User-defined Material Subroutine Research Articles

Experimental and numerical studies on the collapse of Titanium alloy ring-stiffened cylinder

This paper investigates the collapse of ring-stiffened Titanium alloy cylinders used for subsea resource exploration under hydrostatic pressure through experimental and numerical methods. Extensive material tests were first conducted on Titanium alloy specimens to obtain the fundamental mechanical properties and variation characteristics. Then, a 4236 mm-long test cylinder was fabricated with an inner radius of 650 mm and a thickness (ts) of 18.85 mm. The initial geometric imperfection was measured at evenly-spaced positions in the axial direction and 48 locations around the circumference by dial gauges. Afterward, the test cylinder was transferred into a custom hyperbaric pressure vessel and pressurized to collapse. As to the numerical analysis, a user-defined material subroutine implementing the incremental J2 deformation theory was developed to predict plastic bifurcation pressure. Moreover, a nonlinear finite element (FE) model, which incorporated the measured geometric imperfection and material nonlinearity, was used to reproduce the experiment. The numerical results were found to exhibit reasonably good agreement with the test data. In addition, parametric studies were conducted regarding material properties, geometric parameters, and imperfection sizes on the load-carrying capacity of Titanium alloy ring-stiffened cylinders.

Characterization of the stress-state dependent ductile fracture behavior for Q960 ultra-high-strength structural steel

Ultra-high-strength structural steels are gaining popularity in civil engineering due to their exceptional strength-to-weight ratio. However, their inherent lower ductility compared to normal strength structural steel warrants particular attention and further investigation for safety assessment. In this study, a total of 24 specimens made of Q960 ultra-high-strength steel (UHSS), including 21 tensile specimens with various stress states and 3 three-point bending specimens, were tested to assess its mechanical behavior undergoing large deformation. Subsequently, the impact of stress state on plasticity, damage evolution and fracture initiation was analyzed and characterized. The Bai-Wierzbicki yield surface with a deviatoric associated flow rule was identified to characterize the complex plasticity behavior of Q960 UHSS. With respect to damage evolution, the stress-state dependent fracture energy Gf was proposed in this study to describe the damage softening behavior. The uncoupled Hosford-Coulomb locus with a non-linear weight function was utilized to predict the fracture initiation behavior. Finally, the utilized stress-state dependent constitutive models for Q960 UHSS, with the calibrated parameters and user-defined material subroutine VUMAT, were successfully verified through the three-point bending test with good agreement. Moreover, based on a parametric analysis of radius-to-thickness ratio, the bendability design regarding the minimum radius-to-thickness ratio of Q960 UHSS was proposed.

Nonlinear mechanical behaviour and visco-hyperelastic constitutive description of isotropic-genesis, polydomain liquid crystal elastomers at high strain rates

The mechanical behaviour of isotropic-genesis, polydomain liquid crystal elastomers (I-PLCEs) at various strain rates is systematically investigated via experiments, theoretical analysis, and numerical modelling. Experiments encompassing SEM (scanning electron microscope), DSC (differential scanning calorimetry), TGA (thermogravimetric analyzer), quasi-static and dynamic (SHPB - split Hopkinson pressure bar) mechanical tests, as well as drop-weight impact tests, are undertaken to identify the nonlinear, large-strain, rate-dependent relationship between compressive stress and deformation of the I-PLCEs studied. Subsequently, a three-dimensional compressible visco-hyperelastic constitutive model for the material is established based on the summation of Cauchy stress components. The as-used model yields good agreement with experimental data, particularly an excellent description of the mechanical responses at high strain rates of 103∼104 s−1. The fully-calibrated constitutive model is implemented in the commercial finite element code ABAQUS via a virtual user-defined material (VUMAT) subroutine. The inhomogeneous deformation processes of the I-PLCEs, corresponding to impact by a hemispherically-tipped drop weight, which induces complex stress states, are also well described. Finally, when evaluated by two dimensionless physical parameters, the I-PLCEs demonstrate a more pronounced strain rate sensitivity in terms of dynamic strength and impact toughness compared to other commonly used materials, highlighting their superior performance in dynamic loading scenarios. The present study is helpful for the design and development of impact-resistant LCE-based materials and structures.

