User Subroutine Research Articles

Synthesis, Characterization, and Modeling of Aligned ZnO Nanowire-Enhanced Carbon-Fiber-Reinforced Composites.

This paper presents the synthesis, characterization, and multiscale modeling of hybrid composites with enhanced interfacial properties consisting of aligned zinc oxide (ZnO) nanowires and continuous carbon fibers. The atomic layer deposition method was employed to uniformly synthesize nanoscale ZnO seeds on carbon fibers. Vertically aligned ZnO nanowires were grown from the deposited nanoscale seeds using the low-temperature hydrothermal method. Morphology and chemical compositions of ZnO nanowires were characterized to evaluate the quality of synthesized ZnO nanowires in hybrid fiber-reinforced composites. Single fiber fragmentation tests reveal that the interfacial shear strength (IFSS) in epoxy composites improved by 286%. Additionally, a multiscale modeling framework was developed to investigate the IFSS of hybrid composites with radially aligned ZnO nanowires. The cohesive zone model (CZM) was implemented to model the interface between fiber and matrix. The damage behavior of fiber was simulated using the ABAQUS user subroutine to define a material’s mechanical behavior (UMAT). Both experimental and analytical results indicate that the hierarchical carbon fibers enhanced by aligned ZnO nanowires are effective in improving the key mechanical properties of hybrid fiber-reinforced composites.

Experimental and numerical analysis on mode II fracture toughness of CFRP adhesive joints using a nonlinear cohesive/friction coupled model

In this work, a nonlinear cohesive/friction coupled model was proposed by combining experiments and numerical simulations for mode II fracture analysis of hybrid carbon nanofillers toughened carbon fiber composite bonded joints. First, end-notched flexure tests were performed to investigate the toughening effects of multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) hybrids modified adhesive on mode II fracture toughness of adhesive joints. Testing results show that 0.75 wt% MWCNTs/GNPs hybrid has an improvement of 45% compared to pure adhesive. Secondly, a phenomenological progressive rising friction model was introduced into an exponential cohesive zone model for delamination analysis of adhesive layer under shear and normal compression loadings. Finally, the proposed model was implemented in ABAQUS/Explicit via user subroutine VUMAT and model parameters were determined by fitting experimental results of end-notched flexure tests for adhesive joints.

A Hierarchical Single-Surface Model for an Unsaturated Soil-Structure Interface

In this paper, according to the shear test of the interface between unsaturated soil and steel plate, the modified apparent roughness is used to describe interface roughness. And considering the influence of matrix suction and interface roughness, the stress variable is constructed to improve the yield and plastic potential functions. Then a modified hierarchical single-surface elastoplastic interface constitutive model (HISS-Unsaturated-Int model) is set up. The finite element calculation of HISS-Unsaturated-Int model is achieved by compiling and calling the user subroutine FRIC of ABAQUS. The interface shear test between unsaturated soil and structure with different water contents and the pull-out shear test between unsaturated soil and soil nail are simulated. The calculation results are consistent with those of experiments, which shows the modified interface constitutive model can effectively predict the shear mechanical properties of soil-structure interface under different soil moisture content, normal stress and interface roughness.

Finite element modelling of low velocity impact test applied to biaxial glass fiber reinforced laminate composites

• High-speed imaging and DIC are used to track damage progression during impact test • A proposed 3D FE model allows predicting the impact behavior of laminate composites • [0/90] 3s laminates failed due to fiber breakage at high compression impacted region • [±45] 3s laminates exhibited matrix cracking and delamination caused by high tension The aim of this paper is to describe the dynamic behavior of biaxial glass fiber reinforced laminate composites under low velocity impact test through finite element modelling. Experimental investigations by impact test performed using an instrumented drop weight testing machine were conducted on three-point bending composite samples in order to assess their impact damage resistance. Moreover, the experimental setup allowed the visualization of real-time damage progression of the impacted laminate composite via high-speed camera Phantom V2512 enabling to capture 83000 frames per second. Dynamic strain fields were extracted by Digital Image Correlation (DIC) method. Based on the experimental results, a numerical study of the impacted specimens was developed as a user subroutine VUMAT integrated to ABAQUS/Explicit in order to precisely capture the progressive dynamic failure of the laminate composite under impact test. In the proposed model, the damage and failure of each ply are accounted by a Hashin 3D damage-based behavior, and a cohesive zone model is employed to capture the onset and progression of inter-laminar delamination. A good experimental-numerical correlation was obtained for peak force and failure modes.

