Intermediate Strain Rates Research Articles

Quasi-static and dynamic responses of gradient hexachiral auxetics: Experimental and numerical analysis

In the current paper, the mechanical responses of polymer gradient hexachiral auxetic metamaterials were experimentally and numerically investigated under quasi-static and intermediate strain rate loadings. Five gradient design strategies were explored, including uniform, positive, negative, center positive, and center negative gradients. The deformation/failure mechanism, Poisson's ratio, and energy absorption capability were experimentally analyzed, together with the effects of impact velocity and gradient distribution. Experimental results revealed that the gradient design can mitigate the shear deformation of hexachiral auxetics, which efficiently remain its negative Poisson's ratio effect under dynamic loading. Compare with the quasi-static loading, earlier failure of auxetics was observed under dynamic loads due to the strain rate and inertial effect, which diluted their negative Poisson's ratio effect and energy absorption capability. Among the designs, auxetics with negative gradient configurations showed the best energy absorption under dynamic loading. Numerical analysis further supported the experimental findings and provided insights into the influence of geometric parameters. It shows that thicker configurations and non-uniform node gradient series with coefficients of 1.15 (positive, center positive) and 0.87 (negative, center negative) provided optimal energy absorption under the dynamic loading. This work offers profound insights into enhanced performance mechanisms and provides avenues for optimizing structural design of auxetic metamaterials.

High strain-rate fracture behavior of a hollow projectile

In this paper, a hollow projectile made of AISI4340 steel was fabricated based on an arbitrary warhead shape, and high-speed impact tests were performed according to various target encounter conditions including velocity and angle. In order to characterize the dynamic fracture behavior, tensile tests were performed on various notch specimens under three strain rates (10–3 s-1, 100 s-1, 103 s-1), and the stress state histories of the specimens were calibrated through a hybrid experimental-numerical analysis. In particular, for the intermediate (100 s-1) and high (103 s-1) strain rate conditions, the local temperature rose due to adiabatic heating until fracture was experimentally measured, and the thermal softening behavior determined from the inverse optimization technique was considered for the dynamic constitutive model. For the comparison with the high-speed impact test results, a rate-dependent ductile fracture model was utilized for numerical simulation. Considering that the model parameters were calibrated with the thermal softening effect, the proposed fracture model implicitly takes temperature into account. The deformation and fracture modes of the projectile from experimental and numerical study showed very good agreement under all impact conditions. It was confirmed that the softening of a material by adiabatic heating should be considered along with strain rate hardening in dynamic fracture simulation.

Experimental and analytical study on the compressive behavior of PMI foam with different densities and cell sizes under intermediate strain rates

Polymethacrylimide (PMI) foam materials have great potential for applications in the field of protective energy absorption owing to their superior compressive properties. Existing studies on the mechanical behavior of PMI foams are limited, particularly for loading at intermediate strain rates. This paper presents a comprehensive experimental study of PMI foam with four different densities under quasi-static and intermediate strain rate compression (0.001 s−1 to 100 s−1). Each density included three different cell sizes to determine their effects on the compressive performance. Loads parallel and perpendicular to the foam rise direction were considered to investigate the anisotropic behavior. Intermediate strain rate tests were conducted using a high-speed hydraulic servo testing machine, which achieved a stable strain rate while ensuring consistent specimen sizes in both quasi-static and dynamic tests. In all the experiments, the compression process was captured using a high-speed camera and the macroscopic deformation mode and microscopic deformation mechanism were analyzed by combining Digital Image Correlation (DIC) and Scanning Electron Microscopy (SEM). The PMI foam with finer cell sizes exhibited an enhanced compression performance. As the strain rate increased, the strain rate effect became more evident in the high-density specimens and an increase in cell size diminished this effect. A modified analytical model incorporating the cell-size correction term was developed based on Gibson and Ashby's model. The modified model exhibited an average error of 4.9 % in the predicted plateau stress of PMI foam with different cell sizes at the same density.

