Recent advances in additive manufacturing have facilitated the development of versatile multimaterial structures constructed by combining flexible and rigid constituents. The mechanical behavior of these structures is heavily dependent on the chemical and mechanical properties of their constituent materials and their bonding during layer-wise fabrication. There exists a lack of a generalizable approach to quantify the mechanical properties of these multi-material structures and the strength of their interfaces. This work investigates the use of standard test methods to characterize the stress-strain characteristics and interface strength of 3D printed single- and bimaterial structures constructed from polylactic acid (PLA) and thermoplastic polyurethane (TPU). Geometrically modified standard tensile samples with interfacial designs intended to reflect normal, shear, and combined loading conditions are fabricated and tested. The test sample geometries are selected to simulate the interface stress conditions observed in T-Peel and lapshear tests. Digital image correlation (DIC) is used to quantify the strain fields developed in the samples. The full-field strain response is then utilized to understand the underlying failure mechanisms at the material interfaces, further highlighting the divergence between the strain at the interface and the average strain fields across the samples. Postmortem analysis of the samples reveals significant plastic deformation at the materials interfaces in the single-materials samples, which are not observed in the bi-material samples. As a consequence of the minimal deformation of the adherends in the bi-materials samples, it is possible to directly compare their mechanical responses across sample classes through energy methods. The modified tensile samples successfully captured the interface failure of single material samples. Additionally, the novel testing protocols successfully captured the macroscale behavior of multi-material T-Peel and lap-shear samples.

Soft materials and structures enable large motions and complex stability profiles through careful geometric designs that are useful for many soft robotics applications. Advancements in additive manufacturing have simplified the fabrication of novel structures with complex geometries, continuing the path to unlocking the full potential of soft structures. In this work, we develop and characterize single print pressure-driven linear extensile soft robotic actuators with a stackable bistable architecture. The actuators are designed through a two-step design process consisting of physical iterations and simulations. Force-displacement characterizations quantify the stackable bistable response of the actuators. The effects of the number of segments with blocked and free-displacement boundaries are discussed in detail for different starting states. Actuator performance is compared to actuators of similar morphologies from the literature. This work provides a recipe for the simple design and fabrication of stackable bistable soft actuators for improved functionality in soft robotic systems.

Polyimide (PI) is recognized as a highly promising dielectric material for high-temperature superconductor (HTS) power transmission cables, owing to its excellent insulation properties, thermal stability, and mechanical properties. To improve its performance, this study explores PI-based nanocomposites incorporating 3-aminopropyltriethoxysilane (APTES)-functionalized SiO2 nanoparticles to enhance dielectric and mechanical performance. Five compositions were synthesized, including pristine PI and PI with 2, 4, 6, and 8 wt% APTES-modified SiO2 nanofillers. The nanocomposites were fabricated by controlled thermal imidization to ensure maximum conversion from polyamic acid (PAA) to PI and strong filler-matrix adhesion. Comprehensive characterization was carried out to evaluate the dielectric and mechanical performance of the prepared nanocomposites. Scanning electron microscopy (SEM) was used to analyze the surface morphology of post-breakdown samples, while Fourier-transform infrared spectroscopy (FTIR) confirmed the successful imidization from PAA to PI. The PI/4 wt% APTES-coated SiO2 nanocomposite possessed a DC dielectric strength of 298.2 kV/mm at room temperature (RT) and 408.2 kV/mm at 93 K, both of which were significantly higher than those of sol-gel-derived SiO2 composites. Incorporating APTES-modified SiO2 resulted in improvements of similar to 85.2% at RT and similar to 52.4% at 93 K compared to their sol-gel SiO2 counterparts. In addition, the PI/4 wt% APTES-SiO2 composite showed enhanced mechanical properties relative to pure PI. These results are attributed to the strong interaction between the silica particles and the polymer enabled by the APTES coating, which improved the material's ability to withstand electrical field stress and reduced trapped charge buildup.

