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High-entropy alloys (HEAs) have attracted tremendous attention owing to their controllable mechanical properties, whereas additive manufacturing (AM) is an efficient and flexible processing route for novel materials design. However, a profound appraisal of the fundamental material physics behind the strengthening of AM-printed HEAs upon low/intermediate-temperature annealing is essential. In this work, CoCrFeNiMn HEAs have been prepared using laser-engineered net shaping (LENS) and subsequently annealed at different temperatures. The CoCrFeNiMn HEA annealed at intermediate-temperature (873 K) exhibits a strong strain hardening capability, resulting in ultimate strength of 725 MPa and plasticity of 22%. A ternary heterogeneous strengthening mechanism is proposed to explain this phenomenon, in which equiaxed grains, columnar grains, and σ precipitates play different roles during tensile deformation. The resultant excellent strength and ductility can be ascribed to the heterostructure-induced mismatch. The equiaxed grains provide adequate grain boundaries (GBs), which induce dislocation plugging-up and entanglement; the columnar grains induce the onset and arrest of the dislocations for plastic deformation; and the σ precipitates hinder the movement of slip dislocations. The results provide new insights into overcoming the strength-ductility trade-off of LENS-printed HEAs with complex geometries.

In this paper, a series of static/dynamic tensile tests are performed for glass fiber reinforced plastic (GFRP) composites. Using the combination of high-speed photography and digital image correlation (DIC) technology, true stress–strain curves in different directions and strain rates are obtained. We also obtained the dynamic failure strain of the material in different directions, which are used to accurately describe the dynamic tensile and failure behavior of the material. The experimental results show that there is a stiffness change point N in three directions under different strain rate (10−3 s−1, 10 s−1, 100 s−1) tensile conditions. The stiffness before and after N point is recorded as Einitial and Echanged respectively. The values of Echanged in weft direction and warp direction are about 30% to 50% of Einitial, while Echanged in tilt direction is only about 10% of Einitial. The fiber has the highest strength in the weft direction and the tilt direction has the lowest strength. With the combination of high-speed photography and DIC technology, the dynamic failure parameters of different directions under the strain rate of 100 s−1 are obtained. The dynamic failure strains in three directions are 0.245, 0.373 and 0.341, respectively. The parameters are verified by impact three-point bending test. These works can more accurately describe the dynamic mechanical behavior of glass fiber reinforced plastic (GFRP) composites and provide reference for the design of GFRP structures.

Steel matrix syntactic foam is a promising lightweight and energy absorbing material. In this study, an infiltration casting technology is developed to prepare syntactic foams, which brings in alum...

The structural and mechanical responses of Ti-based and Zr-based metallic glasses (MGs) treated by the nanosecond laser shock wave were investigated. A rejuvenation transition was observed, due to the structural rearrangement of neighboring atoms in the instantaneous shock process. Plenty of embryonic shear bands occurred along the shock-affected zone. A damped model of the shock pressure was established to describe the characteristic of the embryonic shear bands. The results showed that the shear band initiation in the nanosecond scale along the length direction was divided into two stages: the initial low propagation stage and the following fast propagation stage, because of the reduction of the shock pressure. The hardness of the shock-affected zone displayed a maximal decrease of 25% in the critical zone between the two propagation stages, due to the combined effect of the structural rejuvenation and the compressive residual stress. A schematic of the energy landscape was proposed to explain the relationship between the rejuvenation and the formation of the embryonic shear bands. The maximum tensile plasticity increased by 0.46%. The results demonstrated that the MGs possessed different shear band propagation modes in the initiation stages, which deepened the understanding of the deformation mechanism in the MGs.

In this study, we focus on the dynamic failure property of A6N01S-T5 aluminum alloy use for high-speed trains. The method of split Hopkinson tensile bar (SHTB) and three-dimensional (3D) digital image correlation (DIC) was put forward to find the dynamic mechanical properties and dynamic failure strain of A6N01S-T5 aluminum alloy, and on the basis of this, Johnson–Cook model constitutive parameters and dynamic failure strain parameters were obtained through a series of static and dynamic tests. An important character of this method was that the sandwich structure from the true high-speed train was used in penetration test, followed by the numerical calculation of the same working condition using LS-DYNA. Then we compare the experimental results with simulation results mentioned above in terms of failure morphology in structure and the bullet speed throughout the entire process to verify the accuracy of the parameter. The experimental results provide a data basis for the crash simulation model of high-speed trains, in turn to optimize the structural design and whole efficiency.

