Generation Of Tensile Stress Research Articles

Influence of mechanical stress on biomass allocation in three species (Pachira aquatica Aubl., Sextonia rubra (Mez) van der Werff and Simarouba amara Aubl.) with contrasting posture control mechanisms

Abstract Key message The posture control in Pachira aquatica, Sextonia rubra and Simarouba amara is achieved through contrasted mechanisms involving tensile stress in wood only, bark only or both. This study evidenced that the restoration of verticality does not imply an overall cost for trees but modifies growth kinetics and biomass allocation to organs and tissues, improving the posture control of the trees in the three species. Context All trees need a motor system to correct their position through the generation of asymmetric tensile stress around the stem, leading to active bending. In angiosperms, depending on the species, tensile stress is generated either in wood (tension wood), in bark or in both. Aims Here, we investigated how gravitropic stimuli (tilted stems without any movement) may affect growth and biomass allocation and whether this process depends on the posture control mechanism of the species. Methods Tree growth kinetics, final biomass allocation and wood and bark proportion, localisation and density were measured on young tilted plants and straight plants as controls. Pachira aquatica, Sextonia rubra and Simarouba amara were selected according to the location of their their motor system within bark only, wood only or both wood and bark, respectively. Results In response to tilting, trees from the three species increased their diameter and decreased their slenderness, but the total biomass (including stem and roots) was not different from that in the control trees, suggesting that reaction to artificial tilting does not imply a specific cost for the plant. However, the species exhibited strong differences in growth kinetics, in the amount and organisation of the tissues or in biomass allocation to different organs (root vs shoot, wood vs bark), adapted to the specificity of the posture control mechanisms and improving their motor function. Conclusion Whatever the posture control mechanism, uprighting does not modify the total biomass invested. However, allocation of biomass to different organs is strongly modified to obtain an efficient control of the posture.

Insights into the Fragmentation of Aluminum Hydride and Its Effect on Combustion and Agglomeration of HTPB Propellant.

AlH3 has gained considerable attention as a fuel additive due to its ability to offer high specific impulse and superior combustion performance. However, few studies have focused on the fragmentation and agglomeration behavior of AlH3. This study investigated the effects of fragmentation of AlH3 and AlH3/PVDF particles on the thermal decomposition, ignition, agglomeration, and combustion of HTPB propellants. Thermal analysis indicated that AlH3 and AlH3/PVDF can accelerate the decomposition of ammonium perchlorate by abundant active sites for the adsorption of the decomposition intermediates. Single-particle combustion uncovered the mechanism behind the directional spray of molten Al from the AlH3 particle and the fragmentation of the AlH3/PVDF particle. The melting of porous Al induces particle shrinkage due to solid-liquid interfacial tension and the structural restoration of the oxide shell, which consequently results in the sealing of cracks in the oxide shell of AlH3. Additionally, the accumulation of internal tensile stress leads to the reopening of these cracks and the directional ejection of the molten Al. The flexible oxide shell contributes to a smaller minimum normalized diameter of the AlH3/PVDF particle, aiding in the generation of internal tensile stress, while the sublimation of AlF3 induced the fragmentation. Synchrotron-based X-ray imaging revealed the formation of aggregates promoted by molten Al, the splitting of AlH3 aggregates due to hydrogen explosion, and the enhanced fragmentation of AlH3/PVDF due to the synergistic effect of hydrogen explosion and the sublimation of AlF3. Compared to raw particles, the CCPs (condensed combustion products) of SP2 propellant display a 48% reduction in average size (D50 = 24.5 μm), whereas there is an over 89% decrease in particle size for the CCPs of SP3 propellant (D50 = 5.14 μm). This study contributes to understanding the fragmentation of AlH3 and AlH3/PVDF upon ignition and combustion, providing valuable insights for the development and optimization of propellants containing AlH3.

