Adiabatic Heating Research Articles

Adiabatic self-heating determination for Ti6Al4V at different temperatures

Nowadays, numerical models are one of the most widely used techniques to predict material performance subjected to different manufacturing processes. However, to obtain accurate predictions, these models require reliable input data from thermomechanical tests. Nevertheless, during the test performance the material is self-heated due to a phenomenon known as adiabatic self-heating. Despite the proven relevance of a proper characterization, adiabatic self-heating is not properly taken into account during thermomechanical tests. In addition, in the literature, two different definitions were found under the umbrella of adiabatic-self heating. On the one hand, it could be defined as the ratio between the heat spent to heat the sample to the plastic work, value commonly taken as 0.9. On the other hand, many authors define the adiabatic heating as the ratio between the heat experimentally measured to the total plastic work. This second approach, although seems easier, is neglecting heat losses. These two different approaches could lead to misunderstandings once this parameter is implemented in the models. This paper aims to clarify this issue. Moreover, the techniques found in literature aiming to measure this parameter are usually based on 2D approaches at low temperatures. In this paper, a 3D methodology to measure adiabatic self-heating is presented which considers all possible heat losses (conduction, convection, radiation and mass flux) through infrared measurements and Digital Image Correlation (DIC) technique. The adiabatic self-heating was measured for a widely used alloy (Ti6Al4V) obtaining promising results.

Heating Effects of Supra-arcade Downflows on Plasma above Solar Flare Arcades

We deliberately select three flares to investigate heating effects of supra-arcade downflows (SADs) on the surrounding fan plasma. Prior work found in one flare that the plasma around most SADs tends to heat up or stay the same temperature, accompanied by discernible signatures of the adiabatic heating due to plasma compression as well as viscous heating due to viscous motions of plasma. We extend this work to more flares and find that the heating effects of the SADs are also present in these events. The adiabatic heating is dominant over the viscous heating in each event. The adiabatic heating in the two M1.3 flares, being on the order of about 0.02–0.18 MK s−1, is fairly comparable. In the more energetic X1.7 flare, the adiabatic heating is on the order of 0.02–0.3 MK s−1, where we observe a more pronounced temperature increase during which dozens of SADs descend through the fan. As SADs constantly contribute to the heating of the surrounding fan plasma, the areas where SADs travel through tend to cool much slower than the areas without SADs, and the plasma of higher temperature ends up concentrating in areas where SADs frequently travel through. We also find that the cooling rate of areas without SADs is ∼1000 K s−1, much slower than would be expected from normal conductive cooling. Instead, the cooling rate can be interpreted nicely by a process where conductive cooling is suppressed by turbulence.

Energy consumption and surface roughness maps for low and moderate speed machining of Aluminum alloy 2014: An experimental study

<abstract> <p>The rising energy prices and soaring environmental concerns have put an immense pressure on the wide usage of machining processes. The total power consumption during machining includes the power consumed by the machine itself and the power used to remove the material from the workpiece. An accurate prediction of energy consumption during the machining process is the basis for energy reduction. In this study, the specific cutting energy and surface finish for low and moderate-speed orthogonal machining of the aluminum alloy 2014 are evaluated. The measured values for the specific cutting energy and surface roughness are presented as maps on a grid, which is based on the machining parameters including the following: (1) cutting speed and (2) undeformed chip thickness. The specific cutting energy map depicts low energy consumption values of 0.52 J/mm<sup>3</sup> for the aluminum alloy 2014 at medium speed machining. The roughness maps depict high roughness values at high cutting speeds. Both maps help in optimizing the machining process to achieve a required surface roughness with minimal energy consumption. A review of a specific cutting energy map demonstrates that energy consumption decreases by increasing the cutting speeds. The decrease in energy consumption at moderate speeds corresponds to the low cutting forces. This potentially happens as a result of thermal softening of the material caused by adiabatic heating. This subsequently leads to an increase in the machinability of the aluminum alloy 2014 at moderate cutting speeds. Furthermore, the decreasing chip thickness and increasing shear angle as a result of increasing the cutting speed confirms the increased machinability of the workpiece at moderate speeds.</p> </abstract>

