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Grain boundary (GB) strengthening of metallic materials faces limitations as grain sizes are reduced to the nanoscale, primarily due to the transition from the strengthening to the softening effects. Understanding the intrinsic mechanisms behind such pronounced transitions is essential for optimizing the mechanical properties of nanocrystalline (NC) alloys. In this work, the critical grain size responsible for this transition is determined for NC CoCrNi medium-entropy alloys using molecular dynamics simulation, and the underlying mechanisms concerning varying temperatures and strain rates are systematically investigated for the samples with the critical grain size. The transition from Hall-Petch (HP) strengthening to Inverse Hall-Petch (IHP) softening is established by decreasing the average grain sizes from 21 nm to 3 nm. The critical grain size for the current alloys is estimated to be 12 nm, and the underlying deformation mechanisms are illustrated as follows: In the IHP regime, GB-mediated deformation becomes dominant, leading to softening. Conversely, in the HP regime, dislocation slip dominates the deformation mode, contributing to strengthening. It is found that an increase in temperature from 77 K to 1100 K leads to a decrease in the average flow stress and Young's modulus. Meanwhile, the deformation mechanisms change from dislocation slip (77K–700K) to GB-mediated deformation (700 K–1100 K). Furthermore, the underlying mechanisms correlating to the critical strain rate are uncovered. It can be attributed to the shift in deformation mechanisms from dislocation slip at relatively low strain rate regimes (5 × 107 s−1-2 × 109 s−1) to dislocation drag at relatively high strain rate regimes (2 × 109 s−1-5 × 109 s−1). Our atomistic insights into the tensile response of NC CoCrNi may provide a crucial basis for designing superior properties to meet the growing demands of advanced engineering fields.

To study temperature and rate-dependent behaviors of molybdenum-rhenium (Mo–Re) alloys, compressive tests at variable temperatures and strain rates were conducted. After the same machining and thermal annealing processes, fully recrystallization happens in Mo and Mo–Re alloys with low Re content (5 wt %), while recovery occurs in Mo–Re alloys with high Re content (14 wt% and 42 wt %). The strength increases as Re content rises at all temperatures (300 °C, 600 °C and 1200 °C), meaning sufficient strengthening effect of Re. The rate-dependent behaviors, however, show different trends between Mo and Mo–Re alloys. Positive strain rate sensitivity is detected in Mo and low Re Mo–Re alloys in all temperatures, which is consistent to other research. While, near zero strain rate sensitivity is found in high Re Mo–Re alloys at 600 °C. Such an unusual phenomenon may be contributed to high stored dislocation density during mechanical processes. The stored dislocations are activated at a proper temperature to accommodate plastic flow, which weakens the influence of strain rate on strength.

The static and dynamic compressive properties of AlON ceramics were investigated using a universal testing machine and a split Hopkinson pressure bar (SHPB) device. The test results indicated that AlON ceramics exhibit nearly identical compressive strength under quasi-static loading. However, a significant strain rate strengthening effect was observed under dynamic loading. Elongated fragments were obtained when the specimens were subjected to quasi-static loading, while the fragments were finely granular for the specimens exposed to dynamic loading. During the compression of AlON ceramics, the pre-existing twins substantially impeded crack propagation and dislocation motion, and the fracture surfaces not only display conventional intergranular and transgranular fractures but also include the characteristics of secondary cracks, fragmented regions, dislocation networks, dislocation arrays, and even subgrain boundaries. Multiple dislocation motion modes and the formation of subgrain boundaries indicated a plastic behavior of AlON ceramics under high-strain-rate dynamic loading. AlON ceramics exhibit the complexity of dislocation behavior in response to impact force. [ABSTRACT FROM AUTHOR]

This study focuses on investigating the properties of a cement-based piezoelectric composite modified with 0.5 % Multi-Walled Carbon Nanotubes (MWCNTs) under varying stress conditions. Through calibration methods, a critical point named G was identified at around 60.4 MPa. Below this point, the force-electric response displayed a linear relationship, characterized by an average sensitivity of 117 pC/N. Conversely, above this point, non-linear behavior surfaced due to internal damage within the piezoelectric ceramic, leading to diminished piezoelectric properties. The frequency response characteristics highlighted efficient conversion of pressure signals to electrical signals with minimal delay, showcasing stability across diverse cyclic loading amplitudes. In addition, a dynamic compression experiment revealed sensitivity to strain rate and the notable contribution of domain switching to total strain. Moreover, an analysis of the stress–strain curve under impact load provided insights into the composite's non-linear growth and its response to strain rate variations.

