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Determination of dynamic tensile response of materials has been a challenge because of experimental difficulty. The split Hopkinson tensile bar (SHTB) is one of the most widely used devices for characterization of various materials under dynamic-tensile loading conditions. Since one-dimensional wave propagation in bars is disturbed by specimens and grips, however, SHTB measurement accuracy may not be guaranteed. This means that the stress–strain curve of the specimen that is calculated using strains at bars may not indicate the real stress–strain relation of the specimen. In this study, simulations for the SHTB test were carried out to investigate the effects of thread pitch, specimen length, specimen diameter, and thread inner diameter of the specimen on the measurement accuracy for two types of metals with medium and high yield strengths. Finally, specimen shapes are recommended for accurate measurement of the stress–strain relation of tantalum and tungsten carbide.

This study provides an analysis of the hot embossing process with poly methyl methacrylate (PMMA) film. The hot embossing process engraves a fine pattern on a flexible film using a stamp, applied heat and pressure. As the quality of the embossing pattern varies according to various process variables, the mechanism of making the embossed shape is complicated and difficult to analyze. Therefore, analysis takes much time and cost because it usually has to perform a lot of experiments to find an appropriate process condition. In this paper, the hot embossing process was analyzed using a computational analysis method to quickly find the optimal process. To do this, we analyzed the embossing phenomenon using the finite element method (FEM) and arbitrary Lagrangian–Eulerian (ALE) re-mesh technique. For this purpose, we developed a constitutive model considering the strain, strain rate, temperature-dependent stress and softening of the flexible film. Work hardening, strain softening, and temperature-softening behavior of PMMA materials were well described by the proposed method. The developed constitutive model were applied in the embossing analysis via user-subroutine. This proposed method allowed a precise analysis of the phenomenon of film change during the hot embossing process.

Characteristics of the stress pulse generated by impact of a hollow striker on the flange of a split Hopkinson tension bar are investigated via an explicit finite element analysis. Design guidelines are extracted for the hollow striker and flange from the viewpoint of eliminating spurious waves located between the incident and reflected pulses. According to design guidelines, it is desirable to have a striker cross-sectional area the same as that of the flange. It is also desirable to make the cross-sectional area of the striker (flange) the same as that of the bar. As for the flange length, it is recommended to be comparable to the diameter of the bar. The magnitude and duration of the primary stress pulse are consistent with the results of a one-dimensional analysis even when spurious waves are present; meanwhile, overly long spurious waves should be avoided to eliminate their superposition with the reflected pulse. Spurious waves appear when general impedance of the striker is higher than the bar. The origin of spurious waves is a series of step-wise residual pulses generated by multiple cycles of striker impact that make the striker keep compressing the flange after the first cycle of impact. Step-wise residual pulses appear in two forms (continuous waves and discrete waves) in spurious waves due to the secondary impacts during the entrance process of step-wise residual pulses to the flange. The consequences of spurious waves in the use of split Hopkinson tension bars are discussed.

The specimen strain rate in the split Hopkinson bar (SHB) test has been formulated based on a one-dimensional assumption. The strain rate is found to be controlled by the stress and strain of the deforming specimen, geometry (the length and diameter) of specimen, impedance of bar, and impact velocity. The specimen strain rate evolves as a result of the competition between the rate-increasing and rate-decreasing factors. Unless the two factors are balanced, the specimen strain rate generally varies (decreases or increases) with strain (specimen deformation), which is the physical origin of the varying nature of the specimen strain rate in the SHB test. According to the formulated strain rate equation, the curves of stress–strain and strain rate–strain are mutually correlated. Based on the correlation of these curves, the strain rate equation is verified through a numerical simulation and experiment. The formulated equation can be used as a tool for verifying the measured strain rate–strain curve simultaneously with the measured stress–strain curve. A practical method for predicting the specimen strain rate before carrying out the SHB test has also been presented. The method simultaneously solves the formulated strain rate equation and a reasonably estimated constitutive equation of specimen to generate the anticipated curves of strain rate–strain and stress–strain in the SHB test. An Excel® program to solve the two equations is provided. The strain rate equation also indicates that the increase in specimen stress during deformation (e.g., work hardening) plays a role in decreasing the slope of the strain rate–strain curve in the plastic regime. However, according to the strain rate equation, the slope of the strain rate–strain curve in the plastic deformation regime can be tailored by controlling the specimen diameter. Two practical methods for determining the specimen diameter to achieve a nearly constant strain rate are presented.

