Strain Rate Levels Research Articles

Dynamic response of UHPFRCs in direct-shear tests

The comprehension of the phenomena caused by fast-transient-loading regimes on structures, such as impact or blast, plays a capital role in order to prevent catastrophic failures. In fact, very complex failure modes can occur at these regimes where compression, tensile and shear strengths are involved. The present study investigates the dynamic response of commercial Ultra High Performance Fibre Reinforced Concretes (UHPFRCs) in direct-shear. Four UHPFRCs with different percentages of fibre reinforcement (1%, 2%, 3% and 4%) were tested at the same high strain-rate level. These results were compared with those obtained on their matrix. They showed that increasing the percentage of fibres enhances the direct-shear strength. The experiments were carried out by means of a Modified Hopkinson Bar device installed in the DynaMat Laboratory.

Serrated flow and failure behaviors of a Hadfield steel at various strain rates under extensometer-measured strain control tensile load

A Hadfield steel has been investigated to clarify the serrated flow and to explore the failure behavior at room temperature. The tensile experiments were performed under extensometer-measured strain control, rather than under conventional cross-head displacement control, at strain rates ranging from 6 × 10−3 s−1 to 6 × 10−6 s−1. Three types of serrations, including type A, B and C ones, are observed. The occurrence of different types of serrations depends on both strain rate and strain level. The type C serration is identified in Hadfield steels at room temperature for the first time. At high strain rates, there is substantially higher serration density and reduction in stress compared with that observed at low strain rates. Furthermore, two different initiation modes of deformation bands are revealed. The fracture crack nucleates at a position with dense twins, and propagates primarily in the direction perpendicular to the tensile axis and deflects frequently due to the interaction with the boundary of grains and twins.

Fibrous soft tissues damage evaluation with a coupled thermo-visco-hyperelastic model

We model the thermo-visco-hyperelastic behavior of soft tissues using a thermodynamic framework. For this purpose, we develop a rheological model which can predict the stress–stretch behavior as well as the temperature variation due to visco-hyperelasticity and rate-dependent damage of the matrix and fibers. In a second stage, we carry out a set of cyclic uniaxial tensile tests to provide experimental data and calculate the material constants for the constitutive model utilizing the genetic algorithm. Later, with the purpose of validation, we determine the stress–stretch relation, and determine the temperature change in another series of experiments, and draw comparisons with the model predictions. The results show that the material behavior strongly depends on the temperature, strain rate and also the fiber’s direction. We determine that the temperature change is more significant at high strain rate and initial temperature levels and that it is necessary to use the developed coupled model.

Impact of notch geometry on dynamic strength of materials

Abstract The resilience of materials from the notch effect together with subsequent operating life and safety are important factors for the choice of material in industrial manufacture as well as for crash scenarios. The aim of this investigation is to study stress caused by the notch effect for HCT780XD and X5CrNi18-10 at different tensile test strain rates as well as due to various stress concentration factors during a high-speed tensile test. The notch geometries chosen are round, trapezoid, sharp and bore. They are compared to a test specimen without notch. The stress distribution analysis of notches was performed by the finite-element-method (FEM) in order to determine stress concentration factors. The high-speed tensile test was recorded with a high-speed camera. For this purpose, the test specimens were marked with a speckle-pattern, essential for the identification of strain rate via Dantec Dynamics Istra 4D. The influence of the stress concentration rate on parameters like energy, strength and the elongation of break was established and will be reported in the following. The level of the concentration factor leads to negative effects while the strain rate level delivers positive results. On other hand, the stress concentration factor is more important on the parameter than the strain rate. By contrast, Poisson's ratio does not affect the parameters observed.

