Abstract This study deals with additive manufacturing (AM) products. Employing AM technology, com... more Abstract This study deals with additive manufacturing (AM) products. Employing AM technology, complex-shaped, and lightweight parts can be fabricated using the aluminum alloy, AlSi10Mg. Typically, AM of this alloy is performed by laser powder bed fusion (LPBF). The mechanical properties of LPBF products are crucial for many engineering applications. Therefore, there have been efforts to measure the dynamic and quasi-static properties of such products. However, both the dynamic and quasi-static shear behaviors of this alloy are yet to be investigated. The present study is focused on experimentally investigating the shear behavior of LPBF AlSi10Mg. Quasi-static shear tests were performed according to the ASTM B565 protocol, whereas dynamic shear tests were conducted using a standard split Hopkinson pressure bar equipped with an innovative sample holder that generates pure shear in a sample. The results of the performed tests showed that as the shear load rate increased, the shear strength considerably increased. In addition, in the quasi-static regime, the shear strength was practically independent of the product build direction. In contrast, under the dynamic shear conditions, samples built horizontally failed at a shear strength approximately 10% lower than that of those manufactured vertically. To investigate the build orientation effect on the shear behavior, this study conducted extensive microstructural characterization along with fracture analysis. Crack nucleation and propagation were analyzed in view of the effects of the unique microstructural characteristics of the LPBF AlSi10Mg, including the morphologies of the melt pool boundaries. The results obtained in this study suggested that for engineering applications in which dynamic loads are expected, the build orientation of AlSi10Mg parts produced using LPBF technology must be considered.
International Journal of Impact Engineering, Apr 1, 2023
Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Ther... more Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Therefore, investigation of their behavior under such extreme conditions is crucial. A recent review highlighted a significant gap in understanding the mechanical behavior of threads under impact loads [Warren et al. J Constr Steel Res, 2022;194]. Therefore, this study investigates the effect of thread geometries on stress waves propagating through the threads using two parallel approaches: experimental and numerical simulations. Different threads were manufactured from steel alloys with varying lengths of pitch and tooth geometries. Experiments using a split Hopkinson pressure bar (SHPB) system and a numerical simulation using the LS-DYNA code were performed to characterize the propagation of a stress wave when crossing different threads. The investigation revealed that increasing the tooth height of the threads and the number of teeth reduced the distortion of stress waves produced by the thread. It was also shown that a fine thread could reduce the level of the transmitted stress under dynamic loading, similar to the effect observed when using a porous material. With this feature, it will be possible to design thread connections with the ability to mitigate or transmit dynamic impulse loads.
Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Ther... more Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Therefore, investigation of their behavior under such extreme conditions is crucial. A recent review highlighted a significant gap in understanding the mechanical behavior of threads under impact loads [Warren et al. J Constr Steel Res, 2022;194]. Therefore, this study investigates the effect of thread geometries on stress waves propagating through the threads using two parallel approaches: experimental and numerical simulations. Different threads were manufactured from steel alloys with varying lengths of pitch and tooth geometries. Experiments using a split Hopkinson pressure bar (SHPB) system and a numerical simulation using the LS-DYNA code were performed to characterize the propagation of a stress wave when crossing different threads. The investigation revealed that increasing the tooth height of the threads and the number of teeth reduced the distortion of stress waves produced by the thread. It was also shown that a fine thread could reduce the level of the transmitted stress under dynamic loading, similar to the effect observed when using a porous material. With this feature, it will be possible to design thread connections with the ability to mitigate or transmit dynamic impulse loads.
Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Ther... more Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Therefore, investigation of their behavior under such extreme conditions is crucial. A recent review highlighted a significant gap in understanding the mechanical behavior of threads under impact loads [Warren et al. J Constr Steel Res, 2022;194]. Therefore, this study investigates the effect of thread geometries on stress waves propagating through the threads using two parallel approaches: experimental and numerical simulations. Different threads were manufactured from steel alloys with varying lengths of pitch and tooth geometries. Experiments using a split Hopkinson pressure bar (SHPB) system and a numerical simulation using the LS-DYNA code were performed to characterize the propagation of a stress wave when crossing different threads. The investigation revealed that increasing the tooth height of the threads and the number of teeth reduced the distortion of stress waves produced by the thread. It was also shown that a fine thread could reduce the level of the transmitted stress under dynamic loading, similar to the effect observed when using a porous material. With this feature, it will be possible to design thread connections with the ability to mitigate or transmit dynamic impulse loads.
Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Ther... more Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Therefore, investigation of their behavior under such extreme conditions is crucial. A recent review highlighted a significant gap in understanding the mechanical behavior of threads under impact loads [Warren et al. J Constr Steel Res, 2022;194]. Therefore, this study investigates the effect of thread geometries on stress waves propagating through the threads using two parallel approaches: experimental and numerical simulations. Different threads were manufactured from steel alloys with varying lengths of pitch and tooth geometries. Experiments using a split Hopkinson pressure bar (SHPB) system and a numerical simulation using the LS-DYNA code were performed to characterize the propagation of a stress wave when crossing different threads. The investigation revealed that increasing the tooth height of the threads and the number of teeth reduced the distortion of stress waves produced by the thread. It was also shown that a fine thread could reduce the level of the transmitted stress under dynamic loading, similar to the effect observed when using a porous material. With this feature, it will be possible to design thread connections with the ability to mitigate or transmit dynamic impulse loads.
Constitutive modeling is the mathematical description of material behavior under various loadings... more Constitutive modeling is the mathematical description of material behavior under various loadings. This is one of the most intensely researched fields within solid mechanics because of the importance of accurate constitutive models for practical engineering problems. A wide range of available constitutive models for concrete were investigated. One successful model implemented in the hydrocode LS-DYNA, is the Concrete Damage model, which is able to predict the behavior of confined plain concrete. This model is calibrated in this work for a wide range of concrete strengths, with respect to triaxial compression test data found in literature. The adjustment of parameters was performed as function of the only parameter known to most users: the unconfined compressive strength of a specific concrete. This enables an occasional user to input only this parameter, while the rest will automatically be calculated. This calibrated model named herein BGU, provides a flexible and powerful method to predict the performance of confined plain concrete. It is shown that the present calibration performed well when compared to the automatic parameters generation option currently available in the code.
In spite of the worldwide recognition of the importance of testing blast effects on dummy humans,... more In spite of the worldwide recognition of the importance of testing blast effects on dummy humans, there is a lack of blast simulators that are capable of generating realistic blast conditions in the laboratory. The objective of the present study was to design, construct and test a blast tube that is able to accurately reproduce loading histories of actual explosions in the laboratory. The design combines some advantages of existing blast-wave generating facilities. Using numerical simulations, a 5-m long blast tube was designed. The blast tube is large enough to enclose dummies including torso and head, wearing vests and/or helmets. The system generated blast waves equivalent to those of a spherical explosion of about 3.5 kg TNT with an over pressure of 0.64 bar and a positive phase of 4 ms. The repeatability of the experiments was very good. The blast tube’s open end is square of 1.57 m × 1.57 m and although designed for experiments on human dummies, it could be used for testing even full-scale structural components. High quality high-speed photography was demonstrated through the designed windows. Our preliminary study on the effect of a helmet on a dummy’s head revealed that the tested helmet amplified by a factor of 2 and more the peak pressure in the back side of the head. The newly designed blast tube is capable of simulating close range blast waves, manifested with short positive durations. Experiments with and without helmets revealed the importance of blast testing for improving helmet design.
