Dynamic Behavior Of Materials Research Articles

Dynamic energy absorption behavior of lattice material filled with shear thickening fluid

In the present research the dynamic energy absorption behavior of lattice material filled with shear thickening fluid (STF) is studied by the modified split Hopkinson pressure bar (SHPB) apparatus. The nominal engineering stresses versus strain curves are measured to analyze the dynamic energy absorption behavior of the combined material. The dynamic behavior of lattice material is also measured as a comparison. The results show the dynamic strength of the lattice material is about 3 MPa, which is a little higher than the static strength of about 2.3 MPa because of the inertial effect during impact. However, when filled with STF, the combined material shows extraordinary energy absorption behavior when compared to the empty lattice material. The stress of the lattice material filled with STF increase almost linearly to about 9 MPa and then increases slowly with the increase of strain. The excellent energy absorption behavior of the lattice material filled with STF could be explained by the interaction between the lattice cores and the STF during compression. For the lattice material, while some of cores buckling, it will lose the load capacity. While filled with STF, if the initial buckling of some of the lattice cores happens during impact, the strong lateral drag force is generated by the ambient STF due to the fast lateral velocities of the cores, leading to the great increase of its dynamic energy absorption behavior.

Dynamic Mechanical Behavior of Rock Materials

Dynamic rock mechanics investigates the mechanical behavior of rock under dynamic loading conditions and change in mechanical properties of the rock. Loading techniques were almost used for both intermediate and high strain rate tests. In this work, dynamic tests and dynamic mechanical behavior of rock materials were studied. Dynamic tests were discussed to predict the stress-strain behavior. Different dynamic mechanical properties of rock materials including uniaxial and triaxial compressive strength, tensile strength, shear strength and fracture toughness were summarized. The effect of pressure, temperature and water saturation as well as microstructure, size and shape of rock on the mechanical properties of rock materials was considered.

Design of an Experimental Program to Assess the Dynamic Fracture Properties of a Dual Phase Automotive Steel

The dynamic behavior of materials must be considered when determining the crashworthiness of a vehicle and the safe design of the vehicle components. Series of mechanical tests at wide ranges of stress and strain rates are essential to identify the material's damage and fracture behavior identical to realistic conditions. In the present contribution, an extensive experimental program has been developed to assess the dynamic fracture properties of dual phase steel (DP-K 1000). Various tensile specimen geometries covering wide range of stress states are employed for testing at quasi static conditions. The limitations imposed on the sample geometries by clamping technique and requirements of high strain rate test based on split Hopkinson bar test principle restrict the use of static geometries for the dynamic range. An optimization approach based on finite element simulations has been adopted to determine the most suitable dimensions for various tensile specimen geometries quantified by stress triaxiality. The possibility of introducing a standard for dynamic sample geometries has also been investigated. Moreover, the influence of transition zone on the deformation of the specimen has been analyzed and incorporated into the optimization strategy. The optimized specimen geometries are finally adopted for dynamic material testing so as to derive enough inputs for constitutive material modeling and to facilitate fundamental material research.

A Damaged Constitutive Model for Rock under Dynamic and High Stress State

The main research work of this paper focuses on the theoretical prediction of the constitutive relationship for rock, concrete, and other quasi-brittle materials under dynamic and complex stress state and the influence of dynamic mechanical behavior of rock on practical engineering problems was studied. A damaged elastoplastic model (DEPM) is established for the investigation and prediction of static or dynamic mechanical behavior of rock material. The mechanical behavior (brittleness or plasticity) and dynamic response (due to underground impact pressure and high-velocity impact of projectile) of rock under high in situ stress were investigated via the DEPM combined with the explicit finite element method. This paper suggests the influence of the brittle or plastic mechanical behavior of rock material on deep underground rock engineering.