Strain rate sensitivity of rotating-square auxetic metamaterials

This study provides an in-depth analysis of the mechanical behavior of rotating-square auxetic structures under various strain rates. The structures are fabricated using stereolithography additive manufacturing with a flexible resin. Mechanical tests performed on structures include quasi-static, intermediate, and high strain rate compression tests, supplemented by high-speed optical imaging and two-dimensional digital image correlation analyses. In quasi-static conditions (5 × 10–3 s-1), multiscale measurements reveal the correlation between local and global strains. It is shown that cell hinges play a significant role in structural deformation and load-bearing capacity. In drop tower impact conditions (intermediate strain rate of ca. 200 s-1), the auxetic structures display significant strain rate hardening compared to loading at quasi-static rates. The thin-hinge structures maintain a Poisson's ratio of approximately -0.8, showing higher auxeticity than slow-rate compression tests. High strain rate conditions (ca. 2000s-1) activate additional deformation mechanisms, including a delayed state of equilibrium exemplified by a heterogeneous distribution of lateral strains, possibly due to stress wave interactions and inertial stresses. The study further reveals nonlinear correlations between Poisson's ratio, strain, and strain rate, indicating reduced auxeticity at higher strain rates. These observations are discussed in terms of complex wave interactions and the strain rate hardening characteristics of the base polymer.

A New Method for the Dynamics Analysis of Super-Elastic-Plastic Foams under Inhomogeneous Loading and Unloading Conditions.

In this research, a new computational method was proposed for describing the mechanical behavior of super-elastic-plastic foams under inhomogeneous compressive impacts. The method regarded the foam material as composed of two typical mechanical properties superimposed multiple times: one was the hyper-elastic layer, and the other was the elastoplastic layer. The hyper-elastic layer and the elastoplastic layer were interwoven and overlapped, divided into double-layer, four-layer, and six-layer configurations to characterize the foam material. After the equivalent layering of the foam, by comparing the results of the four-layer and six-layer divisions, it was found that when the layering reached four layers, the foam performance curve had already converged. The study utilized the HYPERFOAM model and Mullins effect in the ABAQUS software to establish the constitutive relationship of the hyper-elastic layer. It adopted the Crushable foam model to develop the constitutive relationship of the elastoplastic layer. Under uniaxial compression conditions, quasi-static and intermediate strain rate compression tests were performed on polyethylene (PE) foam materials with three different densities. Based on the experimental results, the parameter values of the hyper-elastic-plastic foam model in the ABAQUS code were determined. By comparing the computational results and the experimental results, the established finite element (FE) model was validated using the mechanical behavior of indentation and compression tests. The results showed that this method could effectively describe the complex mechanical behavior and residual deformation of hyper-elastic-plastic foam packaging materials under non-uniform compression, and the experimental and simulation results agreed well, proving the reliability of this method.

Dynamic tensile behavior and rate-dependent constitutive model of 304 + Q235 bimetallic steel

This paper aims to elucidate the rate-dependent behavior of 304 + Q235 bimetallic steel under intermediate strain rates. Tensile tests were conducted on 18 sets of specimens to obtain the engineering stress-strain curves at various strain rates. The experimental results demonstrate that the strain rate significantly influences both the yield strength and ultimate strength of the 304 + Q235 bimetallic steel, albeit with different trends. The yield strength exhibits an upward trend with increasing strain rates, whereas the ultimate strength demonstrates a downward trend. Additionally, the strain-hardening effect diminishes proportionally with the increase in strain rate. The parameters of the Cowper-Symonds model associated with the dynamic yield strength increase factor (DIFy) were determined based on the experimental findings. A bilinear rate-dependent constitutive model was established to accurately predict the dynamic ultimate strength increase factor (DIFu) of 304 + Q235 bimetallic steel. The established bilinear model was utilized to enhance the Johnson-Cook model for accurate prediction of the true stress-strain behavior of 304 + Q235 bimetallic steel under intermediate strain rates. The calculated results obtained from the modified Johnson-Cook model exhibit good agreement with the experimental results.