Four-dimensional (4D) printing is a unique application of additive manufacturing (AM) which enables additional shape transformations over time. Although 4D printing is an interesting and attractive phenomenon, it still faces several challenges before it can be used for practical applications: (i) the manufacturing cost should be competitive, and (ii) the shape transformations must have high dimensional accuracy and repeatability. In this study, an irreversible and repeatable thermoresponsive shape transformation method was developed using a material extrusion type AM process and a plain thermoplastic polymer (ABS) without a shape-memory function. Various types of annular discs were additively manufactured using printing paths programmed along a circular direction, and additional heat treatment was conducted as a thermal stimulus. The programmed circumferential anisotropy led to a unique 2D-to-3D shape transformation in response to the thermal stimulus. To obtain more predictable and repeatable shape transformation, the thermal stimulus was applied while using a geometric constraint. The relevant dimensional accuracy and repeatability of the constrained and unconstrained thermal deformations were compared. The proposed shape transformation method was further applied to AM and to the in situ assembly of a composite frame–membrane structure, where a functional membrane was integrated into a curved 3D frame without any additional assembly procedure.

To reveal the stress transfer mechanism of the flange in a split Hopkinson tension bar, explicit finite element analyses of the impact of the hollow striker on the flange were performed across a range of flange lengths. The tensile stress profiles monitored at the strain gauge position of the incident bar are interpreted on a qualitative basis using three types of stress waves: bar (B) waves, flange (F) waves, and a series of reverberation (Rn) waves. When the flange length (Lf) is long (i.e., Lf &gt, Ls, where Ls is the striker length), the B wave and first reverberation wave (R1) are fully separated in the time axis. When the flange length is intermediate (~Db &lt, Lf &lt, Ls, where Db is the bar diameter), the B and F waves are partially superposed, the F wave is delayed, then followed by a series of Rn waves after the superposition period. When the flange length is short (Lf &lt, ~Db), the B and F waves are practically fully superposed and form a pseudo-one-step pulse, indicating the necessity of a short flange length to achieve a neat tensile pulse. The magnitudes and periods of the monitored pulses are consistent with the analysis result using the one-dimensional impact theory, including a recently formulated equation for impact-induced stress when the areas of the striker and bar are different, equations for the reflection/transmission ratios of a stress wave, and an equation for pulse duration time. This observation verifies the flange length-dependent stress transfer mechanism on a quantitative basis.

The Split Hopkinson tensile Bar (SHTB) is one of the most widely used methods to study various material behaviors under tensile loading and high nominal strain rate. Since the specimens and grips with thread may disturb the wave propagation between bars and specimen, the stress-strain relation of the specimen, which is calculated from the strains of the incident and transmitter bars, may not match with the stress-strain relation measured directly from the specimen. In order to ensure a high accuracy of SHTB system, specimen calibration is usually carried out. To ensure the no wave distortion in the thread region, the stress wave should be entirely transmitted from the incident bar to the transmitter bar in tests using the calibration specimen (not the real specimen) of the same material and the same diameter with the transmitter and incident bars. Therefore, the strain signals at the incident and transmitter bars need to be the same. This study investigated the wave transmit characteristics through threads in SHTB test. To investigate the effect of the thread on the wave transmission, a specimen with the same diameter of the incident and the transmitter bars is used in the SHTB test. The effects of the thread inner diameter, yield stress, thread type, and strike velocity on the wave propagation characteristics are investigated. Based on the analysis results for various conditions, a proper design guide for a thread shape to secure the measurement accuracy in the SHTB test is proposed.

Three types of surfaces in the Schroeder–Webster (SW) theory, i.e., sliding, mixed, and sticking surfaces, have been verified via finite element analysis of an axisymmetric compression test for a metallic specimen. Judging from (i) the radial profile of the pressure at the top elements and (ii) the radial displacement at the top nodes, the three types of SW surfaces are not manifested in the numerical simulation. However, the SW friction compensation model developed for the SW-sliding surface is remarkably reliable in predicting the measured stress–strain curve of the barreled specimen down to the height-to-diameter ratio of 0.1. The origin of this reliability is discussed along with recommendations for using the SW friction compensation model for the SW-sliding surface.

