Average Plastic Strain Research Articles

Corrected Method for Scaling the Structural Response Subjected to Blast Load

In scale-down tests of ship structures subjected to a blast load, the accuracy of the predicted response of a prototype is affected by the material substitution and geometric distortion between a scaled model and a full-size structure; this is known as incomplete similarity. To obtain a more accurate response from a prototype during small-size tests, a corrected method for scaling the response of thin plates and stiffened plates under a blast load was derived. In addition, based on numerical simulations of explosion responses by employing the elastic–plastic model and the Johnson–Cook constitutive model, it was found that using the average yield stress derived from the equivalent plastic strain energy in the ideal elastic–plastic model can obtain consistent structural responses. Moreover, a method for calculating the distortion factor caused by the yield stress of different materials was proposed. Furthermore, it was demonstrated that the average effective plastic strain between the prototype and the corrected model is equal, and based on this, a similarity prediction method was established to correct the distortions caused by yield stress and the thickness of blast loaded plates. The results indicate that the proposed correction method can compensate for the differences caused by distorted factors of yield stress and thickness, with the maximum error in the structure’s peak displacement being less than 3%.

Greatly improved room temperature formability of Al-Zn-Mg-Cu-Fe alloy sheets via coupling distribution of domains with different strengths

Using high strength Al-Zn-Mg-Cu-Fe alloy sheets as structural parts is capable of realizing lightweight of automobiles, but the low room temperature formability seriously limits their widespread use in automotive industry. In this study, we propose a new strategy to construct the coupling distribution of domains with different strengths by an innovate short-flow thermomechanical processing route. Both the compatible deformation capacity and room temperature formability of alloy sheets can be greatly enhanced by the coupling action of dislocations with domains. Finally, the average plastic strain (r̅-value) of alloys with a suitable coupling distribution of domains with different strengths can be greatly increased to 0.7171, which is a great outclass as those of traditional Al-Zn-Mg-Cu alloy sheets. Furthermore, the evolution mechanisms of domains with different strengths and the coupling action mechanisms of dislocations with domains have been discussed in detail.

Blast mitigation effects of TPU polymers-based inflated membrane structures under external explosion

TPU polymers-based inflated membrane structures are a potential technology to protect pre-existing lightweight buildings, with the advantages of lightweight and simple building process. In this study, a series of field blast tests were performed for the structural response and blast load parameters to study the blast mitigation effect of the inflated membrane structures. The corresponding numerical models of the tests were then developed and validated to reproduce the fluid-structure interaction. Thin aluminum plates were used as the protected structure. Based on their maximum displacement, average plastic strain, depth of depression, and permanent deformation, damage factor was defined to evaluate their damage level. Both the experimental and numerical results show that inflated membrane structures can effectively mitigate blast loading and significantly reduce the permanent displacement of the aluminum plate by elastic compression deformation. The effects of the initial height and internal pressure of inflated membrane structures on the blast mitigation effectiveness were clarified by the parametric study. The overpressure-impulse equivalent damage curves were established by combining the damage factor and overpressure-impulse loads, which further confirmed the blast mitigation performance of inflated membrane structures. Our study has verified the blast mitigation effects of inflated membrane structures based on TPU polymers, which is potentially valuable in the field of protective engineering for lightweight construction.

Enhanced formability of Al–Mg–Si–Zn alloy sheet via dislocation structure and texture during cold rolling

