Shell Fracture Research Articles

An adaptive space-time phase field formulation for dynamic fracture of brittle shells based on LR NURBS

We present an adaptive space-time phase field formulation for dynamic fracture of brittle shells. Their deformation is characterized by the Kirchhoff-Love thin shell theory using a curvilinear surface description. All kinematical objects are defined on the shell's mid-plane. The evolution equation for the phase field is determined by the minimization of an energy functional based on Griffith's theory of brittle fracture. Membrane and bending contributions to the fracture process are modeled separately and a thickness integration is established for the latter. The coupled system consists of two nonlinear fourth-order PDEs and all quantities are defined on an evolving two-dimensional manifold. Since the weak form requires $C^1$-continuity, isogeometric shape functions are used. The mesh is adaptively refined based on the phase field using Locally Refinable (LR) NURBS. Time is discretized based on a generalized-$\alpha$ method using adaptive time-stepping, and the discretized coupled system is solved with a monolithic Newton-Raphson scheme. The interaction between surface deformation and crack evolution is demonstrated by several numerical examples showing dynamic crack propagation and branching.

Modeling of Fracture of Spatial Concrete Structures Under Impulse Loads

The paper presents the results of a numerical study of the fracture of concrete structures — the protective shell of a nuclear power plant (NPP), under impulse loads. The model of concrete behavior takes into account differences in the limits of compressive and tensile strength, dependence of tensile strengths on the strain rate, plastic properties of concrete, fracture, fragmentation of the shell. Numerical modeling is carried out by finite element method in a three-dimensional statement as part of a phenomenological approach of the mechanics of a continuous medium, using the authors computational software. The dynamics of the fracture of the nuclear power plant protective shell, which has a complex cellular structure and the role of wave processes, is investigated.

Extension of linear stability analysis for the dynamic stretching of plates: Spatio-temporal evolution of the perturbation

Being able to predict fragment distributions in terms of speed and size, following the fracture of thin metallic shells subjected to dynamic expansion, is of major importance for civil and military applications. For ductile metals, this fracture process is initiated by plastic flow instability resulting in necking, i.e. the occurrence of local thinnings where the plastic deformation is localized. Since decades, linear stability analyses have been carried out to study the multiple necking formation via a perturbation of the fundamental state. The underlying assumption related to the linear stability analyses developed so far is the time scale separation (meaning that the development of the instability is much faster than the evolution of the fundamental state), see Fressengeas and Molinari (1994) or Shenoy and Freund (1999). The aim of the work is to propose an extended linear stability analysis which can tackle situations where the time scale separation hypothesis is no more satisfied (i.e. at very large strain rates). The proposed methodology is exemplified by considering the dynamic extension of a plate under plane strain condition; the material behavior being modeled adopting various constitutive laws from rate insensitive to thermo-viscoplastic ones. The role of initial perturbation (or defect) is discussed. While the role of the initial conditions is important at the early stage of the deformation process, their influence on the growth rate and on the dominant mode are negligible at large strain for moderate loading rate. One main feature of the proposed model is the estimation of the amplitude development of each mode. A strong difference in the amplitude predictions is revealed between the new model and the classical linear stability analysis of the literature, even if the growth rates are comparable for both approaches at late deformation stage. However, history effect related to the amplitude of a given mode, originating from the early stage process, may lead to strong amplitude differences which remain visible at the late stage. The benefit of the new theory is the fact that the spatio-temporal evolution of the mechanical and thermal parts of the perturbation can be captured naturally, even for cases where the plastic flow is initially stable and becomes unstable as the deformation progresses.

Heat‐Free Fabrication of Metallic Interconnects for Flexible/Wearable Devices

AbstractExploiting interfacial excess (Γ), Laplace pressure jump (ΔP), surface work, and coupling them to surface reactivity have led to the synthesis of undercooled metal particles. Metastability is maintained by a core–shell particle architecture. Fracture of the thin shell leads to solidification with concomitant sintering. Applying Scherer's constitutive model for load‐driven viscous sintering on the undercooled particles implies that they can form conductive traces. Combining metastability to eliminate heat and robustness of viscous sintering, an array of conductive metallic traces can be prepared, leading to plethora of devices on various flexible and/or heat sensitive substrates. Besides mechanical sintering, chemical sintering can be performed, which negates the need of either heat or load. In the latter, connectivity is hypothesized to occur via a Frenkel's theory of sintering type mechanism. This work reports heat‐free, ambient fabrication of metallic conductive interconnects and traces on all types of substrates.

