Strain Rate Impact Research Articles

Effect of printing parameters on the structure and high strain rate deformation behavior of additively manufactured 316L stainless steel

High strain rate deformation is characterized by strain localization and formation of adiabatic shear bands (ASBs), as opposed to the movement of slip systems seen in quasistatic deformation. Several studies have been done on the effect of printing parameters on the structure and properties of 316L stainless steel, little attention has been paid to the high-strain rate deformation behaviour of this material. In this study, the effect of printing parameters on the microstructure and high strain rate deformation behavior of 3D-printed 316L stainless steel alloy was investigated. Printing parameters such as hatch spacing, laser power, scan speed and build direction, and their effect on the microstructure, density, hardness, and high strain rate compressive behaviour of 316L stainless steel alloy are established. The results show that even though increasing laser power leads to a reduction in part porosity and better part consolidation (density), scan speeds had a significant effect on porosity production regardless of laser power and hatch spacing. A scan speed of 750 mm/s generally resulted in the lowest porosities regardless of laser power and hatch spacing (150 W ≤ P ≥ 250 W and 0.08 mm ≤ h ≥ 0.14 mm respectively). Scan speeds of 1250 mm/s would have to be coupled with higher laser powers (P ≥ 200 W) to get relatively similar porosities obtained at 750 mm/s. Regardless of the laser power and hatch spacing, scan speeds ≥1750 mm/s resulted in higher porosities. In addition, increasing hatch spacing, generally, resulted in increasing levels of porosity in the printed samples, with a hatch spacing of 0.08 mm resulting in the lowest levels of porosities while hatch spacing of 0.14 mm resulted in the highest levels of porosities. In terms of Volumetric Energy Density (VED), VEDs between 44 and 139 J/mm3 resulted in the lowest porosities, highest densities, and hardness, regardless of the build directions. In addition, increasing laser power resulted in thicker and tightly packed sub-grain structures whereas, an increase in hatch spacing and scan speed resulted in thinner/loosely packed sub-grain structures, and thinner/tightly packed sub-grain structures, respectively. Regardless of the printing parameters, samples perpendicular to the build direction had higher maximum flow stresses and lower strains when compared with the samples parallel to the build direction during high strain rate impact. Also, any combination of individual printing parameters that result in finer sub-grain structures resulted in a relatively brittle deformation behaviour during high strain rate impact. The formation of ASBs is characterized by refined grains within the region of the ASBs and coarser grains away from the ASBs region.

Numerical Simulation Study of Expanding Fracture of 45 Steel Cylindrical Shell under Different Detonation Pressure.

Detonation and fragmentation of ductile cylindrical metal shells is a complicated physical phenomenon of material and structural fracture under a high strain rate and high-speed impact. In this article, the smoothed particle hydrodynamics (SPH) numerical model is adopted to study this problem. The model’s reliability is initially tested by comparing the simulation findings with experimental data, and it shows that different fracture modes of cylindrical shells can be obtained by using the same model with a unified constitutive model and failure parameters. By using this model to analyze the explosive fracture process of the cylindrical shells at various detonation pressures, it shows that when the detonation pressure decreases, the cylindrical metal shell fracture changes from a pure shear to tensile–shear mixed fracture. When the detonation pressure is above 31 GPA, a pure shear fracture appears in the shell during the loading stage of shell expansion, and the crack has an angle of 45° or 135° from the radial direction. When the pressure is reduced to 23 GPA, the fracture mode changes to tension–shear mixing, and the proportion of tensile cracks is about one-sixth of the shell fracture. With the explosion pressure reduced to 13 GPA, the proportion of tensile cracks is increased to about one-half of the shell fracture. Finally, the failure mechanism of the different fracture modes was analyzed under different detonation pressures by studying the stress and strain curves in the shells.

