PEEK occupies a special niche in the materials world. This semi-crystalline thermoplastic boasts an unusual combination of strength, toughness, chemical resistance, and thermal stability, which explains its popularity in aerospace structures, biomedical implants, and high-performance bearings. Yet precisely because it is increasingly used in applications where foreign object debris and high-velocity impacts threaten structural integrity, engineers need to know how it behaves when struck at angles rather than head-on. Most ballistic studies focus on normal incidence, where the projectile travels perpendicular to the target surface. Oblique impact, by contrast, is far more common in the real world, from runway debris kicked up at shallow angles to micrometeoroids grazing spacecraft panels, and it produces fundamentally different failure patterns.

The research team, led by J. Y. Hua and S. N. Luo, fired spherical stainless steel projectiles at PEEK plates at an incident angle of 60 degrees, with velocities ranging from roughly 100 to 450 meters per second. This velocity window spans the transition from low-speed denting to high-speed penetration, making it an ideal regime for probing how the material’s response evolves. High-speed imaging and post-impact examination of the targets revealed the dominant failure mode: wear-induced ductile damage. Rather than shattering brittly, the PEEK deformed plastically, material accumulated and smeared along the crater path, and the surface failed through a process reminiscent of erosive wear, consistent with earlier impact and erosion studies of the polymer dating back decades.

Central to the study is a new dynamic constitutive model for PEEK, built from uniaxial compression tests conducted across an unusually wide range of conditions, with strain rates from 0.001 to 5000 per second and temperatures from 300 to 473 Kelvin. Under all these conditions, PEEK displayed classic elastic-plastic behavior, but with two competing trends: strain rate hardening, where the material grows stronger when deformed faster, and thermal softening, where it weakens as temperature rises. The researchers captured the elastic modulus with a sigmoidal function of normalized temperature, anchored around the glass transition, multiplied by a power-law term for strain rate effects. The plastic flow stress was described by combining a Johnson-Cook style strain hardening term with a three-segment strain rate sensitivity model and a linear thermal softening coefficient.

The details of that strain rate sensitivity are revealing. The normalized flow stress of PEEK rises gently at low strain rates, transitions through a smooth curved region between roughly 100 and 316 per second, and then climbs steeply at high rates, with the slope in the high-rate regime more than seven times that of the low-rate regime. This three-segment structure, connected with strict continuity at the turning points, reflects the polymer’s underlying relaxation physics, in which different molecular mechanisms dominate at different timescales. The model also incorporates a Mie-Grüneisen equation of state to handle volumetric compression and accounts for adiabatic heating, assuming that about 60 percent of plastic work converts to heat, a standard assumption for polymers in impact simulations.

With the material model in hand, the team built finite element simulations using a coupled Eulerian-Lagrangian approach, a technique well suited to problems involving extreme deformation where conventional mesh-based methods would tangle. The simulations reproduced the experimental observations closely, capturing crater shapes, rebound behavior, and energy losses. More importantly, the validated simulations became a data factory. By varying incident angles and impact velocities across many virtual experiments, the researchers generated a large dataset of ballistic responses that would have been prohibitively expensive and time-consuming to produce physically.

That dataset then trained a back-propagation neural network, the workhorse architecture of machine learning, to learn the mapping from impact conditions to outcomes. Once trained, the network predicted the full suite of ballistic response quantities: rebound angle, rebound velocity, and the geometric parameters of the crater, including its dimensions along three axes and its volume. The team distilled these predictions into phase diagrams, maps that delineate regimes of behavior as functions of incident angle and impact velocity, showing at a glance where the projectile rebounds steeply, where it skims and gouges, and how crater geometry shifts across the parameter space.

Critically, the researchers did not simply trust their neural network’s internal logic. They validated the machine learning predictions against an independent impact experiment, one not used in training, and found agreement. This closed loop, from physical experiment to physics-based simulation to machine learning and back to experimental verification, represents a template for how modern materials science increasingly operates. Rather than choosing between expensive experiments and idealized models, the workflow lets each element compensate for the weaknesses of the others: experiments anchor the simulations in reality, simulations supply the statistical volume that experiments cannot, and machine learning compresses both into fast, predictive tools.