Energy Absorption Behavior of Carbon-Fiber-Reinforced Plastic Honeycombs under Low-Velocity Impact Considering Their Ply Characteristics.

Honeycomb structures made of carbon-fiber-reinforced plastic (CFRP) are increasingly used in the aerospace field due to their excellent energy absorption capability. Attention has been paid to CFRP structures in order to accurately simulate their progressive failure behavior and discuss their ply designability. In this study, the preparation process of a CFRP corrugated sheet (half of the honeycomb structure) and a CFRP honeycomb structure was illustrated. The developed finite element method was verified by a quasi-static test, which was then used to predict the low-velocity impact (LVI) behavior of the CFRP honeycomb, and ultimately, the influence of the ply angle and number on energy absorption was discussed. The results show that the developed finite element method (including the user-defined material subroutine VUMAT) can reproduce the progressive failure behavior of the CFRP corrugated sheet under quasi-static compression and also estimate the LVI behavior. The angle and number of plies of the honeycomb structure have an obvious influence on their energy absorption under LVI. Among them, energy absorption increases with the ply number, but the specific energy absorption is basically constant. The velocity drop ratios for the five different ply angles are 79.12%, 68.49%, 66.88%, 66.86%, and 60.02%, respectively. Therefore, the honeycomb structure with [0/90]s ply angle had the best energy absorption effect. The model proposed in this paper has the potential to significantly reduce experimental expenses, while the research findings can provide valuable technical support for design optimization in aerospace vehicle structures.

Damage analysis of solid propellants with default defects based on macro-microscopic approach

Solid Rocket Motors (SRMs) rely on solid propellants, directly impacting the energy capacity of launch vehicles. High energy density material (HEDM) propellant exhibits superior energy density owing to the incorporation of a substantial quantity of energetic particles. However, the elevated particle fill ratio makes the formation of pores more prevalent during the manufacturing process of propellants. These pores, representing default defects, are indispensable factors in the investigation of mechanical properties of propellants. This study investigated default defects effects on HEDM propellants, analyzing both microscopic and macroscopic perspectives. Utilizing micro-CT imaging, the microstructure and default defects were characterized. Subsequently, using the cohesive zone model (CZM), microscopic damage evolution was depicted with the representative volume element (RVE) model, allowing in-depth analysis. Furthermore, a damage constitutive model, based on microscopic damage, was developed, with its softening function calibrated using microscopic damage evolution characteristics. Implemented into ABAQUS via user-defined material subroutine (UMAT), this model enabled the prediction of the mechanical response of HEDM propellants with different defects. The results demonstrate that the developed model accurately predicts the mechanical responses of HEDM propellants with different defects, showcasing the potential of macro-microscopic approaches in analyzing propellants with default defects.

Structural integrity assessment of a solid propellant grain considering confining pressure effect

Confining pressure has a significant effect on the mechanical properties of solid propellant, and it is crucial to develop a refined structural integrity analysis and assessment method considering confining pressure effect and nonlinearity of propellant material for the design of propellant grain of a solid rocket motor (SRM), especially for the design of high charge modulus. In this work, an existing viscoelastic-viscodamage constitutive model considering confining pressure effect was implemented as a user-defined material subroutine and verified using a few experiments under various confining pressures. The mechanical responses and safety factor of a typical variable cross-section propellant charge SRM with various charge modulus under ignition pressurization load and room temperature were analyzed and evaluated. The results show that the user-defined material subroutine can well describe nonlinear mechanical responses of solid propellant under different experimental conditions. The Von Mises stress and minimum safety factor considering the confining pressure effect are higher than the value without considering the confining pressure effect, while the Von Mises strain is opposite. In addition, regardless of whether the confining pressure effect is considered or not, the maximum Von Mises stress and strain show a nonlinear increasing relationship with the charge modulus, and the minimum safety factor decreases with charge modulus. However, when analyzing the structural integrity of high charge modulus, using two evaluation methods will yield completely different evaluation results. It is believed that this research can support structural integrity analysis and design of high charge modulus SRM.

