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Soft materials are known for their plethora of biomedical applications, intricate structure–property correlation and nonlinear mechanical response. Multiple length–time scale phenomena and hierarchical structure results in their nonlinearity. Phenomenological and continuum mechanical models have been developed to predict their mechanics, which have mostly been very material-specific with inability to predict the mechanics of different types of soft materials simultaneously. This shortcoming has been addressed in the present work, wherein a generic nonlinear viscoelastic model has been proposed to predict the mechanical response of hydrogels, sponges, and xerogels. A fractal derivative viscoelastic model is proposed considering a fractal Maxwell model in parallel with a nonlinear spring. In particular, this model is chosen to qualitatively mimic the material nonlinearity inherent in soft materials. The fractal dashpot in combination with the nonlinear spring accounts for the power law time-dependent rheology of generic soft materials. These two different aspects in the form of nonlinear stiffness and non-Newtonian rheology account for mechanics of most soft materials. The present model is shown to fit well the existing literature results for mechanical response of a multitude of soft material classes with different test conditions and loading rates, which is one of the salient features of the model, apart from its simplistic mathematical framework. Further, a parametric study is reported on the mechanics of nanocellulose loaded poly(vinyl alcohol) xerogel. The model predictions are observed to be in conjunction with the experimental observations.

1–3 piezocomposites are extensively used for sensor, actuator, naval and aerospace applications owing to their enhanced electromechanical coupling, wider bandwidth and reliability. However, their performance gets adversely affected when they are subjected to combined static and dynamic loads that initiate both creep and fatigue damage, simultaneously. 1–3 piezocomposites undergo damage due to creep–fatigue interactions because of the passive and viscoelastic polymer matrix and inherent domain switching mechanisms. In this work, experiments are conducted to obtain the electromechanical response of 1–3 piezocomposites subjected to electrical fatigue and mechanical compressive creep loads. It is observed that, the electromechanical response degrades with increase in number of electrical fatigue cycles, matrix volume fraction and mechanical compressive pre-stress. A nonlinear phenomenological combined creep–fatigue damage model is developed within a thermodynamic framework to account for the creep–fatigue interactions in 1–3 piezocomposites. The model predictions are found to be in agreement with the experimental observations.

Magnetoelectric core-shell nanocomposites have tremendous applications in biomedical sectors such as targeted and controlled drug delivery for cancer treatment and non-invasive brain stimulation for treatment of Parkinson's disease. The neural activity deep in the brain is triggered with the help of these magnetoelectric nanocomposites by externally applied magnetic fields, since these materials exhibit substantial magneto-mechano-electrical coupling. Also, they posses better connectivity between the ferroelectric and ferromagnetic phases without interface effects. B a T i O 3 (BTO) and C o F e 2 O 4 (CFO) exhibits significant ferroelectric and ferromagnetic properties which are able to induce effective multiferroic properties. Core-shell nanocomposites of C o F e 2 O 4 as core and B a T i O 3 as shell, were synthesized by co-precipitation and sol-gel techniques, respectively. The optimum calcination temperature required to get pure B a T i O 3 phase was observed to be 900 ∘ C . The tetragonal phase of pure B a T i O 3 and the spinal perovskite structure of pure C o F e 2 O 4 was in accordance with the X-ray diffraction patterns. The core-shell type morphology of the synthesized nanocomposites were corroborated using Transmission Electron Microscopic images. The piezoelectric and magnetization response of these magnetoelectric nanocomposites were captured as functions of temperature and volume fractions of individual phases.

Grain size has significant effects on the electromechanical response of piezoceramics. Since the sintering temperature and holding duration affect the grain size and correspondingly their effective piezoelectric response, these issues should be appropriately addressed in order to properly design smart and intelligent systems. In this paper, PMN-PT piezoceramics with different grain sizes are synthesized by using conventional solid state processing and two-stage sintering method. Grain size effects on the hysteresis, fatigue and creep response are analyzed. Dielectric, ferroelectric and piezoelectric material properties of PMN-PT ceramics are investigated. The piezoelectric coefficient and remnant polarization exhibit diverse grain size effects depending on the sintering temperature and holding duration. The relative dielectric permittivity, piezoelectric coupling constant and dielectric constant show maximum values of 4104.2, 559 pC/N 562.291 pC/N and 0.705 for the microstructure with average grain size of 3.8 μm. The myriad effects of grain size on piezoelectric response are reported in details.