The practical implications reach across several industries. For aerospace engineers, PEEK-based composites are attractive for their light weight and toughness, and understanding oblique impact behavior directly informs designs that must tolerate runway debris, hail, and bird strikes. The study’s reference list situates the work within a broader research program by the same groups, which has applied the same experiment-simulation-machine learning pipeline to polycarbonate, ultrahigh molecular weight polyethylene, magnesium alloys, Inconel 718, and high-entropy alloys. Together, these efforts are building a systematic, data-driven science of high-speed impact across material classes, replacing fragmented case-by-case testing with transferable methodology.

There is also a deeper scientific payoff. The constitutive model developed here encodes how PEEK’s molecular architecture, its semi-crystalline structure straddling a glass transition, manifests in macroscopic impact response. The pronounced strain rate sensitivity above the transition region suggests that molecular relaxation processes govern the polymer’s ability to absorb and dissipate impact energy, knowledge that could guide the design of next-generation thermoplastic armor, protective coatings, and impact-tolerant structures. As machine learning tools grow more sophisticated, the ability to generate validated phase diagrams of material behavior on demand may transform how engineers approach impact protection, turning what was once a domain of costly trial and error into a predictive, quantitative discipline. For a polymer that already bridges the operating room and the flight deck, that is a significant step forward.

Subject of Research: Oblique high-speed impact behavior and predictive modeling of polyether ether ketone (PEEK) plates

Article Title: Oblique impact of polyether-ether-ketone (PEEK) plates: ballistic experiments, FEM modeling and machine learning

Article References: Hua, J. Y., Wang, N., Xu, J., Bao, S. H., Yang, T., Sun, Y. F., Cai, Y., & Luo, S. N. (2026). Oblique impact of polyether-ether-ketone (PEEK) plates: ballistic experiments, FEM modeling and machine learning. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13898-z

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13898-z

Keywords: PEEK, ballistic impact, oblique impact, finite element method, machine learning, neural network, constitutive model, polymers, strain rate hardening, thermal softening, crater geometry, aerospace materials

High-entropy alloys are unusual in more ways than one. Because several elements share the crystal lattice almost equally, the atoms interfere with one another’s movement, producing severe lattice distortion, sluggish diffusion, and what materials scientists call the cocktail effect, in which the mixture behaves in ways its individual components do not predict. When such an alloy is designed around elements with high heats of combustion, notably titanium and zirconium, it becomes an energetic structural material: a substance that can hold together as a structural component and, under violent impact, fragment and react with oxygen to release substantial chemical energy. That dual personality is precisely what makes these alloys interesting for impact applications, since they deliver both kinetic penetration and a secondary thermal and overpressure effect that conventional penetrators cannot provide.

The team fabricated three candidate alloys, TiZrWV, TiZrNbWV, and TiZrNbV, using vacuum arc melting. High-purity raw materials were weighed in equiatomic ratios, melted under strong electromagnetic stirring, and remelted six times, with each melt held liquid for twenty minutes to ensure chemical homogeneity. The ingots were then rapidly solidified under argon-assisted forced convection and machined into cylindrical specimens by wire electrical discharge machining. Tungsten was added to boost solid-solution strengthening, while niobium and vanadium were introduced to stabilize the solid-solution phases and refine the grains. The choice of composition was deliberate: each alloy represents a different balance between strength, ductility, and reactivity.

Quasi-static compression tests on an MTS Landmark universal testing machine, run at strain rates of 0.001 and 0.01 per second, revealed a clear hierarchy of mechanical behavior. TiZrNbWV emerged as the strongest of the three, with a yield strength rising from 1418 to 1469 megapascals as the strain rate increased, and a fracture strength reaching 2000 megapascals at the higher rate while still sustaining a fracture strain of roughly thirteen percent. Its yield strength exceeded that of TiZrWV by more than twenty-five percent under identical conditions. TiZrNbV, by contrast, showed the best plastic deformation capability and the highest quasi-static fracture strength, climbing from 1960 to 2021 megapascals, while TiZrWV sat in the middle with intermediate values in both categories.