Microstructural material characterization of hypervelocity-impact-induced pitting damage

In a hypervelocity impact (HVI) between the micrometeoroids/orbital debris (MMOD) and multi-layered shielding mechanisms of spacecraft, the debris cloud, formed by shattered materials of the outer bumper layer and projectile, commits multitudinous pitting craters and cracks that are disorderedly scattered in the rear wall layer. Material degradation due to the pitting damage is a precursor of structural fragmentation and system failure of the space assets. In this study, microscopic material degradation of the rear wall of a typical dual-layered Whipple shield, initiated and intensified by the debris cloud-engendered pitting damage, is characterized using metallographic analysis including optical microscope (OM), laser scanning microscope (LSM), scanning electron microscope (SEM) and X-ray diffraction (XRD). Results have revealed that (1) the degree of material degradation shows difference in the central cratered area, the ring cratered area, and the spray area, respectively; (2) the dynamic recrystallization gives rise to the formation of fine grains adjacent to pitting craters; (3) the extents of recrystallization and dislocation depend on the strain rate levels during HVI; and (4) the temperature elevation, caused by the heat transformed from the adiabatic plastic deformation energy and shock heating, warrants the recrystallization. Two types of damage, namely micro-voids and micro-cracks, are identified beneath the pitting damage area; under the extremely high compressive strain rate induced by HVI, micro-voids are initiated by the nucleation of grains or deteriorate from existing material defects, and these micro-voids further expand at the grain boundaries and within the grains to form micro-cracks under a tensile-type wave converted from the HVI-induced shock wave.

Strain rate-dependent behaviors of mechanical properties of structural steel investigated using indentation and finite element analysis

In this study, a series of experiments, which consist of constant strain rate indentation, and optical microscope examination, and finite element analysis were performed to investigate the strain rate-dependent behaviors of the mechanical properties of SS400 structural steel. The microstructures of SS400 steel was characterized using optical microscope examination. The influences of strain rate indentation on the characteristics of the loading/unloading curves were presented and the results showed a higher applied load, a higher loading curvature, and a higher loading work for a higher strain rate level. The strain rate-dependent behaviors of indentation hardness (H), yield strength (σy), and work hardening (n) were investigated. When the strain rate level increased, both H and σy strongly increased, while work hardening showed an almost linear increase. The strain rate-dependent behaviors of material properties were validated through the experimental and the numerical verifications. The strain rate sensitivity (SRS) value of 0.056 ± 0.02 was reported for SS400 steel and was highly consistent with the general trend reported for several types of structural steel in the literature.

Numerical analysis of the influence of the damping rings' thickness on interrupted dynamic tension results using SiC-reinforced ZC71 magnesium alloy specimens

Abstract. This article discusses the influence of the thickness of the damping rings used for interrupting a dynamic tension experiments on the results of a modified split Hopkinson tension bar (SHTB). In this paper a device enclosed in an external fixture used for interrupting a dynamic tension experiment in a SHTB is studied. The novelty of this manuscript with respect to previous studies lies in the fact that the dynamic tension experiment in a SHTB is interrupted in order to study the mechanical behavior of the material at high strain rates. The role played by such device is to interrupt the experiment at different levels of plastic deformation, particularly when the specimen is about to reach its failure strength. Finite-element (FE) simulations of high-strain-rate tension experiments are accomplished on a particle-reinforced metal matrix composite specimen (namely SiC-reinforced ZC71 magnesium alloy) when varying the thickness of the damping rings. Interrupting the test before the specimen breaks offers the possibility of being able to study in a more detailed way the deformation process of such material at high strain rates. Therefore, this work focuses on the study of the behaviour of materials undergoing high strain rates, developing a tool which allows materials to withstand different levels of strain rates in a controlled manner and providing guidance for future studies. In view of this research, it can be concluded that the thickness of the damping rings is a factor that can resolutely influence the interrupted dynamic tension experiment results avoiding the specimen's failure by optimally buffering the experiment using 0.8 mm thick lead damping rings.

