Quasi-static Strain Rate Research Articles

Effect of microstructural evolution during melt pool formation on nano-mechanical properties in LPBF based SS316L parts

Laser Powder Bed Fusion (LPBF) based SS316L parts have a wide range of industrial applications, mainly due to their high strength and good structural integrity. However, there is a lack of nano-mechanical studies related to microstructural evolution. The SS316L cubes were examined for grain orientation and phase transformation with the help of OM and FESEM with EDX analysis. Cellular and columnar grains were substantiated by FESEM with EDX. Further, the δ ferrite and γ austenite phases occurrence was evident in XRD analysis. The orientation of grains was further examined through EBSD analysis. The microstructural transformation was substantiated by mapping based on modulus. Cellular grains showed a maximum hardness of 6 Gpa, whereas the transverse columnar grains displayed a minimum elastic modulus of 105.8 GPa. Nano-wear and scratch analysis predicted the functions caused by the occurrence of cellular dendritic microstructure, and the topographic wear and scratch tracks confirmed the soft nature of transverse columnar grains by forming more pile-up of materials. Strain Rate Sensitivity (SRS) of the SS316L specimens was analysed based on three different strain rates (4.1 ×10−5 s−1, 2.08 ×10−4 s−1 and 3.3 ×10−4 s−1). It was concluded that the LPBFed SS316L parts possessed a negative strain rate. In contrast, the simultaneous attainment of maximum Ultimate Tensile Strength (UTS) and ductility was possible in the quasi-static strain rate.

A novel fractal-statistical scaling model of rocks considering strain rate

The scaling-dependent behaviors of rocks are significant to the stability and safe operation of the structures built in or on rock masses for practical engineering. Currently, many size effect models are employed to connect laboratory measurements at small scales and engineering applications at large scales. However, limited works consider the strain rate effect. In this study, an fractal-statistical scaling model incorporating strain rate is proposed based on a weakest-link approach, fractal theory and dynamic fracture mechanics. The proposed scaling model consists of 8 model parameters with physical meaning, i.e. rate-dependent parameter, intrinsic material parameter, dynamic strain rate, quasi-static strain rate, quasi-static fracture toughness, micro-crack size, micro-crack intensity and fractal dimension, enabling the proposed scaling model to model the scaling behaviors under different external conditions. Theoretical predictions are consistent with experimental data on red sandstone, proving the reliability and effectiveness of the proposed scaling model. Thus, the scaling behaviors of rocks under dynamic loading conditions can be captured by the proposed fractal-statistical scaling model. The sensitivity analysis indicates that the size effect becomes more obvious with a higher strain rate, larger fractal dimension, smaller micro-crack size or lower micro-crack intensity. Therefore, the proposed scaling model has the potential to capture the scaling behaviors considering the thermal effect, weathering effect, anisotropic characteristic etc., as the proposed scaling model incorporated model parameters with physical meaning. The findings of this study are of fundamental importance to understand the scaling behaviors of rock under dynamic loading condition, and thus would facilitate the appropriate design of rock engineering.

An insight into mechanical response and twinning behavior of bimodal textured AZ31 magnesium alloy under quasi-static and high strain rate compression

In this work, the dynamic, quasi-static response and twinning behavior of bimodal textured Mg–3Al–1Zn alloy is comparatively investigated at room temperature. The relationship between yield stress and bimodal texture is quantified and characterized as an increase of the yield stress with the increasing of the percentage of <0002>⊥ED (extrusion direction) component; the ultimate strength under high strain rate (103 s−1) was quite higher than that under quasi-static condition (10−3 s−1); but, the yield stress was just the reverse. The competition between twin nucleation and growth is significantly affected by a coupled effect of strain rate and texture. Twin nucleation shows positive strain rate sensitivity and is largely enhanced by dynamic deformation. However, the twin growth tendency is bimodal texture and strain rate dependent: it is favorable under quasi-static condition for samples with more soft orientations, while for samples with more hard orientations, dynamic deformation promotes twin growth. It is also found that a higher strain rate can stimulate the activation of more twin variant types in bimodal textured magnesium alloys.

