Strain Rate Sensitivity Of Material Research Articles

On the numerical simulation of sheet metal blanking process

The use of the blanking process has been widely spread in mass production industries. In this technique, the quality of the final product is directly related to the setting parameters of the process and the material response of the sheet. In the present work, a general framework based on the finite element method for the simulation of the sheet metal blanking process is presented. The proposed approach properly addresses all the numerical challenges related to blanking. First, an extension of elasto-viscoplastic constitutive equations for the large strain regime is used to take into account the material strain-rate sensitivity. Then, the inertial effects coming from high velocity operations are considered by means of an implicit time integration scheme. Moreover, the frictional contact interactions are simulated with the classical Coulomb law and an energetically consistent formulation of area regularization. Finally, ductile fracture is modeled thanks to the element deletion method coupled with a fracture criterion. The blanking process is then simulated for different setting parameters. The accuracy of this approach is evaluated by comparing the numerical predictions to experimental results for both quasi-static and dynamic conditions. Good agreement is found between experimental and numerical results for all cases.

Modelling the constitutive behaviour of aluminium alloy B206 in the as-cast and artificially aged states

The constitutive behaviour of the aluminium-copper casting alloy B206 has been investigated in both the as-cast and artificially aged states. For the as-cast material, a unified plastic flow stress model has been developed using experimental data acquired from compression tests performed using a Gleeble 3500 thermo-mechanical simulator. The unified model considers different constitutive models, each for a specific temperature interval, and the transition between these models at an intermediate temperature based on the material's strain rate sensitivity. For the artificially aged material, a linear-fit yield strength evolution model as a function of the heat treatment parameters has been developed using experimental data from tensile tests and hardness measurements. The yield strength model takes advantage of an independently developed microstructure model that specifically describes the precipitation kinetics of the material based on differential scanning calorimetry measurements. Evaluations of the developed models show good fits between the predicted strengths and data from experiments as well as the literature.

Strain rate sensitivity of Cu/Ta multilayered films: Comparison between grain boundary and heterophase interface

Grain boundaries and heterophase interfaces both play important roles in enhancing the strain rate sensitivity (SRS) of engineering materials as they often serve as obstacles for dislocation motion. In this work, however, we carried out nanoindentation tests on Cu/Ta multilayers prepared with a wide range of modulation periods (Λ) and modulation ratios (η) and found negative contribution of incoherent interfaces to SRS. Activation event extension aided by incoherent interface shear was suggested as the dominating mechanism for rate related deformation process in Cu/Ta multilayers. The results provided valuable insights into the fundamental roles of grain boundaries and heterophase interfaces in plastic deformation.

Simplified analytical method for evaluating web girder crushing during ship collision and grounding

The paper presents a simplified analytical method to examine the crushing resistance of web girders subjected to local static or dynamic in-plane loads. A new theoretical model, inspired by existing simplified approaches, is developed to describe the progressive plastic deformation behaviour of web girders. It is of considerable practical importance to estimate the extent of structural deformation within ship web girders during collision and grounding accidents. In this paper, new formulae to evaluate this crushing force are proposed on the basis of a new folding deformation mode. The folding deformation of web girders is divided into two parts, plastic deformation and elastic buckling zones, which are not taken into account for in the existing models. Thus, the proposed formulae can well express the crushing deformation behaviour of the first and subsequent folds. They are validated with experimental results of web girder found in literature and actual numerical simulations performed by the explicit LS-DYNA finite element solver. The elastic buckling zone, which absorbs almost zero energy, is captured and confirmed by the numerical results. In addition, the analytical method derives expressions to estimate the average strain rate of the web girders during the impact process and evaluates the material strain rate sensitivity with the Cowper-Symonds constitutive model. These adopted formulae, validated with an existing drop weight impact test, can well capture the dynamic effect of web girders.

Investigation on activation volume and strain-rate sensitivity in ultrafine-grained tantalum

The activation volume and strain-rate sensitivity (SRS) of a plastic deformation process and their relationship are investigated in the present work. Through theoretical modeling based on the movement of both isolated kinks and kink pairs in a dislocation line, a new form of theoretical relationship between the strain-rate sensitivity and activation volume was developed, which reduces to the conventional and frequently used form of the same relationship when SRS is approaching zero. The strain rate jump test during the necking stage of tensile testing process is validated through combined theoretical analytical approaches, for obtaining strain rate sensitivity of materials with short uniform tensile stage, such as UFG metals. To validate the theoretical approaches and modeling presented in this work, strain-rate jump tests and repeated stress relaxation experiments were conducted, to measure the SRS and activation volume of commercial pure ultrafine-grained tantalum prepared by equal channel angular pressing. The experimental relationship between the strain-rate sensitivity and the activation volume of these tantalum samples fits well with the new form of theoretical relationship developed in this investigation. Experimental data from literature on nickel confirms the new form as well. The new form of theoretical relationship between strain-rate sensitivity and activation volume can be applied for both bcc and fcc metals as long as the major plastic deformation mechanism is dislocation gliding.

