High Strain Rate Mechanical Behavior Research Articles

High Strain Rate Mechanical Behavior of Polyamide 66 and Polyamide 66-Glass Fiber Reinforced

High Strain Rate Mechanical Behavior of Polyamide 66 and Polyamide 66-Glass Fiber Reinforced

High strain rate mechanical behavior of Ti-6Al-4V with micro–macro correlation under compressive loading

The application of Ti6Al4V in lightweight structures subjected to impact loads necessitates a comprehensive understanding of its dynamic response under high strain rates to ensure the safety and reliability of these components. In this paper, the macroscopic compressive mechanical behavior of Ti6Al4V under high strain rates was investigated through the split-Hopkinson pressure bar (SHPB) technique at both room temperature and 200 °C, followed by microstructure analysis of the compressed specimens, carried out employing a combination of optical microscopy (OM) and electron backscatter diffraction (EBSD), and elastic modulus and hardness measurements, performed by a nanoindentation instrument. At room temperature, both the modulus and strain hardening exponent of Ti6Al4V exhibited an initial increase before the strain rate reached 1000s-1, after which they decreased at 3400s-1. Meanwhile, the flow stress continued to rise with increasing strain rate. Microscopic observations revealed that twinning played a substantial role in the strain hardening effects. Notably, the basal slip system of the hcp (hexagonal close packed) α phase did not display a more favorable trend, while the c+a type slip systems became more active. Under a strain rate of 5000s-1, discontinuous dynamic recrystallization emerged as the dominant mechanism, resulting in an increased frequency of low-angle grain boundaries, as well as a reduction in grain size and dislocation density. These results provide a basis for explaining the mechanical behavior of Ti6Al4V under high strain rates and offer insights for the establishment of material models that take into account microstructure deformation mechanisms.

High strain rate mechanical behaviour of uniform and hybrid metallic TPMS cellular structures

Four different geometries of uniform triply periodic minimal surface (TPMS) cellular structures (Diamond, Gyroid, IWP, and Primitive) and a hybrid Gyroid–Diamond TPMS structure were fabricated and experimentally tested under different compressive strain rates ranging from quasi-static (at 0.005 s−1) to very high strain rates (at 11,000 s−1). Three different testing apparatuses were used: a universal servo-hydraulic testing machine, a direct impact Hopkinson bar (DIHB), and a powder gun. Limited strain rate hardening was observed at quasi-static, low- and intermediate-dynamic deformation, while the shock deformation mode showed significant strain rate hardening of all tested structures. The computational models were developed and validated with the experimental results and then used to predict the TPMS structure’s behaviour even at higher strain rates (up to 35,000 s−1), not attainable by used experimental devices. The Diamond and IWP structures exhibited up to five times higher (an increase from 6.9 to 40.8 J/g) specific energy absorption (SEA) during very high strain rate loading than during the quasi-static and lower dynamic testing. The tests of hybrid samples showed that the sample orientation significantly affects the mechanical high strain rate response, which can be either progressive (less stiff structure at sample impact end) or regressive (stiffer structure at sample impact end).

Effect of Strain Rate on Single Tau, Dimerized Tau and Tau-Microtubule Interface: A Molecular Dynamics Simulation Study.

Microtubule-associated protein (MAP) tau is a cross-linking molecule that provides structural stability to axonal microtubules (MT). It is considered a potential biomarker for Alzheimer’s disease (AD), dementia, and other neurological disorders. It is also a signature protein for Traumatic Brain Injury (TBI) assessment. In the case of TBI, extreme dynamic mechanical energies can be felt by the axonal cytoskeletal members. As such, fundamental understandings of the responses of single tau protein, polymerized tau protein, and tau-microtubule interfaces under high-rate mechanical forces are important. This study attempts to determine the high-strain rate mechanical behavior of single tau, dimerized tau, and tau-MT interface using molecular dynamics (MD) simulation. The results show that a single tau protein is a highly stretchable soft polymer. During deformation, first, it significantly unfolds against van der Waals and electrostatic bonds. Then it stretches against strong covalent bonds. We found that tau acts as a viscoelastic material, and its stiffness increases with the strain rate. The unfolding stiffness can be ~50–500 MPa, while pure stretching stiffness can be >2 GPa. The dimerized tau model exhibits similar behavior under similar strain rates, and tau sliding from another tau is not observed until it is stretched to >7 times of original length, depending on the strain rate. The tau-MT interface simulations show that very high strain and strain rates are required to separate tau from MT suggesting Tau-MT bonding is stronger than MT subunit bonding between themselves. The dimerized tau-MT interface simulations suggest that tau-tau bonding is stronger than tau-MT bonding. In summary, this study focuses on the structural response of individual cytoskeletal components, namely microtubule (MT) and tau protein. Furthermore, we consider not only the individual response of a component, but also their interaction with each other (such as tau with tau or tau with MT). This study will eventually pave the way to build a bottom-up multiscale brain model and analyze TBI more comprehensively.

