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Quasi-static peeling of a finite-length, flexible, horizontal, one-dimensional (1D) plate (strip, thin film) from a horizontal, thin, elastomeric layer (foundation) is considered. The displaced end of the plate is subjected to an upward deflection or to a rotation. The top of the interlayer is perfectly bonded to the plate, and its lower surface is bonded to a rigid, flat substrate. A transversality (debonding) condition is derived for peeling, based on the common fracture mechanics approach. Whereas debonding from a Winkler foundation can be expressed in terms of the displacement (or equivalently the foundation stress2) at the bond termination, the sixth-order formulation required for elastomeric foundations involves a more complex debonding criterion. Transversality relationships are used to describe this limit state (here the onset of debonding) in terms of co-state variables, herein the deflection and slope at the peel front. In the analysis, bending is assumed to be paramount, linear Kirchhoff–Love (classical) plate theory is used to model the deformation, and therefore displacements are assumed to be small. The foundation is linearly elastic and incompressible. The effects of the work of adhesion, the length of the plate, and the initial nonbonded length of the plate are investigated. The results are compared to those for a Winkler foundation. By replacing the shear modulus of the interlayer by viscosity, and displacements by their time derivatives, the results are expected to apply to viscous liquid interlayers as well.

The vagina undergoes large finite deformations and has complex geometry and microstructure, resulting in material and geometric nonlinearities, complicated boundary conditions, and nonhomogeneities within finite element (FE) simulations. These nonlinearities pose a significant challenge for numerical solvers, increasing the computational time by several orders of magnitude. Simplifying assumptions can reduce the computational time significantly, but this usually comes at the expense of simulation accuracy. This study proposed the use of reduced order modeling (ROM) techniques to capture experimentally measured displacement fields of rat vaginal tissue during inflation testing in order to attain both the accuracy of higher-fidelity models and the speed of simpler simulations. The proper orthogonal decomposition (POD) method was used to extract the significant information from FE simulations generated by varying the luminal pressure and the parameters that introduce the anisotropy in the selected constitutive model. A new data-driven (DD) variational multiscale (VMS) ROM framework was extended to obtain the displacement fields of rat vaginal tissue under pressure. For comparison purposes, we also investigated the classical Galerkin ROM (G-ROM). In our numerical study, both the G-ROM and the DD-VMS-ROM decreased the FE computational cost by orders of magnitude without a significant decrease in numerical accuracy. Furthermore, the DD-VMS-ROM improved the G-ROM accuracy at a modest computational overhead. Our numerical investigation showed that ROM has the potential to provide efficient and accurate computational tools to describe vaginal deformations, with the ultimate goal of improving maternal health.

Here, we report for the first time that resonance in dynamic-mode cantilever sensors persists in hydrogels and enables the real-time characterization of hydrogel viscoelastic properties and the continuous monitoring of sol-gel phase transitions (i.e., gelation and dissolution processes). Real-time tracking of piezoelectric-excited millimeter cantilever (PEMC) sensor resonant frequency (fair = 55.4 ± 8.8 kHz; n = 5 sensors) and quality factor (Q; Qair = 23.8 ± 1.5) enabled continuous monitoring of high-frequency hydrogel shear storage and loss moduli (G′f and G″f, respectively) calculated by sensor data and fluid–structure interaction models. Changes in the sensor phase angle, quality factor, and high-frequency shear moduli obtained at the resonant frequency (G′f and G″f) correlated with low-frequency moduli obtained at 1 Hz using dynamic mechanical analysis. Characterization studies were performed using physically and chemically crosslinked hydrogel systems, including gelatin hydrogels (6–10 wt. %) and alginate hydrogels (0.25–0.75 wt. %). The sensor exhibited a dynamic range from the rheological properties of inviscid solutions to hydrogels with high-frequency moduli of 80 kPa and low-frequency moduli of 26 kPa. The sensor exhibited a limit of detection of 260 Pa and 1.9 kPa for changes in hydrogel storage modulus (E′) based on the sensor’s phase angle and quality factor responses, respectively. We also show that sensor data enable quantitative characterization of gelation process dynamics using a modified Hill model. This work suggests that cantilever sensors provide a promising platform for the sensor-based characterization of hydrogels, such as quantification of viscoelastic properties and real-time monitoring of gelation processes.

A general testing approach is presented via a fracture mechanics study on the interfacial delamination behavior in overmolded composite materials using a variant of the double cantilever beam (DCB) geometry. Overmolding, a common injection molding process, is used to fabricate asymmetric DCB test specimens with Lexan™ 3414 resin overmolded onto commercially available TenCate Cetex® FST woven glass fiber/polycarbonate laminates. An analytical beam theory model is employed to partition the planar fracture modes at the overmold interface into mode I and mode II components, which are functions of material properties and relative beam thicknesses. Specimen curvature measurements are integrated into the beam theory model to estimate the residual stress effects on fracture mode mixity. We use the overmold thickness as a tunable variable to control fracture mode mixity, and target near mode I fracture conditions, where we find mode I fracture energy ( GIc) values of approximately 1 kJ/m2. Fiber bridging across the failure interface is observed, which is not expected at the nominal polymer/polymer overmold interface. Complementary scanning electron microscopy images of the failure surfaces indicate crack initiation at the overmold interface, followed by a change in locus of failure to the nearby polymer/glass fiber interface in the top layer of the composite laminate. Fiber bridging is observed in all specimens tested over a modest range of mode mixity, including specimens modified to the single leg bending geometry, suggesting that the polymer/glass interface is more susceptible to crack propagation than the desired overmold interface, which likely derives its strength from molecular interdiffusion during the overmolding process.

