Shear Stiffening Research Articles

Determination of In-Plane Shear Properties of Laminate with V-Notch Rail Shear Test and Digital Image Correlation

Abstract This article presents the results of the application of Digital Image Correlation (DIC) to measurements of in-plane shear modulus and strength of three different carbon fiber reinforced laminates. Three different approaches to shear strain calculations via DIC are evaluated and compared with standard strain gage measurements. Calculation of shear strain based on averaging DIC strain values of strain gages area in most cases yielded results closest to strain gages, while measurements based on single point strain measuring differed the most from strain gages. These results are attributed to shear strain distribution in the center area of the specimen. Thermoplastic matrix fabric reinforced composite had the lowest shear strength at 5% of shear strain, but the highest ultimate shear strength and strain at failure. Of thermosetting materials, laminate reinforced with unidirectional carbon fiber had shear modulus about 10% lower, than fabric reinforced laminate, but higher ultimate strength and strain at failure. This behavior is attributed to the presence of weaves in fabric reinforcing the laminate, causing shear stiffening of the material, but lowering its ability to deform under shear loading.

Nonlinear Finite Element Analysis Formulation for Shear in Reinforced Concrete Beams

This study suggests a novel beam-column element formulation that utilizes an equilibrium-driven shear stress function. The beam shear is obtained from the bi-axial states of micro-planes, through matrix condensation and zero vertical traction assumptions. This properly remedies the shear stiffening of a one-dimensional beam-column element, keeping its degrees of freedom to a minimum. For verification of the proposed method, a total of seven shear test results of reinforced concrete (RC) beams were collected from the literature, in which the key variables were the reinforcement ratio, the presence of shear reinforcement, and section shape. The advantages are clearly shown in the shear stresses distributions being accurately described and the global load-displacement relations being successfully obtained and matching well with various test results. The proposed model shows satisfactory descriptions of the monotonic load-displacement response of the RC beams failing in multiple modes that vary from diagonal-tension to flexural-compression. In addition, more accurate and reliable information of sectional responses including sectional shear deformation and stresses is collected, leading to better prediction of a potential shear failure mode. Finally, the advantages of the proposed model are demonstrated by comparing the analysis results of an RCT-beam by using the different shear assumptions that include the constant and parabolic shear strains, constant shear flow, and the proposed shear stress function.

Rheology of crosslinked entangled polymers: Shear stiffening in oscillatory shear

ABSTRACTPolyborosiloxane (PBS) was synthesized from boric acid and hydroxyl‐terminated polydimethylsiloxane (PDMS). The oscillatory shear behavior of PBS formed by PDMS with different molecular weight was studied. The relaxation time of PBS was calculated by Doi‐Edwards model. Finally, the shear‐stiffening mechanism of reversibly crosslinked entangled polymer was obtained. Shear stiffening occurs at lower shear frequencies, which is mainly due to crosslinking bonds and friction between molecules hindering the movement of molecules. The increase in storage modulus at high frequencies is attributed to the resistance caused by entanglement in the stretching process of molecular chains. In addition, the molecular weight is greater and the degree of shear stiffening is higher. Such a conclusion is useful to further study the application of shear stiffening. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48421.

Self-healing ability and application of impact hardening polymers

To realize intelligent and low-cost protective materials, the development of compounds with smart stress-responsive properties and self-healing abilities over a wide temperature range is a major research goal. In this study, a series of impact hardening polymers (IHPs) with stress-responsive properties and self-healing ability in low temperature were synthesized via the condensation copolymerization of silicone polymers with various chain lengths and boron contents. The results of rheological analysis indicate that the IHPs have self-healing ability at low temperatures (healing times of less than 30 h and healing efficiencies better than 80% at −25 °C). Moreover, the relative shear stiffening effect (RSTe), which describes the stress-responsive ability of the IHP, exceeded 2000. In addition, the self-healing mechanism of IHP was investigated by Fourier transform infrared spectroscopy microscopy and pulsed 1H-NMR measurements. The results suggested that reversible dynamic cross-linking arising from the boron compound and the reduction in the threshold entanglement density owing to the longer length of the flexible polymeric backbone is crucial to impart low-temperature self-healing ability and stress-responsive properties to IHP.