Effectiveness of external prestressing in enhancing the non-ductile hanger failure mechanism in reinforced concrete inverted T-beams

Recently, inverted T-beams have been used in reinforced concrete (RC) bridges to support transverse precast stringers. Inverted T-beams, contrary to practice with conventional beams, are loaded on the flanges upper surface. This loading configuration causes hanger failure due to the generation of vertical tensile stresses near the bottom of the web. The key purpose of this study is to investigate the efficiency of vertical external prestressing stainless-steel bars in mitigating non-ductile hanger failure in reinforced concrete inverted T-beams. An experimental study on six inverted-T beams, including two un-strengthened specimens, was carried out. The study showed that the value of the prestressing level had a considerable impact on the performance of hanger mechanism in relation to crack pattern, ultimate loads, cracking behavior, load–deflection, strains, and ductility. The experimental results indicated that the suggested method for strengthening inverted T-beams had efficacy in reducing the seriousness of the non-ductile hanger failure and resulted in a strength increase of up to 53% when compared to that of the un-strengthened specimen. Additionally, two analytical models for estimating the hanger capacity and the average crack width of the strengthened RC inverted T-beams were proposed. The models that were proposed exhibited a high degree of agreement with the experimental results.

Biophysical Electrical and Mechanical Stimulations for Promoting Chondrogenesis of Stem Cells on PEDOT:PSS Conductive Polymer Scaffolds.

The investigation of the effects of electrical and mechanical stimulations on chondrogenesis in tissue engineering scaffolds is essential for realizing successful cartilage repair and regeneration. The aim of articular cartilage tissue engineering is to enhance the function of damaged or diseased articular cartilage, which has limited regenerative capacity. Studies have shown that electrical stimulation (ES) promotes mesenchymal stem cell (MSC) chondrogenesis, while mechanical stimulation (MS) enhances the chondrogenic differentiation capacity of MSCs. Therefore, understanding the impact of these stimuli on chondrogenesis is crucial for researchers to develop more effective tissue engineering strategies for cartilage repair and regeneration. This study focuses on the preparation of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) conductive polymer (CP) scaffolds using the freeze-drying method. The scaffolds were fabricated with varying concentrations (0, 1, 3, and 10 wt %) of (3-glycidyloxypropyl) trimethoxysilane (GOPS) as a crosslinker and an additive to tailor the scaffold properties. To gain a comprehensive understanding of the material characteristics and the phase aggregation phenomenon of PEDOT:PSS scaffolds, the researchers performed theoretical calculations of solubility parameters and surface energies of PSS, PSS-GOPS, and PEDOT polymers, as well as conducted material analyses. Additionally, the study investigated the potential of promoting chondrogenic differentiation of human adipose stem cells by applying external ES or MS on a PEDOT:PSS CP scaffold. Compared to the group without stimulation, the group that underwent stimulation exhibited significantly up-regulated expression levels of chondrogenic characteristic genes, such as SOX9 and COL2A1. Moreover, the immunofluorescence staining images exhibited a more vigorous fluorescence intensity of SOX9 and COL II proteins that was consistent with the trend of the gene expression results. In the MS experiment, the strain excitation exerted on the scaffold was simulated and transformed into stress. The simulated stress response showed that the peak gradually decreased with time and approached a constant value, with the negative value of stress representing the generation of tensile stress. This stress response quantification could aid researchers in determining specific MS conditions for various materials in tissue engineering, and the applied stress conditions could be further optimized. Overall, these findings are significant contributions to future research on cartilage repair and biophysical ES/MS in tissue engineering.

Effects of annealing on the interfacial microstructure and mechanical properties of explosively welded AZ80 magnesium alloy and A6005C aluminium alloy

Effects of annealing on the interfacial microstructure, mechanical properties, and residual stress of the explosively welded AZ80 magnesium alloy and A6005C aluminium alloy cladding plate were investigated. By explosive welding, a thin interlayer composed by an intermetallic compound, i.e. γ-Mg17Al12 phase, was formed at the interface of the cladding plate. After annealing at both 373 K and 473 K, the thickness of the interlayer increased. After annealing at 473 K, the interlayer changed from a single layer of γ-Mg17Al12 phase to a double layer of γ-Mg17Al12 phase and β-Al3Mg2 phase, resulting in a decrease in shear strength. As a result of nanoindentation measurement at the interface, the hardness was remarkably high in the β-Al3Mg2 phase. It was suggested that this phase became the crack initiation site for brittle fracture and the shear strength is decreased. Measurements of the residual stress using synchrotron radiation X-rays at the interface of cladding plate revealed the tendency of the generation of tensile residual stress on AZ80 magnesium alloy side and compressive residual stress on A6005C aluminium alloy side. After annealing at 473 K, residual stress in AZ80 magnesium alloy side and A6005C aluminium alloy side changed to compressive and tensile stresses, respectively, and these stress values became smaller in both cases. On the other hand, after annealing at 373 K, compressive residual stress was observed in both AZ80 magnesium alloy side and A6005C aluminium alloy side.

