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Abstract BackgroundPressure-sensitive adhesives (PSAs) are integral to various industrial applications, yet a significant gap remains in accurately assessing their impact properties under dynamic conditions. This limitation hampers the optimization of PSAs for specific uses where impact resistance is critical. ObjectiveThis study aims to develop an experimental method to evaluate the impact properties of PSAs, providing a reliable and reproducible technique to assess their performance. MethodWe designed an experimental setup to simulate real-world impact conditions, incorporating high-speed cameras and an image analysis algorithm to capture the adhesive's behavior under sudden loads. The method's novelty lies in its ability to quantify maximum failure load and adhesion failure mechanisms in the dynamic loading of PSAs. ResultsThe experimental results reveal critical insights into the impact resistance of various PSA formulations, highlighting significant differences in energy dissipation and failure patterns. ConclusionThese findings offer new data not previously available in the literature, enabling a more precise evaluation of PSA performance. The developed method provides a robust framework for assessing the impact properties of PSAs, offering valuable guidance for the design and selection of adhesives in applications requiring enhanced impact resistance. This work bridges the gap between quasi-static testing and realistic dynamic performance, contributing to the advancement of PSA technology. 

Additively manufactured auxetics (structures exhibiting a negative Poisson’s ratio) offer a unique combination of enhanced mechanical strength and energy absorption. These properties can be further improved through strategic material placement and architectural design. This study investigates the feasibility of fabricating bi-material rotating-square auxetic structures composed of flexible and rigid constituents in their squares and hinges. Rotating-square auxetic structures are manufactured via material extrusion using rigid polylactic acid (PLA) and flexible thermoplastic polyurethane (TPU) to explore the effects of material distribution on mechanical performance and failure characteristics at the macro (i.e., component) and meso (i.e., cell) scales. Baseline tests are conducted to quantify single- and bi-material interfacial strength and failure modes under normal, shear, and combined loading conditions. Upon validation of interface integrity, single- and bi-material auxetic structures are fabricated and tested in uniaxial compression. Relative to the TPU single-material structure, the PLA square-TPU hinge structure provides a 33% increase in structural stiffness, increases energy absorption, delays the global densification strain by 10%, yields a structural Poisson’s ratio at least 0.3 lower than its single-material counterpart through global axial strains of 20%, and demonstrates partial shape recovery. Multiscale experimental analyses supplemented by a kinematic model reveal the rotation-dependent stiffening mechanisms of these structures, highlighting the benefits of flexible hinge materials. Bi-material structures with flexible hinges are shown to have bilinear trends in structural stiffness and energy absorption, not intrinsic to their single-material counterparts. These findings highlight the potential of bi-material design strategies in advancing the functionality and tunability of auxetic structures for the next generation of mechanical metamaterials. 

The advent of additive manufacturing, i.e., 3D printing, has enabled the flexibility to realize complex shapes and structures, such as triply periodic minimal surface (TPMS) structures, which are desirable in many engineering applications due to their unique mechanics. Common applications include protective armor or structural reinforcement for military or civilian uses. In this report, three TPMS structures (gyroid, Schwarz diamond, and Schwarz primitive) were fabricated using a hyperelastic photocurable resin and vat photopolymerization (VPP) technique. Additional sets of the same structures were fabricated with geometrical porosity to ascertain the mechanical response of each porous structure as a function of different strain rates (quasi-static and low-velocity impact), i.e., the effect of higher surface area to volume ratio. The results showed that irrespective of geometry, including pores in the TPMS structures causes reduced stress and truncated strain levels achieved under quasi-static loading. Gyroid structures outperformed the other TPMS structures, resulting in higher deformation, irrespective of porosity level. Alternatively, the drop impact results indicated that adding porosity decreased the stress levels and extended the plateau region, achieving greater strains than neat resin structures. The effects of porosity and glass microballoon reinforcement were investigated under the same loading regimes for the gyroid structure. The response of the dual hybridized structures proved to increase the impact efficacy of the gyroid structures compared to all other variations investigated. The results of this paper indicate the potential of additively manufactured TPMS structures made of hyperelastic materials and decorated with stochastic pores for improved impact response. 

Head impacts are a major concern in contact sports and sports with high-speed mobility due to the prevalence of head trauma events and their dire consequences. Surrogates of human heads are required in laboratory testing to safely explore the efficacy of impact-mitigating mechanisms. This work proposes using polymer additive manufacturing technologies to obtain a substitute for the human skull to be filled with a silicone-based brain surrogate. This assembly was instrumentalized with an Inertial Measurement Unit. Its performance was compared to a standard Hybrid III head form in validation tests using commercial headgear. The tests involved impact velocities in a range centered around 5 m/s. The results show a reasonable homology between the head substitutes, with a disparity in the impact response within 20% between the proposed surrogate and the standard head form. The head surrogate herein developed can be easily adapted to other morphologies and will significantly decrease the cost of the laboratory testing of head protection equipment, all while ensuring the safety of the testing process. 

Auxetic mechanical metamaterials show significant potential to impact many engineering fields and have been a topic of considerable research interest in recent years. Existing literature on the topic often aims to achieve larger negative Poisson's ratios or tailorable responses by carefully designed and distributed unit cells. Herein, it is aimed to investigate the relationships between global and local strain fields in rectangular center‐symmetric perforated planar structures, thus highlighting the role of local morphology on the macroscopic material response. Additively manufactured samples with hyperelastic constitutive behavior are characterized under tension. The structures are designed and developed with several perforation aspect ratios, leading to various degrees of auxeticity. Global and local strain fields are characterized using a multiscale digital image correlation measurement approach. The local rotation and in‐plane strain fields generated within the solid portions of the unit cells are correlated with the global strain fields and macroscopic Poisson's ratios for a range of cell geometries. The interplay between cell rotation and strain at the meso (unit cell) scale is shown to be the dominant factor in the strain‐dependent evolution of the Poisson's ratio in the structures.