Strain Rate Sensitivity Of Material Research Articles

Dynamic response analysis of ship reef hard grounding

Abstract With the rapid growth of the world economy, the shipping industry has developed rapidly on a global scale, and the water traffic has become increasingly busy. Therefore, the risk of ship transportation has become bigger and bigger, and ship grounding and collision occupy about 50% of the total number of all accidents, among which the consequences of oil tanker grounding accidents are more serious, which not only jeopardize the safety of the personnel but also cause a large area of pollution to the sea. Therefore, it is particularly important to study the ship grounding and collision of oil tankers. Based on the nonlinear finite element software ABAQUS, a calculation model for oil tanker grounding is established, and simplified boundary conditions that can be used for grounding simulation analysis are determined. At the same time, material strain rate sensitivity and material hardening properties are considered. Based on the quasi-static tensile test data of Q235 steel used in ships, material parameters that meet the requirements of collision simulation calculation are calibrated and calculated. The dynamic response of hard grounding is analyzed from both external and internal mechanisms. The research results can provide a reference for improving the grounding collision performance of oil tankers.

Experimental study and crystal plasticity modeling of additive manufacturing IN718 superalloy considering negative strain rate sensitivity behavior

Revealing underlying mechanisms of IN718 superalloy's negative strain rate sensitivity behavior at elevated temperatures is significantly important for its further applications in extreme conditions. This work investigates the microstructural mechanisms of the temperature-dependent strain rate sensitivity using experimental methods and crystal plasticity characterization model. Firstly, Electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) techniques are utilized to observe microstructural characteristics of additively manufactured IN718 alloy at different temperatures. The influence of solute precipitation and dislocation slip interaction on the mechanical properties is discussed. Subsequently, the representative volume element (RVE) model is constructed by using acquired orientation, grain size, and dislocation density data. Finally, the crystal plasticity model incorporating dynamic strain aging and dislocation density evolution is developed to characterize the relationship between strain rate sensitivity and temperature. The model can effectively simulate the transition of strain rate sensitivity of additive manufacturing materials. The results indicate that the increase in dislocation pinning caused by intensified carbon element precipitation at high temperatures is the primary reason of the strain rate sensitivity change observed in IN718 metal materials. The carbon precipitated at the dislocation nucleus induces serration flow in IN718 superalloy under high strain. This study contributes to understanding and characterizing the microscopic mechanisms of deformation in metal materials subjected to thermal environments.

Novel nanoindentation strain rate sweep method for continuously investigating the strain rate sensitivity of materials at the nanoscale

We introduce a new nanoindentation method to continuously measure the hardness while sweeping through orders of magnitudes of strain rates within a single experiment. While nanoindentation already allows the determination of the strain rate sensitivity of materials by means of strain rate jump tests, these are typically limited to few discrete strain rates. With the new method, the strain rate sensitivity can be measured continuously as a function of the strain rate. Applications to fused silica, Zn-22 %Al superplastic alloy, single crystalline aluminum, various nanocrystalline metals and a palladium-based metallic glass are shown. Besides some discrepancy with the reference measurements, the new method seems only affected by the presence of a strong nanoindentation size effect. Provided this indentation size effect is not excessively large and can be corrected for accurately, the method proves robust, with no suggestion that the direction of the strain rate sweep affects the evaluation of the strain rate sensitivity.

Behaviour of expansion tubes under impact load: Deformation mode and mechanical response

The expansion tube is a promising energy absorption structure in spacecraft and airplanes, where it is often exposed to impact stress. Despite prior studies on the deformation mode of expansion tubes have been conducted, the majority of them have relied on quasi-static load. This research has utilized theoretical, computational, and experimental approaches to concentrate on the behaviors of expansion tubes exposed to dynamic impacts. The experimental results show that the strain-rate effect and the downward velocity discrepancy could even cause the deformation mode transition of the expansion tube. The dynamic theoretical model of steady-state expansion force is derived and used to predict the normalized compressive force. A normalized compressive force more than 0.85 would promote buckling deformation of the expansion tube. The normalized compressive force is then used to predict the deformation mode diagrams of the expansion tube for a range of geometric parameters; this analysis reveals that the semi-angle, expansion rate, and radius/wall-thickness of the tube all play a role in determining the deformation mode, while the effect of length/radius is less evident. The expansion tube's buckling resistance is enhanced by the parent material's strain rate sensitivity and diminished by the strain hardening effect.