Experimental modal analysis of RC beams strengthened with SHCC subjected to shear under impact strain rates

Strain Hardening Cement-based Composites (SHCC) holds great potential to be a remarkably effective material for shear strengthening of reinforced concrete beams in case of impact scenarios due to its great energy dissipation capability through the formation of multiple very fine cracks (∼100 μm of width). The effectiveness of this technique is investigated in the paper at hand through the observation of the changes in the dynamic properties in terms of natural frequencies, mode shapes, and damping ratios. Considering a non-destructive free vibration excitation of the specimens before the experiment (undamaged/intact state) to their condition after the destructive impact of an accelerated steel projectile. In this sense, two types of reinforcement configurations and two types of strengthening layers were investigated: reinforced concrete beams with and without stirrups were strengthened with SHCC made with polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (UHMWPE) and tested under increasing levels of kinetic energy varying between 2.1 kJ and 6.4 kJ by using an accelerated impactor inside a drop tower facility. The parameter variation enabled conclusions with respect to the influence of the stirrup reinforcement and the SHCC strengthening contributions in the case of dynamic scenarios.

Deformation-activated recrystallization twin: New twinning path in pure aluminum enabled by cryogenic and rapid compression

SummaryBulk aluminum rarely forms deformation or annealing twins owing to its high stacking fault energy. We report a novel twinning mechanism mediated by dynamic recrystallization in 6N pure aluminum under high strain rate (∼1.3 × 104 s−1) impact at a cryogenic temperature (77 K). Discontinuous dynamic recrystallization occurs during rapid severe plastic deformation and generates inhomogeneous microstructures exhibiting low-angle and high-angle grain boundaries. Unexpectedly, Σ3 twin boundaries were able to develop during dynamic recrystallization. Although these recrystallization twins have similar morphology as that of annealing twins, their formation relies on deformation activation instead of thermal activation, which was suppressed by the cryogenic experiment. Besides, strong orientation dependence was observed for formation of these novel twins. Beyond annealing and deformation twin, deformation-activated recrystallization twin is a new path for pure aluminum twinning.

Enhanced high-strain-rate impact resistance of helicoidal composites by fused deposition modelling

Composites with both lightweight and high strength have tremendous demands in aerospace, rail transit, and military, etc., especially for ones with strong high-strain-rate dynamic impact resistance. Among them, bioinspired helicoidal composites is one type of them. Currently, due to the limitation of fabrication, only 0, 45, and 90° helicoidal composites have been investigated to enhance impact resistance. Other angle helicoidal composites have not been completely explored yet. Here, we have fabricated a series of helicoidal composite specimens with 0°–90° helicoidal angles by Fused Deposition Modeling (FDM), tested their high strain rate impact resistance using Split Hopkinson Pressure Bar (SHPB), high-speed camera images and Digital Image Correlation (DIC), and analyzed the relationship of the strain rate and dynamic stress–strain. In order to verify the experimental results, the numerical simulations have been carried out. The results indicate that dynamic experiments have revealed remarkable strain rate sensitivity, the maximum stress for 60° helicoidal composite is higher than others at the strain rates are 700 s−1 and 1000 s−1. The underlying mechanism of the enhancements lies in that helicoidal angles remedy effective the weakness of adhesion between adjacent filaments. Helicoidal modes effectively improve the anisotropy. Our findings here provide a method to enhance the high strain rate impact performance and broaden the application of 3D printing.

Uncertainty bounds for multivariate machine learning predictions on high-strain brittle fracture

Simulation of the crack network evolution on high strain rate impact experiments performed in brittle materials is very compute-intensive. The cost increases even more if multiple simulations are needed to account for the randomness in crack length, location, and orientation, which is inherently found in real-world materials. Constructing a machine learning emulator can make the process faster by orders of magnitude. There has been little work, however, on assessing the error associated with their predictions. Estimating these errors is imperative for meaningful overall uncertainty quantification. In this work, we extend the heteroscedastic uncertainty estimates to bound a multiple output machine learning emulator. We find that the response prediction is accurate within its predicted errors, but with a somewhat conservative estimate of uncertainty.

A computational investigation on the influence of the l/d ratio and strain rate on the deformation behavior of rolled and homogeneous armor steel in the split Hopkinson pressure bar test process

