Large Deformation Region Research Articles

An adaptively coupled DEM–FEM algorithm for geometrical large deformation analysis of member structures

An adaptively coupled discrete element method (DEM)–finite element method (FEM) algorithm is proposed to utilize the respective DEM and FEM advantages. The key problems of the algorithm, such as meeting consistency requirements, determining the adaptive criterion, transferring information between the DEM and FEM and dynamic coupled interface processing, are examined in detail. The adaptively coupled algorithm scheme is formulated. The proposed algorithm is subsequently verified by examples. In the algorithm, the identification of large deformation regions is an automatic process, and the DEM and FEM calculation domains can be changed interactively in every load step in real time according to the structural response. When the response of an arbitrary element in the FEM domain exceeds the predetermined adaptive criterion, the element will be moved to the DEM domain and analysed using the DEM. Similarly, when the response of an arbitrary element in the DEM domain is smaller than the predetermined adaptive criterion, the element will be moved to the FEM domain and analysed using the FEM. The transition of elements between the two domains achieves the objective of using the DEM for large deformation regions and the FEM for small deformation regions.

The failure mechanism at adiabatic shear bands of titanium alloy: High-precision survey using precession electron diffraction and geometrically necessary dislocation density calculation

The difficulty of obtaining key information, such as the crystal orientation and geometrically necessary dislocation (GND) density distribution, from a large deformation region in an adiabatic shear band (ASB) has hindered further study of the ASB failure mechanism in titanium alloys. In this work, the crystal orientation information of the failure position and surrounding region in ASB of a Ti-5Al-2.5Cr-0.5Fe-4.5Mo-1Sn-2Zr-3Zn alloy was obtained via transmission electron microscopy (TEM) and precession electron diffraction (PED) with a high spatial resolution. The GND density distribution was calculated and the adiabatic shear failure mechanism was revealed from the collected data. ASBs (original width: ∼3–4 µm) formed in the cylinder sample during dynamic compression (strain rate: ∼3000 s−1). In the transition region at the edge of ASB, the grains were severely elongated along the direction parallel to the ASB boundaries with an average grain size on the order of μm and an average GND density of 5.5374 × 1015/m2 in the α phase. Relatively strong < 0001 > textures and weak < -12–10 > textures were observed. However, significant dynamic-recrystallization-dominated grain refinement in ASB, especially at the crack tip, resulted in many ultrafine equiaxed recrystallized grains (10 nm level) and weakened textures. The highest average GND density (8.6242 × 1015/m2, in α phase) occurred in the crack tip region. Moreover, the vicinity of the "primary microcracks" in the main crack extension line was characterized by combinations of an extremely high GND density work hardening region and a group of low GND density recrystallized grains. This indicates that cracks in the ASB were initiated by the deformation incompatibility between the antecedent recrystallization region and the surrounding high work hardening region.

Prevention of network destruction of partially hydrolyzed polyacrylamide (HPAM): Effects of salt, temperature, and fumed silica nanoparticles

Polymer flooding is one of the most effective enhanced oil recovery (EOR) methods. High temperature and a high salt content in oil reservoirs significantly decrease the performance of polymer flooding. In this work, the viscoelastic properties of a partially hydrolyzed polyacrylamide (HPAM) solution with and without salt (NaCl) and at two different temperatures (35 °C and 70 °C) were evaluated using rheological approaches. Two fumed silica nanoparticles (NPs) featuring different surface chemistries were used, and their ability to prevent destruction of the polymer network structure against salt addition and temperature increase was investigated. Linear rheological tests (frequency sweep, creep, and creep recovery) and nonlinear rheological tests (large amplitude oscillatory shear) were employed to evaluate the network structure of these systems. The results showed that either adding salt or increasing the temperature destroyed the mechanical integrity of the HPAM 3-dimensional elastic network. However, the introduction of both types of NPs at a sufficient concentration maintained the network structure of HPAM solutions in the small deformation region. In the large deformation region, it was shown that the extent of intra-cycle shear-thickening behavior in the HPAM solution (T = 35 °C and without any salt) decreased by incorporating salt or by increasing the temperature. Moreover, upon incorporating either of the NPs to the HPAM solution, the nonlinear viscoelastic behavior dramatically changed, and the critical strain (linear to nonlinear transition) decreased to a much lower strain amplitude. The outcomes of this study will help petroleum scientists to design more efficient EOR methods.