Strain hardening exponent and strain rate sensitivity exponent of dual-phase steels at quasi-static strain rates during tensile testing

Strain hardening exponent and strain rate sensitivity exponent of dual-phase steels at quasi-static strain rates during tensile testing

Effect of freezing conditions on the microstructure and compressive response of alumina/epoxy nacre-type composites

An attempt is made to fabricate nacre-type alumina/epoxy (cerepoxy) composites using ice-templated alumina scaffolds. A polydimethylsiloxane (PDMS) wedge with varying angles (0°, 5°, 10°, and 20°) is used to modulate the temperature gradients during the freezing. While a 2D nucleation is observed in uni-directional freezing (0° wedge angle), bi-directional freezing in non-zero wedge angles shows 1D nucleation. The lamellae thickness and interlamellar spacing increase with the increase in PDMS wedge angle. Moreover, the density of scaffolds varies along the freezing direction, with denser microstructure recorded at the bottom location (closer to the cold finger). The quasi-static and high strain rate compression testing reveals the anisotropy in the compressive mechanical properties of cerepoxy samples. The compressive strength and energy-absorbing capacity increase at higher loading rates. Finally, the present study establishes the structure-property relationship and shows that bi-directional freezing is an efficient route to design and fabricate cerepoxy composites having nacre-type microstructure.

Unveiling the Strain Rate Sensitivity of G18NiCrMo3-6 CAST Steel in Tension/Compression Asymmetry

This research was devoted to unveiling the strain rate sensitivity (SRS) of G18NiCrMo3-6 cast steel in tension/compression asymmetry. For that purpose, detailed mechanical characterization tests were conducted providing a process window covering quasi-static and medium strain rate regimes (0.001, 0.1, 10 [s−1]) in tension and compression states. Through this experimental effort, the SRS of the material could be extracted by a function of strain and the strain rate which enabled us to create a mathematical expression to easily be implemented as a state variable for constitutive material modeling. Finally, a pressure- and rate-dependent constitutive material model on the basis of the Cocks’89 yield locus definition was created using a subroutine (UMAT) file and the material parameters were verified with respect to the experimental data. The UMAT file also takes into account the tension/compression asymmetry in yielding to handle the effect of the porous media plasticity concept. The predictions of the proposed material model are in line with the experimental outputs.

Strain rate and temperature dependence of short/unidirectional carbon fibre PEEK hybrid composites

Short fibre and hybrid carbon fibre PEEK composite materials were tested in tension and compression under quasi-static and high strain rate conditions to observe the strain rate dependence. Multiple temperatures including room temperature, ＋85 and −50 °C were used to investigate the temperature dependence of the materials. The hybrid laminate comprised a consolidated short fibre core reinforced with outer UD plies in the 0°orientation to provide maximum reinforcement whilst minimising the quantity of expensive UD composite used. Under compression, the beneficial effect of the hybridisation strategy was observed for all high-strain rate testing conditions, where the hybrid laminate outperformed the response of the individual constituents in terms of strength and strain rate dependency. The outer unidirectional (UD) layers contributed to confining the short fibre core, providing superior structural integrity. Under tension, the response was dominated by the UD layers with a 288% increase in strength at room temperature over the short fibre material. However, in the high temperature quasi-static case, the strength was dramatically reduced, by 64%, due to the debonding of the UD reinforcement. This study shows the suitability of hybrid composites for impulsive applications and provides material parameters for the future design of composite structures subjected to impact events.

Effect of strain rate on the deformation of 6061-T6 aluminum alloy at cryogenic temperature

This study investigates the tensile behavior and strain rate sensitivity (SRS) of a commercial Al6061-T6 Al alloy at various temperatures (i.e., 20, −50, and −150 °C). Uniaxial tensile tests were performed at the quasi-static strain rates of 10−2, 10–3, and 10−4 s−1. A significant improvement in mechanical properties was found at the cryogenic temperature (CT), due to the enhanced work-hardening behavior during CT deformation. The SRS exponent changed from a slightly positive value at RT to a negative one as the temperature is lowered. The dislocation density was evaluated for the samples strained to 5 and 8% at 20 °C and −150 °C using X-ray diffraction to probe the strengthening mechanisms during RT and CT deformation. The dislocation density was higher for the strain rate of 10−2 s−1 at RT and 10−4 s−1 for CT. In addition, electron back scatter diffraction (EBSD) based kernel average misorientation (KAM) and coincidence site lattice (CSL) boundary distribution analysis were conducted on the initial and 8% strain-interrupted samples at both temperatures to explain the effect of temperature and strain rate on work hardening behavior. Transmission electron microscopy (TEM) was applied to investigate the dislocation structures under different strain rate and temperatures. The cryogenically deformed sample exhibited a distinctive dislocation tangling phenomenon in proximity of the grain boundary, In contrast, the room temperature deformed sample demonstrated dislocation penetration occurring at the grain boundary. The character of SRS and improved work-hardening behavior can be attributed to changing dislocation kinetics with decreasing stacking fault energy (SFE) at cryogenic temperature.