Scanning electron microscopy of the fractured specimens explained these differences at the microstructural level. TiZrWV failed through quasi-cleavage planes characteristic of brittle fracture. TiZrNbWV displayed a layered, stepped surface with folds and tear ridges, indicating a mixed brittle-ductile process in which crack propagation was accompanied by significant plastic flow. TiZrNbV showed river-like patterns and abundant ductile dimples of moderate depth, the signature of fracture driven by the nucleation, growth, and coalescence of microvoids. In all three alloys, cracks tended to propagate along shear planes oriented at roughly forty-five degrees to the compression axis, and bright flashes of light were recorded at the fracture surfaces, evidence that even slow, quasi-static loading can trigger measurable energy release.

Dynamic compression on a split Hopkinson pressure bar pushed the strain rates into the 1000 to 3100 per second range and exposed pronounced strain-rate strengthening in every alloy. TiZrWV saw its yield strength jump by more than fifty percent relative to its quasi-static values, reaching 1723 megapascals at a strain rate of 2105 per second, while TiZrNbWV reached 1967 megapascals at nearly 3000 per second. The relative strengthening followed the order TiZrWV, then TiZrNbV, then TiZrNbWV, a pattern the authors attribute to the different ways niobium and tungsten contribute to baseline strength and strain-rate sensitivity. Under the microscope, the dynamically fractured specimens told an even more dramatic story: all three alloys developed localized molten structures on their fracture surfaces, from large melted blocks in TiZrWV and TiZrNbWV to fine molten droplets and flow traces in TiZrNbV, confirming that adiabatic heating during high-strain-rate shear had raised local temperatures high enough to melt metal.

The centerpiece of the study was a series of ballistic experiments in which cylindrical flat-nosed projectiles of each alloy were fired from a 14.5-millimeter-bore ballistic gun at a novel three-layer spaced target: a five-millimeter-thick 2A12 aluminum front plate, a 1.6-millimeter circuit board populated with conventional electronic components, and a three-millimeter aluminum rear plate, with 100-millimeter gaps between layers. A high-speed camera running at 30,000 frames per second captured the impacts at velocities ranging from roughly 548 to 1012 meters per second. The results showed a striking division of labor. Penetration of the front aluminum plate was dominated by kinetic effects, with classic plug-shear failure and circular perforation, and many reactive fragments passed through unreacted, producing only weak flashes. The main energy-release reactions occurred later, when the fragmented projectile struck the rear circuit board, shattered further, and generated a high-temperature fragment cloud that reacted violently with atmospheric oxygen.

To quantify this behavior, the researchers introduced two optical indicators drawn from the high-speed imagery: flash duration, which reflects how long the energy release persists, and flash intensity, a normalized grayscale measure of instantaneous emission. TiZrNbWV proved the most persistent reactor, with flash durations reaching 73.92 milliseconds at high impact velocity, nearly eighteen times its low-velocity value. TiZrWV followed with a maximum of 55.08 milliseconds, while TiZrNbV lasted only 15.02 milliseconds even at 984 meters per second, showing little sensitivity to velocity. The intensity picture inverted at different speeds: at low velocities TiZrNbV produced the highest peak flash intensity and the steepest rise, indicating strong instantaneous release, whereas at high velocities TiZrWV delivered the most intense and rapid response. TiZrNbV’s intensity curves at 856 and 984 meters per second even showed a bimodal profile, attributed to secondary fragmentation generating a second wave of reactions.

The damage left behind on the targets provided macroscopic confirmation of these differences. The third-layer aluminum plates perforated by TiZrWV and TiZrNbWV projectiles bore irregular, jagged, tear-like holes with rough boundaries, consistent with dispersed fragment populations and sustained, spatially nonuniform loading. The TiZrNbV projectile, with its better deformability and shorter energy-release duration, left a comparatively regular circular perforation with smooth edges, indicating a more concentrated residual fragment population. At the highest velocities, all three alloys produced a quasi-detonation phenomenon, in which extensive fragmentation enlarged the reactive surface area and triggered intense thermochemical release within a very short time, and in several cases energy release continued behind the fully perforated target as a pronounced post-penetration effect.