Expansion Fracture Behavior of Metallic Cylindrical Shell Caused by Explosive Detonation

To investigate the dynamic behavior of expansion-induced fracture of a metal cylindrical shell driven by the detonation of condensed explosives, detonation driving tests were designed and implemented under two working conditions, namely, (i) the metal cylindrical shells made up of different metals were driven by the same explosive, and (ii) the cylindrical shells made up of the same metal were driven by different explosives. The entire expansion process was recorded using a high-speed scanning camera, and the curves of expansion displacement and velocity vs. time were plotted. The applicability of the parameters characterizing the detonation driving capability of explosive was also analyzed. The SEM images of the fragment fracture sections of the metal shells show that the metal fracture mode is related to the driving capability of explosive and the properties of metals. The shells made up of metals with a suitable ductility showed mainly the ductile tensile fracture with local brittle fractures, while the shells made up of metals with a poor ductility showed mainly the brittle shear fracture.

Microfluidic fabrication of microcapsules tailored for self-healing in cementitious materials

Autonomic self-healing in cement-based infrastructure materials has recently emerged as a promising strategy for extending the service life of concrete infrastructure. Amongst the various self-healing systems being developed, the use of microcapsules has received significant attention partly because of its ease implementation. Up to date, microcapsules for self-healing applications have been mainly manufactured using bulk emulsifications polymerisation techniques. However this methodology raises concerns regarding shell dimensions and interfacial bonding. This study proposes for the first time the fabrication of microcapsules with tailored characteristics for mechanically triggered self-healing action in cement-based composites. For this, a microfluidic device was used to produce a double emulsion template for the formation of microcapsules, containing both aqueous and organic liquid core. In addition, a novel method has been proposed to functionalize the microcapsules' surface with hydrophilic groups in order to increase the interfacial bond with the cementitious host matrix. The core retention was studied using EDX and TGA, and their mechanical triggering was investigated via SEM of the microcapsules embedded in the cement paste. The results demonstrated the capability of microfluidics to produce microcapsules with liquid organic core, thin shell, hydrophilic surface and appropriate fracture strength for use in mechanically triggered self-healing of cementitious materials.

Thermal Energy Storage P(BeA-co-MMA) Microcapsules with Excellent Thermal Stability

Latent heat storage systems are a highly efficient and environmentally friendly way to use residual heat and store renewable energy. Phase change materials, which can absorb or release the enthalpy of phase changes in certain temperature ranges, are considered one of the most reliable latent heat storage and thermoregulation materials. Phase change materials have been successfully used in the fields of solar energy, aerospace and aviation, environmental control and textiles. Among phase change materials, paraffin and n-alkane are the most popular due to their characteristics of high phase change enthalpy, high energy storage density, relative low volume change ratio, good physical and chemical stability, and no super-cooling or phase segregation behavior. More importantly, there are wide selective working temperature ranges which greatly extend their operating conditions. Microcapsules are considered micro scale containers to prevent core from leaking, isolate from the surroundings and maintain chemical and functional stability in service. There are several methods to synthesize microcapsules: spray dying, complex coacervation, sol-gel, in-situ polymerization, suspension polymerization and emulsion polymerization. Among these, suspension polymerization has the advantages of being simple, cheap, stable, eco-friendly and being easily controlled, which makes it the most popular chemical method to synthesize microcapsules. The majority shell materials of traditional microcapsules are synthetic macromolecules. The properties of shell materials impact the application and processing conditions of the final energy storage composite materials greatly. Nevertheless, the low thermal stability and physical strength of polymers in nature strongly restrict application fields of microcapsules. What’s more, the evaporation or sublimation of core materials causes the low thermal stability. In order to overcome these shortages of shell fracture, core leakage, core evaporation or sublimation existing in traditional microcapsules, we design a copolymer microcapsule with strong interaction force between shell and core parts, which has an unseparated surface layer between shell and core. The strong interaction force between shell and core allows the extreme improvement of thermal stability of microcapsules. The copolymer microcapsule is synthesized by suspension polymerization of behenyl acrylate (BeA) and methyl methacrylate (MMA). In this paper, we report the synthesis, size control and structure analysis of P(BeA-co-MMA) microcapsules which have both excellent thermal stability and high thermal energy storage capacity.

