High Strain Rate Compression Tests Research Articles

Strain rate sensitivity of rotating-square auxetic metamaterials

This study provides an in-depth analysis of the mechanical behavior of rotating-square auxetic structures under various strain rates. The structures are fabricated using stereolithography additive manufacturing with a flexible resin. Mechanical tests performed on structures include quasi-static, intermediate, and high strain rate compression tests, supplemented by high-speed optical imaging and two-dimensional digital image correlation analyses. In quasi-static conditions (5 × 10–3 s-1), multiscale measurements reveal the correlation between local and global strains. It is shown that cell hinges play a significant role in structural deformation and load-bearing capacity. In drop tower impact conditions (intermediate strain rate of ca. 200 s-1), the auxetic structures display significant strain rate hardening compared to loading at quasi-static rates. The thin-hinge structures maintain a Poisson's ratio of approximately -0.8, showing higher auxeticity than slow-rate compression tests. High strain rate conditions (ca. 2000s-1) activate additional deformation mechanisms, including a delayed state of equilibrium exemplified by a heterogeneous distribution of lateral strains, possibly due to stress wave interactions and inertial stresses. The study further reveals nonlinear correlations between Poisson's ratio, strain, and strain rate, indicating reduced auxeticity at higher strain rates. These observations are discussed in terms of complex wave interactions and the strain rate hardening characteristics of the base polymer.

Mechanical response and damage evolution of CF/PEEK‐reinforced Al metal‐composite hybrid structure at high strain rate

AbstractA CF/PEEK‐reinforced Al metal‐composite hybrid structure (CF/PEEK/Al) and CF/PEEK bar were subjected to high strain rate compression tests with a split Hopkinson pressure bar (SHPB). With the increase in strain rates, both CF/PEEK and CF/PEEK/Al showed the strain rate strengthening effect, and the microscopic fracture surface of CF/PEEK changes from rough to smooth. In the plastic rheological stage of stress–strain curve, CF/PEEK shows a plateau effect, while CF/PEEK/Al shows the strain weakening effect. When the strain rate exceeds 3800 s−1, the stress–strain curve exhibits a secondary wave peak. The peak values of the secondary peak in CF/PEEK/Al are larger than CF/PEEK. In terms of peak stress, yield stress, and specific energy absorption, CF/PEEK/Al outperforms CF/PEEK. Compared with other structures, the CF/PEEK/Al metal‐composite hybrid structure has certain advantages in terms of yield strength and yield strain. The finite element method (FEM) and DIC system were applied to observe the dynamic damage process in the compression impact test. It was found that in CF/PEEK/A1, the Al thin‐walled tube was damaged and cracked at the contact interface, and the inner CF/PEEK began to be damaged at the contact center with the input bar. And at 335 μs after contact began, the shear band ran through to the side of the samples.Highlights A CF/PEEK‐reinforced Al metal‐composite hybrid structure (CF/PEEK/Al) was proposed, which is superior to CF/PEEK. When the strain rate exceeds 3800 s−1, the stress–strain curves of CF/PEEK and CF/PEEK/Al exhibit a secondary wave peak. The peak values of the secondary peak in CF/PEEK/Al are larger than CF/PEEK. With the increase in strain rates, both CF/PEEK and CF/PEEK/Al showed the strain rate strengthening effect, and the microscopic fracture surface of CF/PEEK changes from rough to smooth. In CF/PEEK/A1, the Al thin‐walled tube was damaged and cracked at the contact interface, and the inner CF/PEEK began to be damaged at the contact center with the input bar. And at 335 μs after contact began, the shear band ran through to the side of the samples.