The authors are careful to note the limits of the work. The ballistic tests covered a restricted velocity range, and flash duration and intensity are image-based comparative parameters rather than direct measurements of absolute energy, so the quantitative link between microstructure, fragmentation, and reaction kinetics remains to be established. Even so, the study delivers a practical design guideline: TiZrNbWV suits applications demanding high structural strength, strong penetration, and sustained post-impact energy release; TiZrWV fits scenarios that prize rapid, intense release under high-velocity impact; and TiZrNbV offers a balance of deformability, concentrated penetration, and fast response. By turning the fleeting glow of an impact into numbers, the work gives designers of energetic structural materials a quantitative language for tuning composition to the mission at hand.

Subject of Research: Mechanical properties and impact-induced energy release of TiZr-based high-entropy alloy projectiles against multi-layer circuitry targets

Article Title: Mechanical properties of reactive high-entropy alloys and energy-release characteristics of projectiles impacting a multi-layer circuitry target

Article References: Shi, Z., Wu, C., Qiao, W., Mao, L., Pang, Z., & Liu, J. (2026). Mechanical properties of reactive high-entropy alloys and energy-release characteristics of projectiles impacting a multi-layer circuitry target. Mechanical Sciences, 17(2), 901-916. https://doi.org/10.5194/ms-17-901-2026

Image Credits: AI Generated

DOI: 10.5194/ms-17-901-2026

Keywords: high-entropy alloys, energetic structural materials, TiZr-based alloys, ballistic impact, energy release, vacuum arc melting, split Hopkinson pressure bar, flash duration, flash intensity, penetration mechanics, strain-rate strengthening, multi-layer target

The study, led by Professor Taekyung Lee of Pusan National University’s School of Mechanical Engineering in collaboration with Seoul National University and Kyungpook National University, was made available online on September 26, 2026, and will appear in Volume 24 of the Journal of Magnesium and Alloys on November 1, 2026. Its central finding is deceptively simple. When hot-rolled AZ31 plates were struck at high velocity along their normal direction, the plates consistently absorbed 6.5 to 6.7 percent more energy and fractured symmetrically upon perforation. When the very same plates were struck along their rolling direction, the response changed dramatically: deformation concentrated into narrow shear bands, adiabatic heating intensified, and the plates failed with asymmetric, elliptical fracture patterns.

To understand why a few percentage points of impact direction can matter so much, it helps to start with the crystal structure itself. Magnesium atoms arrange themselves in a hexagonal close-packed lattice, one of the least symmetric common metal structures. Unlike the cubic lattices of iron or aluminum, which offer many equivalent slip systems in nearly every direction, a hexagonal lattice has a strongly preferred slip plane: the basal plane, the flat hexagonal face of the crystal unit cell. Dislocations move far more easily along that plane than across it. When a polycrystalline metal is processed by rolling, the thousands of tiny grains that make up the plate do not end up randomly oriented. Rolling tends to align them so that their basal planes lie roughly parallel to the sheet surface, producing what metallurgists call a strong basal texture.

That texture is the hidden variable in ballistic performance. In a randomly oriented material, an impact from any direction would sample roughly the same distribution of easy and hard slip systems. In a textured AZ31 plate, an impact along the normal direction encounters a fundamentally different crystallographic landscape than an impact along the rolling direction. The Pusan team set out to quantify exactly how that landscape shapes deformation, stress localization, and fracture at impact velocities near 884 meters per second, using plates ranging from 5 to 20 millimeters in thickness.

