Plastic Energy Dissipation Research Articles

Revealing the Dominant Forming Mechanism of Single Point Incremental Forming (SPIF) by Splitting Plastic Energy Dissipation

Over the years, a number of studies have been presented which investigate the forming mechanisms of single point incremental forming (SPIF). Different research groups revealed that membrane, bending or shear deformation prevail under the conditions they investigated. The current paper moves a step forward and quantifies the respective contribution of the each forming mechanisms, i.e. membrane stretching, bending, through-thickness shear, involved in the SPIF process. For this purpose, the plastic energy dissipation during the SPIF process is split up into membrane, bending and shear energy terms. A validated numerical model of the SPIF process is used to analyze the internal energy at various part locations. Using an analytical approach, this internal energy is decomposed into the membrane, bending and shear contributions. Further, a parametric study based on FE simulations is used to analyze the sensitivity of the deformation mechanisms to the SPIF process variables. At any particular location on the geometry of a part formed with SPIF, the deformation mechanism is always a combination of these three deformation modes. Dominance of one particular deformation mode over the other two is dependent on the process variables, for example, the bending mode of deformation dominates at larger tool diameters and shear dominates at increasing sheet thickness. The prevalence of each deformation modes as a function of SPIF process variable is discussed. Process outcomes and part quality in SPIF can be designed for maximum formability and geometrical accuracy, by controlling the contribution from each deformation mode through adjusting the process variables.

A unique spinning method for grain refinement: repetitive shear spinning

Abstract Grain refinement is an effective way to improve the strength of non-heat treatment aluminium alloy. To obtain much refined grains in the spun parts made of 3003 aluminium alloy, a unique spinning method, i.e., repetitive shear spinning, is proposed. The new process is composed of two shear spinning passes conducted along the two sides of sheet metal sequentially. By using finite element (FE) simulation and physical experiment, the grain refinement in the repetitive shear spinning is investigated. The results indicate that, under the same thickness reduction ratio, both the larger equivalent plastic strain and the larger shear strain can be obtained in the repetitive shear spinning than those obtained in the traditional single-pass and two-pass shear spinning processes. Meanwhile, the density of plastic dissipation energy (DPDE) in the repetitive shear spinning process is larger than that in two-pass shear spinning and doubled compared with that in single-pass shear spinning. This makes more deformation energy to be transformed into the stored energy thus provides more driving force for grain refinement. The observation of microstructure indicates that after the repetitive shear spinning, a great number of refined grains are generated with the average grain size refined from 48.6μm of the initial microstructure to 3.77μm, which is smaller than the grain size of 7.17μm and 4.91μm in single-pass and two-pass shear spinning, respectively. With the grain refinement, the micro hardness and its homogeneity of spun part after repetitive shear spinning is improved obviously. Compared with the average standard deviation of micro hardness obtained by the single-pass and two-pass shear spinning, it is decreased by 70.27% and 61.41% under the same thickness reduction ratio. The developed repetitive shear spinning process thus has the good capability for grain refinement and enhancement of mechanical properties of the spun parts and the promising application potential in industries.

Low velocity bending impact behavior of foam core sandwich beams: Experimental

Abstract This experimental study addresses the low velocity impact bending response of sandwich beams with expanded polystyrene (EPS) foam core reinforced by aluminum face-sheets using adhesive bonding technique. The deformation behaviour and the effects of the geometric and mechanical design parameters were investigated to improve the impact energy absorbing capability. The bending impact tests were performed for different foam density, foam and plate thicknesses for three impact energy levels: 4.45 13.05, 26.78 J. The photographs of the permanent deformation geometries of the sandwich beams were presented and the contact force and kinetic energy histories were discussed. The specimens with higher foam core density and thicker foam core resulted in the lowest permanent central deflections and contact force variations. As the foam core density and thickness were increased, the energy absorbing capability was improved. The specimens having thinner face-sheets exhibited higher contact force variations and the lower permanent deflection than those of the specimens with thicker face-sheet even for the lowest impact energy. However, as the impact energy was increased, the specimen with thinner face-sheet exhibited lower contact force levels and higher permanent deflections than those of the specimens having thicker face-sheets. In addition, the plastic dissipation energy was increased in the face-sheets. The face-sheet thickness was more effective on the permanent deflections, whereas the foam core density was more effective on the energy absorbing capability.