Inelastic mechanical behaviour of an additively manufactured titanium alloy: a statistical continuum mechanics theory perspective

Statistical continuum mechanics theory was used to simulate the inelastic stress of polycrystalline materials using two-point statistics. For the experimental part, the Electron beam melting (EBM) technique (Arcam EBM Q10 additive machine) was used to fabricate cylindrical rods of Ti-6Al-4V both in horizontal and vertical directions. Electron backscatter diffraction (EBSD) technique was employed to achieve statistically reliable orientation maps of vertically and horizontally printed samples. In this study, high strain rate compression tests at six different strain rates were performed, and the stress–strain curves were generated. This work is amongst the first attempts to model the microstructure of additively manufactured hexagonal alloys under compressive loadings using the statistical continuum mechanics theory. The model is capable of simulating reasonably large microstructures (statistically representative) with a practical computational cost and accuracy, unlike numerical models that require a high computational cost. It should be noted that in additive manufacturing, due to large grains and high anisotropy, microstructures used in the simulations should be large enough to include sufficient information from the material’s structure. Therefore, using finite element models would be very challenging here. On the other hand, the statistical continuum mechanics theory uses the statistical representation of the material’s characteristics for solving the governing equations with Green’s function that enables this methodology to use more microstructure characteristic information without having a noticeable change to the computational cost. The proposed model in this study uses different microstructure characteristics such as crystal grain orientation, total slip systems, active slip systems, gain morphology, and chemical phases that are obtained from EBSD images for simulating the inelastic mechanical behavior of polycrystalline materials. Although this model simulates polycrystalline materials by considering various crystal and grain information, unlike numerical methods, it doesn’t simulate the grain interactions well and we cannot study local deformation and crack nucleation sites. This model works very well for simulating the overall behavior of material instead of each individual grain and failure analysis. This model has shown a good combination of computational cost and accuracy in which the error between the simulated and experimental strength for vertical and horizontal samples was 6.21% and 8.07%, respectively.

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.

Study of dynamic mechanical behavior of 1060-H112 aluminum alloy: Experimental and numerical simulation

1060-H112 aluminum alloy with high ductility is widely used in industrial engineering. The study of its dynamic mechanical behavior has theoretical and engineering application value. In this paper, quasi-static tensile tests from room temperature to 250°C and high strain rate compression tests were conducted using a universal material testing machine and a Hopkinson compression bar. By using a hybrid test-numerical simulation method, a modified Johnson-Cook (MJC) strength model and Cockcroft-Latham (C-L) fracture criterion parameters were calibrated. Subsequently, Taylor impact tests were performed on 1060-H112 aluminum alloy specimens with a diameter of 12.66 mm and a length of 50.64 mm in the range of 176.3–483.03 m/s. Upsetting and tensile tearing were observed in the tests. A 12.68 mm diameter blunt nosed projectile impact test on 2 mm 1060-H112 aluminum alloy plate was also conducted with a light gas gun system, and the speed related parameters and failure modes were obtained. Finally, a three-dimensional model corresponding to the test was established in ABAQUS/Explicit finite element simulation software, and the failure modes of the Taylor rod and the velocity parameters and failure modes of the target impact test were predicted. The results show that the MJC strength model and the C-L fracture criterion can predict the experimental results of the two tests accurately. It shows that the MJC strength model and C-L fracture criterion have high accuracy, and they will play an important role in the application of 1060-H112 aluminum alloy in industrial engineering.

Modelling of friction stir welded AA2139 aluminium alloy panels in tension and blast

Predicting the performance of welds under blast loading is challenging, particularly in alloys where the weld region is associated with significant material property gradients. This situation arises for friction stir welds of aluminium alloy AA2139, which represents an example case of a high strength aluminium alloy. To address this problem, a novel multiscale model has been developed that captures the effect of welding on the microstructure and links this to the constitutive material behaviour. The properties of the local weld regions are determined by generating equivalent microstructures in specimens of sufficient size to perform representative tensile and high strain rate compression tests. In this way, the local material properties can be obtained based on the fundamental controlling microstructure, independent of the weld configuration. The constitutive behaviour informs a finite-element model at the macro-scale in which material property gradients arising from microstructural changes are captured. The model has been demonstrated to accurately predict the local strain evolution across the weld zone and demonstrates that strain localization occurs in the heat affected zone region for both cross-weld tensile tests and air blast loading. It is shown that the gradual change in strength between weld zones must be correctly accounted for to predict the correct strain localization behaviour. The work highlights the importance of an accurate description of the variation in local material properties in determining the response of structures under blast loading.

