In-plane Stretch Research Articles

Numerical investigation of elastomeric solid cutting: Enhancing cut initiation for minimally invasive biological tissue cutting

Abstract Minimizing tissue damage during blade cutting is vital for optimal surgical outcomes. However, the elastomeric properties of tissues require that they be considerably deformed before cut initiation, resulting in physical damage. Thus, the blade indentation depth required for cut initiation must be reduced by enhancing the cut-initiation ability of a process. In this study, factors that influence the cut initiation of elastomeric solids are identified by investigating the tensile stress states beneath the blade that trigger cut initiation. Finite element simulations are used to analyze interfacial interactions between the blade and workpiece and their relation to the stress states. Results show that the distribution of the in-plane stretch of the workpiece surface along the blade surface plays a key role in determining the stress states and the resulting cut-initiation ability. The effects of process parameters, including interfacial friction, blade tip geometry, blade motion, and workpiece size, are examined and discussed by analyzing the corresponding in-plane surface stretch distribution. This study offers a fundamental understanding of cut initiation in elastomeric solid cutting for improving surgical cutting tasks.

Microscopic Raman study of fungal pigment using the genetically amenable rock inhabitant Knufia petricola as a model organism

Fungal pigments such as melanin and carotenoids are distinctive markers of animal and plant pathogenic fungi as well as their environmental relatives. These complex pigments play important roles in pathogenicity and stress tolerance while also being useful as biomarkers. Accordingly, it is important to be able to identify in situ the pigments in black fungi, a group of clinical and environmental importance. In this study, wild-type and genetically modified strains of Knufia petricola A95 and wild fungal cells attached to ancient rock were investigated for their spectroscopic and microscopic Raman features and morphological appearance. Knockout mutants of melanin synthesis genes pks1 (polyketide synthase), sdh1 (scytalone dehydratase), and both pks1 and the carotenoid synthesis gene phd1 (phytoene desaturase) were studied We applied two different Raman microscopes using two lasers, with 633 nm and 488 nm wavelengths. We analyzed and compared Raman spectra between the measured reference substances and the mutant and wild-type strains. In the wild strain WT:A95, the peaks close to melanin peals were found at 1353 cm−1 and 1611 cm−1. There are no characteristic melanin peaks at 1580–1600 cm−1 and around 1350 cm−1 at the spectrum of the Δpks1/Δphd1 mutant and the Δsdh1 mutant. The Δpks1 mutant spectrum has the peaks at the beta-carotene v2 C-C in-plane stretch at 1155 cm−1 and v3 C-CH3 deformation at 1005 cm−1. The peaks of carotenoids and melanin were found in all mutants and the wild strain, except the Δpks1/Δphd1 mutant. Raman spectra allow for discrimination between the various pigments. Hence, interactions between natural fungal melanin, as well as other protective pigments, and complex environmental matrices can be characterized on a range of spatial and temporal scales.

Generalized interfaces enabling macroscopic modeling of structural adhesives and their failure

This contribution presents a geometrically linear and thermodynamically consistent model for generalized mechanical interfaces. The interface’s response is kinematically non-coherent, i.e. allows for a displacement jump and for anisotropic cohesive failure, and kinetically non-coherent, i.e. allows for traction jumps across the interface and membrane damage. The traction jump ensues from an interfacial (in)elasticity formulation which introduces additional membrane stiffness and membrane stresses under in-plane stretch in the interface. The cohesive and membrane degradations are coupled via damage variables determined by the different deformation modes to account for their interaction. A numerical example applies the presented model to brittle interfaces adhesively bonding two substrates. By subjecting the interface to a small in-plane stretch prior to performing a peel test, the influence of damage coupling on the mechanical response is illustrated.

Soft Capacitive Pressure Sensors: Trends, Challenges, and Perspectives.

