Direct Impact Hopkinson Pressure Bar Research Articles

Investigating the high-impact deformation behavior of induced-nano precipitation hardened in718 alloy

Superalloys are the preferred materials for modern high-rotational complex components due to their exceptional ability to maintain critical properties such as strength, oxidation/corrosion resistance, even under extreme temperatures and dynamic impacts. This study investigates the dynamic impact response of nanoprecipitation-hardened 718 alloy (NPH-718 alloy) under various loading conditions. The wrought nickel-based NPH-718 alloy was transformed into a nanostructured state through a precise heat treatment process involving controlled cooling rates (28℃∙s‒1 to 30℃∙s‒1). A mechanical compression test, leading to failure, was employed to assess the alloy's ability to withstand dynamic impacts. The compressive dynamic behavior of the alloy at high strain rates (4000 s‒1 to 7500 s‒1) and temperatures ranging from ‒180℃ to 750℃ was evaluated using a custom-built direct impact Hopkinson pressure bar apparatus. The flow data obtained for NPH-718 alloy exhibited sensitivity to thermally activated processes. Consequently, as the temperature increased at a constant high loading rate, both flow stress and adiabatic effect increased. Conversely, at a constant deformation temperature, the flow characteristics exhibited an increase as the loading rate decreased. This study establishes key trends in the flow stress, adiabatic effect, temperature, and the strain rate sensitivities of NPH-718 alloy, offering valuable insights for design and performance evaluation purposes. It underscores the significant influence of temperature and strain rate on the flow behavior of NPH-718 alloy, further solidifying its reliability in demanding applications.

Performance of additively manufactured polylactic acid (PLA) in prolonged marine environments

Additive manufacturing has seen rapid growth in a variety of sectors due to its advantages in cost and lead-time reductions. However, there is still skepticism regarding the ability for additively manufactured materials to withstand extreme conditions. In this report, AM polylactic acid (PLA) was submerged in seawater at several temperatures to deduce long-term effects of marine environments for these materials. An accelerated aging model was developed to predict life of PLA over years with only weeks or days of exposure. Samples were submerged in artificial seawater between one and ten weeks with temperatures ranging from 22 °C to 60 °C. Several mechanical tests were then conducted on the submerged samples, such as quasi-static tensile, nanoindentation, and direct impact Hopkinson pressure bar tests to deduce different mechanical properties after aging. For the seawater samples, diffusion data was collected to measure water absorbed by the specimens. An Arrhenius relationship was then studied, and a model for accelerated aging was developed with resulting acceleration factors. Results indicated clear aging in PLA over the timeframe studied, and it was deduced that the acceleration factors of aging between 22 °C and 40 °C, 22 °C and 50 °C, and 22 °C and 60 °C, were 1.91, 3.56, and 5.57, respectively. Results demonstrated a clear aging in PLA; it can be predicted that PLA will lose 0.5% ductility at around 5166 h (30.75 weeks) and will suffer a 10 MPa drop in yield strength at around 2084 h (12.4 weeks) while submerged in 17 °C seawater.

Effects of cell size vs. cell-wall thickness gradients on compressive behavior of additively manufactured foams

Graded foams show great potential in impact protection and blast resistance applications but a limited experimental study on their compressive behavior has been reported. Thus, this paper investigates the gradient effect on the compressive behavior of foams experimentally and numerically. The cell size graded foam (SGF) and the cell-wall thickness graded foam (TGF) are both built by the Voronoi method and then fabricated by the additive manufacturing technique. Meanwhile, uniform foams with different cell sizes (SUF) and cell-wall thicknesses (TUF) are also produced to be compared with graded foams. Quasi-static and dynamic compressive tests are conducted respectively by using a universal testing device and a direct-impact Hopkinson pressure bar. Experimental results reveal that SGFs deform continuously from lower to higher density regions and hence possess a gradually increasing plateau stage in the stress-strain curve. TGFs show three stepwise plateau stages because of their three layers with different cell-wall thicknesses. Moreover, SUFs and TUFs with constant relative density present similar mechanical properties despite their different cell morphologies. Several empirical formulae are proposed for uniform foams and fit well with experimental data. Further simulation is verified by experimental results and indicates that TGFs with adequate layers also possess a gradually increasing plateau stage just like SGFs. It means that the strength of each layer in a graded foam depends on its local density rather than cell morphology.