Investigation on Discrepancy of Dynamic Responses between Two Constitutive Models of Aluminum Alloy BRBs in Structures

In the study, the employment of central aluminum alloy buckling restrained braces were attempted for damage control of a single-deck steel frame. Smart metal of A5083-H111 aluminum alloy was chosen to use as the core material of aluminum alloy buckling restrained braces. The discrepancy of structural dynamic responses between two constitutive models of aluminum alloy buckling restrained braces was studied. Two different cross section areas of aluminum alloy buckling restrained braces were adopted for comparative study. Displacement, reaction, stress, strain of critical area and plastic energy dissipation were investigated under one strong ground motion. Suggestions for further academic research and engineering application were given.

Modeling nanostructural surface modifications in metal cutting by an approach of thermodynamic irreversibility: Derivation and experimental validation

Performance and operational safety of many metal parts in engineering depend on their surface integrity. During metal cutting, large thermomechanical loads and high gradients of the loads concerning time and location act on the surfaces and may yield significant structural material modifications, which alter the surface integrity. In this work, the derivation and validation of a model of nanostructural surface modifications in metal cutting are presented. For the first time in process modeling, initiation and kinetics of these modifications are predicted using a thermodynamic potential, which considers the interdependent developments of plastic work, dissipation, heat conduction and interface energy as well as the associated productions and flows of entropy. The potential is expressed based on the free Helmholtz energy. The irreversible thermodynamic state changes in the workpiece surface are homogenized over the volume in order to bridge the gap between discrete phenomena involved with the initiation and kinetics of dynamic recrystallization and its macroscopic implications for surface integrity. The formulation of the thermodynamic potential is implemented into a finite element model of orthogonal cutting of steel AISI 4140. Close agreement is achieved between predicted nanostructures and those obtained in transmission electron microscopical investigations of specimen produced in cutting experiments.

Change aspects of microstructure and mechanical behavior of Bi and Zn-doped Sn–0.5Cu solders for microelectronic applications

Sn–Cu alloys were recommended as a promising substitute for traditional Sn–Pb alloy in wave soldering applications. In the present study, the change of microstructure, thermal and mechanical behavior associated with alloying of Bi and Zn into Sn–0.5Cu solder have been investigated. DSC analysis reveals that large kinetic undercooling arising from the rapid solidification condition is the vital mechanism to realize the eutectic lamellar structures for hypoeutectic Sn–Cu solder. Unlike Bi addition, Zn doping is very effective in reducing undercooling and the onset temperature, although the pasty range is slightly increased. Correlations between thermal analysis, microstructure, and mechanical behavior were established. One conspicuous feature of Zn addition is that Zn promotes the nucleation of fine β-Sn grains, which in turn increases the ductility, but the yield strength, ultimate tensile strength and Young’s modulus are decreased. These properties emerge the plastic energy dissipation ability of solder joints at room temperature. In contrast, the higher mechanical strength of Bi-containing solder is contributed by the solid solution effects and precipitations hardening mechanism of Bi.

Seismic analysis of a tall metal wind turbine support tower with realistic geometric imperfections

