High Strain Rate Behavior Research Articles

Mechanical characterization of Araldite LY 564 epoxy: creep, relaxation, quasi-static compression and high strain rate behaviors

A comprehensive mechanical characterization of a commercially available hot curing epoxy system (Araldite® LY 564 with Aradur® 2954, HUNTSMAN Corp.) is performed including compression tests at quasi-static and high strain rates, as well as creep and relaxation tests. High strain rate tests (770/s and 875/s) are conducted by using a split-Hopkinson pressure bar (SHPB) apparatus and compared with the quasi-static compression tests at three strain rates (0.001/s; 0.01/s; 0.1/s). Elasticity modulus and yield stress increase with increasing strain rate, for both quasi-static and high strain rate conditions. The change in the mechanical behavior from quasi-static to high strain rate conditions is quite prominent. While strain rate changes from 0,001/s to 875/s, elasticity modulus increases from 1465 to 3193 MPa (118% increase) and yield stress from 119 to 204 MPa (72% increase). The evident exponential hardening at the viscoplastic region seen on quasi-static stress–strain curves is not observed at high strain rates. Creep tests are performed for three different stress levels (50, 100 and 200 MPa) and 120 min (7200 s). In addition, relaxation tests are done for three different strain levels (3.16, 7.15 and 35.5%) also for 120 min (7200 s). Creep compliance and relaxation modulus are calculated and plotted. It is observed that creep strain values at the stress levels lower than the yield point are lesser compared to creep strain values around yield strength since rate dependency is increasing with increasing stress. Creep at the hardening region is not prominent due to polymer chain entanglement. Stress drop during relaxation is increasing with the increasing strain level.

Polymer-based nanocomposites for impact loading: A review

The aim of this article is to review the solution and challenges associated with high strain rate behavior of nanocomposites. Nanoclay, carbon nanotube, graphene, and inorganic nanofillers are most commonly used for developing nanocomposites with superior dynamical properties against high strain rate loading. Higher specific properties, thermal stability and large number of design variables makes nanofillers a potential candidate to replace conventional fibers in many applications that includes nanocomposites for space structures, defense vehicles and automobiles. Despite the many advantages associated with nanocomposites, their applications are still limited, due to inadequate knowledge of failure mechanism under different types of loading scenarios that includes static and dynamic behavior. In the present article, a comprehensive review of characterizing techniques used with nanocomposites is presented in conjunction with failure governing mechanism of organic as well as inorganic nanofillers reinforced nanocomposites.

Predicting the high strain rate behaviour of particulate composites using time-temperature superposition based modelling

Polymeric particulate composites are widely used in engineering systems where they are subjected to impact loading – at a variety of temperatures – leading to high strain rate deformation. These materials are highly rate and temperature dependent, and this dependence must be well understood for effective design. It is not uncommon for many of these materials to display mechanical responses that range from glassy and brittle to rubbery and hyperelastic [1-3], due to their polymeric constituents. This makes accurate measurements and modelling not only necessary, but challenging. This is made more difficult by experimental artefacts present when traditional tools such as the split Hopkinson pressure (SHPB) or Kolsky bar are used to interrogate the high rate response of low-impedance materials. The transition from isothermal to adiabatic conditions as the rate of deformation increases also has an effect on the mechanical response, which cannot be neglected if the high rate behaviour is to be accurately predicted. In this paper, time-temperature superposition based frameworks that have enabled the high rate behaviour of neoprene rubber [4] and (plasticised) poly(vinyl chloride) [5] to be captured, will be extended to explore the high strain rate behaviour of unfilled natural rubber and several grades of glass microsphere filled natural rubber particulate composites.

Strain rate dependence of SM490B at room temperature and application to Cowper-Symonds constitutive models

Steel is widely used as a constituent material for various structures such as automobiles and ships. To perform high precision analysis including high strain rate behavior, an understanding for the strain rate dependence of material strength becomes very important. The purpose of this study is to evaluate the strain rate dependence of material strength with rolled steels for welded structure, JIS SM490B (ASTM E). We investigated the deformation characteristics at room temperature by performing compression tests at a wide range of strain rates and applied the obtained experimental results to the material constitutive model using an optimization method of Nelder-Mead method. The quasi-static tests were conducted using a universal testing machine at the strain rate of 10-3, 10-2 and 10-1 s-1. The impact test was conducted using a split Hopkinson pressure bar apparatus at the strain rate of approximately 103 s-1. As the results of the compression tests, it was confirmed that SM490B has a positive strain rate dependence of material strength. The Cowper-Symonds constitutive model showed good agreement with the experimental results up to the strain of 20%. However, the error became larger between experimental results and CS approximation as the strain increases to 20% or more.