Capability of New High Strength ADI‐Materials for Automotive Components under Crash Loading

The development of new high strength ADI (Austempered Ductile Iron) materials and their evaluation for light weight design purposes are parts of the German research project LEA. The Fraunhofer IWM is investigating the capabilities of ADI materials under crash loading situations by conducting high rate tensile tests which quantify the material's strain rate sensitivity. These investigations result in a reduction of the austenitic phase content of initially about 35%, decreasing to 5–10% after fracture (as characterized by X‐ray diffraction), indicating a phase transformation comparable to a TRIP effect. Several ADI 1000 and ADI 1200 samples show an increase in strength of about 100 MPa up to crash‐relevant strain rates of 100 s−1, and the elongation at fracture is constant or increasing, in some cases up to more than 20%. Thus, the newly developed high strength ADI materials exhibit good potential for crash loaded automotive components.

Dynamic Crushing of All-Metallic Corrugated Panels Filled With Close-Celled Aluminum Foams

Under quasi-static uniaxial compression, inserting aluminum foams into the interstices of a metallic sandwich panel with corrugated core increased significantly both its peak crushing strength and energy absorption per unit mass. This beneficial effect diminished however if the foam relative density was relatively low or the compression velocity became sufficiently high. To provide insight into the varying role of aluminum foam filler with increasing compression velocity, the crushing response and collapse modes of all metallic corrugate-cored sandwich panels filled with close-celled aluminum foams were studied using the method of finite elements (FEs). The constraint that sandwich panels with and without foam filling had the same total weight was enforced. The effects of plastic hardening and strain rate sensitivity of the strut material as well as foam/strut interfacial debonding were quantified. Three collapse modes (quasi-static, transition, and shock modes) were identified, corresponding to different ranges of compression velocity. Strengthening due to foam insertion and inertial stabilization both acted to provide support for the struts against buckling. At relatively low compression velocities, the struts were mainly strengthened by the surrounding foam; at high compression velocities, inertia stabilization played a more dominant role than foam filling.

Response of Semi-Buried Structures Subjected to Multiple Blast Loading Considering Soil–Structure Interaction

Dynamic response of semi-buried structures subjected to non-contact multiple blast loadings resulting from conventional weapon detonation is investigated. The structure is modelled using shell elements, wherein, the effects of soil–structure interaction are incorporated by modelling the soil using frequency independent spring–dashpot–mass model. The strain rate sensitivity of structural steel material is considered using Johnson–Cook material model. A detailed investigation is carried out to understand the effects of standoff distance, explosive amount, strain rate and provision of stiffeners on the dynamic behaviour of semi-buried structure. The present investigation is focused on soil–structure interaction and multiple blasts which are the realistic scenarios. The finite element analysis is carried out using commercially available finite element package ABAQUS/Explicit V6.7. Results indicate that the buried depth of structure, soil–structure interaction and strain rate governs the dynamic behaviour of the structure. It is concluded that blast design of structure is governed by the time interval between successive blasts, and not merely by single blast of the given amount of explosive.

Strain rate sensitivity of Al-based composites reinforced with MnO2 additions

Fine-grained Al-based composites reinforced with MnO2 particles were manufactured by means of powder metallurgy (PM) and mechanical alloying (MA) methods. It was found that the applied powder consolidation methods, including KOBO extrusion, did not induce any chemical reaction between thermodynamically unstable components. However, it was shown that addition of magnesium to the Al-matrix initiated a reaction in the vicinity of MnO2 particles that resulted in the nucleation and growth of nano-sized aluminum–magnesium oxides. This led to a local refining of structural components. The most intense refining of structural components was observed for the MA Al–MnO2 composite. Strain rate sensitivity (SRS) of as-extruded materials was tested in compression in the range 293–773K. SRS was determined by making a rapid change in the basic true strain rate from ε̇=1.2·10-3 to ε̇=1.2·10-2. It is found that SRS did not practically depend on strain. The highest value of SRS was observed for the PM Al–MnO2–Mg composite. SRS of PM materials evidently increases with deformation temperature; however, it becomes smaller above a temperature of ∼600K. For the MA Al–MnO2 composite, tested at high temperatures, primary mechanical alloying resulted in relatively low increase of SRS with temperature that also becomes smaller above ∼600K. Suppression of the increase in SRS at high temperatures can be attributed to the specific features of grain boundaries created by the adhesive bonding between powder particles. This could hinder grain boundary sliding mechanisms and micro-cracks development at the compound interfaces.