Effect of Strain Rate on Deformation Behaviors of Ti-12.1Mo -1Fe Metastable Beta Alloy

The addition of expensive elements to β titanium alloys, which are widely used in the aerospace and automobile industries, causes their price to increase. Low-cost, high-strength alloy (Ti-12.1Mo-1.0Fe) was designed in this study and the compressive behaviors of this alloy were evaluated for application to automobile parts, etc. As a result of ambient compression in the quasi-static and dynamic strain rate range (1×10&lt;sup&gt;-4&lt;/sup&gt;/s~6×10&lt;sup&gt;3&lt;/sup&gt;/s), twinning occurred at all strain rates, and in particular, adiabatic shear bands were observed, which cause cracks and fractures under dynamic strain rate conditions. In addition, when the relationship between these bands and mechanical characteristics was evaluated, an increase of compression strength and Vickers hardness was confirmed to occur, due to the strain rate hardening effect under compression loading. Some twins were formed by the deformation behavior during compression plastic deformation of the Ti-12.1Mo-1.0Fe alloy, and it exhibited excellent compression strength characteristics, showing a very high strain rate hardening effect at a high strain rate.

Additive Manufacturing Benchmark 2022 Subcontinuum Mesoscale Tensile Challenge (CHAL-AMB2022-04-MeTT) and Summary of Predictions.

This additive manufacturing benchmarking challenge asked the modelling community to predict the stress-strain behavior and fracture location and pathway of an individual meso-scale (gauge dimensions of approximately 200 μm thickness, 200 μm width, 1mm length) tension specimen that was excised from a wafer of nickel allow IN625 manufactured by laser powder bed fusion (L-PBF). The data used for the challenge questions and answers are provided in a public dataset (https://data.nist.gov/od/id/mds2-2587). Testing models against the data is still possible, although a good-faith blinded prediction should be attempted before reading this article, as the results are contained herein. The uniaxial tension test was pin loaded, conducted at quasi-static strain rates under displacement control, and strain was measured via non-contact methods (digital image correlation). The predictions are challenging since the number of grains contained in the thickness of the specimen are sub-continuum. In addition, pores can be heterogeneously distributed by the L-PBF process, as opposed to intentionally seeded defects. The challenge provided information on chemical composition, grain and subgrain structure (surface-based measurements via electron backscatter diffraction and scanning electron microscopy) and pore structure (volume-based measurements via X-Ray computed tomography) along the entire gauge length for the tension specimen. During the challenge, prediction responses were collected from six different groups. Prediction accuracy compared to the measurements varied, with elastic modulus and strain at ultimate tensile strength consistently over-predicted, while most other values were a mix of over- and under-predicted. Overall, no one model performed best at all predictions. Failure-related properties proved quite challenging to predict, likely in part due to the data provided as well as the inherent difficulty in predicting fracture. Future directions and areas of improvement are discussed in the context of improving model maturity and measurement uncertainty.

Scaling effects on the strain rate sensitivity of unidirectional and [+45/−45]s laminates under tensile loading

The effects of geometric scaling on the strain rate sensitivity of unidirectional and [+45/−45]s laminates under uniaxial tensile loading has been investigated experimentally. Two material systems, Toray T800/3900-2B carbon/epoxy unitape and Newport NB321/7781 fiberglass/epoxy fabric, were used in the study. The nominal strain rates investigated ranged from quasi-static (0.0002 s−1) to moderate strain rates of 50 s−1 across the scaled specimen geometries. The geometric scaling effects at different strain rates were quantified in terms of the Weibull modulus. At each strain rate, the average failure stress of [0]4 carbon, [0]4 fiberglass, and [±45]s fiberglass showed a declining trend with increasing specimen size. However, the percentage of the strength reduction was less significant at higher strain rates compared to the quasi-static strain rate. In contrast to the other stacking sequences, [+45/−45]s carbon specimens showed a maximum percentage in strength reduction at a high strain rate compared to the quasi-static strain rate, indicating increased scaling effect with strain rate. The magnitude of Weibull modulus ( m) for the specimens increased with strain rate indicating diminishing scaling effects, while [+45/−45]s carbon specimens exhibited an opposite trend.