Verification of the master curve concept (ASTM E1921) and inhomogeneity analysis of a German RPV weld for various loading rates

Fracture-mechanical investigations were carried out on the weld metal of a German reactor pressure vessel steel 22NiMoCr3-7. The crack tip loading rate and test temperature were varied in the brittle-ductile transition region, while the results were evaluated using the Master Curve methodology (ASTME1921), including inhomogeneity analyses in Appendix X5. The highly inhomogeneous datasets could be uniformly described with a bimodal distribution function with good accuracy compared to the standard procedure. However, this agreement decreases with increasing crack tip loading rate. In contrast, a modified Master Curve with a shape factor of p = 0.03/°C progressively matches the data better with increasing crack tip loading rate. The weld metal has a significantly higher toughness than the base material. SEM analysis suggests that the inhomogeneity is due to the stochastic distribution of sharp microcracks, while the observed inhomogeneity is additionally superimposed by dynamic effects (adiabatic heating and local crack arrest) at increased crack tip loading rates. It is argued that the microstructural inhomogeneity is hereby »neutralized« and could instead be interpreted as a possible artifact of the standard Master Curve under dynamic conditions.

The Asian–Pacific oscillation over the past millennium in PMIP3 and PMIP4

Atmospheric oscillations in the upper troposphere can exert considerable impacts on regional and global climate change. The Asian–Pacific Oscillation (APO), a recently identified upper-tropospheric teleconnection is not yet well documented during the last millennium. Here we use 13 models from Paleoclimate Modeling Intercomparison Project phase 3 and 4 (PMIP3 and PMIP4, respectively) to first assess their ability in simulating the climatology and interannual variability of the APO in recent decades, and then examine the changes in APO over the last millennium and attribute the associated sources. The evaluation indicates that the models show high fidelity to the observed climatological horizontal and vertical distributions of the boreal summer APO, with the PMIP4 models outperforming the PMIP3 ones. And the majority of the models can reasonably reproduce the out-of-phase teleconnection pattern over the Asia–North Pacific sector and the interannual variability of the APO index (APOI). During the last millennium, the APOI exhibits a remarkable decreasing trend, and the indices averaged in the MCA (LIA) are significantly larger (smaller) referring to the whole millennium. In warm/cold periods, APO changes over East Asia can be attributed to the Tibetan Plateau heating-induced condensation latent heat and the northern part of the East Asian summer monsoon-associated latent heat, whereas the changes over the North Pacific originate from dry adiabatic heat and the superposed weak offsetting latent heat. Additionally, the APOI reveals a notable interannual scale cycle at around five years.

Linkages between Amplified Quasi-Stationary Waves and Humid Heat Extremes in Northern Hemisphere Midlatitudes

Abstract Humid heat extremes, taking account of both temperature and humidity, have adverse impacts on society, particularly on human health. It has been suggested that quasi-stationary waves (QSWs) with anomalously high amplitudes contribute to the occurrence of near-surface precipitation extremes and temperature extremes in the midlatitudes of the Northern Hemisphere. However, little attention has been paid to the linkages between amplified QSWs and humid heat extremes. Using the ERA5 dataset, we identify amplified QSWs of zonal wavenumbers 5–7 (Wave 5–7) in summer months from 1979 to 2020. These amplified QSWs show clear circumglobal wave patterns horizontally and nearly barotropic structure vertically. Linking amplified Wave 5–7 to wet-bulb temperature (WBT) extremes, we find that amplified QSWs preferentially induce prominently prolonged WBT extremes in specific regions: north-central North America for amplified Wave 5; western United States, south-central Asia, and eastern Asia for amplified Wave 6; and western Europe and the Caspian Sea region for amplified Wave 7. Analyses of physical processes indicate that, accompanied by the amplification of Wave 5–7, the changes in horizontal temperature advection, adiabatic heating associated with descending motion, downward solar radiation, moisture transport and moisture flux convergence, and surface latent heat fluxes largely account for the increase in persistence of WBT extremes. Significance Statement In the context of climate change, humid heat extremes exhibit different changes and impacts from high-temperature extremes, but the physical processes that may cause them remain unclear. This study aims to explore the atmospheric dynamic processes leading to the concurrence of humid heat extremes, which may exacerbate the risk from heat stress in today’s interconnected world. In specific regions over the Northern Hemisphere midlatitudes, durations of humid heat extremes are found to be elevated simultaneously by amplified quasi-stationary waves. We further identify the physical connections between amplified quasi-stationary waves and humid heat extremes over targeted regions. This would help in better understanding the role of changing atmospheric circulations in the humid heat extremes.