The vagina is a highly inhomogeneous, anisotropic, and viscoelastic organ that undergoes significant deformations in vivo. The mechanical attributes of this organ facilitate important physiological functions during menstruation, intercourse, and birthing. Despite the crucial mechanical role that the vagina plays within the female reproductive system, the deformations that the organ can sustain over time under constant pressure, in both the longitudinal direction (LD) and circumferential direction (CD), have not been fully characterized. This experimental study focuses on quantifying the creep properties of the vagina via ex vivo inflation testing using the rat as an animal model. Toward this end, rat vaginas were subjected to three consecutively increasing constant luminal pressures (28, 55, and 83 kPa) using a custom-built experimental setup and the resulting inhomogeneous deformations were measured using the digital image correlation (DIC) method. The vagina was found to deform significantly more in the CD than the LD at any constant pressure, suggesting that the organ primarily adapts to constant pressures by significantly changing the diameter rather that the length. The change in deformation over time was significantly higher during the first inflation test at a constant pressure of 28 kPa than during the second and third inflation tests at constant pressures of 55 and 83 kPa, respectively. The findings of this study on the mechanical behavior of the vagina could serve to advance our limited knowledge about the physiology and pathophysiology of this important reproductive organ.

The broad versatility of the T-peel test, widely used for characterizing adhesion across a plethora of adhesives, adherends, and geometries, results in a range of responses that may complicate meaningful interpretation. This research effort, involving several specific specimen types, was undertaken to investigate concerns that commonly used configurations may not always result in plateaus in the force-displacement response. An experimental and numerical study is conducted on debonding of T-peel specimens having 75 mm bond length and 0.81 mm thick adherends made of either 6061 aluminum (Al) or one of the three steels (G70 70U hot dip galvanized, E60 elctrogalvanized (EGZ), 1010 cold-rolled steel) bonded with either LORD® 406 or Maxlok™ acrylic adhesive. For the EGZ and the Al adherends, specimens with a bond length of 250 mm and adherend thickness of 1.60 mm are also examined. Effects of adherend materials and thicknesses, bond lengths, and adhesives on test results are examined using three metrics to interpret the T-peel bond performance. A limited correlation is found between the commonly used “T-peel strength” and the energy dissipated per unit debond area. For these two metrics, the relative performances of the CRS and the Al specimens are quite different. Quasi-static plane strain deformations of the test specimens are analyzed by the finite element method (FEM) and a cohesive zone model using the commercial software, ABAQUS, to help interpret the test data. Numerical results provided energies required to elastically and plastically deform the adherends, and help determine the transition from non-steady-state to steady-state debonding. The FE simulations also facilitate determination of the fraction of the crosshead displacement at which steady-state debonding occurs. Results reported herein could help engineers select appropriate peel specimen dimensions for extracting meaningful data for the adhesive performance and comparisons.

Fused filament fabrication (FFF), a form of filament-based material extrusion additive manufacturing (AM), is an extremely useful technique for the rapid production of highly customized products; however, empirical evidence is heavily relied upon for understanding of the process. Initial modeling attempts have traditionally focused on predicting heat transfer and either interlayer diffusion and adhesion or stress development but have not taken a combined approach to analyze all three components simultaneously in a multiphysics model. In this study, we implement finite difference models to examine the combined heat transfer, polymer diffusion represented as degree of healing (Dh), and residual stress development in FFF of poly(ether imide) (PEI). Printing with PEI is of great interest because of its desirable mechanical properties and high use temperatures, but it also creates a more challenging modeling problem with higher thermal gradients and greater potential thermal processing window compared to traditionally modeled AM materials, such as acrylonitrile-butadiene-styrene (ABS) and polylactic acid (PLA). The larger processing window can potentially allow for more processing options but can also significantly complicate the optimization process. In this study, experimental analyses including trouser tear tests and part warpage measurements provide correlation to predicted Dh and stress levels. The models suggest that the temperature of a layer is influenced by the subsequent printing of up to at least three layers in the geometry studied. The results of this study further demonstrate the sensitivity of the molecular mobility and degree of healing to the reptation time (τrep), such that a small change in the τrep on the order of 2–3x can result in an order of magnitude difference in the time before interfacial healing can begin, culminating in significantly less healing occurring. Furthermore, the reptation time and subsequent healing predictions are highly reliant on the extrapolation method used to extend the reptation time to temperatures below those at which it was measured resulting in significantly different predictive results, even if the same experimental data is used.

Capitalizing on features including high resolution, smooth surface finish, large build volume, and simultaneous multi-color/multi-material printing, material jetting additive manufacturing enables the fabrication of full-scale anatomic models. The ability to print materials that resemble relevant, compliant tissues has especially motivated implementation of material jetting for patient-specific surgical planning or training models. In an effort to broaden the material selection for the material jetting process, and to provide materials that more closely mimic the functional needs for a wider variety of tissues, a composite material system is explored that uses non-curing fluid dispersed into a photo-curable medium. The material properties of the composites are examined through both thermal and mechanical analysis (dynamic mechanical analysis, Shore hardness testing, puncture testing, and tensile testing). Higher contributions of non-curing fluid generally reduce part strength and stiffness, and exponential and second-order polynomial expressions are appropriate fits for many of the mechanical properties as functions of non-curing fluid concentration. Through the fundamental exploration of the impact of an added diluent on material properties, the study advances knowledge on the process-property relationship for multi-material jetting. Additionally, better understanding of the mechanical property space offered by these materials will expand the capabilities of material jetting in the context of biomedical applications. The collection of mechanical properties serve as reference data sets to facilitate quicker screening for tissue-mimicking, medical models.