High Strain Rate Response of A356/Al2O3 Aluminum Alloy MMCs Using Ls-Dyna

Aluminum alloy Metal Matrix Composites (MMCs) have evoked a keen interest in recent times for potential applications in aerospace, defense, marine and automotive industries owing to their superior strength to weight ratio and high temperature resistance despite the higher manufacturing costs. The ability to withstand dynamic loading is an important design criteria for many structures in above applications. The Split Hopkinson Pressure Bar (SHPB) also referred to as Kolsky bar is a commonly used setup for high strain rate test in strain rate range from 102 to 104 s-1. The high strain rate data is required for safety and structural integrity assessment of structures subjected to dynamic loading. As high strain rate data is not easily available in open literature need was felt for setting up such high strain rate testing. Four distinctly different A356/Al2O3 Aluminum alloy MMCs reinforced with 0, 4, 8 and 12% fiber volume fraction of aluminum oxide (Al2O3) particles were prepared using stir casting process. These four different MMCs were numerically characterised with SHPB test setup using Ls-Dyna software. The compressive stress-strain behavior was obtained at different strain rates ranging from 100 to 1200 s-1. The high strain rate results of compressive SHPB for all specimens were obtained in the form of stress-strain graphs, which strongly indicates that the high strain rate affects on the dynamic mechanical behaviour of MMC materials. These results are helpful to select the MMCs for structural applications where materials go under dynamic loading.

Investigating LES Turbulence Model in Modeling the Velocity Distribution Around the Hydraulic Structure Using ANSYS

Dynamic rock mechanics investigates the mechanical behavior of rock under dynamic loading conditions and change in mechanical properties of the rock. Loading techniques were almost used for both intermediate and high strain rate tests. In this work, dynamic tests and dynamic mechanical behavior of rock materials were studied. Dynamic tests were discussed to predict the stress-strain behavior. Different dynamic mechanical properties of rock materials including uniaxial and triaxial compressive strength, tensile strength, shear strength and fracture toughness were summarized. The effect of pressure, temperature and water saturation as well as microstructure, size and shape of rock on the mechanical properties of rock materials was considered.

Value Quantification of Electrochemical Capacitor Active Material

Performance parameters and relationships typically reported about the electrodes in an electrochemical capacitor generally do not allow active material value to be determined. This is primarily due to non-ideal dynamical behavior of these storage materials, which typically is a result of having high surface area. We show that electrochemical capacitor active material value can be quantified using a derived equivalent circuit model in combination with circuit simulations. The circuit model embodies electrical characteristics of the active material in device form and SPICE simulations allow model validation and storage system design, which is necessary to establish active material value. We demonstrate this approach starting with test data from a small activated-carbon, aqueous-electrolyte capacitor and proceed to design a regenerative braking-energy storage system for a 20-ton city transit bus. A value is then assigned to the active material in the capacitor based on its functional performance. For a specific energy storage application, this approach can provide value quantification of various active materials and thus allow for legitimate comparisons.

Thermal effect of the welding process on the dynamic behavior of the HSS constitutive materials of a fillet welded joint

Welded joints, due to their manufacturing process, are commonly weakened areas. This study provides a pragmatic methodology to analyze the dynamic behavior of the Base Metal (BM) and the Heat-Affected Zone (HAZ) of an HSS (High Strength Steel) fillet welded joint. Firstly, a specific approach has been developed to generate the HAZ material using a thermal treatment. Hardness and grain size are used to validate the replicated HAZ. This approach appears efficient and repeatable. Secondly, the true stress-strain quasi-static and dynamic behaviors up to failure of the BM and the HAZ have been determined. This characterization was performed thanks to a video tracking procedure and Bridgman-LeRoy correction. The comparison between these two materials shows that the thermal field of the welding process increases the HAZ yield stress and hardening while decreasing the strain at failure. It appears that the base metal is not rate sensitive from quasi-static up to 1350s-1 . On the contrary, the heat affected material appears to be rate sensitive, but by softening. This unexpected dynamic material softening requires further analyses. A first Finite Element (FE) numerical analysis on a welded substructure submitted to impact loading shows a strong influence of the HAZ on the initiation of failure mechanisms.

