Strain Rate Levels Research Articles

Drop Tower Adaptation for Medium Strain Rate Tensile Testing

A novel drop tower modification was designed and implemented in order to enable tensile coupon testing at medium strain rate regime (1–200/s), using a drop weight apparatus, instead of intermediate strain rate servo-hydraulic tensile machines. The developed tensile device, which consists of one movable and one rigid frame, has the ability to transform the compression loading of a drop tower machine into tension loading on the specimen. A simulation model of the proposed concept has been developed in the explicit FE code LS-DYNA and validated by experimental measurements of load and displacement histories. During the development phase, the model was used for the device preliminary design, i.e. the selection of the optimal acquisition sensor locations and the introduction of an absorber material in order to avoid undesired vibrations, as well as for the sizing of the main components of the device. During the testing phase, the numerical model was used for the determination of the appropriate testing parameters which lead to the desired testing conditions (velocity, strain rate and load level). The final design of the tensile device was implemented in an Instron drop tower machine and initial experimental tests were performed for the assessment of the proposed method. Details of the material types and specimen geometries that were tested, as well as impact testing parameters, such as range of strain rate, energy and velocity are comprehensively described in this paper. It was demonstrated that the proposed device can serve as a cost effective alternative of servo-hydraulic tensile machines, is compatible to Digital Image Correlation optical devices due to the good optical access to the tested specimen and does not introduce significant ringing effects in the piezoelectric load cell; therefore, it is suitable for medium strain rate tensile testing.

Polymeric foams for cryogenic temperature application: Temperature range for non-recovery and brittle-fracture of microstructure

In the present study, low-temperature material characteristics of polyurethane foam, glass fiber-reinforced polyurethane foam, and polyisocyanurate foam were investigated. These foams are key candidate materials for use in membrane-type liquefied natural gas insulation systems. Liquefied natural gas insulation systems should be able to withstand severe environmental conditions, such as fluid induced impact loading under cryogenic temperatures. For the robust design of insulation system, both failure characteristics and deformation recovery capacity of the materials must be evaluated. Therefore, the failure characteristics and the deformation recovery ratio of the foams were investigated at various temperatures and strain rate levels. The present study revealed that the fracture behavior and the recovery ratio were significantly affected by decreases in temperature. As a newly obtained insight, it has been found that the recovery ratio for all tested materials significantly decreased at −163°C. This finding is the opposite of the behavior that typically occurs at low temperatures. In particular, polyurethane foam specimen after experiment at the temperature of −163°C was easily broken into pieces even under the application of small loads. In addition, it has been revealed that polyisocyanurate foam showed superior mechanical characteristics at cryogenic temperatures owing to the ring structure of isocyanurate.

Fracture characterization in cancellous bone specimens via surface difference evaluation of 3D registered pre- and post-compression micro-CT scans

In recent years, increasingly stringent passenger vehicle safety requirements have led to a renewed interest in the fracture properties of bone. It has been shown that cancellous bone architecture is strongly linked to its overall behavior (Follet et al. 2011; Prot et al. 2015). Micro-fracture mechanisms have been resolved by time-consuming direct microscopy (Prot et al. 2012) or by the use of calcein (Lambers et al. 2014; Hernandez et al. 2014). Furthermore, the application of CT scanners, along with the development of registration algorithms, has allowed separated portions of fractured specimen to be registered to the pre-compression scan so as to quantify the difference between 3D shapes as a mean to characterize the fracture behavior (Tassani & Matsopoulos 2014). However, this method is operator-dependent in the case of multiple fracture zone identification and requires sufficient deformation of the specimen to obtain distinct registration sets. In addition, built-in micro-compression testers, developed by CT scanner manufacturers, are limited to loading at low levels of strain rate, which does not allow measurement over a range that is representative of daily life. The goal of this study was to identify distinct fracture patterns based on micro-CT scans of cancellous bone specimens, loaded over a large range of strain rates, without the need for specimens that have broken into separate pieces.