The effects of cold rolling deformation on microstructure, texture, and formability of Al–Mg–Si–Zn alloy were systematically investigated using OM, SEM, TEM, XRD, and tensile tests. The results indicated that the effect of cold rolling deformation on the recrystallized microstructure and texture of the sheets mainly depended on the differences in dislocation structure and texture distribution. With the increase of cold rolling deformation, the dislocation structure gradually changed from a mixture of density dislocation walls and microbands to a lamellar bands structure, the large-angle boundary gradually increased, and the size of dislocation cells gradually decreased, which indicated a gradual increase of the stored deformation energy. In addition, the deformation texture increased gradually with the raise of cold rolling deformation. After T4P treatment, the recrystallized grain size gradually reduced with the increase of cold rolling deformation. The main recrystallization textures in the sheets with 41 % and 55 % deformation were Cube {001}<100> and CubeRD {310}<001> textures, respectively. The CubeND {001}<310> texture was predominantly present in the sheets with 69 % and 82 % deformation, meanwhile some P {011}<233> texture was also observed in the sheets with 82 % deformation. It was found that the dislocation structure and deformation texture distribution formed in the sheet with 82 % cold rolling deformation contributed to smaller recrystallized grains and weaker P texture after solution heat treatment, which enhanced the average plastic strain ratio r value and improved the formability.

The development of a W-shaped channel extrusion for fabricating magnesium alloy shells by combining high amplitude shear strain with a shorter process

This paper introduced a novel single-pass severe plastic deformation method, W-shaped channel extrusion (WCE), for the direct manufacturing of magnesium alloy shells. Based on a strategically installed mandrel and die with a sloping asymmetric step structure, the WCE constructed a narrow severe plastic deformation gap specially reinforced by a backpressure-shear structure and an asymmetric shear-extrusion section. Therefore, slender bar with large height-to-diameter ratio (i.e. small diameter, large height) could easily be extruded into larger diameter shell in narrow gap through only one pass, and an impressive average plastic strain of over 4.1 was concurrently achieved, evidenced by upper bound theory calculation. The WCE process was further experimentally conducted on AZ80 alloy at 340 °C, demonstrating remarkable improvements in yield strength, ultimate tensile strength, and elongation of the extruded part by factors of 2.4, 2.2, and 4.3 over the as-cast condition. The consecutive multi-stage shear and high hydrostatic pressure catalyzed extreme grain refinement, favored high matrix dislocations and concurrently facilitated the early dynamic conception of granular β-Mg17Al12 particles in subgrains stage, all of which served the superior performance. Moreover, no prominent anisotropy detected in the extruded part due to the development of a special bimodal basal texture under shear stress, where its synergistic effect with grain refinement evidently promoted the balance of strength and plasticity. Finally, the mechanism of bimodal texture formation associated with specific slip activations during WCE and anisotropic weakening was revealed in detail.

Material variability effects on automotive part production process

The current efforts to reduce the carbon footprint throughout the chain in the automotive industry by increased use of recycled materials poses new challenges for materials production and their use. The increase of steel scrap fraction in the current primary steel making processes, used for producing steel sheet metal for automotive components, possibly affects the material properties variability beyond the limits observed in the materials produced today despite mitigating actions in steel production. In this paper material variability increase was modelled by selecting deterministic values outside the range of the material grade used to design and manufacture an automotive part. The values were selected from an experimental data set representing the cold rolled mild steels material class range. The effects were studied numerically on a reverse engineered model of an existing automotive part production process. It was found that the manufacturing feasibility in this particular case is mainly affected by the weighted average plastic strain ratio and less by the degree of planar anisotropy.

Integral Hydro-Bulge Forming Method of Spherical Pressure Vessels Using a Triangle Patch Polyhedron

AbstractThis paper proposes an integral hydrobulge forming (IHBF) method using a triangular patch polyhedron as the closed preform shell. When triangular flat parts are welded along the edges in sequence, triangular patch polyhedra are naturally formed. From the radius of the spherical pressure vessel, a design formula was derived to calculate the side lengths of the triangular flat plate parts. The water pressure, water volume, average strain of molding, and amount of springback after molding, which are necessary for implementing the IHBF for practical use, were also formulated. To verify the forming performance of the spherical pressure vessel using IHBF method, the finite element method was carried out, and a stainless-steel spherical pressure vessel with a thickness of 1.0 mm and a diameter of approximately 500 mm was fabricated using the proposed IHBF method. As a result, the measured shape error expressed as roundness to diameter ratio was 0.52%, and the calculated average plastic strain was 0.02, which was approximately 1/19 times of the forming limit strain of the material. The amount of springback after forming by calculation was approximately 0.7 mm, indicating that the amount of water required for IHBF was 5.90% of the volume of the spherical pressure vessel, while the required water pressure was no bigger than 2.3 MPa. The process directly utilizes triangular flat plate parts, eliminating the need for molds to process closed preform shells resulting in a low average plastic strain during forming, thereby improving the quality of the formed spherical pressure vessels.