A physics-informed impact model refined by multi-fidelity transfer learning

Impact performance is a key consideration when designing objects to be encountered in everyday life. Unfortunately, how a structure absorbs energy during an impact event is difficult to predict using traditional methods, such as finite element analysis, because of the complex interactions during high strain-rate compression. Here, we employ a physics-based model to predict impact performance of structures using a single quasistatic experiment and refine that model using intermediate strain rate and impact testing to account for strain-rate dependent strengthening. This model is trained and evaluated using experiments on additively manufactured generalized cylindrical shells. Using transfer learning, the trained model can predict the performance of a new design using data from a single quasistatic test. To validate the transfer learning model, we extrapolate to new impactor masses, new designs, and a new material. The accuracy of this model allows researchers to quickly screen new designs or leverage pre-existing databases of quasistatic test data. Furthermore, when impact tests are necessary to validate design selection, fewer impact tests are necessary to identify optimal performance.

Tensile behavior of CoCrFeMnNi high-entropy alloy with intermediate strain rate included

High-entropy alloys (HEAs) have garnered considerable attention for their exceptional mechanical properties, positioning them as promising candidates for diverse industrial applications. This study investigates the mechanical properties of CoCrFeMnNi HEAs over a wide range of strain rates. Based on a novel electromagnetic loading device equipped with high-speed photography, intermediate strain rate tests of the HEAs are conducted. The intermediate strain rate test results show the effectiveness of the experiments. Experimental results exhibit a remarkable strain rate sensitivity of the HEA during tension tests. Microstructural analysis reveals the collaborative interplay between deformation twins and dislocations, enhancing strength-plasticity relationships under tension loading. Finally, a constitutive model (M-JCK) integrated by Johnson-cook model and KHL model is proposed to describe the mechanical behavior of HEAs. The results demonstrate that the M-JCK model outperforms traditional models, providing enhanced accuracy in capturing the complex responses of HEAs across varying strain rates.

On the effect of strain rate during the cyclic compressive loading of liquid crystal elastomers and their 3D printed lattices

Nematic liquid crystal elastomers (LCEs) are a unique class of network polymers with the potential for enhanced mechanical energy absorption and dissipation capacity over conventional network polymers because they exhibit both conventional viscoelastic behavior and soft-elastic behavior (nematic director changes under shear loading). This additional inelastic mechanism makes them appealing as candidate damping materials in a variety of applications from vibration to impact. The lattice structures made from the LCEs provide further mechanical energy absorption and dissipation capacity associated with packing out the porosity under compressive loading.Understanding the extent of mechanical energy absorption, which is the work per unit mass (or volume) absorbed during loading, versus dissipation, which is the work per unit mass (or volume) dissipated during a loading cycle, requires measurement of both loading and unloading response. In this study, a bench-top linear actuator was employed to characterize the loading-unloading compressive response of polydomain and monodomain LCE polymers and polydomain LCE lattice structures with two different porosities (nominally, 62% and 85%) at both low and intermediate strain rates at room temperature. As a reference material, a bisphenol-A (BPA) polymer with a similar glass transition temperature (9 °C) as the nematic LCE (4 °C) was also characterized at the same conditions for comparing to the LCE polymers. Based on the loading-unloading stress-strain curves, the energy absorption and dissipation for each material at different strain rates (0.001, 0.1, 1, 10 and 90 s-1) were calculated with considerations of maximum stress and material mass/density. The strain-rate effect on the mechanical response and energy absorption and dissipation behaviors was determined. The energy dissipation ratio was also calculated from the resultant loading and unloading stress-strain curves. All five materials showed significant but different strain rate effects on energy dissipation ratio. The solid LCE and BPA materials showed greater energy dissipation capabilities at both low (0.001 s−1) and high (above 1 s−1) strain rates, but not at the strain rates in between. The polydomain LCE lattice structure showed superior energy dissipation performance compared with the solid polymers especially at high strain rates.