Semi-analytical solution for thermo-poro-elastic stresses in a wellbore cement plug and implications for cement properties that minimize risk of failure

In high temperature and high pressure environments, inducing pore pressure changes in low permeability cement can be problematic. Despite the typical design goal of achieving very low permeability in cement plugs for better isolation, this work reveals through a new semi-analytical TPE stress solution that lower permeability is not always ideal. In fact, such materials are unable to rapidly drain excess pore pressure compared to the rate at which it builds due to thermal changes may lead to the generation of internal tensile stress and increased risk of damage. This work identifies the parameter groups linked to the “permeability penalty” by utilizing dimensional and pairwise bivariate analysis through Monte Carlo sampling. It proposes two dimensionless parameter groups that divide into three distinct regions based on the likelihood of producing tensile effective stresses in a plug. This work provides a new wellbore cement design concept focuses on achieving the lowest permeability without risking failure from pore pressure buildup, rather than simply achieving the lowest possible permeability. Furthermore, this work for the first time highlights the important role of cement's specific heat in mitigating pore pressure buildup, paving a new path for enhancing this property in future cement design.

Interaction of two offset parallel flaws in geomaterials under unloading conditions

Rock excavation operations (also known as unloading) in high rock slopes, tunnels, and underground engineering cause large and extensive stress relaxation, even accompanied by the generation of tensile stress. Nevertheless, there have been few studies on the theoretical damage mechanism of rock under unloading conditions. In the present work, based on the superposition principle and the classic Kachanov method, we first proposed a theoretical calculation method for the stress intensity factor (SIF) at the tips of two parallel-offset flaws under unloading conditions. Then, a theoretical analysis of the effect of various geometric factors, such as flaw staggered distance, rock bridge length, flaw length and inclination angle, was carried out. The interaction between two offset flaws was captured. The results support the following findings: 1) The smaller the staggered distance or the rock bridge length is, the larger the flaw length, and the interaction between the two flaws becomes more significant. 2) With increasing inclination angle, the tendency of tensile failure becomes more obvious, while that of shear failure weakens. 3) The longer the rock bridge length, the more significantly the flaw interaction is affected by the inclination angle. Finally, discrete element simulations were carried out to verify the theoretical analysis results, and good consistency was found. These research results are helpful for understanding the failure mechanism of fractured rock masses caused by excavation (unloading condition).

Dynamic tensile failure of a V-shaped canyon induced by vertically travelling SV waves

Seismic-induced slope instability is a threat to transportation infrastructures in V-shaped canyons. The seismic-induced tensile cracks are commonly observed in the upper part of slopes and promote the seismic-induced slope instability. It is necessary to reveal the mechanism of tensile cracking in the upper part of a slope during an earthquake. The boundary integral equation method is employed to calculate the stresses induced by the self-weight and the vertically travelling SV wave. The stress development process shows that the self-weight induced stress plays an essential role in the development of tensile stress. Besides, the diffracted Rayleigh wave travelling along the ground surface is the major contributor to the generation of tensile stresses on the ground surface near the canyon. The effects of parameters, such as excitation frequency, canyon depth and slope inclination, on stress development are discussed. Parametric analyses indicate that the ground surface near a steeper slope more likely suffers tensile cracking. Higher-frequency waves prioritize producing the tensile cracks on the ground surface near a canyon. The steep slope of a deeper canyon allows the seismic waves within a wider frequency range to induce the tensile cracks on the ground surface near a canyon in priority.