Superplastic tensile test was performed on a β-rich α+β titanium alloy (SP700 alloy) sheet in the temperature range of 740–800 °C and strain rate range of 1 × 10−4-1 × 10−2 s−1. The effects of the deformation parameters on the flow curves, strain rate sensitivity (m-value), thermal activation energy (Q-value) and microstructural characteristics were investigated to elucidate the deformation mechanism. The results showed that the examined SP700 alloy exhibited excellent superplasticity with fracture elongation up to approximately 1800% deformed at 760 °C and 1 × 10−3 s−1, with the maximum m-value of 0.55. The shapes of the flow curves were dependent on the strain rate and showed a type of curves without an obvious steady-state flow at relatively high strain rates. For this β-rich α+β titanium alloy, when deformed at 740–780 °C and medium strain rates (approximately 5 × 10−3-5 × 10−4 s−1), the controlling deformation mechanism was the grain boundary sliding accommodated by dislocation slip. When deformed at 800 °C and a strain rate of 1 × 10−4 s−1, m-value decreased to 0.1 and the main deformation mechanism was dislocation slip with a high β volume fraction over 50% and average grain size over 5 μm. Meanwhile, the dominant texture changed from to or because of the grain rotation caused by the grain boundary sliding.

This study focuses on investigating the properties of a cement-based piezoelectric composite modified with 0.5 % Multi-Walled Carbon Nanotubes (MWCNTs) under varying stress conditions. Through calibration methods, a critical point named G was identified at around 60.4 MPa. Below this point, the force-electric response displayed a linear relationship, characterized by an average sensitivity of 117 pC/N. Conversely, above this point, non-linear behavior surfaced due to internal damage within the piezoelectric ceramic, leading to diminished piezoelectric properties. The frequency response characteristics highlighted efficient conversion of pressure signals to electrical signals with minimal delay, showcasing stability across diverse cyclic loading amplitudes. In addition, a dynamic compression experiment revealed sensitivity to strain rate and the notable contribution of domain switching to total strain. Moreover, an analysis of the stress–strain curve under impact load provided insights into the composite's non-linear growth and its response to strain rate variations. [ABSTRACT FROM AUTHOR]

Undeformed tungsten suffers from a brittleness that makes it unsuitable for applications at low temperatures. Cold-worked tungsten materials such as drawn wires or rolled plates can however show considerable ductility even at low temperatures. The reason for this behaviour is so far not understood. We investigated a series of potassium-doped tungsten wires that were subsequently drawn from one sintered ingot, making them chemically identical. Hence, the properties of the wires could be studied without the influence of different impurity levels. Using transient mechanical tests, namely repeated stress relaxation experiments and strain-rate jump tests, the effective activation volumes V e f f and strain-rate sensitivities m of the wires were determined at room-temperature. Based on the obtained results, it is deduced that the motion of (a 0 / 2) 〈 111 〉 screw dislocations by formation and dissociation of kink-pairs is controlling the rate of plastic deformation in all wires that show plasticity at room temperature. It is hence concluded that the ductility of drawn tungsten wires at low temperatures is not due to a change in the rate-controlling deformation mechanisms, but should be a consequence of the microstructural and textural changes during wire drawing. [ABSTRACT FROM AUTHOR]

The high-temperature deformation behavior of Ti-6.5Al-2Sn-4Zr-4Mo-1W-0.2Si alloy with lamellar starting structure was investigated by carrying a series of isothermal compression tests at temperatures of 850-1030 °C and strain rates of 0.001-10 s−1 on the Gleeble-3500 simulator. Strain-rate sensitivity exponent and deformation activation energy have been analyzed associated with the dynamic softening behaviors. Meanwhile, different identification approaches like power dissipation distribution and work-hardening derivative have been adopted to identify the softening behaviors corresponding to different deformation parameters. Then constitutive models based on the dislocation evolution have been employed to characterize the flow curves of Ti-6.5Al-2Sn-4Zr-4Mo-1W-0.2Si alloy. Finally, microstructure observation has been carried out to verify the dynamic softening behaviors occurred under different deformation conditions. [ABSTRACT FROM AUTHOR]