Flow stress behavior, constitutive modeling, processing map and microstructure evolution in hot deformation of Fe-7.1Al-0.7Mn-0.4C-0.3Nb alloy

The automotive industry has experienced rapid growth, with lightweight materials becoming a significant research focus. Among these materials, low-density Fe–Mn–Al–C alloys show promise for this kind of application. To optimize the hot workability parameters of these alloys for practical applications, it is essential to explore the hot deformation behavior of these alloys. In this study, we investigated the hot deformation behavior of the low-density alloy Fe-0.77Mn-7.10Al-0.45C-0.31Nb using hot plane-strain compression tests for several deformation temperatures (850, 950, 1050, and 1150 °C) and nominal strain rates (0.01, 1, and 10 s−1). Unusual flow stress curves were observed for low (0.01 s−1) and intermediate (1 s−1) strain rates, with discontinuous yielding and multiple stress peaks. This high-temperature behavior is associated with non-homogeneous strain partitioning in duplex microstructure (δ-ferrite + γ). Using the Zenner-Hollomon constitutive modeling approach, we estimated the thermal activation energy for hot deformation as 297 kJ/mol from the experimental peak flow stresses. Additionally, processing maps and microstructure correlation analysis revealed that the alloy exhibits good hot workability in the 975–1075 °C temperature range under low and intermediate strain rates (0.01–0.3 s−1). Under these processing conditions, the δ-ferrite microstructure is submitted to dynamic recovery, while the austenitic phase undergoes dynamic recrystallization.

True stress-strain identification accounting for anisotropy of sheet metals

Sheet metals for the automotive industry are subjected to continuous research efforts aiming at ever increasing mechanical performance. A remarkable feature of modern high strength sheet metals is their anisotropy, intrinsic in the technologic process of their production. When the effect of anisotropy on the mechanical response of a material cannot be neglected, specimens along different directions are usually tested, possibly under different stress states, to assess the flow curves and the deforming ratios for each direction and each loading mode. Such data are then used to calibrate many possible plastic anisotropy models available in the literature. In this work, the experimental procedures for determining the stress-strain curve and the anisotropic straining ratio are studied in detail, referring to representative tensile tests along the rolling direction of two anisotropic sheet metals, respectively PHS-1800 steel and 6181 aluminium alloy. Both alloys are ductile and exhibit remarkably long post-necking phases in tension, revealing that, in such cases, the standard procedures for the experimental derivation of the hardening curves and of the anisotropic strain ratios are limited to the very early phases of the material life and miss to cover the major part of the strain range up to failure. Different alternative procedures for the derivation of experimental data and for their postprocessing are considered and compared to each other, identifying a set of guidelines for achieving a good engineering accuracy up to failure in deriving both the stress-strain curves and the anisotropic strains ratios. The above analyses are made on the results of tensile tests at static, intermediate and high strain rate, confirming the generality of the identified procedure.

Temperature Dependent Dynamic Response of Open-Cell Polyurethane Foams

BackgroundPolyurethane foams have many uses ranging from comfort fitting seats and shoes to protective inserts in helmets and sports equipment. Current military helmet designs employ foam pads of varying densities and bulk material properties to help absorb energy from impacts ranging from quasi-static to ballistic level strain-rates.ObjectiveThis study aims to analyze the thermomechanical uniaxial compression behavior of a high density liner foam pad and a low density liner foam pad used in the Advanced Combat Helmet. These experiments were conducted under strain-rates of 102\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^2$$\\end{document} s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document} and under temperature conditions ranging from -20 to 40 °C. This temperature range was chosen to simulate desert and arctic conditions, with a strain-rate regime chosen to represent loads that would occur often throughout the life of the helmet, such as drops, bumps from riding in a vehicle, or heavy collisions from falling.MethodMultiple experimental apparatuses were used in this study, including a Shimadzu TCE-N300 thermostatic chamber (used to create the varying temperature environments) and a custom-built drop-test system (used to induce intermediate strain-rates). Every experiment was paired with two accelerometers and a high speed camera used for Digital Image Correlation (DIC) to analyze sample deformation and resultant acceleration. The foam’s mechanical response and energy absorption properties were investigated from the measured stress-strain curves. Additionally, each foam composition was analyzed with X-ray computed micro-tomography (XCT) to investigate microstructure properties pre and post-mortem.ResultsResults show that temperature decreased the energy absorption of the low density composition by 48% ± 5% as temperature changed from -20 °C to 40 °C, while energy absorption increased by 53% ± 16% for the high density composition over the same temperature.ConclusionA comparison between the loading response and the material’s density characteristics revealed that the foam’s mechanical properties are heavily dependent on strain-rate applications, as well as environmental factors including temperature. Several important characteristics surrounding each foam composition’s deformation mechanics and damage tolerance as a result of temperature are discussed.

