Electron irradiation-induced change of structure and damage mechanisms in multi-walled carbon nanotubes

Owing to their unique structure and excellent electrical property, carbon nanotubes （CNTs） as an ideal candidate for making future electronic components have great application potentiality. In order to meet the requirements for space appli- cation in electronic components, it is necessary to study structural changes and damage mechanisms of multi-walled carbon nanotubes （MWCNTs）, caused by the irradiations of 70 and 110 keV electrons. In the paper, the changes of structure and damage mechanisms in the irradiated MWCNTs, induced by the irradiations of 70 and 110 keV electrons, are investigated. The changes in surface morphology and structure of the irradiated MWCNT film are characterized using scanning electron microscopy （SEM）, x-ray photoelectron spectroscopy （XPS）, Raman spectroscopy, x-ray diffraction analysis （XRD）, and electron paramagnetic resonance （EPR） spectroscopy. It is found that the MWCNTs show different behaviors in structural changes after 70 and 110 keV electron irradiation due to different damage mechanisms. SEM results reveal that the irra- diation of 70 keV electrons does not change surface morphology of the MWCNT film, while the irradiation of 110 keV electrons with a high fluence of 5 x 1015 cm-2 leads to evident morphological changes, such as the formation of a rough surface, the entanglement of nanotubes and the shrinkage of nanotubes. Based on Raman spectroscopy, XPS, and XRD analyses, it is confirmed that the irradiation of 70 keV electrons increases the interlayer spacing of the MWCNTs and disorders their structure through electronic excitations and ionization effects, while the irradiation of 110 keV electrons obviously reduces the interlayer spacing of the MWCNTs and improves their graphitic order through knock-on atom dis- placements. The improvement of the irradiated MWCNTs by 110 keV electrons is attributed to the restructuring of defect sites induced by knock-on atom displacements. EPR spectroscopic analyses reveal that the MWCNTs exposed to both 70 keV electrons and 110 keV electrons suffer ionization damage to some extent.

Experimental and numerical study of hypervelocity impact damage on composite overwrapped pressure vessels

Ground-based tests are important for studying hypervelocity impact(HVI)damage to spacecraft pressure vessels in the orbital debris environment.We analyzed the damage to composite overwrapped pressure vessels(COPVs)in the HVI tests and classified the damage into non-catastrophic damage and catastrophic damage.We proposed a numerical simulation method to further study non-catastrophic damage and revealed the characteristics and mechanisms of non-catastrophic damage affected by impact conditions and internal pressures.The fragments of the catastrophically damaged COPVs were collected after the tests.The crack distribution and propagation process of the catastrophic ruptures of the COPVs were analyzed.Our findings contribute to understanding the damage characteristics and mechanisms of COPVs by HVIs.

Damage Mechanism of Ultra-thin Asphalt Overlay(UTAO) based on Discrete Element Method

Aiming to analyze the damage mechanism of UTAO from the perspective of meso-mechanical mechanism using discrete element method(DEM),we conducted study of diseases problems of UTAO in several provinces in China,and found that aggregate spalling was one of the main disease types of UTAO.A discrete element model of UTAO pavement structure was constructed to explore the meso-mechanical mechanism of UTAO damage under the influence of layer thickness,gradation,and bonding modulus.The experimental results show that,as the thickness of UTAO decreasing,the maximum value and the mean value of the contact force between all aggregate particles gradually increase,which leads to aggregates more prone to spalling.Compared with OGFC-5 UTAO,AC-5 UTAO presents smaller maximum and average values of all contact forces,and the loading pressure in AC-5 UTAO is fully diffused in the lateral direction.In addition,the increment of pavement modulus strengthens the overall force of aggregate particles inside UTAO,resulting in aggregate particles peeling off more easily.The increase of bonding modulus changes the position where the maximum value of the tangential force appears,whereas has no effect on the normal force.

A creep model for ultra-deep salt rock considering thermal-mechanical damage under triaxial stress conditions

To investigate the specific creep behavior of ultra-deep buried salt during oil and gas exploitation,a set of triaxial creep experiments was conducted at elevated temperatures with constant axial pressure and unloading confining pressure conditions.Experimental results show that the salt sample deforms more significantly with the increase of applied temperature and deviatoric loading.The accelerated creep phase is not occurring until the applied temperature reaches 130℃,and higher temperature is beneficial to the occurrence of accelerated creep.To describe the specific creep behavior,a novel three-dimensional(3D)creep constitutive model is developed that incorporates the thermal and mechanical variables into mechanical elements.Subsequently,the standard particle swarm optimization(SPSO)method is adopted to fit the experimental data,and the sensibility of key model parameters is analyzed to further illustrate the model function.As a result,the model can accurately predict the creep behavior of salt under the coupled thermo-mechanical effect in deep-buried condition.Based on the research results,the creep mechanical behavior of wellbore shrinkage is predicted in deep drilling projects crossing salt layer,which has practical implications for deep rock mechanics problems.