A Method for Plotting Failure Envelopes of Unidirectional Polymer Composite Materials under Different Strain Rates

This article emphasizes the significance of investigating the nonlinear behavior and strength characteristics of polymer composite materials under various strain rates. The study utilizes test results of a unidirectional (UD) composite material subjected to compression at different angles relative to the reinforcement direction, using quasi-static, static, and dynamic strain rates. The analysis focused on a UD layer experiencing compressive stresses perpendicular to the fiber reinforcement and in-plane shear stresses. A novel model is presented, enabling the calculation and prediction of the strength of a UD composite under uniaxial loading at different angles to the fiber direction, considering various strain rates. The developed model facilitates the derivation of equations for the failure envelopes of UD Carbon Fiber-Reinforced Polymers (CFRPs) under quasi-static, static, and dynamic loading conditions. To construct the failure envelopes of CFRPs, it is necessary to acquire experimentally determined values of tensile and compressive strength in the direction perpendicular to the reinforcement, as well as the ultimate strength in uniaxial compression of a specimen with reinforcement at a 45° angle to the loading axis. The failure envelopes generated using the proposed model exhibit excellent agreement with experimental data, with coefficients of determination ranging from 0.864 to 0.957, depending on the deformation rate. Consequently, the developed model holds promise for predicting the strength of other UD polymer composite materials.

Thermo-visco mechanical behavior of glass fiber reinforced thermoplastic composite

Thermoplastic composite materials are being increasingly used in many domains, such as automotive, marine and aeronautics. This growing interest is due to the relatively good mechanical properties and the recycle-ability of these materials. In various heat assisted forming technologies, thermoplastic composites may be subjected to severe conditions such as high temperatures, complex strain paths and potentially significant strain rates. This is expected to highly influence the overall behavior of these materials and make the characterization of their mechanical behavior more complicated. In this paper, the mechanical behavior of discontinuous glass fiber reinforced polypropylene composite is investigated at temperatures going from room temperature (RT) to 120°C and strain rates ranging from quasi-static conditions to 10 s−1 by means of uniaxial tensile tests. The effect of material orientation is also investigated at RT and quasi-static strain rate of 0.001 s−1 pointing out a clear anisotropic behavior. The tensile properties including Young’s modulus E, ultimate tensile strength σ ult and strain at failure ɛ f are determined based on the Standard ISO-527 reporting an obvious sensitivity of the material to temperature and strain rate. Based on literature review, the phenomenological isotropic constitutive model of G’Sell and Jonas, originally designed for unreinforced semi-crystalline polymers, is selected to describe the mechanical behavior of the studied material. A modification of the original constitutive equation is proposed for a better representation of the overall behavior of the material at different temperature and strain rate conditions. Finally, the validity of this phenomenological law calibrated by means of the uniaxial tests, is evaluated for an equi-biaxial loading state close to that encountered by the material during heat assisted sheet forming process. The significant disagreement highlighted between numerical and experimental results proved that the uniaxial tensile tests are not sufficient to characterize the behavior of the material when a complex loading condition is imposed.