A Practical Inverse Identification of Johnson–Cook Parameters at Intermediate Strain Rates Using Split Hopkinson Pressure Bar Test

A practical inverse method based on the hybrid experiment-finite element (FE) simulation is proposed for identifying strain rate sensitivity of a metal covering intermediate to dynamic loading conditions. The methodology uses the dynamic split Hopkinson pressure bar (SHPB) test for measuring mechanical responses at medium strain rates by optimizing temperature increase, non-uniform strain rate distributed in the non-standard notched SHPB specimens. From the standard dynamic SHPB test, the thermal softening index of the Johnson–Cook (JC) model is first determined by fitting the FE simulation to temperature changes in the specimen. The discrepancy between the measured and predicted flow stresses with the conventional JC model can be attributed to the assumption of constant strain rate sensitivity. Therefore, the new approach using the notched SHPB specimens under dynamic loadings is introduced to identify mechanical responses covering a broader range of strain rate. Finally, the strain rate sensitivity parameter in the JC model as a function of strain rate is evaluated through the inverse FE scheme, in which the sigmoidal function is determined to be optimum by predicting the flow stresses under wider range of strain rate, especially in the intermediate range of strain rate. The present study provides a new methodology based on hybrid experiment and numerical simulation to fill the gap in predicting mechanical responses between quasi-static and dynamic tests using commonly available tensile test and SHPB test.Graphical

Microstructure and Mechanical Behavior Comparison between Cast and Additive Friction Stir-Deposited High-Entropy Alloy Al0.35CoCrFeNi.

High-entropy alloys (HEAs) are new alloy systems that leverage solid solution strengthening to develop high-strength structural materials. However, HEAs are typically cast alloys, which may suffer from large as-cast grains and entrapped porosity, allowing for opportunities to further refine the microstructure in a non-melting near-net shape solid-state additive manufacturing process, additive friction stir deposition (AFSD). The present research compares the microstructure and mechanical behavior of the as-deposited AFSD Al0.35CoCrFeNi to the cast heat-treated properties to assess its viability for structural applications for the first time. Scanning electron microscopy (SEM) revealed the development of fine particles along the layer interfaces of the deposit. Quasi-static and intermediate-rate compression testing of the deposited material revealed a significant strain-rate sensitivity with a difference in yield strength of ~400 MPa. Overall, the AFSD process greatly reduced the grain size for the Al0.35CoCrFeNi alloy and approximately doubled the strength at both quasi-static and intermediate strain rates.

The Effects of Strain Rate and Anisotropy on the Formability and Mechanical Behaviour of Aluminium Alloy 2024-T3

The present study focuses on the mechanical behaviour and formability of the aluminium alloy 2024-T3 in sheet form with a thickness of 0.8 mm. For this purpose, tensile tests at quasi-static and intermediate strain rates were performed using a universal testing machine, and high strain rate experiments were performed using a split Hopkinson tension bar (SHTB) facility. The material’s anisotropy was investigated by considering seven different specimen orientations relative to the rolling direction. Digital image correlation (DIC) was used to measure specimen deformation. Based on the true stress–strain curves, the alloy exhibited negative strain rate sensitivity (NSRS). Dynamic strain aging (DSA) was investigated as a possible cause. However, neither the strain distribution nor the stress–strain curves gave further indications of the occurrence of DSA. A higher deformation capacity was observed in the high strain rate experiments. The alloy displayed anisotropic mechanical properties. Values of the Lankford coefficient lower than 1, more specifically, varying between 0.45 and 0.87 depending on specimen orientations and strain rate, were found. The hardening exponent was not significantly dependent on specimen orientation and only moderately affected by strain rate. An average value of 0.183 was observed for specimens tested at a quasi-static strain rate. Scanning electron microscopy (SEM) revealed a typical ductile fracture morphology with fine dimples. Dimple sizes were hardly affected by specimen orientation and strain rate.