Effectiveness of thermoviscoplastic material models in predicting the thermomechanical behavior of rolled homogenous armor steel

This paper studies the dynamic and elevated temperature behavior of rolled homogeneous armor steel, which is crucial for applications such as blast, impact, crash, and ballistic resistance. The deformation behavior is analyzed through tensile tests. The tensile tests of RHA steel are conducted at quasi-static 10−4s−1≤εṗ≤10−1s−1 to dynamic 10−1s−1≤εṗ≤36.439s−1 strain-rates and at high temperatures ranging from room temperature (RT,27°C) to 500°C. Phenomenological Johnson-Cook (JC) models and semi-physical Rusinek-Klepaczko (RK) material models are employed in finite element (FE) simulations of tensile tests. The JC failure model, with a small modification, is used along with the material models to simulate the loss of load-bearing capacity of the component. All necessary material parameters are extracted from the test data. The material models and the damage model are integrated into ABAQUS CAE FE software through a user-defined material subroutine (UMAT). The simulated outcomes acquired from diverse material models are validated with relevant experimental findings, and the effectiveness of each material model is evaluated through qualitative and quantitative assessments. Observations reveal limitations in the existing models to accurately replicate material behavior under tension, particularly in the dynamic strain rate, and elevated temperature range. Consequently, a modified version of the JC model (MJC) is explored to better account for the impact of temperature on strain hardening and strain rate hardening. The proposed modification in the material model significantly enhances the predictive accuracy of FE simulations of tensile deformation across the entire spectrum of studied strain rates and temperatures compared to the original JC and RK models.

Free Vibration Analysis of Curvilinearly Tapered Axially Functionally Graded Material Beams

The study focuses on the free vibration analysis of beams made of axially functionally graded materials (AFGM) with curvilinear variable cross-sections along their length. The beams encompass various shapes, including concave and convex conic sections, with axial material properties varying according to polynomial and exponential laws. The equations of motion are derived using Hamilton’s principle within the framework of Timoshenko beam theory. These governing equations, subjected to various boundary conditions, are solved using the differential transform method (DTM). The proposed solution technique is validated by comparing computed natural frequencies with the existing literature and results obtained using three-dimensional finite element analysis in ABAQUS. The incorporation of material gradients into the beam finite element models was achieved using the user-defined material subroutine (UMAT). Additionally, a comprehensive study is conducted to examine the influence of various factors on the natural frequencies of functionally graded beams. These factors include parameters of material laws, types of variable beam shapes, slenderness ratio, and specific boundary conditions. This study provides a thorough understanding of the modal dynamics of the considered beams, offering valuable insights into the behavior of FGM structures.

Numerical modeling of composite spherical-roof contoured core (SRCC) sandwich panels subjected to blast loading

This article investigates the blast response of spherical-roof contoured core (SRCC) and novel spherical-roof contoured core (NSRCC) sandwich panels under blast loading, which involves the core design and material types. Due to the practical difficulties associated with investigating explosive materials and replicating blast loads, finite element (FE) modeling is a favorable method as a functional and convenient alternative. FE models are established under different charge masses of used trinitrotoluene (TNT), which is used in aluminum (AL) and carbon fiber-reinforced plastic (CFRP) cores of sandwich panels. The Johnson-Cook model and the user-defined material subroutine (VUMAT) predicted the blast performance of AL and CFRP materials on SRCC and NSRCC types with three patterns. The results showed that as the mass of TNT increased from 1 kg to 3 kg, the internal energy was also linearly increased. It was shown that the NSRCC type provided higher internal energy than the SRCC type. The maximum internal energy was shown on the NSRCC2 pattern, which was 41.20 kJ with 3 kg TNT. Moreover, SRCC pattern 1 with AL material provided the minimum deflection compared with the other five patterns. It was indicated that SRCC pattern 1 showed better blast protection performance on 1, 2, and 3 kg TNT. It is highlighted that the SRCC with two patterns can have potential applications on protective structures under blast loading.