Precipitation behaviour in an Al-Zn-Mg-Cu alloy subjected to high strain rate compression tests

A peak aged (T6 heat treatment: 743 K /1.5 h + 393 K/24 h) Al-Zn-Mg-Cu alloy was subjected to compression tests at different strain rates from 1.0 × 10 −3 s −1 to 5.0 × 10 3 s −1 . The yield strength of the Al alloy is increasing with strain rates from 1.0 × 10 −3 s −1 to 4.0 × 10 3 s −1 and is decreasing with increasing strain rate from 4.0 × 10 3 s −1 to 5.0 × 10 3 s −1 showing strain rate sensitivity (SRS). A large number of Al 3 Zr, GP zones, η’, T (Al 2 Mg 3 Zn 3 ) and S (Al 2 CuMg) precipitates are present in the Al alloy after compression at 1.0 × 10 −3 s −1 and Al 3 Zr, η’, η, T and S phases are dominant after high strain rate compression from 3.5 × 10 3 s −1 to 5.0 × 10 3 s −1 . The precipitates average size is increased after compression at 1.0 × 10 −3 s −1 . High strain rate compression leads to the growth of precipitates and overlapping. Coarse precipitates are mainly found along the grain boundaries and dislocations. The increasing SRS with increasing strain rates up to 4.0 × 10 3 s −1 owes to the growth of precipitates which are hindering dislocations motion and generating more dislocations. Increasing strain rates to 5.0 × 10 3 s −1 , the coarse precipitates are no longer effective in pinning dislocations, leading to a decrease in yield strength and a negative SRS value. • The precipitates average size is increased after compression at 1.0×10 -3 s -1 . • The growth of precipitates and overlapping are dominant after high strain rate compression. • Coarse precipitates are formed either in the vicinity of dislocations or across the grain boundaries. • The SRS increases with increasing strain rates from 1×10 -3 s -1 to 4.0×10 3 s -1 and becomes negative after 5.0×10 3 s -1 .

Texture evolution during high strain-rate compressive loading of maraging steels produced by laser powder bed fusion

Dynamic mechanical behavior and texture evolution in maraging steels, manufactured using the laser powder bed fusion (LPBF) technique, was investigated at strain rates of 2000 s −1 , 2300 s −1 , 3200 s −1 for the as-built samples, and 650 s −1 , 680 s −1 , 950 s −1 for the heat-treated samples. Split-Hopkinson Pressure Bar (SHPB) was used for high strain rate compression tests. The texture of the deformed samples was measured using the electron backscattering diffraction (EBSD) technique. A strong strain rate dependence on the crystallographic texture of as-built LPBF-maraging steel samples was observed. However, strain rate does not have any significant effect on texture evolution in the heat treated maraging steel samples. • Maraging steels rod samples were produced using laser powder bed fusion technique (LBBF) and subjected to heat treatment at 490 °C for 6 h. • High strain rate compressive behavior of LPBF-maraging steel samples in as-built and heat treat conditions were investigated by Split Hopkinson pressure bar apparatus. • Texture of deformed as-built and heat-treated samples were studied by electron backscattering diffraction technique (EBSD) where significant texture development in as-built samples was observed and no texture change for heat-treated samples was seen.

A pseudo-strain energy density function for mechanical behavior modeling of visco-hyperelastic materials

In this paper, a constitutive phenomenological framework is proposed for the mechanical behavior of the visco-hyperelastic materials. This phenomenological framework is based on the concept of developing the schemas of the classical linear theory to the nonlinear regime. For this purpose, at first, the structure of the constitutive law of a viscoelastic material is constructed in the linear behavior case. In this construction, it can be observed that the stress can be decomposed into the sum of the elastic part and viscous part with a time function. In generalization of the rebuilt framework of the linear theory to a nonlinear regime to model the mechanical behavior of visco-hyperelastic materials, a pseudo-strain energy density function including elastic and viscous parts is assumed. The hyperelastic behaviors of the developed nonlinear model is organized via a Hookean-type constitutive equation in terms of the power law and exponential strain measures which have a high flexibility for capturing the mechanical behaviors with different trends from the S-shaped to J-shaped forms. The time part of the developed framework is proposed such that it can model the long-term memory fading principle. For this purpose, the new weight functions are used such that they can capture the rate behavior and recognize the relaxation curves with different starting points from stretch and rate of stretch. Finally, in order to do the comprehensive evaluation of the performance of the proposed model, the high strain rate tensile and compression tests, separately and simultaneously; and also, the relaxation tests at different levels from strain and strain rate beside the moderate rate tensile tests are examined to calibrate the model for different mechanical behaviors. It is observed that there is a good accordance between the test data and predictions of the model for the mechanical behavior of visco-hyperelastic materials.

