Strain Rate Impact Research Articles

IMPACT TESTING OF ORDNANCE GELATINE UNDER MODERATE STRAIN RATE CONDITIONS

An experimental study on energy absorption capabilities and strain rate sensitivity of ordnance gelatine was performed. Strain energy density under quasi static compression and moderate strain rate impact tests was compared. In the study two types of material were tested, bulk ordnance gelatine and polymeric open-cell meshwork filled with ordnance gelatine. From the results a significant strain-rate effect was observed in terms of ultimate compressive strength and strain energy density. In comparison of the deformation behaviour under quasi static conditions and drop weight test the difference was very significant, however slight increase in both strength and strain energy density was observed even between different impact energies and velocities during the impact testing. The peak acceleration was significantly reduced in polymer meshwork filled by gelatine in comparison to the bulk gelatine.

Crushing Behavior of Graded Auxetic Structures Built from Inverted Tetrapods under Impact

In this study, the crushing behavior of auxetic structures built from inverted tetrapods with linearly graded porosity under impact is analyzed both experimentally and computationally. The auxetic samples with linearly graded porosity are fabricated from Ti‐6Al‐4V alloy powder by the selective electron‐beam melting method. A detailed geometrical analysis of the fabricated samples based on micro computed tomography (microCT) scans is performed to determine the variation of strut thickness and other fabrication imperfections. The samples are subjected to high strain rate compression loading in two specimen orientations (specimens with low or high porosity facing the impacting plate) using the powder gun. Appropriate computational models are later developed and validated to provide a more detailed insight into the crushing mechanism of auxetic structures during impact. The experimental and computational results show dependency of the plateau stress inclination (progressive/regressive) on the gradation of the auxetic cellular structures (specimens with low or high porosity in front). These initial results prove that it is possible to tailor the mechanical behavior of a porous structure with introduction of graded porosity, especially in the case of high strain rate impact, where the main deformation occurs at the impacting front.

Ballistic Limit of Laminated Panels with Different Joining Materials Subjected to Steel-Hardened Core Projectile

This paper presents the computational-based ballistic limit of laminated metal panels comprised of high-strength steel and aluminium alloy Al7075-T6 plates to necessitate a weight reduction of 25% in the existing armour steel plate using three different joining materials. Numerical models of the triple-layered panels were developed using the commercial Explicit Finite Element code and were impacted by a 7.62-mm armour-piercing projectile at velocities ranging from 400 m/s to 1000 m/s. The ballistic performance of each configuration plate in terms of the ballistic limit velocity, depth of penetration and end of penetration, was quantified and considered. It was found that the panels with joining materials exhibited a better ballistic limit on an average of 1.5% than that of the panel without a joining material. The penetration depth of the panel joined by polyurethane possessed the lowest depth of 22 mm with a higher contact duration compared to the panel without a joining material. This happened because the polyurethane adhesive was better able to absorb energy at a high strain rate impact than the other joining materials. Thus, based on the investigation that was carried out, polyurethane seems to be the most interesting option for joining different metals of Ar500 and Al7075-T6 as a laminated panel for armoured vehicle applications.

Effect of strain rate, off-axis loading and high-velocity impact damage on the compressive strength of C/SiC composite

The compressive strength of C/SiC composite at different strain rates, off-axis orientations and after high-velocity impact was studied. The compressive strength was found to be 137 ± 23, 130 ± 46 and 162 ± 33 MPa at a strain rate of 3.3 × 10−5, 3.3 × 10−3, 3.3 × 10−3 s−1, respectively. On the other hand, the compressive strength was found to be 130 ± 46, 99 ± 23 and 87 ± 9 MPa for 0°/90°, 30°/60° and 45°/45° fibre orientations to loading direction, respectively. After high-velocity impact, the residual compressive strength of C/SiC composite was found to be 58 ± 26, 44 ± 18 and 36 ± 3.5 MPa after impact with 100, 150 and 190 m/s, respectively. The formation of kink bands in fibre bundles was found to be dominant micro-mechanism for compressive failure of C/SiC composite for 0°/90° orientation. On the other hand, delamination and the fibre bundles rotation were found to be the dominant mechanism for off-axis failure of composite.