On the Horizontal Deviation of a Spinning Projectile Penetrating into Granular Systems

The absence of a general theory that describes the dynamical behavior of the particulate materials makes the numerical simulations the most current powerful tool that can grasp many mechanical problems relevant to the granular materials. In this paper, based on a two-dimensional soft particle discrete element method (DEM), a numerical approach is developed to investigate the consequence of the orthogonal impact into various granular beds of projectile rotating in both clockwise (CW) and counterclockwise (CCW) directions. Our results reveal that, depending on the rotation direction, there is a significant deviation of the x-coordinate of the final stopping point of a spinning projectile from that of its original impact point. For CW rotations, a deviation to the right occurs while a left deviation has been recorded for CCW rotation case.

A nondeterministic GHM based model applied to sandwich beams

In order to obtain computational models to aid the design of viscoelastic sandwich structures used as passive damping treatment, many aspects should be considered. A crucial aspect is the incertitude inherent to experimental characterization tests of the complex modulus. Usually the computational model parameters are determined by curve fitting mean values of experimental dispersions. Considering that these dispersions are inherent to experimental tests and also those small variations in the fitted parameters lead to considerable changes on the dynamic behavior of a Viscoelastic Material (VEM), hence a nondeterministic model is more suitable than the usual deterministic ones. In that way, starting from dynamic properties of a VEM, a nondeterministic numerical model, which takes into account incertitudes in the VEM curve fitting procedure, is proposed.

An experimental method for determination of dynamic mechanical behavior of materials at high temperatures

An experimental method for measuring dynamic behavior of materials at high temperatures (up to 1600℃) was proposed in this work. The experimental system includes a classical split Hopkinson pressure bar, a MoSi2 heating source for achieving high temperature, and two piston rods added to complement the double-synchronous assembled system. During the experiments, the specimen can be supported by asbestos and semi-Alumina ceramic tube. To estimate the thermal conduction of the tested specimen during the cold contact time (CCT), the time during which the hot specimen is in contact with the cold bars before being compressed, the CCT was measured experimentally based on an on-off circuit, and the finite element method (FEM) was also employed to calculate the thermal conduction of the tested specimen. High speed camera was employed to record images of the specimen during testing through a window in the heating furnace. For better understanding of the influence of oxidation of specimens, the system was also equipped with an argon supply system to prevent the specimen from oxidation at the high temperatures. To verify the ability of the proposed method to operate at high temperatures, experiments were conducted on an TC4 alloy at test temperatures ranging from 20 to 1400℃ at the strain rate of 2000 s−1, and on SiC at temperatures ranging from 20 to 1600℃ at the strain rate of 250 s−1.

Damping behavior investigation and optimization of the structural layout of load-bearing biological materials

Load-bearing biological materials, such as bone and nacre, exhibit superior mechanical properties to support and/or protect biological functionalities. To achieve this, an important ability of such materials is to effectively shield external dynamic impacts. The damping behavior of the biological materials mostly stems from their architecture and also the organic portion which intrinsically shows viscoelastic-plastic behavior. The loss modulus, a crucial representative of dynamic behavior, is a key index on how strong the bio-composite is able to damp the dynamic energy within its structure. This work aims to study the damping behavior of mostly seen staggered architectures of biological materials such as regular, offset, stairwise, herringbone and random staggering patterns to highlight the optimal layout, geometry, and feature sizes of the bio-composite in which the energy dissipation within the structure is maximal. Our studies show that not only the stairwise staggering arrangement overall offers the highest loss modulus amongst other existing architectures but also the aspect ratio of the platelet leading to the maximum loss modulus is remarkably in excellent agreement with experimental observations which might explain why this kind of arrangements is extensively observed in nature. Our findings here not only gain valuable insights into the dynamic behavior of load-bearing biological materials, but also provide useful guidelines to the design of bio-inspired engineering materials.

Discrete Model for Concrete Fracture: Numerical Study of Dynamic Response

Abstract The contribution presents simulations on concrete specimens. The discrete meso-scale particle model with random geometry based on Voronoi tessellation is used. The model was enhanced with dynamic solver based on implicit Newmark method. Model is tested on cantilever beam loaded by a force at the free end to verify the ability of the model to simulate the dynamic behavior of a simple linear elastic material. Results computed with different time discretization and model settings are compared. The behavior of the model in nonlinear regime is investigated on concrete specimens loaded at different displacement rates. The constitutive law used within this contribution is insensitive to strain rate.