SummaryThe global growth in wind energy suggests that wind farms will increasingly be deployed in seismically active regions, with large arrays of similarly designed structures potentially at risk of simultaneous failure under a major earthquake. Wind turbine support towers are often constructed as thin‐walled metal shell structures, well known for their imperfection sensitivity, and are susceptible to sudden buckling failure under compressive axial loading.This study presents a comprehensive analysis of the seismic response of a 1.5‐MW wind turbine steel support tower modelled as a near‐cylindrical shell structure with realistic axisymmetric weld depression imperfections. A selection of 20 representative earthquake ground motion records, 10 ‘near‐fault’ and 10 ‘far‐field’, was applied and the aggregate seismic response explored using lateral drifts and total plastic energy dissipation during the earthquake as structural demand parameters.The tower was found to exhibit high stiffness, although global collapse may occur soon after the elastic limit is exceeded through the development of a highly unstable plastic hinge under seismic excitations. Realistic imperfections were found to have a significant effect on the intensities of ground accelerations at which damage initiates and on the failure location, but only a small effect on the vibration properties and the response prior to damage. Including vertical accelerations similarly had a limited effect on the elastic response, but potentially shifts the location of the plastic hinge to a more slender and, therefore, weaker part of the tower. The aggregate response was found to be significantly more damaging under near‐fault earthquakes with pulse‐like effects and large vertical accelerations than far‐field earthquakes without these aspects. Copyright © 2016 John Wiley & Sons, Ltd.

Finite element analysis of the effect of process-induced voids on the fatigue lifetime of a lead-free solder joint under thermal cycling

Process-induced voids remain one of the key concerns in thermo-mechanical reliability of solder alloys. Previous studies reported that the void effect on fatigue failure reliability of solder joints depends on the void configuration and some other specific characteristics of the electronic package. This paper investigates the void effect on the solder material layers used in power modules subjected to thermal passive cycles. The Anand's visco-plastic model of the solder alloy is identified based on experimental data obtained with a micro-tester. The constitutive model is then used in a finite element analysis to study the behaviour of Innolot Pb-free solder joint used in an electronic assembly. An algorithm called Monte Carlo Representative Volume Element Generator is used to generate, based on the statistical probability law for the diameters, the 2D disk distribution of the voids (thereafter extruded in the form of cylinders) within the solder layer. The dissipated plastic energy is considered as a damage variable indicator representing the void effect on the fatigue lifetime of the solder. Results suggest that the fatigue reliability of solder joints depends not only on the size, location and ratio of the voids but also on their statistical distribution. The critical sites for damage are located at the corners of the joint, as well as at the border of voids. Fatigue lifetime of the solder joint decreases as the volume fraction of voids increases. Moreover, voids near the critical sites facilitate initiation of damage significantly. On the contrary, the solder joint behaviour is almost not affected by voids located far from the critical sites.

Mechanical plasticity of cells.

Under mechanical loading, most living cells show a viscoelastic deformation that follows a power law in time. After removal of the mechanical load, the cell shape recovers only incompletely to its original undeformed configuration. Here, we show that incomplete shape recovery is due to an additive plastic deformation that displays the same power-law dynamics as the fully reversible viscoelastic deformation response. Moreover, the plastic deformation is a constant fraction of the total cell deformation and originates from bond ruptures within the cytoskeleton. A simple extension of the prevailing viscoelastic power-law response theory with a plastic element correctly predicts the cell behaviour under cyclic loading. Our findings show that plastic energy dissipation during cell deformation is tightly linked to elastic cytoskeletal stresses, which suggests the existence of an adaptive mechanism that protects the cell against mechanical damage.

Cell wall material strain hardening on dynamic responses of closed-cell foams

Abstract The paper focuses on the effects of the cell wall material strain hardening on the mechanical behavior of closed-cell foams subjected to the constant velocity loading. Three-dimensional Voronoi models were established to present the closed-cell foams, and the simulations were performed by employing Ls-dyna 971. The investigation found that the cell wall strain hardening could postpone the onset of the localized collapse bands and retard deformation mode transformation. Strain hardening convergence phenomenon was observed in the closed-cell foams; increasing the impact velocity could weaken the effect of the material strain hardening. The crushing stress at both impact and stationary sides were analyzed in detail. The material strain hardening could always enhance the plastic energy dissipation. Furthermore, the strain hardening convergence was also observed in the plateau stress at the impact side and plastic energy dissipation but not in the densification strain and the plateau at the stationary side.