Despite the application of the Additive Manufacturing process and the ability of parts' construction directly from a 3D model, particular attention should be taken into account to improve their mechanical characteristics. In this paper, we present the effect of individual process variables and the strain-rate sensitivity of Onyx (Nylon mixed with chopped carbon fiber) manufactured by Fused Filament Fabrication (FFF), using both experimental and simulation manners. The main objective of this paper is to present the effect of the selected printing parameters (print speed and platform temperature) and the sensitivity of the 3D-printed specimen to the strain rate during tensile behavior. A strong variation of tensile behavior for each set of conditions has been observed during the quasi-static tensile test. The variation of 40 °C in the platform temperature results in a 10% and 11% increase in Young's modulus and tensile strength, and 8% decrease in the failure strain, respectively. The variation of 20 mm·s−1 in print speed results in a 14% increase in the tensile strength and 11% decrease in the failure strain. The individual effect of process variables is inevitable and affects the mechanical behavior of the 3D-printed composite, as observed from the SEM micrographs (ductile to brittle fracture). The best condition according to their tensile behavior was chosen to investigate the strain rate sensitivity of the printed specimens both experimentally and using Finite Element (FE) simulations. As observed, the strain rate clearly affects the failure mechanism and the predicted behavior using the FE simulation. Increase in the elongation speed from 1 mm·min−1 to 100 mm·min−1, results in a considerable increase in Young's modulus. SEM micrographs demonstrated that although the mechanical behavior of the material varied by increasing the strain rate, the failure mechanism altered from ductile to brittle failure. [ABSTRACT FROM AUTHOR]

The present paper deals with the consideration of the rate-sensitivity mechanical behavior of metallic materials, in the framework of mean field and full field homogenization approaches. We re-examine the possibility of describing properly this rate sensitivity with a simple and widely used power law expressed at the level of the slip system, and we propose a methodology to accelerate the identification of the global material constitutive law for Finite Element (FE) simulations. For such an aim, simulations of a tensile test are conducted, using a simple homogenization model (the Taylor one, used in a relaxed constraint form) and an FE code (Abaqus), both using the same single-crystal rate-dependent constitutive law. It is shown that, provided that the identification of this law is performed with care and well adapted to the examined case (rate-sensitive or insensitive materials, static and/or dynamic ranges), the simple power law can be used to simulate the macroscopic behavior of polycrystalline aggregates in a wide range of strain rate (including both static and dynamic regimes) and strain-rate sensitivity values (up the rate-insensitive limit).

In the case of aluminium structures, welding may introduce weak zones, so-called heat-affected zones (HAZ), close to the weld. Such zones should be accounted for when predicting the overall behaviour of structures since the deformation tends to localise within such weak zones leading to the initiation of failure. Whereas the effect of welding on mechanical properties such as yield stress and work hardening is known, the ductility of a HAZ and how to model it in numerical simulations remain uncertain. To address this challenge, this article presents a combined experimental and numerical study on ductile fracture modelling of an aluminium alloy AA6063-T6 exposed to complex heat treatments. The first part includes preparation of specimens and uniaxial tension testing. A Gleeble machine is used to generate well-defined heat treatments involving high peak temperatures and heating rates. Subsequently, uniaxial tension tests at low and high strain rates are conducted. The results show the effect of the complex heat treatments on strength, strain-rate sensitivity, and fracture. The numerical part of this study concerns two widely used damage models, namely the Cockcroft–Latham fracture criterion and the Gurson–Tvergaard model. Calibration of material models and a parameter study are carried out based on analyses of the tension tests. To this end, a case study is conducted elaborating on the importance of the damage parameters within an idealised heat-affected zone and plane-strain tension. The main objective of this study is to obtain a more general understanding of ductile fracture modelling of aluminium alloys exposed to complex heat treatments representative of welding. • Ductile fracture modelling of heat affected zones in an aluminium alloy. • Tensile ductility of heat-affected zones at low and high strain rates. • Dedicated Gleeble experiments generating heat treatments representative of welding. [ABSTRACT FROM AUTHOR]

Strain-rate dependence in face centered cubic high entropy alloys still remains controversial despite extensive efforts on this topic. Strain-rate sensitivity reflects underlying thermally-activated deformation and the controversy boils down to the deformation mechanism. There has been disagreement even on the experimental values of the strain-rate sensitivity and activation volume. This study reviewed and analyzed the differences in experimental values and proposed mechanisms to resolve the controversy over the nature of thermal obstacles in CrMnFeCoNi alloy. TEM study on CrMnFeCoNi supports the presence of nanoscale heterogeneity elucidating the nature of rate-controlling mechanism in the alloy.