Exploring the influence of strain rate on BTRM tensile behaviour

Extensive research has been conducted on textile-reinforced mortars (TRM); however, a significant gap exists in understanding how strain rate influences mechanical performance. TRM composites reinforced with basalt textiles (BTRM) have become increasingly popular due to the remarkable sustainable profile of basalt fibres. However, there is limited knowledge regarding the tensile response of this composite under intermediate strain rates. The present study examined a BTRM composite consisting of a bi-directional basalt fibre textile and a commercially available mortar strengthened with short glass fibres. Coupon specimens with aluminium tabs were tested with a high-speed servohydraulic equipment at strain rates ranging from 0.5/s to 10/s. Strain and failure propagation were captured with a high-speed camera and analysed using the digital image correlation (DIC) technique. Tests were also performed under quasi-static conditions for comparison. The mechanical characteristics of the BTRM composite were assessed based on its tensile strength, ultimate strain, toughness, stress-strain behaviour, and failure mode. The findings demonstrated that the BTRM composite exhibited sensitivity to changes in strain rate. Increasing strain rates enhanced tensile strength, ultimate strain, and toughness. Furthermore, the specimens examined under dynamic conditions displayed distinct stress-strain characteristics compared to similar specimens subjected to quasi-static loadings.

Forced ignition of cool, warm and hot flames in a laminar non-premixed counterflow of DME versus air

Recently, there has been great interest in the ignition of cool and warm flames (controlled by low-temperature chemistry, LTC, and intermediate-temperature chemistry, ITC, respectively) as they may have significant influence on the ignition and subsequent propagation of hot flame. However, it is still unclear how the flow affects the ignition of the cool/warm flame and their transition to the hot flame in a non-premixed fuel/oxidizer system. In this study, we conduct 2D simulations on the forced ignition of cool, warm and hot flames in a laminar, axisymmetric, non-premixed counterflow of dimethyl ether (DME) versus air. The objective is to assess the effects of flow on the ignition of cool, warm, and/or hot flames and the transition among them. Different ignition energies and strain rates are considered. It is found that the transient ignition process is mainly controlled by the extinction/transition limits and the minimum ignition energies of cool, warm and hot flames. When the strain rate is lower than the transition limits of cool and warm flames, all three flame types can be directly initiated but they eventually evolve into a hot flame. At intermediate strain rate between the transient limit and the extinction limit, quasi-steady cool and warm flames can be obtained without further transition to a hot flame. When the strain rate exceeds the extinction limit of cool and warm flames, only the hot flame can be ignited. An ignition regime diagram in terms of ignition energy and strain rate is proposed, in which different regimes for cool, warm and hot flames as well as transition among them are identified. Furthermore, the ignition of cool and warm flames in a non-premixed counterflow is shown to be very sensitive to ignition position as well as strain rate and ignition energy.

Thermomechanical Processing of V-N Structural Plate Steels in Low Temperature Austenite

Austenite restoration during thermomechanical (TM) rolling of typical vanadium-microalloyed structural steels was studied to optimize strength in the as-rolled and air-cooled condition. Multi-pass plate rolling simulations were performed on V-N microalloyed and CMn steels to compare recrystallisation behaviour in various temperature regions. Included were a conventional schedule ending at high temperature and two TM schedules with mill exit temperatures in the intermediate and low austenite regions. Increasing delay periods after roughing enhance the suppression of recrystallisation after the start of finishing thereby increasing both nucleation site density and nucleation rate for ferrite formation and refinement in grain size. Good agreement was found between microstructures after industrial TM rolling and those obtained from laboratory simulations. Although precipitation of vanadium carbonitrides is an effective strengthening mechanism, appreciable gains in yield strength due to grain refinement can be achieved by rolling in the lower austenite region. Low nitrogen contents in V steels produce coarser final ferrite grain sizes and lower strengths probably due to a larger precipitate size. V-N steels display similar flow behaviour to CMn grades down to approximately 825°C at low to intermediate strain rates but may experience alternate regions of work hardening and dynamic softening at lower temperatures in austenite.