Wear mechanism of nanotwinned cBN tools in nano-cutting Ni-Cr-Fe alloy by molecular dynamics simulation

In this paper, the molecular dynamics method is used to study the tool wear of nanotwinned cBN (nt-cBN) tool during nano-cutting of Nickel-based superalloys. It is determined that the main wear mechanism of the nt-cBN tool in the cutting process is diffusion wear. In the region where tool diffusion wear occurs, the phenomenon of nt-cBN tools changing from zinc amphibole cBN SP3 hybridization to layered hBN SP2 hybridization occurs. The workpiece atoms that diffuse into the tool act as metal catalyst atoms to achieve the transfer of electrons from the SP3 hybridized B atom to the N atom, realizing the transition from SP3 hybridization to SP2 hybridization. This structural transformation affects the performance of the tool, resulting in lower tool strength and more diffuse wear phenomena. Diffused workpiece atoms make the tool lattice distort and lead to the generation of tensile stresses in the tool cutting edge, which can promote the occurrence of tool diffusion wear and provide favorable conditions for structural phase transformation of the tool. Tool temperature and diffusion wear interact. The high cutting temperatures leads to more severe diffuse wear and diffusion wear affects tool thermal conductivity. Diffusion wear occurs in areas where high temperatures are concentrated and makes tool wear worse.

Study on Influence of Joint Locations and Hydraulic Coupling Actions on Rock Masses’ Failure Process

Distribution of joints and fissures under hydraulic coupling condition is particularly critical to the stability of surrounding rock masses in underground engineering construction. In this paper, DDARF (Discontinuous Deformation Analysis for Rock Failure) and RFPA (Rock Failure Process Analysis) are compared and analyzed firstly based on laboratory tests. Then using preferred software RFPA, the failure process, stress state, acoustic emission characteristics and energy dissipation laws of rock masses with different joint locations are analyzed under the hydraulic coupling condition. Results show that a large tensile stress region is generated on both ends of the original joint with the micro-cracks’ propagation, water pressure in cracks promotes the generation of tensile stress to a certain extent, damage effect angle increases gradually from the rock specimen with the middle joint to that with the marginal joint; the same water pressure has a certain auxiliary effect on the main crack failure when the joint is close to the middle part of the specimen, and has a dominant effect on the local crack failure when the joint is far away from the middle; the maximum water pressure shows the “U” shaped distribution. At low initial water pressure, stresses of specimens with symmetrical joint locations have similar evolution trends, while at high initial water pressure, the water pressure in cracks has significant dissipation and thus the maximum water pressure in the system does not exceed the initial value. The length of the main crack path is positively proportional to the number of acoustic emissions and the energy accumulation capacity, and evolution of the damage variable basically shows a development trend of steady growth-rapid growth-steady growth.

Stress and distortion of the 10 mm thick plate EH40 and 316L different butt joints in 10 kW laser welding

Residual stress has a negative influence on the service life of different welded joints between high-strength steel and stainless steel, especially in corrosive environment, it is very easy to cause fracture failure of welded joints. In this study, three types of medium-thick plate butt joints in single-pass full penetration laser welding, including EH40 & 316L, EH40 & EH40 and 316L & 316L butt joints, were deeply studied and verified by thermo-elastoplastic finite element method and experiments. The peak exponentially increasing double cone heat source model was used to calculate the transient temperature field. The relevant data of longitudinal, transverse residual stresses and angular distortion in three cases were extracted for comparative analysis. At the same time, the influence of the same material and different materials on the evolution mechanism of nodes stress in the weld was also discussed. It had been found that the geometric profile average error between the simulation and experiment in three cases was less than 0.5%. The stress state changes of the weld and its vicinity nodes in EH40 & 316L butt joint were faster than EH40 & EH40 and 316L & 316L butt joints. Compared with the welded joints of the same material, the welded joints of EH40 and 316L in the weld and its vicinity were remarkable differences in transverse residual stress, equivalent residual stress and angular distortion. The transverse residual stress gradient value of the EH40 side in EH40 & 316L was almost twice that of EH40 & EH40. The mismatch material properties of EH40 & 316L aggravated the generation of transverse tensile residual stress on the EH40 side. The results obtained in the current study were beneficial to the assessment of the integrity in the butt joints and the control of potential risks.

