Conditional Generative Adversarial Network Enabled Localized Stress Recovery of Periodic Composites

Structural damage in heterogeneousmaterials typically originates frommicrostructures where stress concentration occurs.Therefore,evaluating the magnitude and location of localized stress distributions within microstructures under external loading is crucial.Repeating unit cells(RUCs)are commonly used to represent microstructural details and homogenize the effective response of composites.This work develops a machine learning-based micromechanics tool to accurately predict the stress distributions of extracted RUCs.The locally exact homogenization theory efficiently generates the microstructural stresses of RUCs with a wide range of parameters,including volume fraction,fiber/matrix property ratio,fiber shapes,and loading direction.Subsequently,the conditional generative adversarial network(cGAN)is employed and constructed as a surrogate model to establish the statistical correlation between these parameters and the corresponding localized stresses.The stresses predicted by cGAN are validated against the remaining true data not used for training,showing good agreement.This work demonstrates that the cGAN-based micromechanics tool effectively captures the local responses of composite RUCs.It can be used for predicting potential crack initiations starting from microstructures and evaluating the effective behavior of periodic composites.

Fatigue Crack Initiation Potential from Defects in terms of Local Stress Analysis

The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue（VHCF） behavior, which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects. Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions（elastic modulus lower than the matrix）. The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions（elastic modulus higher than the matrix）. It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter. There exists an inclusion size independent region for crack incubation, outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one. As for elliptical inclusions, reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation, whereas the long-axis length controls the site of peak stress concentration. The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions. Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime. The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime, and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.

Extended Kantorovich method for local stresses in composite laminates upon polynomial stress functions

The extended Kantorovich method is employed to study the local stress concentrations at the vicinity of free edges in symmetrically layered composite laminates subjected to uniaxial tensile load upon polynomial stress functions. The stress fields are initially assumed by means of the Lekhnitskii stress functions under the plane strain state. Applying the principle of complementary virtual work,the coupled ordinary differential equations are obtained in which the solutions can be obtained by solving a generalized eigenvalue problem. Then an iterative procedure is established to achieve convergent stress distributions. It should be noted that the stress function based extended Kantorovich method can satisfy both the traction-free and free edge stress boundary conditions during the iterative processes. The stress components near the free edges and in the interior regions are calculated and compared with those obtained results by finite element method（FEM）. The convergent stresses have good agreements with those results obtained by three dimensional（3D） FEM. For generality, various layup configurations are considered for the numerical analysis. The results show that the proposed polynomial stress function based extended Kantorovich method is accurate and efficient in predicting the local stresses in composite laminates and computationally much more efficient than the 3D FEM.

Local Stress Field in Wafer Thinning Simulations with Phase Space Averaging

From an ingot to a wafer then to a die,wafer thinning plays an important role in the semiconductor industry.To reveal the material removal mechanism of semiconductor at nanoscale,molecular dynamics has been widely used to investigate the grinding process.However,most simulation analyses were conducted with a single phase space trajectory,which is stochastic and subjective.In this paper,the stress field in wafer thinning simulations of 4H-SiC was obtained from 50 trajectories with spatial averaging and phase space averaging.The spatial averaging was conducted on a uniform spatial grid for each trajectory.A variable named mask was assigned to the spatial point to reconstruct the shape of the substrate.Different spatial averaging parameters were applied and compared.The result shows that the summation of Voronoi volumes of the atoms in the averaging domain is more appropriate for spatial averaging.The phase space averaging was conducted with multiple trajectories after spatial averaging.The stress field converges with increasing the number of trajectories.The maximum and average relative difference(absolute value)of Mises stress was used as the convergence criterion.The obtained hydrostatic stress in the compression zone is close to the phase transition pressure of 4H-SiC from first principle calculations.