CuCrZr alloy has been acknowledged as one of the most promising candidate materials for the heat sink of plasma facing components in nuclear fusion devices with its excellent thermal conductivity and specific mechanical strength. For the safety concern, it is vital to provide appropriate safety assessment on these mechanical structures with CuCrZr alloy, such as divertor and blanket, under high strain rate loading conditions like displacement events, disruption, and so on. In this work, the dynamic compressive behavior of the CuCrZr alloy over wide range of strain rates and temperatures was investigated using the split Hopkinson pressure bar technique. Based on the experiment results, a constitutive equation is built by using Johnson-Cook model to describe the dynamical property of CuCrZr alloy. However, the results indicated that the influences of the strain, strain rate and temperature on the dynamic behavior are not independent. Therefore, a modified Johnson-Cook model was further developed to take the coupled effects of strain, strain rate and temperature into account. Compared with the original Johnson-Cook model, the modified Johnson-Cook model calculated results show good agreement with the experimental data.

To solve the problem of insufficient protective performance of the existing body armor, an impact-resistant shear stiffening gel (SSG) with excellent flexibility and formability was innovatively used as buffer plate for body armor, and then combined with ultra-high molecular weight polyethylene (UHMWPE) ballistic panel to form a UHMWPE/SSG composite body armor. Based on the ballistic impact experiment and numerical simulation, the effect of buffer plate on the protective performance, and the dynamic behavior and protective mechanism of UHMWPE/SSG were analyzed systematically. The results indicated that the flexible SSG buffer plate could quickly stiffen under ballistic impact, thereby producing the strong “Jamming” effect to resist impact deformation and absorb impact energy. Moreover, the “Jamming” effect enhanced with the increase of impact load, and showed a distribution mode that weakened from the ballistic impact area to the periphery. Compared with traditional buffer materials, more impact energy was dispersed and absorbed by SSG, but less impact energy was transmitted to the human body. Therefore, the impact strength of UHMWPE/SSG on the human body and backface signature of human body were also significantly reduced (up to 50%), which showed that the composite body armor with SSG buffer material exhibited superior protective performance.

The free surface particle velocity of NiTi target shocked with a pulsed laser beam was measured with a photonic Doppler velocimetry (PDV) system to study the dynamic deformation behavior of NiTi alloys at ultra-high strain rate of 10(6)similar to 10(7)/s. Through the analysis of the particle velocity profiles, the shock wave intensity was found to have the influence on the process of austenite-martensite transformation. Theoretical analysis of shock wave propagation showed that the first plateau in the velocity profile was caused by martensitic transformation and the second plateau if existing is caused by the subsequent plastic yielding of shock induced martensite. Residual martensite of the NiTi, which exhibited as needlelike structures, was observed in the laser shocked region. Based on the present results, and the studies by Nemat-Nasser et al. (2005), Liao et al. (2012), and Wang et al. (2013), we concluded that laser induced shock can cause the martensitic transformation as long as the laser induced shock pressure reaches a critical value. The dynamic transition stress and the dynamic tensile strength of NiTi alloys were also determined from the experimentally measured surface velocity profile. (C) 2017 Elsevier Ltd. All rights reserved.

Laser shock peening is a promising effective approach for improving mechanical properties of metallic glass. In this work, laser shock peening was employed to study the surface morphologies of metallic glasses with different toughness. Numerous localized circular-or arc-shaped structures, with the size of 5~20 μm, were observed in the shock treated surface. The number of these unique localized structures has a close correlation to the ability of metallic glass to accommodate plastic deformation. In addition, the surface morphology evolution of Zr-based metallic glasses with different crystalline degrees is also discussed, indicating that the circular-or arc-shaped structures only appear in fully amorphous system.