Identification of the elastic constant values for numerical simulation of high velocity impact on dyneema® woven fabrics using orthogonal experiments

Dyneema® fibres and fabrics are widely used for ballistic protection due to its lightweight and super mechanical properties against high strain rate impact, and finite element (FE) simulation and analysis are used to study the response to the impact in parallel to the experimental-based research methods. However, elastic constants of the yarn except the Young’s modulus were difficult to obtain and were basically assigned based on assumptions and approximations in the FE modelling, which caused some inaccuracies. This paper reports a study on the influence of each elastic constant of Dyneema® yarn model in modelling a single layer Dyneema® woven fabric against ballistic impact using the orthogonal experiment method. Orthogonal table L25 (56) was employed to analyse six factors (i.e. E11, E33, ν, G13, G23, and their interactions) with each having five levels. The ballistic modelling results were validated against the experimental results, viz. energy absorption, failure time of the first yarn broken and number of failed yarns. According to the orthogonal analysis, G13 was shown as the most significant in influencing the simulated results, with a confidence level of more than 95%, and ν was the least significant. Through the orthogonal study, the combination of levels of the elastic constants that led to a significant agreement between the FE and practical results was identified.

ANALISA EKSPERIMENTAL MODULUS ELASTISITAS BAHAN KOMPOSIT GLASS FIBER REINFORCED PLASTIC (GFRP) BERDASARKAN VARIASI DIAMETER SERAT AKIBAT BEBAN IMPAK LAJU REGANGAN TINGGI

<span class="longtext"><em><span>GFRP composite is one of the alternative materials that the most common as a replacement for metal. This material has a light weight, easy to shape, good strength, and the production cost is relatively cheaper. This study aims to investigate the effects of fiber diameter variation of mechanical behavior, particularly on the load of high strain rate impact. The application of impulse momentum theory in the elastic region is the basic science for this case. The AGC apparatus is equipment that used for this test because it could generate the high strain wave rate in the single direction loading. Specimen is placed in the position of tight end against the end of the input bar. The results showed that the increase in fiber diameter variations affect the value of E that is produced. Thus, it is become a recommendation for the use of this material in certain circumstances.</span></em></span>

Constitutive models for the dynamic behaviour of direct metal laser sintered AlSi10Mg_200C under high strain rate shock loading

Metal additive manufacturing has been a revolutionary step in designing new complex shapes with a faster and cleaner building capacity in comparison to subtractive manufacturing processes. The ability to print lightweight alloys such as aluminum has made metal 3D printing attractive to different industries from aerospace to energy sector and bioengineering. Of particular interest in this study, are the mechanical properties of AlSi10Mg_200C alloys under high strain rate regimes. In this work, samples in horizontal and vertical printing directions were additively manufactured using an EOS M290 machine through the Direct Metal Laser Sintering (DMLS) technique. High strain rate impact tests were then carried out using a split-Hopkinson pressure bar to investigate the mechanical properties and dynamic behaviour of DMLS-AlSi10Mg_200C in both build directions. Furthermore, a constitutive model was developed for both printing directions to encompass the results, which used varying strain rates to better understand the high strain rate behaviour of these alloys. Finally, a parametric study followed to demonstrate the effect of each parameter for both models.

Experimental study on dynamic compressive properties of fiber-reinforced reactive powder concrete at high strain rates

Dynamic compression tests were performed on reactive powder concrete (RPC) using split Hopkinson pressure bar (SHPB) apparatus. A total of 102 RPC samples including plain RPC (PRPC), steel fiber-reinforced RPC (SFRPC) with 2% and 5% volumetric fractions of steel fiber, and hybrid fiber reinforced RPC (SPFRPC) with 0.2% polypropylene (PP) fiber and 2% steel fiber were characterized at high strain rate of 72–317 s–1. The characteristics of wave transmission, dynamic compressive strength, compressive deformation, energy absorption ability, and dynamic increase factor (DIF) of RPC under high strain rate impact were systematically investigated. Results revealed that the dynamic compressive strength of PRPC was more sensitive than that of SFRPC. With the addition of 0.2% PP fiber, the dynamic compressive strength of SFRPC exhibited a slight decrease. The critical strain increased with increasing strain rate. In all cases of RPC, the critical damage variable decreased with increasing fiber content subjected to same strain rate impact. The energy absorption of RPC increased with increasing content of steel fiber, which was found to be higher than normal strength concrete and high strength concrete. It was observed that application of CEB formula to calculate DIF of RPC was not safe. Thus, an empirical formula for DIF of RPC was proposed based on test data of this study and existing literature results.