An improved creep damage mechanics model and its application to evaluate creep strain design codes for hot outlet manifold in steam reformer furnace

Considering the effect of carbide coarsening and cavity nucleation and growth on the creep damage of Fe–Cr–Ni-based austenitic stainless steels, an improved creep damage constitutive model is established based on the hyperbolic sine function. The material parameter optimisation procedure and the associated ABAQUS user subroutine CREEP of the model are also developed. In addition, the improved model is calibrated and validated by comparing the extrapolated failure times for the three creep damage models, and applied to an idealized pipe bend to evaluate the inelastic strain limits in the ASME-NH code. The results show that the proposed model is capable to reflect the creep behaviour of a hot outlet manifold steel under different stress levels at 890 °C. During the long-term creep exposure of the manifold component, the limits of 1% creep membrane strain and 2% creep membrane strain + bending strain ruled in ASME-NH code may not be conservative. If only the cumulative inelastic strain is considered in the structural design, membrane strain is the primary rule for investigating the failure of the pipe bending.

Development of the Concurrent Multiscale Discrete-Continuum Model and Its Application in Plasticity Size Effect

A concurrent multiscale model coupling discrete dislocation dynamics to the finite element method is developed to investigate the plastic mechanism of materials at micron/submicron length scales. In this model, the plastic strain is computed in discrete dislocation dynamics (DDD) and transferred to the finite element method (FEM) to participate in the constitutive law calculation, while the FEM solves the complex boundary problem for DDD simulation. The implementation of the whole coupling scheme takes advantage of user subroutines in the software ABAQUS. The data structures used for information transferring are introduced in detail. Moreover, a FE mesh-based regularization method is proposed to localize the discrete plastic strain to continuum material points. Uniaxial compression tests of single crystal micropillars are performed to validate the developed model. The results indicate the apparent dependence of yield stress on sample size, and its underlying mechanisms are also analyzed.

Cure process evaluation of CFRP composites via neural network: From cure kinetics to thermochemical coupling

This work focuses on the comprehensive modeling of the cure kinetics and the thermochemical coupling to investigate the cure process of CFRP composites. Neural network (NN) is instead of the general cure kinetics model to find the relationships between the cure kinetics parameters where the temperature rate is also involved. Large data sets from non-isothermal differential scanning calorimetry (DSC) are used for the network training and validation. For comparison, the NN model and general model are respectively written as user subroutine and combined with the thermochemical coupled model to implement the finite element (FE) cure analysis. Compared with the general model, the NN model significantly improves the prediction accuracy of cure behavior. The reaction rates at various temperature rates from the NN model are also coincide with the corresponding experiments. Furthermore, it is also found that the NN model can capture the local trends of the composites during cure by taking into account the effect of the temperature rate on the cure kinetics. The temperature and degree of cure (DoC) gradients of the composite during cure process are reduced than the general model, which provides a new insight for the cure and cure related characteristics analysis.

Numerical simulation of the near-fault spontaneous rupture and its influence on dynamic soil-structure interaction

Based on the sub-model technology and the secondary development of the user subroutine in the finite element software ABAQUS, a numerical simulation method to model the dynamic process of near-fault spontaneous rupture is established and verified. The physical nature of near-fault spontaneous rupture is revealed by the proposed numerical simulation method. In addition, the influence of near-fault pulse-type ground motion on the dynamic soil-structure interaction (DSSI) is investigated, which is compared with that of the far-fault ground motion. The results indicate that the acceleration of near-fault ground motion has rich low-frequency components, and the peak vertical acceleration approximately is 1.5 times greater than peak horizontal acceleration, which usually is 2/3 as seismic codes deal with the far-fault ground motion. Finally, it is observed that the distributions of responses of the superstructure and the soil foundation under near-fault ground motion and far-fault ground motion are similar with each other. The responses under near-fault ground motion can be 254.5% greater than that under far-fault ground motion, which means that the near-fault ground motion is more destructive to the structure.

Nonlinear finite element analysis on timber-concrete composite beams

To support the parametric analysis of timber-concrete composite (TCC) beams and the structural analysis of TCC frame systems, the computational effectiveness of various dimensional finite element (FE) models for TCC beams were evaluated in this study, including the one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D). It was found that the 1D frame FE model achieved by the fibre beam elements and the 2D model formed by the layered shell elements show apparently higher computational efficiency than the 3D FE model with solid elements. The FE models all exhibit good consistence with the test results in terms of the macroscopic bending performance of the TCC beams. Owe to the developing of the ABAQUS user subroutines in this study, the 3D model established in the ABAQUS/Explicit module could almost perfectly present the failure modes of the TCC beams, including the fracture of timber, failure of end connections, and cracks on concrete slab, and show higher accuracy of the numerical results than the 1D and 2D models. Additionally, the FE numerical interfacial strain and slip distributions are slightly larger than the test ones, due to various shortcomings of the definitions of the connections in the FE models. Generally, the FE models were validated to be feasible sufficiently for further studies on TCC structures.