Experimental, numerical, and theoretical studies of the response of short cylindrical stainless steel tubes under lateral air blast loading

This paper presents a series of experimental, numerical and theoretical analyses of cylindrical tubes subjected to lateral external air blast loading. The cylindrical tubes are laterally loaded in the mid span using cylindrical TNT with a charge mass of 20, 25, 30, and 40 g detonated at a stand-off distance of 50, 100, and 150 mm. The resultant impulses produce failures ranging from large inelastic deformation of the cylindrical tube to tearing of the structure. The results are compared with a modal solution and numerical simulations carried out in ABAQUS 6.12-1, where good correlations with the numerical simulations are observed. The experimentally obtained deformed cross-section can be divided into three parts: a flat top section, two arcs near the flat top and an undeformed circular arc at the bottom. The axial deformation section can also be divided into three parts: the first part is a large plastic deformation region at the center of the structure that divides two outward moving plastic hinges from the second part, which can be defined as a rigid section moving around the plastic hinge at the support, and the third part is the undeformed boundary region. The above deformation patterns were used to develop a theoretical model based on a rigid-plastic analysis. Since the experimental pressure of the cylindrical explosive was more localized than the theoretically defined pressure distribution, the modal solution together with the upper bound yield condition of the tube used in the analysis, overestimated the resistance of the tube.

Combination of DEM/FEM for Progressive Collapse Simulation of Domes Under Earthquake Action

In this paper, a coupled DEM/FEM model is presented to overcome the disadvantages generated by individually using discrete element method (DEM) or finite element method (FEM). The model divides the domain into two independent parts: the DEM sub-domain and the FEM sub-domain. First, the governing equations of the coupled model are established according to the virtual work equation and variational principle, the formula of interface-coupled force is derived based on the penalty function method, and the corresponding program is coded. Second, the key problems involved in the coupled model are investigated, which are the selection of the penalty parameter, the determination of the time-step, and the formulation of the governing equations of the system in the DEM and FEM domains. Finally, the coupled model is applied to progressive collapse simulation of single-layer reticulated domes subjected to severe earthquakes. Both the collapse process and failure mechanism of the numerical results are consistent with the shaking table test phenomena, thereby verifying the validity of the coupled model proposed. The method proposed realizes the balance between computational efficiency and accuracy, that is, the small deformation region is simulated by FEM, while the DEM is applied to discretize the large deformation or failure region. Additionally, it is of high computational efficiency and of reliable precision.

Spin singlet and spin triplet pairing correlations on shape evolution in sd -shell N=Z Nuclei

We study the shape evolution of $N=Z$ nuclei $^{24}\mathrm{Mg},^{28}\mathrm{Si}$, and $^{32}\mathrm{S}$ in the axially symmetric deformed Woods-Saxon model, taking into account both $T=0$ and $T=1$ pairing interactions. We find the coexistence of $T=0$ and $T=1$ superfluidity phases in the large deformation region $|{\ensuremath{\beta}}_{2}|&gt;0.3$ in these three nuclei. The interplay between the two pairing interactions has an important effect on determining the deformation of the ground states in these nuclei. The self-energy contributions from the pairing correlations to the single particle (s.p.) energies are also examined.

Experimental Nonlinear Model Identification of a Highly Nonlinear Resonator

In this work, two model identification methods are used to estimate the nonlinear large deformation behavior of a nonlinear resonator in the time and frequency domains. A doubly clamped beam with a slender geometry carrying a central intraspan mass when subject to a transverse excitation is used as the highly nonlinear resonator. A nonlinear Duffing equation has been used to represent the system for which the main source of nonlinearity arises from large midplane stretching. The first model identification technique uses the free vibration of the system and the Hilbert transform (HT) to identify a nonlinear force–displacement relationship in the large deformation region. The second method uses the frequency response of the system at various base accelerations to relate the maximum resonance frequency to the nonlinear parameter arising from the centerline extensibility. Experiments were conducted using the doubly clamped slender beam and an electrodynamic shaker to identify the model parameters of the system using both of the identification techniques. It was found that both methods produced near identical model parameters; an excellent agreement between theory and experiments was obtained using either of the identification techniques. This follows that two different model identification techniques in the time and frequency domains can be employed to accurately predict the nonlinear response of a highly nonlinear resonator.