The present investigation deals with the finite-element analysis of the high strain rate deformation behavior of the quenched and tempered armor-grade rolled and homogeneous armor steel. The rolled and homogeneous armor steel is extensively used in civil and military structures such as battle tanks, armament combat vehicles, combat helicopter, etc. The dynamic deformation behavior of rolled and homogeneous armor steel, that is, resistance against ballistic circumstances relates to its mechanical behavior under high strain rate conditions. In the present research work, a finite-element analysis investigation (using Abaqus finite-element analysis code) has been carried out to evaluate the influence of specimen l/d ratios and high loading strain rates on the deformation behavior and stress–strain responses of the rolled and homogeneous armor steel. Further, an attempt has also been made to check the high strain rate and specimen l/d ratio influence on the strain amplitudes of incident, reflected, and transmitted pulses. The numerical investigation has been carried out with the rolled and homogeneous armor steel specimen with l/d ratios of 1, 0.8, and 0.6. In addition, three high impact strain rates of 2130, 2907, and 3105 s−1 are considered to evaluate the stress–strain responses. The results revealed that the l/d ratio and strain rate have a significant influence on the specimen stress–strain response and the strain amplitudes of incident, reflected, and transmitted pulses. The peak stress value is increased with the increase in the l/d ratio and strain rate. The developed finite-element analysis model has predicted the stress–strain responses with <3% percentage error. The obtained finite-element analysis results have been validated with the experimental investigation with an l/d ratio of 0.6 and a strain rate of 3105 s−1 for rolled and homogeneous armor steel.

Impact-induced twinning in a magnesium alloy under different stress conditions

We investigate deformation twinning in a highly textured magnesium alloy plate of Mg-3.1%Al-0.9%Zn-0.4%Mn under edge-on impact with in situ, synchrotron-based, ultrafast X-ray diffraction measurements, and corresponding stress states are simulated with the finite element method. Deformation twinning and its anisotropy under the triaxial stress condition are explained by a statistical analysis of resolved shear stress. The critical resolved shear stress criterion is applicable under complicated stress conditions induced by high strain rate impact loading. Three typical simple stress conditions are further explored as verification and application cases: uniaxial-stress, uniaxial-strain, and plane-stress. Extension twinning in the magnesium alloy is prone to occur for impact loading applied perpendicular to the crystallographic c-axis, regardless of the exact stress conditions.

Energy Absorption Characteristics of PVC Coarse Aggregate Concrete under Impact Load

In this paper, polyvinyl chloride (PVC) coarse aggregate with different mixing contents is used to solve the problems of plastic pollution, low energy absorption capacity and poor damage integrity, which provides an important reference for PVC plastic concrete used in the initial support structures of highway tunnels and coal mine roadway. At the same time, the energy absorption characteristics and their relationship under different impact loads are studied, which provides an important reference for predicting the energy absorption characteristics of concrete under other PVC aggregate content or higher impact speed. This study replaced natural coarse aggregate in concrete with different contents and equal volume of well-graded flaky PVC particles obtained by crushing PVC soft board. Also, slump, compression, and splitting strength tests, a free falling low-speed impact test of steel balls and a high-speed impact compression test of split Hopkinson pressure bar (SHPB) were carried out. Results demonstrate that the static and dynamic compressive strength decreases substantially, and the elastic modulus and slump decrease slowly with the increase of the mixing amount of PVC aggregate (0–30%). However, the energy absorption rate under low-speed impact and the specific energy absorption per MPa under high-speed impact increase obviously, indicating that the energy absorption capacity is significantly enhanced. Regardless of the mixing amount of PVC aggregate, greater strain rate can significantly enhance the dynamic compressive strength and the specific energy absorption per MPa. After the uniaxial compression test or the SHPB impact test, the relative integrity of the specimen is positively correlated with the mixing amount of PVC aggregate. In addition, the specimens are seriously damaged with the increase of the impact strain rate. When the PVC aggregate content is 20%, the compressive strength and splitting strength of concrete are 33.8 MPa and 3.26 MPa, respectively, the slump is 165 mm, the energy absorption rate under low-speed impact is 89.5%, the dynamic compressive strength under 0.65 Mpa impact air pressure is 58.77 mpa, and the specific energy absorption value per MPa is 13.33, which meets the requirements of shotcrete used in tunnel, roadway support and other impact loads. There is a linear relationship between the energy absorption characteristics under low-speed impact and high-speed impact. The greater the impact pressure, the larger the slope of the fitting straight line. The slope and intercept of the fitting line also show a good linear relationship with the increase of impact pressure. The conclusions can be used to predict the energy absorption characteristics under different PVC aggregate content or higher-speed impact pressure, which can provide important reference for safer, more economical, and environmental protection engineering structure design.