Dynamic mechanical properties and constitutive model of photosensitive resin specimens at different temperatures

In order to reveal the dynamic mechanical properties of resin-molded parts prepared from photosensitive resin composition at different temperatures, four typical service temperatures (26 °C, 50 °C, 70 °C and 90 °C) were selected, and the mechanical properties of photosensitive resin specimens under quasi-static and high strain rate (1200 s−1, 1500 s−1 and 1800 s−1) loading were tested by universal material testing machine and split Hopkinson pressure bar (SHPB) experimental device. The stress–strain data of the material were obtained. Results show that the stress of photosensitive resin specimens decreases with the increase of temperature under quasi-static and high strain rate loading conditions, reflecting a certain temperature softening effect. Two typical stages of strain softening and strain hardening exist in the quasi-static compression process of the specimens at room temperature, while the specimens only exhibit strain hardening at 50 °C, 70 °C and 90 °C. Under dynamic loading, the elastic modulus, peak stress and peak strain of the photosensitive resin increase with the increase of the strain rate, reflecting an obvious effect of strain rate strengthening. The nonlinear thermo-viscoelastic constitutive model can better describe the mechanical behavior of the material under high strain rates and service temperatures, and the experimental values are in good agreement with the fitted values of the model. The results can provide theoretical model and method support for the design and development of resin-based materials and the optimization of their mechanical properties.

Experimental study on dynamic behaviour of High Strength Low Alloy Steels at cryogenic temperatures

The dynamic behaviour of the High Strength Low Alloy (HSLA) Steel before and after heat treatment was examined experimentally by performing quasi-static and high strain rate testing at room temperature and cryogenic temperatures. The high strain rate tensile testing of the material was performed on a Split Hopkinson Tensile Bar (SHTB) setup. A customized liquid nitrogen chamber equipped with UTM and SHTB was used to perform the cryogenic temperature testing. The stress–strain response of the material under different loading conditions was recorded and a modified Johnson–Cook material model was derived. It was observed that the flow stress of the material increased when tested at a high strain rate and it further increased at a cryogenic high strain rate loading environment. The microstructure of the material before and after heat treatment was also observed to identify the dominant phase change in microstructure contributing towards improvement in strength after heat treatment. The failure analysis of each category of the samples was examined under the Scanning Electron Microscope (SEM) after the testing. The formation of small and deep dimples for as-received samples and bigger and shallow dimples for heat-treated samples distinguished the type of failure between the quasi-static, high strain rate and cryogenic high strain rate material testing.

MECHANICAL ANISOTROPY OF COLD-ROLLED ST-37 STEEL PLATE UNDER HIGH STRAIN RATES LOADINGS

Cold rolling process in metal could increase its strength and hardness, but also induces mechanical anisotropy. This is caused by unresolved plastic strain and microstructural changes caused by plastic deformation exerted by pair of rolls at a relatively low temperature. This research aims to provide understandings on anisotropic and strain rate sensitive behaviour of St-37 mild steel. The microstructure of rolled and unrolled St-37 plate were observed in 3 directions relative to the rolling direction. The cylindrical specimens were then prepared from rolled plate with 33.3% reduction in the orientation of 0o, 45o, and 90o relative to the rolling direction. Specimens were also prepared from unrolled plate as comparison in the same directions. These specimens were then tested in compression loading, first in quasi-static condition, and then by using Split-Hopkinson Pressure Bar (SHPB) at the strain rate of 1200 s-1. The experimental results in the form of stress-strain curves are used to obtain the parameters of Simplified Johnson-Cook viscoplastic model. The anisotropy of yield strength in rolled specimens could be seen on both quasi-static and high strain rate condition, where the highest strength found on the specimens perpendicular to the rolling direction. In addition, the effect of cold rolling to the strain rate sensitivity of the material were also captured successfully in this study, where specimens from rolled plate show less strain rate sensitivity compared to the unrolled specimens.

Carbon-induced negative strain-rate sensitivity in a quenching and partitioning steel

The present work reports an abnormal negative strain-rate sensitivity (SRS) in a room-temperature quenching and partitioning (RT-Q&P) steel. The mechanisms responsible for such negative SRS were systematically investigated. Continuous and interrupted tensile tests at quasi-static (10− 3s − 1) and high strain rate (600 s − 1) were performed. It is found that the flow stress after yielding exhibits abnormal negative SRS. Interestingly, similar martensitic transformation occurs at both 10− 3s − 1 and 600 s − 1, implying that transformation induced plasticity (TRIP) effect is not responsible for the negative SRS. The designed interrupted rate-change tests and interrupted high-strain-rate load-unload-load tests were performed to reveal the significant effect of interstitial carbon atoms on the negative SRS. Moreover, the Cottrell atmospheres with carbon atoms segregated at dislocations were further confirmed by atom probe tomography (APT). The Cottrell atmosphere can be continuously rebuilt during deformation at quasi-static strain rate, but cannot be rebuilt at high strain rate since there is no sufficient time for carbon atoms to diffuse to the high-velocity dislocations. The lack of Cottrell atmospheres at high strain rate results in (1) absence of extra carbon dragging force and (2) a lower dislocation density, both of which contribute to a lower flow stress at high strain rate and therefore the negative SRS.