A novel composite silicone foam with enhanced safeguarding performance and self-healing property

A novel hybrid silicone foam was successfully prepared by mixing shear stiffening gel (STG) with polymethylvinylsiloxane (MVQ) using expandable microsphere as a blowing agent. Storage modulus of STG/MVQ composite foam increased from 0.045 MPa to 0.38 MPa, when the shear frequency shifted from 0.1 Hz to 100 Hz, presenting a remarkable shear stiffening characteristic. The tensile result demonstrated that the elongation at break of the hybrid foam was as high as 814%. Besides, the hybrid foam gained excellent self-healing performance due to the cold-flow feature and viscous state of STG. More importantly, the drop hammer test indicated the safeguarding performance enhanced as the STG contents increased, with the residual peak force decreasing from 36.72 KN to 31.50 KN. In addition, a possible mechanism was proposed that it was the dynamic “BO bonds” that played a significant role in shear stiffening.

Multifunctional triboelectric nanogenerator towards impact energy harvesting and safeguards

The widespread impact kinetic energy always causes injury and property loss in daily life. Conventional safeguarding systems protect human beings by passively insulating and dissipating impact energy but have ignoring exploring them. Triboelectric nanogenerator (TENG) proves to be a favorable device in harvesting mechanical energy but shows no protection property. Thus, developing novel safeguarding material with energy-harvesting performance is essential in future generation personal security field. Here, a multifunctional TENG capable of collecting impact kinetic energy with safeguarding property is reported by assembling Al foil with hybrid polymer matrix. The matrix is fabricated by incorporating polytetrafluoroethylene (PTFE) nano-particles into shear stiffening (SST) gel and polydimethylsiloxane (PDMS). The optimized TENG (6 mm) after introducing 1% PTFE nano-fillers into hybrid polymer attains the maximum electrical voltage, current and output power of 44.20 V, 8.84 μA and 390.73 μW, respectively. More importantly, SST/PDMS/PTFE-based TENG generates an instantaneous power density of 135.57 μW/m2 under low velocity impact. The impact force simultaneously decreases from 1971.90 to 839.72 N, exhibiting a remarkable safeguarding property. Finally, a wearable TENG-based electronic skin can capture external impact energy into electrical energy, resist the impact and precisely map force distribution which demonstrates promising application in next generation smart wearable systems.

A liquid metal-based triboelectric nanogenerator as stretchable electronics for safeguarding and self-powered mechanosensing

In recent years, high performance triboelectric nanogenerator (TENG) shows distinguished energy-harvesting property which guarantees its potential applications as a power source. However, collision and harsh impact, occurred in daily life, always injure human beings. Developing novel multifunctional TENG with self-powered mechanosensing and protection properties from violent impact is an urgent and meaningful work. Here, a multifunctional TENG with energy-harvesting, safeguarding and self-powered force-sensing properties was fabricated by assembling shear stiffening gel/polydimethylsiloxane (SSG/PDMS) matrix with GaInSn liquid metal. Operating at 10 Hz, TENG with a size of 50 × 50 × 4 mm3 obtains an output voltage of 35.72 V, current of 5.10 μA, and maximum output power of 182.17 μW. In addition, TENG device generates enhanced output power of 323.97 μW under tensile strain of 80%. Besides harvesting mechanical energy, TENG with fast stimuli-responsive character, has been proven as a self-powered sensor to monitor varieties of physiological movements. More importantly, the TENG device with high damping property enables to dissipate 66.43% of external impact force which provides favorable protection effect. Correspondingly, the distinguished output voltage signals can also reveal and assess different impulsive loads. This work proposes a multifunctional TENG device with energy-harvesting, and utilizing the flexible TENG for safeguarding and self-powered mechanosensing.

Novel Safeguarding Tactile e‐Skins for Monitoring Human Motion Based on SST/PDMS–AgNW–PET Hybrid Structures

AbstractA novel versatile electrical skin (e‐skin) with safeguarding and multisensing properties based on hybrid structures is developed by assembling Ag nanowires (AgNWs), polyester (PET) film with hybrid shear stiffening polymer/polydimethylsiloxane (SST/PDMS) matrix. The hybrid SST/PDMS polymer shows stable configuration. Storage modulus of the SST/PDMS increases from 5.5 kPa to 0.39 MPa when the shear frequency changes from 0.1 to 100 Hz, exhibiting typical rate‐dependent behavior. e‐Skin functions as a human‐monitoring device by detecting various motions such as gentle touching, stroking, elbow bending, as well as speaking. More importantly, due to the shear stiffening characteristic, e‐skin with high damping capacity exhibits safeguarding performance, which can dissipate impact force from 720 to 400 N and increase buffer time (from 0.9 to 2 ms). Meanwhile, distinguishable resistance values can reveal the level of harsh impact applied on the e‐skin. In addition, the visible thermosensation effect of e‐skin similar to chameleon epidermis is convenient for assessing environmental temperature. e‐Skin arrays can precisely map the dynamic impact location and pressure distribution. Finally, the high electrical sensitivity and shear stiffening performance are attributed to the disturbance of AgNW effective conductive paths and dynamic BO bonds, respectively.