High strain rate compression of epoxy micropillars

The study of the high strain rate mechanical behaviour of materials using micropillar compression tests has been hindered so far due to the lack of suitable instrumentation. In the present study, a novel high strain rate micropillar compression set-up is introduced. The high speed tests are carried out with the impact configuration of a pendulum-based nanoindentation device that was instrumented with force-sensing capability by means of a piezoelectric load cell. This is necessary due to the considerable inertia effects that make the actual applied force on the sample much higher than the actuated one. The proposed novel technique was successfully applied to study the mechanical behaviour of an epoxy resin over a wide range of strain rates. The results reveal a significant strain rate sensitivity that correlates well with that obtained for the same material at the macroscale, validating the novel set-up.

High strain rate mechanical behavior of Ti-6Al-4V octet lattice structures additively manufactured by selective laser melting (SLM)

Dynamic mechanical response of additively manufactured 12 mm × 8 mm size octet lattice structures of Ti-6Al-4V was investigated. Cylindrical specimens composed of a single unit cell of 36 trusses and 13 nodes sandwiched between two support plates were built using selective laser melting. The radius of the trusses was incrementally varied from 0.1 mm to 0.5 mm. The interior of the trusses and nodes examined using electron microscope was found to be fully densified with no porosity of other defects and demonstrated a uniform microstructure throughout the lattice composed of acicular martensitic needles. High strain rate compression of these lattices was conducted using split-Hopkinson pressure bar at a displacement rate of 7500 mm/s corresponding an effective strain rate of ~ 1000 s−1. The peak compressive strength and energy absorption capacity of the lattices were found to vary almost linearly with relative density of the cell, which increased proportional to the square of the truss radius. Specific strength and absorption however were found to be significantly higher in the lattice with smallest truss radius. The dynamic compression was recorded using a high speed camera at 100,000 fps capture rate and provided insight into the deformation processes: the initial elastic wave loading, the inertia driven continued loading followed by relaxation for elastic strain recovery. These mechanisms which significantly diversified with the truss radius are presented.

High strain rate mechanical behavior of seashell-mimetic composites: Analytical model formulation and validation

Seashells (esp. Nacre, or mother-of-pearl) have attracted a substantial attention of the scientific community over the last decade. What makes it as a role model for bio-inspiration is its strength and specifically, superior toughness owing to the synergistic effect of its inorganic and organic components and its unique architecture at its most elementary level, i.e., brick and mortar architecture. Note that preponderance of the studies in literature is limited to quasi-static loading scenario. This is intriguing given the fact that both these materials are fundamentally used as a basic structural material for synthesizing and in orchestrating complex load bearing/shielding structures against predatory impact/crush attack. In our current investigation, an attempt has been made to predict the strength of these types of composites under dynamic regime by development of an analytical model, followed by experimental verification under high-strain rates using Split Hopkinson Pressure Bar (SHPB). Two types of ceramic–polymer composites were tested which were prepared via freeze casting technique. A reasonable agreement has been observed between the analytically predicted and experimentally observed values. Manifestation of dynamic self-stiffening behavior by these types of composites will find particular relevance in application of these types of composites as protective material when subjected to ballistic impact. Understanding the mechanical behavior of these types of composites under dynamic rates of loading will provide insights on the applicability of hybrid bio-inspired composites as protective materials.

Characterization and numerical modeling of high strain rate mechanical behavior of Ti-15-3 alloy for machining simulations

Abstract Metastable beta titanium alloys combine light weight, high strength, and excellent corrosion and fatigue resistance, and can therefore be very useful in many demanding applications. However, they are also difficult to machine and the machining costs of titanium components can be significant compared to the overall costs of the component. Finite element simulations can be used to optimize the cutting conditions and reduce the machining costs. However, any attempt to simulate the rather complex machining processes needs reliable material models that can only be generated when the mechanical behavior of the material is understood well. In this work, the mechanical properties and behavior of titanium 15-3 alloy was studied in a wide range of strain rates and temperatures, and a constitutive model was generated for simulating orthogonal cutting of the alloy. The strain-hardening rate of Ti-15-3 is a strong function of strain rate, and it decreases rapidly as the strain rate is increased. Also, the strain rate sensitivity of the material was found to depend strongly on temperature. Johnson–Cook plasticity model, based on isothermal stress–strain curves, was used to model the behavior of the material. The isothermal stress–strain response was calculated from the experimental data, and the model used in the simulations was modified to account also for the adiabatic heating and consequent thermal softening of the material. The current model is able to simulate the serrated chip formation frequently observed for titanium alloys at high cutting speeds.