Dynamic properties of additively manufactured tantalum

In this study a series of gas gun plate impact experiments have been conducted on samples of wire arc AM tantalum to investigate the high strain rate behaviour of the material, including tensile failure (spall) mechanisms. Conventionally processed tantalum was fielded alongside the AM material to provide a direct comparison under identical loading conditions. In-situ velocimetry data was supported by pre-shock characterisation of the samples to supply information on the initial material microstructure, and to link these features to the observed differences in dynamic behaviour. Additional post-shock analysis of the damaged region in the samples provided further insight into the failure process.

Estimation of mechanical response of 2-phase oxide ceramic composites under high strain rate

The high strain-rate behaviour of ceramic composites (CCs) results from the complexity of their internal microstructure. The CCs are built of mixtures of different phases joined by various technological processes. Usually, CCs contain small amounts of porosity and technological defects. The phases can exhibit purely elastic behaviour or elastic–plastic one under a high level of loading. The paper describes experimental tests performed with Split Hopkinson Pressure (SHPB) of exemplary ceramic samples and proposes a correlation based on mesocrack formation with this type of loading. Therefore, a multiscale model of microcracking processes for high strain rate is proposed and related to time.

A Rapid Throughput System for Shock and Impact Characterization: Design and Examples in Compaction, Spallation, and Impact Welding

Many important physical phenomena are governed by intense mechanical shock and impulse. These can be used in material processing and manufacturing. Examples include the compaction or shearing of materials in ballistic, meteor, or other impacts, spallation in armor and impact to induce phase and residual stress changes. The traditional methods for measuring very high strain rate behavior usually include gas-guns that accelerate flyers up to km/s speeds over a distance of meters. The throughput of such experiments is usually limited to a few experiments per day and the equipment is usually large, requiring specialized laboratories. Here, a much more compact method based on the Vaporizing Foil Actuator (VFA) is used that can accelerate flyers to over 1 km/s over a few mm of travel is proposed for high throughput testing in a compact system. A system with this primary driver coupled with Photonic Doppler Velocimetry (PDV) is demonstrated to give insightful data in powder compaction allowing measurements of shock speed, spall testing giving fast and reasonable estimates of spall strength, and impact welding providing interface microstructure as a function of impact angle and speed. The essential features of the system are outlined, and it is noted that this approach can be extended to other dynamic tests as well.

High strain-rate dynamic compressive behavior and energy absorption of distiller’s dried grains and soluble composites with paulownia and pine wood using a split Hopkinson pressure bar technique

Novel bio-based composite wood panels (CWPs) that consisted of distiller’s dried grains and solubles (DDGS) flour adhesive bound to a wood filler/reinforcement were subjected to high strain-rate compression loading, and their behavior was investigated. Specimens of DDGS-Paulownia wood (PW) or DDGS-pinewood (Pine) composites made using DDGS with fractions of 10%, 15%, 25%, and 50% were tested at high strain-rates using a modified compression Split Hopkinson Pressure Bar (SHPB). Both DDGS-PW and DDGS-Pine composites displayed strain-rate sensitivity, and DDGS-PW had a 25% fraction, which showed the highest ultimate compressive strength of 655 MPa at approximately 1600/s. The 90%-PW had the highest specific energy of 19.24 kJ/kg at approximately 1600/s when loaded via dynamic compression. The CWPs constructed of DDGS-PW had higher strength and energy absorption than DDGS-Pine with the exception of the 50% DDGS composites.

On the Cover of this Issue: High Strain Rate Behavior of a Viscoelastic Gel Under High-Velocity Microparticle Impact by D. Veysset et al.

On the Cover of this Issue: High Strain Rate Behavior of a Viscoelastic Gel Under High-Velocity Microparticle Impact by D. Veysset et al.

