User Subroutine Research Articles

Numerical implementation of BBM in FE package for solving unsaturated soil boundary value problems

Unsaturated soil mechanics is of keen relevance when dealing with soil above the water table orcompacted soils. Barcelona Basic Model (BBM) is one of the most widely used constitutive models for describing unsaturated soil behaviour and is available as a soil model in software like PLAXIS and CODE_BRIGHT (developed by UPC). The present study demonstrates a numerical framework to implement BBM within the ABAQUS package as a user-defined soil model using a user subroutine called UMAT (User Material). The backward Euler integration scheme coupled with Newton Raphson iterative algorithm is used to construct the model. A convergence test is conducted to check the accuracy and stability of the integration. UMAT's inability to manage pore pressure led to proxy diffusion to depict suction variation. The heat equation is used as a proxy for diffusion in three-dimensional space and time. Anoedometer test is simulated in which the soil is allowed to consolidate and then set to saturate at constant vertical stress, which leads to wetting collapse. The results demonstrate a reasonable behaviour of wettinginduced compression of soil subjected to loading with varying saturation level.

Finite Element Implementation of the Exponential Drucker–Prager Plasticity Model for Adhesive Joints

This paper deals with the numerical implementation of the exponential Drucker-Parger plasticity model in the commercial finite element software, ABAQUS, via user subroutine UMAT for adhesive joint simulations. The influence of hydrostatic pressure on adhesive strength was investigated by a modified Arcan fixture designed particularly to induce a different state of hydrostatic pressure within an adhesive layer. The developed user subroutine UMAT, which utilizes an associated plastic flow during a plastic deformation, can provide a good agreement between the simulations and the experimental data. Better numerical stability at highly positive hydrostatic pressure loads for a very high order of exponential function can also be achieved compared to when a non-associated flow is used.

Numerical simulation of composite grid sandwich structure under low-velocity impact

Abstract The low-velocity impact finite element model of the carbon fiber-reinforced composite grid sandwich structure was established by ABAQUS. Its panels and grid are both carbon fiber-reinforced composite laminates. The constitutive relation of composite laminates is written into the VUMAT user subroutine using the Fortran language. Simulation of intralaminar failure behavior of composite laminates using the three-dimensional Hashin failure criterion. The quadratic stress criterion and the B-K energy criterion were used to simulate the interlaminar failure behavior, and the delamination damage of the composite panel and the interface debonding damage were simulated. The finite element models of four different types of composite grid sandwich structures, including quadrilateral configuration, triangular configuration, mixed configuration, and diamond configuration, were established. The influence of the single grid width and the height of the grid on the impact resistance of each composite grid configuration was studied. Compared with other geometric configurations, triangular grid sandwich structure provides the best energy absorption characteristics, and T-6-10 has the highest fracture absorption energy (15816.46 mJ). The damage propagation law of carbon fiber-reinforced composite grid sandwich structure under impact load is analyzed.

Investigating the impact of different machinability processes and fibre architecture on the bearing performance of pin-loaded textile structural composites for automotive applications: Experimental and finite element analysis

The present study aims to investigate the effect of different machinability processes such as drilling, abrasive water jet machining (AWJM), and laser beam machining (LBM) along with different fibre architectures on the behavior of machined hole, bearing strength (joint performance), and failure mechanism of different textile fibre-reinforced structural composites (TFRSC) fabricated using glass, basalt, and sisal fibres suitable for automotive applications. Different structural preforms such as chopped fibre, unidirectional (UD), bidirectional (2D), and three-dimensional (3D) woven orthogonal structures were developed and subsequently converted to their respective composite forms using the vacuum-assisted resin transfer molding (VARTM) process. Morphological damage evaluation and fractography of the developed composite materials were carried out by optical microscopic analysis. The experimental observations revealed that the basalt fibre reinforced textile structural composite (BFRTSC) specimens exhibited the highest bearing strength compared to glass fibre reinforced textile structural composites (GFRTSC) and sisal fibre reinforced textile structural composites (SFRTSC) for all the processes. The bearing strength of composites was in the order of 3D > chopped > 2D > UD, respectively. Additionally, a novel methodical mechanics-based approach was introduced to develop the finite element modeling (FEM) mesoscale model using SOLIDWORKS to study the bearing response of pin-loaded TFRSC. The well-established 3D Hashin's failure and Puck's failure models were used to predict the woven-based composite material damage modes, and it was implemented through a user subroutine along with LS-DYNA. The bearing response predicted by the FEM simulation was found in a good agreement with the experimental observations.