Experimental investigation on stapled and nailed connections in light timber frame walls

Regarding shear wall, the use of staples to connect sheathing panels to wooden studs and horizontal members is increasing because of a faster manufacturing process. In order to enhance the knowledge of the behavior of such connection, this paper presents experimental results for monotonic and cyclic tests (for two levels of strain rate) on stapled and threaded nailed connections. Concerning monotonic tests, stapled connections behavior oriented at 90° and nailed ones is similar; also the component strength is impacted by the orientation of the staples. Concerning cyclic tests, in addition to observations on the monotonic tests, the increase of strain rates leads to an effective decrease of the connections’ ductilities.

In situx-ray diffraction of fast compressed iron: Analysis of strains and stress under non-hydrostatic pressure

Series of high-pressure x-ray diffraction patterns of iron and its high-pressure polymorphs were collected with 0.1--0.2-s exposure time utilizing a membrane diamond anvil cell (DAC) for compression at various loading and unloading rates to a maximum pressure of 70 GPa. Strain rates of ${10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at a maximum pressurization rate of 4.1 GPa/s were achieved in non-hydrostatic compression of hcp Fe. Linewidth analysis was used to retrieve strain and uniaxial stress of Fe as a function of pressure upon both compression and decompression. Analysis of the lattice parameters ratio $c/a$ of hcp Fe indicates the presence of complex non-hydrostatic stress states, which developed as a function of strain rate, relaxation time, and various levels of hydrostaticity. Our results emphasize the importance of a controlled pressurization in DACs because the experimental loading rate strongly influences the stress state of the sample, particularly on decompression. Our time-resolved x-ray diffraction of the phase transition from bcc Fe to hcp Fe reveals residual grains of bcc Fe capable of surviving to very high pressures ($>35$ GPa) for a few minutes after the transition.

Effect of loading velocity on the seismic behavior of RC joints

The strain rate of reinforced concrete (RC) structures stimulated by earthquake action has been generally recognized as in the range from 10-4/s to 10-1/s. Because both concrete and steel reinforcement are rate-sensitive materials, the RC beam-column joints are bound to behave differently under different strain rates. This paper describes an investigation of seismic behavior of RC beam-column joints which are subjected to large cyclic displacements on the beam ends with three loading velocities, i.e., 0.4 mm/s, 4 mm/s and 40 mm/s respectively. The levels of strain rate on the joint core region are correspondingly estimated to be 10-5/s, 10-4/s, and 10-2/s. It is aimed to better understand the effect of strain rates on seismic behavior of beam-column joints, such as the carrying capacity and failure modes as well as the energy dissipation. From the experiments, it is observed that with the increase of loading velocity or strain rate, damage in the joint core region decreases but damage in the plastic hinge regions of adjacent beams increases. The energy absorbed in the hysteresis loops under higher loading velocity is larger than that under quasi-static loading. It is also found that the yielding load of the joint is almost independent of the loading velocity, and there is a marginal increase of the ultimate carrying capacity when the loading velocity is increased for the ranges studied in this work. However, under higher loading velocity the residual carrying capacity after peak load drops more rapidly. Additionally, the axial compression ratio has little effect on the shear carrying capacity of the beam-column joints, but with the increase of loading velocity, the crack width of concrete in the joint zone becomes narrower. The shear carrying capacity of the joint at higher loading velocity is higher than that calculated with the quasi-static method proposed by the design code. When the dynamic strengths of materials, i.e., concrete and reinforcement, are directly substituted into the design model of current code, it tends to be insufficiently safe.

A comparative study on the restrictions of dynamic test methods

Dynamic behavior of materials is investigated using different devices. Each of the devices has some restrictions. For instance, the stress-strain curve of the materials can be captured at high strain rates only with Hopkinson bar. However, by using a new approach some of the other techniques could be used to obtain the constants of material models such as Johnson-Cook model too. In this work, the restrictions of some devices such as drop hammer, Taylor test, Flying wedge, Shot impact test, dynamic tensile extrusion and Hopkinson bars which are used to characterize the material properties at high strain rates are described. The level of strain and strain rate and their restrictions are very important in examining the efficiency of each of the devices. For instance, necking or bulging in tensile and compressive Hopkinson bars, fragmentation in dynamic tensile extrusion and petaling in Taylor test are restricting issues in the level of strain rate attainable in the devices.