A response prediction method for clamped circular plates subjected to repeated blast loading

The aim of the present investigation is to study the behavior of metal circular plates that are clamped under the repeated blast loading at stand-off distances. In this paper, the Johnson-Cook constitutive equation involving the effects of material strain hardening and strain rate sensitivity is introduced into the empirical analysis equations used to predict the midpoint displacement of circular plates. Thus, a group of non-dimensional empirical equations are proposed for predicting the midpoint displacement of clamped metal circular plates subjected to both single and repeated blast loading. The proposed non-dimensional empirical analysis equations are verified with the simulation analysis and experimental results of previous papers. It indicates that the proposed non-dimensional midpoint displacement prediction equations demonstrate good consistency in predicting the deformation of clamped metal circular plates under both single and repeated blast loading conditions.

STRAIN RATE DEPENDENCY OF NEOHOOKEAN MATERIAL USING KELVIN-VOIGT VISCOELASTIC MODEL

This research paper examines the strain rate dependency of a Neo-Hookean material using the Kelvin-Voigt viscoelastic model. The Neo-Hookean model is widely used to describe the mechanical behavior of rubber-like materials, but it fails to consider the timedependent response observed in many materials. To overcome this limitation, the Kelvin-Voigt viscoelastic model, which incorporates a viscosity term into the NeoHookean model, is employed to capture the strain rate dependency. The paper commences by introducing the Neo-Hookean material model and its assumptions. It then elucidates the rationale behind incorporating the Kelvin-Voigt model to account for the time-dependent behavior. To validate the strain rate dependency of the KelvinVoigt model, tensile test is performed using experimental data. The experimental data comprises stress-strain curves, from which the material parameters of both the Neo-Hookean and Kelvin-Voigt models are determined using curve-fitting techniques. The study presents and analyzes the results, with a specific focus on the material's strain rate sensitivity. The stress-strain curves obtained from the Kelvin-Voigt model are compared with the experimental data, and the model's accuracy is evaluated. Additionally, the material parameters obtained from the curve-fitting process are examined to explore the influence of strain rate on the mechanical response of the material.

Integrating HIP and homogenisation heat treatment and its effect on the workability of a conventional peritectic TiAl alloy

The aim of this study was to understand how consolidation and homogenisation, as well as the possibility of integrating the two processes, influences the pre-compression microstructure and subsequent compression behaviour of an industry accepted, peritectic solidifying titanium aluminide ingot alloy; 45XD.This study differs from existing work with its emphasis on understanding the effect of integrating consolidation and homogenisation on workability, as well as how these processes influence the pre-and post-compression microstructure individually. This was assessed by the material's strain rate sensitivity in primary and secondary compression. The target microstructure was identified from the literature as being capable of a strain rate sensitivity of ≥0.3.The results presented here show that it is possible to integrate the consolidation and homogenisation stages, and this proved beneficial, namely achieving a high lamellar content and elementally homogeneous microstructure presented prior to compression. Subsequent deformation generated high values of dynamic recrystallisation fraction and globularisation returned upon primary compression, and the highest strain rate sensitivity at secondary compression, in comparison to material processed by the traditional approach. This is thought to be due to pressure and temperature during the integrated process reducing both segregation, seen through casting, and so the B2/β0 content compared to consolidated material. The integrated consolidation and homogenisation approach also prevented the grain growth seen in the traditional route; this is anticipated to be due to the two slow cooling stages involved in the traditional process rather than the effect of isostatic pressure.