Microstructure and residual stress evolution in nanocrystalline Cu-Zr thin films

Grazing incidence X-ray diffraction (GIXRD) and scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS) were employed to study the microstructure evolution and stress development in the nanocrystalline Cu100−X-ZrX (2.5 at% ≤ x ≤ 5.5 at%) alloy thin films. Small Zr additions to Cu led to significant lattice parameter anisotropy in the as-deposited Cu-Zr thin films both due to macroscopic lattice strain and stacking faults in the Cu matrix. Strain free lattice parameters obtained after the XRD stress analysis of Cu-Zr thin films confirmed formation of a supersaturated substitutional Cu-Zr solid solution. For the first time, the study of film microstructure by XRD line profile analysis (XLPA) confirmed progressive generation of dislocations and planar faults with increasing Zr composition in Cu-Zr alloy films. These microstructural changes led to the generation of tensile stresses in the thin films along with considerable stress gradients across the films thicknesses which are quantified by the traditional dψhkl−Sin2ψ and GIXRD stress measurement methods. The origin of tensile stresses and stress gradients in the Cu-Zr film are discussed on the basis of film growth and heterogeneous microstructure with changing Zr composition.

Influence of microstructure on the failure of ultra-high molecular weight polyethylene composite beams impacted by blunt projectiles

Increasing the fibre tensile strength is well-known to improve the ballistic performance of ultra-high molecular weight polyethylene (UHMWPE) fibre composite beams. Here we investigate modifying ply microstructure as an alternate way to enhance ballistic performance. Specifically, we numerically investigate the mechanisms via which failure is initiated in [0o/90o]nlaminates comprising UHMWPE tape plies and compare with the commonly used UHMWPE fibre composites. Each ply of the beam is discretely modelled using a pressure-dependent crystal plasticity framework with the deformation mechanisms of the tape and fibre-based microstructures modelled by recognising differences in the available slip system in these two material systems. The numerical calculations are used to construct failure mechanism maps for a range of impact velocities, ply shear strengths and ply tensile strengths. Three dominant failure mechanisms emerged from the study: (i) failure of plies immediately under the projectile via an indirect tension mechanism in which compressive stress imposed normal to the plies by the projectile induces tensile in-plane ply stress due to the anisotropic expansion of the alternating 0o/90o plies; (ii) failure due to tensile straining at the rear of the impacted beam resulting from a combination of beam bending and stretching and (iii) a shear plugging like failure mode due to tensile straining at the edge of the impact site. Tensile stress generation by indirect tension is significantly reduced in the tape-based microstructures due to the unavailability of the slip systems that result in lateral plastic expansion. In fact, our calculations suggest that over the entire range of parameters modelled here the tape-based microstructures outperform the fibre-based microstructures. This suggests an alternate route to enhancing the ballistic performance compared to the traditional methods that rely mainly on increasing fibre strength.

Analysis of abrasive belt wear effect on residual stress distribution on a grinding surface

Owing to elastic grinding and other favorable processing characteristics, belt grinding has been widely used in many industrial applications, including machining titanium alloys of difficult-to-cut metals. Abrasive belt wear inevitably occurs in the grinding process, but its mechanism and effects have not been fully understood, especially its effect on the residual stress (RS). To overcome this shortcoming, this study experimentally investigates the abrasive wear influence on the surface RS distribution of titanium plate belt grinding through a synthetic cause analysis. First, abrasive belts with different wear stages are prepared according to the wear law throughout the abrasive belt life cycle, and their wear degrees are quantitatively evaluated. Then, comparative grinding experiments of the TC4 flat plate are conducted using the prepared abrasive belts. Finally, the surface RS distribution of the grinding traces and stress differences induced by the abrasive belt wear are analyzed and discussed comprehensively. The results indicate that the more severe the abrasive belt wear is, the greater the accumulation of grinding heat and the greater the ratio of the grinding energy will be, which will further lead to residual tensile stress (RTS) generation, especially in the grinding direction. In addition, owing to the fast heat dissipation on both sides of the contact wheel, the surface RS along the grinding traces' vertical direction obeys an approximately symmetrical distribution, having low values on the sides and a peak in the middle. The strong accumulation of grinding heat caused by severe wear makes the difference between the stress distribution extremums increase with the abrasive belt wear.