Soft pressure sensors are critical components of e-skins, which are playing an increasingly significant role in two burgeoning fields: soft robotics and bioelectronics. Capacitive pressure sensors (CPS) are popular given their mechanical flexibility, high sensitivity, and signal stability. After two decades of rapid development, e-skins based on soft CPS are able to achieve human-skin-like softness and sensitivity. However, there remain two major roadblocks in the way for practical application of soft CPS: the decay of sensitivity with increased pressure and the coupled response between in-plane stretch and out-of-plane pressure. In addition to existing strategies of building porous and/or high dielectric constant soft dielectrics, are there any other promising methods to overcome those bottlenecks? Are there any further considerations for the widespread deployment of e-skins? This perspective aims to shed some light on those topics.

The Unified Theory of Tubesheet Design—Part I: Theoretical Foundation

Abstract The unified theory of tubesheet (TS) design for fixed TS heat exchangers (HEXs), floating head and U-tube HEXs is developed by removing the midplane symmetry (MPS) assumption, which assumes a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the HEX or one TS need be considered. The unified theory can be successfully extended to all common types of HEXs, with arbitrary combinations of TS configurations for two TSs, and with arbitrary material properties and temperature for each component due to less assumptions. The effects of unperforated annual plate, TS flange, gravitational (e.g., dead-weight of tubes, catalyst inside tubes, shell side fluid and tube side fluid) and fluid flow pressure loss, bending stiffness of the tubes, TS in-plane stretch, pressure in TS perforations as well as temperature gradient across TS are also evaluated by the unified theory. Theoretical analysis shows that the existing theories of TS design can be derived from the unified theory as special deductions. The limitations and problems faced in the existing theories and codes are systematically reviewed and solutions to these limitations and problems are also provided by the unified theory. Numerical comparisons indicate that predictions given by the unified theory agree well with finite element analysis (FEA), while ASME results are not accurate or not correct and may lead to unsafety design.

Universal Strain Energy-Mediated Dynamic Porosity in Physically Networked Elastomers and Their Applications.

Mechanical and physical properties of porous polymers are highly dependent on the arrangement of their internal pores, which once synthesized are widely considered static. However, here we introduce an unconventional dynamic porosity strategy in physically networked elastomer polymers, irrespective of their chemistry. This strategy allows for an omnidirectional and reversible reconfiguration of porosity in response to applied mechanical deformations, even at ambient conditions. In particular, the normal contact pressure between human fingers (just 0.62 MPa) applied on thin elastomer films results in a permanent reversion of the pores to a denser and more solid state. The porous-to-solid transition leads to a 3 order of magnitude reduction in pore density and up to a 22% relative volumetric shrinkage of the films, resulting in an opaque-to-transparent transition (OTT) that acts as a visual indication of porosity state (porous vs nonporous). It is shown that the pore reversion pressure onset is dependent on the average pore-to-pore distance that is controllable through process-specific parameters. Furthermore, the porosity transition is reversible for multiple cycles when the through-plane compression activation is coupled with an in-plane stretch (ε = 700%). A strain energy-mediated thermodynamic model is successfully implemented to confirm the effects of mechanical deformations on pore reversion and generation. Finally, applications of the newfound dynamic porosity concept are exploited for pressure indication, on-demand modulation of materials' mechanical and thermal characteristics, and flexible photomasks.

Tribological properties of suspended hexagonal boron nitride under electric field

Hexagonal boron nitride (h-BN) has huge potential applications in micro-nano electromechanical system due to its good lubricity and insulation. In this paper, a microporous array is prepared on a SiO<sub>2</sub>/Si substrate by the substrate etching process, and then the h-BN is transferred to the microporous substrate to form a suspension structure. The effect of electric field on tribological properties of suspended h-BN is studied by atomic force microscopy. The results show that the friction of the suspended h-BN is smaller than the friction on the h-BN supported by the substrate, because the greater in-plane stretch weakens the puckering effect. The electric field increases the friction of the suspended h-BN, and the influence of positive bias is greater than that of negative bias. The application of the electric field increases the electrostatic force on the tip, thereby increasing the additional load and the interface barrier in the friction process. The electric field causes the stick-slip behavior to change from single-slip to multi-slip. Compared with the h-BN supported by the substrate, h-BN in the suspended state is strongly affected by the electric field. The reduction of the interface distance and the absence of the substrate oxide layer lead the electrostatic force to increase. This paper proposes a method to adjust h-BN’s friction by electric field, which provides theoretical guidance for studying the friction characteristics of two-dimensional materials.