Rheological analysis of core-stabilized Pluronic F127 by semi-interpenetrating network (sIPN) in aqueous solution

A poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic F127) micelle system was stabilized using an ultraviolet-induced semi-interpenetrating network (sIPN). The sIPN structure within the micelle cores was found to stabilize the micelles against low temperatures, but affected the resulting material properties. In this study, the rheological properties of Pluronic F127 with sIPN (F127-sIPN) and without sIPN (F127) were compared. The presence of the sIPN structure increased the gelation temperature (Tgel) at the same concentration, and unlike F127, F127-sIPN exhibited strong heating rate dependent and thermodynamically irreversible behaviors. Hard gels containing various concentrations of F127-sIPN and F127 were investigated at 40 °C. At concentrations above 18 wt. %, both F127-sIPN and F127 exhibited similar linear viscoelastic properties due to the tight, ordered core–shell micelles packing, but the two systems exhibited different behaviors below 18 wt. % concentration. To investigate this difference, hard gels with 16 wt. % F127-sIPN and F127 were selected, and two types of nonlinear rheological tests were conducted, i.e., large amplitude oscillatory shear (LAOS) and strain-rate frequency superposition (SRFS) tests. The cage modulus of F127-sIPN obtained from LAOS testing showed it maintained its elastic contribution over the large deformation region meaning that a loose core network still existed. The relaxation time spectrum of F127-sIPN obtained by SRFS testing indicated it had two relaxation modes (fast and slow) whereas that for F127 had only a fast mode. The slow relaxation mode of F127-sIPN is associated with crosslinking of the sIPN. Since these behaviors were not observed in linear rheological tests, it was concluded that nonlinear rheological tests provide more structural information about hard gels.

Large Indentation Method to Measure Elasticity of Cell in Robot-Integrated Microfluidic Chip

Robot-integrated microfluidic chip is a promising tool to realize mechanical characterization of a single cell with high throughput and high accuracy. The microfluidic chip used in our system has a pair of mechanical microprobes to measure deformation and reaction force of cells. In our previous studies, we measured the elasticity of cells in this measurement system. However, there were some problems on the measurement method limited in small deformation regions of cell. To avoid these problems, we changed the measurement method to utilize large deformation regions of cell. First, we proposed a differential-type sampling moire method to extend the range of the force or displacement measurement. Second, the mechanical model of the cell was improved to express the deformation characteristics up to large deformation region. The deformation model was derived from high-molecular theory. Therefore, the measured elasticity was related to the intracellular filament network. As a result, we accomplished in measuring the elasticity of the cells using the experimental data in a large deformation region. This study provided us with a more practical and reliable method to measure cellular elasticity.

Experimental and numerical analysis of micromechanical damage for DP600 steel in fine-blanking process

In fine-blanking process, metallographic inspection shows that highly elongated and densely packed grains extend along the shear direction within a narrow large deformation region, meanwhile micro-cracks initiate and grow up. By taking the effect of shear loading into account, the recent modified GTN model proposed in 2014 is implemented and validated. The shear damage model combines the damage mechanics concept with the porous plasticity model and is applied to simulate fine-blanking process to verify its accuracy in predicting material damage and tearing fracture. The formation and propagation of shear band in fine-blanking process are studied. Both numerical results and experimental observations reveals that the effect of shear damage plays a significant role in inducing tearing, which eventually leads to fracture failure in the fine-blanking process.

大ひずみ領域における金属コア圧電ファイバ／アルミニウム複合材料 の出力電圧特性

Large deformation in structures occurs by disasters such as earthquakes, the lifetime of the structures is degraded by the deformation. Structural Health Monitoring (SHM) technique, structural soundness monitoring by measuring the deformation using sensors attached to the structures, has attracted attention in recent years. In SHM technique, output behavior at large deformation region and robustness of the sensor are very important. In this research, output voltage characteristic in large strain region of metal-core piezoelectric fiber/aluminum composite was investigated by tensile vibration test. As a result, output voltage generated from the composite was decreased with increasing applied strain. The output voltage decreasing was occurred by reduction of stress transfer between matrix and piezoelectric layer of the fiber because of the fracture of the piezoelectric layer by applying large strain. However, the output voltage was verified at about 20% strain, the composite demonstrated very high robustness compared to conventional piezoelectric ceramics, and it shows that the composite was expected to apply to the SHM technique.