Winter surface air temperature variation over Pakistan during 1970–2014 and its principal drivers in the tropical ocean

This study examined the leading modes of Dec–Feb (DJF) month's surface temperature (DJFTs) and associated mechanisms of the most significant drivers affecting temperature variability during the season from 1970 to 2014. EOF1 of DJF temperature reveals uniform spatial patterns over most of Pakistan, except extreme north, accounting for 84.38% of the total variance. The typical warm (cold) years were 1970, 1981, 1988, 1990, 1993, 2004, 2006, 2009, 2010 (1972, 1974, 1975, 1984, 2008, 2012). Favorable atmospheric conditions for warm temperature events (DJFTs) in Pakistan is an anomalous anticyclone spread over northeast Pakistan to Tibet at low to upper levels. This condition is controlled by the south and western part, of which there are significant southerly and convergent flows at low level with ascending motion over the study area. Further, examination revealed a close relationship of warm DJFTs with Western Indian Ocean warming and Eastern Indian Ocean indices but insignificant relation with the Nino3.4 index. Ocean warming related to Dipole Mode Index over eastern Indian Ocean (DMIE) is more influential (r = 0.51) than over the western Indian Ocean (DMIW) index (r = 0.37). The Ocean warming related to DMIE (EIOW), especially in Nov-Dec-Jan (NDJ) season, forced a significant ascending motion locally and then induced warm DJFTs-associated circulation pattern, propagating northward to Europe. By the dynamic diagnosis, the adiabatic and diabatic heating for the warm DJFTs is not remarkable. However, the horizontal temperature advection is positive and significant over most parts of Pakistan, both in the DJF and NDJ season, from the surface to the top of the troposphere. It suggests that the warm temperature advection is the possible prominent way of the EIOW on the warm DJFTs.

Thermal conductivity of iron and nickel during melting: Implication to the planetary liquid outer core

We report the measurements of the thermal conductivity (κ) of iron (Fe) and nickel (Ni) at high pressures and high temperatures near the core–mantle boundary (CMB) of Mercury and Mars. κ values are estimated from the temperature measurements across the sample surface in a laser-heated diamond anvil cell (LHDAC) and using the COMSOL software. In both metals, values of κ drop suddenly at the melting temperature and can be used as another helpful technique to estimate melting temperatures at high pressures. We have obtained the thermal conductivity values of Fe and Ni at the CMB of Mercury to be about (60–70) ± 28 W m−1 K−1 and (65–70) ± 28 W m−1 K−1, respectively. Similarly, in the case of Mars CMB pressure and temperature conditions, the thermal conductivities of Fe and Ni are found to be 80 ± 32 W m−1 K−1 and (65–70) ± 28 W m−1 K−1, respectively. Using the obtained thermal conductivity values of Fe and Ni, we have estimated the adiabatic heat conduction at the outer core of Mercury and Mars, to be about (59–69) ± 28 mW m−2 and (61–72) ± 29 mW m−2, respectively. Both these values are much larger than the values predicted recently from the estimation of electronic thermal conductivity.