This work investigates how the size and spatial distribution of cells influence the apparent Poisson's ratio of ordered and unordered (stochastic) flexible cellular structures at constant relative densities. A unifying semi-empirical model that captures the structures' mechanical behavior is first developed. The model assumes that adding cells with various diameters alters the apparent Poisson's ratio in an exponentially decaying or growing fashion. The model is parameterized by combinations of cell volume fractions that capture cell morphology and response, an exponential constant, and the apparent Poisson's ratio of a structure with equal cell sizes for a structure class. To validate the model, flexible structures with ordered and unordered cell arrangements are designed, manufactured, and characterized using experimental full-field measurements and finite element simulations. Experimental results validate that the apparent Poisson's ratios and elastic moduli of the ordered structures decrease with an increase in the cell size distribution in the linear deformation regime. Unordered structures show smaller systematic changes in apparent mechanical parameters with a change in cell size distribution. Full-field deformation analyses are performed at various length scales to discuss the contributions of the model parameters and structural deformation mechanisms. Experimental and finite element data are used to fit models for known cell volume fraction functions. Numerical simulations are then performed using the model to study the influence of key model parameters on the apparent Poisson's ratio behaviors in broad classes of cellular solids.

The mechanisms governing the initiation and progression of transverse cracking in composites, particularly under cyclic loading, are inherently multiscale and therefore challenging to investigate experimentally. This work introduces and implements an experimental methodology for the characterization and quantification of fiber-matrix interfacial debonding, with emphasis on the mechanisms driving damage evolution and stiffness degradation under cyclic loading. Leveraging recently developed macro-fiber-based approaches, composite representative volume elements are fabricated by embedding several dozens of glass macro fibers (1mm diameter) within an epoxy matrix. The specimens are subjected to both monotonic and cyclic loading, while fiber-matrix debonding and associated deformation and failure mechanisms are examined using optical digital image correlation. A key outcome of this study is the ability to quantify the number of debonded fibers as a function of strain and cycle number under monotonic and cyclic loading, respectively. The correlations established between microscale damage and macroscale performance metrics reveal the underlying mechanisms linking interfacial debonding to stiffness reduction and provide experimental evidence supporting percolation-type transverse crack formation in unidirectional composites. Furthermore, cyclic loading results indicate that the kinematics and kinetics of the fiber debonding process differ significantly between peak and minimum stresses within each cycle.

This work investigates the applicability of polymer cold spray as a fast and efficient process for repairing impact damage in glass fiber-reinforced composites. Controlled projectile impacts applied by a gas gun instrument are first used to generate varying damage morphologies and depths in woven glass fiber-reinforced composites. The damaged areas are then repaired using two different thermosetting powders applied via cold spray, followed by short post-curing cycles. The thermosetting powders include a polyester-based and an epoxy-based powder reinforced with chopped glass fibers. Differences between the various repair methods are discussed in terms of their effectiveness in restoring lost properties and the apparent adhesion of the repair material to the composite substrate. Moreover, the efficacy of cold spray repair in restoring flexural strength due to impact is quantified for both feedstocks and compared to the conventional resin repair method. Results show that the thermosetting cold spray feedstock can restore up to 80

Until recently, the concept of soft robot safety was an informal notion, often attributed solely to the fact that soft robots are less likely to damage their operating environment than rigid robots. As the field moves toward feedback control for practical applications, it becomes increasingly important to define what safety means and to characterize how soft robots can become unsafe. The unifying theme of soft robotics is to achieve useful functionality through deformation. Consequently, limitations in constitutive model accuracy and risks of material failure are inherent to all soft robots and pose a key challenge in designing provably safe controllers. This work introduces a formal definition of material safety based on strain energy functions and provides a controller that enforces it. We characterize safe and unsafe sets of an incompressible hyperelastic material and demonstrate that safety can be enforced using a high-order control barrier function (HOCBF) with quadratic program-based feedback control. As a case study, we consider material safety enforcement on theoretical models of soft actuators with a tubular geometry having inertial effects, first-order viscous effects, and full-state feedback. Simulation results verify that the proposed methodology can enforce the material safety specification.