Changes of residual stresses on the surface of leucite-reinforced ceramic restoration luted with resin composite cements during aging in water

The objective of this study was to compare the changes in the residual stresses present on the surface of leucite-reinforced dental ceramic restorations luted with a self-adhesive and a conventional resin composite cement during aging in water. Ring specimens made of a leucite-reinforced ceramics were luted to ceramic cylinders using a self-adhesive (Panavia SA Luting Plus) or a conventional resin composite cement (Panavia V5) in dual-cure or self-cure mode. Residual stresses on the ring surface were measured using indentation fracture method at 1 h, 1, 3, 7, 14 and 28 days of the 37 °C water immersion. Water sorption, water solubility and elastic modulus of the cements were also measured. Compressive stress was generated on the surface of the ceramic rings by the polymerization of the resin composite cements, and the stresses appeared to decrease over time by water sorption of the cements. The dual-cured conventional resin composite cement remained compressive stresses on the ceramic surface, while only the self-cured self-adhesive cement, which demonstrated the greatest water sorption, generated tensile stresses during the four weeks of aging in water. The elastic moduli of cements did not significantly change through the immersion, suggesting that the stresses were less affected by the modulus. To prevent the generation of tensile stresses on the leucite-reinforced ceramic restoration, self-adhesive cements exhibiting small water sorption should be clinically selected.

In Situ Stress Evolution on an Au Thin Film Cathode during Charging/Discharging of a Lithium-Oxygen Battery

The theoretical energy density of lithium-oxygen batteries is by far the closest competitor to gasoline compared to other battery configurations. Unfortunately, the commercial implementation of this technology is hindered by the poor cycling performance, mostly due to the slow oxygen evolution/reduction kinetics. The sluggish oxygen evolution kinetics are typically more severe and cause the slow oxidation of lithium peroxide. This can cause the irreversible formation of a passivating lithium peroxide layer on the cathode surface. The interfacial dynamics due to the growth of lithium peroxide and the formation of cathode-electrolyte interface (CEI) layers remains largely unknown. It is crucial to elucidate how the nucleation of discharge products on the cathode surface affects the electrochemical performance in order to design new electrolytes and cathodes for lithium-oxygen batteries.In this study, we propose to investigate the surface stress generation associated with the formation of discharge products and CEI layers on the cathode. Thin Au films were deposited via e-beam evaporation onto a borosilicate glass substrate. These substrates were used as a cathode material. The electrolyte used was 1 M LiTFSI in DMSO, and it was saturated with oxygen prior to performing electrochemical measurements. In situ stress measurements were conducted using a multiple beam optical sensor (MOS). A custom cell was designed to monitor the physical response of the cathode during cycling while enabling optical access to the back of the substrate for stress measurements.The in situ curvature evolution on an Au coated borosilicate glass cantilever is monitored during one discharge/charge cycle at 10 µA/cm2. Tensile stress generation is observed during the formation of the lithium peroxide discharge product on the Au cathode. A large stress relaxation into the compressive regime is observed at the onset of charging. From our results, the nucleation of lithium peroxide onto the cathode surface causes tensile stress, while the oxidation of lithium peroxide causes compressive stress. Revealing the mechanical response of the charge/discharge products on the cathode will shed light into the formation mechanisms of these products, which is crucial to design both new electrolytes and cathode materials for lithium-oxygen batteries.Acknowledgement: This work was supported by the Binational Science Foundation (#2018327), and we are thankful to both Dr. Malachi Noked and Rosy for fruitful discussions.

Surface Integrity of Turned 6061 Aluminum Alloy using Liquid Nitrogen as Cooling Medium

Aluminum alloy is widely used in automobile, aerospace and other fields due to its excellent strength-to-weight ratio. And machined surface integrity is an important factor affecting the service performance of parts. In this paper, surface integrity of turned 6061 aluminum under liquid nitrogen cooling condition was studied. The results shows that, cryogenic cooling cutting with liquid nitrogen could effectively reduce the surface roughness, inhibit the generation of residual tensile stress and improved the surface microhardness compared with dry and emulsion cooling cutting. Cryogenic cooling processing has great advantages in cutting machining.