1-3 piezocomposites are often subjected to mechanical pre-stress under elevated temperatures in naval and offshore applications, actuators, and so on, where the presence of the ductile epoxy matrix causes considerable creep in the system. This deteriorates the system efficiency and its overall performance. To address this issue, experiments are performed to capture the electromechanical response for thermo-mechanical creep of 1-3 piezocomposites for different fiber volume fractions under various thermal loads, wherein mechanical depolarization is observed. The piezo-coupling coefficient is observed to degrade. Higher thermal loads resulted in an increase in creep strain coupled with a decrease in creep polarization. A Kelvin–Voigt fractal derivative viscoelastic model is used to capture the creep strain. The creep strain is decomposed into ferroelastic and anelastic parts. The evolution of creep polarization is obtained using the ferroelastic component of the creep strain. The model predictions are found to be in agreement with the experimental observations.

Xerogels are porous networks of crosslinked polymers that are useful for biomedical applications such as drug delivery, scaffold engineering, tissue regeneration, cell culture and wound dressing. However, inferior mechanical properties curtail their applications to a considerable extent. Nanocellulose fibers and crystals are often added into the polymer matrix to improve their mechanical strength. Here, nanocellulose in the mass ratios of 7%, 13% and 18% are loaded into polyvinyl alcohol (PVA) matrix followed by thermo-morpho-mechanical characterization. With increase in nanocellulose content, thermal degradation occurs at a lower temperature. It is observed that addition of higher quantity of nanocellulose crystals leads to the formation of weak cellulose-rich regions causing xerogel rupture. This is predominantly observed for xerogel loaded with 18% nanocellulose crystals. Similarly, addition of higher quantity of nanocellulose fibers increase brittleness of the xerogels causing fracture. This is predominantly observed for xerogel loaded with 18% nanocellulose fibers. Creep strain and stress relaxation is observed to decrease with addition of nanocellulose loading owing to molecular chain restriction and polymer chain immobility.

Piezocomposites find extensive use in sensors and actuators, naval applications, etc. owing to their characteristic coupling between electro-mechanical domains. These materials being relatively soft and ductile show a viscoelastic response to applied thermo-mechano-electrical loads. This causes a drift in the system response and creep is observed, which affects system efficiency. During prolonged service periods, self-heating occurs. Often, piezocomposites are subjected to mechanical pre-stresses prior to electrical input. To understand the response of 1–3 piezocomposites under these intricate loading conditions, experiments are performed to study the electro-mechanical response of 1–3 piezocomposites under thermo-mechano-electrical creep loads. Mechanical depolarisation is observed during the creep process. Creep strain is observed to increase with matrix volume fraction, mechanical pre-stress, electric field and temperature. A decrement in creep polarisation is seen with the increase in matrix v...

Experimental and theoretical characterization on creep behavior of 1 − 3 piezocomposites subjected to a constant electric field and mechanical pre-stress is carried out. For curved surfaces or systems undergoing large deformations, piezo-ceramics are unsuitable since these are brittle in nature. Hence, piezo-composites are used as replacements. Creep is an important aspect with these systems due to a softer viscoelastic matrix involved. Electrical and electro-mechanical creep are studied for these systems. Electrical creep is observed to be maximum at the coercive field. Also, it is inferred that the volume fraction of fiber in 1 − 3 piezocomposites affects the system performance. The electro-mechanical creep is observed to reduce exponentially with increasing compressive pre-stress. Mechanical depolarization effect is observed during compressive loading. A thermodynamically-consistent macro-mechanical model is proposed to capture the creep behavior of 1 − 3 piezo-composites. It is found that the model predictions are in agreement with the experimental observations.