On the longitudinal shifts of the Agulhas retroflection point

The Agulhas system is the strongest western boundary current system in the Southern Hemisphere and plays an important role in modulating the Indian-to-Atlantic Ocean water exchange by the Agulhas leakage.It is difficult to measure in situ transport of the Agulhas leakage as well as the Agulhas retroflection position due to their intermittent nature.In this study,an innovative kinematic algorithm was designed and applied to the gridded altimeter observational data,to ascertain the longitudinal position of Agulhas retroflection,the stability of Agulhas jet stream,as well as its strength.The results show that the east-west shift of retroflection is related neither to the strength of Agulhas current nor to its stability.Further analysis uncovers the connection between the westward extension of Agulhas jet stream and an anomalous cyclonic circulation at its northern side,which is likely attributed to the local wind stress curl anomaly.To confirm the effect of local wind forcing on the east-west shift of retroflection,numerical sensitivity experiments were conducted.The results show that the local wind stress can induce a similar longitudinal shift of the retroflection as altimetry observations.Further statistical and case study indicates that whether an Agulhas ring can continuously migrate westward to the Atlantic Ocean or re-merge into the main flow depends on the retroflection position.Therefore,the westward retroflection may contribute to a stronger Agulhas leakage than the eastward retroflection.

Complex variable approach in studying modified polarization saturation model in two-dimensional semipermeable piezoelectric media

A modified polarization saturation model is proposed and addressed math- ematically using a complex variable approach in two-dimensional （2D） semipermeable piezoelectric media. In this model, an existing polarization saturation （PS） model in 2D piezoelectric media is modified by considering a linearly varying saturated normal electric displacement load in place of a constant normal electric displacement load, applied on a saturated electric zone. A centre cracked infinite 2D piezoelectric domain subject to an arbitrary poling direction and in-plane electromechanical loadings is considered for the analytical and numerical studies. Here, the problem is mathematically modeled as a non-homogeneous Riemann-Hilbert problem in terms of unknown complex potential functions representing electric displacement and stress components. Having solved the Hilbert problem, the solutions to the saturated zone length, the crack opening displace- ment （COD）, the crack opening potential （COP）, and the local stress intensity factors （SIFs） are obtained in explicit forms. A numerical study is also presented for the proposed modified model, showing the effects of the saturation condition on the applied electrical loading, the saturation zone length, and the COP. The results of fracture parameters obtained from the proposed model are compared with the existing PS model subject to electrical loading, crack face conditions, and polarization angles.

PROBABILISTIC METHODOLOGY OF LOW CYCLE FATIGUE ANALYSIS

The cyclic stress-strain responses (CSSR), Neuber's rule (NR) and cyclic strain-life relation (CSLR) are treated as probabilistic curves in local stress and strain method of low cycle fatigue analysis. The randomness of loading and the theory of fatigue damage accumulation (TOFDA) are considered. The probabilistic analysis of local stress, local strain and fatigue life are constructed based on the first-order Taylor's series expansions. Through this method proposed fatigue reliability analysis can be accomplished.

Application of the local approach to the fatigue assessment for welded joints

The fatigue behavior of welded structures is currently determined by means of recommendations defined in terms of S-N curve corresponding to the detail classes of welded joints without taking account of the actual geometry of the weld. A new fatigue strength assessment method based on Dang Van multiaxial fatigue limit criterion was introduced, which is named the local approach and presented by Institut de Soudure recently. The local approach has advantages in taking welding residual stresses and the geometry of the weld toe and weld root into consideration. The application of the local approach to the fatigue strength assessment of low carbon steel Q235B welded joints was studied. The fatigue tests and finite element analysis results show that the local approach parameters recommended by Institut de Soudure were incorrectly for low carbon steel Q235B welded joints. With aluminum alloy welded joints being used widely, the parameters of the local approach used for aluminum alloy welded joints were obtained and verified on bases of the fatigue tests and finite element analysis.

Exploring the Interplay between Local Chain Structure and Stress Distribution in Polymer Networks

The mechanical behavior of polymer networks is intrinsically correlated with the local chain topology and chain connectivity.In this study,we delve into this relationship through the lens of coarse-grained molecular dynamics(CG-MD)simulations.Our aim is to illuminate the intricate interplay between local topology and stress distribution within polymer monomers,cross-linkers,and various components with distinct cross-link connections,thereby elucidating their collective impact on the mechanical properties of polymer networks.We mainly focus on how specific local structures contribute to the overall mechanical response of the network.In particular,we employ local stress analysis to unravel the mechanics of these structures.Our findings reveal the diverse responses of individual components,such as junctions,strands,cross-linkers between junctions,and dangling chain ends,when subjected to stretching.Notably,we observe that these components exhibit varying degrees of deformation tolerance,underscoring the significance of their roles in determining the mechanical characteristics of the network.Our investigations highlight junctions as primary contributors to stress accumulation,and particles with higher local stress showing a stronger correlation between stress and Voronoi volume.Moreover,our results indicate that both strands and cross-linkers between junctions exhibit heightened stress levels as strand lengths decrease.This study enhances our understanding of the multifaceted factors governing the mechanical attributes of cross-linked polymer systems at the microstructural level.