An experimental investigation into mechanical behaviour of Basalt PEI laminates at various strain rates

High temperature blast and ballistic resistant light weight fibre reinforced plastic composites are very much in demand. Accordingly, the present investigation is focused on the quasi-static and high strain rate behaviour of Basalt/Polyetherimide (PEI) composite. Nine different types of composite samples were fabricated by compression moulding. The moulding pressures and temperatures were varied in the range of 15–25 bars and 325 °C−375 °C, respectively. Quasi-static testing was done at the rate of 3 mm/min to establish the optimum moulding parameters. High strain rate compression tests in the strain rate range of 900/s−3200/s were performed on the optimum moulded composite specimens. High strain rate testing revealed that 40 layered Basalt/PEI composite moulded at 25 bar pressure and 350 °C attained highest stress of 932 MPa whereas, the same was found to attain 446 MPa under quasi-static loading conditons. Thus, establishing the rate depedency of Basalt/PEI composite. Damage studies revealed that Basalt/PEI undergoes brittle damage, typically resulting in shear plane angle of 42°−45°. SEM micrographs revealed brittle fibre failure and matrix free fibre bundles, indicating the importance of process parameter optimiation.

Incorporation of the effect of strain rate in Mie-Gruneisen equation of state for polyethylene

Volume change versus pressure is expressed through an equation of state (EOS) such as the well-known Mie-Gruneisen equation. Equation of state is an essential requirement to be defined for numerical simulation of high rate events such as impact. All EOSs have some coefficients which are identified by experiment and are usually considered constant and strain rate independent. In this study, the effect of strain rate on the coefficients of Mie-Gruneisen equation is obtained for polyethylene by experiment and numerical simulation. The low and high strain rate compression tests are conducted using Instron testing machine and Hopkinson bar, respectively. The load-displacement and load-volume change curves are obtained from the experiments. The strain rate dependent constants of Mie-Gruneisen equation of state are obtained through a combined experimental/numerical/optimization technique. The compression test is simulated using Ls-dyna hydrocode. The results show that the coefficient γ is not affected by strain rate but the coefficients C and S1 are severely strain rate dependent. The latter varies with strain rate in a linear fashion and the former varies cubically with strain rate.

Experimental Study and Numerical Model of Spruce and Teak Wood Strength Properties Under Compressive High Strain Rate Loading

Spruce and teak wood as anisotropic materials have complex behavior, particularly in the relationship between strain-rate and strength. High strain-rate compression tests between 590 s-1 and 3300 s-1 were carried out using two types of split Hopkinson pressure bar (SPHB) in order to measure the behavior of the wood along three principal axes with respect to fiber direction and growth rings. Numerical simulation using finite element software of the wood materials under high strain rates was performed and showed results with only a difference of 10% to the experimental results. The strain rate affects the strength of materials. In this case, it follows the power function, which means the higher the strain rate, the stronger the material.

Effect of Stress-Relieving Heat Treatment on the High Strain Rate Dynamic Compressive Properties of Additively Manufactured Ti6Al4V (ELI)

A study was undertaken on the compressive high strain rate properties and deformation behaviour of Direct Metal Laser-Sintered (DMLS) Ti6Al4V (ELI) parts in two separate forms: as-built (AB) and stress relieved (SR). The high strain rate compression tests were carried out using a Split Hopkinson Pressure Bar test system at ambient temperature. The average plastic strain rates attained by the system were 400 s−1 and 700 s−1. Comparative analyses of the performance (flow stresses and fracture strains) of AB and SR specimens were carried out based on the results obtained at these two plastic strain rates. Microstructural analyses were performed to study the failure mechanisms of the deformed specimens and fracture surfaces. Vickers microhardness test values were obtained before and after high strain rate compression testing. The results obtained in both cases showed the strain rate sensitivity of the stress-relieved samples to be higher in comparison to those of as-built ones, at the same value of true strain.