Piezoelectric materials like bulk PZT are often subjected to mechanical creep loads. But being brittle, these undergo premature failure. Hence, piezocomposites are used instead of bulk PZT. Piezocomposites are viscoelastic in nature owing to the presence of a ductile epoxy matrix. In this paper, mechanical creep is studied for 1–3 piezocomposites of different fiber volume fractions under various levels of compressive prestress. Experiments are performed to understand the difference in their electromechanical response. The creep strain is found to increase with the increase in matrix volume fraction and stress level. The creep polarization is observed to decrease with the decrease in fiber volume fraction and stress level. The degradation in the piezocoupling coefficient is observed to be the maximum for the piezocomposite 35% fiber volume fraction and the least for the bulk PZT. This is attributed to the time-dependent permanent re-orientation of ferroelastic domains by 90 $$^\circ $$ during the mechanical creep phenomenon. Finally, a viscoelastic model comprising of two parallel Kelvin–Voigt elements is proposed to capture the creep strain. The creep strain is decomposed into the ferroelastic strain and anelastic strain. Creep polarization is computed from the ferroelastic strain. The model predictions are found to be in agreement with the experimental results.

Xerogels are attractive candidates for scaffolds in tissue engineering and drug delivery. Polyvinyl alcohol xerogels are extensively used for biomedical applications owing to their ease of synthesis, biocompatibility and biodegradability. However, these materials are challenged in terms of their weak mechanical properties and structural instabilities. This work is aimed to look into the morpho-mechanical response of transversely isotropic freeze-dried PVA xerogels under mechanical compressive loads. Microstructurally aligned PVA xerogels are synthesized using unidirectional freeze-drying technique. Uniaxial compressive tests are performed alongside viscoelastic characterization. The effect of mechanical compressive loads upon the xerogel microstructure is studied. Microbuckling of material fibers, creasing and wrinkling of the bulk xerogels are some of the microstructural and surface instabilities discussed.

Polyvinylidene fluoride (PVDF) composite films have witnessed widespread applications in biomedical and flexible electronic devices in recent years. These applications require a material that has enhanced electrical properties, mechanical flexibility along with sufficient strength. A lot of research has been carried out to enhance the piezoelectric and dielectric properties of the PVDF composite films. However, the improvements in the mechanical properties of nano filler reinforced PVDF composite films has not received due attention. In this regard, the present work focuses towards enhancing the mechanical strength and load-bearing capability of PVDF, by preparing composites of PVDF films impregnated with Cellulose Nanocrystals (CNC) and Barium Titanate (BTO) nanoparticles. Solution casting method was adapted to fabricate the PVDF/BTO/CNC films. X-ray diffractograms and FTIR (Fourier Transform infrared spectroscopy) corroborate the presence of ferroelectric γ phase in PVDF. Mechanical and viscoelastic measurements have been carried out to examine the influence of CNC and BTO nano fillers in the PVDF matrix. The addition of CNC and BTO in PVDF film has improved the mechanical strength significantly. The stress achieved for PVDF/5% BTO and PVDF/5% BTO with 0.9% of CNC at 5% strain was 17% and 130% higher than the pristine PVDF, respectively. This study can be helpful for the design engineers to meet custom/specific requirements for a myriad of end-user applications such as piezoelectric nanogenerators.

The paper presents a methodology in the SPH framework to analyze physical phenomena those occur in detonation process of an explosive. It mainly investigates the dynamic failure mechanism in surrounding brittle rock media under blast-induced stress wave and expansion of high pressure product gases. A program burn model is implemented along with JWL equation of state to simulate the reaction zone in between unreacted explosive and product gas. Numerical examples of detonation of one- and two-dimensional explosive slab have been carried out to investigate the effect of reaction zone in detonation process and outward dispersion of gaseous product. The results are compared with those obtained from existing solutions. A procedure is also developed in SPH framework to apply continuity conditions between gas and rock interface boundaries. The modified Grady–Kipp damage model for the onset of tensile yielding and Drucker–Prager model for shear failure are implemented for elasto-plastic analysis of rock medium. The results show that high compressive stress causes high crack density in the vicinity of blast hole. The major principal stress (tensile) is responsible for forming radial cracks from the blast hole. Spalling zones are also developed due to stress waves reflected from the free surfaces.