Source Mechanisms of Recent Earthquakes in Southern Saudi Arabia: Detecting of New Seismic Sources

Recently in 2020, in southern Saudi Arabia three felt earthquakes occurred in Asir region, in the Khamis Mushait, Ahad Rafidah, and AL-Shuqiq area, of magnitude 3.45, 3.1, and 3.5, respectively. The most interesting event was the earthquake that occurred in Khamis Mushait area, along a lake formed behind the Tadhah Dam (~7 km), fearing any damage to the dam’s body and the consequent destruction. Moment tensors for each event were computed for determining fault plane solutions, seismic moment, moment magnitude (Mw) and the CLVD ratio. In addition, the frequency contents in the waveforms of each event were identified. The obtained focal mechanisms represent different styles of faulting, normal movement with strike slip and strike slip with reverse. These tectonic movements on faults parallel to the Red Sea refer to the tensional forces due to the Red Sea rift system. These events occurred due to a natural tectonic movement, with considering the Khamis Mushait event as an induced event because of the lake behind the Dam. Many previous seismic hazard assessment studies have been conducted in southern Saudi Arabia without considering these recent seismic sources. Thus, our study provides new information related to detecting of new active seismic sources, which contributes to updating studies of seismic risk assessment in this region. In addition, our study pushes us to establish other additional seismic stations around these new seismic sources. This in turn will play a pivotal role in controlling seismic sources and then reassessing the seismic hazard in southern Saudi Arabia.

On the formation of shear bands in a metallic glass under tailored complex stress fields

The formation of shear bands in metallic glasses(MGs)was examined by tailoring localized complex stress fields(LCSFs).The findings have shown that the LCSFs in MGs can increase the localization of strained atoms and accelerate the release of accumulated deformation energy for initiating a shear band in confined and thin-layered regions.The findings not only add more knowledge to the formation mechanisms of shear bands in MGs,but also provide possible rationale for the discrepancies in the mechanical properties of different-sized MGs.As compared with the bulk samples,the higher strength and larger elastic limits in nanoscaled MGs could be attributed to the elimination of stress-concentrators,which can serve as LCSFs.

A NEW METHOD IN GROUNDWATER FLOW MODELING

One of the challenges in groundwater modeling is the prediction of hydraulic head related to local stress fluctuations with a regional scale model. Typical applications of numerical models require extensive field information for input data and for calibration If we can model the change directly, we may not have to know all the modeling parameters because sometimes the changes only depend on fewer parameters. In this article, we present an improved methodology for groundwater modeling related to local stress fluctuations using a perturbation approach. Our results demonstrate that this approach is capable of matching an exact solution for drawdown in both confined and unconfined aquifers. The results suggest that this perturbation method can provide an accurate representation of head in a large-scale hydrogeological system.

Examining the influence of the loading path on the cracking characteristics of a pre-fractured rock specimen with discrete element method simulation

Damage in a rock mass is heavily dependent on the existence and growth of joints,which are also influenced by the complex stress states induced by human activities(e.g.,tunneling and excavation).A proper representation of the loading path is essential for understanding the mechanical behaviors of rock masses.Based on the discrete element method(DEM),the influence of the loading path on the cracking process of a rock specimen containing an open flaw is examined.The effectiveness of the model is confirmed by comparing the simulation results under a uniaxial compression test to existing research findings,where wing crack initiates first and secondary cracks contribute to the failure of the specimen.Simulation results confirm that the cracking process is dependent upon both the confining pressure and the loading path.Under the axial loading test,a higher confining pressure suppresses the development of tensile wing cracks and forces the formation of secondary cracks in the form of shear bands perpendicular to the flaw.Increase of confining pressure also decreases the influence of the loading path on the cracking process.Reduction of confining pressure during an unloading test amplifies the concentration of tensile stress and ultimately promotes the appearance of a tensile splitting fracture at meso-scale.Confining pressure at the failure stage is well predicted by the Hoek-Brown failure criterion under quasi-static conditions.