Strain rate response of mechanical behaviors for Fe-8Mn-6Al-0.4C duplex low-density steels with different austenite stabilities

Duplex low-density steels exhibit excellent mechanical properties under quasi-static tensile tests with the help of the transformation-induced plastic (TRIP) effect produced by the deformation-induced martensitic transformation (DIMT), which is sensitive to the strain rate. Therefore, adjusting the microstructure to accommodate the deformation at different strain rates is crucial for the optimal design of high-performance duplex low-density steels. In this study, Fe-8Mn-6Al-0.4C low-density steel was subjected to intercritical annealing (IA) treatment to obtain duplex microstructures with an inhomogeneous distribution of austenite grain sizes and tensile tests were performed at two strain rates. The response of the mechanical behavior to the strain rate was investigated, and the mechanical stability of austenite was evaluated using a method based on the grain size. The results show that the diverse mechanical behaviors of experimental steel depend mainly on the mechanical stability of austenite rather than the stacking fault energy (SFE). The mechanical stability of austenite increased with decreasing annealing temperature and was further enhanced by adiabatic heating at a higher strain rate. Accordingly, the DIMT mode changed from a stress-induced transformation to a strain-induced transformation until the DIMT was inhibited by adiabatic heating. The improvement in the mechanical properties of the experimental steel is attributed to the sustainable DIMT with a reasonable mechanical stability of austenite.

Performance of a compressed-air energy storage pile under various operation conditions

Compressed air energy storage (CAES) has been re-emerging over the last decades as a viable energy storage option, and the authors have recently explored the idea of utilizing building foundations (termed as CAES piles) as small-scale storage media for the air charge and load-bearing elements under simplified conditions. This paper extends the authors' preceding work and conducts a series of finite element analyses to investigate the impact of various operation conditions, such as heat management, ground profiles, and pile-head fixities, on the performance of a CAES pile. The analysis results show that a greater axial pile displacement and a greater tensile stress are induced in the CAES pile during the air charge as the surrounding soil becomes weaker. Adiabatic and diabatic heat management conditions yield greater axial pile displacements but smaller tensile stresses in the pile during the air charge than an ideal isothermal condition. Having a restraint at the pile head (i.e., fixed-head condition) or at the pile base (i.e., hard rock layer beneath the pile base) leads the CAES pile to move toward a direction where there is a less restraint and such a constraint induces compressive stress in the pile during the air charge, eventually reducing the magnitude of the tensile stress in the pile when fully pressurized. The maximum tensile stress developing in the pile during the air charge does not exceed the lower bound of yield strengths of steels commonly used for conventional pipe piles but the analysis results suggest that higher-grade steels would be more advantageous for CAES piles to secure some safety margins.

The strain rate history effect in a selective-laser-melt 316L stainless steel

The strain rate history effect in a selective laser melt 316L (SLM-316L) alloy was investigated through quasi-static (10−3 s−1) and high strain rate (1600-3200 s−1) interrupted and reloading compression tests. The specimens pre-tested until about prescribed strains at quasi-static and high strain rates were reloaded dynamically and quasi-statically, respectively. The results revealed that the flow stress depended on strain and strain rate as well as strain-rate history. Quasi-static reloading the dynamically pre-tested specimens until about prescribed strains induced a higher flow stress than the specimens tested quasi-statically. The strengthening was ∼70 MPa at 0.11 pre-strain and decreased as the dynamic test pre-strain was increased due to adiabatic heating. On the other side, reloading the quasi-statically pre-tested specimens dynamically at 0.11 pre-strain resulted in ∼60 MPa lower flow stress than the specimens tested dynamically. The grains of the quasi-statically tested specimens until 0.11 strain were shown to have a lower Taylor factor for twinning and geometrically necessary dislocation density, indicating more potential for twinning than dynamically tested specimen. Although, quasi-statically and dynamically tested specimens were deformed predominantly by the twinning induced plasticity, a higher fraction of twin boundaries was shown microscopically in the dynamically pre-tested specimens until 0.11 pre-strain. This phenomenon of boundary strengthening could be used as a tool of strengthening of SLM-316L alloy at low strains.