Determining the stress–strain behaviour at large strains from high strain rate tensile and shear experiments

To characterise the high strain rate mechanical behaviour of metals, split Hopkinson bar experiments are frequently used. These experiments basically yield the force and elongation history of the specimen, reflecting not only the specimen material behaviour but also the specimen structural behaviour. Calculation of the real material behaviour from this global response is not straightforward, certainly for materials such as Ti6Al4V where due to low strain hardening, the specimen deformation is very inhomogeneous. However, for fundamental material research and constitutive material modelling, knowledge of the true effective stress versus plastic strain, strain rate and temperature is essential. In this contribution, a combined experimental-numerical approach for extraction of the strain rate and temperature dependent mechanical behaviour from high strain rate experiments is presented. The method involves the identification of the material model parameters used for the finite element simulations. The technique is applied to determine the stress–strain behaviour of Ti6Al4V using both high strain rate in-plane shear and tensile test results. For the tensile tests, even stress–strain data beyond diffuse necking are retrieved. A comparison is made between the material behaviour extracted from the tensile and the shear experiments. The material behaviour is modelled with the Johnson–Cook constitutive relation. It is found that the simultaneous use of tensile and shear tests to identify the model parameters gives a more generally applicable model. Validation of the material model and the finite element simulations is done by local strain measurements in the shear and tensile test by means of digital image correlation.

Experimental and Numerical Investigation of High Strain Rate Mechanical Behavior of a [0/45/90/ - 45] Quadriaxial E-Glass/Polyester Composite

Quasi-static (10−3–10−1s−1) and high strain rate (∼900s−1) compression behavior of an E-Glass fiber woven fabric reinforced Polyester matrix composites was investigated by using a Shimadzu AG-I testing machine and a Split Hopkinson Pressure Bar apparatus in the Dynamic Testing and Modeling Laboratory of Izmir Institute of Technology. During the experiments, a high speed camera was used to determine deformation behavior. In both directions, modulus and failure strength increased with increasing strain rate. Higher strain rate sensitivity for both elastic modulus and failure strength was observed in the in-plane direction. Based upon these experimental data, a numerical model was developed using the commercial explicit finite element code LS-DYNA to investigate compressive deformation and damage behavior of composites. Excellent agreement was demonstrated for the case of high strain rate loading. Also, the fracture geometries were successfully predicted with the numerical model.

High strain rate mechanical behavior of epoxy under compressive loading: Experimental and modeling studies

Investigations on high strain rate behavior of epoxy LY 556 under compressive loading are presented. Compressive Split Hopkinson Pressure Bar (SHPB) apparatus was used for the experimental investigations. The studies are presented in the strain rate range of 683–1890 per second. It was generally observed that the compressive strength is enhanced at high strain rate loading compared with that at quasi-static loading. During SHPB testing of the specimens, it was observed that the peak force obtained from the strain gauge mounted on the transmitter bar is lower than the peak force obtained from the strain gauge mounted on the incident bar. Further, an analytical method is presented based on variable rate power law for the prediction of compressive strength at high strain rate loading for epoxy LY 556. Using the analytical method, high strain rate compressive stress–strain behavior is presented up to strain rate of 10,000 per second.

High strain rate mechanical behavior of polyurea

Stress–strain measurements are reported for an elastomeric polyurea in uniaxial tension over a range of strain rates from 0.06 to 573 s −1. The experiments were carried out on a new drop weight test instrument, which provides mechanical data at strain rates up to 1000 s −1, filling the gap between conventional low speed instruments and split Hopkinson bar tests. The tensile data obtained herein are compared with recent high strain rate compression data on the same material [Yi et al. Polymer 2006;47:319–29]. Advantages of the present measurements include a more uniform strain rate and the ability to ensure homogeneous strain.

Experimental investigation of the spatial variation in viscoplastic response of free-machining steels in hot rolling conditions

In this paper the high temperature, high strain rate mechanical behaviour of two free-machining steels is reported. The effect of bloom position on material ductility is investigated, and a physically-based parameter is created to link microstructure characteristics at various bloom locations with observed material ductility.

Consolidation and high strain rate mechanical behavior of nanocrystalline tantalum powder

High ductility and strength exhibited in nanograined materials can potentially be exploited in explosively formed penetrator liner applications. Both coarse and nanocrystalline tantalum powder were consolidated by Plasma Pressure Compaction (P 2C) to study the effect of grain size on dynamic compression properties. The powders were consolidated rapidly with 1 minute of isothermal holding time to retain initial microstructure. The P 2C consolidated specimens were cut by electric discharge machining (EDM), polished for SEM characterization, and tested in dynamic compression using a Kolsky apparatus. The effect of grain size on yield stress and strain was investigated at various strain rates for a coarse grained and a nanograined specimen. Especially, the high strain rate response of nanocrystalline tantalum is discussed in this paper.