Introduction of rolling motion at the tool-tip in metal cutting

In metal cutting, severe sliding contact at the tool-chip interface is unavoidable by a conventional cutting tool, which results in sloth motion of chip, higher chip thickness, formation of stagnation zone, higher power consumption, and poor surface finish. To overcome this limitation, rolling motion is introduced at the tool-chip interface by mounting a roller at the tip of a cutting tool – termed as Rotating Tip Cutting Tool (RTC-tool). The roller in RTC-tool rotates during cutting and establishes rolling-sliding contact. A model in-situ experimental configuration is used to study the plane strain flow characteristic of pure copper, notoriously known for higher chip thickness and power consumption, during cutting at low speed. The performance of RTC-tool is compared with sharp cutting-edge and blunt cutting-edge tools (fixed curvature and stationary-roller-tip tool). It is found that rolling motion at the tool-tip decreases the chip thickness and average plastic strain within the chip by about two times than sharp tool and more than two times than the blunt tools. Additionally, there is a significant improvement in surface roughness than the sharp tool. The digital image correlation techniques reveal that flow characteristics within the chip and near the tool-tip interface (retardation region) are influenced by the non-laminar plastic flow of materials. As opposed to the unstable retardation region and periodic cracking at the tool-tip of sharp tool, additional rolling motion at the tool-tip cuts the chip at the incipient stage, forms a stable retardation region, and increases the average velocity of the material in this region. Large retardation region and sloth motion of materials within the retardation region produce large chip thickness during cutting by blunt edge tool. As the chip thickness rather power consumption of the sharp tool is less than the blunt tool, force measurement in nearly orthogonal cutting configuration of RTC-tool compared with the sharp cutting edge. These tests are performed at moderate speed range in a lathe machine. The force measurement data and post-cutting characterization are aligned with the in-situ observations. As the frictional resistance of the roller controls the chip thickness further improvement in the performance of the RTC-tool is possible by reducing the frictional resistance of the roller.

Memory surfaces separating the processes of monotonous and cyclic loads

The processes of elastoplastic deformation of structural materials can consist of a sequence of monotonous and cyclic loading regimes, in which peculiar effects and features arise. Mathematical modeling of such processes, as well as resource assessment and forecasting, is a very difficult task. In addition, the analysis of transient processes from cyclic to monotonous and from monotonous to cyclic shows the need to separate these processes. Based on the analysis of the results of experimental studies of samples of stainless steel 12Х18Н10T under a rigid (controlled deformation) deformation process, which is a sequence of monotonic and cyclic loading modes, under conditions of uniaxial tension-compression at normal temperature, the features and differences in the processes of monotonic and cyclic loading are revealed. To describe these features and separate the processes of monotonic and cyclic loading modes in the theories of plastic flow with combined hardening, various variants of memory surfaces are introduced. An analysis of the results of experimental studies of stainless steel showed that in the space of the plastic strain tensor, the size of the memory surface is determined by the range of plastic strains, and the position of the center is determined by the values of average plastic strains under cyclic loading. Various variants of the memory surface are considered, their capabilities and disadvantages are identified, and the most adequate variant of the memory surface is determined. To confirm the operability of this version of the memory surface, together with the equations of the Bondar plasticity model, the calculated and experimental results were compared and a reliable agreement was obtained between these results both in terms of the kinetics of the stress-strain state and in terms of the number of cycles to failure.

Effect of Geometrical Parameters of Microscale Particles on Particle-Stimulated Nucleation and Recrystallization Texture of Al-Si-Mg-Cu-Based Alloy Sheets.