With digital-image correlation techniques, it is now possible to measure the forming-limit diagram, FLD, of metal sheet using both strains outside (Bragard-type analysis) and inside (temporal, correlation-coefficient calculation) of a necking instability. We performed these measurements using the Marciniak and Kuczynski, MK, specimen geometry on three metals having very different strain-rate sensitivities: Zn20, a Zn-Cu-Ti alloy; a cold-rolled steel; and an AA6061-T4 aluminum alloy. The relationship between the Bragard type and temporal FLDs was very different depending on the metal's strain-rate sensitivity. For the highly strain-rate sensitive Zn20, m = 0.075, the temporal FLD was well above the Bragard type for all strain states, from uniaxial tension to balanced-biaxial deformation. In the case of the cold-rolled steel, m = 0.015, the two analyses were equivalent in balanced-biaxial deformation, but the temporal results were higher in plane-strain and uniaxial tension, by 25 and 40%, respectively. The two types of FLD curves were equivalent for all strain states for the AA6061-T4 aluminum alloy, m = zero. In addition, we found that the strain paths followed by the three metals were different for the same MK sample geometries. These differences were due to different shapes of the yield/flow loci, as confirmed based on visco-plastic self-consistent simulations. These results indicate that engineers should account for the different FLDs for positive strain-rate sensitive metals, possibly as upper and lower bounds. In addition, it appears that for metals with yield/flow loci like that of the AA6061-T4 aluminum alloy, certain strain paths between plane strain and balanced-biaxial deformation are difficult to attain when using the MK-type sample geometry. [ABSTRACT FROM AUTHOR]

Elastomeric polymer foams are widely used in sports and other protective padding applications due to their unique properties, such as excellent cushioning and relatively high‐energy absorption to weight ratio. This work investigates the mechanical and energy absorption performance of an elastomeric hybrid structure polyurea foam in response to low‐velocity impact. The examined polyurea foams are synthesized using a novel self‐foaming process that leads to the development of a semi‐closed cellular structure. The quasi‐static response of the foam is first characterized by measuring the global stress–strain and energy absorption characteristics. The evolution of the foam's Poisson's ratio is also characterized by in situ digital image correlation (DIC) measurements. The same properties are also studied in dynamic loading conditions by subjecting the foam samples to controlled impact tests. A strain‐dependent rate sensitivity parameter is used to quantify differences between the quasi‐static and dynamic strength and energy absorption responses of the foam. The examined foam shows significant enhancement in strength at increased strain rates while retaining its excellent energy absorption capacity. This unique characteristic of the examined foam is discussed in terms of the concurrent effects of entrapped gas and the rate sensitivity of the parent polymer. [ABSTRACT FROM AUTHOR]

Nanocrystalline nickel, having a grain size of about 40 nm, was fabricated by electrolytic deposition and strain-rate jump tests were performed at temperaures between 77 and 473 K to obtain the strain-rate sensitivity m and the activation volume V ∗ . The values of m changed from about 0.05 to about 0.005 with increasing stress, indicating that the deformation is governed by the motion of glide dislocations. V ∗ increased as the temperature increased from 77 to 200 K, but decreased with further increase in temperature. It is concluded that the rate-controlling deformation mechanism in nanocrystalline nickel is temperature sensitive and changes from forest dislocation cutting to dislocation bowing-out and depinning from grain boundaries as the temperature increases. [ABSTRACT FROM AUTHOR]

The excellent mechanical properties of Ni-based superalloys rely on their unique two-phase microstructure. Here we probe the influence of interfacial dislocation network on strain-rate sensitivity in Ni-based single crystal superalloys. The interfacial dislocation could generate a weak long-range interaction to the matrix dislocations at low stress, and react with the incoming dislocation to form junctions at high stress. The weak interaction results in the dislocation pile-up around the interphase boundary, while the strong dislocation reaction makes the gradual loss of interface coherency. With the activation energies, we predict the dependence of strain-rate sensitivity on temperature in a regime with two bounds correlated with these two kinds of interactions. The predicted values match well with that measured in experiments at elevated temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper presents a study on plastic deformation behavior of Cu–50Ta alloy at temperatures of 286–473 K and strain-rate of 0.01–6200 s ^−1 . The effects of temperature, strain-rate, and strain on the yield strength, flow stress, and strain-rate sensitivity coefficient were determined. A phenomenological model was established to predict variation of the strain-rate sensitivity coefficient for Cu–50Ta alloy under dynamic compression. A Johnson–Cook constitutive model was established to predict the equivalent stress–equivalent plastic strain relationship under extreme deformation (high temperature and strain-rate). The results showed that the plastic deformation behavior of Cu–50Ta alloy was affected by temperature, strain-rate, and strain. The material exhibited obvious strain-rate strengthening and thermal softening. As the strain-rate increased, the yield strength logarithmically increased. At a temperature of 286 K, the strain-rate increased from 0.01 s ^−1 to 6200 s ^−1 , and the yield strength increased from 543.75 MPa to 881.13 MPa. In addition, the yield strength linearly decreased as the deformation temperature increased. Under conditions of dynamic deformation, the variation of strain-rate sensitivity coefficient could be expressed as a function of strain-rate and strain. The phenomenological model accurately described the variation of the strain-rate sensitivity coefficient of Cu–50Ta under dynamic deformation conditions. The Johnson–Cook constitutive parameters, calibrated by experimental data, described the plastic deformation behavior of the alloy under high-velocity impact.