A comparison study on the impact failure behavior of laminate and woven composites with consideration of strain rate effect and impact attitude

The anisotropy and heterogeneity of carbon fiber–reinforced polymer (CFRP) composites contribute to their complex failure characteristics, posing challenges for predicting their impact failure behavior. In this study, high-fidelity finite element models are established to predict the performance of laminated and woven composite panels impacted by a metallic projectile, with consideration of strain-rate effect and impact attitudes. The numerical models are experimentally validated and are applied to investigate the impact resistance of both composites under various impact conditions. The results indicate that the maximum ratio of delamination of laminated panel is about 0.4, which is greater than 0.2 of the woven panel. Woven panels can better withstand impact damage, while laminates are more prone to delamination under high-speed impacts loads.

High strain rate impact effect and failure behavior of 3D six-directional braided composites

The high strain rate impact effect and failure behavior of three-dimensional six-directional (3D6d) braided composites with different braiding angles are reported in this paper. The results show that 3D6d braided composites exhibit typical strain rate sensitivity. Under out-of-plane dynamic compression, the stress–strain curves rise linearly and present obvious softening characteristics. With the increase of strain rate, both the strength and modulus of composites increase, while the failure strain decreases. Moreover, the composites with larger braiding angle exhibit higher dynamic curves and better out-of-plane high strain rate properties. The fracture morphology shows that composites exhibit different dynamic damage and failure modes at different strain rates and braiding angles. As the strain rate increases, the composites show clearer shear fracture feature and more severe brittle failure. The damage is mainly manifested as shear of the sixth and braiding fibers, tear of the fifth fibers on the shear plane, interfacial debonding and matrix cracking. As the braiding angle increases, the damage and fracture of composites are reduced obviously and the composites become less brittle failure at high stain rates.

Evaluation of impact ductile fracture behaviour of low carbon austenitic stainless steel SUS304L using damage mechanics model

To clarify the applicability of the Gurson-Tvergaard-Needleman (GTN) model for impact ductile fracture behaviour, SHB test was reproduced by finite element analysis (FEA). The strain-rate dependence of the strength for austenitic stainless steel JIS SUS304L was obtained by tensile tests at quasi-static strain rate and impact strain rate. The CowperSymonds power law, which takes into the strain-rate dependence of strength, and the GTN model implemented in the commercial FEA code were used to simulate for impact ductile-fracture behaviour. GTN model parameters were determined by minimizing the difference between the simulated and measured stress-strain curve using response surface method. SHB test was simulated using GTN model obtained from quasi-static tensile test result. Simulation results did not occur the necking and fracture on the specimen. The fracture surfaces were observed by SEM micrograph. The appearance of ductile fracture since dimples are observed, regardless of the strain rate. It is necessary to adjust the parameter to accelerate the nucleation of the void. By identifying the GTN parameters in consideration of the strain rate dependence including impact strain rate. It would be possible to improve the simulation accuracy of impact ductile fracture behaviour.

Micro CT Image-based Simulations of Concrete under High Strain Rate Impact using a Continuum-Discrete Coupled Model

A continuum-discrete coupled modelling approach is developed in LS-DYNA to simulate complicated fragmentation phenomena of concrete under high strain-rate impact, using realistic mesoscale finite element models converted from micro X-ray computed tomography (XCT) images. In this approach, the Johnson Holmquist concrete (JHC) constitutive law based on continuum damage plasticity is used to simulate plasticity and crushing of elements, while a node-split method with contact is used to simulate discrete fracture between elements. This avoids using the popular element erosion technique, which suffers from unreal losses in mass and energy, especially under dynamic loadings with high strain rates. After the new approach is validated, extensive Monte Carlo simulations (MCS) of 93 2D FE models and one 3D model from XCT images of a 37.2mm concrete cube, consisting of aggregates, mortar, interfaces, and pores, are conducted under compressive impact with strain rates ranging from 0-1000/s and different end friction conditions. It is demonstrated that the developed modelling approach can simulate realistic failure mechanisms such as discrete fracture propagation under strain rates of 0-200/s and immediate crushing, fragmentation and prilling under higher strain rates of 500-1000/s. The predicted compressive dynamic increase factor of strength (CDIF)–strain rate curve is found well within the range of experimental data and very close to other empirical curves. Quantitative statistical calculations show that the meso-structure is the main contributor to the dynamic strength increase or reduction when the strain rate is lower than 10/s, but the inertial effect becomes dominant when the strain rate is higher than 100/s. The end friction confinement makes only 3-5% difference.