Effect of Surface Modification on Strain Rate Sensitivity of Polypropylene/Muscovite Layered Silicate Composites

Surface modification is one of the treatment methods that can be implemented to improve the strain rate sensitivity of composite materials. In this study, both untreated and treated polypropylene/muscovite layered silicate composites were tested under static and dynamic loading up to 1100 s-1 using the universal testing machine and the split Hopkinson pressure bar apparatus, respectively. Muscovite particles were treated with lithium nitrate and cetyltrimethylammonium bromide as a surfactant through ion exchange treatment. Results show that the treated polypropylene/muscovite specimens with fine state of dispersion level shows better rate of sensitivity as compared to untreated polypropylene/muscovite specimens under a wide range of strain rate investigated. Apart from that, the rate of sensitivity of both tested polypropylene/muscovite layered silicate composites also show great dependency on the strain rate sensitivity was steadily increased with increasing strain rate. Unfortunately, the thermal activation values show contrary trend. Key words: Ion exchange treatment; Strain rate sensitivity; Muscovite particles; Split Hopkinson pressure bar apparatus; Strain rates

Experimental and modeling study of dynamic mechanical properties of cement paste, mortar and concrete

Understanding the strain rate effects on cement-based materials are important for accurately modeling concrete structure damage to high-velocity impact and blast loads. This paper reports experimental results of the strain rate effect on paste, mortar and concrete. A pulse-shaped split Hopkinson pressure bar (SHPB) was employed to determine the dynamic compressive mechanical responses and failure behavior of paste, mortar and concrete under valid dynamic testing conditions. Quasi-static experiments were conducted to study material strain rate sensitivity. Strain rate sensitivity of the materials is measured in terms of the stress–strain curve, elastic modulus, compressive strength and critical strain at peak stress. Empirical relations of dynamic increase factor (DIF) for the material properties are derived and presented. A compressive constitutive model with strain rate and damage effects was modified to accurately describe the dynamic compressive stress–strain curves of the materials.

Modeling of strain rate effects for concrete with viscoelasticity and retarded damage

Especially the tensile strength of concrete may by far exceed the quasistatic values under high strain rate conditions. Due to present knowledge this is predominantly caused by the higher resistance of water in the capillary system of concrete on one hand and by the limited speed of the evolution of micro and macro cracks on the other hand. This contribution combines a viscoelastic approach for the first case with a gradient damage approach for the second, whereby damage is retarded due to inertial effects under high strain rates. The combined material law includes quasistatic behavior as a special case and covers dynamic material behavior up to a strain rate in a magnitude of 103 s−1. The material law is implemented in a Finite Element Method and first of all applied to uniaxial wave propagation problems. Thus, typical experimental results for dynamic tensile strength increase factors may be reproduced within a calibration procedure for the three dynamic material parameters of the material law. Furthermore, the structural behavior of a plain concrete beam under impact loading is investigated. The effects of structural mass inertia and the material's strain rate sensitivity to the dynamic load bearing capacity are exemplarily outlined. Finally, it is demonstrated how the dynamic load bearing capacity depends on load duration beneath load amplitude.

Dynamic buckling of an inclined strut

The dynamic compressive response of a sandwich plate with a metallic corrugated core is predicted. The back face of the sandwich plate is held fixed whereas the front face is subjected to a uniform velocity, thereby compressing the core. Finite element analysis is performed to investigate the role of material inertia, strain hardening and strain rate hardening upon the dynamic collapse of the corrugated core. Three classes of collapse mode are identified as a function of impact velocity: (i) a three-hinge plastic buckling mode of wavelength equal to the strut length, similar to the quasi-static mode, (ii) a ‘buckle-wave’ regime involving inertia-mediated plastic buckling of wavelength less than that of the strut length, and (iii) a ‘stubbing’ regime, with shortening of the struts by local fattening at the front face. The presence of strain hardening reduces the regime of dominance of the stubbing mode. The influence of material strain rate sensitivity is evaluated by introducing strain rate dependent material properties representative of type 304 stainless steel. For this choice of material, strain rate sensitivity has a more minor influence than strain hardening, and consequently the dynamic collapse strength of a corrugated core is almost independent of structural dimension.

Analysis of anisotropy and strain rate sensitivity of open-cell metal foam

This paper addresses numerical and experimental analysis of the m.pore® aluminium foam. Numerical models are based on computed tomography data in order to capture the complex material meso-structure. Uni-axial experimental tests were performed for quasi-static loading and an excellent agreement with numerical results has been obtained. Numerical analyses were extended for characteristic strain rates in order to analyse the strain rate sensitivity and anisotropy. Both, the micro-inertia and the base material strain rate sensitivity have an influence on the dynamic behaviour of the cellular metal

Numerical Simulation of Steel Bolted Beam-Column Connections Subjected to Dynamic Loading

This study presents the development of a finite element model with the capability to simulate and analyse the response of steel bolted beam-column connections subjected to rapid rate of applied loading. The effect of material strain-rate sensitivity was taken into account and a shear failure criterion was adopted to predict the failure of the connection. By comparing the simulation results with the experimental observed responses, it is found that the numerical techniques and the material model used here can predict the structural behaviour of connections reasonably well.