Thermal Convection in Vesta’s Core from Experimentally-Based Conductive Heat Flow Estimates

Electrical resistivity measurements of Fe-5 wt% Ni were made in situ under pressures of 2–5 GPa and temperatures up to 2000 K in a cubic-anvil press. The thermal conductivity was calculated from the measured electrical resistivity data using the Wiedemann–Franz law. Comparison of these data with previous studies on pure Fe and Fe-10 wt% Ni shows that a change in the Ni content within the range 0–10 wt% Ni has no significant effect on electrical resistivity of Fe alloys. Comparing the estimated adiabatic core heat flux of ~331 MW at the top of Vesta’s core to the range of estimated heat flux through the CMB of 1.5–78 GW, we infer that the mechanism stirring Vesta’s liquid outer core to generate its surface magnetic field tens of millions of years ago in its early history was thermal convection.

Strengthening of pretreated aluminum during ultrasonic additive manufacturing

Strengthening effects in materials bonded by the high strain rate plastic deformation process, Ultrasonic Additive Manufacturing (UAM), were investigated. Aluminum (Al 6061) was pretreated by tempering and annealing prior to bonding through UAM. Following UAM, multiscale material characterization was performed. Tensile testing in the rolling (x) direction demonstrated the material became harder after the UAM process, and nanoindentation demonstrated the foil-foil interfaces became harder than the bulk foil material. The strengthening effects are a result of microstructure changes at the interfaces and in the bulk foil regions which were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. These microstructure changes result from dynamic recrystallization, dynamic recovery, adiabatic heating, and precipitate dissolution. This study signifies the metallurgical features creating mechanical strength increases, rather than decreases, in UAM builds.

Influence of Strain History on Dynamic Strain Localization and Stress State During High-Rate Tensile Loading of Titanium Alloys: Experiments, Modeling, and Analytical Methods

Abstract The determination of the mechanical response of engineering materials subjected to high loading rates plays an important role in determining their performance and application. The high strain-rate tensile response of metals is usually investigated by means of the split-Hopkinson tension bar (SHTB) apparatus. The interpretation of the obtained results is, however, subjected to analogous stress and strain uniformity challenges present during quasi-static tensile experiments. Beyond the onset of necking, strains cease to be uniform along the gauge length and localize around the necking zone. Consequently, the nominal strain rate underestimates the effective strain rate experienced by the material. The analysis of the effective strain rate and stress state beyond the onset of necking has received considerable attention in the literature. Several research efforts have focused on the optimization of the geometry of specimens to be employed for the characterization of the dynamic tensile response using the SHTB. The present work investigates, systematically, the effects of strain history and adiabatic heating on the stress state during dynamic loading. A series of monotonic and various strain history experiments were conducted and analyzed. The diameter evolution, effective strain rate, and temperature histories were measured for all conducted experiments. Numerical simulations were carried out to examine the stress state during strain localization and to accurately reproduce engineering and local thermos-mechanical variables. The effectiveness of existing postnecking corrections for high-rate experiments is assessed. A modified postnecking correlation taking into account the effects of adiabatically induced thermal softening is proposed.

Electrical resistivity of the Fe–Si–S ternary system: implications for timing of thermal convection shutdown in the lunar core

The composition of the lunar core has been suggested to be Fe-rich with varying amounts of lighter elements, such as Si and S. Presence of Si and S affects electrical and thermal transport properties and thus influences core thermal processes and evolution. Paleomagnetic observations constrain a high intensity magnetic field that ceases shortly after formation of the moon (~ 3.5–4.2 Ga year ago), and thermal convection in the core may contribute to generation of this field. In this study, the electrical resistivity of Fe-14 wt% Si-3 wt% S was measured in both solid and molten states at pressures up to 5 GPa and thermal conductivity was calculated via the Wiedemann–Franz Law from the electrical measurements. The results were used to estimate the adiabatic conductive heat flux of a molten Fe-14 wt% Si-3 wt% S lunar core and compared to a Fe-2-17 wt% Si lunar core, which showed that thermal convection of either core composition shuts down within the duration of the high intensity magnetic field: (1) 3.17–3.72 Ga year ago for a Fe-14 wt% Si-3 wt% S core; and (ii) 3.38–3.86 Ga years ago for a Fe-2-17 wt% Si core. Results favouring compatibility of these core compositions with paleomagnetic observations are strongly dependent on the temperature of the core-mantle boundary and time-dependent mantle-side heat flux.

