaging karsts, caves, sinkholes, and clandestine tunnels.

Failure Modes of 15 kV SiC SGTO Thyristors during Repetitive Extreme Pulsed Overcurrent Conditions

SiC SGTO thyristors are an advanced solution for increasing the power density of medium voltage power electronics. However, for these devices to replace Si thyristor technology in industrial applications their characteristics and failure modes must be understood. This letter presents the failure modes of two 15-kV SiC SGTO thyristors during repetitive overcurrent conditions. The devices were evaluated with 2-kA (3.85 kA/cm2) square pulses of 100 ${\boldsymbol \mu}$ s duration using a pulse forming network. Throughout testing, each devices’ static characteristics were analyzed for signs of degradation; upon degradation, testing was ceased and the physical failure mode was determined through imaging with a scanning electron microscope (SEM) in conjunction with a focused ion beam. The electrical results demonstrate the failure modes of both SiC SGTO thyristors during pulsed overcurrents electrically manifested themselves as a conductive path through the gate-anode junction and an increased device on-state voltage. SEM imaging revealed one SiC thyristor formed an approximately ${10\hbox{-}\mu{\rm m}}$ wide cylindrical void, and the second SiC thyristor formed an approximately ${200\hbox{-}\mu{\rm m}}$ long crack. However, the experimental results demonstrate these 15-kV SiC SGTO thyristors’ robust ability to repetitively switch at extreme high current density for tens of thousands of cycles.

Numerical simulations of cylindrical void growth in Mg single crystals

In this work, growth of cylindrical voids corresponding to different stress states in Mg single crystal under plane strain conditions is analyzed using a crystal plasticity based finite element procedure which accounts for twinning through a quasi-slip approximation. Two lattice orientations are considered with the c-axis along the thickness in one orientation and major principal stress direction in the other. Profuse tensile twinning occurs in the second orientation. To understand its influence clearly, this orientation is also analyzed with tensile twinning intentionally suppressed. The results show a pronounced effect of texture hardening triggered by tensile twinning in slowing down void growth. However, once lattice reorientation occurs, multiple slip systems become active causing more rapid void growth. The simulations also predict a transition in void coalescence mechanism from internal ligament necking to shear localization along an inclined band with reduction in biaxiality stress ratio when tensile twinning occurs.

Comparison of voiding mechanisms in semi-crystalline polyamide 6 during tensile and creep tests

Behaviour of a semi-crystalline polymer, polyamide 6, described by loading curves, as well as necking and whitening phenomena, is related to its micro-structural evolution in terms of void morphology and distribution during both tensile and creep tests. Notched bars have been subjected to creep tests interrupted at the onset of the tertiary creep stage and at the onset of final rupture. Inspections of these specimens using Synchrotron Radiation Tomography have been coupled with statistical image analysis treatment to obtain spatial distributions of void length and void volume fraction. Cavitation mechanisms observed and quantified during creep and tensile tests were similar: from penny shaped voids (diameter larger than height) perpendicular to the drawing direction to cylindrical voids (diameter equal to the height) arranged in columns during the neck extension. The void volume fraction distributions along radial and axial directions presented an inverted parabolic profile with a maximum located at the centre of the sample.

On the relationship of minimum detectable contrast to dose and lesion size in abdominal CT

CT dose optimization is typically guided by pixel noise or contrast-to-noise ratio that does not delineate low contrast details adequately. We utilized the statistically defined low contrast detectability to study its relationship to dose and lesion size in abdominal CT. A realistically shaped medium sized abdomen phantom was customized to contain a cylindrical void of 4 cm diameter. The void was filled with a low contrast (1% and 2%) insert containing six groups of cylindrical targets ranging from 1.2 mm to 7 mm in size. Helical CT scans were performed using a Siemens 64-slice mCT and a GE Discovery 750 HD at various doses. After the subtractions between adjacent slices, the uniform sections of the filtered backprojection reconstructed images were partitioned to matrices of square elements matching the sizes of the targets. It was verified that the mean values from all the elements in each matrix follow a Gaussian distribution. The minimum detectable contrast (MDC), quantified by the mean signal to background difference equal to the distribution’s standard deviation multiplied by 3.29, corresponding to 95% confidence level, was found to be related to the phantom specific dose and the element size by a power law (R^2 > 0.990). Independent readings on the 5 mm and 7 mm targets were compared to the measured contrast to the MDC ratios. The results showed that 93% of the cases were detectable when the measured contrast exceeds the MDC. The correlation of the MDC to the pixel noise and target size was also identified and the relationship was found to be the same for the scanners in the study. To quantify the impact of iterative reconstructions to the low contrast detectability, the noise structure was studied in a similar manner at different doses and with different ASIR blending fractions. The relationship of the dose to the blending fraction and low contrast detectability is presented.