Investigation of the Texture and Dislocation Behavior of AZ31 Magnesium Alloy Under Different Strain Rate Conditions

This study is concerned with the influence of strain rate on the plastic deformation behavior and microstructure variables of AZ31 magnesium alloy sheet. The tensile properties were measured at room temperature at a strain rate of 0.001 and 100 s-1 using a universal tensile testing machine and servo-hydraulic high-speed tensile test machine respectively. The microstructure was observed at strains of 5, 10, 15%, produced by tensile elongation. As the strain rate increased, the flow stress and the yield strength increased. The EBSD analysis showed that a texture of the (0001) basal plane was formed parallel to the rolled plate and texture was formed at random in the (101¯0) non-basal plane. Double twinning was not observed and extension and contraction twinning were observed at all strain rate levels. The fraction of twin boundaries were increased as strain rate increased. TEM analysis determined that the dislocation density of the specimen strained by 1% at a strain rate of 100 s-1 was higher than that of a strain rate of 0.001 s-1. In addition, the <c+a>dislocation was found at all strain rate levels. These results show that the deference in elongation of the AZ31 Mg alloy is dependent on strain rate. Key words: AZ31 magnesium alloy, high-speed tensile test, EBSD analysis, texture, TEM analysis, pyramidal slip

A Method to Estimate Effective Viscous Damping Ratio and Restoring Force From the Dynamic Response Data of Structures

Dynamic response of structures is a complex process that is not well understood. Seismic design codes allow structural systems of buildings to behave inelastically during strong ground shaking. While hysteretic energy dissipation due to inelastic behaviour is rather well understood, there exist other energy dissipation mechanisms which are not as well understood. Energy dissipation due to mechanisms other than material nonlinearity are often modelled in the form of a single, velocity proportional damping mechanism with an equivalent viscous damping ratio. In seismic design and dynamic analysis of structures, this equivalent viscous damping ratio is generally taken to be constant (e.g. 2% or 5% of the critical) regardless the response is elastic or inelastic. Instead of making such a strong assumption about the viscous damping ratio, which may have large influence on the peak response levels, it is advisable to use an effective viscous damping estimate based on studying the actual response of real structures responding to dynamic loads. Use of such obtained viscous damping will allow extraction of restoring forces from dynamic force estimates. At low strain-rate levels, such as those observed during seismic response, restoring forces match the resistance that develops during quasi-static loading, a method to estimate effective viscous damping and restoring force empirically from dynamic response of a structure is presented. The method considers inelastic response explicitly, i.e. no linearization assumptions are made for the load-deformation behavior of the structure. The presented method is tested on several computational simulation models with various hysteretic behaviors and a preset constant viscous damping ratio to verify that the algorithm 1) estimates a damping ratio close to the value used in the simulations, and accordingly, 2) captures the hysteretic behavior accurately. The method is illustrated using data obtained from reinforced concrete test specimens subjected to design-level base excitations on an earthquake simulator.

Thermomechanical response of Mg AZ31 at different levels of temperatures and strain rates

Magnesium alloys’ mechanical behavior has received increasing attention because of its high strength to weight ratio making them ideal for various industrial applications, such as vehicle components, transportation and aerospace. The objective of this work is to closely investigate the thermo-mechanical properties of magnesium alloy AZ31 at different strain rates and temperatures. Tensile tests are conducted on a 30 mm gauge length MgAZ31 specimens at two quasi-static strain rates (1.11x10−3 s−1 and 0.28 s−1) at a range of temperatures between 25 ºC and 250 ºC. Digital Image Correlation (DIC) system was used to calculate the true strain and provide quantitative assessment of the localized deformation response at high levels of deformation. The stress-strain responses of MgAZ31 show that the yield stress as well as the ultimate stress decreases as temperature increases and strain rate decreases. Moreover, the difference between the yield and ultimate stresses at both strain rates increases rapidly as temperature increases. The material shows a significant increase in ductility as temperature increases while the modulus of elasticity remains independent of change in strain rates.