This study aims to experimentally quantify fiber-matrix debonding and transverse crack evolution in single-fiber and double-fiber composite specimens, with particular emphasis on hybrid glass/carbon configurations. Neat epoxy, single macro fiber, and double macro fiber model composites with controlled inter-fiber spacing, orientation, and fiber material are fabricated and tested. A full-field experimental measurement framework, enabled by digital image correlation (DIC), has been developed to quantify local displacement and strain fields at and in the vicinity of fiber-matrix interfaces as functions of inter-fiber distance, orientation angle, and fiber material. Novel aspects of this experimental approach include in situ, high-resolution strain measurements enabling direct visualization of damage initiation, shear band formation and crack coalescence, and fiber-to-fiber interaction effects. The experimental findings suggest that carbon macro fibers exhibit highly localized strain fields and delayed debonding, while glass macro fibers lead to more diffuse and rapid debonding initiation. The approximate global stress levels at the onset of fiber-matrix debonding were determined to be 12 MPa and 8 MPa for carbon and glass macro fibers, respectively. In double-fiber specimens, inter-fiber distance and angle strongly influenced shear band formation and microcrack coalescence. In double-fiber specimens fabricated with the same fiber material, shear band coalescence was observed at shorter inter-fiber distances and orientation angles below 45 degrees. In contrast, hybrid glass/carbon configurations promote shear band coalescence at larger inter-fiber distances and orientation angles exceeding 45 degrees. This study provides new experimental insights into interfacial mechanics and fiber-fiber interactions in hybrid composites, enabling robust validation of micromechanics-based models.

Auxetic (negative Poisson’s ratio) structures made from rotating squares have attracted considerable attention due to their tunable shape control, strength, and strain energy absorption capacity. The present study aims to explore the interrelations between mesoscale kinematics and the macroscopic mechanical behavior of additively manufactured rotating-square auxetics under compressive loads. Specifically, correlations between the rotational degree of freedom of the squares, mechanical deformation of the cell hinges, and the macroscopic nonlinear mechanical and Poisson’s behaviors are investigated using experimental measurements supplemented by mathematical models. Structures with variable cell hinge thicknesses are fabricated by stereolithography additive manufacturing technique and then subjected to compressive loads applied at quasi-static and dynamic conditions with several orders of magnitude difference in strain rate. Multiscale mechanical deformation of the structure in each case is analyzed using digital image correlation (DIC). Experimental characterizations indicate strongly nonlinear and rate-sensitive auxetic behaviors in the examined structures. The role of cell hinge thickness is discussed in terms of the mechanical constraint that these components impose on the rotational degree of freedom of the solid squares in the structure, concurrently causing a nonlinear strain hardening behavior.

Complex structures with unique mechanics are pivotal to advancing additive manufacturing, enabling applications where traditional methods are impractical. This study presents a novel 3D auxetic S-shaped monostructure designed for scalability, tunability, and printability using vat photopolymerization. Unit cell geometries were fabricated and experimentally evaluated under quasi-static loading conditions, with full-field analyses providing insights into their structural performance. Benchmarking against common auxetic structures (re-entrant and star topologies) highlighted the superior capabilities of the proposed design. The S-shaped monostructures exhibited geometric insensitivity in their force-displacement responses, with a stiffness of similar to 180 N/m, withstanding large displacements of 11 mm without fracture or self-contact and supporting forces up to 1.8 N (i.e., 95 times their weight) before fully recovering upon unloading. Computational and experimental results demonstrated robust spatial auxeticity, persisting up to 85 % of axial global displacement due to geometry-driven rigid body motion, independent of base material properties. The S-shaped structures achieved superior auxetic performance (nu(maxti) approximate to -0.43) compared to re-entrant (nu(maxti) approximate to -0.30) and star (nu(maxti) approximate to - 0.05) counterparts, with a monotonic and reversible auxetic response throughout loading. Strain contour analyses from digital image correlation validated the reduced stress concentrations and rigid body-dominated mechanism. The exceptional auxeticity and mechanical resilience of the S-shaped monostructures suggest promising applications in advanced designs, including 3D stackable configurations for impact mitigation applications.