Separation of an elastomeric layer sandwiched between two flexible molds is a common processing step in the fabrication of soft articles such as ophthalmic lenses. The demolding process is typically engineered for the interfacial separation to preferentially occur at a desired interface by applying displacements at suitably selected locations. This study addresses the potential to predispose debonding along the desired interface by exploring roles of (i) the relative flexibilities of the two molds, (ii) differential preheating of the two molds and the interlayer, and (iii) the interlayer curvature. These objectives are accomplished by numerically analyzing the axisymmetric deformations of an elastomeric layer of uniform thickness sandwiched between the flexible molds, the flanges of which are pried apart by applying displacements along the axis of symmetry. The debonding at each interface between the molds and the interlayer is modeled by using a bilinear traction-separation (TS) relation. It is found that (i) debonding initiates preferentially from the edge of the interface between the elastomer and the mold of lower bending rigidity; (ii) preferential debonding can be engineered by differentially preheating the assembly; (iii) edge debonding is inherently biased to the interface with the smaller radius (inner) mold interface, and (iv) the occurrence of the preferential edge debonding at an interface can be attributed to a bias in the opening shear towards that interface. However, when the overall confinement of the system is large enough as dictated by the flexural rigidities of the molds for an elastomeric layer of given modulus and thickness, debonding may nucleate internally and complicate the progression of the separation process. Internal debonding may also occur when the temperature differential between the two molds is large enough at a given absolute level of temperature increment. Sensitivities of the predicted debonding mechanisms to prescribed biases in the TS parameters are assessed. These results could prove useful in design of demolding processes to achieve desired separation characteristics.

We study analytically the case of cylindrical buckling of a plate resting on an incompressible, elastomeric foundation, which is subjected to in-plane uniaxial compression. Our analysis differs from the classical Hetenyi solution for a plate resting on a Winkler foundation in that we consider the incompressibility and the continuity of the elastomer foundation. The critical buckling force is derived in terms of the flexural rigidity and the length of the plate, and the modulus and the thickness of the elastomer foundation. The critical buckling force scales with these parameters in a different manner than in the case of a Winkler foundation. The ratio of the critical buckling load in the present problem to that predicted for its Winkler counterpart is shown to depend strongly on the lateral confinement of the elastomer foundation.

Fracture, the propagation of a dominant flaw, is an important mode of failure in engineering materials, including adhesively bonded joints. The growth of pre-existing or service-induced cracks, flaws, or damage, whether through rapid propagation or slow advances due to cyclic fatigue or extended load duration, can lead to unsatisfactory performance and even catastrophic failures. The stress intensity factor or the strain energy release approach, which is more commonly used for bonded joints, can be used to quantify the likelihood of failure when compared to critical values of these parameters. Polymeric adhesives are inherently viscoelastic, so time or rate, temperature, and other factors such as moisture content and other environmental influences can affect fracture resistance, sometimes significantly. Fracture energies measured in bonded systems can depend on the mode mixity (opening and sliding components), adhesive thickness, and other factors as well.

In automated layup manufacturing processes of fiber-reinforced polymer composites, the quality of the manufactured part is strongly dependent on frictional behavior. Improper control of frictional forces can lead to defect formation. Frictional sliding rheometry tests provide an innovative methodology to accurately characterize the tool-ply friction of unidirectional (UD) prepreg employing unique annular plate geometries. The effect of processing parameters (temperature, velocity, and normal force) on the frictional response of a carbon fiber prepreg was studied. Moreover, utilizing custom designed plate geometries coupled with optically transparent fixtures allowed for in-situ quantification of the prepreg-rigid surface contact area along with simultaneous characterization of the process parameter-dependent frictional mechanisms. Our findings highlight the reduction in frictional forces with increasing temperature, attributed to the increased resin flowability, while increases in sliding rates resulted in a pronounced increase in the frictional forces. The effect of applied load on the frictional characteristics was more complicated due to contributions from both the adhesive and normal forces. Finally, the results were interpreted in light of the contact area measurements performed at different temperatures, normal force, and sliding rate.

Vaginal tearing at childbirth is extremely common yet understudied despite the long-term serious consequences on women's health. The mechanisms of vaginal tearing remain unknown, and their knowledge could lead to the development of transformative prevention and treatment techniques for maternal injury. In this study, whole rat vaginas with pre-imposed elliptical tears oriented along the axial direction of the organs were pressurized using a custom-built inflation setup, producing large tear propagation. Large deformations of tears through propagation were analyzed, and nonlinear strains around tears were calculated using the digital image correlation technique. Second harmonic generation microscopy was used to examine collagen fiber organization in mechanically untested and tested vaginal specimens. Tears became increasingly circular under pressure, propagating slowly up to the maximum pressure and then more rapidly. Hoop strains were significantly larger than axial strains and displayed a region- and orientation-dependent response with tear propagation. Imaging revealed initially disorganized collagen fibers that aligned along the axial direction with increasing pressure. Fibers in the near-regions of tear tips aligned toward the hoop direction, hampering tear propagation. Changes in tear geometry, regional strains, and fiber orientation revealed the inherent toughening mechanisms of the vaginal tissue. STATEMENT OF SIGNIFICANCE: Women's reproductive health has historically been understudied despite alarming maternal injury and mortality rates in the world. Maternal injury and disability can be reduced by advancing our limited understanding of the large deformations experienced by women's reproductive organs. This manuscript presents, for the first time, the mechanics of tear propagation in vaginal tissue and changes to the underlying collagen microstructure near to and far from the tear. A novel inflation setup capable of maintaining the in vivo tubular geometry of the vagina while propagating a pre-imposed tear was developed. Toughening mechanisms of the vagina to propagation were examined through measurements of tear geometry, strain distributions, and reorientation of collagen fibers. This research draws from current advances in the engineering science and mechanics fields with the goal of improving maternal health care.