Influence of Evolution in Anisotropy During Strain Path Change on Failure Limits of Sheet Metals

Effects of evolution in anisotropy during plastic deformation under strain path changes on the formability and failure were investigated in the present study. The evolution in anisotropic property of the extra deep drawing steel was considered by implementing the non-quadratic anisotropic yield function Yld2000-2d as a function of effective plastic strain, and the corresponding forming behaviour in two-step forming processes was analysed. For the strain path effect, pre-strain was applied under biaxial mode using Marciniak in-plane stretch forming set-up, followed by the secondary deformation using the out-of-plane stretch forming tool. For failure prediction of the proposed two-step forming, different failure limit approaches were investigated. First, a strain based forming limit diagram (FLD), proposed as the Marciniak–Kuczynski model was modified to include the evolution in anisotropic yield function. The dynamic shift in FLD was also determined by taking strain path change into consideration. In addition, the influence of evolution of yield function on the strain path independent failure limit criteria was also assessed in terms of stress based forming limit diagram. Finally, the prediction accuracy of the failure limit criteria was compared among different models in terms of failure location and limiting dome height (LDH). It was observed that the incorporation of evolution in anisotropic yield surface improved the prediction of formability in terms of the LDH and strain distribution for the investigated material.

Influence of anisotropy evolution on strain path independent failure limits

Stress based forming limit diagram (SFLD) and polar effective plastic strain (PEPS FLD) diagram are two techniques reported to be independent of strain path change. Thus, these techniques are capable of predicting failure in pre-strained sheet metal samples. Therefore, in this study evolution in anisotropy properties of sheet metals were considered in terms of evolution in yield shape during conversion of conventional strain based forming limit diagram (FLD) to SFLD and PEPS-FLD. The evolution in anisotropic properties were modelled as a function of effective plastic strain depending on which the anisotropic coefficients of non-quadratic yield model Yld2000-2d were calculated. Further, to analyse the strain path change effect, metal sheets of extra deep drawing quality steels were first deformed under biaxial condition in Marciniak in-plane stretch forming setup. The pre-strained samples were subsequently deformed with hemispherical punch till the necking appears. For failure prediction, finite element modelling of the complete procedure was performed considering evolution in the yield shape. Finally, both the failure criteria were implemented for failure prediction. Implementation of the evolution in yield function in finite element analysis and failure criteria provided better results.

Modeling and Simulation of Thin Layered Composites Under Non-mechanical Stimuli

This study presents a nonlinear deformation analysis of a thin composite plate undergoing shape reconfigurations triggered by electric field inputs. The composite plate consists of electro-active and inactive (substrate) layers. Prescribing electric field inputs to the electro-active layer induces large curvatures in the composites, while the in-plane stretch is relatively small. This has a consequence that only shapes with zero (or nearly zero) Gaussian curvature can be attained by prescribing the electric field, which leads to snap-through shape reconfigurations in order to achieve developable surfaces. When the substrate is made of an elastic layer, the shape returns to its original planar configuration upon removal of the electric field. In order to fully or partially retain the reconfigured shape upon removing the electric field input, a substrate layer made of a shape memory polymer is considered. Shape reconfigurations and retentions of the composites having elastic substrate and shape memory substrate are simulated.

Multimodal Capacitive and Piezoresistive Sensor for Simultaneous Measurement of Multiple Forces.