Cohesive zone model prediction of debonding failure in CFRP-to-steel bonded interface with a ductile adhesive

A cohesive zone model (CZM) with triangular traction-separation law can predict the progressive fracture damage of the CFRP-to-steel bonded interface well with a brittle adhesive. Since a distinct plateau stage in the traction-separation relationship exists for a ductile adhesive, the triangular CZM cannot provide a valid prediction in this case. In the present article, a trapezoidal CZM using the finite element (FE) computational method was developed and coded in the user subroutine (UMAT) of ABAQUS to define the material property model of the cohesive layer. The debonding failure of the CFRP-to-steel bonded interface under single-mode and mixed-mode loadings were studied through FE analysis. The load-displacement relationship, the shear stress distribution, and the strain distribution obtained from FE simulation generally have good agreement with experimental results acquired from published papers. The effectiveness of the proposed FE model to simulate the progressive fracture damage of the CFRP-to-steel bonded interface with a ductile adhesive was validated.

Wear and leakage assessments of canted coil Spring–Energized polytetrafluoroethylene seals under Ultra-High cycle operations

Spring-energized polytetrafluoroethylene (PTFE) seals are typically utilized to meet requirements of long service life due to their excellent temperature and chemical resistance. After high-cycle operations, the contact pressure between the seal lip and the associated counterpart possibly decrease because of the wear on the contact surface and this eventually leads to loss of the sealing capability. Evaluations of the wear and resulting sealing performance of the seal are rather essential, however, the corresponding experiments are generally time consuming and costly. Simulation procedures for the seal based on the finite element analysis were therefore developed in the current study. Experimental and numerical investigations of mechanical behaviors of the canted coil spring and PTFE materials were first performed. A wear analysis model for the spring-energized PTFE seal used in a glue dispenser with reciprocating motions for semiconductor equipment was subsequently constructed. The modified Archard model was implemented into a self-coded user subroutine of commercial finite element software to assess the amount of wear on the seal. Simulation results revealed that the geometry dimensions of the worn seal are in fair agreement with those of the corresponding samples. Contact pressure distributions of the seal lip were also examined during the operations. Peak of the contact pressure was found to gradually decrease along with the increase of cycles due to the wear, and potential fluid leakage was then assessed via the Reynolds equation. Numerical calculations showed that no leakage would occur after ultra-high cycle operations which was consistent with practical observations.

The application of FEM-BEM coupling method for steady 2D heat transfer problems with multi-scale structure

This paper presents a finite element method and boundary element method (FEM-BEM) coupling scheme to study the steady-state 2D heat transfer problem for structures with multi-scale features. Based on the geometric information or material properties, the considered model is divided into FE sub-domain and BE sub-domain. The closed BE sub-domain in the mixed FEM-BEM model is defined as a finite super-element for which the effective stiffness matrix and effective fluxes are obtained by the self written BE code and hence can be assembled to the FE systems by the user subroutine (UEL) provided in ABAQUS. This method offers a number of key improvements compared with current analysis methods available for multi-scale structures. These improvements are: (i) the flexibility and convenience by using the powerful pre/post-processing in ABAQUS; (ii) the higher accuracy of the solution over the classical FEM; and (iii) the computational cost and time can be highly reduced by using the coupling method. Numerical results are compared with reference solutions and demonstrate the accuracy and effectiveness of the proposed approach.

An Engineering Prediction Model for Stress Relaxation of Polymer Composites at Multiple Temperatures.

This study develops an engineering prediction model for stress relaxation of polymer composites, allowing the prediction of stress relaxation behaviour under a constant strain, over a range of temperatures. The model is based on the basic assumption that in the stress relaxation process the reversible strain is transformed to irreversible strain continuously. A strain-hardening model is proposed to incorporate nonlinear elastic behaviour, and a creep rate model is used to describe the irreversible deformation in the process. By using stress relaxation data at different temperatures, under different strains, the dependence on temperature and initial strain of the model parameters can be established. The effectiveness of the proposed model is verified and validated using three polymer composite materials. The performance of the model is compared with three commonly used stress relaxation models such as the parallel Maxwell and Prony series models. To ease the use of the proposed model in realistic structural problems, a user subroutine is developed, and the stress relaxation of a plate structure example is demonstrated.