Thermo-mechanical coupled three-dimensional finite element simulation analysis of drilling thermoplastic braided carbon fiber composite and optimization of process parameters

Optimizing the process parameters of thermoplastic composites during drilling is paramount for mitigating tearing, delamination, and other forms of damage, while simultaneously enhancing the tensile strength and extending the long-term service life of the overall structure. This study focuses on exploring a thermal-mechanical coupling method for predicting dynamic mechanical progressive failure and determining optimal process parameters during the drilling of Braided Carbon Fiber Reinforced Polyether Ether Ketone (BCF/PEEK). Initially, a thermal conduction constitutive model of BCF/PEEK was developed based on the proposed thermal distribution ratio calculation method. Concurrently, a user-defined material subroutine VUMAT and a bilinear cohesive element model, were implemented on the ABAQUS/Explicit platform to simulate the delamination behaviors of drilling BCF/PEEK using a tapered drill-reamer. Subsequently, a comprehensive BCF/PEEK drilling experiment platform was constructed, and the simulation accuracy of the drilling finite element (FE) model was verified through temperature, thrust force, and hole-wall morphology analyses. Finally, response surface regression models were established for process parameters, and the optimal parameters were predicted and validated. The results indicate that minimum thrust force and temperature can be achieved using the tapered drill-reamer with a spindle speed set at 4878.79/3000 r/min and a feed speed of 34.04/30 mm/min, respectively, with a maximum error of only 8.78 %.

Bending shape memory properties and multi-scale viscoelastic behaviors of knitted-fabric reinforced polymer composites

Knitted fabrics with easily deformable loop structure have the potential in the development of shape memory polymeric composites with large recovery deformations. The knitted fabric reinforced shape memory epoxy polymer composites (SMPC) were prepared in this work. The effects of loop densities, orientations and bending radii on shape memory properties of SMPC were investigated. The shape fixity ratio and shape recovery ratio of SMPC subjected to U-shaped bending radius of 5 mm are above 98 %. The shape recovery force of SMPC can reach up to 5.9 N. The thermodynamic properties of SMP were also characterized to obtain mechanical parameters and a user-defined material subroutine (UMAT) of shape memory epoxy polymer (SMEP) was written. Based on viscoelastic theory and the multi-scale geometrical structures, the macroscopic homogeneous thermodynamic model and mesoscopic thermodynamic model of knitted fabric reinforced SMPC were established to study the macro-scale stress distribution and meso-scale deformation evolution during shape memory process, respectively. The neutral surface position of SMPC during bending deformation is offset inner. The unique anisotropic loop structure of knitted fabric determines the shape memory behavior of the SMPC. Finally, micro-CT characterizations of knitted fabric reinforced SMPC were conducted to further understand the loop deformation mechanism during shape memory process. This study will provide important theoretical and technical support for large deformation structure design and deformation prediction of smart composites.