Quantitative information on the magnitudes and directions of multiple contacting forces is crucial for a wide range of applications including human-robot interaction, prosthetics, and bionic hands. Herein we report a highly stretchable sensor integrating capacitive and piezoresistive mechanisms that can simultaneously determine multiple forces. The sensor consists of three layers in a sandwich design. The two facesheets serve as both piezoresistive sensors and electrodes for the capacitive sensor, with the core being a porous structure made by using a simple sugar particle template technique to give them high stretchability. The two facesheets contain segregated conductive networks of silver nanowires (AgNWs) and carbon nanofibers (CNFs). By measuring the changes in the electrical resistance of the facesheets and the capacitance between the facesheets, three separate mechanical stimuli can be determined, including normal pressure, in-plane stretch, and transverse shear force. The newly developed multidirectional sensor offers a significant opportunity for the next generation of wearable sensors for human health monitoring and bionic skin for robots.

Effect of material model on strain localization analysis in in-plane stretch bending of high-strength steel sheets

Effect of material model on strain localization analysis in in-plane stretch bending of high-strength steel sheets

Identification of important normal modes in nonadiabatic dynamics simulations by coherence, correlation, and frequency analyses.

Nonadiabatic dynamics simulations of molecules with a large number of nuclear degrees of freedom become increasingly feasible, but there is still a need to extract from such simulations a small number of most important modes of nuclear motion, for example, to obtain general insight or to construct low-dimensional model potentials for further simulations. Standard techniques for this dimensionality reduction employ statistical methods that identify the modes that account for the largest variance in nuclear positions. However, large-amplitude motion is not necessarily a good proxy for the influence of a mode on the electronic wave function evolution. Hence, we report three analysis techniques aimed at extracting from surface hopping nonadiabatic dynamics simulations the vibrational modes that are most strongly affected by the electronic excitation and that most significantly affect the interaction of the electronic states. The first technique identifies coherent nuclear motion after excitation from the ratio between total variance and variance of the average trajectory. The second strategy employs linear regression to find normal modes that have a statistically significant effect on excitation energies, energy gaps, or wave function overlaps. The third approach uses time-frequency analysis to find normal modes, where the vibrational frequencies change during the dynamics simulation. All three techniques are applied to the case of surface hopping trajectories of [Re(CO)3(Im)(Phen)]+ (Im = imidazole; Phen = 1,10-phenanthroline), but we also discuss how these techniques could be extended to other nonadiabatic dynamics methods. For [Re(CO)3(Im)(Phen)]+, it is shown that the nonadiabatic dynamics is dominated by a small number of carbonyl and phenanthroline in-plane stretch modes.

Effect of alkali ions on optical properties of flavins: vibronic spectra of cryogenic M+lumichrome ions (M = Li-Cs) in the gas phase.

The photochemical properties of flavins depend sensitively on their environment and are strongly modified by coordination with metal ions. Herein, the electronic spectra of cold complexes of the smallest flavin molecule (lumichrome, LC, C12N4O2H10) with alkali ions (M+LC, M = Li-Cs) are measured by photodissociation in the visible range (VISPD) in a cryogenic ion trap coupled to a tandem mass spectrometer and an electrospray ionization source. The observed vibronic spectra of all ions are assigned to the optically bright S1 ← S0 (ππ*) transition of the most stable O4 isomer of M+LC by comparison with quantum chemical calculations at the PBE0/cc-pVDZ level coupled to multidimensional Franck-Condon simulations. The rich vibronic spectra indicate substantial geometry changes upon S1 excitation. Large red shifts of the S1 origins upon metal complexation and progressions in the intermolecular in-plane metal stretch and bend modes demonstrate that the strength of the metal-flavin interaction in M+LC(O4) strongly increases by S1 excitation. The stronger M+LC bond in the S1 state of M+LC(O4) is rationalized by the charge reorganization upon ππ* excitation of the LC chromophore. The computations confirm that the optical properties of LC can be strongly modulated by metalation via both the type and binding site of the metal ion.