Density-graded polyurea foams represent an emerging class of impact-mitigating materials that exhibit excellent energy absorption properties attributed to the inherent hyperviscoelastic characteristics of the base polymer. However, the specific role of the interfacing strategy has remained largely unexplored; hence, this research aimed to elucidate the contributions of interface and gradation configurations on mechanical performance. Two sets of discretely graded samples were fabricated in-house with adhered and seamless interfaces. All samples were subjected to quasi-static uniaxial tensile loading accompanied by 3D digital image correlation to capture full-field deformation responses and relevant mechanical properties as a function of sample configuration. Our findings indicated that in the elastic regime, stretching and bending deformation mechanisms significantly contributed to the overall mechanical behavior regardless of the interfacing or gradation strategies employed. Furthermore, the type of interface played a critical role in strain localization and transduction across the interlayer region. Specifically, seamless density-graded polyurea foam structures predominantly exhibited an intralayer tearing failure mode, whereas their adhered counterparts consistently failed at the interface via delamination. The outcomes of this research underscore the significant impact of seamless interfaces on the overall mechanical performance of density-graded foams, simultaneously unlocking the potential for continuously graded structures in the next generation of protective padding for sports, civilian, and military applications.

An approach is proposed to determine the optimal initiation crack shapes when using the Coupled Criterion (CC) to assess 3D debonding initiation at a fiber-matrix interface. The optimal crack shapes are determined among potential crack shapes described by their length along the fiber-matrix interface, their angle at the free surface and the shape of the debonding front. The optimal crack shapes actually correspond to the crack shapes maximizing the energy criterion, regardless of the interface fracture properties. Stressisocontours fail to replicate the debonding shapes that were experimentally observed on the side of a single glass fiber-transparent epoxy sample, unlike the energy-based crack shapes. Both methods yield similar interface strength identification, however, the energy-based shapes result in larger identified critical energy release rates.

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.

Auxetic (negative Poisson's ratio) foams with reentrant cell structures exhibit enhanced mechanical properties such as superior strength, energy absorption, and fracture resistance, compared to their nonauxetic counterparts. A well-established method for inducing auxeticity in cellular solids involves permanently changing the cell ribs that are buckled under compressive loads. This permanent change can be achieved by heating a deformed foam for a specific duration. In this study, a thermoforming process is developed to convert closed-cell hyperelastic polyurea foams into auxetic structures. The approach relies on rationally identifying critical compression ratios by assessing key mechanical performance attributes of the pristine foam. Auxetic transformation is achieved by applying compressive strains exceeding a defined threshold, with lateral confinement provided by a custom-designed thermoforming die. Microstructural observations and mechanical testing, including stress-strain and Poisson's ratio measurements, confirm the successful auxetic transformation in the foam. Notably, the transition occurs at compression ratios near the nominal densification strain of the original foam. The resulting auxetic foams demonstrate negative Poisson's ratios approaching -0.6 and exhibit energy absorption capacities several times greater than those of the pristine foam. The simplicity and scalability of the proposed thermoforming method underscore its potential for broader application in the development of the next-generation energy-absorbing structures.

The nonlinear mechanical response of 45 degrees woven composites is studied using full-field strain measurements. First, carbon fabric samples without an epoxy matrix are prepared and tested to evaluate the deformation response of the fiber tows in the absence of internal resistance imposed by the epoxy matrix. Next, the effect of the epoxy matrix is examined by fabricating and testing 45 degrees single-ply carbon fiber reinforced composite laminas. Homogeneous strain fields with minimal resistance against external loads are observed in fabric samples without epoxy. The in-plane rotation of fiber tows in the carbon fabric samples varies linearly with the applied global strain. On the other hand, the development of scissoring action in single-ply composite laminas is shown to cause severe shearing of the epoxy entrapped between the orthogonal fiber tows, leading to the outward protrusion of epoxy from the surface of the lamina. The scissoring effect is correlated with different stages of deformation in a highly nonlinear stress-strain response. Finally, similar tests performed on three-ply laminates reveal that the abovementioned matrix protrusion leads to accelerated delamination in 45 degrees laminates at global strains lower than those in single-ply laminas. The underlying mechanisms for scissoring-induced matrix protrusion are discussed using a simple analytical model.