Laser‐Induced Reversible and Irreversible Surface Nanostructuring

AbstractHarnessing the 4D transmission electron microscopy (4D‐TEM) technique capable of in situ study of light‐material interactions with high spatiotemporal resolution, the authors are able to demonstrate the formation of a reversible laser‐induced periodic surface structure (RLIPSS) on the top few‐nanometer surface of free‐standing amorphous thin films of silicon nitride and silicon dioxide. Compared with commonly reported permanent laser‐induced periodic surface structures (LIPSS) grown on thermal ablation of materials, highly ordered RLIPSS here develop in a nonthermal manner by femtosecond laser irradiation with unprecedented low intensity down to 10 000 times lower than their reported ablation thresholds. This reversible structure can undergo a transition to permanent structure with increasing laser intensity and number of laser shots. The orientation of the RLIPSS exhibits an unexpected memory of laser polarization, making it a potential candidate for future ultrafast rewritable photo‐image recording. These experimental results exclude the widely accepted formation mechanism of LIPSS based on the interference between the incident light with the surface scattering waves or the surface plasmon polaritons, and suggest a novel mechanism for surface laser micromachining, relevant to the laser‐induced production of charged point defects and the polarization‐dependent generation of tensile stress induced by electrostatic repulsion.

Stress control in thick AlN/c-Al2O3 templates grown by plasma-assisted molecular beam epitaxy

Stress evolution was studied in up to ~3 µm thick AlN templates, comprising ∼65 nm thick AlN nucleation layers (NLs) and thick buffer layers (BLs), grown using different growth modes and conditions by plasma-assisted molecular beam epitaxy (PA MBE) on c-Al2O3. Growth of both the NL and BL in a standard PA MBE mode at N-rich conditions (at flux ratio Al/N2* ∼ 0.5) led to instant generation of a relatively high tensile stress (∼1.5 GPa) which is maintained throughout the entire growth. On the contrary, NLs, grown using a migration-enhanced epitaxy (MEE), demonstrated a transition from the initial compressive stress to stress-free growth, which is usually observed in the Volmer–Weber films. Further growth of thick AlN BLs on the MEE–NLs at various Al/N2* ratios revealed a wide variety of stress evolution processes. The BL growth by using metal-modulated epitaxy (MME) at Me-rich conditions with Al/N2* ∼ 1.33 led to a gradual decrease in the initial compressive stress in the 2D AlN layers, whereas standard PA MBE growth of 3D BL at N-rich conditions (Al/N2* ∼ 0.92) exhibited a fast transition from the initial compressive stress to tensile stress. Moreover, we succeeded in achieving the quasi-stress-free growth of a 3.1 µm thick AlN BL using the MME growth mode at the optimum flux ratio Al/N2* = 1.05. These results were compared with the results of other authors and explained using a kinetic approach to description of stress evolution during PA MBE of AlN/c-Al2O3 templates, taking into account several simultaneously acting competitive mechanisms of continuous generation of tensile and compressive stresses.

Application of dynamic fracture mechanics to dry snow slab avalanche release

Field observations and measurements show that dry snow slab avalanches initiate by propagating shear fractures within a relatively thin weak layer sandwiched between a planar, stronger, thicker slab above and stronger material below. After initiation, the weak layer fracture can propagate up and down slope for distances which range from about 10 to 100′s of meters to cause tensile fracture through the body of the slab which results in avalanche release. In this paper, dynamic fracture mechanics is applied to slab tensile fracture after which avalanche release is imminent. Two mechanisms for production of tensile stress are explored employing field measurements of slab properties and lab measurements. The first considers inertial effects related to quasi-brittle fracture near the tip of a propagating weak layer shear fracture. The second is concerned with the tensile stress generation from the stress drop behind the crack front as the fracture propagates in the weak layer. Analysis suggest that both mechanisms can contribute to produce the tensile fracture line which precedes avalanche release. Even though both mechanisms may operate together, they are analyzed separately in this paper.