PHAETHON: Software for Analysis of Shear-Critical Reinforced Concrete Columns

Earthquake collapse of substandard reinforced concrete (RC) buildings, designed and constructed before the development of modern seismic design Codes, has triggered intense efforts by the scientific community for accurate assessment of this building stock. Most of the proposed procedures for the prediction of building strength and deformation indices were validated by assembling databases of RC column specimens tested under axial load and reversed cyclic lateral drift histories. Usually a column structural behavior is assessed by considering all involving mechanisms of behavior, namely flexure with or without the presence of axial load, shear and anchorage. In the present paper a force-based fiber beam/column element was developed accounting for shear and tension stiffening effects in order to provide an analytical test-bed for simulation of experimental cases such as the lightly reinforced columns forced to collapse. Their peculiar characteristics are the outcome of the shear – flexure interaction mechanism modeled here based on the Modified Compression Field Theory (MCFT) and the significant contribution of the tensile reinforcement pullout from its anchorage to the total column’s lateral drift. These features are embedded in this first-proposed stand-alone Windows program named “Phaethon” -with user’s interface written in C++ programming language code- aiming to facilitate engineers in executing analyses both for rectangular and circular substandard RC columns.

Preparation of dual-stimuli-responsive shear stiffening polymer composite and study on its mechanic-magnetic coupling performance

A kind of innovative dual-stimuli-responsive shear stiffening polymer composite (DSR-SST) with excellent pre-magnetized enhanced (PME) effect and ideal adhesion properties was developed by dispersing NdFeB particles into shear stiffening (S-ST) polymer matrix. Owing to its S-ST characteristic, the storage modulus ( G′ ) of DSR-SST can increase from 102 Pa to 106 Pa when the shear frequency of applied stress varies from 0.1 to 100 Hz. Meanwhile, the stiffness of the multi-functional DSR-SST can also be precisely tunable by shear frequency and magnetic field. More importantly, DSR-SST with high remanence exhibits excellent PME effect in which the G′ can be directionally increased from 1.64 MPa to 2.34 MPa through 1200 mT of pre-magnetization process in short time. Additionally, the mechanism of high residual magnetization and the movement of NdFeB particle chains are proposed and discussed. Finally, the ideal cyclic stability, plastic and adhesive characteristics guarantee the composite with reliable mechanical performance and longer lifespan in safeguarding and damping.

The combined effects of pore structure and pore fluid on the acoustic properties of cracked and vuggy synthetic rocks

Many experimental observations show that the shear modulus of carbonate rocks changes in some cases upon fluid saturation. This observation is inconsistent with the presumption inherent to Gassmann's model that the shear modulus is independent of the fluid saturant. If the shear modulus change after saturation is ignored, the reliability of analyses of amplitude-versus-offset (AVO) and time-lapse seismology would be affected. So the elastic properties and velocities of saturated carbonates with complex pore structure are investigated. We made synthetic carbonate cores containing either void cracks or vuggy pores by first embedding predesigned aluminum foils or NaCl grains into the two-component matrix (carbonate cuttings and epoxy) and then leaching them out. We measured the ultrasonic P-and S-wave velocities of eight synthetic cores at different saturation conditions, and compared our measurements with theoretical predictions. The results indicate that changes in the shear modulus after saturation depend on both the pore structure and the pore saturant. Samples with cracks show shear stiffening because the viscous coupling is more active in narrow pores. Regardless of the pore structure, the shear moduli at kerosene saturation are larger than that at brine saturation, because the completely wetting kerosene provides a more shear-resistant solid-fluid boundary. The dynamic moduli changes with saturation increase as the secondary porosity increases. Gassmann's equation underestimates the velocities at shear stiffening and overestimates the velocities at shear weakening. The velocity deviation between Gassmann's prediction and experimental observations increases as the increase of shear modulus change.

Flexural and shear performance of steel-concrete-steel sandwich slabs under concentrate loads

This paper studies the flexural and shear performance of steel-concrete-steel (SCS11SCS: steel-concrete-steel.) sandwich slabs. Six simply supported slabs with different shear spans, section depths and steel configurations were tested under concentrate loads applied at the center. After the tests parts of the steel plates were taken off to observe the crack distribution. The observed modes of failure included flexural yielding and shear punching. The former was initiated by tensile yielding of the bottom steel plate, while the latter was primarily due to punching shear failure of the concrete core. After flexural yielding or concrete punching, the carrying capacities could further increase under large deflections, owning to the shear stiffening of the top steel plate and the membrane action of the bottom steel plate. The top steel plate showed significant shear contribution after concrete punching. A theoretical model is developed to predict the resistance of SCS slabs under concentrate loads. The flexural capacity is calculated with the yield-line method, and the punching shear resistance is analyzed with the radial sector model. The shear contribution of the tie bars is also analyzed experimentally and theoretically.