The size-dependent hardness, strain-rate sensitivity (SRS) and shear transformation zone (STZ) volume of co-sputtered Cu–Zr metallic glass films with film thickness h spanning from 1000 to 3000 nm were systematically investigated by nanoindentation creep tests at room temperature. Experimental results manifested an exceptional decrease in hardness and a monotonic increment in SRS (or a monotonic decline in STZ volume) with reducing h. The deformation mechanism could be rationalized in light of more free volume combined with smaller STZs in thinner films, which likely trigger the strain-softening behavior. Moreover, STZs and deformation-induced devitrification promote the homogeneous-like plastic flow.

Modern nanoindentation devices are capable of dynamic experimentations, which allow us to exploit instrumented hardness tests extensively. Beside the assets of recording mechanical properties continuously over displacement, there are ongoing debates whether the superimposed force alters the material’s hardness. We will show for a broad range of materials that significant hardness differences are noted between dynamic and static tests, even for large displacements. Those mainly result from a changing indentation strain-rate during the hold segment at peak load. This fact must be implicitly considered in studies using static indentation tests to guarantee comparability of obtained nanoindentation hardness values and derived quantities.

A theoretical analysis method is developed to study dynamic responses of clamped strain-rate-sensitive sandwich beam subjected to impulsive loading based on Cowper and Symonds constitutive model. The front sheet of the sandwich beam is normally loaded by a uniformly distributed impulse. The effect of strain-rate sensitivity is characterized by an influence factor independent of time history but related to initial impact kinetic energy. The relationship between this influence factor and initial kinetic energies is analyzed for three different materials. The dynamic response process of sandwich beam is divided into two stages: core compression stage and overall beam deformation stage. The strain-rate sensitivity of materials is considered only in the second stage of dynamic response analysis. From the comparison of the results of sandwich beam with that of monolithic beam, we can conclude that this method of introducing influence factor to characterize the strain-rate-sensitive effect is valid to analyze the dynamic responses of impulsively loaded sandwich beams. [ABSTRACT FROM AUTHOR]

Strain-rate dependence in face centered cubic high entropy alloys still remains controversial despite extensive efforts on this topic. Strain-rate sensitivity reflects underlying thermally-activated deformation and the controversy boils down to the deformation mechanism. There has been disagreement even on the experimental values of the strain-rate sensitivity and activation volume. This study reviewed and analyzed the differences in experimental values and proposed mechanisms to resolve the controversy over the nature of thermal obstacles in CrMnFeCoNi alloy. TEM study on CrMnFeCoNi supports the presence of nanoscale heterogeneity elucidating the nature of rate-controlling mechanism in the alloy. Strong temperature dependence of flow stress, high strain-rate-sensitivity and small activation-volume are key features for deformation of CrMnFeCoNi. TEM analysis supports presence of nanoscale heterogeneity acting as the rate-controlling barrier. [ABSTRACT FROM AUTHOR]

The mechanisms of deformation during the dynamic in-plane compression of [ 0 o / 90 o ] n (cross-ply) ultra-high molecular weight polyethylene (UHMWPE) fibre composites with polymeric matrices have been investigated for strain rates in the range 0.01 s − 1 to 4000 s − 1 . The measured strain rate sensitivity was mild for strain rates less than about 100 s − 1 , but increased sharply at higher rates. X-ray computed tomography and optical microscopy revealed that over the range of strain rates investigated here, the deformation mechanism was kinking (micro-buckling) of the plies with a kink band width of about 1 mm. Ply delamination was also observed, but only during softening phase of the response after the peak strength had been attained. To gain a mechanistic understanding of the observed strain rate sensitivity, finite element (FE) simulations were used to model the compression experiments. For these calculations, each specimen ply was explicitly modelled via a pressure-dependent crystal plasticity framework that accounts for the large shear strains and fibre rotations that occur within each ply in the kink band. Calculations were conducted in the limits of perfectly-bonded and completely un-bonded plies. Good agreement between measurements and predictions was obtained when plies were assumed to be perfectly bonded, confirming the hypothesis that ply delamination plays a small role in setting the peak strength as well as the compressive response of the composite at moderate levels of applied strain. The calculations also show that misalignment of the specimen between the compression platens strongly influences the compression response and especially the initial stiffness. Importantly, the FE calculations reveal that over the range of strain rates investigated here, inertial stabilisation has a negligible contribution to the strong rate sensitivity observed for strain rates above 100 s − 1 and that this sensitivity is primarily associated with the strain rate sensitivity of the polymeric matrix. • In-plane dynamic and static compressive deformation of UHMWPE composites occur by ply level kink band formation. • Strong strain rate sensitivity is observed for strain rates in excess of 100 s−1. • FE simulations using a ply-based pressure-dependent crystal plasticity capture the compressive response with good accuracy. • Delamination plays a minor role in influencing the compressive response, at-least at moderate levels of strain. • The composite strain rate sensitivity is primarily governed by the matrix rate sensitivity rather than the inertial effects. [ABSTRACT FROM AUTHOR]