Mechanical response and microscopic damage mechanism of pre-flawed sandstone subjected to monotonic and multilevel cyclic loading:A laboratory-scale investigation

This study aims to investigate the mechanical response and acoustic emission(AE)characteristic of pre-flawed sandstone under both monotonic and multilevel constant-amplitude cyclic loads.Specifically,we explored how coplanar flaw angle and load type impact the strength and deformation behavior and microscopic damage mechanism.Results indicated that being fluctuated before rising with increasing fissure angle under monotonic loading,the peak strength of the specimen first increased slowly and then steeply under cyclic loading.The effect of multilevel cyclic loading on the mechanical parameters was more significant.For a single fatigue stage,the specimen underwent greater deformation in early cycles,which subsequently stabilized.Similar variation pattern was also reflected by AE count/energy/b-value.Crack behaviors were dominated by the fissure angle and load type and medium-scale crack accounted for 74.83%–86.44%of total crack.Compared with monotonic loading,crack distribution of specimen under cyclic loading was more complicated.Meanwhile,a simple model was proposed to describe the damage evolution of sandstone under cyclic loading.Finally,SEM images revealed that the microstructures at the fracture were mainly composed of intergranular fracture,and percentage of transgranular fracture jumped under cyclic loading due to the rapid release of elastic energy caused by high loading rate.

Energy‐based analysis of seismic damage mechanism of multi‐anchor piles in tunnel crossing landslide area

To study the damage mechanism of multi‐anchor piles in tunnel crossing landslide area under earthquake,the damping performance of multi‐anchor piles was discussed.The energy dissipation springs were used as the optimization device of the anchor head to carry out the shaking table comparison test on the reinforced slope.The Hilbert spectrum and Hilbert marginal spectrum were proposed to analyze the seismic damage mechanism of the multi‐anchor piles,and the peak Fourier spectrum amplitude(PFSA)was used to verify the effectiveness of the method.The results show that the seismic energy is concentrated in the high‐frequency component(30-40Hz)of the Hilbert spectrum and the low‐frequency component(12-30 Hz)of the marginal spectrum.This indicates that they can be combined with the distribution law of the PFSA to identify the overall and local dynamic responses of the multi‐anchored piles,respectively.The stretchable deformation of the energy‐dissipation springs improves the coordination of the multi‐anchor piles,resulting in better pile integrity.The damage mechanism of the multi‐anchor piles is elucidated based on the energy method:local damage at the top and middle areas of the multi‐anchor piles is mainly caused by the low‐frequency component(12-30 Hz)of the marginal spectrum under the action of 0.15g and 0.20g seismic intensities.As the seismic intensity increases to 0.30g,the dynamic response of the slope is further amplified by the high‐frequency component(30-40 Hz)of the Hilbert energy spectrum,which leads to the overall damage of the multi‐anchor piles.

“Standby” EMT and “immune cell trapping” structure as novel mechanisms for limiting neuronal damage after CNS injury

The central nervous system （CNS） contains the two most important organs, the brain and spinal cord, for the orchestration of the mental and physical activities of life. Because of its importance, the human body has evolved barrier systems to protect CNS tissue from the external environment. This barrier is a membrane composed of tightly apposed cells and is selectively permeable to specific molecules by way of membrane transporters.

Thermal stress damage mechanism in single-crystal germanium caused by 1080 nm laser irradiation

The process of thermal stress damage during 1080 nm laser ablation of single-crystal germanium was recorded in real time using a high-speed charge-coupled device.A three-dimensional finite element numerical model based on Fourier's heat conduction equation,Hooke's law and the Alexander–Hasson equation was developed to analyze the thermal stress damage mechanism involved.The damage morphology of the ablated samples was observed using an optical microscope.The results show that the cooling process has an important influence on fracture in the laser-irradiated region of single-crystal germanium.Fracture is the result of a combination of thermal stress and reduction in local yield strength.