Dynamic behavior of magnetically responsive shear-stiffening gel under high strain rate

The dynamic mechanical behavior of the magnetically responsive shear-stiffening gel (MSTG) was investigated by using a modified Split Hopkinson Pressure Bar (SHPB) system. It was found that the elastic modulus of the MSTG increased with increasing strain rate and magnetic field. The elastic modulus of the MSTG with 45 wt% carbonyl iron (CI) particles reached to 126.6 MPa at the strain rate of 7236 s−1, while it was merely 160 Pa without excitation. Under a 300 mT magnetic field, the elastic modulus also increased from 116.5 MPa (no magnetic field) to 255.5 MPa at the strain rate of 2900 s−1. The shear stiffening performance of the MSTG was stable and its maximum yield strain was 17.2%, which was very important for its practical application. The magnetically strengthened mechanisms of the high strain-rate-dependent mechanical properties were proposed. It was found that the enhanced shear-stiffening behavior was attributed to the phase transitions from viscous-liquid state to elastomeric state to glassy state under impact.

Stretchable Polyurethane Sponge Scaffold Strengthened Shear Stiffening Polymer and Its Enhanced Safeguarding Performance.

A simple and scalable "dip and dry" method was developed for fabricating stretchable polyurethane sponge-based polymer composite with excellent shear stiffening effect, creep resisting and adhesion property. The stiffness of the composite was tunable, the storage modulus (G') could automatically increase 3 orders of magnitude with the increasing of shear frequency, and the G'max could reach to as high as 1.55 MPa. Importantly, the composite with ideal damping capacity reduced the impact force by 2 orders of magnitude even under 26 cycles of consecutive dynamic impact loading with no obvious mechanical degradation. Moreover, an enhancing mechanism was proposed and it was found the "B-O cross bond" and the entanglement of polymer chains were attributed to the shear stiffening characteristic. Finally, the excellent adhesion ability and hydrophobicity guarantee the composite with reliable mechanical performance and longer lifespan.

Evolution of elastic and thermal properties during TMOS-gel formation determined by ringing bottle acoustic resonance spectroscopy, impulsive stimulated scattering, photopyroelectric spectroscopy and the hot ball method

The evolution of the elastic and thermal properties of a tetramethylorthosilicate (TMOS)-based gel that exhibits an extraordinary ringing effect when enclosed in a bottle is investigated during the sol–gel transition. The results demonstrate the feasibility of three proposed experimental methods for monitoring of gels during their formation. The shear stiffening evolution during gelation is monitored by ringing bottle, resonant acoustic spectroscopy and by an ultrasonic technique using piezo electric excitation and detection. The evolution of the longitudinal modulus and the thermal diffusivity of the gel during stiffening are simultaneously determined by a combined photoacoustic and photothermal method based on heterodyne diffraction detection of impulsive stimulated scattering by, respectively, a propagating acoustic wave grating and a decaying thermal expansion grating that were both thermo elastically generated using a pulsed laser. Also, the feasibility of an inverse photopyroelectric method and a hot ball technique to monitor the thermal transport efficiency and thermal impedance of a forming gel by tracking the thermal conductivity, the thermal diffusivity, and the thermal effusivity is demonstrated. The network polymerization and stiffening during the sol–gel transition in TMOS-gel corresponds with substantial changes in the shear acoustic velocity and in all thermal properties, while the longitudinal acoustic velocity is only weakly affected.

The Static and Dynamic Mechanical Properties of Magnetorheological Silly Putty

A novel magnetorheological material defined as magnetorheological Silly Putty (MRSP) is prepared by dispersing soft magnetic particles into Silly Putty matrix with shear stiffening property. Static mechanical properties including creep and stress relaxation and dynamic rheological properties of MRSPs are tested by rheometer. The experimental results indicate that the external magnetic field exerts significant influence on the creep and relaxation behaviors. Moreover, the storage modulus of MRSPs increases sharply in response to the external stimuli of increasing angular frequency automatically and can be enhanced by external magnetic field. Besides, temperature plays a key role in shear stiffening and magnetorheological effect of MRSPs. Furthermore, considering the obstruction to the particle chains formation induced by Silly Putty matrix, a nonperforative particle aggregated chains model is proposed. The model curve is in consistency with experimental data, which means it can describe magnetoinduced behavior of MRSPs well.