Polypropylene nano-composites at high strain rate impacts: Characterization, failure modes, and modeling

As a major challenge, development of light-weight fibre reinforced polymer (FRP) composite body armour, characterization of candidate matrix polymers at high strain rate impact is the focus in this research. Polypropylene (PP) and the nano-composites with 1-5% by weight of NC (nanoclay) platelets are the candidates considered. In the characterization phase, high strain rate impact and quasi-static loading tests were performed to figure out the limiting (failure) responses. Comparison between the material systems is, subsequently, made to nominate one matrix configuration. Enhancements of mechanical properties with increase in weight percentage of the nanoparticles are observed at both quasi-static and dynamic loadings. Observations of dispersed imposed failure modes, development of novel model for failure modulus and evaluation of peak strength values are also attempted. Keywords : Nano-composite, Impact behaviour, Analytical modelling, Polypropylene, Failure mode.

Impact Fatigue Life Prediction for Notched Specimen of Steel AerMet100 Subjected to High Strain Rate Loading

In this study, the ultra-low cycle fatigue (ULCF) behavior of a high-strength-ductile steel AerMet100 exposed to repeated high strain rate impact loadings is investigated. Three types of coupon-level experiments were performed, in which a three-point bending (TPB) specimen with a through-thickness notch was employed for multi-impact test. A special ratchet effect associated with complex stress state, dynamic impact loading and other physical mechanisms was observed through measured principal strain variations with a specific decay rate at the notch root surface. An improved ULCF predictive model based on the continuum damage mechanics was developed to quantify the relationship between the fatigue damage and the input of localized impact energy to the notch root. The model expressed in terms of damage growth rate introduces a new exponential term for better predictive accuracy and reduced number of nonlinear dynamic response analysis. As a computational efficient tool, the proposed model can predict impact fatigue life in acceptable timeframe for multiple critical locations in a complex engineering component.

Dynamic behaviour of Al-Mg aluminum alloy at a wide range of strain rates

The effect of strain rate on mechanical properties of Al-2.3wt.%Mg alloy (AA5021) and commercial pure aluminum (purity 99.7wt.%: A1070) was investigated at room temperature. The tensile tests were conducted at strain rates from 1.0×10−4 to 1.0×103 s−1. The universal testing machine was used for strain rate 1.0×10-4 to 1.0×10−1 s−1. For the strain rate 1.0×100 s-1, the servohydraulic testing machine, which was developed by our laboratory, was used. The impact strain rate 1.0×103 s−1 was obtained using the split Hopkinson pressure bar method. The pure aluminum showed positive strain rate dependence of material strength at the investigated strain rates. In contrast, the Al-2.3wt.%Mg alloy showed the negative strain rate dependence at strain rates from 1.0×10−4 to 1.0×100 s−1. However, Al-2.3wt.%Mg alloy showed the positive strain rate dependence at strain rates from 1.0×100 to 1.0×103 s−1. It was surmised that the effect of dislocation locking by the solute Mg atoms became negligible at strain rate of approximately 1.0×100 s−1. It was confirmed that material properties for the Al-Mg alloy at the strain rate of 1.0×100 s−1 were important, since the strain rate dependence changed negative to positive around this strain rate.

Development of a constitutive model for DPX2 explosive

There is a significant challenge in simulating the behaviour of PBXs under high strain rate impact loading. A Porter-Gould physically based constitutive model has been developed for the DPX2 explosive. A series of quasi-static compression and tensile tests over a range of temperatures were performed together with DMA tests to calibrate the model. In particular tests were performed for different L/D ratios to understand the complex localisation and damage behaviour of the material. High rate tests on the compression Split Hopkinson Pressure Bar (SHPB) for a range of temperatures were then used for validation of the model under idealised stress states. Some model development is still required, particularly at lower temperatures near the glass transition temperature. In addition a series of classical Taylor Tests were used to validate the model under impact loading conditions at room temperature. The DYNA3D simulations gave very good results compared to the experiments for these impact conditions.