Motion Analysis of A Wind-Wave Energy TLP Platform Considering Second-order Wave Forces

Offshore wind energy has become a major energy source, and various studies are underway to increase the economic feasibility of floating offshore wind turbines (FOWT). In this study, the characteristics of wave-induced motion of a combined wind-wave energy platform were analyzed to reduce the variability of energy extraction. A user subroutine was developed, and numerical analysis was performed in connection with the ANSYS-AQWA hydrodynamic program in the time domain. A platform combining the TLP-type FOWT and the Wavestar-type wave energy converter (WEC) was proposed. Each motion response of the platform on the second-order wave load, the effect of WEC attachment and Power take-off (PTO) force were analyzed. The mooring line tension according to the installation location was also analyzed. The vertical motion of a single FOWT was increased approximately three times due to the second-order sum-frequency wave load. The PTO force of the WEC played as a vertical motion damper for the combined platform. The tension of the mooring lines in front of the incident wave direction was dominantly affected by the pitch of the platform, and the mooring lines located at the side of the platform were mainly affected by the heave of the platform.

Linear woodcutting of European beech: experiments and computations

Hardwood species are becoming increasingly important with the growing need for a diversity of forests that have recently been facing global temperature changes or conifer pests. This further leads to the growth of its potential as a building material that may originate from sustainable production. As hardwoods need to be properly processed, the article deals with the disintegration of European beech. The influence of wood grain direction, uncut chip thickness and cutting speed on the cutting force magnitudes was experimentally investigated using the device with a rotating arm of approximately 4 m in diameter. Then, the disintegration process was modelled using the finite element method in Abaqus/Explicit. The developed material model consisting of orthotropic elasticity and plasticity with rate-independent and rate-dependent tensile–compressive failure asymmetry was implemented through the user subroutine, while the crack initiation and propagation were realized using the element deletion technique. The computationally predicted average values of cutting forces and chip shapes were, except for a few tests, in good agreement with the experiments. It means that the model may be used for further investigation, such as the influence of tool wear.

Progressive failure analysis and burst mode study of type IV composite vessels

ABSTRACT The lightweight high-pressure hydrogen storage vessels composed of polymer liner and composite reinforced layers have attracted more and more attention. In this research, a 2D axisymmetric finite element model was proposed to predict the damage evolution, failure strength and failure mode of the composite overwrapped pressure vessels, in which a user subroutine UMAT based on Hashin failure criteria was introduced to simulate the progressive damage properties of the composite material. The FE models are validated by comparison with the experimental results, and the error between the numerical simulation results and the experimental results was within 10%. It is found that vessels with different lay-up schemes might burst in different locations. Parametric studies are performed to obtain a critical ratio of hoop and helical layers to keep the vessel burst in safe mode.

Fatigue Crack Propagation Study of Bridge Steel Q345qD Based on XFEM Considering the Influence of the Stress Ratio

In the past, fatigue cracks have appeared in the orthotropic steel decks of bridges shortly after they opened to traffic. Previous studies have shown that high tensile welding residual stress exists in welded joints of steel bridges, which significantly changes the average stress and stress ratio of the joints. However, traditional fatigue crack propagation (FCP) calculations based on the Paris equation do not consider the influence of the stress ratio. Steel Q345qD is a common material used in bridges. Therefore, it is meaningful to study the influence of the stress ratio on the FCP life of steel Q345qD. In this paper, an FCP equation based on the energy release rate considering the influence of the stress ratio is first derived and named the da/dN-ΔG-R equation. Next, three material parameters in the equation are determined based on the results from tests of steel Q345qD under different stress ratios. Then, a user subroutine based on the extended finite element method (XFEM) that considers the influence of the stress ratio is defined and the effects of mesh size are analyzed. Finally, the effects of the stress ratio on FCP are discussed and the adaptability of the da/dN-ΔG-R equation is verified by comparing the values obtained from the equation with experimental results. The results show that: with a 95% guarantee rate, three material parameters in the da/dN-ΔG-R equation are: log(C) = −10.71, m = 2.780, and γ = 0.957; in the numerical simulation, a mesh size of 1 mm is more appropriate than other mesh sizes as it shows better accuracy and efficiency; under the same energy release rate range, the crack growth rate decreases as the stress ratio increases; under the same loading amplitude and cycles, the fatigue life decreases as the stress ratio increases; and finally, the numerical results considering the influence of stress ratio based on the da/dN-ΔG-R equation are close to the test results, while the results without considering the stress ratio based on the Paris equation are inaccurate.