Smashing mantis shrimp strategically impact shells.

Many predators fracture strong mollusk shells, requiring specialized weaponry and behaviors. The current shell fracture paradigm is based on jaw- and claw-based predators that slowly apply forces (high impulse, low peak force). However, predators also strike shells with transient intense impacts (low impulse, high peak force). Toward the goal of incorporating impact fracture strategies into the prevailing paradigm, we measured how mantis shrimp (Neogonodactylus bredini) impact snail shells, tested whether they strike shells in different locations depending on prey shape (Nerita spp., Cenchritis muricatus, Cerithium spp.) and deployed a physical model (Ninjabot) to test the effectiveness of strike locations. We found that, contrary to their formidable reputation, mantis shrimp struck shells tens to hundreds of times while targeting distinct shell locations. They consistently struck the aperture of globular shells and changed from the aperture to the apex of high-spired shells. Ninjabot tests revealed that mantis shrimp avoid strike locations that cause little damage and that reaching the threshold for eating soft tissue is increasingly difficult as fracture progresses. Their ballistic strategy requires feed-forward control, relying on extensive pre-strike set-up, unlike jaw- and claw-based strategies that can use real-time neural feedback when crushing. However, alongside this pre-processing cost to impact fracture comes the ability to circumvent gape limits and thus process larger prey. In sum, mantis shrimp target specific shell regions, alter their strategy depending on shell shape, and present a model system for studying the physics and materials of impact fracture in the context of the rich evolutionary history of predator-prey interactions.

Dropping the hammer: Examining impact ignition and combustion using pre-stressed aluminum powder

Pre-stressing aluminum (Al) particles by annealing and quenching Al powder alters particle mechanical properties and has also been linked to an increase in particle reactivity. Specifically, energy propagation in composites consisting of aluminum mixed with copper oxide (Al + CuO) exhibits a 24% increase in flame speed when using pre-stressed aluminum (PS Al) compared to Al of the same particle size. However, no data exist for the reactivity of PS Al powders under impact loading. In this study, a drop weight impact tester with pressure cell was designed and built to examine impact ignition sensitivity and combustion of PS Al when mixed with CuO. Both micron and nanometer scale powders (i.e., μAl and nAl, respectively) were pre-stressed, then combined with CuO and analyzed. Three types of ignition and combustion events were identified: ignition with complete combustion, ignition with incomplete combustion, and no ignition or combustion. The PS nAl + CuO demonstrated a lower impact ignition energy threshold for complete combustion, differing from nAl + CuO samples by more than 3.5 J/mg. The PS nAl + CuO also demonstrated significantly more complete combustion as evidenced by pressure history data during ignition and combustion. Additional material characterization provides insight on hot spot formation in the incomplete combustion samples. The most probable reasons for higher impact-induced reactivity of pre-stressed particles include (a) delayed but more intense fracture of the pre-stressed alumina shell due to release of energy of internal stresses during fracture and (b) detachment of the shell from the core during impact due to high tensile stresses in the Al core leading to much more pronounced fracture of unsupported shells and easy access of oxygen to the Al core. The μAl + CuO composites did not ignite, even under pre-stressed conditions.

Core-Shell Fibers Electrospun from Phase-Separated Blend Solutions: Fiber Formation Mechanism and Unique Energy Dissipation for Synergistic Fiber Toughness.