Abstract Induced heating in a vacuum environment is the most common method for high precision and... more Abstract Induced heating in a vacuum environment is the most common method for high precision and purity casting. A significant disadvantage in this process is that it is designed for heating in a crucible of uniform diameter. However, it is not suited for the heating of a crucible with stepped diameter. The use of a variable diameter crucible enables to place a larger amount of raw material into the wider diameter section of the crucible. After the material melts, the liquid fills the narrow part of the crucible and enables better control of liquid flow into the casting mold. Hence, in this research we present a novel solution for vacuum inductive heating in a stepped diameter crucible by using a secondary heating coil (SHC). In order to examine the SHC solution, numerical models were developed to describe the temperature distribution and the heat generation in the crucible. Experiments on a vacuum induction furnace were conducted to validate the numerical models of the SHC. A good match was acquired between the numerical and experimental results. The results of the simulations and experiments have shown great improvement in the capability of heating a stepped diameter crucible with an SHC.
An elastic cantilever beam subjected to weak impact is investigated with focus on its ability to ... more An elastic cantilever beam subjected to weak impact is investigated with focus on its ability to absorb the impact energy. It was found that the location of impact along the beam has a significant influence on the kinetic energy absorbed. It was established in both, using an experimental setup and numerical computations. A reduced degrees of freedom analytical model of this simple continuous system is analyzed in order to explain its quite complicated behavior. The same impact spot, at which the maximum impact energy was absorbed, was predicted by the analytical model. At this location 90% of the kinetic energy is absorbed in the form of elastic beam vibrations. The results of this demonstration may be used for designing an elastic kinetic energy absorbing system.
Abstract This study deals with additive manufacturing (AM) products. Employing AM technology, com... more Abstract This study deals with additive manufacturing (AM) products. Employing AM technology, complex-shaped, and lightweight parts can be fabricated using the aluminum alloy, AlSi10Mg. Typically, AM of this alloy is performed by laser powder bed fusion (LPBF). The mechanical properties of LPBF products are crucial for many engineering applications. Therefore, there have been efforts to measure the dynamic and quasi-static properties of such products. However, both the dynamic and quasi-static shear behaviors of this alloy are yet to be investigated. The present study is focused on experimentally investigating the shear behavior of LPBF AlSi10Mg. Quasi-static shear tests were performed according to the ASTM B565 protocol, whereas dynamic shear tests were conducted using a standard split Hopkinson pressure bar equipped with an innovative sample holder that generates pure shear in a sample. The results of the performed tests showed that as the shear load rate increased, the shear strength considerably increased. In addition, in the quasi-static regime, the shear strength was practically independent of the product build direction. In contrast, under the dynamic shear conditions, samples built horizontally failed at a shear strength approximately 10% lower than that of those manufactured vertically. To investigate the build orientation effect on the shear behavior, this study conducted extensive microstructural characterization along with fracture analysis. Crack nucleation and propagation were analyzed in view of the effects of the unique microstructural characteristics of the LPBF AlSi10Mg, including the morphologies of the melt pool boundaries. The results obtained in this study suggested that for engineering applications in which dynamic loads are expected, the build orientation of AlSi10Mg parts produced using LPBF technology must be considered.
International Journal of Impact Engineering, Apr 1, 2023
Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Ther... more Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Therefore, investigation of their behavior under such extreme conditions is crucial. A recent review highlighted a significant gap in understanding the mechanical behavior of threads under impact loads [Warren et al. J Constr Steel Res, 2022;194]. Therefore, this study investigates the effect of thread geometries on stress waves propagating through the threads using two parallel approaches: experimental and numerical simulations. Different threads were manufactured from steel alloys with varying lengths of pitch and tooth geometries. Experiments using a split Hopkinson pressure bar (SHPB) system and a numerical simulation using the LS-DYNA code were performed to characterize the propagation of a stress wave when crossing different threads. The investigation revealed that increasing the tooth height of the threads and the number of teeth reduced the distortion of stress waves produced by the thread. It was also shown that a fine thread could reduce the level of the transmitted stress under dynamic loading, similar to the effect observed when using a porous material. With this feature, it will be possible to design thread connections with the ability to mitigate or transmit dynamic impulse loads.
Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Ther... more Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Therefore, investigation of their behavior under such extreme conditions is crucial. A recent review highlighted a significant gap in understanding the mechanical behavior of threads under impact loads [Warren et al. J Constr Steel Res, 2022;194]. Therefore, this study investigates the effect of thread geometries on stress waves propagating through the threads using two parallel approaches: experimental and numerical simulations. Different threads were manufactured from steel alloys with varying lengths of pitch and tooth geometries. Experiments using a split Hopkinson pressure bar (SHPB) system and a numerical simulation using the LS-DYNA code were performed to characterize the propagation of a stress wave when crossing different threads. The investigation revealed that increasing the tooth height of the threads and the number of teeth reduced the distortion of stress waves produced by the thread. It was also shown that a fine thread could reduce the level of the transmitted stress under dynamic loading, similar to the effect observed when using a porous material. With this feature, it will be possible to design thread connections with the ability to mitigate or transmit dynamic impulse loads.
Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Ther... more Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Therefore, investigation of their behavior under such extreme conditions is crucial. A recent review highlighted a significant gap in understanding the mechanical behavior of threads under impact loads [Warren et al. J Constr Steel Res, 2022;194]. Therefore, this study investigates the effect of thread geometries on stress waves propagating through the threads using two parallel approaches: experimental and numerical simulations. Different threads were manufactured from steel alloys with varying lengths of pitch and tooth geometries. Experiments using a split Hopkinson pressure bar (SHPB) system and a numerical simulation using the LS-DYNA code were performed to characterize the propagation of a stress wave when crossing different threads. The investigation revealed that increasing the tooth height of the threads and the number of teeth reduced the distortion of stress waves produced by the thread. It was also shown that a fine thread could reduce the level of the transmitted stress under dynamic loading, similar to the effect observed when using a porous material. With this feature, it will be possible to design thread connections with the ability to mitigate or transmit dynamic impulse loads.
Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Ther... more Threaded links are widely used in many structures designed to sustain extreme dynamic loads. Therefore, investigation of their behavior under such extreme conditions is crucial. A recent review highlighted a significant gap in understanding the mechanical behavior of threads under impact loads [Warren et al. J Constr Steel Res, 2022;194]. Therefore, this study investigates the effect of thread geometries on stress waves propagating through the threads using two parallel approaches: experimental and numerical simulations. Different threads were manufactured from steel alloys with varying lengths of pitch and tooth geometries. Experiments using a split Hopkinson pressure bar (SHPB) system and a numerical simulation using the LS-DYNA code were performed to characterize the propagation of a stress wave when crossing different threads. The investigation revealed that increasing the tooth height of the threads and the number of teeth reduced the distortion of stress waves produced by the thread. It was also shown that a fine thread could reduce the level of the transmitted stress under dynamic loading, similar to the effect observed when using a porous material. With this feature, it will be possible to design thread connections with the ability to mitigate or transmit dynamic impulse loads.
Constitutive modeling is the mathematical description of material behavior under various loadings... more Constitutive modeling is the mathematical description of material behavior under various loadings. This is one of the most intensely researched fields within solid mechanics because of the importance of accurate constitutive models for practical engineering problems. A wide range of available constitutive models for concrete were investigated. One successful model implemented in the hydrocode LS-DYNA, is the Concrete Damage model, which is able to predict the behavior of confined plain concrete. This model is calibrated in this work for a wide range of concrete strengths, with respect to triaxial compression test data found in literature. The adjustment of parameters was performed as function of the only parameter known to most users: the unconfined compressive strength of a specific concrete. This enables an occasional user to input only this parameter, while the rest will automatically be calculated. This calibrated model named herein BGU, provides a flexible and powerful method to predict the performance of confined plain concrete. It is shown that the present calibration performed well when compared to the automatic parameters generation option currently available in the code.