Polyvinyl alcohol (PVA) hydrogels have recently emerged as suitable bio-mimetic materials for various applications such as scaffold engineering, controlled drug delivery and cell culture. In this paper, PVA hydrogels with different concentrations of PVA have been synthesized using uni-directional freeze-drying technique. Unidirectional freeze-dried hydrogels exhibit transverse isotropy. The microstructurally aligned porous architecture of the hydrogels has been confirmed from Scanning Electron Microscope (SEM) images. Experiments have been conducted to characterize their transversely isotropic and viscoplastic behavior. These materials have been observed to behave non-linearly when subjected to compressive mechanical loads. Theoretical models are proposed to predict the mechanical behavior of the transversely isotropic microstructurally aligned hydrogels. The first model is a visco-elasto-plastic model that incorporates the effect of non-Newtonian behavior effectively using a fractal dashpot. The second model is an anisotropic finite viscoplastic model. The predictions of both the models are found to be in close agreement with the experimental results.

1-3 piezocomposites are excellent candidate materials for sensor, actuator and transducer applications owing to their remarkable dielectric properties, enhanced piezoelectric coupling constants and improved hydrostatic performance along with tunable acoustic impedance, high bandwidth and reliability. These materials find extensive use in aerospace, naval and biomedical sectors. 1-3 piezocomposites show a linear response when subjected to low electric fields and/or mechanical stresses. In such cases, linear models are sufficient for predicting their linear response. But, when high electro-mechanical loads are applied to these materials, they show nonlinearity owing to the presence of a passive and viscoelastic polymer matrix phase and inherent hysteretic damping in the piezoceramic fibers. This is when it becomes mandatory to understand both their linear and nonlinear behavior under different magnitudes of thermo-electro-mechanical static and dynamic loads. Linear response is modeled using linear piezoelectric constitutive equations. Nonlinearities in the form of hysteresis, depolarization, fatigue and creep occurs in 1-3 piezocomposites, which drastically affects their accuracy, precision and efficiency. In order to understand these nonlinearities, analytical and numerical methods have been proposed by several researchers in the past. An endeavor has been undertaken to review some of the attempts made earlier in these directions. Effective properties of these inhomogeneous media are evaluated through different hypotheses and assumptions. Most often, experimental routes are undertaken to predict material properties of 1-3 piezocomposites; nevertheless, the experimental evaluation of a few material properties is quite difficult and sometimes impossible, even with the best state-of-the-art experimental facilities. This motivates researchers to develop theoretical models to predict these material properties. Nonlinearities in 1-3 piezocomposites have been studied by researchers earlier with different theoretical modeling approaches and experimental techniques. This review paper is an endeavor to discuss the progress regarding the study of effective properties and nonlinearities in 1-3 piezocomposites in a coherent and holistic manner.

Hydrogels are soft materials which comprise of trapped solvent within entangled polymeric chains. Although these materials are highly compliant, they are prone to structural and surface instabilities. Buckling, rupture, wrinkling and fracture are some consequences of these instabilities. Under the influence of self-weight and external axial loads, slender hydrogels show viscoelastic buckling. In this paper, the time-dependent response of hydrogels under these conditions has been modelled by considering the energy on account of bending, the work done due to self-weight and external loads, and the viscous damping effects. A Prony series has been used to characterize the compliance and capture the viscoelastic nature of the hydrogels. The parameters of the Prony series were calculated by assuming the hydrogel to be a 3–3 composite. The governing differential equations have been derived using a variational technique. Components of the model have been verified using recorded experimental findings. The model predictions have been reported and a parametric study has been carried out to understand the effect of different system parameters such as the axial load, density and compliance on the viscoelastic response of slender hydrogels.