Study of localization in a void-sheet under stress states near pure shear

A recent study of a porous ductile material under pure shear has indicated that no failure is predicted, whereas a number of micro-mechanical analyses for simple shear have shown that a maximum shear stress is reached and failure occurs. Both simple shear and pure shear are characterized by zero stress triaxiality and in both types of stress states the analyses show that the voids collapse to micro-cracks. The possibility of failure in pure shear is further investigated here by studying the effect of an initial imperfection in the form of a row of circular cylindrical voids inclined to the principal tensile stress. A number of previous investigations have shown that such imperfections can lead to plastic flow localization in a shear band and subsequently lead to void-sheet fracture inside the band. The present analyses confirm that the imperfection results in localization failure, even at moderate or rather low stress triaxiality. But in pure shear, with zero stress triaxiality, no failure is predicted. Initially the imperfection results in increasing shearing along the row of voids, but this tendency towards increasing shearing is interrupted if the voids collapse to micro-cracks. For the largest imperfection considered, i.e. a relatively large ratio of the void radius to void spacing, there is still a range of stress states in the vicinity of pure shear, where no localization is predicted, and for smaller imperfections this range is larger.

Bioinspired toughening mechanism: lesson from dentin

Inspired by the unique microstructure of dentin, in which the hard peritubular dentin surrounding the dentin tubules is embedded in the soft intertubular dentin, we explore the crack propagation in the bioinspired materials with fracture process zone possessing a dentin-like microstructure, i.e. the composite structure consisting of a soft matrix and hard reinforcements with cylindrical voids. A micromechanical model under small-scale yielding conditions is developed, and numerical simulations are performed, showing that the rising resistant curve (R-curve) is observed for crack propagation caused by the plastic collapse of the intervoid ligaments in the fracture process zone. The dentin-like microstructure in the fracture process zone exhibits enhanced fracture toughness, compared with the case of voids embedded in the homogeneous soft matrix. Further computational simulations show that the dentin-like microstructure can retard void growth, thereby promoting fracture toughness. The typical fracture mechanism of the bioinspired materials with fracture process zone possessing the dentin-like structure is void by void growth, while it is the multiple void interaction in the case of voids in the homogeneous matrix. Based on the results, we propose a bioinspired material design principle, which is that the combination of a hard inner material encompassing voids and a soft outer material in the fracture process zone can give rise to exceptional fracture toughness, achieving damage tolerance. It is expected that the proposed design principle could shed new light on the development of novel man-made engineering materials.

Equilibrium Distribution of Semiflexible Polymer Chains between a Macroscopic Dilute Solution Phase and Small Voids of Cylindrical Shape

The equilibrium partition coefficients () are calculated for self‐avoiding, semiflexible chains with the same finite contour length () and a variety of persistence length () values, partitioning between a macroscopic dilute solution phase and confined solution phases in cylindrical voids in the steric exclusion limit. It is found that, for the range of ‐values most relevant to polymer separation in size exclusion chromatograph (), the geometric mean of the radius of gyration () and the mean span () performs better than or alone in collapsing the partition curves of the various linear semiflexible chains onto a single curve. The dependence of the confinement free energy () on the persistence length is also examined. For those chains having the same , the pore diameter is found to be the necessary and sufficient condition for to decrease with the increase of .