The uniaxial tensile experiments for Ti5Al2.5Sn alloy were performed at strain rates ranging from 10−3–10+3 s−1 and test temperatures of 153–873 K. Experimentally measured stress-strain responses indicate the yield strength exhibits positive strain-rate dependency, while the yield strength increases as the test temperature is decreased. To understand the thermomechanical coupling of dynamic plastic deformation, a specially developed single-tensile-pulse loading technique was used, and the isothermal stress-strain curves for the rates of 180 and 450 s−1 were obtained at temperatures of 203, 298 and 573 K. The plastic strain hardening measurements obtained here are essentially athermal and largely independent of strain rate, consistent with titanium and its alloys being bcc-structure-like in mechanical behaviour. Based on the experimentally obtained plastic deformation features of the alloy, the physically based Voyiadjis-Abed constitutive relationship was modified to model the dynamic tensile deformation of the Ti5Al2.5Sn alloy at low and high temperatures. [ABSTRACT FROM AUTHOR]

The paper addresses the investigation of high-strain rate compressive behavior of Al foams subjected to impact at the intermediate striking velocity ranged from 40 to roughly about 80 m/s. Relatively ductile AlSiMg foam and high-strength AlZnMg foam, whose cell walls contain numerous brittle eutectic domains, are used in the experiments. Strain-rate sensitivity for different structural kinds of Al foams is determined by comparison of the plateau stress achieved at the dynamical and quasistatic compression. Difference in the dynamical response of these Al foams is revealed and clarified based on the strain rate and inertia effects under conditions of plastic cell collapse or brittle damage of the cell wall material induced by cracking of eutectic domains. [ABSTRACT FROM AUTHOR]

A version of the model of elasto-visco-plastic deformation is developed taking into account the strain-rate sensitivity. This model leads, as a special case, to the equations of the Prandtl-Reuss-Hill flow theory resolved for stress rates. This mechanical model is used in deriving the structural relations describing inelastic deformation of spatially reinforced composite media based on an algorithm of time steps. In addition, a mathematical model is constructed for elasto-visco-plastic bending behavior of reinforced plates when the possible weak resistance to transverse shear is taken into account in the framework of the traditional nonclassical Hambardzumyan theory and the geometric nonlinearity of the problem is given in the Karman approximation. The solution to the initial boundary value problem is based on an explicit numerical cross-type scheme. The bending inelastic dynamic deformation is investigated for plane and spatially reinforced fiberglass and metal composite rectangular plates under the action of a load caused by an air blast wave. It is shown that the neglect of the strain-rate sensitivity of the composition components most commonly leads to an overestimation of the calculated deflections and the strain state characteristics of these components. It is demonstrated that for relatively thick structures the replacement of the flat reinforcement structure by the spatial structure implies a decrease in the intensity of the binder deformation by tens of percent as well as to a decrease in the plate deflections (it is insignificant in the case of metal composite structures and is on the order of tens of percent in the case of fiberglass plates). For relatively thin structures the replacement of the plane reinforcement structure by the spatial reinforcement structure does not result in a decrease in their flexibility in the transverse direction and in a decrease of the strain state characteristics of the composition components. [ABSTRACT FROM AUTHOR]

Load-controlled nanoscale deformation experiments were carried out in conjunction with post-mortem electron microscopy of nanoscale deformation region involving the use of focused-ion beam, to obtain nanomechanical insights on the strain rate sensitivity of ultrafine grained structure (UFG) and compare with the coarse-grained (CG) counterpart. The UFG steel had a grain size of ∼530 nm and spherical precipitates of ∼80 nm. Both yield strength (σ y) and ultimate tensile strength (σ UTS) increased significantly with increased strain rate from 0.0015 to 0.15 s−1, with consequent decrease in elongation-to-failure (ɛ f) during tensile straining. Nanoscale deformation experiments indicated an increase of hardness from 3.02 ± 0.05 GPa to 3.36 ± 0.05 GPa on increasing the strain rate from 0.05 to 0.5 s−1 with positive strain rate sensitivity (SRS) of 0.025, which was larger than the CG counterpart and is in striking contrast to the observed phenomena in bcc metals. From the theoretical analysis, it is envisaged that the small activation volume of ∼25 b3 (b -Burgers vector) is a consequence of combination of UFG and nanosized precipitates that prevented the movement of dislocations leading to a peculiar hardening behavior, which was responsible for the abnormal SRS and activation volume of UFG ferritic steel. [ABSTRACT FROM AUTHOR]