Extended GTN model for predicting ductile fracture under a broad range of stress states

Continuous damage accumulation resulting from void nucleation, growth, and coalescence leads to the final ductile fracture. The Gurson–Tvergaard–Needleman (GTN) model and its extensions, which model these mechanisms, are famous coupled constitutive models for predicting material failure. In this study, the GTN model was extended by incorporating two different damage variables: void volume fraction and shear damage; this was done to describe damage accumulation and ductile fracture under a wide range of stress states from compression-dominated to tension-dominated ones. The shear damage was composed of the shear void nucleation related to secondary voids and shear deformation damage. In addition, the shear void nucleation and void closure effect resulting from compression deformation were introduced to model the change in void volume under different loading conditions. The Lode angle dependence function was updated to express the various stress states and competition mechanism between two damage variables. Furthermore, the effective damage and its incremental form was used to define the extent of damage accumulation and fracture initiation. Subsequently, the proposed model was implemented in finite element software through user subroutines, and three distinct experiments related to high, low, and negative stress triaxiality were performed to calibrate the model parameters by the inverse analysis method for 1045 steel and 2024-T351 aluminum alloy. The experimental results under proportional and path-changing loading conditions were used to analyze the damage evolution and validate the proposed model. The good agreement between the numerical prediction and experimental results for the displacement and location at fracture, indicates good predictive ability of formulated model under arbitrary loading conditions.

Buckling response of laminated FG-CNT reinforced composite plates: Analytical and finite element approach

A 3D finite element model using suitable user subroutines in ABAQUS finite element software and an analytical solution based on the third-order shear deformation laminated plate theory combined with von Kármán's geometric nonlinearity are proposed to simulate the functionally graded Carbon Nanotubes (CNTs) throughout a plate's thickness. The former is used for the estimation of the buckling response and the latter is used for the estimation of both buckling and post-buckling response of laminated nanocomposite plates under uniform and parabolic loading. In the analytical approach, the governing equations are solved by the Galerkin method and Airy's stress function to obtain the buckling coefficient of FG-CNT reinforced composite plates. The effect of various factors, such as boundary conditions, types of loads, distribution type and volume fraction of CNTs, geometrical parameters, etc on the buckling and post-buckling response is investigated. It has been found that a higher volume fraction of CNTs increases the buckling and post-buckling strength of laminated composite plates, whereas the laminated plate with angle-ply orientations equal to 45 degrees has the maximum buckling load factor among the CNT fiber orientations considered.

Mixed Cohesive Zone Modeling of Interface Debonding between Propellant and Insulation

High-quality interface between propellant and insulation is the strict requirement difficult to quantify in solid rocket motor. In this study, the mixed mode delamination process of propellant and insulation interface is considered in double cantilever sandwich beam (DCSB) and single lap-joint (SLJ) test. The PPR cohesive zone model (CZM) and bilinear CZM in ABAQUS are introduced in this mixed fracture progress. In order to implement the PPR model in ABAQUS, user subroutine user element (UEL) is programmed for the novel CZM. Two simple pure mode I and mode II fracture problems are designed to check the accuracy of the UEL, and the result of verification is excellent. DCSB and SLJ test and their corresponding results are used again in the same inverse analysis with the two typical effective displacement-based and potential-based CZM. Base on the results, a series discussion and some conclusions are made. The debonding progress of the propellant and insulation interface in DCSB and SLJ test are mixed mode. The PPR CZM is prior in simulation than the bilinear CZM in ABAQUS because the PPR CZM is much more flexible with changeable traction-separation shape. The real normal and tangential displacement at damage initiation shows the unreasonable change in bilinear CZM in ABAQUS under mixed mode fracture. The PPR CZM and bilinear CZM in ABAQUS are all thickness-dependent model. The real initial stiffness and the critical displacement in the bilinear CZM and the real maximum traction in PPR model are dependent on the thickness of cohesive element. The different thickness dependence of the two model is caused by the implementation method.

Counterexample-trained neural network model of rate and temperature dependent hardening with dynamic strain aging