Through single-tube electrospinning, the biodegradable core-shell fibers of poly(3-hydroxybutyrate) (PHB) and poly(d,l-lactic acid) (PDLLA) were obtained from blend solutions with different compositions at a total polymer concentration of 7 wt %. Regardless whether PHB is the major or minor component (PHB/PDLLA = 90/10, 75/25, 50/50, and 25/75 wt. ratio), these phase-separated solutions all yielded core-shell fibers with PHB as core and PDLLA as shell. A new scenario of core-shell fiber formation was proposed on the basis of the relative magnitude of the intrinsic relaxation rate of fluids and external extension rate during electrospinning. The effects of blend compositions on the morphologies of the Taylor cone, whipping jet, and as-spun fibers were investigated. The diameters of core-shell fibers can be tailored by simply varying the PHB/PDLLA ratios. Two scaling laws describing the apparent viscosity (ηo) dependence of the outer fiber diameter (dfo) and core fiber diameter (dfc) were derived. That is, dfo ∼ ηo0.38 and dfc ∼ ηo0.86. The microstructures of the as-spun fibers were determined by differential scanning calorimetry, Fourier transform infrared spectroscopy, and synchrotron wide-angle and small-angle X-ray scatterings. Results showed that the PDLLA component was in the amorphous state, and the crystallizability of PHB component remained unchanged, except the amorphous 10/90 fibers electrospun from a miscible solution state. The synergistic mechanical properties of the core-shell fibers were obtained, along with the ductile PDLLA shell enclosing the brittle PHB core. The enhanced toughness was attributed to the fragmentation of the brittle PHB core and necking fracture of the ductile PDLLA shell, which served as an effective route for energy dissipation. Compared with the neat PHB fiber, the 90/10 and 75/25 core-shell fibers possessed larger elastic moduli, which was attributed to the high PHB crystal orientation in their core sections despite the reduced PHB crystallinity. By contrast, the crystal c-axis of PHB in the 25/75 core-shell fibers was preferentially perpendicular to the fiber axis, suggesting the significant stretching of developing PHB crystals during electrospinning.

Preparation of Human Inner Ear Structures for High Resolution Imaging Studies

Background: Introduction of the novel imaging modalities such as optical coherence tomography and scanning laser optical tomography allow the morphological and functional evaluation of the intracochlear structures up to the histological details. The prerequisite of these modalities is the preparation of the membranous labyrinth by removing the dense otic capsule to enhance the optical penetration depth. In the present study, a combination of the chemical decalcification and bone drilling method was explained for the preparation of the human inner ear structures for further studies. Methods: In this study, nine human temporal bones were used and trimmed in cubes containing middle and inner ear structures. The samples were immersed in 5% nitric acid, 20% EDTA, and 10% EDTA solutions, respectively. The samples were brought out and rinsed every 90 minutes, and mechanical removal of the softened bone was performed with surgical drill system until the complete decalcification of the samples was confirmed by X-ray imaging. The prepared samples were evaluated by microcomputed tomography imaging for anatomical distortions caused by the preparation process. Results: Complete decalcification and preparation of the samples were obtained in average of 10.5 ± 0.5 hours. No obvious morphological changes were observed in microcomputed tomography images, except for fracture of the remnant bony shell at the apex of cochlea in one sample. Conclusions: The combination of the chemical decalcification and mechanical removal of the softened bone can accelerate the sample preparation for high resolution imaging studies without significant morphological changes.

Influence analysis of aging of spinal segment on the mechanical behavior of vertebral cortex

The risk of vertebral cortical shell fracture increases with aging. However, it remains unclear how aging contributes to cortex fracture at present. The aim of this study is to make understanding of the mechanism of how the spinal aging influences the cortical shell strain. Two finite element (FE) models of spinal segments (mildly and fully aged) were created, and then were compared to the FE models of the healthy spinal segment. The FE models of the aged spinal segments were generated by modifying both the geometry of the intervertebral disc (IVD) and the material properties of the spinal components. To find out under which case the cortical shell strain was influenced more, we created two types of FE model comparison methods: one with changes only in the spinal material properties and the other with changes only in the IVD geometry. The results showed that the cortical shell strains increased with aging and that compared to the changes of IVD geometry, the changes of spinal material property have a higher influence on the cortical shell strains. This study may suggest that for the prevention and treatment of vertebral cortex fracture, the augmentation of the vertebral body is a more effective treatment.