Hierarchical structure and compressive deformation mechanisms of bighorn sheep (Ovis canadensis) horn.

Bighorn sheep (Ovis canadensis) horns show remarkable impact resistance and energy absorption when undergoing high speed impact during the intraspecific fights. The present work illustrates the hierarchical structure of bighorn sheep horn at different length scales and investigates the energy dissipation mechanisms under different strain rates, loading orientations and hydration states. These results demonstrate how horn dissipates large amounts of energy, thus provide a new path to fabricate energy absorbent and crashworthiness engineering materials.

An experimental study of NiTi alloy under shear loading over a large range of strain rates

This paper describes a complete experimental study on a NiTi SMA alloy at the ambient temperature (Pseudoelastic behavior) under a double in-plane shear loading over a large range of strain rates from 10−4 to 103s-1. Under the quasi-static loading (10−4–10−2s-1), the optical full-field measurement revealed to be necessary because of the very small total displacement of 0.3mm. The intermediate loading rate (10−1–101s-1) is realized with a modified MTS machine able to load at the speed of 300mm/s. A high-speed camera is needed to follow such test. Finally, the Split Hopkinson bars are used to perform tests at impact loading rates (102–103s-1).The tests are continually performed over 7 decades of the strain rate. Not only the nominal stress-strain curves, but also the detailed observation of transformation band evolution are measured. The main findings are as follows: (i) A regular stress increase with the strain rate is observed over 7 decades of the strain rates; (ii) the strain field is not homogeneous under in-plane shear conditions, even at the very low strain rate of 10−4s-1. One band at 10° from the shear direction is visible under the lower strain rate. (iii) At the impact strain rate of 102s-1, two separated bands are observed.

High strain rate dynamic compressive behaviour of Al6061-T6 alloys

This paper investigated the compression behaviour of Al 6061-T6 alloys at various high strain rates through split Hopkinson pressure bar (SHPB) experiments. The compressive strength (σc) of Al 6061-T6 alloy was measured in the present work as a function of high strain rate ε̇. The measurements were done in the strain rate regime of 3×103 to 6.5×103s−1. The results showed that the corresponding (σc) increased exponentially with increase in (ε̇). But, the increase in strain with increasing ε̇ was primarily linear. The work hardening coefficient decreased with the increase of strain. However, the decrease of work hardening coefficient with increasing strain rate did not follow a similar trend. With the increase of ε̇ the strain rate sensitivity increased. As expected, the thermal activation volume decreased with increase in ε̇. Using the Zerilli-Armstrong equation attempt was made to evaluate the related material constants to predict the stress-strain curves. The predicted stress-strain behaviours matched reasonably well with those measured experimentally in the current SHPB experiments. The implication of these results in engineering design and simulation in the context of resisting high strain rate impact was also discussed.

High strain rate behavior of STF-treated UHMWPE composites

This study systematically investigates the effect of Shear Thickening Fluid (STF) treatment on the high strain rate properties of UHMWPE (Ultra High Molecular Weight Poly Ethylene) composites. Spherical nanosilica particles of size 100nm were used for the synthesis of STF. The high strain rate impact studies were accomplished on in-house designed and fabricated Split Hopkinson Pressure Bar (SHPB) experimental set-up. Compression moulded UHMWPE variant Gold Shield® was used as the ballistic composite. Both STF-treated as well as neat Gold Shield® specimens were subjected to high strain rate impact testing. From the experimental results and Fractography studies it was revealed that STF treatment enhanced the ballistic resistance of Gold Shield® composite material. In the SHPB experiments, the improved ballistic performance of STF-treated Gold Shield® specimens was manifested in terms of higher peak stress, specimen strain rates and impact toughness.

