High-speed Deformation Research Articles

On structural transformations in a material under nonstationary actions

A model of material of complex crystalline structure consisting of two lattices coupled by nonlinear interaction forces that ensure several stable equilibrium configurations is considered. The continuum model is compared with the discrete model whose analysis reveals the effect, which has been observed in high-speed deformation experiments, of decrease in the initial pulse under nonstationary actions.

Mechanisms of high-speed deformation of polycrystalline copper

The mechanisms of high-speed deformation of ultrafine-grained copper produced during severe plastic deformation by equal-channel angular pressing were analyzed using numerical modeling in comparison with those in the case of coarse-crystalline copper. The activity of annihilation processes during nonconservative motion and double cross slip of dislocations was estimated. Their effect on the macroscopic behavior of samples is shown.

1 GPa級高強度TRIP型複合組織鋼の高速引張変形挙動

1 GPa級高強度TRIP型複合組織鋼の高速引張変形挙動

Microstructure and strain-stress analysis of the dynamic strain aging in inconel 625 at high temperature

Serrated flow is a result of unstable plastic flow, which occurs during tensile and compression tests on some dilute alloys. This phenomenon is referred as the Portevin Le-Chatelier effect (PLC effect). The aim of this research was to investigate and analyze this phenomenon in Inconel 625 solution strengthened superalloy. The tested material was subjected to tensile tests carried out within the temperature range 200-700 °C, with three different strain rates: 0.002 1/s, 0.01/s, and 0.05 1/s and additional compression tests with high deformation speeds of 0.1, 1, and 10 1/s. The tensile strain curves were analyzed in terms of intensity and the observed patterns of serrations Using a modified stress drop method proposed by the authors, the activation energy was calculated with the assumption that the stress drops’ distribution is a direct representation of an average solute atom’s interaction with dislocations. Subsequently, two models, the standard vacancy diffusion Bilby-Cottrell model and the realistic cross-core diffusion mechanism proposed by Zhang and Curtin, were compared. The results obtained show that the second one agrees with the experimental data. Additional microstructure analysis was performed to identify microstructure elements that may be responsible for the PLC effect. Based on the results, the relationship between the intensity of the phenomenon and the conditions of the tests were determined.

Experimental and numerical analysis of the high-speed deformation and erosion damage of the titanium alloy VT-6

The temporal characteristics of the dynamic fracture of the titanium alloy VT-6 have been investigated under high-speed loading conditions. A relationship has been established between the process of dynamic tension of the specimen according to the Kolsky method and the surface erosion damage. A numerical analysis of the experimental data has been carried out. The method of erosion loading has been further developed as a tool for the dynamic testing of strength properties of materials.

A Measurement of Volume Resistivity in Fe-Based Shape Memory Alloy under Tensile Deformation and its Evaluation of Strain Rate Sensitivity

Fe-based shape memory alloy (Fe-SMA) shows the smaller shape memory effect (SME) compared with the widely-used NiTi alloy. However, because its production cost is much lower than the NiTi alloy, Fe-SMA is challenged to be applied in civil engineering fields such as vibration absorbers and joints. A key of the SME is stress-induced martensitic transformation. Thus, it is important to evaluate an amount of martensite, which can control such excellent performance of Fe-SMA, for increasing a reliability of the Fe-SMA. However, until now, it is quite hard to find studies to evaluate the amount of martensite in Fe-SMA experimentally during deformation at various strain rates, especially during high speed deformation. Instead of the evaluation, it is convenient to capture change in volume resistivity, which has a correlation with the amount of martensite, at various strain rates. In the past, the volume fraction of α’ martensite is evaluated by using a resistance measurement based on the four point-probes method. The advantages of the method are quite simple and relatively high precision, however, its disadvantages are a requirement of strictly-precise reference resistor and power supply, and it is easily affected from noise. In this study, at first, a circuit of Kelvin double bridge with a higher precision is assembled. Then, the rate sensitivity of volume resistivity in Fe-28Mn-6Si-5Cr alloy, which is a kind of Fe-SMA, is experimentally estimated by using the assembled circuit of Kelvin double bridge during tensile testing at various strain rates.