The effects of the shapes (needle and round) and volume fractions (low and high) of microscale particles in Al-Si-Mg-Cu-based alloys on recrystallization behavior, texture evolution, mechanical properties, and formability are investigated. The recrystallized grain size decreases as the size and volume fraction of the particles decrease and increase, respectively, regardless of the particle shape. The investigated alloys with a relatively low volume fraction of 0.7 to 2.4 vol.% exhibit higher efficiency particle-stimulated nucleation (PSN) than alloys with a high volume fraction of 6.0 to 21.0 vol.%. This is because the interaction between the particles and dislocations cannot be greatly promoted when the volume fraction of the particles is large enough to form agglomerates. The sheets with round-shaped particles exhibit higher yield strength (YS) and elongation (EL) than sheets with needle-shaped particles. The improvement in YS is due to the combined effects of grain refinement and particle strengthening, and the EL is improved by reducing the probability of cracking at the tips of round-shaped particles. The sheets with round-shaped particles exhibit relatively higher average plastic strain ratio (r¯) and planar anisotropy (∆r) than the sheets with needle-shaped particles, owing to the development of Goss {110}&lt;001&gt; or rotated-Goss {110}&lt;110&gt; orientations.

Evading the low formability dilemma in Al-Zn-Mg-Cu alloys via heterogeneous structure

In order to evade the low formability dilemma in Al-Zn-Mg-Cu alloys, the formation mechanism of heterogeneous structure and its effect on the formability of Al-Zn-Mg-Cu alloys were systematically investigated. The results show that a heterogeneous structure with a co-existence of soft and hard microdomains can be formed by controlling and optimizing the thermomechanical processing. The formability of the alloys can be greatly enhanced, i.e., the average plastic strain ratio−r is increased from 0.6744 to 0.7502, which is mainly contributed to the coordinated deformation of soft and hard microdomains, and the weakened textures. In addition, both the formation mechanism of heterogeneous structure and high formability mechanism were put forward in this paper.

Enhanced grain refinement and texture weakening in Al–Mg–Si alloy through a novel thermomechanical processing

A novel thermomechanical processing including solution treatment, first rolling with large deformation and overaging based on particle-stimulated nucleation of recrystallization (PSN) was proposed to obtain an Al–Mg–Si alloy sheet with fine grain structure, weak texture and high plastic strain ratio. The difference between conventional thermomechanical processing and novel thermomechanical processing with two overaging time parameters was investigated by texture and microstructure characterization and tensile tests. The results show that the finest grain structure with an average size of 17 µm, the weakest recrystallization texture with a volume fraction of 8.4 %, the highest average plastic strain ratio r of 0.69 and the lowest planar anisotropy Δr of − 0.001 were acquired by the novel thermomechanical processing with a long overaging time. In comparison with conventional thermomechanical processing, the novel thermomechanical processing can generate completely different particle distributions and tends to develop a weak texture consisting of the CubeND {001}<310>orientation. Increasing the overaging time is effective in coarsening the particles and thus refining the grain structure, weakening the texture, and improving the average r and Δr values. The genetic characteristics of texture evolution and the particle distribution determine the final recrystallization texture.

Thermomechanical analysis of induction assisted friction stir welding of Inconel 718 alloy: A finite element approach

Friction stir welding (FSW) of hard and high melting point materials such as Nickel and Titanium based alloys is challenging due to excessive tool wear and low weld quality. Various auxiliary energy sources have been proposed in the past decade as an assistance to conventional FSW process. In this work, a modified 3D Finite element (FE) model was developed to simulate the induction preheated FSW of Inconel 718 alloy using ABAQUS/Explicit software package. Investigation of temperature distribution and residual stress generation was carried out for induction assisted FSW (IAFSW) process and compared with those of FSW process. Features such as Arbitrary Lagrangian–Eulerian (ALE) formulation, adaptive meshing technique, mesh sensitivity analysis and mass scaling techniques have been utilised in order to develop a reliable and computationally efficient FE model. Experimental study was carried out with Induction assistance as a preheating method to FSW process. The thermal history of the numerical model was fairly validated with experimental results. Results showed that temperature increased by 138 °C in IAFSW process and was more uniformly distributed as compared to FSW. Residual stresses decreased by 15% on an average in the IAFSW joints. In addition to that, plastic strain and tool reaction forces were also predicted using the FE model proposed in this work. Plastic strain values were higher in IAFSW process with a maximum difference of 55% over the average plastic strain value of FSW at the beginning of the traverse period when temperature was highest. Reaction forces on probe tip reduced by 23% in IAFSW process during the plunge stage.