Deformation and recrystallization behavior of the new Ni-Co based superalloy ingot material prepared by electron beam smelting layered solidification

The hot deformation behavior of a Ni-Co based superalloy prepared by electron beam smelting layered solidification (EBSL-Ni-Co superalloy) was studied by hot compression tests under the temperature range of 1070–1180 °C and the strain rate range of 0.001–1 s−1 with a true strain of 0.693. The results show that the flow stress increases with the increase of strain rate and the decrease of deformation temperature, and the friction effect and adiabatic heating effect make the measured value of material flow stress higher than the real value. The hyperbolic sine Arrhenius equation was developed based on the γ' phase solution temperature, and the plastic flow stress model of the EBSL-Ni-Co superalloy was established. The deformation activation energies of the alloy in the γ + γ′ dual-phase and γ single-phase regions were 621.6 kJ/mol and 368.6 kJ/mol, respectively. The processing map of the superalloy was constructed based on the dynamic material model (DMM). Combined with the variation of microstructure, the optimum hot deformation processing conditions of the superalloy were determined. The increase of strain rate and the decrease of deformation temperature, the proportion of low-angle grain boundaries (LAGBs), and the reduction of recrystallization. Additionally, the hot deformation behavior of the EBSL-Ni-Co superalloy was found to be greatly influenced by the coarsening of γ' phase solution temperature.

Responses of East Asian Climate to SST Anomalies in the Kuroshio Extension Region during Boreal Autumn

Abstract Sea surface temperature anomalies (SSTAs) in the Kuroshio Extension (KE) region play a key role in influencing midlatitude climate variations. This study investigates the impacts of SSTAs in the KE region (KE-SSTA) on East Asian climate during boreal autumn. The results reveal that positive KE-SSTA, changing the meridional temperature gradient in the lower troposphere, contributes to the formation of an anomalous quasi-barotropic anticyclonic circulation (AC) over the KE region. Such a configuration tends to be affected by wind through thermal wind adjustment. Simultaneously, it is also impacted by eddy activity related to the strengthened atmospheric baroclinicity through transient eddy feedback. By both means, obvious descending motion and warm advection are generated, which increases tropospheric temperature through adiabatic heating and further maintains the abnormal AC over the KE region. This anomalous circulation changes wind in the east of the East Asian trough, which favors the transportation of warm and wet air from the midlatitude northwest Pacific and further causes the increase of temperature in northeastern East Asia. Moreover, it also leads to the convergence and divergence of water vapor fluxes in Northeast Asia and eastern North China, favoring the increase and decrease of precipitation in the two regions, respectively. The above responsive characteristics of the midlatitude atmospheric circulation to positive KE-SSTA can be confirmed by numerical experiments. These results suggest that anomalous SST in the KE region can be used as a potentially effective predictability source for autumn climate variations in the mid- to high latitudes of East Asia. Significance Statement Anomalous variations of sea surface temperature (SST) in the Kuroshio Extension (KE) region play an essential role in influencing midlatitude climate variations. Considering the importance of autumn climate in East Asia to agricultural production, this study aims to explore the relationship between the East Asian climate and autumn SST anomaly in the KE region (KE-SSTA), and to understand how KE-SSTA effects East Asian climate during boreal autumn. Our results suggest that KE-SSTA can be used as one of the effective predictors of East Asian climate, which will contribute to the improvement of short-term climate prediction levels over East Asia.