The load–displacement curves of an aluminum alloy and tantalum were determined using a hat-type specimen in the compression test. Based on the results of finite element analysis, the employed geometry of the hat-type specimen was found to yield a load–displacement curve that is nearly independent of the friction between the specimen and the platen. The flow stress–strain curves of the alloy and tantalum were modeled using the Ludwik and Voce constitutive laws, respectively; furthermore, simulation of the compression event of the hat-type specimen was performed by assuming appropriate constitutive parameters. The constitutive parameters were varied via an optimization function built in matlab until the simulated load–displacement curves reasonably fit the experimental curve. The optimized constitutive parameters obtained in this way were then used to construct friction-free flow stress–strain curves of the two materials.

Supplemental material, Supplemental Material2 for Evolution of specimen strain rate in split Hopkinson bar test by Hyunho Shin and Jong-Bong Kim in Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science

We investigated the protection capability of a plate against high speed projectiles demonstrating collision and penetration behaviors by finite element analysis. The element erosion method was used for penetration analysis, which showed that the speed of the projectile was slightly reduced by the collision with the protection plate. Protection capability was measured by the projectile’s attitude angle change because the damage of our tanks by projectiles was also dependent on the projectile-tank collision angle. When the length of the protection plate was sufficiently long, the projectile was severely deformed and incapacitated. In the case of a small plate, the projectile was deformed only in the collision region. Thus, projection capability was investigated by the change of attitude angle. The effect of collision angle, velocity, and length of the plate on the rotational and vertical velocities of the projectile was investigated.

The effects of barreling of a particulate compact specimen caused by the end restraint and rubber membrane during a conventional triaxial test on the yield surface of a pressure-dependent and linearly elastic-perfectly plastic constitutive model are investigated via finite element analysis. The combined influence of the end restraint and rubber membrane results in barreling of the specimen which causes artifacts in the experimental deviator stress vs. strain curve determined in the triaxial test. Values of the shear-failure deviator strengths at varying confinement pressures have been modified to exclude the artifact caused by the specimen barreling, followed by the refinement of the yield-surface parameter set of the constitutive model. The refined set predicts the experimental curve better than the unrefined one does.

This study considers process design in forming a hinge bracket. A thin hinge bracket is typically produced by bending a sheet panel or welding a hollow bar into a sheet panel. However, the hinge bracket made by bending or welding does not have sufficient durability in severe operating conditions because of the stress concentration in the bended region or the low corrosion resistance of the welded region. Therefore, this study uses forming to produce the hinge bracket part of a foldable container and to ensure durability in difficult operating conditions. An extrusion process for a T-shaped hinge bracket is studied using finite element analysis. Preliminary analysis shows that a very high forging load is required to form the bracket by forging. Therefore, extrusion is considered as a candidate process. Producing the part through the extrusion process enables many brackets to be made in a single extrusion and through successive cutting of the extruded part, thereby reducing the manufacturing cost. The design focuses on reducing the extrusion load and on ensuring shape accuracy. An initial billet is designed to reduce the extrusion load and to obtain a geometrically accurate part. The extruded part is bent frequently because of uneven material flow. Thus, extrusion die geometries are designed to obtain straight parts.

1 서울과학기술대학교 기계자동차공학과 (Department of Mechanical and Automotive Engineering, Seoul National University of Science and Technology)2 한국과학기술원 기계공학과 (Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology)3 서울과학기술대학교 기계시스템디자인공학과 (Department of Mechanical System Design Engineering, Seoul National University of Science and Technology) Corresponding author: wjchung@seoultech.ac.kr, Tel: +82-2-970-6390

The reason for the observation of an anomalously long pulse duration time of a signal transmitted from a soft specimen in a Kolsky bar experiment is investigated. A systematic numerical experiment based on explicit finite element analysis has been carried out using elastic specimens with varying elastic moduli. The soft specimen is in a compressive state even after the passage of the pulse because it cannot push the bars, such that a fairly long transmitted pulse is monitored at the gage position of the output bar. The arrival of the second pulse (tensile) from the right-end surface of the output bar releases the first transmitted signal (compressive) coming from the compressed state of the specimen, which puts an end to the fairly long transmitted pulse. Because the observation of a prolonged transmitted pulse is a natural phenomenon, the prolonged transmitted pulse should not annoy researchers in the process of confirming the validity of experiments on soft specimens. Our concern in verifying the validity of experiments can be limited to other aspects such as the process of measurement and the amplification of weak transmitted signals coming from soft specimens.