On the possibility of fabricating fully austenitic sub-micron grained AISI 304 stainless steel via equal channel angular pressing

The paper concerns possibility of increasing yield and ultimate stress of austenitic stainless steels by equal channel angular pressing (ECAP). To this end, an AISI 304 stainless steel subjected to eight passes of ECAP in a 105◦ die at 350◦ C. The evolution of microstructure as a function of induced plastic strain (number of passes) was studied in different length scales using light and scanning transmission electron microscope (STEM). An insight into fine details of dislocation structure was possible via X-ray diffraction profile analysis. Changes in the structure of processed samples were correlated with their mechanical properties as determined by tension tests and microhardness measurements. Light microscopy revealed a shear banded microstructure. STEM observations demonstrated co-existence of three types of grain structure: (a) short elongated grains/subgrains of 71 nm average width formed within the shear band regions, (b) a mixture of coarse (200–350 nm in size) and nanocrystalline (15– 70 nm) equiaxed grains developed outside the shear bands, and (c) nano-twin lamellae of 3–20 nm width inside nanocrystalline grains. XRD peak profile analysis using modified Warren–Averbach approach yielded dislocation density as ρ = 4.71 × 1015 m−2 after eight passes. ECAP-induced changes in the microstructure led to a remarkable increase in yield strength of the alloy (YS=1498 MPa, about 3.5 times the initial amount). At the same time, a moderate ductility of εf = 12% was maintained, primarily due to a large post-necking elongation. This behavior was explained by an increase in the strain rate sensitivity of ECAP-ed material in combination with the contribution from transformation induced plasticity, TRIP effect, caused by formation of ∼ 22 vol% martensite during room temperature straining.

Research on Dynamic Nonlinearity of Materials Based on Tensile Test

The collision impact process of a structure is a complex nonlinear dynamic response process, and the key factor to ensure the accuracy of the calculation results when calculating the simulation collision problem is the definition of the dynamic nonlinear parameters of the material during the simulation analysis, and there is no uniform treatment for the collision impact problem. Based on Q235 steel, quasi-static tensile and high-speed tensile tests were carried out, and the material parameters were calibrated to meet the requirements of the collision simulation calculation based on the test data, considering the material strain rate sensitivity and material hardening properties. The research results can be used as a reference for the simulation of large structure impact problems.

Forming of Multiple Straight Convex Shapes on Aluminum Sheet Using Impulsive Water Pressure

Several types of multiple straight convex shapes were formed on a thin aluminum sheet with a grooved die using impulsive water pressure. The maximum pressure was 160 MPa in the high-speed forming, wherein a drop hammer testing machine was used, whereas it was 100 MPa in the low-speed forming because of the limitations of the press machine. The effects of the forming speed, cross-sectional shape, and pitch of the grooves on the deformation behavior were investigated. The increase in the impulsive water pressure was found to be affected significantly by the compressibility of water. The symmetricity of the convex shape in the cross-section decreased at both ends for a smaller pitch because of the imbalance of the material flow at both peripheries of the groove. The concave surface profile of the pressure side was more rounded in the high-speed forming than that in the low-speed forming when semicircular and rectangular grooved dies were used. This may be attributed to the fact that the plastic deformation becomes more uniform owing to the positive strain rate sensitivity of the test material. In the forming with rectangular grooves, fracture occurred under the low- and high-speed conditions, wherein the maximum pressure was set to 100 MPa. However, the material did not fracture during high-speed forming with a pressure of 160 MPa, where the convex shape was higher and the material contacted the bottom of the groove. This behavior may be because the dislocation density of the material did not increase rapidly owing to the strain rate being maintained high until the material suddenly stopped deforming in the latter condition. In forming with a trapezoidal grooved die, the formed profiles were considerably similar under all conditions because the strain was considerably smaller than that with the other grooves.