Tensile behaviour of geopolymer-based materials under medium and high strain rates

Geopolymers are a promising class of inorganic materials typically obtained from an alluminosilicate source and an alkaline solution, and characterized by an amorphous 3-D framework structure. These materials are particularly attractive for the construction industry due to mechanical and environmental advantages they exhibit compared to conventional systems. Indeed, geopolymer-based concretes represent a challenge for the large scale uses of such a binder material and many research studies currently focus on this topic. However, the behaviour of geopolymers under high dynamic loads is rarely investigated, even though it is of a fundamental concern for the integrity/vulnerability assessment under extreme dynamic events. The present study aims to investigate the effect of high dynamic loading conditions on the tensile behaviour of different geopolymer formulations. The dynamic tests were performed under different strain rates by using a Hydro-pneumatic machine and a modified Hopkinson bar at the DynaMat laboratory of the University of Applied Sciences of Southern Switzerland. The results are processed in terms of stress-strain relationships and strength dynamic increase factor at different strain-rate levels. The dynamic increase factor was also compared with CEB recommendations. The experimental outcomes can be used to assess the constitutive laws of geopolymers under dynamic load conditions and implemented into analytical models.

Quantitative stress echocardiography in pediatric patients with type-1 diabetes mellitus

s of the 51st Workshop for Pediatric Research 51st Workshop for Pediatric Research Gottingen, Germany 16-17 April 2015 This supplement has not been sponsored. Meeting abstracts Aims Type-1 diabetes mellitus (T1DM) is among the most common chronic diseases in children. Serious long-term complications include both ultra-morphologic and functional cardiac impairment. Whether these pathologic processes already take place in the early phase of T1DM yet has to be determined. Aim of this prospective, blinded, clinical study was to assess the effect of T1DM on left ventricular (LV) myocardial function in pediatric patients using quantitative stress echocardiography. Methods We included 40 patients with T1DM (aged 11.53 ± 3.1 years) and 44 sex-matched healthy controls (11.43 ± 2.9 years; p = 0.91). Conventional and quantitative echocardiography was performed both at rest and during physical exercise on a bicycle. Myocardial strain and strain rate were measured using speckle tracking echocardiography. Results Children with T1DM had increased myocardial contractility both at rest and at all levels of physical stress testing when compared to healthy volunteers. At rest, T1DM patients had significantly higher circumferential strain rate (-2.05 ± 0.35 s -1 ) than healthy controls (-1.86 ± 0.25 s -1 ; p = 0.016). During exercise, diabetic children showed more negative values for longitudinal strain rate (-2.59 ± 0.47 s -1 ) than healthy controls (-2.32 ± 0.41 s -1 ; p = 0.021). Furthermore, in T1DM patients, disease duration inversely correlated with myocardial strain rate and serum HbA1c levels correlated with strain both at rest and during stress testing. Conclusion Long-term impairment of cardiac function in diabetic cardiomyopathy may be preceded by a transient phase of increased myocardial contractility early in the course of T1DM. The extent of hyperglycemia in childhood correlates with the alteration of myocardial contractility.

Assessment of atrial fibrillation and vulnerability in patients with Wolff-Parkinson-White syndrome using two-dimensional speckle tracking echocardiography.

PurposeThe aim was to assess atrial fibrillation (AF) and vulnerability in Wolff-Parkinson-White (WPW) syndrome patients using two-dimensional speckle tracking echocardiography (2D-STE).MethodsAll patients were examined via transthoracic echocardiography and 2D-STE in order to assess atrial function 7 days before and 10 days after RF catheter ablation. A postoperative 3-month follow-up was performed via outpatient visit or telephone calls.ResultsResults showed significant differences in both body mass index (BMI) and supraventricular tachycardia (SVT) duration between WPW patients and DAVNP patients (both P<0.05). Echocardiography revealed that the maximum left atrial volume (LAVmax) and the left ventricular mass index (LVMI) in diastole increased noticeably in patients with WPW compared to patients with DAVNP both before and after ablation (all P<0.05). Before ablation, there were obvious differences in the levels of SRs, SRe, and SRa from the 4-chamber view (LA) in the WPW patients group compared with patients in the DAVNP group (all P<0.05). In the AF group, there were significant differences in the levels of systolic strain rate (SRs), early diastolic strain rate (SRe), and late diastolic strain rate (SRa) from the 4-chamber view (LA) both before and after ablation (all P<0.05). In the non-AF group, there were decreased SRe levels from the 4-chamber view (LA/RA) pre-ablation compared to post-ablation (all P<0.05).ConclusionOur findings provide convincing evidence that WPW syndrome may result in increased atrial vulnerability and contribute to the development of AF. Further, RF catheter ablation of AAV pathway can potentially improve atrial function in WPW syndrome patients. Two-dimensional speckle tracking echocardiography imaging in WPW patients would be necessary in the evaluation and improvement of the overall function of RF catheter ablation in a long-term follow-up period.