3D Characterization of corneal deformation using ultrasound speckle tracking.

The three-dimensional (3D) mechanical response of the cornea to intraocular pressure (IOP) elevation has not been previously reported. In this study, we use an ultrasound speckle tracking technique to measure the 3D displacements and strains within the central 5.5 mm of porcine corneas during the whole globe inflation. Inflation tests were performed on dextran-treated corneas (treated with a 10% dextran solution) and untreated corneas. The dextran-treated corneas showed an inflation response expected of a thin spherical shell, with through-thickness thinning and in-plane stretch, although the strain magnitudes exhibited a heterogeneous spatial distribution from the central to more peripheral cornea. The untreated eyes demonstrated a response consistent with swelling during experimentation, with through-thickness expansion overriding the inflation response. The average volume ratios obtained in both groups was near 1 confirming general incompressibility, but local regions of volume loss or expansion were observed. These results suggest that biomechanical measurements in 3D provide important new insight to understand the mechanical response of ocular tissues such as the cornea.

Contact of a spherical probe with a stretched rubber substrate.

We report on a theoretical and experimental investigation of the normal contact of stretched neo-Hookean substrates with rigid spherical probes. Starting from a published formulation of surface Green's function for incremental displacements on a prestretched, neo-Hookean, substrate [J. Mech. Phys. Solids 56, 2957 (2008)JMPSA80022-509610.1016/j.jmps.2008.07.002], a model is derived for both adhesive and nonadhesive contacts. The shape of the elliptical contact area together with the contact load and the contact stiffness are predicted as a function of the in-plane stretch ratios λ_{x} and λ_{y} of the substrate. The validity of this model is assessed by contact experiments carried out using an uniaxally stretched silicone rubber. For stretch ratio below about 1.25, a good agreement is observed between theory and experiments. Above this threshold, some deviations from the theoretical predictions are induced as a result of the departure of the mechanical response of the silicone rubber from the neo-Hokeean description embedded in the model.

Experimental investigation of stretch flangeability of high-strength steel sheets by means of in-plane stretch bending

Abstract An in-plane stretch bending test for metal strips has been developed as a new stretch flangeability testing method. The in-plane stretch bending tests were performed using several high-strength steel (HSS) sheets and forming limit strains at the outer edge of the bent strips were measured. The experimental results were compared with those of the conventional hole-expansion test. The stretch flangeability in the in-plane stretch bending test was found to be lower than that of the hole-expansion test using conical punches. It was also found that the forming limit strain at the sheet edge linearly increased with increasing the strain gradient around the edge in the both tests. It should be emphasized that the conventional limit hole expansion ratio does not necessarily represent the stretch flangeability in the in-plane stretch bending.

Implementation of YLD-96 Plasticity Theory in Formability Analysis of Bi-axial Pre-strained Steel Sheets

In the present work, 15% bi-axial pre-strain with 0.98 strain ratio was induced in extra deep drawing (EDD) steel sheets using an in-plane stretch forming set-up. Subsequently the quasi-static uni-axial tensile responses were evaluated experimentally, and the YLD-96 plasticity theory was implemented to estimate the anisotropic coefficients of both as-received and pre-strained materials. The strain based forming limit diagrams (ɛ-FLD) were evaluated by deforming sheet materials under six different strain paths ranging from 0.94 to -0.31 in a limiting dome height (LDH) test set-up. It was found that the failure strains of the pre-strained material decreased significantly with a shift towards tension-tension region, and hence, the polar effective plastic strain-FLD (PEPS-FLD) was estimated using the formulated YLD-96 plasticity theory to restrict the dynamic shift of failure strains. The FE models of LDH testing of both as-received and pre-strained materials were developed, and the formability was predicted successfully by incorporating PEPS-FLD as a damage model within an error of approximately 6%.