Comparison of two CDM-based models for necking and fracture limits of metal sheets under hot stamping conditions

A unified viscoplastic constitutive model based on continuum damage mechanics (CDM) has been recently developed for the prediction of forming limit curves (FLCs) and fracture forming limit curves (FFLCs). In this CDM model, two damage variables which are strain path dependent (so called strain-based model) have been introduced to model the necking and fracture limits in sheet metal forming. In this study, the two damage variables have been modified by replacing the strain components with stress components to model stress path dependence (so called stress-based model). Then the two sets of CDM models have been analysed and compared in respect to their computational accuracy and efficiency. For this purpose, all the material constants in the models have been calibrated using the recently published data of 22MnB5 boron steel sheet under hot stamping conditions. Subsequently, the two models together with the calibrated material constants have been implemented into the commercial software ABAQUS using a user subroutine VUMAT, and applied to biaxial tests for the computation of both the necking and the fracture limit strains at hot stamping temperatures. Computational results show that there is little difference between the two CDM models with respect to the computed limit strain values, but the strain-based CDM model is more time-efficient.

Influence of key-slot structure on interior ballistic performance of low-velocity aircraft

To investigate the interior ballistic performance of the cooperation of the guide slot and the guide key at a low velocity, a pressure-structure coupling method was put forward to obtain the effect of different structures. A lumped parameter model of an interior ballistic was utilized to govern the propellant combustion. The mechanical behaviors were predicted based on the engineering software ABAQUS. We coupled the interior ballistic parameters and the structures through moving boundaries by using a user subroutine interface VUAMP in ABAQUS. The results show that the coupling approach can accurately predict the movement process of the aircraft in the barrel, and different numbers of slots and different guide key structures were simulated. The fewer the slots, the smaller the resistance of the aircraft, but the corresponding guiding performance is poor. Inversely, more slots lead to the better guiding performance. Furthermore, different guide key structures have a greater impact on the exit velocity of the aircraft, but have little effect on the guiding performance. The guide key with edge rounding-off in side is a better structure case, which can obtain a stable rotational velocity and high launch speed on the premise of ensuring rigidity. The result can be applied to provide an important reference in determining reasonable numbers of slots and guide key structures for the application in engineering.

Modelling of ductile damage in single point incremental forming process using enhanced CDM model

Investigation of the ductile damage in single point incremental forming (SPIF) is necessary for improving the formability during the manufacturing process. In this paper, an enhanced Continuum Damage Mechanics (CDM) model was applied for ductile damage prediction in SPIF process. In particular, the stress state dependence was considered in the damage evolution to improve fracture prediction capability at various loading paths. The enhanced CDM model was implemented into finite element code ABAQUS/Explicit through user subroutine VUMAT. The accuracy of the finite element model to predict the failure during SPIF process was investigated, good agreements were found between the simulation results and the experimental observations.

Coupled CEL-FDEM modeling of rock failure induced by high-pressure water jet

The coupled Euler-Lagrange (CEL) method and finite-discrete element method (FDEM) are adopted to simulate the rock fracture process induced by high-pressure water jet and to solve the problems of highly nonlinear, fluid-solid coupling and large deformation. The water is characterized by the CEL model, and water can flow freely in the Euler mesh, which can better simulate the actual shape of water jets. The rock is integrated into the FEDM model, which is produced by embedding zero-thickness cohesive elements. The traction-separation criterion of the cohesive element is used to simulate the rock damage. In the simulation process of rock fracture by the water jet, infinite elements are used to realize the non-reflection boundary conditions, simulate real working conditions, and improve the calculation reliability. Furthermore, the VUSDFLD user subroutine is used to delete the elements outside the box and reduce the impact of the fallen rock debris on simulation speed, thus improving calculation efficiency. The formation process of rock impact pits and slits, as well as the velocity and distribution of water jets, are studied by comparing the simulation and experiments of water jet fixed-point impact and moving cutting. Subsequently, the fracture mechanism of rock under the water jet is revealed, presenting new possibilities for studies into rock fractures by high-pressure water jet.

A Multiaxial Fatigue Damage Model Based on Constant Life Diagrams for Polymer Fiber-Reinforced Laminates.