Nonlinear FEM based high-speed shell shattering simulation for shelled edible agricultural products: Pecan fruit shattering

This paper introduces an advanced engineering simulation procedure for the nonlinear finite element method (FEM) based high-speed shattering case of shelled edible agricultural products. A high-speed impactor which is targeted at the Pecan fruit (kernel-in-shell) was considered in this case study. Physical compression tests were conducted on Pecan fruit specimens and experimental deformation characteristics were utilized to describe realistic material models in the FEM based engineering simulation. Subsequently, a reverse engineering approach was employed in the solid modeling stage and the Pecan shell shattering case under high-speed loading was simulated, considering the explicit dynamics approach. The effect of the high loading rate on the deformation characteristics of the Pecan fruit components was observed. Visual outputs from the simulation revealed the shattering behavior of the Pecan fruit components under defined boundary conditions. In addition to useful visual simulation outputs, time-dependant stress distributions on the Pecan fruit under high-speed loading rates were represented using graphs. Simulation results have revealed that maximum equivalent stress values were 7.1 (MPa), 5.1 (MPa), and 0.336 (MPa) for shell, packing material, and kernel, respectively. Maximum reaction force at impact was calculated as 996,000 (N). This work contributes to further research into the use of nonlinear numerical method based high-speed deformation simulation studies for shelled edible agricultural products.

Formation mechanism of the high-speed deformation characteristic microstructure based on dislocation slipping and twinning in α-titanium

Abstract

Effects of Initial Micro-Structures on Deformation Behaviors of Commercial Pure Titanium

In this paper, effects of initial micro-structures on deformation behaviors of commercial pure titanium were elaborated by investigating the evolution of dislocation boundary and its adiabatic shear sensitivity. At the low to medium stain rates, the main plastic deformation mechanism of as-annealed commercial pure titanium is dislocation slipping. Meanwhile, geometrically necessary boundaries (GNBs) with different directions are generated and crossed with each other. However, new dislocation boundaries are formed in as-cold rolled plates, which are parallel to the initial ones induced by cold rolling. When the strain rate is up to 1000 s-1, the initial dislocation boundary playes an adverse role in the adiabatic shear sensitivity of commercial pure titanium. The adiabatic shear band is the high-speed deformation characteristic micro-structure in commercial pure titanium. In addition, dynamic recrystallized grains are generated in the center of an adiabatic shear band, which is consistent with the sub-grain rotation mechanism.

High-speed deformation of a high-strength coated thin plate

The punch with cone-shaped working part impact into aluminum alloy thin-walled structures is mathematical modelled applying thermo-mechanical coupling of the physical processes during their deformation. The nonlinear physical properties contain a nonlinear stress-strain rate dependence of temperature. To solve the contact problem the sliding boundary conditions (friction) on moving bodies introduced. Numerical simulation of impact process performed using the finite element method and the independent Lagrange-Euler approach for two variants of the thin-walled box structure. The solution of the dynamic viscous plastic contact problem allowed to determine and to design the stress-strain fields in the thin-walled box section. For comparison, the deformation process of the similar aluminum construction with an additional rib considered when the punch with the conical working part impact. Analysis of stress-strain state for two various types of structural geometries demonstrated that mounting the additional medial rib into its design let redistribute the stress fields and significantly reduce the plastic deformation area during the impact because of four fold increasing in its contact stiffness. Such modification of the structure rigidity may improve the strength properties of the entire composite security device applying changes only to its part.