On Void Coalescence Under Combined Tension and Shear

A micromechanics-based yield criterion is developed for a porous ductile material deforming by localized plasticity in combined tension and shear. The new criterion is primarily intended to model void coalescence by internal necking or internal shearing. The model is obtained by limit analysis and homogenization of a cylindrical cell containing a coaxial cylindrical void of finite height. Plasticity in parts of the matrix is modeled using rate-independent J2 flow theory. It is shown that for the discontinuous, yet kinematically admissible trial velocity fields used in the limit analysis procedure, the overall yield domain exhibits curved parts and flat parts with no vertices. Model predictions are compared with available finite-element (FE) based estimates of limit loads on cubic cells. In addition, a heuristic modification to the model is proposed in the limit case of penny-shape cracks to enable its application to materials failing after limited void growth as well as to situations of shear-induced void closure.

A model for porous single crystals with cylindrical voids of elliptical cross-section

This work presents a rate-dependent constitutive model for porous single crystals with arbitrary number of slip systems and orientations. The single crystal comprises cylindrical voids with elliptical cross-section at arbitrary orientations and is subjected to general plane-strain loadings. The proposed model, called modified variational model (MVAR), is based on the nonlinear variational homogenization method, which makes use of a linear comparison porous single crystal material to estimate the response of the nonlinear porous single crystal. The MVAR model is validated by periodic finite element simulations for a large number of parameters including general in-plane crystal anisotropy, general in-plane void shapes and orientations, various creep exponents (i.e., nonlinearity) and general plane strain loading conditions. The MVAR model, which at the present state involves no calibration parameters, is found to be in good agreement with the finite element results for all cases considered in this work. The model is then used in a predictive manner to investigate the complex response of porous single crystals in several cases with strong coupling between the anisotropy of the crystal and the (morphological) anisotropy induced by the shape and orientation of the voids.

Three-dimensional simulations of dynamics of void collapse in energetic materials

This work presents the response of a porous heterogeneous energetic material subjected to severe loading conditions. Spherical voids are embedded in an otherwise homogeneous material with the mechanical properties of condensed phase explosives. The effect of imposed shocks on spherical (three-dimensional) voids is compared with the cylindrical (two-dimensional) voids studied in an earlier work, in terms of energy deposition and the maximum temperature reached in the material as the void collapses. It is observed that there is a significant rise in maximum temperature of the energetic material in the presence of spherical voids compared to cylindrical voids. In addition to increasing the maximum temperature, the three-dimensional effects also influence the energy distribution as the void collapses. This study also compares mutual void–void interactions by analyzing different relative positions between two voids for both cylindrical and spherical shapes. Apart from the comparison, this study reinforces the importance of micro-scale dynamics in understanding and quantifying the response of an energetic material to shock loading.

Numerical limit analysis and plasticity criterion of a porous Coulomb material with elliptic cylindrical voids

The paper is devoted to a numerical Limit Analysis of a hollow cylindrical model with a Coulomb solid matrix (of confocal boundaries) considered in the case of a generalized plane strain. To this end, the static approach of Pastor et al. (2008) [18] for Drucker–Prager materials is first extended to Coulomb problems. A new mixed—but rigorously kinematic—code is elaborated for Coulomb problems in the present case of symmetry, resulting also in a conic programming approach. Owing to the good conditioning of the resulting optimization problems, both methods give very close bounds by allowing highly refined meshes, as verified by comparing to existing exact solutions. In a second part, using the identity of Tresca (as special case of Coulomb) and von Mises materials in plane strain, the codes are used to assess the corresponding results of Mariani and Corigliano (2001) [13] and of Madou and Leblond (2012) [11] for circular and elliptic cylindrical voids in a von Mises matrix. Finally, the Coulomb problem is investigated, also in terms of projections on the coordinate planes of the principal macroscopic stresses.