A constitutive law for dielectric elastomers subject to high levels of stretch during combined electrostatic and mechanical loading: Elastomer stiffening and deformation dependent dielectric permittivity

We construct a constitutive law for the response of dielectric elastomers subject to high levels of stretch during combined electrostatic and mechanical loading. The constitutive law is based on a statistical mechanics analysis of a freely jointed chain, due to Kuhn and Grün [1–3], that relates the force of extension and polarizability anisotropy of a polymer chain to its fractional extension, r/nl, through the inverse Langevin function. We utilize a Padé [4] approximant that accurately represents the inverse Langevin function through the entire range of fractional extensions. Thereafter, we cast this machinery into the 8-chain lattice [5,6] and model an elastomer as a heavily interpenetrated network of 8-chain lattices. We assume that the motions of each lattice are affine with the overall deformation of the elastomer. In this fashion, the fractional extension of each chain, r/nl, is linked to the stretch ratios. With such an approach, we obtain a materially objective free energy density and an expression for the dielectric permittivity of the elastomer that depends on the current state of deformation and the overall stretch level. The elastic free energy density depends on two parameters, the small deformation shear modulus and the chain extensibility limit. We observe that the present model and the well established Arruda and Boyce [5], Gent [7], and neoHookean models are all special cases of the eight chain model of the elastic free energy density presented in this work. The isotropic part of the dielectric permittivity and the electrostrictive coefficient depend on the dilatation. The dielectric permittivity remains isotropic under a pure dilatation, but otherwise becomes anisotropic during deformation. The form of the permittivity resembles that of the deformation dependent permittivity presented by Jiménez and McMeeking [8]. However, in the model presented in this work, the electrostrictive coefficient is not only affected by dilatation but also becomes a function of the current level of deformation through the first invariant of the left Green-Cauchy tensor. We utilize the free energy density of the dielectric elastomer to compute the response of a thin film actuator subject to electrostatic and mechanical loading. In this model, the actuator is allowed to have different levels of in-plane limit stretch, and the through thickness permittivity is allowed to increase or decrease with in-plane extension of the actuator. We establish a parameter space map, extensibility limit versus electrostrictive coefficient of the elastomer, for which our constitutive law is relevant to the behavior of dielectric elastomers. With this approach, we study the actuation, electric charge storage, and stability characteristics of the actuator. From the results of our calculations we clearly identify two types of actuator behavior: actuators that exhibit electromechanical instability (type A), and actuators that do not exhibit this instability (type B). We establish that type A actuators develop hysteresis loops in a similar manner to those identified by Zhao, Hong and Suo [9] and Jiménez and McMeeking [8], for dielectric elastomers with constant isotropic permittivity that stiffen during straining, in the case of the former, and for dielectric elastomers that do not stiffen but exhibit a through thickness permittivity that increases/decreases with straining, in the case of the latter. Finally, we show that, while pre-stretch reduces the electric potential and electric charge levels required to operate the actuator, and simultaneously enhances the sensitivity of the actuator to electric potential, it has a detrimental effect on the sensitivity to electric charge.

Fabrication of Deep Cup with Flange by In-Plane Stretch Forming Applying Compressive Force in Thickness Direction

This paper presents a new stretch forming method that applies compressive force for forming a deep cup with a flange. In this method, a punch and a die having a hole are used, and the main parameters are the depth of the die hole, ddh, and the clearance between the punch and the die, c. The effect of ddh and c was investigated by using an aluminum blank of thickness 2 mm in an experiment and a finite element analysis (FEA). When ddh was too small, the material flow could not be controlled appropriately, and when ddh was too large, a local thinning occurred during initial stretching into the die hole. When c was set at large, the side wall thickness of the formed cup was uneven, but a deep cup could be obtained by setting c below a half of the blank thickness. As a result, a deep cup of height 8.3 mm and with a flange was formed successfully under the condition that ddh was 1.5 mm and c was 0.5 mm.