The experimental design combined four complementary techniques. High-velocity impact tests measured projectile velocity loss, energy absorption, penetration behavior, and bulging on the rear face of the target. Microstructural characterization then mapped where and how the material had deformed at the grain scale. Crystallographic analysis identified which deformation mechanisms, slip or twinning, had operated in specific regions of the impacted plates. Finally, finite-element simulations tied the observations together by computing stress localization, plastic dissipation, and damage accumulation throughout the impact event, revealing the localized conditions under which shear bands form and fracture initiates.

The mechanism behind the two contrasting responses is a story about twinning. Extension twinning is a deformation mode unique in importance to hexagonal metals: under tension along the crystal’s c-axis, a slice of the lattice shears and reorients itself in a mirror-symmetric way, allowing the crystal to accommodate strain that dislocation slip alone cannot provide. In the ND-impacted plates, the researchers found that extension twinning activated in a relatively uniform, homogeneous way across the deformation zone. Because twinning reorients crystals progressively, it spreads plastic work over a wider volume, keeps stresses distributed, and delays the onset of catastrophic localization. The result is symmetric perforation in thin plates and, in thicker plates that resisted full penetration, controlled bulging on the back face rather than cracking, a signature of a material absorbing energy by deforming over a broad region.

Rolling-direction impacts told the opposite story. Struck along the rolling direction, the textured plate could not activate twinning uniformly. Instead, deformation proceeded through heterogeneous slip combined with patchy twinning, concentrating plastic work into narrow bands. In those bands, the intense localized shearing generated heat faster than it could conduct away, a phenomenon known as adiabatic heating because it occurs on timescales too short for thermal exchange. The softened, heated material shears even more easily, feeding a runaway instability: the adiabatic shear band. Within these bands, the researchers observed dynamic recrystallization, in which the severe strain and heat generate new, fine grains that further localize flow. The end result was the asymmetric, elliptical fracture morphology that distinguished RD impacts from their ND counterparts.

What makes the finding practically significant is that it points to performance gains that require no new alloy development and no added weight. As Professor Lee put it, instead of inventing a new alloy or adding mass, engineers can boost ballistic resistance simply by orienting the plate so that impacts arrive along the direction in which its texture promotes uniform, symmetric deformation, describing this as essentially free performance extracted from material that already exists. For military vehicle panels, aircraft structures, and other weight-sensitive protective applications, where every kilogram of armor trades directly against payload or fuel efficiency, a 6.5 percent improvement in energy absorption achieved through orientation-aware design alone is a meaningful margin.

The work also carries a broader lesson for the lightweight materials community. Magnesium alloys are the lightest structural metals in widespread engineering use, roughly three-quarters the density of aluminum, and they combine that low density with high specific strength and excellent damping capacity. Yet their adoption in impact-critical applications has been slowed by exactly the kind of complexity this study illuminates: a hexagonal lattice, processing-induced texture, and deformation mechanisms like twinning that have no direct analogue in cubic metals. By linking crystal-scale deformation physics to stress localization and macroscopic fracture, the Pusan team has provided a quantitative framework that connects what a metallurgist sees in a pole figure to what an armor engineer measures at the target range.

The authors are careful to note that further research will be needed to determine how the orientation-based approach performs under more complex, real-world impact conditions, including oblique strikes, multi-hit scenarios, and the varied loading paths of actual service environments. Still, the study establishes crystallographic texture and component orientation as first-class design variables for textured magnesium alloys under high-velocity impact, alongside composition, thickness, and heat treatment. For a field racing to make vehicles and aircraft lighter without making them more vulnerable, the message is clear: before specifying a magnesium armor plate, check which way its crystals are facing, because in a hexagonal metal, direction is destiny.

Subject of Research: Orientation-dependent ballistic response and fracture mechanisms of textured AZ31 magnesium alloy plates

Article Title: Pusan National University study reveals why mg alloy ballistic performance depends on impact direction

Article References: Pusan National University study reveals why mg alloy ballistic performance depends on impact direction. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: magnesium alloys, AZ31, ballistic impact, crystallographic texture, hexagonal close-packed, extension twinning, adiabatic shear banding, dynamic recrystallization, finite-element simulation, lightweight armor, anisotropy, Pusan National University