A Mechanism for the Evolution of Hot Compression Cracking in Inconel 625 Alloy Ingot With Respect to Grain Growth

Hot compression cracking of as‐cast Inconel 625 alloy is investigated utilizing high temperature and high strain rate compression tests. The formation mechanism of the hot crack is systematically investigated based on grain growth and crystal slipping. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) analysis are used to characterize the crack morphology and microstructure. SEM results showed that the hot cracking during hot compression of Inconel 625 alloy is determined to be quasi‐cleavage fracture. The formation of carbide (NbC) plays a key role in the formation and propagation of the hot crack. The various slip systems activation strongly contributes to the overall growth of the dynamic recrystallization (DRX) grains. In addition, subgranular boundary formation within the grown grains is attribution to various slip systems activation in different local regions. The initiation and propagation of the hot compression cracks in Inconel 625 alloy depend on the development of the subgranular boundary within the grown grain.

High strain rate compression testing of intra-ply and inter-ply hybrid thermoplastic composites reinforced with Kevlar/basalt fibers

In this study, the influence of hybridization on the compression response of thermoplastic matrix-based composites under high strain rate loading was investigated. The intra-ply and inter-ply hybrid composites were manufactured with Kevlar/Basalt yarns as the reinforcements with Polypropylene as a matrix. Cylindrical composite specimens were laser cut from the flat compression moulded laminates. The composite specimens were loaded under high strain rate using split-Hopkinson pressure bar setup at strain rates ranging from 2815/s to 5481/s. The study revealed differences in the rate-dependent growth of peak stress, peak strain and toughness with the strain rate. Intra-ply hybrid composites with alternate weaving of Kevlar and basalt yarns exhibited highest peak stress as compared to the Inter-ply hybrid composites (alternate layers of Kevlar and basalt fabrics) and another intra-ply composite containing Kevlar in the warp and basalt in the weft direction. Whereas in inter-ply hybrid composite, with Kevlar as the loading face attained higher stress, while composite with Basalt as the loading face attained higher strain. SEM micrographs revealed that Kevlar on the loading face can bear the impact with lesser delamination as compared to the Basalt on the loading face. Damage studies revealed that Kevlar fiber surface loading results in higher stress as compared to basalt (brittle) surface loading with lower overall damage.

Effect of adiabatic heating on microstructure evolution in Ti-6Al-4V during high strain rate forging

The effect of adiabatic heating on microstructure evolution during high strain rate subtransus forging of a Ti-6Al-4V alloy having equiaxed initial microstructure was studied through experiments and modelling. Ø45 × 67.5 mm cylindrical billets with embedded thermocouples were forged at four different α+β temperatures on a Schuler 2100t screw press to evaluate the extent of adiabatic heating in different parts of the billet. In the centre of the billet, the highest temperature increase due to adiabatic heating exceeded the β transus (~1000 °C) during forging. The microstructure of the forged billets was examined for any changes in β phase volume fraction due to adiabatic heating. The forging process was then simulated in Deform 2D/3D software. High strain rate compression testing in the α+β and β temperature fields was carried out using a Phoenix forge simulator to generate input mechanical properties for the model. The effect of the billet size on the α-β phase transformation during forging and post-forge cooling is also discussed.