On the conical indentation response of elastic auxetic materials: Effects of Poisson's ratio, contact friction and cone angle

The linear elastic analytical solution of an axisymmetric probe indenting a semi-infinite half-space forms the backbone of most indentation data analysis protocols. It has been noted in the literature that the theoretical solution relies on a boundary condition that is ill-posed which leads to discrepancies from the actual response that depends, among other parameters, on the Poisson's ratio of the indented material. While correction factors have been proposed, prior studies have concentrated on the positive Poisson's ratio regime and have neglected an exciting and developing class of materials: the auxetic systems. The finite element method is used to simulate the conical indentation response of elastic materials with Poisson's ratios covering the whole thermodynamically possible range, − 1 ≤ ν ≤ 0.5 . Consistent with theoretical predictions, the indentation resistance and hardness of auxetic materials is enhanced compared to their non-auxetic counterparts. The stress profiles and contact details are systematically analyzed and the increase in resistance is traced to the shear stiffening and the reduction of contact area compared to conventional materials. Furthermore, it is shown that the analytical linear elastic solution falls short in accurately describing the indentation response, especially for negative Poisson's ratio materials. In contrast to the theoretical prediction, the contact area reduces as the Poisson's ratio increases resulting in increased required force to penetrate the material and an enhanced pressure distribution beneath the indenter. The analytical solution is corrected for the whole ν range and best fit polynomials are proposed for ease-of-use. The effects of contact-friction and indenter cone-angle are also studied and quantified.

Dynamic Centrifuge Tests to Evaluate Reinforcing Mechanisms of Soil-Cement Columns in Liquefiable Sand

Four centrifuge tests were performed to investigate the reinforcing mechanisms of soil-cement columns in liquefiable sand. Two unimproved baseline models and two models improved with soil-cement columns were subjected to sine sweep and earthquake base motions of varying intensities to observe acceleration, pore pressure, lateral displacement, and settlement responses. The dynamic records were processed to derive the effective natural frequency of the profiles and to obtain the dynamic stress-strain responses for unimproved and improved soil. It was found that the shear reinforcement mechanisms of columns was not effective in reducing cyclic stress ratios in the treated soil; liquefaction triggering occurred nearly at same time for both unimproved and improved soil cases and the magnitude of the resulting soil settlement was not significantly reduced. When the bases of the columns were free to rotate, the columns rocked within the soil and produced negligible shear stiffening of the soil profile. When the bases of the columns were fixed against rocking, the columns deformed primarily in shear and flexure but the observed increase in natural frequency of the improved profile was much smaller than predicted by assuming shear strain compatibility between the columns and soil. Design equations that account for shear strain incompatibility are shown to give reasonable estimates for the observed natural frequency of the improved soil profile, and these equations suggest that the associated reduction in cyclic shear stress in the improved soil is small. The soil-cement columns did, however, remain intact and would have provided a means for supporting overlying structures even after liquefaction was triggering in the soil.

Rate-dependent and self-healing conductive shear stiffening nanocomposite: a novel safe-guarding material with force sensitivity

A novel rate-dependent and self-healing conductive composite with well defined shear stiffening (S-ST) effect was facilely fabricated by dispersing the multi-walled carbon nanotube (MWCNT) into a shear stiffening polymer matrix.

In pursuit of light intermetallic hydrides

The search for a “magic” intermetallic compound, absorbing reversibly and easily copious quantities of hydrogen, has been pursued since the discovery of the first intermetallic hydrides, forty–fifty years ago. Obviously, such intermetallic compounds should contain light metal elements like Li, Mg, Al, Ca, etc. We summarize in this work the experimental evidence that sheds light on the hydrogen absorption abstinence of Al-rich intermetallic compounds. The elastic moduli and/or bonding properties of Al-containing pseudobinary compounds initially decrease and then increase as a function of the Al composition. The shear and the bulk moduli behave differently in the Al-rich portion, where they present significantly higher and lower values, respectively, than the corresponding moduli in the Al-poor region. We claim that the extensive shear stiffening inhibits the hydrogen absorption in the Al-rich compounds. An explanation for the choice of the LaSn0.22Ni0.78 compound as a sorption cryocooler in the Planck mission is suggested in view of its shear softening with respect to LaNi5. Recent attempts to find pseudobinary intermetallic hydrides, containing Li and Ca, are also presented. These attempts are conducted by considering some of the factors which regulate hydrogen absorption in intermetallic compounds.