Selective laser melting (SLM) is a mature method in the fabrication of structures bearing static loading and small strains; however, structures experiencing large deformation under impact loading remains an issue. In this paper, the author fabricates a 316L stainless steel thin-walled circular tube with preset internal circumferential rectangular groove defects using the SLM method. MTS compression and Split Hopkinson Pressure Bar tests are performed to judge the material behavior of SLM printed 316L stainless steel, and Johnson-Cook constitutive model parameters are fitted from the results. The crushing behavior of the SLM printed tube is studied experimentally and numerically via the drop hammer test and the finite element analysis. There are two stages of the crushing process of the tube as the results show: the buckling stage and the splitting stage. The internal grooves have effects on controlling the initial buckling position and fracture position during the buckling stage. The double buckling-splitting crushing mode in one simple structure, provides a new energy absorption approach for engineering application.

Metal matrix syntactic foams have been attracting considerable attention in recent years due to their potential applications in lightweight structures and energy absorption. In this paper, CF-8 cast austenitic stainless steel with hollow Al2O3 spheres’ syntactic foams was prepared by infiltration casting technology. The effects of preheat temperature of ceramic mold and diameter of hollow ceramic spheres on the forming ability of infiltration casting of syntactic foams were investigated and the results showed that suitable preheat temperature of ceramic mold is a necessary prerequisite to produce the syntactic foams. And then, the quasi-static compressive behavior of these foams was studied by experimental and numerical simulation and the energy absorption capacity is evaluated.

Laser shock peening (LSP) has been shown to be an effective technique to improve the plasticity of metallic glasses (MGs) by inducing residual stress and structural rejuvenation. However, the microstructural evolution and viscoelastic properties of MGs after LSP remain largely unknown. In this paper, the dynamic mechanical properties of Zr-based MG before and after LSP treatment were investigated. An abnormal increase of storage modulus near the glass transition temperature (Tg) was observed after LSP. An increased deviation between the dynamic mechanical result and the quasi-point defect (QPD) theory prediction was also observed after the LSP treatment. A competing mechanism between the liquid-like region change and the pre-existing fringe-like precipitations is proposed to explain the unusual dynamic behaviors of MGs after LSP treatment. The results provide insightful information about the microstructural changes and viscoelastic properties of the MGs after LSP.

The progressive buckling behavior of a gradient grooved tube (GGT) designed for high temperature gas cooled reactor (HTR) is systematically studied in this paper. Based on the plastic hinge formation process, a sectionalized theoretical model is established to predict the quadratic upward trend of crushing force of GGT. The ordered and stable energy absorbing process of GGT is examined by low-speed axial impact using drop hammer test machine. The effect of proactive regulation parameters, including non dimensional groove width W/(h(0)+h), groove depth h(0)/h and half wavelength (H+W-h)/root Dh on buckling modes and force-displacement curve is determined by experiments and FE simulation. The non-dimensional groove width W/(h+h(0)) is determined as pi/2 according to the geometry coordination of bending grooves. The non-dimensional groove depth h(0)/h influences local buckling behavior and the critical value 0.6 for regular bending has been determined by numerical simulation. The quadratic trend force-displacement curves of GGT can be actively controlled by (H+W-h)/root Dh within a certain range less than 1.2. Free-fall impact experiments show that GGT has a better stability than perfect tube when used as an energy absorber to protect the graphite in HTR from slender control rod impact. (C) 2016 Elsevier Ltd. All rights reserved.

Loosening of threaded connection under impact loading is important in engineering structures such as the fuze on a projectile. It is difficult to predict the loosening behavior of threaded connection under impact loading due to their high nonlinearities introduced by friction and discontinuity. This paper investigated the loosening response of a threaded connection under a series of impact loadings using Hopkinson bar experiments. The loosening phase diagram was constructed by measuring the preload variation. The correlation between the preload variation and the loosening rotation angle was established by using high speed camera and digital image correlation technique. A 3D finite element model was developed to analyze the loosening mechanism of threaded connection. The transient behavior of threaded connection shows a tightening-loosening alternation process. This study provides an understanding of the loosening process of threaded connection under impact loading and is helpful for optimizing structural design under extreme conditions.