Strain-rate sensitivity of cement composites: Insights from field's metal nano-inclusions

In this study, we investigated the effects of Field's metal (FM) nanoparticles (NPs) on the mechanical behaviour of cement paste under quasi-static (QS) and dynamic strain rates. We found the highest mechanical improvement under QS loading with 1 % FM dosage, beyond which boosting the dosage diminished its contribution to the mechanical performance. Conversely, under dynamic loadings, boosting the dosage from 1 % to 4 % FM significantly and progressively enhanced the overall mechanical performance. These results indicated that the mechanical impacts that ensue from the microstructural modification effects of FM differ with dosage and strain rates. Under QS loading, the pore-refining effect of the NPs was predominant, whereas, under dynamic loadings, the reinforcing effect of the NPs was the primary factor. Furthermore, by comparing the effects of FM and nanosilica NPs, our findings underscored the pronounced effect of the nanomaterial physical and mechanical properties on the strain-rate sensitivity of cementitious nanocomposites.

Study on master curve of tensile strength of thermal aging composite modified double base propellant

In order to study the mechanical properties of thermal aging composite modified double-base propellant (CMDB), the tensile properties of CMDB propellant were investigated at four temperatures (323, 293, 273, 253 K) and different strain rates (3.3 × 10-5- 3.3 × 10-2 s−1). Uniaxial tensile tests were carried out on CMDB propellant samples with different aging time (0,10,20,35,50,65,80,100 d). The mechanical properties of CMDB propellant after aging were studied. The results show that the maximum tensile strength decreases with the aging time, and the strength value fluctuates within a certain range (Δσ<30 %). The master curve of the maximum tensile strength of CMDB propellant was obtained by using the time-temperature equivalence principle (TTSP), and the master curve equation of the aging strength was established. The equation can be used to predict the maximum tensile strength of CMDB propellant with different aging time in the range of quasi-static strain rate.

Development of the Johnson-Cook flow stress and damage parameters for the impact response of polycarbonate: Experimental and numerical approach

The Johnson and Cook (JC) flow stress and damage model parameters of a polycarbonate (PC) plate were determined by the mechanical tests and numerical simulations of the tests. The experimental tests included quasi-static and high strain rate tension and compression, quasi-static notched-specimen tension, quasi-static indentation (QSI), low velocity impact (LVI) and projectile impact (PI). Initially, five different quasi-static flow stress-strain equations were extracted from the experimental and numerical tests. The flow stress equation determined from the experimental average true stress-true strain curve well agreed with the effective stress-strain obtained from the quasi-static numerical tension test. The numerical QSI force-displacement curve based on the experimental average true stress-true strain equation was further shown to be very similar to that of the experiment. The LVI and PI test simulations were then continued with the experimental average true stress-true strain equation using five different flow stress-strain rate relations: JC, Huh and Kang (HK), Allen-Rule and Jones (ARJ), Cowper-Symonds (CS) and the nonlinear rate approach (NLA). The rate sensitivity parameters of these relations were extracted from the quasi-static and high strain rate tests. The LVI test simulations using the stress-strain rate relations exhibited force-displacement curves higher than those of the experiments. The detected almost no strain rate sensitivity in the LVI tests was ascribed to low strain rate dependency of the flow stress at these intermediate strain rates and large strains involved. On the other side, all the stress-strain rate relations investigated nearly predicted the experimental damage types: dishing at 100 and 140 m s−1 and petalling at 160 m s−1, except the CS relation which predicted the fracture of the plate at 140 m s−1. The experimental average projectile exit velocity at 160 m s−1 was further well predicted by the used stress-strain rate relations while the experimental average petal thicknesses were under estimated by the models. The absorbed energy at 160 m s−1 PI test was determined 1.6 times that of the QSI test, which proved an increased energy absorption capability of the tested PC at the investigated impact velocities.