Influence of conditioning on the high strain rate compression response of Kevlar thermoplastic composites

AbstractKevlar reinforced composite material systems are widely used for personal body armors. Due to aging or minor damage while in service, body armor may get exposed to external weather conditions, including moisture. The present study investigates the effect of moisture on the high strain rate behavior of Kevlar/Polypropylene (K‐PP) composite. Flat K‐PP composite laminate was manufactured using a vacuum‐assisted compression molding followed by laser machining. Dynamic compressive loading tests were performed using split Hopkinson pressure bar setup. The phenomenological modeling approach was adopted to characterize the rate‐dependent behavior of dry and wet composites. For identical dynamic compressive loading, different strain rates, strains, and stresses were attained by the dry and wet composite specimens. Macroscopic and microscopic imaging was done to expose the variation in damage behavior as a function of moisture absorption. Significant property reduction as a function of moisture absorption emphasizes the need for moisture proofing and protecting the armor products from minor damages leading to sites for moisture ingress.

Comparative study of impact response of GFRP and CFRP composite plates against blunt nosed projectile

Three-dimensional finite element simulations were carried out to explore the perforation behavior of the unidirectional GFRP and CFRP plates against the blunt nosed projectile within sub-ordnance velocity regime. The ballistic performance of both the composite plates of 140 mm × 140 mm × 3.3 mm comprising of four layers (each 0.825 mm thick) having unidirectional fibres was evaluated. The mass, diameter and length of projectile was kept constant as 52.5 gm, 19 mm and 50.8 mm respectively. To carry out the numerical simulations, commercial finite element code Ansys/Ls-Dyna was employed. To simulate the high strain rate behavior of GFRP composite material, enhanced composite damage material model was considered whereas for CFRP, Composite failure material model offered better results. The performances of both the plates were evaluated based on residual velocity, ballistic limit and energy absorption. The numerical results were found to be close enough with the experimental results available in the literature. It was found that GFRP plates underperformed than that of CFRP plates.

An Experimental Study on the Dynamic Behavior of an Ultra High-Strength Concrete

Ultra-high-strength concrete is a newly developed construction material that has a minimum 120 MPa or higher compressive strength. Recently, the usage of high-strength and ultra-high-strength concretes has become widespread due to the enhancement of the concrete technology. Many civil engineering structures constructed by using concrete materials are usually subjected to, in addition to static loads, dynamic loads due to earthquakes, wind and storm, impact and blast, which take place under high energy and high strain rate values. The effects of such loadings on the structure must be understood thoroughly. In recent years, the withstanding of a structure on these loading conditions has become a crucial issue for its impact on the economy and human safety. One of the approaches to fulfill these requirements is to develop high-strength or ultra high-strength concretes (UHSCs). In this study, an ultra-high-strength concrete with a compressive strength of 135 MPa was designed and developed. In order to determine the dynamic behavior of this UHSC, the specimens at three height/diameter ratios (approximately, 0.6, 1.0 and 1.2) were extracted from the prepared concrete mixtures. These concrete specimens were tested to determine both the quasi-static and dynamic compressive behaviors of the developed concrete. In the quasi-static compression tests, cylindrical specimens and a conventional compressive testing machine were used. In order to study the dynamic compressive behavior, a Split Hopkinson Pressure Bar (SHPB) test setup was used. In this test system, the time variations of compressive strength, the strain and strain rates under uniaxial pressure loading were experimentally evaluated and the deformation and fracturing processes of the specimens were recorded using a high-speed camera. The test results, based on the testing of 21 different specimens, have shown that the dynamic compressive strength values of the developed concrete varied in the range of 143 to 253 MPa, while the strain rate values varied in the range of 353 s−1 to 1288 s−1. Using the data generated in the SHPB tests, the parameters present in a Johnson–Holmquist–Cook concrete material model, which is used in numerical studies on the high strain rate behavior of concretes, were evaluated.