In the last decade, fatigue damage models for fiber-reinforced polymer composites have been developed assuming the fracture energy equivalence hypothesis. These models are able to predict a fatigue life of composite laminates, but their identification requires a significant number of off-axial tests for various stress ratios. The present study proposes the stress ratio dependent model, which phenomenologically adopts a decrease in stiffness and residual strength of a unique ply according to experimental constant life diagrams. Hashin, Tsai-Hill, and the maximum stress failure criteria are utilized for damage initiation considering the residual strength of the ply. The obtained results indicate a sufficiency of using S-N curves for UD 0°, UD 45°, and UD 90° for identification of the model. The model was verified by S-N curves for UD 10°, UD 15°, and UD 30° and its applicability was demonstrated for prediction of a fatigue life of composite laminates with an arbitrary lay-up. The model is implemented into ABAQUS finite element software as a user subroutine.

Experimental study on fatigue behavior of notched concrete beams with CFRP external bonding

This paper investigated the low cycle fatigue and post-fatigue behavior of the interface between CFRP and concrete by the four-point loading on notched beams with CFRP fabric external bonding. Twelve specimens were subjected to fatigue tests at the stress levels of 0.70, 0.75, 0.80, and 0.85, respectively, to investigate low cycle fatigue behavior. Nine specimens were subjected to fatigue load at the stress levels of 0.70, 0.75, and 0.80, respectively, then were subjected to monotonic loading tests to investigate post-fatigue behavior. Based on the bilinear cohesion model, a FE model was established to simulate the low cycle fatigue behavior of the CFRP-concrete interface through the user subroutine of Abaqus, and the FE results were checked against the test results to verify the model's accuracy. The test results showed that the failure mode of the specimens was that the concrete near the notch cracked while the CFRP peeled off. The fatigue life decreased with the increase in the stress level, and the logarithm of the fatigue life was linear with the stress level. Fatigue loading would cause specimens' CFRP debonding, and the monotonic ultimate bearing capacity of the specimen decreased when the CFRP residual bond length was less than the effective bond length, while that remained unchanged when the CFRP residual bond length was not less than the effective bond length.

A physics-based life prediction model of HP40Nb heat-resistant alloy in a coupled creep-carburisation environment

This paper reports the development of a physics-based life prediction model that incorporates three micro-damage processes. Both the particle area fraction and size, as determined using scanning electron microscopy, are used as the model input to predict the precipitate coarsening damage. Moreover, the carbon concentration profile, as measured using electron probe micro-analyser, are used to derive the carburisation damage distribution index. Compared to the Kowalewski-Dyson model, the present model can predict the stress dependent creep fracture strain and rupture time. The model, compiled into finite element via the user subroutine, can predict the heterogeneous damage accumulation of the cracking furnace tube made of HP40Nb alloy in a coupled creep-carburisation environment. The predicted total damage is comprised of precipitate coarsening and carburisation as the predominance, while the creep cavitation contributing much less due to the low internal pressure. The distribution of the precipitate coarsening is relatively homogeneous through the thickness, whereas the carburisation causes a steep increased damage value close to the inner surface. With regard to time, the precipitate coarsening damage is predominant in the initial stage, while the total damage is governed by the carburisation process with further exposure. Eventually, the total damage reaches the critical value at 59600 h, with the thickness of carburised layer exceeding 50% of the tube thickness, and the tube fails in the inner surface which agrees with the operational experience. The parametric sensitivity study reveals the importance of determining the Cr-rich carbide area fraction for the life prediction.

Understanding tack behaviour during prepreg-based composites’ processing

Prepreg tack is a critical material property that plays a key role in Automated Fibre Placement (AFP) manufacture-induced defect generation. However, there is a gap in current state-of-the-art manufacturing process modelling that includes tack phenomena and experimental data that shows the influence of multiple factors. In this paper, a modified probe test method, which is capable of inter-ply and ply-tool tack measurement, is proposed. The influence of multiple factors, consistent with the AFP deposition process (i.e. pull-off rate, contact time, pressure and temperature) on tack is studied. Further, a comprehensive modelling framework accounting for all the factors is developed and implemented as a user subroutine for a commercial finite element (FE) package. The work highlights the complexity of tack behaviour at play and shows promise for filling the gap between the tack data/model scarcity and simulation of manufacturing processes such as AFP and forming.

