The integration of auxetic cellular structures and shape memory polymers presents a novel approach that could introduce new paradigms for the design and application of these structures across various industries. This paper aims to explore the feasibility and potential of combining these two technologies through simultaneous analysis of the mechanical properties and shape memory performance of the 4D‐printed auxetic cellular structures. The specimens were fabricated from PETG using the FDM 3D printing method and investigated in terms of energy absorption and compressive strength, as well as shape fixity ratio and shape recovery ratio for evaluating mechanical properties and shape memory characteristics. The results indicated that auxetic cellular structures can almost fully recover and maintain their mechanical properties over multiple loading cycles, thanks to the capabilities of 4D printing technology. This finding opens up possibilities for the development of reusable impact and energy absorbers, as well as personal protective gear. Furthermore, PETG's exceptional printability, shape memory effect, and formability enabled both 3D and 4D printing under high applied strain at ambient temperature (cold programming). Two effective strategies were proposed and tested to tackle the issue of low shape fixity—the only limitation of cold programming. These strategies involved increasing the applied strain to 70% and implementing a load‐holding time. Both methods proved effective, enhancing the shape fixity ratio by 16–21% while achieving a high shape recovery rate of over 95%.

Mental fatigue has been studied extensively in relation to its impact on aerobic-, strength-, and motor-based tasks, but anaerobic power-based tasks have received limited attention. Interdisciplinary research investigating the underlying mechanisms by which mental fatigue influences physical performance has been called for. In two studies, the effects of mental fatigue on maximal power jump and endurance jump performance as well as kinetics and kinematics during jump performance were examined. Samples of collegiate volleyball players (Study 1; N = 14) and recreationally active students (Study 2; N = 27) completed two 30 min experimental manipulations (high vs. low cognitive control exertion) before performing three maximal power squat jumps followed by 15 repeated countermovement jumps, with measurements of kinetics and kinematics. For Study 1, the maximal power performance was significantly lower under a mentally fatigued condition, but no differences were observed for repeated jump performance, which may have been attributable to alterations in jump mechanics. For Study 2, no between-condition differences were observed for the maximal power performance, although repeated jump performance was significantly lower under a mentally fatigued condition. Collectively, these findings suggest that the impacts of mental fatigue on power-based performance tasks may depend on the task demands as well as the training status of the individual.

While American football games are played across a wide range of temperatures, current helmet rating protocols assess performance primarily at room temperature. This study evaluates how temperature influences impact attenuation performance of American football helmets and their padding systems. Six top-ranked NFL helmet models and their padding units were tested at four controlled temperatures (− 10, 5, 23, and 38 °C). Padding units were evaluated using drop tests (4.9 kg, 2.4 m/s) to measure peak acceleration, transmitted force, strain, and impact duration. Helmets were tested using a linear impactor (15.6 kg, 5.5 m/s) with a Hybrid III headform instrumented for measuring head peak linear accelerations (PLA) and angular velocities (PAV). Head injury criterion (HIC) and DAMAGE metrics were also calculated. Colder conditions (− 10, 5 °C), compared to room temperature (23 °C), stiffened padding responses, significantly increasing peak acceleration and transmitted force by up to 307% and 331%, respectively, while reducing strain and impact duration in drop testing. Similarly, helmet tests showed significant increases up to 56%, 44%, 86%, and 23% in PLA, PAV, HIC, and DAMAGE, respectively, compared to 23 °C. Warm conditions (38 °C) softened padding materials, producing significantly higher strain and impact durations by up to 38% and 56%, respectively, while helmet-level responses showed no significant change across head kinematic or injury metrics, compared to 23 °C. These findings demonstrate that temperature can influence impact attenuation performance of helmets. Incorporating more comprehensive temperature-dependent evaluation into helmet design and rating protocols is essential, ensuring consistent protection across diverse environmental conditions.

Fall is one of the leading causes of traumatic brain injury (TBI), and thus, there is an increasing interest in validated tools and protective devices to prevent fall-related TBI. Developing head protective technologies for fall requires a reliable testing method to realistically mimic kinematics of head impacts due to fall to evaluate the injury attenuation of such protective headgears. The objective of this study is to recommend an appropriate and repeatable testing method for simulating fall-related head impacts due to standing height falls. To that end, several impact testing methods that commonly use to assess the efficacy of protective headgear were evaluated and compared. The four different test methods include: (1) a whole-body anthropomorphic test device (ATD) drop; (2) a drop-tower equipped with a Hybrid III head and neck assembly; (3) ASTM F429/F1446 standard; and (4) a linear impactor equipped with a Hybrid III head and neck assembly. Although the ATD drop system simulates fall-related head impacts realistically by considering the whole-body kinematics during falls from standing height, this method showed low repeatability. Among the three repeatable testing methods, only the drop tower with Hybrid III head and neck assembly showed statistically similar results to the ATD drop system for front and rear head impacts for all parameters examined in this study including peak linear acceleration, Head Injury Criterion, peak angular acceleration and peak angular velocity. The results suggested that drop-tower with Hybrid III head and neck assembly can realistically captured both translational and rotational motions of the head during impact due to standing height falls in a repeatable manner.

Combat helmet padding plays a critical role in mitigating head injury by absorbing impact energy during ballistic and blunt impacts. While existing padding systems rely on foam- or lattice-based architectures, the potential of controlled structural buckling as a crashworthiness mechanism for helmet padding remains largely unexplored. This study introduces and experimentally evaluates a Buckling-Driven combat helmet padding architecture consisting of interconnected pre-curved columns designed to promote progressive buckling, load redistribution, and sustained energy absorption. The proposed architecture was compared with three commercially foam- or lattice-based padding systems. Padding-level crashworthiness was characterized using drop-tower experiments through stress-strain response, strain energy density, and crash load efficiency. Helmet-level performance was subsequently evaluated using NIJ-Level-III-A ballistic testing and blunt impact testing using a pneumatic linear impactor at velocities of 10, 14, and 17ft/s. Head response was quantified using peak linear acceleration (PLA), Head Injury Criterion (HIC), peak angular velocity (PAV), and the DAMAGE metric. The proposed padding exhibited a plateau-dominated compressive response with sustained load-management efficiency over an intermediate strain range. Under ballistic loading, it reduced PLA and HIC relative to foam-based systems and achieved performance comparable to or improved over the lattice-based configuration depending on impact location. Under blunt loading, the response was velocity- and location-dependent: linear impact metrics were generally comparable to those of the lattice system, whereas rotational metrics showed greater sensitivity to impact location. These findings suggest that controlled buckling can improve ballistic impact attenuation while preserving acceptable blunt impact performance, with further geometric optimization needed to enhance blunt responses.