高軸力下で繰返し曲げを受けるH形断面鋼柱の弾塑性座屈モデル

1. Introduction In the heavy plant structures such as thermal power plants composed with the braced frame structures, the H-section columns receive cyclic bending moment under high axial forces, and cause local and overall buckling. However, it is difficult to express the collapse characteristics of columns under such conditions with multi-spring models because the yield areas distributes not only at the ends of the columns but along the member. Modeling such columns as detailed FEM with shell-models is still time-consuming and not practical in structural design. To solve such problem, a single-dimensioned macro-member model composed of fiber elements is proposed in this paper. Each element has the hysteresis reflecting local buckling characteristics and hence can express the local and overall buckling phenomena with simple element which can be easily plugged into time-history analyses. The validity of the proposed element is compared with experiments and FEM analyses with shell elements, followed by discussions on their accuracies. 2. Cyclic bending test of H-section columns under interlocking axial loads Cyclic loading tests with bending moment and interlocking axial loads are carried out on two H-section columns with different width-thickness ratios. FEM analyses were also carried out under the same loading conditions and compared with the test results. Generally the maximum buckling forces observed in the test were higher than FEM analyses with shell elements, however, the deterioration ratios are more significant in the test results. 3. FEM analyses with different width-thickness ratios Cyclic behavior of H-section columns with various width-thickness rations under bending moments and high axial forces are researched with constructed FEM analyses. It is confirmed that the deterioration-starting curvature is reduced with higher width-thickness ratios and axial forces. The overall hysteresis are generally covered with the monotonic load-deformation curves. 4. Proposal for a single-dimensioned macro-member model To construct a simple element applicable to time-history analyses for large structures, a single macro-member models composed of fiber elements whose hysteresis reflects the characteristics of local buckling deteriorations. The hysteresis curves are composed of six stages with different simple functions of normalized axial deformation, and deterioration characteristics of the local buckling is expressed by the parameters of width-thickness ratios. 5. Comparison of macro-model results and shell-model results The behaviors of H-section columns are analyzed with the proposed macro-models, and compared with the results of cyclic loading tests and the results by FEM analyses with shell-models. Generally the results of macro-models agrees well with those of tests and shell-model results, except for relatively earlier deterioration after the local buckling. The positions of local buckling and the dissipated plastic energy by the hysteresis were also agreed well with the tests and shell-model results. The results of H-section columns with various width-thickness by FEM analyses with shell-models are also compared with the analyses with the proposed macro-models. Generally the positions of local buckling and the dissipated plastic energy by the hysteresis were agreed well with the results of shell-model. 6. Conclusions 1) The results of shell-models are generally agreed with the test results when the effects of boundary conditions and residual deformation are taken into account. 2) The results of proposed single macro-member models generally agrees with those of shell-models and modified test results, which are considered to be successful to express the characteristics of H-section columns subjected to bending moment and high axial forces.

Numerical Study of Influence of Mechanical Heterogeneity on Cyclic Plastic Energy Dissipation of Plate with a Center Hole

Numerical Study of Influence of Mechanical Heterogeneity on Cyclic Plastic Energy Dissipation of Plate with a Center Hole

Development and application of a simplified model for the design of a super-tall mega-braced frame-core tube building

Resilience-based earthquake design for next-generation super-tall buildings has become an important trend in earthquake engineering. Due to the complex structural system in super-tall buildings and the extreme computational workload produced when using refined finite element (FE) models to design such buildings, it is rather difficult to efficiently perform a comparison of different design schemes of super-tall buildings and to investigate the advantages and disadvantages of different designs. Here, a simplified nonlinear model is developed and applied to compare two design schemes (i.e., the fully braced scheme and half-braced scheme) of a super-tall mega-braced frame-core tube building, which is an actual engineering project with a total height of approximately 540m. The accuracy of the simplified model is validated through a comparison of the results of modal analyses, static analyses and dynamic time history analyses using the refined FE models. Subsequently, the plastic energy dissipation of different components and the distribution of the total plastic energy dissipation over the height of the two design schemes are compared using the proposed simplified model. The analyses indicate that the fully braced scheme is superior because of its more uniform energy distribution along the building height and the large amount of energy dissipated in the replaceable coupling beams, which enables rapid repair and re-occupancy after an earthquake. In contrast, the potential damage in the half-braced scheme is more concentrated and more severe, and the damage in the core tubes is difficult to repair after an earthquake.