Interface bond degradation and damage characteristics of full-length grouted rock bolt in tunnels with high temperature

Full-length grouted bolts play a crucial role in geotechnical engineering thanks to their excellent stability.However,few studies have been concerned with the degrading performance of grouted rock bolts caused by extensive and continuous heat conduction from surrounding rocks in high-geothermal tunnels buried more than 100 m(temperature from 28C to 100C).To investigate the damage mechanism,we examined the time-varying behaviors of grouted rock bolts in both constant and variable temperature curing environments and their damage due to the coupling effects of high temperature and humidity through mechanical and micro-feature tests,including uniaxial compression test,pull-out test,computed tomography(CT)scans,X-ray diffraction(XRD)test,thermogravimetric analysis(TGA),etc.,and further analyzed the relationship between grout properties and anchorage capability.In order to facilitate a rapid assessment and control of the anchorage performance of anchors in different conditions,results of the interface bond degradation tests were correlated to environment parameters based on the damage model of interfacial bond stress proposed.Accordingly,a thermal hazard classification criterion for anchorage design in high-geothermal tunnels was suggested.Based on the reported results,although high temperature accelerated the early-stage hydration reaction of grouting materials,it affected the distribution and quantity of hydration products by inhibiting hydration degree,thus causing mechanical damage to the anchorage system.There was a significant positive correlation between the strength of the grouting material and the anchoring force.Influenced by the changes in grout properties,three failure patterns of rock bolts typically existed.Applying a hot-wet curing regime results in less reduction in anchorage force compared to the hot-dry curing conditions.The findings of this study would contribute to the design and investigations of grouted rock bolts in high-geothermal tunnels.

An Analysis of the Dynamic Behavior of Damaged Reinforced Concrete Bridges under Moving Vehicle Loads by Using the Moving Mesh Technique

This work proposes a numerical investigation on the effects of damage on the structural response of Reinforced Concrete(RC)bridge structures commonly adopted in highway and railway networks.An effective three-dimensional FE-based numerical model is developed to analyze the bridge’s structural response under several damage scenarios,including the effects of moving vehicle loads.In particular,the longitudinal and transversal beams are modeled through solid finite elements,while horizontal slabs are made of shell elements.Damage phenomena are also incorporated in the numerical model according to a smeared approach consistent with Continuum Damage Mechanics(CDM).In such a context,the proposed method utilizes an advanced and efficient computational strategy for reproducing Vehicle-Bridge Interaction(VBI)effects based on a moving mesh technique consistent with the Arbitrary Lagrangian-Eulerian(ALE)formulation.The proposed model adopts a moving mesh interface for tracing the positions of the contact points between the vehicle’s wheels and the bridge slabs.Such modeling strategy avoids using extremely refined discretization for structural members,thus drastically reducing computational efforts.Vibrational analyses in terms of damage scenarios are presented to verify how the presence of damage affects the natural frequencies of the structural system.In addition,a comprehensive investigation regarding the response of the bridge under moving vehicles is developed,also providing results in terms of Dynamic Amplification Factor(DAFs)for typical design bridge variables.

Incompatible thermal deformation of interlayers and corresponding damage mechanism of high-speed railway track structure

In the service period,the instability of ballastless track bed are mostly related to the damage of interlayers which are mainly resulted from the incompatible thermal deformation of interlayers.The temperature field within the ballastless track bed shows significant non-uniformity due to the large difference in the materials of various structure layers,leading to a considerable difference in the force bearing of different structure layers.Unit Ballastless Track Bed(UBTB)is most significantly affected by temperature gradient.The thermal deformation of interlayers within UBTB follows the trend of ellipsoid-shape buckling under the effect of the temperature gradient,resulting in a variation of the contact relationship between structure layers and a significant periodic irregularity on the rail.When the train travels on the periodically irregular rail,the structure layers are locally contacted,and the contact zone moves with the variation of the wheel position.This wheel-followed local contact greatly magnifies the interlayer stress,causes interlayer damage,and leads to a considerable increase in the bending moment of the track slab.Continuous Ballastless Track Bed(CBTB)is most significantly affected by the overall temperature variation,which may cause damage to the joint in CBTB.Under the combined action of the overall temperature rise and the temperature gradient,the interlayer damage continuously expands,resulting in bonding failure between structural layers.The thermal force in the continuous track slabs will cause the up-heave buckling and the sudden large deformation of the track slab,and the loss of constraint boundary of the horizontal stability.For the design of a ballastless track structure,the change of bearing status and structural damage related to the incompatible thermal deformation of interlayers should be considered.