Relatively few engineering devices and structures are monolithic, as combinations of materials are often needed to meet the necessary functionality, performance, weight, and cost requirements. Progress in additive manufacturing (AM) now allows multiple materials to be produced in a single manufacturing process, opening new opportunities for expeditiously achieving functional and performance targets. Just as interactions at interfaces have long been of interest in the area of adhesive bonding, similar issues need to be addressed for printed composite materials, including how print orientation may affect failure. In this study, acrylic photopolymers were printed in a multi-material jetting process to produce fracture specimens consisting of an elastomeric layer sandwiched between two stiffer strips. Findings are offered as contributions to the three process modeling challenge topics conveyed in a recent NIST/NSF Measurement Science Roadmap (Pellegrino, 2016), as follows: • Specimen configuration guidelines for standards for validation, certification, and qualification of models/parts: Several test specimen configurations based on the double cantilever beam method were fabricated and evaluated to measure the fracture resistance of the assembled layers, leading to a recommended configuration that minimizes material usage and spurious dissipation. • Understanding interfacial science of AM polymers: Substantial differences are reported in the measured fracture energy and locus of failure (though nominally occurring at or near material interfaces) as a function of the print direction and interface architecture. • Non-equilibrium material/process measurements and models: All tests showed increased fracture energy at higher crack propagation rates; power-law relationships proved useful for comparing specimen configurations and describing this rate-dependent nature of the interface and materials involved. These studies, which additionally include encouraging preliminary results with graded multilayers, also suggest opportunities for designing printed interfaces with improved performance and durability for multi-material AM products.

Purpose Material jetting approximates composite material properties through deposition of base materials in a dithered pattern. This microscale, voxel-based patterning leads to macroscale property changes, which must be understood to appropriately design for this additive manufacturing (AM) process. This paper aims to identify impacts on these composites’ viscoelastic properties due to changes in base material composition and distribution caused by incomplete dithering in small features. Design/methodology/approach Dynamic mechanical analysis (DMA) is used to measure viscoelastic properties of two base PolyJet materials and seven “digital materials”. This establishes the material design space enabled by voxel-by-voxel control. Specimens of decreasing width are tested to explore effects of feature width on dithering’s ability to approximate macroscale material properties; observed changes are correlated to multi-material distribution via an analysis of ingoing layers. Findings DMA shows storage and loss moduli of preset composites trending toward the iso-strain boundary as composition changes. An added iso-stress boundary defines the property space achievable with voxel-by-voxel control. Digital materials exhibit statistically significant changes in material properties when specimen width is under 2 mm. A quantified change in same-material droplet groupings in each composite’s voxel pattern shows that dithering requires a certain geometric size to accurately approximate macroscale properties. Originality/value This paper offers the first quantification of viscoelastic properties for digital materials with respect to material composition and identification of the composite design space enabled through voxel-by-voxel control. Additionally, it identifies a significant shift in material properties with respect to feature width due to dithering pattern changes. This establishes critical design for AM guidelines for engineers designing with digital materials.

This paper experimentally investigates adhesive fracture resistance and crack path selection in adhesive joints containing well-defined localized interfacial defects. Several systematic patterns of localized interfacial defects were created on base-acid treated aluminum adherends by physical vapor deposition of copper through a mask. Adhesive joints were prepared using a commercially available, structural epoxy adhesive and the effect of localized interface defects on the performance of adhesive joints was studied. Under mode-I loading conditions, the presence of localized weak interfaces influenced the fracture energy of a propagating debond over a considerable distance. For a crack tip approaching a given weak interface pattern, a falling or reverse R-curve type trend was observed. Within the same DCB specimen, as the crack tip advanced beyond the patterned region, a rising R-curve type trend was observed as fracture energy increased with increasing crack lengths, which were recorded visually and were also inferred using compliance and crack length relationship. In addition, the mode-I fracture energy was found to scale with the area fraction of the weak interfaces according to a rule of mixtures. For the adhesive system and the joint geometry used in this study, it was observed that cohesive failure in the adhesive layer can be obtained even in the presence of exceptionally weak interfaces (similar in size to those detected in mode-I loading) when loaded under mixed mode conditions (achieved through asymmetric loading of DCB-like specimens), that tended to steer the crack away from the interface. When loaded with the opposite mode mixity direction, the crack tip propagated through the defects, though the fracture energy did not exhibit similar R-curve type trends as observed in mode-I tests. The results offer insights into the interaction of propagating cracks in adhesive layers and their interactions with discrete, localized defects, which could lead to improvements in surface preparations and bond integrity or even to joint designs having intentionally placed defects useful in controlled disassembly or for other purposes.