Constitutive models dealing with the thermal and visco-plasticity of metals have seen wide applications in the automotive industry. A basic plasticity and fracture characterization of a 1.5 mm thick DP780 dual phase steel sheet based on uniaxial tensile (UT) experiments with seven distinct material orientations is complemented by low (∼0.001/s), intermediate (∼1/s) and high (∼150/s) strain rate experiments on notched tensile (NT) and shear (SH) specimens at temperatures ranging from 20 °C to 500 °C. At low strain rates, we observe a non-monotonic effect of the temperature on the force-displacement curves, with the highest curve obtained at 300 °C. Contrasting low speed tests, a monotonic effect of temperature is observed for intermediate and high strain rate experiments, with the highest curves obtained at 20 °C for both cases. Strain rate jump tests are performed proving the positive strain rate sensitivity of the steel. A machine-learning based plasticity model is developed to capture the observed complex strain rate- and temperature effect. The material is modeled as elasto-plastic, with a Hill’48 yield surface and a non-associated flow rule. The flow resistance is decoupled into a reference strain hardening term and a neural network term, which is a function of the plastic strain, strain rate, temperature, and an additional dynamic strain aging variable. The plasticity model is implemented into a material user subroutine and identified using a counterexample-guided hybrid experimental-numerical approach. The extracted loading paths reveal a complex rate and temperature effect on the ductility of DP780. A neural network based fracture initiation model is therefore adopted to describe the fracture onset across various stress states, strain rates and temperatures considered.

A novel computational growth framework for biological tissues: Application to growth of aortic root aneurysm repaired by the V-shape surgery

Ascending aortic aneurysms (AsAA) often include the dilatation of sinotubular junction (STJ) and extend proximally into the aortic root, which usually leads to aortic insufficiency. The novel surgery of the V-shape resection of the noncoronary sinus, for treatment of AsAA with root ectasia, has been shown to be a simpler procedure compared to traditional surgeries. Our previous study showed that the repaired aortic root aneurysms grew after the surgery. In this study, we developed a novel computational growth framework to model the growth of the aortic root repaired by the V-shape surgery. Specifically, the unified-fiber-distribution (UFD) model was applied to describe the hyperelastic deformation of the aortic tissue. A novel kinematic growth evolution law was proposed based on existing observations that the growth rate is linearly dependent on the wall stress. Moreover, we also obtained patient-specific geometries of the repaired aortic root post-surgery at two follow-up time points (Post-1 and Post-2) for 5 patients, based on clinical CT images. The novel computational growth framework was implemented into the Abaqus UMAT user subroutine and applied to model the growth of the aortic root from Post-1 to Post-2. Patient-specific growth parameters were obtained by an optimization procedure. The predicted geometry and stress of the aortic root at Post-2 agree well with the in vivo results. The novel computational growth framework and the optimized growth parameters could be applied to predict the growth of repaired aortic root aneurysms for new patients and to optimize repair strategies for AsAA.

The influence of layered soil on ice-seabed interaction: Soft over stiff clay

Ice gouging is one of the main threats for the structural integrity of the offshore pipelines in the territories that floating icebergs can reach. The safe and cost-effective burial depth of pipelines for protection against the iceberg attack depends on accurate assessment of the subgouge soil deformation and the ice keel-seabed contact forces. Layered seabed soil strata that are found in many geographical locations adds to the complexity of assessing the ice keel-seabed interaction process by signifying the effect of strain rate dependence and strainsoftening in the soil layers interaction areas and ice-seabed contact zone. The existing design codes simplify the seabed condition by assuming a uniform soil strata. This in turn affects the accuracy and cost-effectiveness of the design. In this paper, large deformation finite element (LDFE) analysis was conducted using Coupled Eulerian Lagrangian (CEL) approach to investigate the free-field ice gouging in layered (soft over stiff clay) cohesive seabed. A modified Tresca soil model was coded in a user subroutine to account for the effects of strain rate and strain-softening on the mobilised undrained shear strength. A comprehensive parametric study was conducted to examine the effect of gouging features and soil shear strength profiles on the subgouge soil deformation, developed plastic shear strains and the resultant ice keel reaction forces. The analysis results showed that the seabed response to ice gouging is heavily dependent on the soil strata, where the layers interaction effects can override the usual response expected from a uniform seabed. Practical solutions were proposed to improve the pipeline design in layered seabed soils.

Application of kinetic pyrolysis models to the analysis of flax chipboards under fire

This study presents a three-dimensional finite element model to describe the thermomechanical behaviour of flax chipboards under fire conditions. The model is based on kinetic models considering the thermal degradation during the pyrolysis phase and the evolution of the physico-mechanical properties as functions of temperature. The numerical model is integrated into Abaqus via user subroutines (Umat and Umatht) and applied to the analysis of the fire behaviour of panels made of flax chipboards. Thermogravimetric tests are performed on flax particles to serve for the identification of the kinetic parameters of the pyrolysis models. Once these kinetic parameters are determined, they are integrated into a complete numerical model to simulate the behaviour under fire of flax chipboards on small and large scales. The obtained trends in the predicted values indicate good agreements when compared to the measured values. The simulations show that the numerical model is capable of accurately modelling the thermomechanical transfers taking place within the material during exposure to fire.