ANALYSIS OF STRAIN-RATE DEPENDENT MECHANICAL BEHAVIOR OF SINGLE CHONDROCYTE: A FINITE ELEMENT STUDY

Various studies have been conducted to investigate the effects of impact loading on cartilage damage and chondrocyte death. These have shown that the rate and magnitude of the applied strain significantly influence chondrocyte death, and that cell death occurred mostly in the superficial zone of cartilage suggesting the need to further understand the fundamental mechanisms underlying the chondrocytes death induced at certain levels of strain-rate. To date there is no comprehensive study providing insight on this phenomenon. The aim of this study is to examine the strain-rate dependent behavior of a single chondrocyte using a computational approach based on finite element method (FEM). An FEM model was developed using various mechanical models, which were standard Neo-Hookean solid (SnHS), porohyperelastic (PHE) and poroviscohyperelastic (PVHE) to simulate atomic force microscopy (AFM) experiments of chondrocyte. The PVHE showed, it can capture both relaxation and loading rate dependent behaviors of chondrocytes, accurately compared to other models.

High strain rate behaviour of 5-harness-satin weave fabric carbon–epoxy composite under compression and combined compression–shear loading

The strain rate dependent mechanical behaviour was studied for the common out-of-autoclave aerospace textile composite 5-harness-satin carbon–epoxy. End-loaded 15°,30° and 45° off-axis and 90° compression tests were carried out at three different strain rate levels (4×10-4s-1,200s-1 and 1000s-1) to determine the effect of strain rate for transverse compression and combined transverse compression/in-plane shear loading. The dynamic tests were carried out on a split-Hopkinson pressure bar, where high speed photography and digital image correlation allowed a detailed study of the specimen deformation and failure process. Quasi-static reference tests were carried out on an electro-mechanical test machine using the same specimen type and a static DIC system. Pronounced strain rate effects on the axial stress–strain response were observed for all specimen types. Failure envelopes for the combined σ22c-τ12 stress state were derived from the experimental data and compared with the maximum stress criterion, which appears well suited to approximate the experimental failure envelope at all strain rate levels. It was observed that the failure envelope was simply scaled up with increasing strain rate, while the overall shape was found to be strain rate independent.

Application of the Warm Hydroforming Process to the Manufacturing of Pre-Aged 6xxx Series Components Using a Numerical/Experimental Approach

In this work the Warm Hydroforming (WHF) process for the production of a 6xxx series Al alloy component has been investigated using a numerical/experimental approach: both experimental and numerical hydroforming tests were carried out using the alloy AC170PX, a pre aged (T4 condition) Al alloy often adopted for automotive applications. In order to evaluate both the mechanical and strain behaviour of the material, tensile tests were carried out at different temperature and strain rate levels using the Gleeble system 3180, keeping also into account the ageing effect; in addition, formability (Nakazima) tests in warm conditions were performed by means of a specific equipment and the Forming Limit Curves at different temperature levels were evaluated according to the ISO standard 12004-2. Hydroforming experiments were carried out using a prototypal press machine specifically designed for WHF and SuperPlastic Forming tests. Such tests, scheduled by a DoE approach, were aimed at investigating the suitability of using the investigated Al alloy in the WHF process: attention was thus focused on those parameters mainly affecting the aging phenomenon (temperature, heating time and cycle time). In order to overcome the actual physical limitation of the hydroforming facilities, a Finite Element (FE) model of the WHF process was also created implementing experimental data (flow stress curves and FLCs) and tuned using data from preliminary WHF tests. In particular, after setting the Coefficient Of Friction (COF) according to temperature and verifying the robustness of numerical simulations, the FE model was used for investigating: (i) the influence of the Blank Holder Force (neglected in the experimental campaign); (ii) the adoption of quite smaller values of the parameter cycle time (being the aim to determine higher strain rates in the material). Through the definition of proper response variables (Flatness, Bursting Pressure and Thickness Ratio) both experimental and numerical results were analyzed by means of polynomial Response Surfaces in order to evaluate the optimal process conditions.