Highlights • SiC f /Ti17 composites matrix can be regulated to lamellar and equiaxed structure. • Equiaxed structure performances higher hardness value than lamellar structure. • Lamellar structure possesses lower m for superior deformation resistance ability. • Deformation is impeded by large amount of phase interfaces in lamellar structure. Abstract Typical lamellar and equiaxed microstructure of matrix in SiC fiber reinforced Ti17 alloy composites (SiC f /Ti17 composites) can be obtained through matrix composition tuning. The discrepancy of microstructure characteristic results in lower hardness but superior deformation resistance (lower strain-rate sensitivity exponent) in lamellar structure than equiaxed structure. The further microstructure analysis on indent impressions indicates the micro-scale deformation mechanism of lamellar microstructure is principally dominated by dislocation movement in each layer along phase interfaces, rather than in the normal direction due to existence of the large amount of phase interfaces. In contrast, higher hardness comes from dislocation pinning in uniform-distribution grain boundaries in equiaxed microstructure, while quasi-isotropy character makes it more strain-rate sensitivity than lamellar structure. [ABSTRACT FROM AUTHOR]

Abstract The temperature- and rate-dependent yielding of twinning-induced plasticity (TWIP) steels containing various carbon contents are investigated in the present work. The activation volume and the activation energy have been determined. The magnitude of these thermal activation parameters for high-carbon TWIP steels are largely different from those of conventional fcc metals, implying the fundamental role of carbon on the thermally activated dislocation activities in carbon-added TWIP steels. A constitutive model, which rationalizes yielding as the thermally assisted bowing out of dislocations under the pinning effect of carbon solutes, is proposed, and for the first time quantitatively predicts the thermal activation parameters of TWIP steels as a function of carbon content. Based on the modeling results of thermal activation parameters, the overall temperature- and rate-dependent yield stresses of TWIP steels containing various carbon contents are predicted, showing good agreements with experimental results. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Highlights • The crack initiation and velocity at high speed were successfully tested by using the modified I-SEFIM. • The crack velocity increased as the applied strain rate increased. • The fiber reinforcement significantly affected on the crack velocity in the UHPFRCs at static rates, but it slightly did at high strain rates. • The strain-rate sensitivity of UHPFRC is strongly correlated with the dynamic crack growth mechanism. Abstract Loading rate effect on crack propagation in ultra-high-performance fiber-reinforced concrete (UHPFRCs) was investigated using a pre-notched three-point bending specimen in an improved-strain energy frame impact machine (I-SEFIM) and image processing techniques. The crack velocity of up to 984 m/s and the crack initiation strain rate of up to 271 s−1 were observed. Crack velocity in UHPFRCs increased as the applied strain rate increased. Fiber reinforcements significantly affected on the crack velocity in the UHPFRC at static rates, but slightly did at high strain rates. There is a strong correlation between the strain-rate sensitivity and the dynamic crack growth characteristics of UHPFRCs. [ABSTRACT FROM AUTHOR]

Dual-level stress plateaus (i.e., relatively short peak stress plateaus, followed by prolonged crushing stress plateaus) in metallic hexagonal honeycombs subjected to out-of-plane impact loading are characterized using a combined numerical and analytical study, with the influence of the strain-rate sensitivity of the honeycomb parent material accounted for. The predictions are validated against existing experimental measurements, and good agreement is achieved. It is demonstrated that honeycombs exhibit dual-level stress plateaus when bucklewaves are initiated and propagate in cell walls, followed by buckling and progressive folding of the cell walls. The abrupt stress drop from peak to crushing plateau in the compressive stress versus strain curve can be explained in a way similar to the quasi-static buckling of a clamped plate. The duration of the peak stress plateau is more evident for strain-rate insensitive honeycombs. [ABSTRACT FROM AUTHOR]

Tensile testing is widely used for the mechanical characterization of materials subjected to superplastic deformation. At the same time, it is known that the obtained flow data are affected by specimen geometry. Thus, they characterize the specimen rather than the material. This work provides the numerical analysis aimed to study how the material flow behavior affects the results of tensile tests. The simulations were performed by the finite element method in Abaqus software, utilizing user-defined procedures for calculation of forces acting on the crossheads. The accuracy of tensile testing is evaluated by the difference between the input material flow behavior specified in the simulations and the output one, obtained by the standard ASTM E2448 procedure based on the predicted forces. The results revealed that the accuracy of the superplastic tensile test is affected by the material properties. Even if the material flow behavior follows the Backofen power law, which is invariant for the effective strain, the output stress–strain curves demonstrate significant strain hardening and softening effects. The relation between the basic superplastic characteristics and the tensile test errors is described and analyzed.