Effects of anisotropy and pre-strain on the shear fatigue behaviour of DP600 steel

The effects of anisotropy and pre-strain on the low-cycle-fatigue properties of DP600 steel sheet under shear path were investigated. Fully reversed strain-controlled fatigue tests were performed along the 0° and 90° directions with the initial strain direction. The cyclic-loading amplitude determined whether the pre-strained material had a hardening process. The high dislocation density in the crystals of pre-strained material can promote the formation of low-energy substructures, obviously accelerating the cyclic-softening rate and reducing the fatigue life. Average plastic strain energy density was affected by the pre-straining state and loading direction and had a linear relationship with the strain amplitude. Overall, these results can be used to verify the calculation model of structural parts.

Microstructure, Texture, and Formability Control by Initial Hot‐Rolling Temperature of Al–Mg–Si Alloy Sheets for Automotive Applications

The effect of initial hot‐rolling temperature on microstructure, texture, and formability of Al–Mg–Si alloy sheets is investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and tensile test. The results indicate that the textures of the sheet after hot‐rolling comprise most of Copper {112}&lt;111&gt;, Brass {011}&lt;211&gt;, and S {123}&lt;634&gt;. Initial rolling temperature affects the particles distribution before solution treatment, thereby further influencing the recrystallized microstructure and texture after T4P treatment. After T4P treatment, Cube {001}&lt;100&gt; component is the main grain orientation in the sheet with an initial rolling temperature of 530 °C. While CubeND {001}&lt;310&gt; orientation forms after other three initial rolling temperatures (450, 500, and 555 °C). However, the grain orientation distribution is more dispersed with an initial rolling temperature of 555 °C. Within the initial rolling temperature range from 530 to 555 °C, the average plastic strain ratio r value of the sheet is relatively higher. The anisotropy Δr value at an initial rolling temperature of 555 °C is the closest to zero. Therefore, the initial rolling temperature of 555 °C is the most conducive to improving the formability of the alloy sheet.

Data-driven prediction of forging outcome: Effect of preform shape on plastic strain in a magnesium alloy forging

Closed-die cast-forging operations require a deliberate design of forging preforms (work-piece) to encourage process-related grain refinement and good metal flow during deformation. To this end, we propose a computational design framework and use artificial neural networks (ANNs) for generating and predicting the forging response of preform shapes, respectively. First, we develop a parametric CAD model to generate preforms algorithmically. The input space of this parametric CAD model is then uniformly sampled to generate a set of 3D models for offline finite element method (FEM) simulations. Finally, forging simulation data is processed to create a dataset for training feedforward ANNs to predict average effective plastic strain response in spatially varying regions of interest within the forging. In this study, we applied the computational design framework to generate preforms for a closed-die cast-forging operation of a Magnesium (Mg) alloy I-beam, and we predicted the average effective plastic strain response in spatially varying regions of the forging to within ±8% of the ground truth.

Extremely improved formability of Al–Zn–Mg–Cu alloys via micro-domain heterogeneous structure

Here we report a heterogeneous microstructure in Al–Zn–Mg–Cu alloys by coupling control of solute gradient distribution, multiscale iron-rich phases and a novel thermomechanical processing that can produce a greatly improved formability (the average plastic strain ratio r = 0.719). A heterogeneous structure of coarse grains surrounded by fine grains is formed, and an unusual high formability is obtained by the synergy of soft/hard microdomains. The process discovered here is amenable to large-scale automotive industrial production at low cost, and might be applicable to other Al alloy systems.