Exploring the dynamic response and energy dissipation capacity of functionally graded EPS concrete

In this study, the concept of functionally graded expanded polystyrene (EPS) concrete is developed to improve the protective capabilities offered by lightweight concrete. First, laboratory tests including Split Hopkinson Pressure Bar and Direct-impact Hopkinson Pressure Bar were conducted to explore the dynamic responses of four representative EPS graded modes. Second, a mesoscopic model was built and verified based on the experimental data to consider the loading scenarios which are of engineering interest. Third, with the help of the specific energy absorption method, the energy dissipation capacity of functionally graded EPS concrete was discussed. Test results show that, due to the quick establishment of the stress equilibrium state, functionally graded EPS concrete presents a poorer mechanical performance compared with homogenous EPS concrete and no obvious difference was found in the dynamic responses of four graded modes. When subjected to the loading velocity which are of engineering interest, the graded mode that set the stronger layer at the input surface and the weaker layer at the output surface could exhibit a superior impact resistance ability and reduce the damage to the protected structure. The graded mode can be used to design the best energy dissipation capacity of material with respect to a given loading condition. These findings provide valuable suggestions in the investigation of high-performance protective material.

Analysis and constitutive modeling of high strain rate deformation behavior of Haynes 282 aerospace superalloy

Haynes 282 is a relatively new γ′- strengthened superalloy used in static and rotating components of jet turbine engines. An understanding of its high strain rate behavior is required to develop constitutive model to predict and optimize its response under dynamic loading applications. Using the direct impact Hopkinson pressure bar, this study investigates the high strain rate behavior of Haynes 282 at strain rates from 2000/s to 10,000/s and temperatures from 25 to 800 °C under three heat treatment conditions (solutionized, standard aged, and special aged). Experimental results show that the flow stress of Haynes 282 remains nearly constant or decreases with increases in strain rate, a remarkable feature that could increase its resistance to impact damage. This contrasts with Inconel 718, the most-widely used superalloy, which show positive strain rate sensitivity. An Arrhenius-type and modified Johnson-Cook constitutive models are developed to predict the combined effects of work hardening, strain rate softening, and temperature on the high strain rate behavior of Haynes 282. The model prediction shows good agreement with the experimental observations. The modified JC model has a higher correlation coefficient and lower average absolute relative error than the Arrhenius-type model, a statistical indication of higher predictability. Nevertheless, both models present powerful tool for designers to simulate and optimize high impact load-bearing applications.

Deformation and failure modes of Ti-6Al-4V lattice-walled tubes under uniaxial compression

The compressive large deformation response of tubes, comprising walls made of rhombic dodecahedron unit cells, under quasi-static and dynamic loadings is investigated. Six types of tubes with cellular walls that vary with respect to cell density, tube cross-section and cellular tube wall thickness are manufactured by selective laser melting (SLM). The quasi-static and dynamic compression tests are conducted by using a universal testing machine, a drop hammer device and direct-impact Hopkinson pressure bar (DHPB), respectively, to determine the force-deformation responses of the different structures. In addition, the deformation modes and energy absorption capacity of the tubes are identified. The influence of the cross-sectional shape and cellular-walled density gradient on the energy absorption capacity of the various tube configurations is examined. The results indicate that the Ti-6Al-4V lattice-walled tubes with a density gradient exhibit the best absorption efficiency among the tubes studied. The collapse strength of the square cross section tube tested is more significant than that of its cylindrical counterparts.

A study of the dynamic impact behaviour of IN 718 and ATI 718Plus® superalloys

ABSTRACTThe dynamic impact response of IN 718 and ATI 718Plus®, in both the solution heat treated and age-hardened conditions, were investigated at different deformation temperatures and strain rates using a direct impact Hopkinson pressure bar. Analyses of the results provide a vital but previously not reported information that the ATI 718Plus® offers a higher resistance to damage during high strain rate ballistic impact deformation compared to the most widely used Iron-nickel based superalloy, Inconel 718. ATI 718Plus® showed higher strain hardening and strain rate sensitivity, in both heat treatment conditions, than IN 718. The difference in the deformation behaviour of both alloys, in the annealed condition, is attributable to the compositional modification in ATI 718Plus® which has been reported to lower its stacking fault energy and increases the tendency for deformation twinning. However, in the age-hardened condition, the difference is believed to be related to the disparity in the operative strengthening mechanism, of the precipitates present in both alloys. Furthermore, a higher susceptibility to strain location and the formation of adiabatic shear band, in aged IN 718, is attributable to the stronger temperature-softening characteristics observed in the alloy and to the limited strain hardening tendency under dynamic impact loading.