An innovative constitutive material model for predicting high temperature flow behaviour of inconel 625 alloy

Inconel 625 nickel alloy with its attractive high-temperature strength, excellent corrosion and oxidation resistance is mainly used for critical applications in demanding environments, in both as-cast and wrought conditions. Hot processing of this alloy is crucial for achieving its tailored mechanical properties due to the significant variation in microstructural changes with varying process parameters like temperature, strain, and strain rate. In this study, isothermal hot compression tests were carried out at temperatures ranging from 900 to 1100 °C, and under strain rates ranging from 0.01 to 1 s−1. The flow curves revealed three stages of deformation, including a substantial work-hardening stage followed by dynamic recovery and flow softening. Microstructural observations showed the occurrence of discontinuous dynamic recrystallisation (DDRX) as the dominant recrystallisation mechanism during the flow softening. Microstructural analysis suggested that the DRX was more sensitive to the test temperature as compared to the strain rate. An innovative material's constitutive model was developed, by combining Johnson–Cook (JC) and Avrami approaches, to predict work-hardening, dynamic recovery, and flow softening stages of deformation. The predicted flow behavior was in a good agreement with the experimentally measured data. The developed material model was integrated into DEFORM® 3D finite element (FE) simulation software as a user subroutine for the prediction of deformation behaviour in a double truncated cone (DTC) sample. Comparison between the experimentally measured data and the results of FE simulation on the DTC sample showed a very good convergence, indicating the suitability of the proposed material’s constitutive model for large scale simulations.Graphical

Study on dynamic recrystallization-based microstructure evolution mechanism of 40Cr during strengthening grinding

Grind-strengthening represents a novel surface modification process that combines a precision removal process of material and the strengthening function of surface modification. The high heat and severe plastic deformation (SPD) of the ground superficial layer can induce the phase transformation and dynamic recrystallization (DRX) grain refinement behavior. Consequently, the prediction and control of the ground superficial layer microstructure is of considerable significance to improve the wear resistance and fatigue resistance performance of parts. Firstly, the dynamic mechanical properties at high temperature and shock compression were tested by the split Hopkinson pressure bar (SHPB) experiment for 40Cr steel. The material's constitutive relations, critical strain and DRX model can be determined. Subsequently, the finite element (FE) method was proposed to predict the temperature and strain distribution of the ground strengthening layer. The DRX volume fraction and grain size of the ground superficial layer can be realized via Abaqus/explicit user subroutine VUHARD. Finally, a phenomenological simulation of the ground strengthening layer microstructure is realized utilizing the cellular automata (CA) method combined with FE analysis results. With the help of grind-strengthening experiments, the gradient distribution of grain refinement layer at different cutting depths is consistent with the prediction results. The proposed DRX and microstructure predicted model can provide theoretical support for the surface strengthening of steel grinding. The modification behavior of the gradient refinement induced by grind-strengthening has significant application potential in the fields of transmission parts such as gears and shafts.

Effects of fracture models on structural damage and acceleration in naval ships due to underwater explosions

This study examined the structural damage quantitively due to a near-field underwater explosion (UNDEX) using a more reasonable fracture model. Technical backgrounds of the Geers-Hunter UNDEX model and combined HC-LN fracture model based on Hosford-Coulomb (HC) and localized necking (LN) were reviewed. A finite element analysis (FEA) model for which the UNDEX experiment was performed was generated. Two fracture models were applied to the FEA model: constant strain and HC-LN fracture models. A user subroutine was developed to implement the HC-LN fracture model. The material constants for the aluminum alloy that was the model ship material were calculated using tensile tests and their FEAs. The HC-LN fracture model predicted a similar extent of damage to that in the experiment. The constant strain fracture model predicted an unrealistic fracture in the bottom shells that reduced the hull girder bending capacity and thus the increased acceleration history. A damage extent can be estimated accurately only by UNDEX FEAs with a more reasonable fracture model.

Study on off-center impact behavior and damage characterization of carbon fiber reinforced aluminum laminate

This paper aims to explore the off-center low-velocity impact response and damage characteristics of carbon fiber reinforced aluminum laminate (CARALL). Progressive damage finite element models of CARALL considering the stiffness degradation and damage evolution of carbon fiber reinforced plastic (CFRP) were established using a user subroutine and verified by off-center impact tests with different impact energies. The motion propagation, damage, and spatial distribution of dynamic responses of CARALL impacted in different locations are studied. The results indicate that increase in impact distance from center dissipates more energy due to the more severe damage and failure of CFRP, while the off-center impact have a negative effect on the plastic energy absorption of metal compared with center impact. A quantitative expression is proposed to describe the spatial distribution of the maximum transverse displacement.