Laser shock peening (LSP) is a widely used surface treatment technique that can effectively improve the fatigue life and impact toughness of metal parts. Cr5Mo1V steel exhibits a gradient hardened layer after a LSP process. A new method is proposed to estimate the impact toughness that considers the changing mechanical properties in the gradient hardened layer. Assuming a linearly gradient distribution of impact toughness, the parameters controlling the impact toughness of the gradient hardened layer were given. The influences of laser power densities and the number of laser shots on the impact toughness were investigated. The impact toughness of the laser peened layer improves compared with an untreated specimen, and the impact toughness increases with the laser power densities and decreases with the number of laser shots. Through the fracture morphology analysis by a scanning electron microscope, we established that the Cr5Mo1V steel was fractured by the cleavage fracture mechanism combined with a few dimples. The increase in the impact toughness of the material after LSP is observed because of the decreased dimension and increased fraction of the cleavage fracture in the gradient hardened layer.

Experimental and numerical simulations were undertaken to estimate the effects of imperfect conditions on stress waves in split Hopkinson pressure bar (SHPB) experiments.ThephotonicDopplervelocimetry(PDV)mea- surement results show that the rise and fall times of an incident wave increases with an increasing inclination angle; also,thefluctuationsoftheincidentwavedisappeargradually with the increase of inclination angle. The following charac- teristics for various defects in the SHPB were obtained by numerical simulation: (1) the influence of a curved bar was negligible; (2) misalignment modestly affects the fluctuation characteristics, and bending waves were generated at this condition; (3) inclination and indentation of the impact end- surface had a great impact on the incident waves, and both of themincreasetherisetimeoftheincidentwavebyincreasing the degree of defects. In view of the results, misalignment, inclination, and indentation in SHPB experiments should be minimized.

Molecular dynamics simulation was conducted to investigate the phase transformation behaviour of nickel-titanium (NiTi, 50%-50% at.%) nanopillar under uniaxial compression at loading rates varying from 3.30x10(7) to 3.30x10(9)s(-1) and at temperatures varying from 325 to 600K. The phase transformation of NiTi was observed to be sensitive to loading rates and temperatures. The phase transformation stress of B2 -> B19 increased with increasing temperature while it was insensitive to loading rate. The phase transformation stress of B19 -> B19 '-> BCO increased with increasing strain rate and decreasing temperature. In addition, reverse phase transformation was observed during compression due to the interaction between the phase transformation of B19 -> B19 '-> BCO and the deformation twinning/dislocation slide-induced plasticity of the BCO phase, leading to different residual crystal structures after loading. Moreover, a diagram for the phase transformation behaviour of NiTi in the simulated ranges of strain rate and temperature was obtained, from which the contrary experimental observations on the phase transformation behaviour of NiTi from the studies of Nemat-Nasser et al. (Mech. Mater. 37 (2005) p.287) and Liao et al. (J. Appl. Phys. 112 (2012) p.033515) at various strain rates could be well explained.

The effects of nanosecond laser peening on Zr41Ti14Cu12.5Ni10Be22.5 metallic glass were investigated in this study. The peening treatment produced an extra-deep shock-affected zone compared to crystal metal. As opposed to the conventional shear bands, numerous arc shear bands appeared and aggregated in the vertical direction of the laser beam, forming basic units for accommodating plastic deformation. The arc shear bands exhibited short and discrete features near the surface of the material, then grew longer and fewer at deeper peened layer depths, which was closely related to the laser shock wave attenuation. An energy dissipation model was established based on Hugoniot Elastic Limit and shear band characteristics to represent the formation of an extra-deep shock-affected zone. The results presented here suggest that the bulk modification of metallic glass with a considerable affected depth is feasible. Further, they reveal that nanosecond laser peening is promising as an effective approach to tuning shear bands for improved MGs ductility.

In this study, several crashworthiness parameters of a circular, thin tube energy-absorbing structure, which is used in a high-temperature, gas-cooled reactor (HTR), are studied experimentally and numerically at various tube thicknesses, temperatures and impact velocities. The average crushing force is fundamentally dependent on strain hardening, strain rate hardening, and, particularly, temperature softening of the material. The peak forces during buckling are significantly affected by the local strain rate in the material and exhibit a decreasing trend in sequentially formed folds. Reducing the tube thickness is an effective method to weaken the average crushing force, but it does not weaken the maximum crushing force. Additionally, the stress concentration at the edge of the backplate-graphite contact surface is evaluated in detail to ensure the structural security of the energy absorber. (C) 2014 Elsevier Ltd. All rights reserved.