Tensile strength and failure of ultra-high performance concrete (UHPC) composition over a wide range of strain rates

In the fibre reinforced concrete, the geometry, density, distribution and orientation of micro-steel fibres can have a significant effect on mechanical performance. In this study, five different compositions of ultra-high performance concrete (UHPC) with steel fibres were designed and developed for improving the mechanical properties over conventional control concrete. An improvement of around 300% in tensile strength was observed with the addition of micro-steel fibres during quasi-static loading in tension. High strain rate behaviour was also studied using Kolsky bar experiments in tension, which was combined with in-situ imaging to capture failure initiation and progression during loading. From the experimental results, it was found that the binary composition of the UHPC matrix is more rate sensitive than the ternary and quaternary compositions. In addition, the dynamic increase factor (DIF) was found to be lower for specimens with micro-steel fibres in contrast to control with only matrix material. From the fracture and failure analysis of UHPC specimens, it was observed that the micro-steel fibres in the UHPC matrix prevent the splitting failure, and maintain the structural integrity of the specimen beyond peak stress.

Mechanical behavior and texture evolution of WE43 magnesium-rare earth alloy in Split-Hopkinson Pressure Bar and Taylor Impact Cylinder Testing

Mechanical behavior and texture evolution of Mg rare-earth alloy WE43 is investigated for strain-rates 10−3/s to upwards of 105/s for the two material conditions - as-cast (AC) and T6 age hardened, rolled plate (RT6). The high strain-rate behavior is tested using both Taylor cylinder impact tests (TC) and split Hopkinson pressure bar tests (SHB) and bulk textures are obtained using neutron diffraction. Unlike the quasi-static strained material, AC and RT6 SHB retained high hardening rates throughout the test, even up to 30% true strain. Moreover, the high strain-rate data revealed that the RT6 material has a much higher strength than the AC material, but similar hardening rates despite significantly different initial texture. The flow stress near yield increased up to 10% for RT6 and up to 30% for AC as the strain-rate increased six orders of magnitude from quasi-static rates 10−3/s to 103/s. Neither material exhibited significant plastic anisotropy over the broad range of strain rates, despite the fact that the RT6 material had a moderately strong initial texture. In the TC tests, the geometric cross-sectional changes and texture along the cylinder from the cylindrical sample foot to head are measured and from the neutron diffraction texture analysis, upper-bound estimates of twin volume fraction are obtained as well as dislocation density from analyzing diffraction peak broadening. Recorded geometrical changes along several loading directions show that the material has deformed homogeneously under impact. Analysis of deformed textures indicates that {101¯2} extension deformation twinning occurred in the RT6 condition over the range of strain rates, with an upper bound estimate of 40% twin volume fraction for approximately 0.10-0.25 true strain. The peak texture components after the impact have their c-axes closely aligned with the impact direction. These observations are presented and rationalized in the paper.

A phenomenological model utilizing Voce’s strain-hardening model describing high strain rate behavior of alloy 800H during hot working

In this study, a simple mathematical model is introduced to predict the stress–strain curves under the occurrence of minor dynamic recrystallization. In this regard, first, with the assumption of nonlinear variation of strain-hardening rate versus strain, a mathematical equation was derived to obtain the strain-hardening rate. Then, using the Voce’s equation, a new mathematical model was developed to predict the hot deformation behavior of metallic alloys at high strain rates. The most important feature of the proposed model is its simplicity so that the only constant of the stress–strain equation can easily be interpolated at different temperatures and strain rates. Accordingly, it was found that the defined material constant varies linearly and exponentially with strain rate and temperature, respectively. The high correlation factors (average correlation factor, r2 = 0.85) obtained from regression analysis showed that the model can provide a precise estimation of flow stress curves for the defined deformation conditions.

High Strain Rate Behavior of Aluminum Alloy for Sheet Metal Forming Processes

Aluminum alloy sheets are gaining increasing interest in the construction of some or all components of the car body in view of their lightweight properties which can allow significant fuel consumption reduction. In order to be suitable for car body application, aluminum alloy sheets should have sufficient mechanical properties both in static (e.g., structural stability and durability) and dynamic conditions (e.g., crash test). Static and dynamic mechanical tests (strain rates: ε ˙ ≈ 1 × 10−3 s−1 and ε ˙ ≈ 5 × 102 s−1 respectively) were conducted on AA6016 alloy sheet (1 mm thick), in T4 and T6 temper and for the longitudinal, transverse, and diagonal rolling directions by means of standard static tensile test and modified Hopkinson bar dynamic tests. Microstructural and fracture morphology observations are also reported. The results show that the ultimate tensile strength increases by 13−14%, and the elongation at fracture increases by 75−105%, depending on the temper, by increasing the strain rate.