A series of plate impact tests were conducted to evaluate the effect of TiB2 on the mechanical performance of the TiB2‐B4C composites under ultra‐high strain rates. The free surface velocity of sample was monitored by the dual‐laser Doppler pin system. The dynamic response and Hugoniot equation of state of the TiB2‐B4C composites were obtained in shock and impact environments. Experimental results showed that the Hugoniot elastic limit (HEL) of the composites was increased from 14.98 to 16.91 GPa with increasing of loading strain rates. The strain‐rate sensitivity of the TiB2‐B4C composites, as well as the pure TiB2, was positive. The HEL of the composites was close to that of the matrix material (B4C), and was little effected by the additional agent (TiB2 phase) at the strain rates higher than 104 s−1. Compared with previous data, a strong relation was showed between the HEL and the wave impedance of the composites under shock wave loading. [ABSTRACT FROM AUTHOR]

Abstract High-entropy alloys (HEAs) can offer exceptional strain-rate sensitivity (SRS) due to high lattice-friction and strong solid-solution strengthening, and in some cases, low stacking fault energy. In this study, SRS of a single phase FCC HEA- Al 0.3 CoCrFeNi was investigated at two grain sizes. Due to its outstanding Hall-Petch coefficient, grain size becomes a crucial microstructural feature in determination of strength and SRS. Change in SRS due to grain refinement was derived to be inversely proportional to associated strength gain, in coarse microstructures where grain size >> dislocation forest cell size. This correlation was proven using two microstructures with grain sizes of 12 µm and 150 µm, with yield strengths of 313 MPa and 145 MPa, and demonstrated SRS of m = 0.029 and 0.064, respectively. SRS was also derived to increase linearly with strength contribution from thermal short-range obstacles. The slope reflects the maximum upper limit on SRS possible upon elimination of all other obstacles of long-range nature. This limiting value of SRS was derived to be 0.118 for this HEA. [ABSTRACT FROM AUTHOR]

Reflow soldering using a Bi-coated Sn-3.0(wt%)Ag-0.5Cu (SAC305) solder ball was successfully performed at a low peak temperature of 215 °C. The Bi shell promptly transformed into a Sn-58Bi eutectic alloy by rapid Bi diffusion into the inner SAC305 during heating, which resulted in melting at 138 °C. The bonding area on a Cu pad produced by melting increased proportionally with the initial thickness of the Bi shell. Although the overall shear force of the solder bump was lower than those of Sn-58Bi and SAC305 bumps due to the smaller bonding area, the shear force per unit area was the highest. [ABSTRACT FROM AUTHOR]

This paper demonstrates the successful printing of H13 tool steel by a selective laser melting (SLM) method at a scan laser speed of 200 mm/s for the best microstructure and mechanical behavior. Specifically, the nanoindentation strain-rate sensitivity values were 0.022, 0.019, 0.027, 0.028, and 0.035 for SLM H13 at laser scan speeds of 100, 200, 400, 800, and 1600 mm/s, respectively. This showed that the hardness increases as the strain rate increases and, practically, the hardness values of the SLM H13 at the 200 mm/s laser scan speed are the highest and least sensitive to the strain rate as compared to H13 samples at other scan speeds. The SLM processing of this material at 200 mm/s laser scan speed therefore shows the highest potential for advanced tool design. Residual stress is expected to affect the hardness and shall be investigated in future research. [ABSTRACT FROM AUTHOR]

The failure mechanism of ‘indirect tension’ is explored for cross-ply IM7/8552 carbon fibre/epoxy laminates subjected to quasi-static, out-of-plane compressive loading. The sensitivity of compressive response to strain rate and to the state of cure is measured, motivated by the hypothesis that the out-of-plane compressive strength is sensitive to the matrix shear strength. A pressure-sensitive film is placed between specimen and loading platen, and reveals that a shear lag zone of reduced compressive traction exists at the periphery of the specimen, giving rise to a size effect in compressive strength. The width of the shear lag zone reduces with increasing shear strength of the matrix. The laminates fail by the indirect tension mechanism: out-of-plane compressive loading generates tension in the fibre direction for each ply and ultimately induces fibre tensile failure. Finite element (FE) simulations and an analytical model are developed to account for the effect of matrix shear strength, specimen geometry, and strain rate on the out-of-plane compressive strength. Both the FE simulations and the analytical model suggest a recipe for increasing the through-thickness compressive strength. [ABSTRACT FROM AUTHOR]