Trimming line development method of auto panel part with undercutting flange

In most of auto-panel manufacturing, trimming is generally performed before flanging. Therefore, the acquisition of the optimal trimming line is one of the keys to obtain an accurate shape of a product. It is not easy to obtain a reasonable trimming line for auto panel part with undercutting flange. Analytical models can only deal with some simple stretch and shrink flanging parts. Section-based method unfolds flange along part outer boundary by defining many section planes. This method may produce inaccurate results for large deformation regions. The incremental simulation based method can generate more accurate trimming line by iterative strategy. However, it is still not widely accepted in the auto-industry since limitation of time and lack of information in the initial die design stage. In this study, a novel fast method to find feasible trimming line is proposed. One-step inverse finite element method (FEM) is used to analyze the flanging process because the strain paths are simple in flanging. The most difficult task of one-step inverse FEM is the generation of initial guess mesh. Robust generation method of initial guess mesh is presented specially to handle the undercutting flange regions. The new geometry method develops the final 3D part mesh with undercutting flange in triangle mesh parameterization way onto the drawing tool surface. The 3D mesh smoothing method with sliding constraint surface technique is utilized to smooth mesh distortion or overstretching after development. Finally, the 3D trimming line is extracted from the outer boundary after one-step inverse FEM simulation. The present trimming line calculation method is successfully applied to the complex industrial applications such as decklid and front fender. This method shows many benefits since trimming line can be obtained in the early die design stage.

A Coupled Level Set and Particle-In-Cell Method for Penetration Problems

A coupled level set and Particle-In-Cell method is presented to treat the penetration problems. The coupled method has the advantages of level set method and Particle-In-Cell method, and the basic idea is that the important com- putation region such as the large deformation region where more particles are added for calculation, while the continuous transport method is used in the other regions. The steel projectile penetration into concert target is simulated by using the above-mentioned coupled method. The numerical results reveal that the adopted coupled method can effectively track the deformation in the process of penetration and also insure efficiency and precision.

Characterization and modeling of polymeric matrix under multi-axial static and dynamic loading

A polymeric foam commonly used in composite sandwich structures was characterized under multi-axial loading at strain rates varying from quasi-static to dynamic. Tests were conducted under uniaxial compression, tension, pure shear and combinations of normal and shear stresses. Quasi-static and intermediate strain rate tests were conducted in a servo-hydraulic testing machine. High strain rate tests were conducted using a split Hopkinson pressure bar (Kolsky bar) system made of polycarbonate bars having an impedance compatible to that of the foam material. The typical compressive stress-strain behavior of the polymeric foam exhibits a linear elastic region up to a yield point, a nonlinear elastic-plastic region up to an initial peak or “critical stress” corresponding to collapse initiation of the cells, followed by strain softening up to a local minimum (plateau or saddle point stress) and finally, a strain hardening region up to densification of the foam. The characteristic stresses of the stress-strain behavior vary linearly with the logarithm of strain rate. A general three-dimensional elastic-viscoplastic model, formulated in strain space, was proposed. The model expresses the multi-axial state of stress in terms of an effective stress, incorporates strain rate effects and includes the large deformation region. Stress-strain curves obtained under multi-axial loading at different strain rates were used to develop and validate the elastic-viscoplastic constitutive model. Excellent agreement was shown between model predictions and experimental results.

Plastic deformation behavior of the friction stir welded AA2024 aluminum alloy

In order to study the effect of plastic deformation on the banded texture and flashes in friction stir welding process, a three-dimensional Lagrangian incremental finite element model was developed. The plastic deformation of friction stir welding and the forming and healing of transient keyhole during the welding process were studied visually by numerical simulation and validated by corresponding experiments. The simulation results were acceptably in good agreement with the experimental data. As shown in the numerical and experimental results, the flash of FSW joint which could be divided into two layers was mainly distributed on the retreating side. The forming and healing process of transient state keyhole includes outflow of weld materials from the beginning to 1/3 rotation period and backflow into the keyhole after 2/3 period. The distribution of plastic deformation was inhomogeneous upon the transverse cross-section, while the large deformation region was near the top surface and close to the advancing side.

SU‐F‐BRF‐03: Quality Assurance of Deformable Image Registration in Radiotherapy

Purpose:To introduce methods to analyze Deformable Image Registration (DIR) and identify regions of potential DIR errors.Methods:DIR Deformable Vector Fields (DVFs) quantifying patient anatomic changes were evaluated using the Jacobian determinant and the magnitude of DVF curl as functions of tissue density and tissue type. These quantities represent local relative deformation and rotation, respectively. Large values in dense tissues can potentially identify non‐physical DVF errors. For multiple DVFs per patient, histograms and visualization of DVF differences were also considered. To demonstrate the capabilities of methods, we computed multiple DVFs for each of five Head and Neck (H' N) patients (P1–P5) via a Fast‐symmetric Demons (FSD) algorithm and via a Diffeomorphic Demons (DFD) algorithm, and show the potential to identify DVF errors.Results:Quantitative comparisons of the FSD and DFD registrations revealed &lt;0.3 cm DVF differences in &gt;99% of all voxels for P1, &gt;96% for P2, and &gt;90% of voxels for P3. While the FSD and DFD registrations were very similar for these patients, the Jacobian determinant was &gt;50% in 9–15% of soft tissue and in 3–17% of bony tissue in each of these cases. The volumes of large soft tissue deformation were consistent for all five patients using the FSD algorithm (mean 15%±4% volume), whereas DFD reduced regions of large deformation by 10% volume (785 cm3) for P4 and by 14% volume (1775 cm3) for P5. The DFD registrations resulted in fewer regions of large DVF‐curl; 50% rotations in FSD registrations averaged 209±136 cm3 in soft tissue and 10±11 cm3 in bony tissue, but using DFD these values were reduced to 42±53 cm3 and 1.1±1.5 cm3, respectively.Conclusion:Analysis of Jacobian determinant and curl as functions of tissue density can identify regions of potential DVF errors by identifying non‐physical deformations and rotations.Collaboration with Phillips Healthcare, as indicated in authorship.