A novel method in dynamic shear testing of bulk materials using the traditional SHPB technique

This paper aims to propose a novel experimental method to study dynamic shear properties and constitutive behaviors of bulk materials using the traditional split Hopkinson pressure bar (SHPB) technique. In this method, a new kind of double-shear specimen (DSS) and fixture is specially designed for the SHPB system. With this setup configuration, the plastic properties of materials can be obtained conveniently at high strain rates under the state of simple shear. The deformation and failure process of the shear zone can be observed directly by using a high-speed photography. Another advantage of this method is that reloading on the specimen can be prevented for the purpose of post-test observation, with the application of the stress reversal Hopkinson technique (Nemat-Nasser etal., 1991). Using this method, the 603 steel was tested at strain rates from 2500s−1 to 48000s−1. With the aid of finite element analysis (FEA), it's found that the shear stress and shear strain play dominant roles in the stress/strain fields. The distribution of the maximum shear strains in the shear zone showed good consistency with the fracture patterns of the tested samples. Through comparison between the tested and simulated shear stress-shear strain curves, very good agreement is noted for all the tested cases.

LAMPI DI LUCE: DAI MILLISECONDI AGLI ATTO SECONDI

Mankind has always been interested in studying the movement of ever faster objects upon using ever shorter light pulses. As a first example of a study of macroscopic samples, mention will be made about the study of a horse, while galloping, as obtained by the photographer Eadweard Muybridge around the end of years ‘800. He was in fact able to obtain a complete recording of the horse movement upon using an ingenious system of several cameras, placed along the horse racing, equipped with shutters with a record opening time of about 2 millisecond (1 ms=10–3 s). About 50 years later (1930) the introduction of flashlamps allowed the achievement of light pulses with duration of about one microsecond (1 μs=10–6 s). Upon using flashlamps in a repetitive mode, it was then possible to record the movement of much smaller and hence much faster objects (e.g. a bullet while crossing an apple or the fast movement of a golf player while hitting the golf ball). It was, however, only after the invention of the laser (T.H. Maiman, 1960) that people became able to generate much shorter light pulses. From initial pulse durations (middle of years ‘60) of about 10 picosecond (1 ps= 10–12 s), one could then generate (during years ’90) laser light pulses of a few femtoseconds (1 fs=10–15 s) down the present values of about 70 attoseconds (1 as=10–18 s). During all these years after laser invention, the research group at the Physics Department of Politecnico di Milano, leaded by the author of the present paper, has always been at the forefront of the international research to generate and utilize laser pulses of ever shorter duration. After the introduction of the first laser at the Politecnico di Milano at the beginning of 1963 (more than 50 years ago!), the research group was able to obtain light pulses of about 5 picoseconnds in 1969, a record value for that time. The same group then obtained record values of light pulses of about 4,5 femtoseconds, at the end of years ’90, and of only 130 attoseconds around the middle of years 2000. Ultrashort laser pulses have been used, throughout all these years, by a large number of international groups to investigate the ultrafast dynamic behavior of molecules, molecular complexes, and solid state nanoscopic materials (such as quantum wells and quantum dots). As a representative example, some discussion will be presented about a recent work made by the research group of Politecnico about the retinal contained in the rodospin protein, a basic element for human vision.

Scale Transitions in Magnetisation Dynamics

AbstractMultiscale modelling is a powerful technique, which allows for computational efficiency while retaining small-scale details when they are essential for understanding a finer behaviour of the studied system. In the case of materials modelling, one of the effective multiscaling concepts is domain partitioning, which implies the existence of an explicit interface between various material descriptions, for instance atomistic and continuum regions. When dynamic material behaviour is considered, the major problem for this technique is dealing with reflections of high frequency waves from the interface separating two scales. In this article, a new method is suggested, which overcomes this problem for the case of magnetisation dynamics. The introduction of a damping band at the interface between scales, which absorbs high frequency waves, is suggested. The idea is verified using a number of one-dimensional examples with fine/coarse scale discretisation of a continuum problem of spin wave propagation. This work is the first step towards establishing a reliable atomistic/continuum multiscale transition for the description of the evolution of magnetic properties of ferromagnets.