The correction functions that can be used to determine the stress state of a flattened disk specimen in diametral testing are provided by referring to the analytical solutions for arc- and point-loaded circular specimens. The stress components at three important locations in the flattened specimen (with varying degrees of truncation, w) were obtained numerically by finite element analysis, and compared with the analytical solutions at the center of the arc- and point-loaded specimens. The correction functions, which are provided as a function of w, can be useful tools for the analysis of the stress state and for the design of the flattened disk specimen in diametral testing.

As the sizes of electric products, such as mobile phones and watch phones, decrease, the joining bolt for the electric product should also be miniaturized. However, the miniature-sized bolt has to support sufficient joining torque and joining force. The bolt also has to support sufficient anti-releasing torque to keep the product fastened. We investigated a serrated bolt as a candidate for a miniature-sized fastener to increase the anti-release torque. In the serrated bolt, serration shapes are formed on the bottom surface of a bolt head to create an obstacle to releasing. In this study, finite element analyses were carried out on joining and releasing, and the anti-release torque was predicted. Through the joining and releasing analyses for various values of the elastic modulus and yield strength of the joined part, the effect of the mechanical properties of the joined part on the anti-releasing performance were investigated. The analysis results showed that a high strength insert nut is needed to increase the anti-releasing torque when the yield strength of the joined part is low, such as a plastic board in a mobile phone.

A short split Hopkinson bar system with a total bar length of 3 m (a 2 m input bar plus a 1 m output bar), a striker length of 254 mm, and a diameter of 25.4 mm, is designed. Through numerical experiments, the stress vs. strain curve and the rate vs. strain curve of the specimen are obtained from the bar signals. These measured curves are reasonably consistent with the input stress-strain curve of the specimen for the numerical experiment and the prediction by the recently reported rate equation, respectively, verifying the reliability of the designed SHPB system.

In order to analyze necking behavior with a continuum level approach, initial imperfection or bifurcation algorithm should be employed. The initial imperfection is arbitrary and has a great effect on the necking behavior of sheet metals. Most polycrystalline materials have natural imperfection such as orientation mismatches across the grain boundaries. In polycrystal plasticity, orientation mismatch is considered as material imperfection instead of geometric imperfection including thickness unevenness. The main idea of this study is to investigate the possibility that the stress concentration on a grain boundary (caused by orientation mismatch) works as imperfection and finally causes necking. For the consideration of stress concentration along grain boundaries, a crystal plasticity model was introduced in three-dimensional finite element analysis of a tensile test. A small region of a tensile specimen was divided into sufficient number of octahedral grains and each grain was discretized by fine tetrahedron elements. The same orientation angle was allocated for all the elements in each grain, but different orientations for grain by grain. Using this crystal plasticity-based analysis, stress concentration in the grain boundaries can be predicted. Also, four damage models based on the critical strain and stress were proposed to consider material softening due to void initiation and growth under the framework of crystal plasticity theory. The damage parameters for each model were determined based on the stress–strain relations obtained from experiment. Void nucleation, growth and coalescence behaviors during necking were reasonably predicted. Finally, the predicted necking behaviors of AA 6022-T4 were compared with the experimental results in terms of necking strain, deformed shape, and necking direction.

This research was carried out to determine the differences of physiological activites between Grifola frondosa of log cultivation(LC) and Grifola frondosa of bottle cultivation(BC). Total flavonoids content, total phenolics content, electron donating ability(EDA), nitrite-scavenging ability(NSA), SOD-like activity and inhibitory effect of Xanthine oxidase were examined. The highest value of total flavonoid content is in water extract from Grifola frondosa of log cultivation at (LC-W40) but, one of total phenolics compound is in water extract from Grifola frondosa of bottle cultivation at (BC-W40). The EDA using DPPH of BC-W40 extract showed the highest value of . Nitrite-scavenging ability was in extract from Grifola frondosa of BC-W40 at pH 1.2. The value was SOD-like activity showed the highest value of in extract from LC-W40. Xanthine oxidase inhibitory activity was the highest value of in extract from Grifola frondosa BC-W40, and dependent on concentrations. These results showed that a the antioxidant effects of Grifola frondosa is excellent. However, physiological activities of Grifola frondosa were not depend on caltivation method regulary, and were different according to kind of solvents, concentraitions and physialogical factors examined such as EDA, SOD-like activity and NSA.