Discrete dislocation modeling on interaction between type-I blunt crack and cylindrical void in single crystals

The interaction between a type-I blunt crack and a near-tip void were modeled by the two-dimensional discrete dislocation dynamics (2-D DDD), with special emphasis on the void growth, crack-tip deformation and their size effect. Different from the classical crystal plasticity-based FE modeling, the present 2-D DDD modeling clearly indicates that, even for the type-I opening crack, the horizontal slip bands connecting the crack tip and the void surface are easily formed, especially when the ligament between the crack-tip and void surface is not too long. Besides the horizontal slip bands, some asymmetric slip bands which are at an oblique angle to the horizontal x-axis can also be formed and intersect with the micro-void. On two coupled parallel slip bands, some oppositely signed dislocations, which can be regarded as equivalent prismatic dislocation loops, glide toward the void/crack-tip surface and thus drive the void growth and the crack tip further blunting. In addition, the size effect by ligament length and void radius on void growth and crack-void interaction has also been investigated carefully. It was found that the size effect is closely related to the formation of horizontal shear slip bands. There exists a critical void radius, under which the near-tip void tends to change into a flat shape and its area decreases gradually with increasing the applied stress intensity factor (SIF) K. These results might be helpful for a better understanding of the fracture and damage mechanisms in the crack-tip process zone.

The application of floating dies for high speed growth of CsI single crystals by edge-defined film-fed growth (EFG)

Floating graphite and fused silica dies were used to grow both undoped and Na doped CsI crystals by the edge-defined film-fed growth (EFG) method. Both die materials yielded high quality CsI crystals at high growth rates. Under the conditions employed in these growth experiments, a pull rate of up to 45mm/h was possible using a 15mm diameter graphite die. Growth procedures were developed to enhance crystal quality through iterative die design in combination with numerical modeling. The formation of cylindrical voids (bubble tracks) was investigated and methods were developed for their suppression.

SU‐E‐I‐70: Quantification of Noise Reduction Dependency On Spatial Granularity With Adaptive Statistical Iterative Reconstruction

Purpose:Adaptive statistical iterative reconstruction (ASIR) is often used for noise reduction. However, it may not be clear whether the noise reduction is uniform across different spatial scales. As this is important for lesion detection, we attempted to quantify the noise reduction dependency on spatial scales under different ASIR blending fractions.Methods:An abdomen phantom (CIRS TE‐07) simulating the medium sized patient was used. The phantom contains a cylindrical void of 4 cm diameter, filled with an insert containing six groups of cylindrical targets (1% and 2% contrast) of 1.2 mm to 7 mm. Helical scans were conducted using a GE 750HD with 120 kVp, 330 mAs and pitch of 1.375 (CTDI_vol = 18 mGy). The images were reconstructed to 5 mm thickness with filtered‐back projection (FBP) and with ASIR at blending fractions of 10% to 100%. The uniform sections of the contiguous slices were subtracted and the resulted regions were divided into matrices of cells matching the target size from 1.2 mm–7.0 mm. Standard deviations (sd) from the means of all cells were computed as a measure of noise at the target size. The noise reduction factor (NRF) was defined as the ratio of the sd from ASIR to the sd of FBP. The NRF was fitted to ASIR blending percentages (ASIRP) at different target sizes (d).Results:The NRF versus ASIRP was fitted to a linear relationship with an intercept of approximately 1.0 and a slope dependent on d (R square = 0.99). The slope versus d was fitted to a power relationship with a power index of – 0.80 and a proportion factor of −0.005 (R square = 0.978).Conclusion:The noise reduction with ASIR increases nonlinearly as the spatial scale (or target size) decreases. The result can be of help for ASIR use in target‐specific diagnoses.

Composite materials: Textural characteristics

Calculation of pore size distribution of complex materials needs special approaches. • Self-consistent regularization is effective tool to analyze data for complex materials. • Model with slit/cylindrical pores and voids between nanoparticles is appropriate for composites. • The SCV/SCR method gives better results than NLDFT or QSDFT for composites. A self-consistent regularization (SCR) procedure applied to integral adsorption equations based on a complex model with slit-shaped and cylindrical pores and voids between spherical nonporous particles packed in random aggregates (SCV model) for two or three types of components (activated carbon, carbon black, silica gel, fumed silica) in composites gives more clear pictures of pore size distributions (PSD) than nonlocal density functional theory (NLDFT) or quenched solid DFT (QSDFT) methods. Results of this approach for individual adsorbents are comparable with the results of NLDFT or QSDFT methods using complex pore models (slit/cylindrical or slit/cylindrical/spherical pores). The SCV/SCR method is useful to study the textural features of hybrid materials, the PSD of which is difficult to be calculated using standard approaches. The PSD of complex materials can also be estimated using (1) high-resolution microscopic methods ( e.g. HRTEM) with software (Fiji/ImageJ) used for quantitative treatment of images; (2) cryoporometry based on low-temperature 1 H NMR spectroscopy; and (3) small angle X-ray scattering (SAXS). Results of these methods can effectively amplify results of the standard adsorption methods.