An overlapping Domain Decomposition preconditioning method for monolithic solution of shear bands

Metals subjected to high strain rate impact often exhibit a sudden and profound drop in the material’s load bearing capability, a ductile failure phenomenon known as shear banding. Shear bands, characterized as material instabilities, are driven by shear heating and described as narrow regions that have sustained intense plastic deformation and high temperature rise. This coupled thermo-mechanical localization problem can be formulated as a nonlinear system with two balance equations, momentum and energy, and two constitutive laws for elasticity and plasticity.In our formulation, mixed finite elements are used to discretize the equations in space and an implicit finite difference scheme is used to advance the system in time, where at every time step, a Newton type method is used to solve the system monolithically. To that end, a block Jacobian matrix is formed analytically using Gâteaux derivatives. The resulting Jacobian is sparse, nonsymmetric and its sparsity pattern and eigenvalue content vary with the different stages of shear bands formation, consequently posing a significant challenge to iterative solvers.To address this issue, an overlapping Domain Decomposition preconditioner that takes into account the physics of shear bands and the domain in which it forms, is proposed. The key idea is to resolve the shear band domain accurately while only solving the remaining domain approximately, with selective updates when necessary. The preconditioner is formed on the basis of an additive Schwartz method that is applied to a Schur complement partition of the system.The proposed preconditioner is implemented in parallel and tested on three different benchmark problems as compared against off-the-shelf solvers. We study the effect of h- and k-refinement that are obtained from Isogeometric discretizations of the system, the overlapping strategy and its parallel performance. Excellent performance is demonstrated in serial and parallel on all benchmark examples.

Appraisal of Strain Rate Sensitivity of Polypropylene Nanocomposites

Neat polypropylene (PP) and PP reinforced with nano clay platelets were tested at a fixed quasi-static strain rate on UTM and at corresponding limiting strain rates on SHPB (Split Hopkinson Pressure Bar). Enactment with minimum possible thickness of specimens cum maximum impact pressure on SHPB experiments is found important to set limiting strain rate. Combination of stress-strain responses of the material systems from both high strain rate impact and quasi-static loading is used to estimate the representative strain rate sensitivity index ‘m’ values: A new approach more justified than the trend of using strain rates limited the quasi-static range. Particular constant strain value of 0.0125 m/m and room temperature of 21±3 oC at which ‘m’ values are calculated were considered. The increase in weight fraction of nano-reinforcement was found to increase strain rate sensitivity index of PP to the level comparable with the lower bound of the range for superplastic alloys.

Strain and rate-dependent neuronal injury in a 3D in vitro compression model of traumatic brain injury.

In the United States over 1.7 million cases of traumatic brain injury are reported yearly, but predictive correlation of cellular injury to impact tissue strain is still lacking, particularly for neuronal injury resulting from compression. Given the prevalence of compressive deformations in most blunt head trauma, this information is critically important for the development of future mitigation and diagnosis strategies. Using a 3D in vitro neuronal compression model, we investigated the role of impact strain and strain rate on neuronal lifetime, viability, and pathomorphology. We find that strain magnitude and rate have profound, yet distinctively different effects on the injury pathology. While strain magnitude affects the time of neuronal death, strain rate influences the pathomorphology and extent of population injury. Cellular injury is not initiated through localized deformation of the cytoskeleton but rather driven by excess strain on the entire cell. Furthermore we find that, mechanoporation, one of the key pathological trigger mechanisms in stretch and shear neuronal injuries, was not observed under compression.

Experimental Study of Failure Mode Transition in Semi-Crystalline Polymers Polypropylene under High Strain Rate

The brittle to ductile failure mode transition and the formation of adiabatic shear bands (ASB) of polypropylene were observed at high strain rate impact loading. The dynamic experiments were conducted using a split Hopkinson pressure bar (SHPB) set-up with hat-shaped specimens. The post-test observations of the recovered specimens were performed by polarized light microscopy. The mechanical behaviors of specimens are strongly influenced by temperature, the strength of specimen decreases with increasing temperature. Furthermore, the specimen fractures as a brittle solid at room temperature (20°C), while at a high temperature (100°C), the specimen fractures on a ductile model. At high temperature impact tests, shear bands are observed in the shear zone of the hat-shaped specimen, and the cracks formed at the corners propagate along the shear bands.