Thermomechanically bonded nonwoven fabrics contain discrete bonds that are formed from melted and fused fibers. In these fabrics, the fibers are loosely organized although they lie predominantly in the machine direction (MD). Through a custom-built biaxial testing device and simultaneous image capture, the mechanical response of individual bonds in thermomechanically bonded nonwoven fabrics made of polyethylene/polypropylene sheath–core fibers was studied. Toward this end, cruciform specimens ( n = 20) with bonds in the gauge areas and arms aligned in the MD and the cross-direction (CD) were subjected to displacement-controlled equi-biaxial tests. The biaxial force–displacement curves along the two loading directions were found to be different. The average maximum force and average stiffness were significantly higher in the MD than in the CD ( p

Proper crack growth in the investigation of adhesive failure in bonded wood joints requires laborious grain control. By adding auxiliary adherends, this paper introduces and evaluates a novel sample geometry that enables fracture energy characterization of wood veneer-adhesive systems. The proposed method was applied to three wood species to test three commonly used wood adhesives. The proposed sample geometry successfully identified differences in fracture energies associated with the use of different adhesives on the same wood species and offered comparable success rate as grain control method. More importantly, for sawn and planed veneers that were without lathe checks common in rotary-peeled veneers, the success rate was even higher than that of rotary peeled veneers, indicating the significance of our proposed method to wood adhesion. We further explored the effects of crack propagation both parallel and perpendicular to wood grain orientation and designed wax strip specimens to understand rising R-curve behavior observed in certain species.

The role of effective axial compliance on the frictional response of extensible strips is investigated, both experimentally and theoretically. A translational actuator pulled a steel sled resting on top of an elastic strip, bonded only at the leading edge of the sled, across a glass substrate. The friction force and local deformation along the length of the strips were measured using a force sensor and a camera, respectively. By increasing the effective axial compliance of the strip, the static friction force was found to decrease dramatically, while the kinetic friction force increased significantly. For sufficiently soft strips, there was no observable static peak, although there was a slope change in the force-displacement curve at the point where progressive slippage initiated at the leading edge. Possible mechanisms for permanent increase in the kinetic friction are discussed that could be implemented in systems where the kinetic friction is of significant importance. A theoretical model, somewhat analogous to an extension of the classical shear lag model to incorporate elastic-plastic interlayers, is proposed to predict the friction response as a function of effective compliance. The results obtained from the theoretical model are compared with experimental results and shown to be in good agreement. This study provides a better understanding of the effect of axial compliance on the frictional response of materials, paving the way for design and optimization of systems where the static and kinetic friction forces play an important role.

Resilin is a disordered elastomeric protein found in specialized regions of insect cuticles, where low stiffness and high resilience are required. Having a wide range of functions that vary among insect species, resilin operates across a wide frequency range, from 5 Hz for locomotion to 13 kHz for sound production. We synthesize and crosslink a recombinant resilin from clone-1 (exon-1+exon-2) of the gene, and determine the water content (approximately 80 wt%) and dynamic mechanical properties, along with estimating surface energies relevant for adhesion. Dynamic moduli master curves have been developed, by applying the time-temperature superposition principle (TTSP) and time-temperature concentration superposition principle (TTCSP), and compared with reported master curves for natural resilin from locusts, dragonflies, and cockroaches. To our knowledge, this is the first time dynamic moduli master curves have been developed to explore the dynamic mechanical properties of recombinant resilin and compare with resilin behavior. The resulting master curves show that the synthetic resilin undergoes a pronounced transition with increasing ethanol concentrations, with the storage modulus increasing by approximately three orders of magnitude. Although possibly a glass transition, alternate explanations include the formation of intramolecular hydrogen bonds or that the chitin binding domain (ChBD) in exon-2 might change the secondary structure of the normally disordered exon-1 into more ordered conformations that limit deformation.

The finite element method and a cohesive zone model are used to analyze plane strain interfacial debonding of an elastomeric layer from an overhanging deformable plate when it is peeled off by applying a normal displacement at the edge of the overhang. The commercial software, ABAQUS, is employed in this work that is focused on understanding the collective role of the following two non-dimensional parameters: (i) the confinement parameter, α, defined in terms of the flexural rigidity of the plate, and the modulus and the thickness of the interlayer, and (ii) the adhesion parameter, ϕ, defined in terms of the cohesive zone parameters and the modulus to thickness ratio of the interlayer. The interfacial adhesion is characterized by a bilinear traction-separation (TS) relation. Numerical experiments reveal that when α is greater than α c , damage initiates at an interior point on the interface and at the interface corner on the traction-free edge irrespective of the value of ϕ. However, ϕ must be greater than ϕ c for the debonding to become wavy/undulatory. The critical value, ϕ c , of the adhesion parameter agrees with the necessary condition found in our previous work on debonding of an elastomeric layer from a rigid block when it is uniformly pulled outward. For α c , damage/debonding initiates only from the interface corner, and no wavy debonding ensues. The peak peeling force prior to the initiation of an internal debond is found to be a monotonically increasing function of ϕ/α, suggesting its potential use as a design variable and as a guide for determining the TS parameters. Results of a few additional numerical experiments in which the elastomeric layer can debond from both adherends provide insights into designing a demolding process for a sandwiched elastomeric layer.

A generalized scaling law, based on the classical fracture mechanics approach, is developed to predict the bond strength of adhesive systems. The proposed scaling relationship depends on the rate of change of debond area with compliance, rather than the ratio of area to compliance. This distinction can have a profound impact on the expected bond strength of systems, particularly when the failure mechanism changes or the compliance of the load train increases. Based on the classical fracture mechanics approach for rate-independent materials, the load train compliance should not affect the force capacity of the adhesive system, whereas when the area to compliance ratio is used as the scaling parameter, it directly influences the bond strength, making it necessary to distinguish compliance contributions. To verify the scaling relationship, single lap shear tests were performed for a given pressure sensitive adhesive (PSA) tape specimens with different bond areas, number of backing layers, and load train compliance. The shear lag model was used to derive closed-form relationships for the system compliance and its derivative with respect to the debond area. Digital image correlation (DIC) is implemented to verify the non-uniform shear stress distribution obtained from the shear lag model in a lap shear geometry. The results obtained from this approach could lead to a better understanding of the relationship between bond strength and the geometry and mechanical properties of adhesive systems.