Hot workability and microstructural evolution of a nickel-based superalloy fabricated by laser-based directed energy deposition

Recently, an innovative process chain has been investigated and proposed, using laser-based directed energy deposition technology to produce pre-forms for forging. The advantage of this technology is to get high-performance near-formed parts at a low cost, compared traditional processing. Clarifying the hot workability and the microstructural evolution of as-deposited samples is the key to obtaining high performance parts. In present work, the plastic behaviors and dynamic recrystallization mechanisms of as-deposited nickel-based superalloy are studied. The compression curves are corrected by eliminating adiabatic heating and friction effect. And then the hot deformation activation energy and the constitutive equation of as-deposited sample are obtained. Compared to the wrought Inconel 625, as-deposited Inconel 625 has lower hot deformation activation energy (466.511 kJ/mol). The peak stress gradually decreases, while the recrystallized grain volume fraction gradually increases when deformation temperature increases and strain rate decreases. The characterization results of TEM and EBSD show that the discontinuous dynamic recrystallization mechanism nucleating by the bowing of grain boundaries occupies an absolute dominant position. In addition, combined with the microstructural evolution and hot processing map, the best hot working conditions of as-deposited Inconel 625 are obtained (deformation temperature: 1050–1150 ℃, strain rate: 0.001–0.01 s−1).

Influence of Entanglements on Ultrahigh Strain Rate Deformation of Polystyrene Microprojectiles

Micron-sized polystyrene spherical projectiles made from three different molecular weight polymers spanning from below to well above the entanglement molecular weight were launched against a rigid substrate, causing ultrahigh strain rate deformation. Scanning electron microscopy, focused ion beam cross sections, and atomic force microscopy were used to elucidate the deformation mechanisms from the observed morphologies of the deformed specimens and to evaluate the important role of entanglements in the various deformation processes that dissipate the kinetic energy. Due to adiabatic heating from shock compression, the temperature at the bottom region of the polymer projectile is elevated above the glass transition temperature, enabling viscoplastic flow. The cooler portions of the unentangled sample undergo strain localization via micro-shear banding and brittle fracture, while the two entangled samples resist fracture and exhibit extensive viscoplastic flow and, in the upper, cooler regions, extensive shear banding followed by crazing. The interaction of molecular entanglements, temperature, shear rate, and total shear strain on melt viscosity influences the amount of additional plastic work and adiabatic heating occurring in the sample and hence the extent of lateral spreading of the projectile over the substrate.

High Strain Rate Failure Behavior of Polycarbonate Plates due to Hypervelocity Impact

Developing a fundamental understanding of how polymers respond during hypervelocity impact (HVI), a high strain rate process, is crucial for the potential use of polymers in HVI protection systems. Here, we present the high-strain-rate impact behavior of commercially available polycarbonates with two different molecular weights, 42 and 21 kg/mol. A powder gun and a two-stage light-gas gun were used to achieve the velocities (v0) of a 4 mm projectile impacting the polycarbonate targets over the range of 400–6,500 m/s. The deformation behavior of the polymer during the impact event was captured using high-speed cameras. The impact caused a variety of responses, ranging from deflection of the projectile to complete perforation for higher v0, leading to major debris cloud formation. These debris clouds consist of both fluid- and dust-like ejecta. The fluid-like debris cloud indicates the melting of polymer caused by the conversion of kinetic energy to thermal energy and subsequent adiabatic heating. The dust-like response can likely be attributed to the brittle failure behavior of polymers, as the polymer became brittle at a high strain rate. Dynamic mechanical analysis (DMA) and dielectric thermal analysis (DETA) on the polycarbonate samples used here indicate an approximately 40 °C increase of Tg with increasing frequency from 1 Hz to 106 Hz, and such an increase has likely led to brittle behavior. While the leading-edge velocity of the debris clouds and perforation diameters scale linearly with v0, we found negligible differences in the HVI response for the two molecular weights of polycarbonate tested here. This study displays the importance of v0 and thickness contributing to responses of polycarbonates undergoing HVI.