High strain rate compressive behaviors and adiabatic shear band localization of 3-D carbon/epoxy angle-interlock woven composites at different loading directions

When serving as a lightweight structural member in many areas, the dynamic mechanical behavior of fiber-reinforced polymeric matrix composites, especially under different strain rates, means a lot to the optimization of structure design as to high-speed impact. Under different loading rates and directions, we experimentally and numerically investigated the strain rate effect on the impact compressive behavior of 3-D angle-interlock woven composites (3-D AWCs) composed of carbon fiber and epoxy resin. Based on the factual geometrical architecture of 3-D AWC, the meso-scale model, which considers the interfacial damage between the reinforcements and matrix, was established to visually characterize the deformation history and damage morphologies of 3-D AWCs during impact compression process. Further, we performed high strain rate compressive tests on the split Hopkinson pressure bar (SHPB) apparatus integrated with a high-speed photography system for capturing images of progressive damage process to validate the proposed mesoscopic model. The stress-strain curves of various strain rates show strain rate effect varies depending on loading direction. The rate sensitivity on the compressive failure strength exists at weft direction and through-thickness direction except for warp direction. Moreover, both from images and finite element model results, the localized adiabatic shear band induced by intense plastic strain, initiates mainly from epoxy resin matrix and propagates along the interface at all three loading directions.

Dynamic stress-strain compressive behaviour of FDM made ABS and PC parts under high strain rates

This paper presents an investigation on dynamic compressive behavior of ABS and Polycarbonate (PC) parts fabricated by Fused Deposition Modelling (FDM) additive manufacturing process when such parts are subjected to high strain rate loading conditions in engineering applications. Split Hopkinson Pressure Bar was used to carry out high strain rate compression tests on cylindrical test specimens fabricated by the FDM process using different FDM process parameters. The dynamic true stress strain curves at high strain rates were compared with quasi-static curves obtained from static compression tests at low strain rates. Results of dynamic compression tests show that FDM process parameter of build style has significant influence on the dynamic response of FDM made ABS and PC parts under high strain rate loading. FDM made PC materials exhibit higher compressive stress than ABS material under static and dynamic conditions. FDM parameters also affect the static compressive strength for both materials.

Microalloyed Steels Laminated Composites Processed by the High‐Strain Rate Compression Tests

Multilayered metal‐to‐metal (MM) composites have been gathering much interest in the last decades because of its extraordinary mechanical properties. In the present study, the microstructural changes and mechanical behavior of two grades of microalloyed steels, that is, ferrite (M_F) and austenite (M_A), in the MM composite systems have been investigated and analyzed. In the present study, the Surface Mechanical Attrition Treatment (SMAT) is applied to introduce initial gradual microstructural changes in steel plates. Considerable microstructural refinement occurs as a result of the accumulated plastic deformation during the SMAT process. In the next step, specimens are again deformed by compression then stocked and compressed using drop weight test with a high strain rate of 1000 s−1, resulting in equivalent plastic strain of ϵ = 0,8.It is found that plastic pre‐deformation, significantly affected the distributions of the equivalent strains and stresses in both materials, but differently. Bonding quality of the laminated MM composites, using SEM and EBSD technique, is analyzed. The rheological properties of particular layers in the investigated M_F and M_A multilayered composites are defined using hardness measurements. Such a procedure demonstrated the work hardening gradients of the samples that are subjected subsequently to the SMAT, high‐strain rate compression, and bonding processes.

Microstructure evolution within adiabatic shear band in peak aged ZK60 magnesium alloy

Microstructure evolution in adiabatic shear band (ASB) in the peak aged ZK60 magnesium alloy cylindrical tube specimen after explosive radial compression was systematically investigated. High strain rate compression tests were performed by means of the radial collapse of thick-walled cylinder technique to achieve nominal strain rates of about 104s−1. The TEM results indicate that the elongated grains and deformed twins are the major characteristics in the boundary of the shear band. The central region in ASB was found to consists of ultrafine and equiaxed grains with a typical size of 100nm. And it was found that precipitates within ASB were significantly reduced, namely the precipitates instantaneous dissolution during adiabatic shearing. It is proposed that fine equiaxed grains within ASB are the result of rotational dynamic recrystallization during localization. The free energy difference between the precipitates and matrix provided a thermodynamic condition for the dissolution of precipitates. Diffusion rate increased due to high strain rate, high shear stress (large strain) and adiabatic temperature rise, which caused instantaneous dissolution of precipitates.