Non-linear dynamic finite element analysis of micro-strut lattice structures made by laser powder bed fusion

The development of additive manufacturing technologies enables the production of a new type of porous materials for the absorption of mechanical energy. These are, for example, metallic lattice structures produced by laser powder bed fusion. The structures can be made from a wide range of alloys, achieve high specific energy absorption, and can be manufactured as hybrid parts with conventional bulk components. To effectively develop lattice structures, it is necessary to complement experimental tests with simulations using the finite element method (FEM) performed under conditions of increased loading velocities. Therefore, this study focuses on the development of the FEM modelling strategy that reflects the effect of strain rate sensitivity of the base material (SS316L) and the most significant geometrical imperfections of the manufacturing process. The strain rate is reflected by the Cowper-Symonds constitutive law, which parameters are determined by the dynamic tensile test on Hopkinson split bars. The imperfections are captured by optical digitalization. The significance of the Cowper-Symonds parameters and geometric imperfections are studied independently, whereas agreement with the experiment is observed. Tests are performed for several lattice structures with different strut orientations and velocities to evaluate the versatility of the proposed approaches. A good correlation between computational and experimental results in terms of energy absorption is found for structures with an experimentally determined strut diameter and the proposed Cowper-Symonds input parameters.

Dynamic similitude technique for predicting the behaviour of structures subjected to impact loads

To predict the dynamic behavior of large-scale structures subjected to impact loads by similitude theory is an important design tool for engineers. However, structures under impact loads usually do not follow the usual similarity laws due to material strain-rate effects and geometrical distortion. This paper proposed a dynamic similitude method to predicting the behavior of structures subjected to impact loads where material strain-rate sensitivity and geometrical distortion are both considered. In this work, we firstly deduced a loss function of the dynamic yield stress to obtain the scaling correction factor to remove the strain rate effects by correcting initial impact velocity. Then a distortion factor related to the geometrical distortion was introduced to obtain a new velocity correction factor, which made it possible to reduce the effects of material strain-rate sensitivity and geometrical distortion together. The efficiency of the scaling approach presented in this paper was illustrated by the numerical simulations and experiments of a T-beam model under impact loading and the results showed a small deviation between the predictions of the model and the prototype.

Dynamic contact behavior of graphite-like carbon films on ductile substrate under nano/micro-scale impact

Coated components are often subjected to high strain rate and repetitive contact damage in practical service, so how to quickly evaluate the dynamic contact behavior of the thin protective coating is particularly important. Highly resolved single nano-impact and novel multiple micro-scale impact tests were used to investigate the dynamic hardness and fatigue failure of 0.55–1.52 μm thick graphite-like carbon (GLC) films on 316L stainless steel with varied thickness, respectively. By analyzing the impact depth and velocity before and after the indenter first contact with the sample, the dynamic hardness of GLC film/substrate system was obtained reasonably based on the energy approach in single nano-impact tests. Possible reasons for the higher dynamic hardness than quasi-static hardness include overestimation of the plastic absorbed energy W p and the strain rate sensitivity of materials. The thickest film/substrate system studied had a higher dynamic hardness than the thinner films due to its higher load carrying capability. Results with the multiple micro-impact technique showed that a GLC film with intermediate thickness (1.1 μm) was more resistant to the impact fatigue, while the thinnest film, 0.55 μm, exhibited more pronounced radial cracks under the indent and the thickest film , 1.52 μm, showed more significant edge ring cracks, these differences resulting from the combined action of stress distribution, film microstructure and mechanical properties . • Provide an energy-based method to evaluate the dynamic mechanical properties of thin films or film/substrate system. • Improve the understanding of impact failure of hard coatings deposited on ductile substrates by the novel micro-impact. • Investigate the influence of acceleration force on the dynamic contact behavior of hard coatings on ductile substrates. • Discuss the influence of film thickness on their dynamic contact behavior.

Rate-dependent strength and ductility of binder jetting 3D-printed stainless steel 316L: Experiments and modeling

Binder-jetting is an additive manufacturing process for metals which involves the rapid making of a binder bonded metal powder part followed by the removal of the binder and creation of a fully dense part in a subsequent sintering step. This emerging process has the potential of high cost efficiency as compared to selective laser melting (SLM) or shaped metal deposition. Here, we determine the rate-dependent plasticity and fracture properties of binder-jetted stainless steel 316L. Electron Back-Scattered Diffraction (EBSD) analysis and tomography revealed an equi-axed grain structure with an initial porosity of 3%. The latter is due to pores with an average size of 20μm that are clustered in layers perpendicular to the build direction. The observed stress-strain curve of the binder-jetted material always lies below that of wrought stainless steel 316L with a 50% lower initial yield stress and a 30% lower ultimate strength. Both the material anisotropy and strain rate sensitivity may be considered as second order effects in that comparison. The equivalent plastic strain obtained from fracture experiments for different stress states is also always lower for the binder-jetted material. The observations for the binder-jetted material therefore stand in stark contrast to those for SLM-made stainless steel which can provide an even higher yield strength than the wrought material. From a mechanism point of view, the low mechanical properties of the binder-jetted material may be explained by the high initial porosity which is reminiscent of cast metals.