Effect of Loop Defects on the High Strain Rate Behavior of PEGDA Hydrogels: A Molecular Dynamics Study.

The high strain rate behavior of nonideal poly(ethylene glycol) diacrylate hydrogels under uniaxial tension and transient-state shear deformations is investigated using molecular dynamics (MD) simulations. This work specifically focuses on the influence of first-order loop defects, including their effect on topological evolutions. Two approaches are proposed to systematically introduce first-order loops, allowing separate and controllable investigations of effective cross-link functionality and cross-link density. MD simulations confirm that first-order loop defects are elastically inactive, but the topological disruptions caused by the presence of loop defects influence mechanical behavior. For decreasing effective cross-link functionality but constant cross-link density, a weaker tensile stress-strain response and decreasing shear-thickening behavior are observed. This is due to nonaffine translation of cross-link junction positions during deformation. Hydrogels with lower cross-link density but constant effective functionality show a stronger stress-strain response and an earlier transition between entropic and enthalpic deformation regimes. This behavior is correlated to changes in mesh size caused by the introduction of loops within an elastically active network. However, the resulting range of cross-link densities is not sufficient to cause measurable changes in shear-thickening behavior. To conclude, reductions in effective cross-link functionality are more important to high strain rate behavior than reductions in cross-link density.

High strain rate behaviour of stainless-clad bimetallic steel

To study the dynamic mechanical properties and to develop constitutive models of stainless-clad (SC) bimetallic steel at high strain rates, 11 groups of standard specimens (i.e. cylinders of 6 mm in diameter and 6 mm in height) are designed. Four different loading directions and clad ratios varying from 0.2 to 0.5 are considered. 123 compression tests are carried out at six strain rate levels (0.0005 s−1, 1000 s−1, 1500 s−1, 2000 s−1, 3000 s−1 and 4000 s−1). Johnson-Cook (J-C) constitutive model is used to fit the test data, and the fitting results are in good agreement with the experimental results. It can be found that the strain rate hardening has a prominent effect on stress development of the SC bimetallic steel at high strain rate compared with that in the quasi-static condition. With an increase of the clad ratio, the stress of the SC bimetallic steel grows slightly as the strain hardenability and sensitivity of the strain rate decrease. The SC bimetallic steel can be considered as an isotropic material in terms of high strain rate behaviour because different loading directions result in no obvious distinction. In addition, a formula for dynamic increase factor (DIF) for strengths as well as a general constitutive equation of J-C model for the SC bimetallic steel are developed and validated, which can be adopted in structural design under impact loading and promote engineering application of the SC bimetallic steel.

Mechanical behaviour and high strain rate deformation of stainless steel 316L processed by selective laser melting

With growing interest in metal additive manufacturing (AM) for aerospace and defence applications, it is important to understand the influence of processing conditions on high strain rate behaviour. This paper presents research on as-built stainless steel 316L AM parts processed using laser powder bed fusion (L-PBF) also referred to as selective laser melting (SLM). Two sets of varied process parameters were examined as well as different build orientations. Laser scan speed and point-to-point distance are shown to have influenced the high strain rate mechanical properties of 316L AM parts. Additionally, both wrought and SLM produced 316L showed strain rate sensitivity (hardening) at strain rates of 103 s1, but this effect was more pronounced in the wrought material. Findings from this study can be used to determine appropriate part orientations and AM process parameters and their influence on the high strain rate behaviour and impact performance of L-PBF AM parts.

High strain-rate behaviour of a Tungsten alloy

Tungsten alloys are often used in machinery and in other industrial sectors. This study presents the dynamic tensile behaviour of a commercial tungsten alloy. The tests were carried-out at high strain-rate (from 850 to 2200 s_1) by means of a Split Hopkinson Tension Bar device on round specimens having diameter and gauge length of 2 mm and 5 mm, respectively. The dynamic stress versus strain curves were compared with the quasi-static ones in order to investigate the rate-dependency of this alloy. High strain-rate caused a growth of proof and ultimate tensile strengths as well as modulus of toughness and a decrease of the area reduction. Finally, the parameters of the Cowper-Symonds relationship were calibrated.