An elastoplastic damage ice material model based on modified Tsai-Wu yield criterion: Experiment, theory and numerical application

Ice, as a novel green and sustainable building material, has attracted more and more attention in building engineering. Appropriate ice material models are crucial for the performance analysis of the increasing ice structures. There is still challenging in modelling ice responses due to the complexity of ice. This study aims to present a nonlinear elastoplastic damage model for ice material. First, a triaxial compression test of artificial ice is conducted. Based on the test results, the modified Tsai-Wu failure criterion with better performance in the tensile zone and physical meaning for hydraulic strength is established. Then, combining plasticity theory with damage mechanics, an elastoplastic damage constitutive law considering the difference between tensile and compressive properties is proposed. The piecewise damage model and the Weibull exponential damage model are employed for compression and tension damage, respectively. Moreover, the numerical iterative algorithm is developed and a user-defined material subroutine (UMAT) is embedded in the finite element software ABAQUS to simulate the mechanical properties of ice. Furthermore, the constitutive model is verified by comparing the FEM results with the results of uniaxial compression, triaxial compression, and three-point bending tests. The results show that the constitutive model can well describe the stress-strain nonlinear behavior and capture the basic failure mode of ice materials. Finally, considering temperature affecting on the failure surface, a temperature dependent ice material model is developed. The current study would help in the design, operation and maintenance of ice structures.

Low-velocity impact responses and failure of sandwich structure with carbon fiber composite honeycomb cores

The objective of this study is to examine the response and failure of honeycomb sandwich structures made of carbon fiber reinforced polymer (CFRP) composites under low-velocity impacts. Four impact tests with varying energies are performed to induce four distinct damage states in the sandwich structure: no discernible damage, damage to the top face sheet and a portion of the core, damage to the lower face sheet, and total penetration. The damage characteristics of the sandwich structures is analyzed by using industrial tomography technology (CT) without destroying them. A refined finite element model is established to further explain the deformation behavior and energy absorption mechanism of the structure, clarifying the effects of the honeycomb core's structural parameters, such as wall thickness, cell side length, and core height. To enhance the precision of simulation outcomes, a model for the onset and progression of damage in plain woven composites is incorporated into the user-defined material subroutine. The experiments and simulations demonstrate a high level of consistency in terms of peak loads, failure mode, and energy absorption.

Crystal plasticity quantification of anisotropic tensile and fatigue properties in laser powder bed fused Inconel 718 superalloy

The high thermal gradients during the laser powder bed fusion (LPBF) process induce an inhomogeneous microstructure with anisotropic mechanical properties. This study developed a dislocation-based strain-gradient crystal plasticity finite element (CPFE) model to reveal the microstructure-based deformation mechanisms and quantify their strengthening contributions to the mechanical property anisotropy of LPBF-ed Inconel 718 (IN718) superalloy. The analyzed microstructural attributes included 1- crystallographic orientations/misorientations, 2- solid solutions (SS), 3- grain size and morphologies, and 4- dislocation dynamics, such as statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs). The investigated mechanical properties contained tensile strength, fatigue resistance, fracture mechanisms, and their anisotropy under loading towards building (BD) and transverse (TD) directions. The experimentally obtained microstructural features of the as-LPBF-ed IN718 were synthesized into the crystal plasticity representative volume elements (CPRVE) to model the deformation behavior using a user-defined material subroutine (UMAT) in the ABAQUS solver. The energy-based fatigue indicator parameter (FIP) captured the fatigue properties and failure mechanisms. The main slip-strengthening contributions stemmed from SSDs, grain size, SS, and GNDs. in TD. The corresponding order in BD was SSDs, SS, GNDs, and grain size because the grain dimensions were larger in BD than in TD. The grain-size effect had the highest contribution to the tensile property anisotropy, followed by SSDs, GNDs, and SS. Aside from their strengthening effects, crystal texture and grain size induced mechanical property heterogeneity and fatigue sensitivity at grain boundaries, reducing their contributions to fatigue strength relative to tensile. In contrast to the previous findings, local accumulations of SSDs and GNDs at softer sides of grain boundaries during cyclic loading reduced the strength heterogeneity in these areas, improving fatigue properties. The highest slip-strengthening effect for fatigue resistance and anisotropy was under SSDs, followed by GNDs, SS, and grain size.