In this paper, an experimental method for measuring dynamic behavior of materials at high strain rates is developed. The strain and stress of a specimen are obtained using a high-precision photonic Doppler velocimetry system and a sensitive polyvinylidene fluoride thin-film sensor respectively. Based on the assumptions of one-dimensional stress state and negligible volume change at plastic deformation stage, the dynamic stress strain relationship of materials can be determined. Using Al-2024T351 alloy and copper as model materials, the method demonstrates its effectiveness in measuring dynamic behavior of materials at high strain rates. (C) 2014 Elsevier Ltd. All rights reserved.

Laser shock peening is promising surface treatment technique to improve the mechanical properties of metals material through introducing a deep compressive residual stresses region. In this work, LSP was employed to treat Zr based bulk metallic glasses (BMGs). In contrast to the work softening in BMGs, laser shock peened BMG shows significant hardening in the plastic deformed region. Furthermore, the plastic deformation region exhibits unique deformation features, which is dependent on the thickness of the samples. The deformation morphology under the impacted region was also studied. The mechanism for the hardening and formation of unique deformation features is discussed.

Bolted flange joints in fuzes undergo high acceleration during penetration, along with nonlinear responses which are forced reaction, structural vibrations, and shock effects. Vibrations of high frequencies aggregate noises and make it harder for the signal processing of fuzes. This study proposed an effective and innovative method of suppressing vibrations of high frequencies caused by impact loading. Shear thickening fluids (STFs) were stuffed into bolted flange joints. A damper of 57 vol/vol% dense silica particle-ethylene glycol suspension was inserted into gaps between the surfaces of the incident bar and the flange. Based on a modified split Hopkinson pressure bar, pulse widths, amplitudes, and structural frequencies of both impact and vibrational response regions were evaluated to examine the effectiveness of the STF damper. The amplitude and pulse width in the vibrational response region were significantly reduced, since this suspension forms jamming clusters subjected to impulses, attenuating the shockwaves. The STF fillers under various lengths of projectiles from 50 mm–400 mm were discussed to validate effectiveness. Further comparisons with epoxy resin fillers with various curing times indicated that the STF inhibited high frequency oscillations as a protector, and damped the dominant frequency of the original structure. However, experimental data showed that the transmission pulse of the incident bar was similar to joints without protection, indicating that the force transmission ratio was not affected by the fillers. These results show the feasibility of STFs as energy absorbers for vibration reduction of bolted flange joints.

In this study, microstructure and mechanical properties of an austenite NiTi alloy treated with laser shock peening (LSP) was investigated. It was found that the thickness of shock affected layer is about 250–300 μm. The surface hardness of the specimen is increased by approximately 10% after LSP. Laser induced shock introduces slightly residual compressive stress in the peened specimen. The superleastic stress–strain curves of the fully LSP processed NiTi material show no change in phase transition stress, about 100 MPa decrease in martensite yield stress, and a loss of maximum transition strain about 12% after LSP. The ultrahigh-strain-rate plastic deformation by LSP results in dislocation substructure and amorphization underneath the surface which are responsible for the hardness increase and superelastic strain loss.

The mechanical properties of laser welded joints under impact loadings such as explosion and car crash etc. are critical for the engineering designs. The hardness, static and dynamic mechanical properties of AISI304 and AISI316 L dissimilar stainless steel welded joints by CO2 laser were experimentally studied. The dynamic strain-stress curves at the strain rate around 103 s−1 were obtained by the split Hopkinson tensile bar (SHTB). The static mechanical properties of the welded joints have little changes with the laser power density and all fracture occurs at 316 L side. However, the strain rate sensitivity has a strong dependence on laser power density. The value of strain rate factor decreases with the increase of laser power density. The welded joint which may be applied for the impact loading can be obtained by reducing the laser power density in the case of welding quality assurance.