Partial discharge behaviours within a void‐dielectric system under square waveform applied voltage stress

There are many factors, which influence the behaviours of partial discharge (PD) within a void located in a solid dielectric material. One of the important factors is the waveform of the applied voltage on the electrode of a system, such as its amplitude and frequency. In this study, a two-dimensional axial-symmetric model of a cylindrical void in polyethylene layers has been developed using finite element analysis software. The model was used to simulate PD activity in the void under square waveform applied voltage under different amplitudes of the voltage. The model has considered the effect of charge propagation along the void surface during a PD occurrence. The obtained simulation results were compared with the measurement results reported from previous literature to validate the PD model that has been developed in this work. It was found that the simulation and measurement results are within reasonable agreement with only slight disagreement. From the comparison, critical parameters from the model that affect PD behaviours under square waveform applied voltage were found to be the electron generation rate and the inception, extinction and critical charge propagation fields. These findings may increase an understanding of PD behaviours within a void in a dielectric material under square waveform applied voltage, which is important for insulation diagnosis.

Radiographic test problem for MCNP and other mesh-based applications

*Synthetic radiographs may be created with the Los Alamos Monte Carlo code MCNP6. A simple MCNP test problem, essentially a 10-cm high truncated aluminum cone containing three 1-cm radius tungsten spheres, each in turn containing a cylindrical void region, was devised to illustrate effects of various code options. While the radiographic test object (RTO) was initially constructed in MCNP’s traditional combinatorial solid geometry (CSG), two additional mesh-based versions were created to exercise two new “hybrid-geometry” features of MCNP6. This paper describes the RTO, user input to exercise the new features and results from several versions of the RTO geometry. To further illustrate possibilities afforded by these new features, this paper also describes MCNP-simulated proton radiography of the CYCLOPS experiment as modeled by PAGOSA, a three-dimensional multimaterial hydrodynamics computer code developed at Los Alamos. CYCLOPS, essentially a line-wave generated, steel-confined, plastic-bonded high-explosive experiment in cylindrical geometry, resulted in a collection of 20 proton radiographs. Such data is extremely valuable for PAGOSA and MCNP6 code validation efforts; the former in terms of burn fronts and reflected shocks and the latter in terms of x-ray and proton transport.

Effective Yield Criterion Accounting for Microvoid Coalescence

An effective yield function is derived for a porous ductile solid near a state of failure by microvoid coalescence. Homogenization theory combined with limit analysis are used to that end. A cylindrical cell is taken to contain a coaxial cylindrical void of finite height. Plastic flow in the intervoid matrix is described by J2 theory while regions above and below the void remain rigid. Velocity boundary conditions are employed which are compatible with an overall uniaxial straining for the cell, a postlocalization kinematics that is ubiquitous during the coalescence of neighboring microvoids in rate-independent solids. Such boundary conditions are not of the uniform strain rate kind, as is the case for Gursonlike models. A similar limit analysis problem for a square-prismatic cell containing a square-prismatic void was posed long ago (Thomason, P. F., 1985, “Three-Dimensional Models for the Plastic Limit–Loads at Incipient Failure of the Intervoid Matrix in Ductile Porous Solids,” Acta Metallurgica, 33, pp. 1079–1085). However, to date a closed-form solution to this problem has been lacking. Instead, an empirical expression of the yield function proposed therein has been widely used in the literature. The fully analytical expression derived here is intended to be used concurrently with a Gursonlike yield function in numerical simulations of ductile fracture.