Additive manufacturing approaches enable designing and fabricating structures with complex geometries, such as triply periodic minimal surface (TPMS) lattices with unique mechanics. TPMS structures are pursued for impact mitigation for civilian and military applications. Herein, additive manufacturing of TPMS structures (gyroids, Schwarz diamond, and Schwarz primitive) is done using hyperelastic photocurable resin with glass microballoon reinforcements and the strain rate effects on the mechanical responses are investigated. A successful optimization of vat photopolymerization 3D printing is done to realize TPMS structures with modified photocurable resin with up to 20.6 vol% (20 wt%) glass microballoons. An exploratory investigation is performed using a split‐Hopkinson pressure bar to test the impact response of bulk samples and TPMS structures. It is found that glass‐reinforced hyperelastic resins exhibit favorable mechanical and structural behaviors, motivating comprehensive experimental regimens as a function of strain rates, including quasi‐static and low‐ and moderate‐velocity loading scenarios. The results highlight the affinity of gyroid structures to self‐contact and relative sliding, enhancing the performance at low strain rates. The primitive TPMS structures outperform the remaining counterparts in the impact loading scenarios based on the structural performance. The outcomes of this research evidence the potential of 3D‐printed TPMS structures with glass‐reinforced hyperelastic photocurable resins for improved impact efficacy.

It is often easier to manufacture and experimentally characterize a soft robotic actuator than it is to construct a theoretical or finite element model to understand its mechanics. In many cases, soft actuators with reinforcement materials need to be homogenized to ascertain models for modeling and control, or a high fidelity model is needed, which is often computationally intractable to solve in real time. The modeling of soft robotic actuators should consider the micro-and mesoscale mechanics that impact the macroscale actuator response. Building on recent developments in both data-driven control and full-field measurement techniques of local strain deformations, we propose using Digital Image Correlation (DIC) to ascertain observables to relate the length scales in soft robotic actuators for rapid dynamic modeling. Using flexible structures with rotating squares as a demonstrating example, we fabricate tubular actuators and characterize their motions using DIC. Nonlinear models relating the length scales are constructed from observables ascertained via DIC using Sparse Identification of Nonlinear Dynamical Systems (SINDy). We demonstrate the goodness-of-fit of several models and discuss limitations in extending these models for control. This work provides a general recipe for the data-driven identification of dynamic models for soft robotic actuators from observables at multiple length scales.

Fiber-matrix interface debonding is a precursor to transverse matrix cracking at the mesoscopic scale in fiber composites. The mechanisms controlling fiber-matrix interface debonding and subsequent transverse crack formation have been explored primarily by computational methods with limited experimental verification. This study aims to establish an experimental approach for characterizing fiber-matrix interface debonding in model macro fiber specimens that replicate realistic microstructures. The primary goal is to measure strain fields at individual fiber-matrix interfaces using optical digital image correlation (DIC) and link these measurements to the initiation and propagation of transverse cracks. Macro fiber composite specimens are fabricated by embedding dozens of randomly distributed glass macro fibers (1 mm dia.) in an epoxy matrix. These specimens are then subjected to controlled transverse loading, and their local strain fields are monitored and quantified with high-magnification optical DIC. The experimentally obtained kinematic fields are first used to connect global and local deformation responses and to investigate the mechanisms governing matrix cracking between the fibers. The experimental data are then used to set up and validate a modeling framework created based on cohesive zone and phase field formulations to investigate fiber-matrix interface debond initiation and matrix cracking, respectively. The experimental protocols described here provide a practical approach for characterizing deformation and failure at the fiber-matrix interface and tracking their evolution into larger transverse cracks. Complementary simulation studies highlight the significance of boundary conditions and the uncertainty in the fiber-matrix interface fracture properties in realistic and reliable predictions of debonding kinetics and matrix crack formation. The presented approach is transferable to smaller length scales to enable the quantitative assessment of the effects of geometric and morphological factors, such as inter-fiber distance and angle, on transverse crack formation in fiber composites.

Fiber-matrix debonding in single fiber specimen is studied experimentally and numerically based on the coupled criterion for which various 2D and one 3D configurations are used. Debonding initiation and propagation are mainly due to normal opening stresses in a 3D model whereas shear stresses play a minor role contrary to a 2D front model, i.e. in a plane normal to the fiber main axis. The 3D model enables describing the free surface singularity similarly to a 2D side model, i.e. along the fiber main axis. The latter cannot represent the debonding arrest and stable propagation after initiation. Overall, a 2D front model under plane strain assumption provides the best description of debonding initiation loading level compared to the 3D model, yet for a larger debonding opening. Experimental debonding openings are determined using DIC, providing the debonding initiation remote loading and corresponding opening. Tensile strengths and critical energy release rates respectively slightly higher and in the same order of magnitude are identified in 3D, based on the debonding opening, compared to a 2D front model.