The physical meanings of the cap aspect ratio (R) and transition surface parameter (α) of the modified Drucker-Prager cap (MDPC) model have been uncovered in relation to the deviator stress curves of a particulate material in conventional triaxial testing by simulating the curves using varying R and α based on finite element analysis. R controls the rate of the stress rise with the increase of the strain; the smaller the R, the faster the rise of the deviator stress. This phenomenon occurs because, in the p-q plane (p is the mean stress and q is the Mises equivalent stress), the cap with a smaller R needs to move a shorter distance on the p axis to maintain the current stress state: a smaller volumetric plastic strain is required according to the hardening law. R does not influence the maximum value of the deviator stress curve. As for the influence of α, it artificially lowers the true failure surface by an amount that is proportional to α so that a fictitious ultimate failure state is achieved. Therefore, it is desirable to set α as small as possible unless the numerical analysis using the MDPC model does not produce a converged solution. An analytical expression to calculate the maximum deviator stress that can be predicted by the MDPC model is provided in terms of α.

1 서울과학기술대학교 자동차공학과 (Department of Automotive Engineering, Seoul National University of Science and Technology)2 서울과학기술대학교 기계자동차공학과 (Department of Mechanical and Automotive Engineering,Seoul National University of Science and Technology)3 서울과학기술대학교 기계시스템디자인공학과 (Department of Mechanical System and Design Engineering,Seoul National University of Science and Technology) Corresponding author: jbkim@seoultech.ac.kr, Tel: +82-2-970-6434

1 서울과학기술대학교 기계시스템디자인공학과 (Department of Mechanical System and Design Engineering,Seoul National University of Science and Technology)2 서울과학기술대학교 기계자동차공학과 (Department of Mechanical and Automotive Engineering,Seoul National University of Science and Technology) Corresponding author: kpark@seoultech.ac.kr, Tel: +82-2-970-6358

Two types of cap parameters were determined by fitting the experimentally obtained triaxial deviator stress curves found in the literature using the modified Drucker–Prager cap model. The two types of parameters were extracted using the specimen model that deforms uniformly or non-uniformly (barreling). The parameter set extracted from the non-uniformly deforming specimen model reasonably predicted the experimental deviator stress curves obtained from the barreling specimen. However, when the parameter set extracted from the uniformly deforming specimen model was used to simulate the experimental curves from the barreling specimen, the prediction was inferior to the set from the non-uniformly deforming model. Considering versatile types and properties of particulate materials (e.g., ceramic, metal, pharmaceutical powders and soils), we suggest to verify the prediction capabilities of the parameter sets from each specimen model. The stress path of the barreling specimen during the conventional triaxial test was fairly complex depending on the position in the specimen. Pure hydrostatic loading was not achieved even during the hydrostatic loading stage. In the shear loading stage, the position-averaged stress path showed a lower slope in the meridional plane than that of the uniformly deforming specimen with a slope of 3 .

The present study aims to develop a warm forming process for an aluminum alloy sheet to reduce weight of an electric housing part. Considering that the target product is produced through a series of metal forming processes using a progressive die, high frequency induction heating is proposed to heat a specific drawing punch locally and rapidly, out of a series performed using a progressive die. To validate the effect of the induction heating on the formability improvement of aluminum alloys, a fully coupled numerical simulation is proposed: (i) finite element (FE) analyses for the multi-stage deep drawing processes are performed to predict formability at each stage; (ii) electrostatic-thermal coupled FE analysis is performed to predict the temperature rise of the drawing punch; and (iii) thermo-mechanical coupled analysis is conducted to investigate the effect of induction heating on the formability change. Through the proposed coupled simulation, it is proven that the drawing punch can be efficiently heated without a significant increase in cycle time, and the corresponding drawing process using aluminum alloys can be successfully performed without forming failure.

Laser bending of metal sheet is a complex process that is achieved by introducing thermal stress into a metal sheet through irradiation using a defocused laser beam. In a single scanning process, it is important to know the effects of the engineering parameters on the final bending angle, such as the laser power, laser beam diameter, and scanning speed. Furthermore, it is important to investigate the effect of the scanning direction in a multiple scanning process. In this paper, FEM analyses for a laser bending process of DP980 high strength steel sheet under different laser powers, scanning speeds, line energies, and scanning directions are performed to analyze the effects of the engineering parameters and the scanning direction on the final bending angle. The results indicate that the final bending angle of a DP980 steel sheet increases in a linear increase fashion with the increase of the line energy in the single scanning process; it also increases when the scanning direction is changed time by time in the multiple scanning process.