Characterization of Mechanical Heterogeneity and Study of the Mechanical Field at the Tip of the Stationary-Growing Crack in Dissimilar Metal Welded Joints

Abstract The mechanical heterogeneity in local areas of dissimilar metal welded joints and the micro-area mechanical state at the crack tip are key factors in determining Environment-Assisted Cracking (EAC). Traditional methods for acquiring material mechanical properties often result in destructive damage to specimens, while conventional “sandwich” models exhibit abrupt changes in interfacial mechanical properties and a lack of research into the mechanical field at the tip of the stationary or growing crack. In light of these challenges, this study, based on the analysis of microstructures in localized regions of the welded joint and the acquisition of material mechanical properties through indentation tests, developed a user-defined material subroutine (UMAT) to characterize the mechanical properties of non-uniform local areas within the welded joint. Additionally, it investigated the mechanical field at the tip of the stationary—growing crack using an integral method and a de-bond technique. The results indicate that non-destructive indentation tests can accurately acquire the material mechanical properties of local areas in the welded joint. Notably, significant changes in mechanical properties typically occur in the material interface regions, making them vulnerable points for potential failure. Furthermore, under the same load, mechanical heterogeneity significantly influences the distribution of the mechanical field at the crack tip. Crack propagation induces alterations in crack tip stresses, resulting in noticeable residual stresses and strains along the propagation path.

A viscoelastic-viscoplastic thermo-mechanical model for polymers under hypervelocity impact

In hypervelocity impact (HVI) events, the thermo-mechanical behaviors of polymer materials are significantly influenced by viscous effects, thereby affecting energy absorption and shielding performance. A viscoelastic-viscoplastic thermo-mechanical model was newly developed to simulate the mechanical behavior and heat generation of polymer materials under HVI. The viscoelastic-viscoplastic constitutive equation and the Mie-Grüneisen equation of state (EOS) were applied to describe the deviatoric deformation and strongly nonlinear volumetric compression behavior. Moreover, the heat generation was calculated considering contributions from plastic work, irreversible entropy increase, and viscous dissipation which was not considered in the previous HVI numerical model. The temperature increase was calculated through adopting the apparent heat capacity method. The model was coded and seamlessly integrated into the LS-DYNA program through a user-defined material subroutine. Additionally, the FEM-SPH adaptive method was adopted to accurately simulate the fragmentation which always accompanies in the HVI event, wherein the finite element method (FEM) handles contact and deformation behavior, while the smoothed particle hydrodynamics (SPH) method addresses the formation and motion of debris clouds. Ultra-high molecular weight polyethylene (UHMWPE) was selected as the sample material, and its model parameters were calibrated based on the data from quasi-static and dynamic experiments. Good effectiveness and accuracy have been validated through comparing the simulation results and experimental results from the literatures. A noteworthy feature is that the model has the capability to capture the temperature distribution and various energy components induced by the impact, including viscous dissipation, plastic work, and residual energy. Finally, a comparison was made to explore the influence of viscosity on the impact response of UHMWPE plates. The analysis results indicate that material viscosity significantly improves energy absorption capabilities and overall shielding performance.

Finite element simulation of axial crushing of flat and C-channel composite coupons using a modified mesomodel

The present article investigates the axial crushing behavior of IM7/8552 material system. Two types of specimens are considered-(a) flat coupons with saw-tooth trigger and (b) C-channels with 45° chamfer as a trigger. All the crushing test data (load, displacement and velocity history curves) is shared in the Crashworthiness working group (CWG) consortium of the Composite material handbook (CMH17.org). Flat crush coupons are tested at the University of Utah and the C-channels are tested at the John H. Glenn Research Center of the National aeronautics and space administration (NASA). Three types of layups are considered for flat coupons: QI1-[90/±45/0]2S, HL2-[902/02/±45/02] and HL3-[90/45/02/90/-45/02]. Two types of layups are considered for C-channels: HL2-[902/02/±45/02] and HL3-[90/45/02/90/-45/02]. Both the specimens are subjected to crush loading with an initial velocity and lumped mass attached to the impactor. All the ply and interface material properties come from the current authors' testing campaign except for strength and stiffness in the fiber direction. The crushing test cases are simulated in LS-Dyna software. Ply and interface behavior is described via user-defined material subroutines of LS-Dyna software. Each ply is modeled using 3D solid elements and the interfaces are modeled explicitly using cohesive elements. Intraply behavior is described using a novel modified meso model proposed by the present authors. Intraply model accounts for nonlinear damage, plasticity, ply fracture toughness and finite element mesh size. Interply response is modeled using bi-linear law. Predictions from present FE models showed a good correlation with crush test data. The effect of finite element model parameters e.g., filtering frequency, mesh size, interface properties, ply fracture toughness, element erosion criterion, contact stiffness and friction coefficient on crushing predictions is demonstrated by conducting parametric studies.