Some Topics in Recent Advances and Applications of Structural Impact Dynamics

This paper reviews some topics related to the advances and applications of structural impact dynamics in recent years. Dynamic behavior of structural members including tubes, beams and plates under axial or transverse loading, and cellular materials and sandwich structures under impact or blast loading are summarized here. The research methodology involves experimental studies, theoretical modeling, as well as numerical simulations. However, as we mainly focus on the longer time dynamic responses of structures and cellular materials, studies of stress wave propagation and the material's strain-rate sensitivity are not included.

Measurement on Strain Rate Sensitivity and Dynamic Mechanical Properties of Various Polymeric Materials

Strain rate sensitivity and dynamic mechanical properties of polymeric materials are affected to a certain extent especially by the rate of loading. However, there is limited number of works reported on that particular issue. Therefore, the paper presents on static and dynamic mechanical properties of various polymeric materials across strain rate from 10-2 to 10-3 s-1. The specimen were tested using universal testing machine (UTM) for static loading and a conventional split Hopkinson pressure bar (SHPB) apparatus for dynamic loading. From the results, the compression modulus and compressive strength of all tested specimen increased significantly with increasing strain rates. In addition, positive increment in terms of strain rate sensitivity was recorded for all tested polymers over a wide range of strain rate investigated. Meanwhile, the thermal activation volume has decreased as increasing strain rate. Of the three polymers, polypropylene shows the highest strain rate sensitivity at static region. On the other hand, at dynamic region, polycarbonate shows the highest strain rate sensitivity than that of polypropylene and polyethylene.

Mechanical properties of single bovine trabeculae are unaffected by strain rate

For a better understanding of traumatic bone fractures, knowledge of the mechanical behavior of bone at high strain rates is required. Importantly, it needs to be clarified how quasistatic mechanical testing experiments relate to real bone fracture. This merits investigating the mechanical behavior of bone with an increase in strain rate. Various studies examined how cortical and trabecular bone behave at varying strain rates, but no one has yet looked at this question using individual trabeculae. In this study, three-point bending tests were carried out on bovine single trabeculae excised from a proximal femur to test the trabecular material's strain rate sensitivity. An experimental setup was designed, capable of measuring local strains at the surface of such small specimens, using digital image correlation. Microdamage was detected using the bone whitening effect. Samples were tested through two orders of magnitude, at strain rates varying between 0.01 and 3.39s−1. No linear relationship was observed between the strain rate and the Young's modulus (1.13–16.46GPa), the amount of microdamage, the maximum tensile strain at failure (14.22–61.65%) and at microdamage initiation (1.95–12.29%). The results obtained in this study conflict with previous studies reporting various trends for macroscopic cortical and trabecular bone samples with an increase in strain rate. This discrepancy might be explained by the bone type, the small sample geometry, the limited range of strain rates tested here, the type of loading and the method of microdamage detection. Based on the results of this study, the strain rate can be ignored when modeling trabecular bone.

A new micromechanics-based scale transition model for the strain-rate sensitive behavior of nanocrystalline materials

An original two-step “three phase” elastic–viscoplastic scale transition model is developed based on the combined self-consistent and Mori–Tanaka schemes. A coated inclusion is embedded within a matrix, wherein the inclusion represents grain interiors and the coating of the inclusion mimics the effects of grain boundaries and triple junctions. The predominant behavior within the grain interiors is captured through dislocation glide, whereas grain boundary (GB) dislocation emission and absorption, as well as thermally assisted GB sliding, describe the deformation processes within the coating describing the GB affected zone. Furthermore, an imperfect interface is assumed between the inclusion and the coating to account for viscoplastic grain boundary sliding along a stick-slip mechanism. Results and discussion focus on the competitive roles of GB sliding, GB dislocation emission/absorption, dislocation sweep in grain cores and collective dislocation plasticity, and the origins of the pronounced strain rate sensitivity of fcc NC materials.

Load Relaxation Behavior of Metastable Austenitic Stainless Steels at Quasi-Static Strain Rates

Instantaneous strain rate sensitivity of metastable austenitic stainless steel EN 1.4318 was studied at quasi-static strain rates (2 × 10−4 ... 10−1 s−1) using load relaxation tests carried out at room temperature. The results indicate that at the studied deformation conditions the instantaneous strain rate sensitivity of the test material decreases with decreasing strain rate but increases with increasing plastic strain. Rapid strain aging was also observed to occur at the studied deformation conditions.