In spite of the worldwide recognition of the importance of testing blast effects on dummy humans,... more In spite of the worldwide recognition of the importance of testing blast effects on dummy humans, there is a lack of blast simulators that are capable of generating realistic blast conditions in the laboratory. The objective of the present study was to design, construct and test a blast tube that is able to accurately reproduce loading histories of actual explosions in the laboratory. The design combines some advantages of existing blast-wave generating facilities. Using numerical simulations, a 5-m long blast tube was designed. The blast tube is large enough to enclose dummies including torso and head, wearing vests and/or helmets. The system generated blast waves equivalent to those of a spherical explosion of about 3.5 kg TNT with an over pressure of 0.64 bar and a positive phase of 4 ms. The repeatability of the experiments was very good. The blast tube’s open end is square of 1.57 m × 1.57 m and although designed for experiments on human dummies, it could be used for testing even full-scale structural components. High quality high-speed photography was demonstrated through the designed windows. Our preliminary study on the effect of a helmet on a dummy’s head revealed that the tested helmet amplified by a factor of 2 and more the peak pressure in the back side of the head. The newly designed blast tube is capable of simulating close range blast waves, manifested with short positive durations. Experiments with and without helmets revealed the importance of blast testing for improving helmet design.
Abstract Induced heating in a vacuum environment is the most common method for high precision and... more Abstract Induced heating in a vacuum environment is the most common method for high precision and purity casting. A significant disadvantage in this process is that it is designed for heating in a crucible of uniform diameter. However, it is not suited for the heating of a crucible with stepped diameter. The use of a variable diameter crucible enables to place a larger amount of raw material into the wider diameter section of the crucible. After the material melts, the liquid fills the narrow part of the crucible and enables better control of liquid flow into the casting mold. Hence, in this research we present a novel solution for vacuum inductive heating in a stepped diameter crucible by using a secondary heating coil (SHC). In order to examine the SHC solution, numerical models were developed to describe the temperature distribution and the heat generation in the crucible. Experiments on a vacuum induction furnace were conducted to validate the numerical models of the SHC. A good match was acquired between the numerical and experimental results. The results of the simulations and experiments have shown great improvement in the capability of heating a stepped diameter crucible with an SHC.
An elastic cantilever beam subjected to weak impact is investigated with focus on its ability to ... more An elastic cantilever beam subjected to weak impact is investigated with focus on its ability to absorb the impact energy. It was found that the location of impact along the beam has a significant influence on the kinetic energy absorbed. It was established in both, using an experimental setup and numerical computations. A reduced degrees of freedom analytical model of this simple continuous system is analyzed in order to explain its quite complicated behavior. The same impact spot, at which the maximum impact energy was absorbed, was predicted by the analytical model. At this location 90% of the kinetic energy is absorbed in the form of elastic beam vibrations. The results of this demonstration may be used for designing an elastic kinetic energy absorbing system.
Additive manufacturing by selective laser melting (AM-SLM) is an advanced manufacturing approach ... more Additive manufacturing by selective laser melting (AM-SLM) is an advanced manufacturing approach in which a structure is fabricated by successive thin powder layers melted by a focused laser beam. The aerospace and automotive sectors are especially interested in the AM-SLM technology that enables quick production of complex and customized structures. AlSi10Mg alloy has been found to be applicable to AM-SLM mainly because good cast-ability, strong weldability and low shrinkage during solidification. While many studies on the quasi-static mechanical properties and the structure of SLM AlSi10Mg were published, there is limited published research focused on the dynamic properties of SLM AlSi10Mg under high rate strains. In addition to that, the shear strength of SLM aluminium alloys is rarely investigated. This study presents an investigation of the AM-SLM AlSi10Mg static and dynamic shear strength and its dependency on build direction. Experiments included quasi-static shear experiments performed according to the protocol of ASTM B565, and dynamic shear tests performed using a split Hopkinson pressure bar (SHPB), coupled to innovative punch assembly that generates pure dynamic shear loads on the sample. The design of this sample holder has been validated numerically and an experimentally. The quasi-static experiments revealed that the static shear strength is independent of build direction. In contrast, the dynamic tests demonstrated that the dynamic shear strength of vertically built samples is higher by almost 11% than the shear strength of samples built horizontally. This last phenomenon explained with a suggested mechanism based onelectron microscope fractography.