User subroutines platform development for rubber hyperelastic constitutive models and its application in finite element analysis

Aiming at the problem that not enough hyperelastic constitutive models (HCMs) built-in commercial finite element analysis (FEA) software could be utilized, and it is difficult to achieve a high-precision description of the mechanical response behavior of various complex rubber and its composite materials, we developed a UHYPER and UMAT subroutine integration platform based on the user-defined material subroutine function in Abaqus/Standard. The integration platform including 70 HCMs can effectively improve the selection comprehensiveness of HCMs in the material property module of FEA software and the accuracy of simulation results. On the premise that the material parameters are consistent, it is found that the calculation results of the UHYPER subroutine in our integration platform are completely consistent with the calculation results of the built-in HCMs in Abaqus, and the error between the UMAT subroutine result and the built-in model result is within 5.0%. This study indicates the effectiveness and reliability of the user-defined material subroutine integration platform.

Calculation of and Key Influencing Factors Analysis on Equivalent Resilient Modulus of a Submerged Subgrade.

To calculate and analyze the equivalent resilient modulus of a submerged subgrade, a constitutive model considering the effect of saturation and matrix suction was introduced using ABAQUS's user-defined material (UMAT)subroutine. The pavement response under falling weight deflectometer (FWD) load was simulated at various water levels based on the derived distribution of the resilient modulus within the subgrade. The equivalent resilient modulus of the subgrade was then calculated using the equivalent iteration and weighted average methods. Based on this, the influence of the material and structural parameters of the subgrade was analyzed. The results indicate that the effect of water level rise on the tensile strain at the bottom of the asphalt layer and the compressive strain at the top of the subgrade is obvious, and its trend is similar to an exponential change. The equivalent resilient modulus of the subgrade basically decreases linearly with the rise in the water level, and there is high consistency between the equivalent iteration and weighted average methods. The saturated permeability coefficient and subgrade height have the most significant effect on the resilient modulus of the subgrade, which should be emphasized in the design of submerged subgrades, and the suggested values of the resilient modulus of the subgrade should be proposed according to the relevant construction conditions.

Trans-scale analysis of 3D braided composites with voids based on micro-CT imaging and unsupervised machine learning

Voids are unavoidable during the manufacturing of 3D braided composites. This study proposes an unsupervised machine learning method combined with micro-computed tomography (micro-CT) scanning and a progressive damage analysis to analyze defects in these composites at a trans-scale level. The method enables the creation of real multiscale models and the determination of the porosity in both the intra-yarn (1.52 %) and inter-yarn (5.04 %) planes. Here, the unsupervised machine learning method is introduced in a trans-scale damage analysis to reduce calculation dimensions and to visualize the clustering data. A user-defined material subroutine (UMAT) is also developed to implement the trans-scale damage model. The experimental validation of the simulation results demonstrates the effective trans-scale damage analysis, showing the predominant pull-shear damage in the yarns, which is primarily located at the interfaces both between the yarns and between the yarns and the matrix. Finally, based on the scanned geometric data the degradation in modulus and strength of 3D braided composites with porosity is studied.