Damage to aircraft composite structures caused by directed energy system: A literature review

This paper presents a comprehensive review of the research studies on direct energy system effect on aircraft composite structures to develop a good understanding of state-of-the-art research and development in this area.The review begins with the application of composite materials in the aircraft structures and highlights their particular areas of application and limitations.An overview of directed energy system is given.Some of the commonly used systems in this category are discussed and the working principles of laser energy systems are described.The experimental and numerical studies reported regarding the aircraft composite structures subject to the effect of directed energy systems,especially the laser systems are reviewed in detail.In particularly,the general effects of laser systems and the relevant damage mechanisms against the composite structures are reported.The review draws attention to the recent research and findings in this field and is expected to guide engineers/researchers in future theoretical,numerical,and experimental studies.

Mechanical properties and energy evolution of Beishan shallow-layer granite under different unloading paths

Rock has mechanical characteristics and a fracture damage mechanism that are closely related to its loading history and loading path. The mechanical properties, fracture damage features, acoustic emission(AE) characteristics, and strain energy evolution of the Beishan shallow-layer granite used in triaxial unloading tests were investigated in this study. Three groups of triaxial tests, namely, conventional triaxial compression test(Group Ⅰ), maintaining deviatoric stress synchronously unloading confining pressure test(Group Ⅱ), and loading axial pressure synchronously unloading confining pressure test(Group Ⅲ), were carried out for the cylindrical granite specimens. AE monitoring device was utilized in these tests to determine the degree to which the AE waves and AE events reflected the degree of rock damage. In addition, the crack stress thresholds of the specimens were determined by volumetric strain method and AE parameter method, and strain energy evolution of the rock was explored in different damage stages. The results show that the shallow-layer granite experiences brittle failure during the triaxial loading test and unloading test, and the rock has a greater damage degree during the unloading test. The crack stress thresholds of these samples vary greatly between tests, but the threshold ratios of all samples are similar in the same crack damage stage. The Mogi-Coulomb strength criterion can better describe the unloading failure strength of the rock. The evolution of the AE parameter characteristics and strain energy differs between the specimens used in different stress path tests. The dissipative strain energy is the largest in Group Ⅱ and the smallest in Group Ⅰ.

One-step cell biomanufacturing platform:porous gelatin microcarrier beads promote human embryonic stem cell-derived midbrain dopaminergic progenitor cell differentiation in vitro and survival after transplantation in vivo

Numerous studies have shown that cell replacement therapy can replenish lost cells and rebuild neural circuitry in animal models of Parkinson’s disease.Transplantation of midbrain dopaminergic progenitor cells is a promising treatment for Parkinson’s disease.However,transplanted cells can be injured by mechanical damage during handling and by changes in the transplantation niche.Here,we developed a one-step biomanufacturing platform that uses small-aperture gelatin microcarriers to produce beads carrying midbrain dopaminergic progenitor cells.These beads allow midbrain dopaminergic progenitor cell differentiation and cryopreservation without digestion,effectively maintaining axonal integrity in vitro.Importantly,midbrain dopaminergic progenitor cell bead grafts showed increased survival and only mild immunoreactivity in vivo compared with suspended midbrain dopaminergic progenitor cell grafts.Overall,our findings show that these midbrain dopaminergic progenitor cell beads enhance the effectiveness of neuronal cell transplantation.

Damage mechanism at different transpassive potentials of solution-annealed316 and 3161 stainless steels

Electrochemical impedance spectroscopy （EIS）, anodic polarization and scanning electron microscopy techniques were used to investigate the damage mechanism in the transpassive potential region of AISI 316 and AISI 316L solution-annealed stainless steels （SS） with different degrees of sensitization. Depending on the DC potential applied during EIS tests, the AC responses in the transpassive region included three different regions：the first one associated with anodic dissolution of the passive layer, the second one contributed to the disso-lution at the area near grain boundaries, and the last one attributed to pitting corrosion. In addition, the fitting results to experimental data showed that as the DC bias during the EIS test increases the charge transfer resistance （Rct） decreases. Moreover, the Rct values decreased as the sensitization temperature increases but the AISI 316L SS samples exhibited a higher resistance to intergranular corrosion than 316 SS samples.