In this paper, an experimental and theoretical study of the buckling response of slender elastic beams within granular media is performed. Buckling loads of beams with different flexural rigidity, length, and boundary conditions within granular media of different depths are determined. The Ritz approximate method is implemented to model the buckling response of the beams based on the concept of an overhanging beam on an elastic foundation, using a series of springs whose spring constants change linearly with respect to the depth of the grains. There is good agreement between the experimental results and the theoretical model. There is a characteristic penetration ratio where the beams are not able to sense the boundary condition at the embedded end, resulting in a convergence of the buckling loads. This condition happens when the rigidity of the beam is lower than the effective stiffness of granular support, leading to the confinement of the lower portion of the beam inside the grains, and acting as a secondary boundary condition that is independent of the condition at the end of the beam. We derive a scaling law to characterize this characteristic penetration ratio in terms of a dimensionless stiffness parameter, allowing for the characterization of three distinct interactions between the beam and medium based on the ratio of granular support effective stiffness to the beam’s effective stiffness.

Additive friction stir deposition is an innovative solid-state additive manufacturing process that creates near-net-shape 3D components based on deformation bonding rather than melting and solidification. Here, we employ this process for selective cladding on thin Al-Mg-Si sheet metals and evaluate its feasibility for automotive manufacturing based on the cladding quality and substrate distortion. We show that under optimal conditions, additive friction stir deposition can produce high-quality cladding without surface or interface porosity even if the substrate is as thin as 1.4 mm. In addition, the high strength and good formability of the original Al-Mg-Si substrate are preserved in the post-cladding reinforced structure, which exhibits an ultimate tensile strength of 250 MPa and an elongation of 30 %. Despite the mechanical forces imposed by the tool during deposition, no local buckling or wrinkling is observed in the thin substrate. During cooling, however, in situ monitoring shows that mild, global substrate distortion gradually develops. Upon unclamping, the cladding-on-plate system reconfigures itself to minimize the potential energy, leading to an anticlastic curvature. Based on the curvature measurement and classical lamination theory, this work provides the first quantification of the residual stress caused by this newly-developed additive technology, in which the maximum tensile stress is estimated to be around 40 MPa in the rectangular reinforced structure and the interface mismatch strain is 7.37 × 10−4. Both values are low thanks to the solid-state nature of the process.

Wavy or undulatory debonding is often observed when a confined/sandwiched elastomeric layer is pulled off from a stiff adherend. Here we analyze this debonding phenomenon using a cohesive zone model (CZM). Using stability analysis of linear equations governing plane strain quasi-static deformations of an elastomer, we find (i) a non-dimensional number relating the elastomer layer thickness, h, the long term Young׳s modulus, E ∞ , of the interlayer material, the peak traction, T c , in the CZM bilinear traction-separation (TS) relation, and the fracture energy, G c , of the interface between the adherend and the elastomer layer, and (ii) the critical value of this number that provides a necessary condition for undulations to occur during debonding. For the elastomer modeled as a linear viscoelastic material with the shear modulus given by a Prony series and a rate-independent bilinear TS relation in the CZM, the stability analysis predicts that a necessary condition for a wavy solution is that T c 2 h / G c E ∞ exceed 4.15 . This is supported by numerically solving governing equations by the finite element method (FEM). Lastly, we use the FEM to study three-dimensional deformations of the peeling (induced by an edge displacement) of a flexible plate from a thin elastomeric layer perfectly bonded to a rigid substrate. These simulations predict progressive debonding with a fingerlike front for sufficiently confined interlayers when the TS parameters satisfy a constraint similar to that found from the stability analysis of the plane strain problem.

Fiber-reinforced rubberlike materials commonly used in tires undergo large deformations and exhibit different responses in tension and compression along the fiber direction. Assuming that the response of a fiber-reinforced rubberlike material can be modeled as transversely isotropic with the fiber direction as the axis of transverse isotropy, we express the stored energy function in terms of the five invariants of the right Cauchy-Green strain tensor and account for different response in tension and compression along the fiber direction. The constitutive relation accounts for both material and geometric nonlinearities and incorporates effects of the fifth strain invariant, I5. It has been shown by Merodio and Ogden that in shear dominated deformations, I5 makes a significant contribution to the stress-strain curve. We have implemented the proposed constitutive relation in the commercial software, LS-DYNA. The numerical solutions of a few boundary value problems studied here agree with their analytical solutions derived by using Ericksen's inverse approach, in which part of the solution is assumed and unknowns in the presumed solution are found by analyzing the pertinent boundary value problem. However, computed results have not been compared with experimental findings. When test data become available, one can modify the form of the strain energy density and replace the proposed constitutive relation by the new one in LS-DYNA.

Few advanced mechanics of materials solutions have found broader and more enduring applications than Emil Winkler's beam on elastic foundation analysis, first published in 1867. Now, 150 years after its introduction, this concept continues to enjoy widespread use in its original application field of civil engineering, and has also had a profound effect on the field of adhesion mechanics, including for soft matter adhesion phenomena. A review of the model is presented with a focus on applications to adhesion science, highlighting classical works that utilize the model as well as recent usages that extend its scope. The special case of the behavior of plates on incompressible (e.g., elastomeric and viscous liquid) foundations is reviewed because of the significant relevance to the behavior of soft matter interlayers between one or more flexible adherends.