Study of Microstructure Formation of Carbon Steel Under High-Speed and Multicycle Hot Plastic Compressive Deformation Using A Gleeble 3500 Unit

Study of Microstructure Formation of Carbon Steel Under High-Speed and Multicycle Hot Plastic Compressive Deformation Using A Gleeble 3500 Unit

Effect of Hafnium on the Structural and Mechanical Properties of the Surface Layers on the Basis of TiNi

The study contains a complex method of forming the surface-modified layers using materials with shape memory effect by high-speed gas-flame spraying of mechanically activated powders based on TiNiHf, followed by thermal and thermomechanical treatment. This allowed us to form the nanoscale state in the surface layers, which have high levels of functional, mechanical and performance properties. We demonstrated that pre-mechanoactivation of TiNiHf powder has reduced the coatings' porosity (less than 1%) and provided adhesive strength of the coating with the substrate (110 to 120 MPa). We described the formation mechanism of nanostructured state in TiNiHf powders under severe plastic deformation during mechanical activation, comprising the steps of high-speed deformation, polygonization and recrystallization.

Design of New Type High-Speed Cutting Deformation Test Device

To study the mechanism of metal cutting deformation, we invented a new type of explosion rotary quick-off tools test device which was based on absorbing advantages of existing cutting deformation test device. So that the tool can be separated from the workpiece with a larger acceleration and the complete chip root samples can be obtained. This new type of explosion rotary quick-off tools test device composed of foundation, explosion chamber, piston body, pressure plate, locking wedge and customized tools, which has many advantages such as simple structure, convenient operation and high rigidity.

On the synergy of diffusible hydrogen content and hydrogen diffusivity in the mechanical degradation of laboratory cast Fe-C alloys

The present work investigates the influence of hydrogen on the mechanical properties of generic Fe-C alloys by tensile tests on notched samples. Different microstructures, such as pearlite, bainite and martensite are generated in a 0.2%C Fe-C alloy by an appropriate heat treatment. “Pure” iron is used as a reference material and a variation in the carbon content up to 0.4% is established for the bainitic grade. The effect of hydrogen is demonstrated by mechanical tests on both in-situ hydrogen charged and uncharged specimens. At high cross-head deformation speed (5mm/min), the results indicate a considerable, though variable hydrogen effect for different microstructures. The bainitic and martensitic materials both show ductility drops of about 20%, whereas the pearlitic and ferritic grades display a higher sensitivity to hydrogen embrittlement (HE) with a ductility loss of approximately 50%. In order to evaluate the role of the diffusible hydrogen, tensile tests are performed at a lower cross-head deformation speed (0.05mm/min) as well. Next to the correlation between the amount of diffusible hydrogen and HE, the distance over which hydrogen can diffuse during a tensile test, determined by hydrogen diffusion coefficient, seems to play a crucial role as well.

ULTRAFINE-GRAINED STRUCTURE FORMATION IN CARBON STEEL BY HIGH-SPEED COMPRESSION DEFORMATION AT INCREASED TEMPERATURES

The velocity increase of plastic deformation leads to significant changes in the microstructure of metallic materials. The structure and properties of the metal defi ne such factors as pressure value (or impulse), deformation velocity (or the duration of the process) and temperature. The work is devoted to the research of the influence of high-velocity deformation on the microstructure of materials. Using research complex Gleeble 3500 high-speed deformation of steel grade 20 at temperatures of 800, 900, 1000 and 1200 °C was carried out. The microstructure was studied and the microhardness of samples was determined. The principal possibility to provide strain refi nement of low-carbon steel structure up to a grain size of about 400 nanometers during high-speed deformation at temperatures of 800–1000 °C is shown, just as it is achieved at a severe plastic deformation without heating or with a slight heating which doesn’t exceed the recrystallization temperature.

Raising the Fatigue Resistance of Titanium Alloys by High-Speed Deformation in the Range of Polymorphic Transformations

Methods for raising the fatigue limit of critical parts by forming the required properties and structure in preforms from α + β titanium alloys by high-speed pressing at a temperature above the point of polymorphic transformations and subsequent heat treatment are described. Amethod for experimental determination of expedient thermomechanical regimes for raising the fatigue limit of the materials of articles is considered for billets from alloy VT9.