Strain-rate response of 3D printed 17-4PH stainless steel manufactured via selective laser melting

This research work aims at investigating the strain rate effects on the 17-4PH stainless steel produced by Selective Laser Melting (SLM), with the final goal of adding knowledge about its mechanical performance for applications under certain dynamic conditions. Mechanical characterization of 3D-printed samples was performed using a universal quasi-static electro-mechanical machine, a hydro-pneumatic machines, and a split Hopkinson Tensile Bar device for quasi-static, intermediate (5 s−1) and high strain-rates (100 s−1 and 300 s−1), respectively. The use of this technology represents the most interesting breakthrough of the paper, allowing to identify and to evaluate the performance of 3D-printed metals in relation to impacts or other dynamic conditions typical of different areas of engineering, which are impossible to be obtained by other technologies. The conducted tests allowed to identify the main information on the material behaviour, from the quasi-static to the high strain-rate regime, mainly in terms of stress–strain curves and fracture, and the behaviour after fracture was assessed by applying the Bridgman equations.The effect of horizontal and vertical printing directions, as well as of an annealing heat treatment, are also discussed. According to the evaluation of the main mechanical parameters, the strain-rate increase affects the mechanical behaviour both in the horizontal and vertical directions: samples built horizontally show increased yield stress and ultimate strain and decreased ultimate strain, whereas specimens printed vertically exhibit a remarkable increase in yield stress and a decrease of the ultimate stresses and strains. However, the better behaviour observed, with respect to the horizontally printed specimens, for vertically printed specimens in the quasi-static regime was no longer observed for intermediate and high strain rates. Moreover, all heat-treated specimens present higher strain-rate sensitivity than not heat-treated ones.Finally, the applicability of the existing Johnson-Cook material strength model to represent the mechanical behavior of 3D printed 17-4PH stainless steel is examined. The obtained results represent an important contribution to the current literature, because the dynamic behaviour of 17-4PH stainless steel produced by SLM, under the studied strain rates values, have been scarcely investigated so far.

Thermo-mechanical experimental investigations of 3D-printed elastomeric polyurethane from low to intermediate strain rates

Additively manufactured (3D-printed) elastomers have increasing applications in impact resistance devices such as helmets, shoe soles, and shock absorbing architectured metamaterials. These rapidly expanding areas require a proper understanding of the thermo-mechanical behaviours of soft polymers. In this contribution, thermal–mechanical properties of 3D-printed elastomeric polyurethane (EPU) are extensively characterised under low to high strain rates which are missing in the literature. The EPU under investigation is digitally manufactured using a Digital Light Synthesis (DLS) technology and is characterised by tensile experiments with a wide range of strain rates spanning from 0.001/s to 500/s and temperature variations of -20 °C to 60 °C. The experimental results reveal deformation nonlinearity, thermal-sensitivity, and strain rate-sensitivity in the elastomer. Moreover, the study reveals the occurrence of the glass transition phenomenon, which is commonly observed in soft materials under low-temperature and high strain-rate conditions. Various graphical illustrations are presented to depict the effects of temperature and strain rate on the stress response. It is observed that as temperature decreases or strain rate increases, the stress amplifies and becomes more sensitive to variations in temperature or strain rate. Additionally, higher strain levels further enhance the stress sensitivity to these variations. The microscopic mechanisms behind the thermal and strain rate sensitivities are discussed, taking into account the influence of the strain level. Overall, this study contributes to a proper understanding of the thermo-mechanical behaviours of digitally-printed soft polymers, particularly in dynamic scenarios.