A Study on the Effect of Process and Material Variables on the Hot Stamping Formability of Automotive Body Parts

Hot stamping is a method capable of manufacturing high-strength automotive body parts by inducing a martensitic phase transformation through forming and die quenching after heating a metal sheet into a high temperature austenite phase. However, it is not easy to solve various formability problems occurring in the hot stamping process due to the complexity of the process and material behavior during high temperature forming. In this study, fracture-related forming limits and martensite phase ratio were selected as criteria for evaluating hot stamping formability. First, a hot stamping test was performed on a T-type part that simplified the B-pillar, an automotive body part, and the fracture behavior according to the temperature and thickness of the sheet blank was investigated. Additionally, forming analysis was performed on the hot stamping process of mass-produced B-pillar parts by varying the temperature of the sheet blank, the thickness of the sheet blank, the die-blank friction coefficient, and the strain-rate sensitivity of material among various process and material variables. Based on the analysis results, the effect of each process and material variable on the hot stamping formability of B-pillar parts was quantitatively analyzed. By utilizing the results of this study, it will be possible to solve the formability problem that occurs in the mass-production hot stamping process for automotive body parts and improve the quality of parts in the future.

A crystal plasticity finite element model embedding strain-rate sensitivities inherent to deformation mechanisms: Application to alloy AZ31

The fundamental power-law relationship representing the flow rule in crystal visco-plasticity ensures uniqueness in the selection of slip systems accommodating imposed plastic strain-rates. The power-law relationship also introduces an artificially high strain-rate sensitivity in crystal plasticity simulations, unless a high value of the power-law exponent is used. However, the use of high values for the exponent is limited by numerical tractability. This paper presents a numerical method implemented in a crystal plasticity finite element (CPFE) model for embedding any value of the power-law exponent reflecting the true material strain-rate sensitivity. Importantly, the method does not increase computation time involved in the simulations. The enhanced CPFE model is used to interpret and predict a complex strain-rate sensitive response and microstructural evolution of AZ31 Mg alloy. Measured values of strain-rate sensitivity for slip and twinning modes are used in the simulations. Calculations show that the model successfully captures the phenomena pertaining to the effect of changing applied strain-rate on the mechanical response including flow stress and evolution of texture and twinning for a broad range of strain-rates ranging from 10−3 s−1 to 103 s−1 and loading orientations in tension and compression. It is shown that such predictions are a consequence of not only relative amounts of slip and twinning activities driven by a set of accurately characterized hardening law parameters but also values of the strain-rate sensitivities inherent to individual deformation mechanisms. Besides, the model validates the measured strain-rate dependency of deformation mechanisms while accurately reproducing the mechanical data. Hence, the model can be used to verify and further refine or infer measured strain-rate sensitivity per deformation mechanism by reproducing experimental data.

An experimental–numerical study on 3D angle-interlock woven composite under transverse impact at subzero temperatures

Three-dimensional (3D) angle-interlock woven composite may possess different impact properties at subzero temperatures compared with that at room temperatures due to its rate- and temperature-dependent resin. The purpose of this work is to investigate the impact damage behaviors of a 3D angle-interlock woven composite undergoing transverse impact tests at subzero temperatures. The experimental data and high-speed photography obtained from the tests have been compared with numerical results to explore the damage development and temperature rise. A coupled thermo-mechanical constitutive model has been developed to capture the effects of increase in temperature, strain rate sensitivity and fragmentation of the composite material. The proposed model is based on the decomposition of the compliance matrix into elastic and plastic parts. Thermo-mechanical coupling has been considered where temperature rises as a result of plastic work. The proposed model has been implemented in the explicit coupled thermo-mechanical finite element code. The numerical results were compared with transverse impact experiments at room and subzero temperatures.