Flow Curve Modelling of a ZM21 Magnesium Alloy and Finite Element Simulation in Hot Deformation

The present investigation deals with the development of a methodology to predict the flow behaviour of the ZM21 magnesium alloy in given intervals of temperature and strain rate by FEM simulation of torsion testing. Equations based on the hyperbolic sine of flow stress and on the multiple linear regression were proposed and implemented into the finite element code. The flow curve shapes obtained by simulation were compared with experimental ones that were not used in the building phase of the equations. It was found that the simulation of torsion tests allows, under given conditions of temperature, strain rate and deformation levels, to obtain flow curve shapes very similar to those obtained by experiments under conditions not included in the building of the models.

Investigation of the mechanical behaviour of AISI 316L stainless steel syntactic foams at different strain-rates

The mechanical behaviour of stainless steel AISI 316L based syntactic foams containing either 40/60vol.% of hollow glass microspheres (S60HS) or 40vol.% of Fillite cenospheres was investigated. In these materials, the hollow particle shells as third phase besides matrix and voids can provide a strengthening effect with the potential of raising mechanical performance above that of conventional, two phase steel foams. Samples were produced by means of metal powder injection moulding (MIM) and subjected to characterization under compressive load, with special attention dedicated to strain-rate sensitivity. Four strain-rate levels were investigated, covering 6 orders of magnitude from 10−3 to 103s−1. For the highest, a Hopkinson Bar apparatus was used. The influence of density on strength was determined for samples containing glass microspheres and described by a power law relationship. The foams mechanical strength was found to increase with strain-rate in accordance with the behaviour observed for the reference material without hollow particles. The data were compared with those obtained in a previous work, in which Fe99.7 matrix syntactic foams containing similar levels of glass microspheres were investigated. The higher strength of the AISI 316L materials is associated with differences in matrix properties. Differences in strain-rate dependence of mechanical properties between both materials can be explained qualitatively based on the fcc (AISI 316L) and bcc (Fe99.7) lattice structure. The introduction of Fillite cenospheres induced a further increase of specific strength. Under quasi-static conditions, samples of this type were found to reach the same yield strength as the reference material despite the reduction in density. The significantly lower strength of glass microsphere based AISI 316L foams can be related to the observed microstructures: due to the high processing temperature (1200°C), glass microspheres are destroyed during sintering, their remainders forming glass inclusions, whereas thermally more stable cenospheres remain intact and can thus stabilize the pores. Finally, an empirical strain-rate sensitive model was adopted to reproduce the experimental data: the fitting procedure used to obtain the model parameters is explained and the influence of the strain-rate discussed. The model allows property prediction for additive content and strain-rate levels further to those evaluated experimentally.

Fatigue Properties of Cu–Nb Conductor Used for Pulsed Magnets at the WHMFC

The Cu–Nb micro-composite wires are used as the conductors for the pulsed magnet above 80 T at the Wuhan National High Magnetic Field Center (WHMFC). The fatigue properties of the Cu–Nb wires carried out with uniaxial tensile test, uniaxial high-cycle fatigue test and four-point bend test at room temperature are studied. The tensile test indicates that a higher loading rate can enhance the hardening rate of the Cu–Nb wire especially at the initial stage of the plastic deformation, leading to a 4% increment of the tensile strength from the lowest strain rate level to the highest one. The four-point bend test reveals that the ratcheting strain occurs in every cycle after plastic deformation commences, which is believed to be one of the most important factors for fatigue life. Moreover, the ratcheting strain stabilizes after about ten cycles with the stable strain increases as the cyclic load levels up. The result of the uniaxial high-cycle fatigue test is found to be a circumstantial evidence for the effect of the ratcheting strain by illustrating that a higher stress amplitude undermines the fatigue life of Cu–Nb wire at the same maximum stress level.