Shot peening is a widely used surface treatment method by generating compressive residual stress near the surface of metallic materials to increase fatigue life and resistance to corrosion fatigue, cracking, etc. Compressive residual stress and dent profile are important factors to evaluate the effectiveness of shot peening process. In this paper, the influence of dimensionless parameters on maximum compressive residual stress and maximum depth of the dent were investigated. Firstly, dimensionless relations of processing parameters that affect the maximum compressive residual stress and the maximum depth of the dent were deduced by dimensional analysis method. Secondly, the influence of each dimensionless parameter on dimensionless variables was investigated by the finite element method. Furthermore, related empirical formulas were given for each dimensionless parameter based on the simulation results. Finally, comparison was made and good agreement was found between the simulation results and the empirical formula, which shows that a useful approach is provided in this paper for analyzing the influence of each individual parameter.

Underwater blast injury is a common war injury during combat around islands or fighting for beachhead, and is also ordinary in underwater engineering construction blast. Adding an extra underwater blast wave protection layer and Integration with life jacket is a reasonable protection measure. Four different low mass density closed cell polyurethane foams used in life jacket have been tested by quasi-static material testing machine, and the dynamic mechanical behavior at strain rates of 10-3～10-1s-1 has been analyzed. The yield strength has been tested with Split Hopkinson Pressure Bar. Experimental results show that the compressive performance of the polyurethane foam is sensitive to the strain rates. The polyurethane foams’ yield strength shows rising trend along with strain rates increase. Under the condition of the plateau yield strength below human’s allowable pressure, the energy absorption performance of the polyurethane foams has been analyzed and in contrast.

We report an explosive boiling in a Zr-based (Vitreloy 1) bulk metallic glass irradiated by a nanosecond pulse laser with a single shot. This critical phenomenon is accompanied by the ejection of high-temperature matter from the target and the formation of a liquid-gas spinodal pattern on the irradiated area. An analytical model reveals that the glassy target experiences the normal heating (melting) and significant superheating, eventually culminating in explosive boiling near the spinodal limit. Furthermore, the time lag of nucleation and the critical radius of vapor bubbles are theoretically predicted, which are in agreement with the experimental observations. This study provides the investigation on the instability of a metallic glass liquid near the thermodynamic critical temperature.

Laser shocking peening is a widely applied surface treatment technique that can effectively improve the fatigue properties of metal parts. We observe many micro-scale arc plastic steps on the surface of Zr47.9Ti0.3Ni3.1Cu39.3Al9.4 metallic glass subjected to the ultra-high pressure and strain rate induced by laser shock peening. The scanning electronic microscopy and atomic force microscopy show that the arc plastic step (APS) has an arc boundary, 50-300 nm step height, 5-50 mu m radius and no preferable direction. These APSs have the ability to accommodate plastic deformation in the same way as shear band. This may indicate a new mechanism to accommodate the plastic deformation in amorphous metallic glass under high pressure, ultra-high strain rates, and short duration.

A new shedding phenomenon of ventilated partial cavitations is observed around an axisymmetric projectile in a horizontal launching experiment. The experiment system is established based on SHPB launching and high speed photography. A numerical simulation is carried out based on the homogeneous mixture approach, and its predicted evolutions of cavities are compared with the experimental results. The cavity breaks off by the interaction between the gas injection and the re-entry jet at the middle location of the projectile, which is obviously different from natural cavitation. The mechanism of cavity breaking and shedding is investigated, and the influences of important factors are also discussed.

The shock pressure generated by the glass confined regime in laser shock peening and its attenuation in the target material are investigated. First, the particle velocity of the target back free surface induced by laser generated shock pressure of this regime is measured using a photonic Doppler velocimetry system. The temporal profile of the particle velocity at the back free surface, where the elastic precursor is captured, manifests a powerful diagnostic capability of this newly developed photonic Doppler velocimetry system for tracking the velocity on short time scales in shock-wave experiments. Second, a coupling pressure analytical model, in which the material constitutive models of confined layers and target material are considered, is proposed to predict the plasma pressure profile at the surface of target. Furthermore, using the predicted shock pressure profile as the input condition, the dynamic response of the target under the shock pressure is simulated by LS-DYNA. The simulated back free surface velocity profile agrees well with that measured by the photonic Doppler velocimetry system. Finally, the attenuation behavior of stress waves and particle velocities in the depth of the target is analyzed, and it indicates an exponential decay. The corresponding empirical formulas for the attenuation behavior are given based on the numerical results.