Improved dynamic impact behaviour of wire-arc additive manufactured ATI 718Plus®

The dynamic response and impact resistance of wire-arc additive manufactured (AMed) and wrought ATI 718Plus in different heat treatment conditions are characterised by using a direct impact Hopkinson pressure bar system. In addition, microstructural analyses of the alloys, before and after impact, are characterised by using advanced microscopy techniques, including scanning electron and transmission electron microscopies. The experimental results show that the impact resistance of the AMed alloy in the as-processed condition is inferior to that of the wrought alloy. The lower impact resistance is attributed to the presence of eutectic solidification constituents in the interdendritic regions and to the inhomogeneous distribution of the strengthening precipitates in the deposit. After the application of the recommended heat treatment for ATI 718Plus, excessive formation of η-phase particles are observed in the microstructure in addition to Laves phase particles. Since the recommended heat treatment for ATI 718Plus is not sufficient to eliminate the deleterious phases and optimise the properties of the alloy, a novel heat treatment procedure is proposed. Dynamic impact study of the AMed alloy after the application of the proposed approach shows that the alloy exhibits a dynamic response and impact resistance comparable to those of the wrought alloy. Furthermore, under high impact momentum, both the wrought and the AMed alloys fail due to the adiabatic shear band. A transmission electron microscopy analysis of the deformed alloys suggests the dissolution of the γ' precipitates in the shear band as well as in the adjacent regions to the shear band. Increase in the rate of dissolution of the precipitates due to strain-assisted diffusion coupled with an increase in the adiabatic temperature during deformation, are likely causes of the dissolution of the precipitates in the shear band regions.

Microstructural evolution of adiabatic shear bands in pure copper during impact at high strain rates

A systematic study was conducted on copper of commercial purity (99.9% purity) to elucidate the sequence of events that leads to the evolution of adiabatic shear bands and the structure of the evolved shear bands after strain localization during deformation at high strain rates and large strains. A direct impact Hopkinson Pressure Bar was used to deform different specimens at increasing impact momentum and strain rate followed by microstructural characterization using metallographic techniques and transmission electron microscopy. It was observed that sequential occurrence of emergence of dislocations from grain and twin boundaries, formation of dislocation cell structures and substructures with varying cell sizes and cell walls, dislocation-nucleation controlled softening and extensive micro twinning characterized the structure of the evolved adiabatic shear bands as a function of impact momentum and strain rate. The dislocation cell structures and substructures were typically made up of high-density dislocation walls surrounding low-density dislocation cell interiors. The microhardness distribution within the evolved shear bands increased up to a peak value at a critical impact momentum and strain rate (≥ 45kgm/s and ≥6827s−1). Above this threshold, microhardness of the regions within the evolved shear bands decreased because of the occurrence of softening. However, the first specimen that exhibited softening had high-density dislocation cell walls surrounding dislocation-free cell interiors with no observed recrystallized grains. Despite the onset of softening both within and outside the shear bands, the regions within the shear bands were always harder than the regions outside the shear bands. The shear bands that exhibited significant softening were clad with vast distribution of microtwins in addition to evolved refined grains and sub-grains. It is discussed that thermally activated dislocation processes as a result of the rise in temperature during impact, dynamic recovery and dynamic recrystallization do not necessarily result in strain localization in the impacted copper specimens because their effects are observed in the structure of the evolved shear bands at a latter stage when strain localization had already occurred.

Experimental and Theoretical Investigation on Dynamic Properties of Metal Foam

In this study, two kinds of dynamic compression tests were conducted using direct-impact Hopkinson pressure bar (DHPB) facilities, i.e., the bullet with foam specimen was shot on the transmission bar, or the foam specimen was mounted at the end surface of the transmission bar and was hit by a bullet directly. Stress enhancement and localized deformation, as two mainly dynamic properties of metal foam, were observed in the experiments. Then, dynamic locking strain is proposed in order to better describe the feature of foam’s localized deformation field during the impact process. A rigid- perfectly-plastic-dynamic-locking strain model (R-P-P-D-L model) is developed to study the dynamic properties of the foams. The parameters included in this model are determined by 3D numerical Voronoi model and experiments. Comparing the predictions from R-P-P-D-L model with numerical results and experimental results, it is found that the R-P-P-D-L model can capture the main deformation mechanisms of the foam in dynamic compression, and provide a more precise prediction than R-P-P-L model. Furthermore, the stress enhancement of foam with the relative density and the impact velocity are discussed using the R-P-P-D-L model.