A chain of point masses connected by linear springs and sometimes dashpots is considered. The chain hangs in a vertical equilibrium configuration, held by its top mass. Then the top mass is released, and the chain falls. Internal damping, modeled by the dashpots, causes the bottom mass to move faster. As the system falls, upper masses sometimes accelerate faster than gravitational acceleration, and collisions may occur between adjacent masses. The types of collisions treated here include elastic, inelastic, and perfectly inelastic (in which colliding masses often stick together thereafter). The unstretched lengths of the springs, and a compressive force caused by pretensioning, may significantly affect the characteristics of the motion. Analytical and numerical results are presented for cases involving a few masses, and some generalizations are made for systems with an arbitrary number of masses. Also, the vertical chain may be used to model the motion of a falling Slinky after release at its top end. The bottom of the continuous Slinky does not move until the coils above it have collapsed onto it, and the collapse time is estimated here using the discrete chain model. For a metal Slinky with 86 masses, the estimated time is close to that previously obtained by a continuous elastic analysis.

The characterization of fracture energy (GIc) of an adhesive joint as a function of bondline thickness requires multiple specimens covering a range of bondline thicknesses. In this work, DCB specimens with linearly increasing or decreasing bondline thickness were studied for their feasibility to determine fracture energy as a function of bondline thickness. In a combinatorial characterization sense, this approach explores the possibility to characterize the effect of bondline thickness on fracture energy through fewer tests than those required for a “one at a time” characterization approach, thus offering a significant reduction in characterization times. Fracture energies were characterized under mode I loading conditions using corrected beam theory. The results obtained from linearly increasing or decreasing bondline thickness specimens showed good agreement with those obtained from specimens with a range of constant bondline thicknesses.

Background: Needle samples may not provide sufficient diagnostic material for the assessment of mediastinal lymph nodes. Objective: The study compared the specimen size and diagnostic performance of a new 19-G endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) needle to that of a standard 22-G EBUS-TBNA needle in a swine model of granulomatous lymphadenopathy. Methods: Granulomatous inflammation was induced in mediastinal lymph nodes (LNs) of 10 domestic swine by injection of talc slurry. The affected LNs were sampled with the 19- and 22-G needles. Collected core tissue area and volume were determined using a specialized software and known needle internal diameter. The sample's quality was assessed using the biopsy core morphology grade (BCMG) as well as the biopsy diagnostic correlation grade (BDCG). Results: There was a significant increase in the average LN size from baseline (11.6 ± 3.2 to 15.2 ± 3.8 mm; p < 0.001) after talc injection. A total of 132 paired samples were collected from 38 LNs. The average mass and volume of the 19-G needle sample were larger than those of the 22-G needle sample: 33.78 ± 47.48 vs. 25.18 ± 32.08 mg (p < 0.002) and 11.40 ± 13.91 vs. 6.91 ± 6.42 mm3 (p < 0.0004), respectively. The pooled needle biopsy samples for the 19- and the 22-G needles had similar BCMG (1.38 ± 0.86 vs. 1.43 ± 0.87, p > 0.2) and BDCG (1.54 ± 0.93 vs. 1.57 ± 0.93, p > 0.2). The 19-G needle samples tended towards less blood contamination (p = 0.057), more often granuloma identification (46 vs. 32%, p = 0.2) and had more cartilage contamination (0.49 ± 1.46 vs. 4.81 ± 16.49% p < 0.003). Conclusion: In experienced hands, the 19- and the 22-G EBUS-TBNA needles have a similar diagnostic yield in the swine model of granulomatous lymphadenopathy. The samples collected by the 19-G needle are larger and may have less blood contamination.

Fracture mechanics characterization tests for adhesive joints are analyzed and reviewed in order to understand their advantages and disadvantages. Data reduction techniques for analytical methods are summarized to understand the improvements implemented in each test. Numerical approaches are also used complementing tests information. Both linear and non-linear methods to obtain the fracture energy release rate are presented. Pure mode I and mode II tests are described. Simple mixed-mode tests, varying only the specimen geometry, with limited mode-mixity are also presented. Performing a wider mode-mixity range requires sophisticated apparatus that are studied in detail. There is no general agreement about the test suitability for mixed-mode fracture assessment of adhesive joints. A universal test that can easily be performed and give accurate results is essential to optimize the expensive testing at the design stage.

Adhesives have become the method of choice for many structural joining applications. Therefore, there is a need for improved understanding of adhesive joint performance, especially their failure, under a variety of loading conditions. Various numerical methods have been proposed to predict the failure of adhesive bonded material systems. These methods generally use a cohesive zone model (CZM) to analyze crack initiation and failure loci. The CZM incorporates a traction–separation law which relates the jump in surface tractions with the jump in displacements of abutting nodes of the cohesive segment; the area under the curve relating these jumps equals the energy release rate which is determined from experimental data. Values of parameters in the CZM are usually obtained through the comparison of results of numerical simulations with the experimental data for pure mode I and mode II deformations. Here a numerical approach to simulate crack initiation and propagation has been developed by implementing CZM in the meshless method using the symmetric smoothed particle hydrodynamics (SSPH) basis functions, and using the design of experiments technique to find optimal values of CZM parameters for mode I failure. Unlike in the finite element method where a crack generally follows a path between element boundaries, in the meshless method a crack can follow the path dictated by the physics of the problem. The numerical technique has been used to study the initiation and propagation of a crack in a double cantilever beam under mode I and mixed mode in-plane loadings. Computed results are found to agree well with the corresponding experimental findings. Significant contributions of the work include the determination of optimum values of CZM parameters, and simulating mode I, mode II and mixed mode failures using a meshless method with the SSPH basis functions.