Room temperature creep mechanism of a Pb-Sn-Sb lead alloy

Lead alloys are the most common materials adopted for the production of subsea power cable sheathing. The sheathing is a layer of stable and watertight metal, which serves to prevent the electrical failure of the cable. During the predicted operational life of the cables of several decades, these experience strains due to the installation process, the oceanic currents and the thermal expansion of the cable. The low melting temperature of such alloys, around 600 K, imply that creep deformation will occur when subjected to loading even at room temperature. The goal of the present study is to investigate the tensile behavior of the Pb-Sb-Sn alloy of interest in order to predict the correlation between strain rate and stress level. A mechanical characterization was performed through tensile testing at different strain rates of specimens cut from power cable sheathing. Due to the extreme ductility of the material, the use of digital image correlation was necessary to compute an acceptable approximation of the in-plane strain field on the surface of the specimens. The results were implemented in finite element method environment using Abaqus and Isight to calibrate a creep model able to reproduce at best the behavior of the material. Such model was also positively tested in the case of a relaxation test. In addition, a tensile test of several steps at different loads was executed with the aim of extrapolating and interpreting the steady state creep exponents at different creep regimes and the indications that these can provide on the deformation mechanisms of the alloy.

Methodology to extract constitutive equation at a strain rate level from indentation curves

In this study, a methodology to determine constitutive equation at a strain rate level from indentation curves was proposed. The constitutive equation for structural steel consists of elastic modulus (E), yield strength (σy), work hardening (n), and a plastic property (α) that represents the plastic plateau stage of structural steel. Four dimensionless equations that show the basic relationship between constitutive parameters, strain rate, and the characteristics of the loading-unloading curve were established. A finite element model to simulate the indentation process was developed for the necessity of constructing four accurate dimensionless equations in the forms of third order polynomial equations. The proposed methodology was then used to determine constitutive equations at a strain rate level from indentation curves for SS400 and SM490 structural steel. The resultant constitutive equations were validated through loading tensile tests for SS400 and SM490 structural steel. Moreover, the proposed methodology was used to determine the strain rate sensitivity of SM490 steel via nanoindentation experiments.

High strain rate deformation of nanostructured super bainite

Outstanding mechanical properties of nanostructured carbide-free hard bainitic steels, also known as super bainite, with microstructural constituents of less than 100 nm thicknesses make them prone to be used in different engineering applications. However, besides their exceptional strength and ductility combinations during ordinary static or quasi-static deformation modes, it would be also interesting to evaluate their mechanical behavior under high strain rate deformation conditions. This article aims to investigate the mechanical performance of nanostructured super bainite isothermally obtained at 300 °C by applying both tensile and compressive high strain rate deformation modes. Hopkinson compressive and tensile tests were performed up to the maximum strain rate levels of 1.82 × 103 s−1 and 1.040 × 104 s−1, respectively. Results indicated that strength and ductility properties both significantly were dependent on the strain rate values during tension and compression even if the effect of tensile deformation mode was more considerable. The strength level enhanced to almost 3000 MPa at the highest tensile deformation rate. According to the results, the transformation-induced plasticity (TRIP) effect could not be effective in ductility promotion at higher tensile strain rate deformations since the austenite-to-martensite transformation did not take place gradually to produce a proficient transformation-induced plasticity effect. As a result, a premature TRIP effect occurred and resulted in lower energy absorption during deformation processes.

Experimental Study on the Mechanical Behavior of EN08 Steel at Different Temperatures and Strain Rates

The objective of this paper is to investigate the mechanical response of EN08 steel at quasi-static and dynamic strain rates. Uniaxial tensile tests under quasi-static regime (from 0.0015 s−1 to 0.15 s−1) are conducted on EN08 steel at a range of temperatures between 298 K and 923 K. Dynamic compression tests are also performed by using a drop hammer and by considering different masses and heights to study the material response at strain rates up to 800 s−1. Through the stress-strain responses of EN08 steel, a strong dependency of the yield stress as well as the ultimate strength on the strain rate and temperature is recognized. Furthermore, the strain hardening is highly affected by the increase of temperature at all levels of strain rate. The microstructure of the steel is also examined at a fracture by using SEM images to quantify the density of microdefects and define the damage evolution by using an energy-based damage model.