Effect of tempering temperature on expansion and fracture mechanism of 40CrMnSiB steel cylinder shell

The study on the fracture mechanism of steel cylindrical shells under different heat treatment conditions was helpful to understand the dynamic response of metal shell under high strain-rate loading. Therefore, the effect of tempering temperature on the expansion and fracture of metal cylinder shells and the regular pattern of fragmentation mass, size distribution fragments was analyzed by using a high speed photography technique and the recovery experiment of static explosion in well. The results revealed that fracture behavior changed from tensile-shear fracture to shear fracture occurring in a 40CrMnSiB steel cylinder shell internally filled with CL-20 based high explosives with the increasing of tempering temperature, and the different tempering temperature conditions on shell have a marked impact on behavior of fragments mass and size distribution. The radial expansion displacement of 40CrMnSiB steel cylinder shells tempered at 500 °C and 600 °C was increased by 39.43% and 21.46%, respectively, compared with that of the shell tempered at 350 °C. Besides, the fragmentation degree of 40CrMnSiB steel shell decreases and the total number of fragments reduces gradually with increasing tempering temperature, but the fragments numbers which mass was more than 1.0g increase from 105 to 159 gradually, which was increased by 77.4%, which could be helpful to improve the understanding of influence of material properties on the deformation, expansion and fragmentation of warhead, also could provide a constructive guidance for selecting heat treatment technology of warhead material.

The mechanical and thermal responses of colliding oxide-coated aluminum nanoparticles

The aggregation and fracture of oxide-coated metal nanoparticles have a significant influence on their physical and chemical behaviors in synthesis, combustion, or detonation. How does the dynamic loading caused by the impact between nanoparticles affect them? Motivated by this issue, we carried out molecular dynamic simulations of two colliding aluminum nanoparticles to investigate their mechanical and thermal properties and response at impact velocities of 200 m/s, 600 m/s, 1000 m/s, and 2000 m/s. At the relatively low impact velocities (equal to or less than 1000 m/s), it was observed that the particles are mildly deformed and adhere to each other, but the shells do not undergo fracture under the dynamic loading. The metal core and oxide shell behave elastically at 200 m/s and elasto-plastically at 600 m/s. A concentration of dynamic volumetric stress appears but no concentration of shear stress and no formation of a hot spot. Due to the low intensity of the loading and the efficient propagation of the stress wave, the shells fail to fracture. At an impact velocity of 2000 m/s, the impact region of the shell undergoes ductile fracture, and the two particles undergo sintering and form a new particle re-coated with oxide, resulting from the concentration of dynamic shear stress and the formation of a hot spot. At all impact velocities in our simulations, the impact between the nanoparticles improves the aggregation but has a little effect on the fracture of the oxide shell.

Nature's technical ceramic: the avian eggshell.

Avian eggshells may break easily when impacted at a localized point; however, they exhibit impressive resistance when subjected to a well-distributed compressive load. For example, a common demonstration of material strength is firmly squeezing a chicken egg along its major axis between one's hands without breaking it. This research provides insight into the underlying mechanics by evaluating both macroscopic and microstructural features. Eggs of different size, varying from quail (30 mm) to ostrich (150 mm), are investigated. Compression experiments were conducted along the major axis of the egg using force-distributing rubber cushions between steel plates and the egg. The force at failure increases with egg size, reaching loads upwards of 5000 N for ostrich eggs. The corresponding strength, however, decreases with increasing shell thickness (intimately related to egg size); this is rationalized by a micro-defects model. Failure occurs by axial splitting parallel to the loading direction-the result of hoop tensile stresses due to the applied compressive load. Finite-element analysis is successfully employed to correlate the applied compressive force to tensile breaking strength for the eggs, and the influence of geometric ratio and microstructural heterogeneities on the shell's strength and fracture toughness is established.

System approach to calculating processes in the apparatus of combined action

A method of calculating the apparatus of combined action for carrying out a high-temperature reaction in a fluidized bed of the solid reactant particles is proposed based on a system approach. In this bed, the reaction proceeds simultaneously with the continuous grinding of the formed products and the processes of heat and mass transfer are significantly intensified in connection with the constant renewal (denudation) of the reaction surface both due to the impact fracture of the product shell in the zones of the intensive collision in the counterflows of the reacting particles and its abrasion in the body of the fluidized bed.