Simulation and experimental study of high power microwave damage effect on AlGaAs/InGaAs pseudomorphic high electron mobility transistor

The high power microwave （HPM） damage effect on the AIGaAs/InGaAs pseudomorphic high electron mobility transistor （pHEMT） is studied by simulation and experiments. Simulated results suggest that the HPM damage to pHEMT is due to device burn-out caused by the emerging current path and strong electric field beneath the gate. Besides, the results demonstrate that the damage power threshold decreases but the energy threshold slightly increases with the increase of pulse-width, indicating that HPM with longer pulse-width requires lower power density but more energy to cause the damage to pHEMT. The empirical formulas are proposed to describe the pulse-width dependence. Then the experimental data validate the pulse-width dependence and verify that the proposed formula P = 55τ^-0.06 is capable of quickly and accurately estimating the HPM damage susceptibility of pHEMT. Finally the interior observation of damaged samples by scanning electron microscopy （SEM） illustrates that the failure mechanism of the HPM damage to pHEMT is indeed device bum-out and the location beneath the gate near the source side is most susceptible to bum-out, which is in accordance with the simulated results.

Damage evolution model of strain hardening cementitious composites under the uniaxial stress state

The deformation and damage behaviors of strain hardening cementitious composites （SHCC） under the uniaxial stress state were investigated in this paper. Two ductile failure-based constitutive models were introduced to describe the uniaxial tension and compression properties of SHCC only using a few parameters. The computation method of model parameters was developed to ease the simulation procedures. Damage evolution of the SHCC was simulated by the formulation of continuum damage mechanics subsequently. The results show that the proposed models fit the stress-strain curves reasonably well, and the damage variables show different growth rules under uniaxial tension and compression. It is concluded that the proposed method can not only simply simulate the constitutive behavior of SHCC with the reasonable accuracy but also capture the characteristic of material degradation.

Revised damage evolution equation for high cycle fatigue life prediction of aluminum alloy LC4 under uniaxial loading

The fatigue life prediction for components is a difficult task since many factors can affect the final fatigue life. Based on the damage evolution equation of Lemaitre and Desmorat, a revised two-scale damage evolution equation for high cycle fatigue is presented according to the experimental data, in which factors such as the stress amplitude and mean stress are taken into account. Then, a method is proposed to obtain the material parameters of the revised equation from the present fatigue experimental data. Finally, with the utilization of the ANSYS parametric design language （APDL） on the ANSYS platform, the coupling effect between the fatigue damage of materials and the stress distribution in structures is taken into account, and the fatigue life of specimens is predicted. The outcome shows that the numerical prediction is in accord with the experimental results, indicating that the revised two-scale damage evolution model can be well applied for the high cycle fatigue life prediction under uniaxial loading.

Damage and fracture mechanism of 6063 aluminum alloy under three kinds of stress states

To study the damage and fracture mechanism of 6063 aluminum alloy under different stress states,three kinds of representative triaxial stress states have been adopted,namely smooth tensile,notch tensile,and pure shear.The results of the study indicate the following.During the notch tensile test,a relatively higher stress triaxiality appears in the root of the notch.With the applied loading increasing,the volume fraction of microvoids in the root of the notch increases continuously.When it reaches the critical volume fraction of microvoids,the specimen fractures.During the pure shear test,the stress triaxiality almost equals to zero,and there is almost no microvoids but a shear band at the center of the butterfly specimen.The shear band results from nonuniform deformation constantly under the shear stress.With stress concentration,cracks are produced within the shear band and are later coalesced.When the equivalent plastic strain reaches the critical value（equivalent plastic fracture strain）,the butterfly specimen fractures.During the smooth tensile test,the stress triaxiality in the gauge of the specimen remains constant at 0.33.Thus,the volume of microvoids of the smooth tensile test is less than that of the notch tensile test and the smooth specimen fractures due to shearing between microvoids.The G-T-N damage model and Johnson-Cook model are used to simulate the notch tensile and shear test,respectively.The simulated engineering stress-strain curves fit the measured engineering stress-strain curves very well.In addition,the empirical damage evolution equation for the notch specimen is obtained from the experimental data and FEM simulations.

New approach based on continuum damage mechanics with simple parameter identification to fretting fatigue life prediction

A new continuum damage mechanics model for fretting fatigue life prediction is established. In this model, the damage evolution rate is described by two kinds of quantities. One is associated with the cyclic stress characteristics obtained by the finite element （FE） analysis, and the other is associated with the material fatigue property identified from the fatigue test data of standard specimens. The wear is modeled by the energy wear law to simulate the contact geometry evolution. A two-dimensional （2D） plane strain FE implementation of the damage mechanics model and the energy wear model is presented in the platform of ABAQUS to simulate the evolutions of the fatigue damage and the wear scar. The effect of the specimen thickness is also investigated. The predicted results of the crack initiation site and the fretting fatigue life agree well with available experimental data. Comparisons are made with the critical plane Smith- Watson-Topper （SWT） method.