Damage and Failure of Axonal Microtubule under Extreme High Strain Rate: An In-Silico Molecular Dynamics Study

As a major cytoskeleton element of the axon, the breaking of microtubules (MTs) has been considered as a major cause of the axon degeneration. High strain rate loading is considered as one of the key factors in microtubule breaking. Due to the small size of microtubule, the real-time behavior of microtubule breaking is hard to capture. This study employs fully-atomistic molecular dynamics (MD) simulation to determine the failure modes of microtubule under different loadings conditions such as, unidirectional stretching, bending and hydrostatic expansion. For each loading conditions, MT is subjected to extreme high strain rate (108–109 s−1) loading. We argue that such level of high strain rate may be realized during cavitation bubble implosion. For each loading type, we have determined the critical energy for MT rupture. The associated rupture mechanisms are also discussed. We observed that the stretching has the lowest energy barrier to break the MT at the nanosecond time scale. Moreover, the breakage between the dimers starts at ~16% of total strain when stretched, which is much smaller compared to the reported strain-at-failure (50%) for lower strain rate loading. It suggests that MT fails at a significantly smaller strain states when loaded at higher strain rates.

Finite Element Analysis on a Newly-Modified Method for the Taylor Impact Test to Measure the Stress-Strain Curve by the Only Single Test Using Pure Aluminum

In this study, finite element analyses are performed to obtain a stress-strain curve for ductile materials by a combination between the distributions of axial stress and strain at a certain time as a result of one single Taylor impact test. In the modified Taylor impact test proposed here, a measurement of the external impact force by the Hopkinson pressure bar placed instead of the rigid wall, and an assumption of bi-linear distribution of an axial internal force, are introduced as well as a measurement of deformed profiles at certain time. In order to obtain the realistic results by computations, at first, the parameters in a nonlinear rate sensitive hardening law are identified from the quasi-static and impact tests of pure aluminum at various strain rates and temperature conducted. In the impact test, a miniaturized testing apparatus based on the split Hopkinson pressure bar (SHPB) technique is introduced to achieve a similar level of strain rate as 104 s−1, to the Taylor test. Then, a finite element simulation of the modified test is performed using a commercial software by using the user-subroutine for the hardening law with the identified parameters. By comparing the stress-strain curves obtained by the proposed method and direct calculation of the hardening law, the validity is discussed. Finally, the feasibility of the proposed method is studied.

TECHNOLOGY VELOCITY VECTOR IMAGING AND STANDARD ECHOCARDIOGRAPHI TO ASSESS LEFT VENTRICLE IN PATIENTS WITH NON Q MIOCARDIAL INFARCTION

The purpose of the study was to assess the function of fibres of the left ventricle (LV) and its dynamics after revascularization in patients with non Q myocardial infarction using standard Echocardiography (EchoCG) and Velocity Vector Imaging (VVI). Materials and methods: 34 patients with MI were examined. Echocardiography studies were performed on the ultrasound scanner AcusonX 300 (Siemens), a 1–5 MHz transducer before and in early period after coronary artery bypass grafting (CABG). Results. Standard EchoCG showed systolic LV dysfunction before and after CABG, contractile dysfunction in 51 (8%) segment before the operation, with subsequent recovery of the function of the 28 (54%) of them. The effect of MI on LV function using VVI showed reducing strain (S) and normal strain rate (SR) of the longitudinal fibers, decrease S and SR of circular fibers and normal function of radial fibers. After GABG S and SR of longitudinal fibers has not changed, S and SR of the circular fibres are decreased. The function of radial fibers are normal. The study established a relationship between the level of LDH-1 and SR of the circular fibers of the LV prior to CABG. Increased level of LDH-1 in the early period after CABG can be a predictor of reduction of S and SR circular fibers. Conclusion: In patients with it is necessary to analyze not only the longitudinal and radial fibers, but also circular.