Effect of graphene-oxide enhancement on large-deflection bending performance of thermoplastic polyurethane elastomer

Thermoplastic polyurethane (PU) elastomers are used as shoe-sole materials due to many excellent properties but their inelastic deformation is a serious deficiency for such applications. Hence, graphene oxide (GO) was introduced into the synthesized thermoplastic PU to produce a GO/PU composite material with enhanced properties. Plastic behaviour of this composite was assessed in cyclic tensile tests, demonstrating reduction of irreversible deformations with the addition of GO. Additionally, in order to evaluate mechanical performance of PU and the GO/PU composite under conditions of large-deflection bending typical for shoe soles, finite-element simulations with Abaqus/Standard were conducted. An elastic–plastic finite-element model was developed to obtain detailed mechanical information for PU and the GO/PU composite. The numerical study demonstrated that the plastic area, final specific plastic dissipation energy and residual height for PU specimens were significantly larger than those for the GO/PU composite. Besides, the addition of GO into the PU matrix greatly delayed the onset of plastic deformation in PU in a large-deflection bending process. The average residual height and final specific plastic dissipation energy for PU were approximately 5.6 and 17.7 times as large as those for the studied GO/PU composite. The finite-element analysis provided quantification of the effect of GO enhancement on the large-deflection bending performance of PU for regimes typical for shoe soles and can be used as a basis for optimization of real composite products.

Three-dimensional modelling of auxetic sandwich panels for localised impact resistance

Sandwich panels with auxetic lattice cores confined between metallic facets are proposed for localised impact resistance applications. Their performance under localised impact is numerically studied, considering the rate-dependent effects. The behaviour of the composite structure material at high strain rates is modelled with the Johnson-Cook model. Parametric analyses are conducted to assess the performance of different designs of the hybrid composite structures. The results are compared with monolithic panels of equivalent areal mass and material in terms of deformations and plastic energy dissipation. Design parameters considered for the parametric analyses include the auxetic unit cell effective Poisson’s ratio, thickness of the facet, material properties and radius of the unit cell’s struts. Significant reduction in computational time is achieved by modelling a quarter of the panel, with shell elements for facets and beam elements for the auxetic core. With projectile impacts up to 200 m/s, the auxetic composite panels are found to be able to absorb a similar amount of energy through plastic deformation, while the maximum back facet displacements are reduced up to 56% due to localised densification and plastic deformation of the auxetic core.

Material characterisation and finite element modelling of cyclic plasticity behaviour for 304 stainless steel using a crystal plasticity model

Low cycle fatigue tests were carried out for a 304 stainless steel at room temperature. A series of experimental characterisations, including SEM, TEM, and XRD were conducted for the 304 stainless steel to facilitate the understanding of the mechanical responses and microstructural behaviour of the material under cyclic loading including nanostructure, crystal structure and the fractured surface. The crystal plasticity finite element method (CPFEM) is a powerful tool for studying the microstructure influence on the cyclic plasticity behaviour. This method was incorporated into the commercially available software ABAQUS by coding a UMAT user subroutine. Based on the results of fatigue tests and material characterisation, the full set of material constants for the crystal plasticity model was determined. The CPFEM framework used in this paper can be used to predict the crack initiation sites based on the local accumulated plastic deformation and local plastic dissipation energy criterion, but with limitation in predicting the crack initiation caused by precipitates.