Enhancing impact resistance of hybrid ultra-high-performance fiber-reinforced concretes through strategic use of polyamide fibers

This study examines the use of polyamide fibers in improving strain rate sensitivity and impact tensile resistance of hybrid ultra-high-performance fiber-reinforced concrete (hybrid UHPFRC). Two types of long fibers, i.e., smooth steel (S) and polyamide (P), and two types of short fibers, i.e., smooth steel (s) and polyamide (p), were hybridized. The volume content of long fibers was maintained at 1%, whereas that of short fibers varied from 0 to 1%. The blending S and p fibers produced the best performance in term of the ductility and energy absorption capacity of UHPFRC whereas the blending S and s fibers did in term of the ultimate tensile strength. The optimal volume content of short fiber that exhibited improvements in strain rate sensitivity and impact resistance was found to be 0.5%. The p fibers have been successfully used to develop cost-effective hybrid UHPFRC with high impact resistance.

Effect of rubber particles and fibers on the dynamic compressive behavior of novel ultra-lightweight cement composites: Numerical simulations and metamodeling

This paper presents, first, a finite element (FE) model for a rubberized ultra-lightweight cement composite (RULCC), which uses a modified Holmquist-Johnson-Concrete (H-J-C) constitutive law that is calibrated and validated by new Split Hopkinson pressure bar (SHPB) tests on the material. The validated FE model is used then as the core of a cloud computing platform using a multi node cloud simulation framework to carry out the parametric simulations, which generate required data to develop a meta-model to predict the dynamic impact strength of the RULCC. Design of experiment (DoE) and Generic Programming techniques are the main instruments in developing meta-models with reduced size of data. Finally, a meta-model of explicit expression, which is the first of its kind and considers the effect of rubber ratio, fiber ratio and dynamic impact strain rate, is proposed to predict the dynamic impact strength of the RULCC.

Impact Behavior of Additively Manufactured Stainless Steel Auxetic Structures at Elevated and Reduced Temperatures

Metamaterials produced using additive manufacturing represent advanced structures with tunable properties and deformation characteristics. However, the manufacturing process, imperfections in geometry, properties of the base material as well as the ambient and operating conditions often result in complex multiparametric dependence of the mechanical response. As the lattice structures are metamaterials that can be tailored for energy absorption applications and impact protection, the investigation of the coupled thermomechanical response and ambient temperature‐dependent properties is particularly important. Herein, the 2D re‐entrant honeycomb auxetic lattice structures additively manufactured from powdered stainless steel are subjected to high strain rate uniaxial compression using split Hopkinson pressure bar (SHPB) at two different strain rates and three different temperatures. An in‐house developed cooling and heating stages are used to control the temperature of the specimen subjected to high strain rate impact loading. Thermal imaging and high‐speed cameras are used to inspect the specimens during the impact. It is shown that the stress–strain response as well as the crushing behavior of the investigated lattice structures are strongly dependent on both initial temperature and strain rate.

Assessment of impact mechanical behaviors of rock-like materials heated at 1,000°C

AbstractRock acts as a natural brittle material and demonstrates reactions to various dynamic disturbances and high temperature. Mechanical property fluctuations under high temperature and dynamic load for rock materials including marble, sandstone and granite are studied in an underground project of Qinling Mountain in China, exposed to 1,000°C treatments and different strain rate impact loadings. Two main research issues are explored: (1) variations of strength and failure modes of the same high-temperature rock type under different strain rate impact loadings. (2) Comparison of strength and failure modes of three high-temperature rock types under same strain rate impact loadings. Experimental results indicate that both 1,000°C temperature and the strain rate exert significant influence on dynamic compressive strength, and dynamic compressive increase factor and failure modes of three rock types. However, the influences of high temperature and strain rate on different rock types have obvious differences.