Local Measurement of Stress–Strain Behaviour of Ductile Materials at Elevated Temperatures in a Split‐Hopkinson Tension Bar System

ABSTRACTA split‐Hopkinson tension bar system is modified to allow for measuring the stress–strain behaviour of ductile materials at large strains, high strain rates and elevated temperatures. The specimen is heated by induction, and a pyrometer provides a laser‐based temperature measurement that controls the testing temperature in a feed‐back loop. A high‐speed digital camera system and an edge detection algorithm are used to obtain local measures of strain after necking of the axisymmetric specimens. Using the local strain measurements and Bridgman's analytical formulas, it is feasible to find the equivalent stress–strain curve to fracture for different levels of strain rate and temperature. Thermal and thermo‐mechanical finite element simulations of the test set‐up are used to evaluate the validity of the proposed experimental method.

Prediction of DP600 Flow Surfaces at Various Strain-rates Using Yld2004-18p Yield Function

Due to an increasing usage of dual phase (DP) steels in the automotive industry, it is necessary to fully investigate the mechanical response of DP steels during sheet forming processes or crash events. In this work, the mechanical response of DP600 obtained at different strain rates was used to calibrate the Yld2004-18P yield function at each level of strain rate, and the effect of strain rate on the flow surface was evaluated. Results showed that variation in the material anisotropy coefficients as a function of strain rate can alter the shape of the flow surface of DP600. The current modelling approach is shown to provide a frame work for the incorporation of strain rate-dependence of anisotropy within finite element simulations of sheet metal forming and crashworthiness.

Creep crack growth rate predictions in 316H steel using stress dependent creep ductility

Short and long term trends in creep crack growth (CCG) rate data over test times of 500–30 000 h are available for Austenitic Type 316H stainless steel at 550°C using compact tension, C(T), specimens. The relationship between CCG rate and its dependence on creep ductility, strain rate and plastic strain levels has been examined. Uniaxial creep data from a number of batches of 316H stainless steel, over the temperature range 550–750°C, have been collected and analysed. Power-law correlations have been determined between the creep ductility, creep rupture times and average creep strain rate data with stress σ normalised by flow stress σ0·2 over the range 0·2<σ/σ0·2<3 for uniaxial creep tests times between 100 and 100 000 h. Creep ductility exhibits upper shelf and lower shelf values which are joined by a stress dependent transition region. The creep strain rate and creep rupture exponents have been correlated with stress using a two-stage power-law fit over the stress range 0·2<σ/σ0·2<3 for temperatures between 550 and 750°C, where it is known that power-law creep dominates. For temperature and stress ranges where no data are currently available, the data trend lines have been extrapolated to provide predictions over the full stress range. A stress dependent creep ductility and strain rate model has been implemented in a ductility exhaustion constraint based damage model using finite element (FE) analysis to predict CCG rates in 316H stainless steel at 550°C. The predicted CCG results are compared to analytical constant creep ductility CCG models (termed NSW models), assuming both plane stress and plane strain conditions, and validated against long and short term CCG test data at 550°C. Good agreement has been found between the FE predicted CCG trends and the available experimental data over a wide stress range although it has been shown that upper-bound NSW plane strain predictions for long term tests are overly conservative.

Effect of strain rate on tensile and work hardening properties for Al-Zn magnesium alloys

The effect of strain rate on the mechanical behaviour of Al-Zn magnesium alloys was examined at room temperature under tensile loading with wide range of strain rate. Quasi static tensile test was performed in four different strain rates to obtain their effect on tensile properties, work hardening rate, strain hardening exponent and strength coefficient using a round shape tensile sample. Two types of Al-Zn magnesium alloys were used in this research study, i.e., AZ31 and AZ61 magnesium alloys. The yield stress and tensile strength of AZ31 were found to be strain rate dependent but not for AZ61. The elongations of AZ31 were approximately about 15% for all strain rate levels but for AZ61 the elongations were slightly decreased with increasing strain rate. For all strain rate levels, the work hardening rate of AZ61 was found to be higher compared to that of AZ31. The strain hardening exponent was decreased with increasing strain rate. In contrast, the strength coefficient was increased with increasing strain rate for both alloys. The change in fracture mode as observed from the fracture surface implies that the fracture mechanisms in AZ31 change as the strain rate increases.