Modelling of dynamic behaviour of viscoelastic materials in the frequency domain

The modern methods of acoustical and vibration calculus require accurate material models to take properly into account the damping and dynamic elastic properties of the viscoelastic materials used for sound and vibration control. The usual modeling methods (spring-dashpot models, fractional calculus) start out of the stress to strain relations defined in the time-domain, which are then transformed into the frequency domain to find the appropriate material functions. In contrast, the damping and elastic properties are known from measurements made in the frequency domain; therefore, it seems to be reasonable to model the material behavior directly in this domain. The aim of this paper is to show that the typical dynamic behavior of solid viscoelastic materials can efficiently be modeled in the frequency domain. This modeling method is based on simple theoretical considerations, the frequency domain experiences and the causality principle. Two fundamental material models are developed in this way, namely one for the damping increasing with frequency, and the other one for the damping peak, which can be either symmetrical or asymmetrical with respect to the logarithmic frequency. The relation of the frequency domain models to the popular fractional derivative models defined in the time-domain is shown and discussed.

Fatigue cracking failure criterion for flexible pavements under moving vehicles

An improved fatigue cracking criterion for flexible pavements is developed. It relates the allowable number of load repetitions with the maximum tensile strain at the bottom of the asphalt concrete layer that causes cracking there. The criterion is established by combining an empirical relationship providing the number of load repetitions as a function of the layer thicknesses and the axle loading as obtained by the AASHO Road Test with that maximum tensile strain developed at the bottom of the asphaltic layer. Because this maximum tensile strain is computed by a FEM that analyses pavements under moving vehicles and assumes the asphaltic layer to behave as a linear viscoelastic material, the resulting failure criterion takes into account speed and viscoelastic material behavior. Thus, this criterion is an improved version of the existing ones based on static or dynamic loading and linear elastic material behavior.

EXPERIMENTAL STUDY FOR DISPERSION AND ATTENUATION CORRECTION IN VISCOELASTIC SPLIT HOPKINSON PRESSURE BAR ARRANGEMENT

Split Hopkinson pressure Bar (SHPB) arrangement is the most practical and accurate method for studying dynamic mechanical behavior of materials under high strain rate. However in case of studying foams, and soft tissues (low impedance materials) a modified version of SHPB with bars made from a viscoelastic material like Poly methyl methacrylate PMMA acrylate is commonly used .so a good prediction of the stress waves and particle velocity acting at the interface between bars and specimen taking into account the dispersion and attenuation effect is essential. In this study the wave propagation coefficient as a function of the phase velocity and the attenuation coefficient in relation to the frequency are calculated experimentally by a set of free end bar tests using a cylindrical projectile to the circumstances of stepped velocity and different firing pressure. Correction processes for longitudinal stains related to incident and reflected waves are preceded in frequency domain using Fast Fourier Transformer (FFT) technique.

Validation of the Hardening Behaviors for Metallic Materials at High Strain Rate and Temperature by Using the Taylor Impact Test

This paper is concerned with the validation of the dynamic hardening behaviors of metallic materials by comparing numerical and experimental results of the Taylor impact tests. Several uniaxial tensile tests are performed at different strain rates and temperatures by using three kinds of materials: 4130 steel (BCC); OFHC copper (FCC); and Ti6Al4V alloy (HCP). Uniaxial material tests are performed at a wide range of strain rates from 10−3 s−1 to 103 s−1. Moreover, tensile tests are performed at temperature of 25 °C and 200 °C at strain rates of 10−3 s−1, 10−1 s−1, and 102 s−1, respectively. A modified Johnson–Cook type thermal softening model is utilized for the accurate application of the thermal softening effect at different strain rates. The hardening behaviors of the three materials are characterized by comparing the seven sequentially deformed shapes of the projectile from numerical and experimental results of Taylor impact tests.