Ultra-high-strain-rate shearing and deformation twinning in nanocrystalline aluminum

Nanocrystalline aluminum films with grain sizes of 50–100 nm were made through E-beam evaporation. The films were then subjected to high-rate shearing deformations with strain rates of 105–106 s−1, with local shear stresses of the order of 10 GPa. The experimental configuration is that of a compression–torsion Kolsky bar, where the specimen is a thin film (thickness 200 nm) mounted on a silicon wafer ring. Strain rates during the shearing are determined from the measured torsional waves in the bars. Site-specific TEM samples are prepared using focused ion beam micromachining to investigate the regions of large plastic deformation. Deformation twins and stacking faults are found to develop under the high-strain-rate shearing. The formation of twinning in pure aluminum with comparatively large grain sizes and the high-strain-rate promotion of twinning are discussed, and possible mechanisms are considered.

RESEARCH DEVELOPMENTS OF SMOOTHED PARTICLE HYDRODYNAMICS METHOD AND ITS COUPLING WITH FINITE ELEMENT METHOD

The merits and shortcomings are existed simultaneously in finite element method (FEM) and smoothed particle hydrodynamics (SPH) method, in which these methods are established based on Lagrange system. In order to makes use of high computational efficiency of FEM and advantages of SPH method, such as naturally simulation of large deformation of material, the coupling algorithm of FEM with SPH method is presented to calculate region of small deformation by FEM and simulate region of large deformation by SPH method. In this paper, the research and application of FEM, SPH and FEM-SPH coupling algorithm are reviewed. And the problems existing in these methods are also discussed. Finally, the computational accuracy and efficiency of FEM, SPH and FEM-SPH coupling algorithm are investigated by a numerical example, which is a reference for research fellows.

Online tracking of deformable objects under occlusion using dominant points

This paper deals with tracking of deformable objects in the presence of occlusion using dominant point representation of the boundary contour. A novel nonintegral time propagation model for propagating the dominant points is proposed. It uses an initial guess generated from a linear operation and an analytical conjugate gradient approach for online robust learning of the shape deformation and motion model. A scheme is presented to automatically detect and correct the region of large local deformation. In order to deal with occlusion, admissible restrictions on deformation and motion of the object are automatically determined. The proposed method overcomes the need of offline learning and learns the deformation and motion model of the object using very few initial frames of the input video. The performance of the method is demonstrated using varieties of videos of different objects.

Fluid-Structure Interaction Study on Diffuser Pump With a Two-Way Coupling Method

In order to study the effect of the fluid-structure interaction (FSI) on the simulation results, the external characteristics and internal flow features of a diffuser pump were analyzed with a two-way flow solid coupling method. And the static and dynamic structure analysis of the blade was also caculated with the FEA method. The steady flow field is based on Reynolds Averaged N-S equations with standard <TEX>$k-{\varepsilon}$</TEX> turbulent model, the unsteady flow field is based on the large eddy simulation, and the structure response is based on elastic transient structural dynamic equation. The results showed that the effect of FSI on the head prediction based on CFD really exists. At the same radius, the van mises stress on the nodes closed shroud and hub was larger than other nodes. A large deformation region existed near inlet side at the middle of blades. The strength of impeller satisfied the strength requirement with static stress analysis based on the fourth strength theory. The dynamic stress varied periodically with the impeller rotating. It was also found that the fundamental frequency of the dynamic stress is the rotating frequency and its harmonic frequency. The frequency of maximum stress amplitude at node 1626 was 7 times of the rotating frequency. The frequency of maximum stress amplitude at node 2328 was 14 times of the rotating frequency. No matter strength failure or fatigue failure, the root of blades near shroud is the key region to analyse.