Elastomeric gaskets are commonly used between cells within a fuel cell stack to ensure that the reactant gases are isolated. Failure of a fuel cell gasket can cause the reactant gases to mix and can lead to failure of the fuel cell. An investigation of the durability of a hydrocarbon elastomeric seal material developed for proton exchange membrane fuel cells was performed by comparing the tearing energy required for crack propagation of as-received and environmentally aged samples. Tear force was recorded as a function of crosshead displacement rate and the critical strain energy release rate was calculated and plotted against crack growth rate. Data obtained at different temperatures were then used to generate a fracture energy master curve. Additional samples were aged in selected relevant environments and compared to the as-received material to study the effect of environmental aging on tear energy master curves. Comparison of tearing energy master curves for different test conditions showed an increase in the tearing energy for all aging environments. The tear energy master curve for testing carried out in water suggested that the crack propagation rates as low as of 10–8 ms–1 can be seen as the fracture energy approaches 80 Jm–2.

Mixed-mode I + II fracture characterization tests of steel-bonded joints were carried out with the dual actuator load apparatus using a previously developed data reduction scheme in order to obtain the fracture envelope. This test involves independent loading of the specimen arms of a clamped double cantilever beam, which allows for easy variation of the I + II mode mixity in fracture characterization through altering the applied displacement rates. Difficulties inherent to crack monitoring during its propagation and imperfections of initial crack manufacture are well managed with the proposed method. Three different cases corresponding to different mode mixities were tested. The experimental results revealed that a linear energetic criterion performs well in describing the fracture envelope of these bonded joints.

The fabrication of modern microelectronic silicon devices mechanically challenges these thin silicon substrates during manufacturing operations. Melt and solution polyesterification enabled the synthesis of polyesters containing photoreactive o-nitro benzyl ester units for use as a potential photocleavable adhesive. Melt transesterification provided a solvent-free method for synthesis of 2-nitro-p-xylylene glycol (NXG)-containing polyesters of controlled molecular weights. 1H NMR spectroscopy confirmed the chemical composition of the photoactive polyesters. Size exclusion chromatography (SEC) determined the number-average molecular weights (Mn) of the polyesters synthesized in the range of 6000 to 12000 g/mol. 1H NMR spectroscopy confirmed increasing levels of photocleavage of the o-nitro benzyl ester functionality with increasing exposure to broad wavelength UV irradiation, and exposure levels ranged from 0–187 J/cm2 UVA. Photocleaveage of approximately 90% of the o-nitro benzyl ester (ONB) units within ...

The present work is dedicated to development of a crack equivalent data reduction scheme applied to the load jig previously developed by Fernlund and Spelt [1] in order to characterize fracture of bonded joints under mixed-mode I + II loading. The jig allows for easy alteration of the mode-mixity and permits covering the full range of mixed-mode I + II combinations. A data reduction scheme based on specimen compliance, beam theory and crack equivalent concept is proposed to overcome several difficulties inherent to the test analysis. The method assumes that the performed test can be viewed as a combination of the double cantilever beam and asymmetrically loaded end-notched flexure tests, which provide modes I and II fracture characterization, respectively. A numerical analysis including a cohesive mixed-mode I + II damage model was performed considering different mixed-mode loading conditions to validate the proposed data reduction scheme. Issues regarding self-similar crack growth and fracture process zone development are discussed. It was verified that the considered in-plane mix mode fracture criterion is well captured using the proposed data reduction scheme.

Visible deformation on the exterior surface of adhesively-bonded automotive body panels in the vicinity of the adhesive application line is referred to as bond-line readout (BLRO). Differential shrinkage of the adhesive and substrate as an assembly cools from bonding temperature to room temperature is the primary factor responsible for BLRO. The gradual relief of bondline readout over time suggests that relaxation of residual stresses within the adhesive layer occurs. This work addresses determination of viscoelastic and thermal expansion characteristics of representative epoxy and polyurethane adhesives for input into numerical models to predict bondline readout. Besides the constitutive properties, usefulness of the bimaterial curvature technique was also investigated for measuring residual stresses in a coating layer, as well as the stress-free temperature. Knowing the constitutive properties and stress-free temperature, the residual stresses and deformations in a steel-epoxy bimaterial specimen were modeled using a simple recursive approach.

A cohesive zone modeling (CZM) approach with a bilinear traction-separation relation is used to study the peeling of a thin overhanging plate from the edge of an incompressible elastomeric layer bonded firmly to a stationary rigid base. The deformations are approximated as plane strain and the materials are assumed to be linearly elastic, homogeneous, and isotropic. Furthermore, governing equations for the elastomer deformations are simplified using the lubrication theory approximations, and those of the plate with the Kirchhoff–Love theory. It is found that the peeling is governed by a single nondimensional number defined in terms of the interfacial strength, the interface fracture energy, the plate bending rigidity, the elastomer shear modulus, and the elastomeric layer thickness. An increase in this nondimensional number monotonically increases the CZ size ahead of the debond tip, and the pull-off force transitions from a fracture energy to strength dominated regime. This finding is supported by the results of the boundary value problem numerically studied using the finite element method. Results reported herein could guide elastomeric adhesive design for load capacity and may help ascertain test configurations for extracting the strength and the fracture energy of an interface from test data.