A unique design for a vertical tension split Hopkinson bar (TSHB) with steel bars 100 mm in diame... more A unique design for a vertical tension split Hopkinson bar (TSHB) with steel bars 100 mm in diameter and an aluminum hammer is presented. This facility was used to investigate the behavior of concrete under dynamic tension. Experiments conducted with specially shaped concrete samples demonstrate that the testing system is capable of accurately depicting the dynamic tensile strength of concrete. The experimental data were used to calibrate three concrete material models implemented in the numerical code LS-DYNA. In this study, experimental and numerical investigation of the phenomenon of the dynamic increase factor (DIF) of concrete in pure tension was conducted. Although many researchers have published experimental data on the DIF in concrete, only a few have addressed this phenomenon in tension using various dynamic experimental methods. Of these, even fewer used direct tension experiments. This study was conducted using direct tension tests on a TSHB system and accompanied by numerical computations. Thus, enabling full understanding of the experimental method and calibration of three material models. Our results agree very well with the modified CEB DIF curve which shows a sharp increase in the dynamic factor starting from strain rates as low as 1 1/s-1. The numerical part of this work included modification of material models implemented in LS-DYNA, and will be explained in the paper. We demonstrated that only after these modifications do the computed tensile pulses agree well with the pulses measured on the transmitter bar. In our opinion, it is essential to combine the experimental and numerical methods herein proposed.
Keywords: Tensile Split Hopkinson Bar, Dynamic testing, Concrete material models, LS-DYNA, Numerical modeling.
The presented experimental study investigated the geometric wall effects of a structure on the be... more The presented experimental study investigated the geometric wall effects of a structure on the behavior of an internally detonated blast. Hitherto, most blast in tunnel studies have worked to characterize blast profiles in smooth tunnels. This simplification effectively neglects wall effects including viscosity and shock reflections off the obstacles along the wall. The exploding wire technique, a rigorously confirmed method for producing accurate blast profiles, was used to simulate a blast event at the closed end of a scaled-down square tunnel with varying degrees of wall roughness. Results showed that the surface geometry of a tunnel's inner cross section has significant impact on the blast's pressure and impulse time histories. In particular, we found that wall roughness amplifies the maximum impulse in proximity to the blast source, contradicting the generally held belief that wall roughness primarily attenuates the blast wave impulse.
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Papers by Eytan Kochavi
In this study, experimental and numerical investigation of the phenomenon of the dynamic increase factor (DIF) of concrete in pure tension was conducted. Although many researchers have published experimental data on the DIF in concrete, only a few have addressed this phenomenon in tension using various dynamic experimental methods. Of these, even fewer used direct tension experiments. This study was conducted using direct tension tests on a TSHB system and accompanied by numerical computations. Thus, enabling full understanding of the experimental method and calibration of three material models. Our results agree very well with the modified CEB DIF curve which shows a sharp increase in the dynamic factor starting from strain rates as low as 1 1/s-1.
The numerical part of this work included modification of material models implemented in LS-DYNA, and will be explained in the paper. We demonstrated that only after these modifications do the computed tensile pulses agree well with the pulses measured on the transmitter bar. In our opinion, it is essential to combine the experimental and numerical methods herein proposed.
Keywords: Tensile Split Hopkinson Bar, Dynamic testing, Concrete material models,
LS-DYNA, Numerical modeling.