Non-linear Dynamic Analysis of Steel Hollow I-core Sandwich Panel under Air Blast Loading

In this paper, the non-linear dynamic response of novel steel sandwich panel with hollow I-core subjected to blast loading was studied. Special emphasis is placed on the evaluation of midpoint displacements and energy dissipation of the models. Several parameters such as boundary conditions, strain rate, mesh dependency and asymmetrical loading are considered in this study. The material and geometric non-linearities are also considered in the numerical simulation. The results obtained are compared with available experimental data to verify the developed FE model. Modeling techniques are described in detail. According to the results, sandwich panels with hollow I-core allowed more plastic deformation and energy dissipation and less midpoint displacement than conventional I-core sandwich panels and also equivalent solid plate with the same weight and material.

Graded square honeycomb as sandwich core for enhanced mechanical performance

Honeycomb as sandwich core construction has been established as one of the best candidates among ultralight cellular materials for load-bearing and energy absorption applications. In this paper, by introducing non-uniform mass distribution (gradient) in the core, substantially improved structural performance of a fully-clamped sandwich plate with square honeycomb core is achieved for out-of-plane uniform pressure loading. In-plane gradient of core is defined and achieved via two approaches: web thickness variation and cell size variation. Response of sandwich plates with linear in-plane core gradient is systematically investigated with finite element (FE) simulations. For both quasi-static and blast loading cases, it is found that positively graded cores exhibit advantage in stiffness, strength as well as plastic energy dissipation compared to cores without gradient or with negative gradient. Examination of plastic energy dissipation in a graded core reveals that ribs placed next to its symmetry plane are more sensitive to changes in thickness or location compared to those placed adjacent to the clamped edges. With the total mass of the core constrained, the constituent material can be more efficiently utilized by either increasing the thickness of ribs close to symmetry planes or moving ribs towards the symmetry planes.

Energy dissipation and constitutive modeling for a mechanistic description of pad scratching in chemical–mechanical planarization

A thermomechanical model to describe the mechanisms of polishing pad scratching in chemical–mechanical planarization (CMP) has been formulated and investigated. CMP is a necessary process in integrated circuit (IC) fabrication to planarize wafers with nanoscale features after patterned layer deposition. Polishing pad asperities can produce microscale scratches on the wafer surface during CMP, reducing IC manufacturing yields. The constructed thermomechanical model accounts for stresses of the pad and wafer contact and also provides the means to track input energy dissipation during CMP. Tracking energy dissipation offers information about processes that may influence scratch production. This knowledge ultimately produces a greater physical understanding of CMP for the prevention of pad scratching. Polishing pad stress relaxation experiments demonstrate the importance of viscoelastic and plastic strain energy dissipation with its effects on the wafer stress field. Scratch producing ability of the polishing pad is found to decrease with use in CMP, with slurry soaking and increasing polishing time. Mechanical behavior of the polishing pad is demonstrated to differ when in compression and in tension. Compressibility of the pad material is shown to be significant in stress modeling through experimental measurement of polishing pad volume change. Differential scanning calorimetry of used polishing pad samples revealed energy dissipation into the polishing pad surface with increasing polishing time of CMP. Energy dissipation processes influence pad scratching in CMP. Analytical wafer stress field modeling unveils that the scratching ability of a polishing pad decreases when it is less stiff or has a smoother surface.

Finite element analysis of sandwich panels with stepwise graded aluminum honeycomb cores under blast loading

This paper presents details and brief results of an experimental investigation on the response of metallic sandwich panels with stepwise graded aluminum honeycomb cores under blast loading. Based on the experiments, corresponding finite element simulations have been undertaken using the LS-DYNA software. It is observed that the core compression stage was coupled with the fluid–structure interaction stage, and the compression of the core layer decreased from the central to the peripheral zone. The blast resistance capability of sandwich panels was moderately sensitive to the core relative density and graded distribution. For the graded panels with relative density descending core arrangement, the core plastic energy dissipation and the transmitted force attenuation were larger than that of the ungraded ones under the same loading condition. The graded sandwich panels, especially for relative density descending core arrangement, would display a better blast resistance than the ungraded ones at a specific loading region.