Bicon Threaded Acetabular Cup Fracture, Case Report, Mode of Failure and Review of the Literature

Threaded acetabular cups have been equally successful as hemispheric cups. However, based on our case, as well as the reported literature, these thin shelled cups may be at risk of fracture in cases with posterocranial or anterocranial bone defects, even if a reconstruction of the defect is planned. We believe press-fit cups with their thicker walls are preferable in cases with osseous defects. Threaded acetabular cups have been equally successful as hemispheric cups. Despite this success there is always the chance of failure. The commonest mode of failure being polyethylene wear, osteolysis, and loosening. A more implant specific mode of failure of threaded acetabular shells has been cup migration [1-3]. Another rare mode of failure seems to be stress fracture of the acetabular shell. After a review of the literature we found two reports of fractured threaded Bicon acetabular shells [4,5] numbering three cases each. As well as a brief mention in a series of 153 cases, where there were 4 cases of fatigue fracture of the titanium shell [6]. This mode of failure is very rare in hemispheric cups with only one published case of cup fracture [7]. The purpose of this case report is to present our experience with this rare, however, implant specific complication, as an addition to the existing literature, in an effort to shed some more light in the involved mechanism of fracture.

Dynamic Failure Behavior of Cylindrical Glass Fiber Composite Shells Subjected to Internal Blast Loading

The dynamic response of open-ended cylindrical glass fiber composite shells subjected to internal blast loading is studied in the current paper. The experimental observation on response characteristics of cylindrical glass fiber shells is presented, in which failure modes of composite structures are especially concerned. It is found that dynamic buckling may occur in the inner steel liner, which may consequently cause delamination and fiber fracture of the outer glass fiber shell and thus limits the blast loading resistant capability of glass fiber explosion containment vessels. The other failure mode is obvious circular plastic expansion of the inner steel liner and fiber fracture of the outer fiber shell. There exists an interesting case that hoop winding fibers fail but fibers with a winding angle do not fail, based on which the hybrid filament wound method for cylindrical composite containment vessels is proposed. The current study may contribute to further understanding on the design and application of glass fiber composite explosion containment vessels (CECVs).

Numerical Simulation of the Nonlinear Deformation and Progressive Destruction of Composite Cylindrical Shells at Nonaxisymmetric Explosive Actions

On the basis of the applied theory of shells, a resolving system of equations is formulated and a method for the numerical solution of problems of nonlinear nonaxisymmetric deformation and fracture of composite cylindrical shells at explosive loadings of different intensity is developed. A model of progressive destruction of a composite shell based on the degradation of stiffness characteristics of individual layers in a multilayer package is elaborated, which depends on the fracture mode of the binder and fiber. The reliability of the technique considered is proved by comparing calculation results with known experimental data. Results of an analysis of the effect of nonaxisymmetric arrangement of an explosive charge on the fracture behavior of composite cylindrical shells with different reinforcement structures are presented.

Coupled simulation of explosion-driven fracture of cylindrical shell using SPH-FEM method

A coupled Smoothed Particle Hydrodynamics-Finite Element Method (SPH-FEM) method was presented to simulate the dynamic fracture of cylindrical shell subjected to internal explosion. The movement of detonation products was modeled by SPH, and the fracture response of cylindrical shell was modeled by FEM, where they were coupled together by a penalty-based coupling approach. A rate-dependent failure criterion which was proposed and verified in our previous work was employed in the simulation to account for the adiabatic shear failure of the cylindrical shell. Also, the decoupled simulation was conducted to make a comparison with the coupled SPH-FEM simulation above. Results showed the interaction between blast wave and cylindrical shell, the dynamic crack propagation behavior and the leakage of detonation products. It is found that the coupled SPH-FEM method can well handle the fluid-structure interaction (FSI) problem which involves local topology changes and ruptures. Compared with decoupled simulation, the coupled SPH-FEM simulation gives a more reliable prediction of the final fracture morphology of cylindrical shell. The decoupled simulation tends to present a result with severer bulging deformation but smaller fracture size compared with the real situation.