Indentation strain rate sensitivity of ball-milled spark-plasma sintered Cu-C metal matrix composite

Bulk ultrafine grained metal matrix composites (MMC) have attracted much attention of many researchers due to their potential in terms of excellent mechanical properties for engineering applications, such as high strength, which can be two or more times of that of their coarse grained counterpart. Bulk ultrafine grained Cu-3wt.%C MMC samples were produced by Ball-Milling (BM) followed by Spark Plasma Sintering (SPS), at a temperature of 900 °C. The Cu-C MMC was compacted progressively by repeating the BM + SPS procedure without changing the weight ratio between Cu and graphite. The room temperature creep behavior, and the strain rate sensitivity (SRS) were inspected by using nanoindentation measurements. Strain rate ranged 0.0025-to-0.5 s−1, and the contact dwelling times ranged 5-to-300 s. A secondary steady-state regime was reached starting from a dwelling time of 120 s irrespective of the strain rate and Cu-C compaction level. A negative trend of the SRS exponent with Cu-C compaction was obtained, with creep stress exponent as high as 28. These results were discussed according to the microstructure features that differentiated the Cu-3wt.%C MMC obtained by the progressive BM + SPS compaction levels.

Deformation to fracture evolution of a flexible polymer under split Hopkinson pressure bar loading

The deformation-to-fracture evolution of a flexible polymer material under high-strain-rate compressive loading conducted by a split Hopkinson pressure bar (SHPB) setup was investigated. Representative tests were carried out at different strain rate levels, followed by the characterization of dynamic damage after each test. Craze and crack patterns on the end surface of the specimen were carefully analyzed. The failure patterns appear along the radial and circumferential directions. The sequence of their formation with increasing strain/stress level was revealed. The mechanisms resulting in the craze and crack patterns were analyzed. The heterogeneous stress distribution in the specimen and the resultant damage morphologies were demonstrated. This research not only shows the deformation-to-fracture evolution of a flexible polymer material under SHPB loading, but also provides a better clarification of the localized stress distribution in the tested material via SHPB technique.

Novel functional advanced echocardiography for the assessment of myocardial mechanics in children with neurocardiogenic syncope \u2013 a blinded prospective speckle tracking head-up tilt-table challenge study

BackgroundData on left ventricular (LV) function in patients with neurocardiogenic syncope (NS) is conflicting in adults and lacking in children. The aim of this study was to analyze LV myocardial performance in children with NS at rest and during head-up tilt-table (HUTT) testing.MethodsThis is the first study to combine HUTT and speckle-tracking echocardiography (STE) in children with NS. 43 consecutive normotensive pediatric patients with NS (mean age 13.9 ± 2.6 years, 51% female) and 41 sex- and age-matched healthy controls were included in the study. The study groups consisted of 21 patients with a positive HUTT reaction (HUTT+) and 22 with a negative HUTT reaction (HUTT-). STE was used to analyze peak systolic LV myocardial strain and strain rate.ResultsConventional echocardiographic parameters were similar in all analyzed groups. When compared to healthy controls, children with NS had depressed levels of circumferential strain rate (p = 0.032) and significantly depressed longitudinal strain rate (p < 0.001) at rest. Interestingly, during HUTT testing LV global strain and strain rate were similar in both groups. LV strain rate was lowest in HUTT+ followed by HUTT- and control subjects both at rest and during HUTT.ConclusionsResting LV longitudinal strain rate is attenuated in children with NS, especially in those with a positive HUTT response. This is further evidence that NS patients feature altered cardiac mechanics rendering them prone to vasovagal perturbations that can ultimately result in collapse.Trial registrationWitten/Herdecke University ethics committee clinical study number: UWH-73-2014.