Strengthening and failure mechanisms of individual carbon nanotube fibers under dynamic tensile loading

Rate-dependent mechanical properties of individual carbon nanotube (CNT) fibers are essential for developing advanced anti-collision composites. In this work, a free-falling Hopkinson tension bar was designed to measure the dynamic tensile strength of CNT fiber. The strength of pure CNT fiber is found to be more sensitive to strain rate than that of composite CNT fiber. The dynamic failure morphologies of pure and composite CNT fiber were compared with their quasi-static responses. It was observed that the pure CNT fiber tends to break off at the weakest point, leading to unraveling of fiber along the twisted direction. On the other hand, the infiltrated polymer tends to change the failure mode and rate sensitivity of CNT fibers. Furthermore, high speed deformation analysis under dynamic loading revealed that pure CNT fiber tends to fail at the weakest point while the composite CNT fiber breaks into several fragments.

Formation of ultrafine-grain structure in carbon steel by high-temperature high-speed compression

Increasing the rate of plastic deformation considerably changes the microstructure of metals. The structure and properties of metals are determined by factors such as the pressure (or pulse), the strain rate (or duration of the process), and the temperature. The influence of high-speed deformation on the microstructure of materials is studied. After high-speed deformation of steel 20 samples by means of the Gleeble 3500 system, at 800, 900, 1000, and 1200°C, their microstructure and microhardness are determined. In principle, the grain size in low-carbon steel 20 may be reduced to around 400 nm by fast deformation at 800–1000°C, as in profound plastic deformation with little or no heating of the metal (not exceeding the recrystallization temperature).

Determination of processing maps for the warm working of vanadium microalloyed eutectoid steels

Microalloying of an eutectoid steel with V may facilitate formation of dispersed nano-scaled VC particles in the microstructure during thermomechanical processing or subsequent heat treatment. In this research, the constitutive flow behavior of vanadium microalloyed eutectoid steel has been investigated in the temperature range 620–770°C at strain rates in the range 0.01–10s−1. Microstructural characterization of the deformed specimens was conducted using SEM and EBSD techniques. In this context, various deformation mechanisms occurring during warm deformation have been characterized and delineated through construction of a processing map by establishing a power dissipation map and an instability map for the steel and superimposing them. The results show that the pearlitic microstructure exhibits several deformation mechanisms within these warm working conditions. Dynamic spheroidization of cementite lamella takes place in the range 660–720°C and 0.01–0.1s−1 with a power dissipation efficiency of 27–33%, characterizing a safe window of processing this steel. The presence of vanadium carbides at grain boundaries strengthened the pearlitic microstructure and retarded the occurrence of some deformation defects during low temperature, high speed deformation in the range 620–720°C and 1–10s−1.

On the Origin and Handling of the Force Oscillation Phenomenon in Tensile Impact Testing of Polymer Materials

Stress respectively force oscillation in mechanical testing is a phenomenon that can be monitored independent of material, stress state and test setup. This study looks at the phenomenon of force oscillation in tensile impact testing on viscoelastic materials and its dependency on longitudinal stress wave propagation. In addition to the extensive research on stress wave propagation and the force oscillation phenomenon, published within the last decades, the Society of Automotive Engineering (SAE) also gives valuable recommendations regarding tensile impact testing on polymer materials. Existing limitations to tensile impact testing reported earlier can be overcome using the approach outlined in this paper. Furthermore, we give an analysis to stress wave propagation in tensile impact testing and its correlation to force oscillations: We find that (1) longitudinal stress wave propagation in tensile specimens cannot be considered to be the origin of the force oscillation phenomenon - even more so it can be neglected, (2) specimens in tensile impact testing are exposed to a nearly oscillation free loading which is presented for haul-off velocities of up to 10 m/s and (3) it is not possible to achieve force equilibrium in the load train conducting tensile impact tests. In addition, we present an approach to measure a nearly oscillation free force signal in tensile impact testing on polymer materials up to 10 m/s. For this purpose a modification of a standard dumbbell specimen is introduced. Measurements presented in this paper are conducted using state of the art 3D high-speed deformation measurement technology based on Digital Image Correlation (DIC).