Numerical Simulation of the Effects of Strain Hardening Exponent with and without Strain Rate Sensitivity of Material on Normal Elastic Plastic Impact

The material behavior selection is an important factor governing the structural response analysis of transverse impact problems, such as impact force, indentation and local elastic plastic deformation. In the present paper, the effects of strain hardening exponent (n) and strain rate sensitivity (m) on the transient response were investigated for low impact velocity by the aid of finite element simulation using the Abaqus Software. Investigated parameters are impact force histories, indentation and plastic strain (PEEQ). Results showed that the effect of n values on required parameters is more pronounced, unlike strain rate sensitivity which has not a significant influence. As well, material behavior selection is essential for an accurate local plastic strain analysis.

Transient Impact Analysis of Elastic-plastic Beam with Strain-Rate Sensitivity

The impact response of an elastic-plastic beam is highly complicated in nonlinear transient dynamics especially when the material's plasticity is strain rate dependent. To reduce the efforts required for the computational analysis, an analytical-numerical model (hybrid model) is proposed for the analysis of the transient response of the beam to sphere impact. The model couples two sub-models, a combined discretization model and a refined contact model. Both the sub-models incorporate the material strain rate dependence. The combined discretization model discretizes the beam in length direction by a dynamic substructure technique and in thickness direction by a finite difference technique to capture the beam wave propagations along the length and the stress variations along the thickness. The refined contact model extends the BD model [1] to sphere-beam impacts by introducing a new representative contact strain rate and a new unloading law to avoided the force discontinuity at the inception of unloading. The hybrid model is validated by the laboratory test performed for low-velocity impacts and by the three-dimensional finite element (FE) analysis implemented for moderate-velocity and moderate high-velocity impacts. The validations show that the hybrid model is accurate, highly efficient and suitable for the parametric study, especially for the study of impact-induced wave effects. By using the hybrid model, tests and FE analyses, the influences of the material strain rate sensitivity are investigated by comparing to the results of elastic plastic impact simulations ignoring the effect of strain rate. It is found that the influence of the strain rate dependence on the contact force-indentation relation is significant even in the low-velocity impacts. The influences of the strain rate dependence on the contact duration and maximum contact force are small, even for the impact velocity up to 30 m/s. The influences of the strain rate dependence on other impact responses are small in the low-velocity impacts, and increase with increasing of the impact velocity. A parametric analysis is conducted to investigate the effects of the impact conditions on the contact duration and coefficient restitution. Their map structures are complicated, depending upon the mass ratio and the impact position. A jumping/falling phenomenon for the variations of the contact duration and coefficient of restitution is found, which are caused by the multiple impacts.

Temperature, strain rate and anisotropy effects on compressive response of natural and synthetic cellular core materials

The remarkable flexural properties of sandwich structures hinge on the selection of performing core materials with suitable out of plane mechanical properties, i.e. compressive ones. For this reason, this work compares the compressive behaviour of a synthetic foam (polyvinyl chloride) and an environmentally friendly agglomerated cork as a function of density, strain rate, temperature and anisotropy. The strain rate sensitivity of these cellular materials was investigated in a wide range of velocity conditions by using drop weight tower and Split Hopkinson Pressure Bar dynamic compression tests. The results highlighted a remarkable strain rate sensitivity of both materials because of their viscoelastic nature and, in particular, an increase in compressive properties with increasing strain rate. This increment was more pronounced in the medium–high strain rate range than in the low-medium one. An embrittlement effect with decreasing temperature was detected, which compromises core materials crashworthiness determining a reduction of the percentage absorbed energy. Despite a remarkable anisotropy induced by the production processes, this work confirms the feasibility of agglomerated cork as a sustainable alternative to petroleum-based cellular core materials especially in consideration of the significant recovery capabilities that ensure a higher dimensional stability of the sandwich structure.