Application of photon Doppler velocimetry to direct impact Hopkinson pressure bars

Direct impact Hopkinson pressure bar systems offer many potential advantages over split Hopkinson pressure bars, including access to higher strain rates, higher strains for equivalent striker velocity and system length, lower dispersion, and faster achievement of force equilibrium. Currently, these advantages are gained at the expense of all information about the striker impacted specimen face, preventing the experimental determination of force equilibrium, and requiring approximations to be made on the sample deformation history. In this paper, we discuss an experimental method and complementary data analysis for using photon Doppler velocimetry to measure surface velocities of the striker and output bars in a direct impact bar experiment, allowing similar data to be recorded as in a split bar system. We discuss extracting velocity and force measurements, and the precision of measurements. Results obtained using the technique are compared to equivalent split bar tests, showing improved stress measurements for the lowest and highest strains in fully dense metals, and improvement for all strains in slow and non-equilibrating materials.

Effect of tempering conditions on dynamic deformation behaviour of an aluminium–lithium alloy

Abstract The deformation behaviour of an aluminium–lithium alloy heat treated to different tempering conditions was investigated at high strain rate in compression using a direct impact Hopkinson Pressure bar. Detailed microstructural investigation was carried out using electron back scatter diffraction and bulk crystallographic texture was determined using X-ray diffraction. All the impacted samples showed presence of adiabatic shear bands irrespective of the prior ageing condition, however, the extent of grain fragmentation and strength of 〈1 0 1〉 parallel to compression direction texture component was strongly dependent on the tempering condition of the alloy. The naturally aged sample showed less propensity to adiabatic shear band formation and therefore, highest toughness, compared to artificially aged samples. This can be attributed to higher resistance to instability by prolonged strain hardening from dislocation–precipitate interaction in the underaged sample compared to peak and over aged samples under dynamic loading conditions. The single stage peak-aged sample provides the best combination of high toughness with stable microstructure amongst the differently aged samples.

Two-wave photon Doppler velocimetry measurements in direct impact Hopkinson pressure bar experiments

Direct impact Hopkinson pressure bar systems offer many potential advantages over split Hopkinson pressure bars, including access to higher strain rates, higher strains for equivalent striker velocity and system length, lower dispersion and faster achievement of force equilibrium. Currently advantages are gained at a significant cost: the fact that input bar data is unavailable removes all information about the striker impacted specimen face, preventing the determination of force equilibrium, and requiring approximations to be made on the sample deformation history. Recently photon Doppler velocimetry methods have been developed, which can replace strain gauges on Hopkinson bars. In this paper we discuss an experimental method and complementary data analysis for using Doppler velocimetry to measure surface velocities of the striker and output bars in a direct impact bar experiment, allowing similar data to be recorded as in a split bar system, with the same level of convenience. We discuss extracting velocity and force measurements, and improving the accuracy and convenience of Doppler velocimetry on Hopkinson bars. Results obtained using the technique are compared to equivalent split bar tests, showing improved stress measurements for the lowest and highest strains.

Experimental investigation on the strain-rate effect and inertia effect of closed-cell aluminum foam subjected to dynamic loading

Abstract In this paper, a series of experiments were conducted to clarify the effects of the strain rate and inertia on the deformation behavior of closed-cell aluminum foams under impact. The quartz-crystal technique was employed to analyze the stress uniformity of aluminum foam samples under split Hopkinson pressure bar (SHPB) loading. It was revealed that the condition of stress uniformity is hard to reach for a thicker foam sample, and the strength of aluminum foam is sensitive to strain rate. Two different direct-impact Hopkinson pressure bar (DHPB) methods were introduced to validate the three deformation modes, i.e. homogeneous mode, transitional mode and shock mode. Results displayed that the stress at the front surface increased dramatically than that at the back surface as the impact speeds increased from 16 m/s to 113 m/s. The axial-inertia effect became more important than the strain rate effect under high speed impact. The dynamic deformation processes were recorded by a Phantom-675 camera and were analyzed through the digital imaging correlation (DIC) method. The deformation of aluminum foam in homogeneous mode was presented by the evolution of global distributed failure, but it was dominated by the local failure in shock mode.