Investigation on surface integrity in laser-assisted machining of Inconel 718 based on in-situ observation

Inconel 718, a nickel-based superalloy, is widely used in the key parts of aero-engine owing to its high performance at elevated temperatures. Meanwhile, Inconel 718 is a typical difficult-to-cut material which is hard to obtain ideal surface integrity by conventional machining (CM). Laser-assisted machining (LAM) has a good potential to improve the machinability. In order to study the processing mechanism in the LAM of Inconel 718, the in-situ and ex-situ studies were presented in this paper. In the in-situ analysis, LAM and CM experiments were carried out based on in-situ observation and digital image correlation (DIC) technology. The Von Mises strain and equivalent plastic strain rate in the primary shear zone (PSZ) and cutting force were measured. According to the DIC results, under the experimental conditions of this paper, the average Von Mises strain and average equivalent plastic strain rate of PSZ in LAM increase by about 10% and 20% respectively due to the softening of Inconel 718 by laser heating, compared with CM. In-situ analysis shows that Inconel 718 in LAM is more prone to plastic flow, resulting in lower cutting forces. As for the ex-situ characterization, the surface roughness, residual stress and tool wear were measured. The measured results show that LAM can significantly reduce surface roughness and tool wear, and cause compressive residual stress on the machined surface.

ПОСТРОЕНИЕ ПОВЕРХНОСТИ ПАМЯТИ ДЛЯ РАЗДЕЛЕНИЯ ПРОЦЕССОВ МОНОТОННЫХ И ЦИКЛИЧЕСКИХ НАГРУЖЕНИЙ

Non-stationary and asymmetric processes of cyclic deformation are considered, which consist of a sequence of monotonic and cyclic loading modes, in which peculiar effects of landing and ratcheting of plastic hysteresis loops occur. Mathematical modeling of such processes of deformation and damage accumulation is based mainly on variants of plasticity theories belonging to the class of theories of plastic flow with combined (isotropic and anisotropic) hardening. In this paper, mathematical modeling of the processes of deformation and damage accumulation is based on a variant of the theory of plasticity – the Bondar model. Based on the analysis of the results of experimental studies of samples of stainless steel 12Х18Н10T under a rigid (controlled deformation) deformation process, which is a sequence of monotonic and cyclic loading modes, under conditions of uniaxial tension-compression at normal temperature, the features and differences in the processes of monotonic and cyclic loading are revealed. To describe these features and separate the processes of monotonic and cyclic loading modes in the theories of plastic flow with combined hardening, various variants of memory surfaces are introduced. An analysis of the results of experimental studies of stainless steel showed that in the space of the plastic strain tensor, the size of the memory surface is determined by the range of plastic strains, and the position of the center is determined by the values of average plastic strains under cyclic loading. Various variants of the memory surface are considered, their capabilities and disadvantages are identified, and proposed variant of the memory surface.To confirm the operability of proposed version of the memory surface, together with the equations of the Bondar plasticity model, the calculated and experimental results were compared and a reliable agreement was obtained between these results both in terms of the kinetics of the stress-strain state and in terms of the number of cycles to failure.

Numerical Investigation of Plastic Strain Homogeneity during Equal-Channel Angular Pressing of a Cu-Zr Alloy

A three-dimensional finite element method (3D FEM) simulation was carried out using ABAQUS/Explicit software to simulate multi-pass processing by equal-channel angular pressing (ECAP) of a circular cross-sectional workpiece of a Cu-Zr alloy. The effective plastic strain distribution, the strain homogeneity and the occurrence of a steady-state zone in the workpiece were investigated during ECAP processing for up to eight passes. The simulation results show that a strain inhomogeneity was developed in ECAP after one pass due to the formation of a corner gap in the outer corner of the die. The calculations show that the average effective plastic strain and the degree of homogeneity both increase with the number of ECAP passes. Based on the coefficient of variance, a steady-state zone was identified in the middle section of the ECAP workpiece, and this was numerically evaluated as extending over a length of approximately 40 mm along the longitudinal axis for the Cu-Zr alloy.