Magnetic pulse welding of copper to steel tubes–Experimental investigation and process modelling

Abstract Magnetic pulse welding allows joining of metallic tubes by a short electromagnetic pulse and the resulting high strain rate impact and plastic deformation. Since melting of materials is avoided, magnetic pulse welding is an efficient method for joining of dissimilar materials. However, the process occurs very fast with little opportunity for real-time monitoring and control, which is also difficult due to the presence of a high amplitude electromagnetic field. A novel attempt is presented here to examine the underlying phenomena for magnetic pulse welding of copper flyer tubes to steel target tubes by computational process modeling and a focused experimental investigation. A coupled electromagnetic and mechanical finite-element model was created to compute the electromagnetic field and pressure distribution, and the progressive nature of flyer tube impact velocity on the target tube. The consequent progress of the angle of impact and contact length are examined for a range of standoff distances between the flyer and target tubes, the target tube wall thickness. The computed results are validated extensively with corresponding experimentally measured results. Overall, the coupled theoretical and experimental investigation provided a useful quantitative insight of magnetic pulse welding of copper and steel tubes, which is expected to help in the advancement of practical application of the process.

A detailed investigation of the effect of hydrogen on the mechanical response and microstructure of Al 7075 alloy under medium strain rate impact loading

Effects of hydrogen and temperature on impact response and corresponding microstructure of aluminum (Al) 7075 alloy were investigated under medium strain rate impact loading. The specimens were subjected to impact energy of 12 J and 25 J, corresponding to impact velocities of 2.13 m/s and 3.08 m/s, respectively. These energy levels were decided after a couple of impact tests with different impact energy values, such as 6 J, 10 J, 12 J, 25 J. The experiments were conducted at five different temperatures. Electrochemical charging method was used for hydrogen charging. Microstructural observations of hydrogen uncharged and hydrogen charged specimens were carried out by scanning electron microscope. Hydrogen changed the crack propagation behavior of Al 7075 alloy depending on the temperature. Coexistence of several hydrogen embrittlement mechanisms, such as hydrogen enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) were observed under impact loading. The impact response of Al 7075 was significantly deteriorated by the hydrogen charging and changing temperature affected the absorbed energy of hydrogen-charged specimens. In addition, molecular dynamics simulations were conducted to uncover the atomistic origin of hydrogen embrittlement mechanisms under impact loading. In particular, hydrogen decreased the cohesive energy and enhanced the average dislocation mobility. Therefore, the experimental results presented herein constitute an efficient guideline for the usage of Al alloys that are subject to impact loading in service in a wide range of temperatures.

Dynamic mechanical properties and failure characteristics of electron beam melted Ti-6Al-4V under high strain rate impact loadings

This study presents an investigation on the effects of building direction on microstructure, dynamic mechanical properties, and deformation mechanisms of electron beam melted Ti-6Al-4V (EBM-Ti64) cylindrical rods. Microstructural features were characterized using optical microscopy (OM) and scanning electron microscopy (SEM). The initial microstructure in both directions consists of transformed α+β phases and grain boundary-α (αGB)along prior β-grain boundaries. The vertically built cylindrical specimens have a finer grain structure, including lower interlamellar spacing and finer α-laths compared to the horizontally built ones. Dynamic impact tests using Split-Hopkinson Pressure Bar (SHPB) were conducted on both horizontally and vertically built samples at strain rates ranging between 1150 and 2700 s−1. Dynamic mechanical properties are strain-rate sensitive; the maximum flow stress of 1960 MPa (at 2100 s−1) and 2160 MPa (1650 s−1) were obtained for horizontal and vertical specimens, respectively. Horizontal specimens fragmented when deformed at 2700 s−1, whereas the vertical specimens failed at a much lower strain rate (1900 s−1). At a given strain rate, vertical specimens exhibited better dynamic strength and lower strain (total strain) due to their finer microstructure. The temperature rise during deformation primarily governs flow softening at all conditions, which led to the formation of adiabatic shear bands (ASBs). Microstructures of deformed specimens revealed thermal softening features such as voids formation. These voids coalesced and grew, leading to crack initiation and propagation along ASBs. Fractographic examination of the fragmented specimens under impact loading revealed ductile dimples and smoother surfaces, which indicate a combination of both ductile and brittle fracture. The contribution of twinning and pyramidal <c+a> slip systems is the primary deformation mechanism during high strain rate impact loading of EBM-Ti64. The experimentally obtained flow curves are in good agreement with the Chang-Asaro equation-based constitutive modeling results.