The use of the direct impact Hopkinson pressure bar technique to describe thermally activated and viscous regimes of metallic materials

The influence of strain rate over domains involving the thermal activation and the viscous drag behaviour of the dislocations is discussed. While it is recognized that the Koslky-Hopkinson technique or split Hopkinson pressure bar technique can generate data up to the upper strain-rate limit of the thermal-activated regime, it is necessary to use a direct impact Hopkinson pressure bar technique to access the viscous regime. Data generated with this technique are presented for a series of metals, including steel, nickel, copper and tungsten alloys. The motivation to generate such data is provided through three industrial applications.

Effect of prior heat treatment on the dynamic impact behavior of 4340 steel and formation of adiabatic shear bands

Abstract Adiabatic shear bands are observed in many metallic materials when subjected to both high strain rates (>103 s−1) and large strains. These bands are a manifestation of damage and may lead to catastrophic failure because they are susceptible to crack nucleation and propagation due to their high internal stresses. Minimizing their occurrence is important in many applications and processes. In this study, the effect of prior heat treatment on evolution of adiabatic shear bands in a typical heat treatable steel was investigated. Cylindrical AISI 4340 steel specimens were heat treated at 315 °C, 425 °C and 620 °C, for 1 h or 2 h and impacted using the direct impact Hopkinson pressure bar (DIHPB). Tempering the specimens at 315 °C and 425 °C for 1 h led to the formation of distinct and well-developed adiabatic shear bands after impact. Diffused shear bands were formed in the specimens tempered at 315 °C and 425 °C for 2 h. Specimens tempered at 315 °C and 425 °C for 1 h exhibited higher impact resistance than those tempered at 315 °C and 425 °C for 2 h. The 1 h tempered specimens absorbed much of the impact energy without failing at the impact momentums at which those tempered at 315 °C and 425 °C for 2 h were failing. These observations demonstrate that tempering the steel specimens at 315 °C and 425 °C for 1 h may lead to catastrophic failure through the formation of adiabatic shear bands, while they exhibit higher impact resistance than when they are tempered for 2 h. Tempering the steel specimens at 620 °C for 2 h before impact resulted in the formation of deformed shear bands, which are less susceptible to crack nucleation and propagation. However, at very high impact momentum, a distinct transformed shear band was observed, indicating that catastrophic failure through the formation of adiabatic shear bands can be reduced by increasing the tempering temperature of the steel specimens even though at very high impact momentums, failure through the formation of adiabatic shear bands is likely to occur. Thus the initial conditions (tempering temperature and time) of a heat treatable steel before impact are shown to have a significant influence on the formation of adiabatic shear bands.

Microstructure and nanohardness distribution in a polycrystalline Zn deformed by high strain rate impact

Polycrystalline Zn with an average grain size of about 300 μm was deformed by direct impact Hopkinson pressure bar at a velocity of 29 m/s. An inhomogeneous grain structure was found consisting of a center region having large average grain size of 20 μm surrounded by a fine-grained rim with an average grain size of 6 μm. Transmission electron microscopy investigations showed a significant dislocation density in the large-grained area while in the fine-grained rim the dislocation density was negligible. Most probably, the higher strain yielded recrystallization in the outer ring while in the center only recovery occurred. The hardening effect of dislocations overwhelms the smaller grain size strengthening in the center part resulting in higher nanohardness in this region than in the outer ring.

The effect of aspect ratio on the compressive high rate deformation of three metallic alloys

Metallic cylinders of rolled homogeneous armour (RHA) steel, Ti6A14V, and FNC tungsten alloy of four different length l to diameter d ratios (dimensions in mm): 8/4, 4/8, 10/8, 8/10) were deformed at high rates of deformation using a direct impact Hopkinson pressure bar. Highspeed photographic sequences of the deformation were taken using a Hadland Imacon 790 imageconverter camera working at either 1 or 2 x 10 4 frames/s. It was found that titanium alloy cylinders of all four aspect ratios shear-banded and fractured, but that cylinders made from RHA steel and FNC tungsten behaved in a ductile manner when lid 1. We conclude that adiabatic shear banding is not just an inherent material property but that in some materials size effects/geometry can trigger this phenomenon.