Showing papers on "Stress relaxation published in 2014"
TL;DR: The polyimine is presented, which fundamentally behaves like a classic thermoset at ambient conditions yet can be reprocessed by application of either heat or water, and is thus the fi rst reported Covalent network polymers, which offer robust mechanical properties, generally lack the ability to be recycled.
Abstract: DOI: 10.1002/adma.201400317 as dynamers by Lehn, [ 25,26 ] are stimuli-responsive polymers, most notably exhibiting macroscopic responses to changes in pH. [ 27,28 ] Several imine-containing polymers have been demonstrated, including pH-responsive hydrogels [ 20 ] and a working organic light-emitting diode (OLED). [ 23 ] However, the potential of polyimines as malleable, mechanically resilient polymeric materials, as well as their processability, have remained largely unexplored. We envision that imine-linked polymers can take malleability in covalent network polymers to the next level of simplicity, affordability and practicality. Herein, we present the fi rst catalyst-free malleable polyimine which fundamentally behaves like a classic thermoset at ambient conditions yet can be reprocessed by application of either heat or water. This means that green, room temperature processing conditions are accessible for this important class of functional polymers. A crosslinked polyimine network was prepared from commercially available monomers: terephthaldehyde, diethylene triamine, and triethylene tetramine ( Figure 1 a). A polyimine fi lm was obtained by simply mixing the three above components in a 3:0.9:1.4 stoichiometry in the absence of any catalyst in a mixture of organic solvents (1:1:8, v/v/v, CH 2 Cl 2 /EtOAc/EtOH), then allowing the volatiles to evaporate slowly. Alternatively, the polymer can be obtained as a powder by using ethyl acetate as the only solvent. The polymerization reaction was confi rmed by infrared spectroscopy, which revealed that aldehyde end groups were consumed while imine linkages were formed (Figure S2, Supporting Information). The resulting translucent polymer is hard and glassy at room temperature ( T g is 56 °C) (Figure S1, Supporting Information) and has a modulus of near 1 GPa with stress at break of 40 MPa (Figure S3, Supporting Information). The time and temperature dependent relaxation modulus of the polyimine fi lm was tested to characterize the heat-induced malleability. Figure 1 b depicts the results of a series of relaxation tests over a wide range of temperatures (50–127.5 °C) on a double logarithmic plot. Specifi cally, at 80 °C, the bond exchange reaction is initiated and the normalized relaxation modulus is decreased from 1 to 0.11 within 30 min, indicating an 89% release of the internal stress within the thermoset polymer. By shifting each relaxation curve horizontally with respect to a reference temperature at 80 °C, a master relaxation curve was constructed (Figure 1 c), which indicates the stress relaxation of the polyimine follows the classic time-temperature superposition (TTSP) behavior. The plot of time-temperature shift factors as a function of temperature (Figure 1 d) shows that the polyimine’s stress-relaxation behavior exhibits Arrheniuslike temperature dependence. Using the extrapolation, we calculated that while it takes 30 min for the stress to be relaxed by ca. 90% at 80 °C, the same process would take ca. 480 days at room temperature. The polyimine is thus the fi rst reported Covalent network polymers, which offer robust mechanical properties, generally lack the ability to be recycled. [ 1 ] There has been a great deal of research effort to incorporate reversible crosslinks into network polymers in order to obtain mechanically tough materials with self-healing properties. [ 2–13 ] Many have employed non-covalent crosslinks to achieve this goal. Ionic and hydrogen bonds are readily reversible and have been known to achieve effi cient self-healing performances. [ 14–17 ]
578 citations
TL;DR: It is shown that the unstable and high mobility nano-scale liquid-like regions acting as flow units persist in the glass and can be activated by either temperature or external stress.
Abstract: For glasses, the structural origin of their flow phenomena, such as elastic and plastic deformations especially the microscopic hidden flow before yield and glass-to-liquid transition (GLT), is unclear yet due to the lack of structural information. Here we investigate the evolution of the microscopic localized flow during GLT in a prototypical metallic glass combining with dynamical mechanical relaxations, temperature-dependent tensile experiments and stress relaxation spectra. We show that the unstable and high mobility nano-scale liquid-like regions acting as flow units persist in the glass and can be activated by either temperature or external stress. The activation of such flow units is initially reversible and correlated with β-relaxation. As the proportion of the flow units reaches a critical percolation value, a mechanical brittle-to-ductile transition or macroscopic GLT happens. A comprehensive picture on the hidden flow as well as its correlation with deformation maps and relaxation spectrum is proposed.
231 citations
TL;DR: In this article, the effect of a multifunctional epoxide (Joncryl®;ADR-4368) on the interfacial properties of biopolymer blends based on poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) was systematically investigated.
Abstract: The focus of this paper was to gain a true understanding of the impact of a multifunctional epoxide (Joncryl®;ADR-4368) on the interfacial properties of biopolymer blends based on poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT). The effect of Joncryl on the shear rheological, morphological, and interfacial properties of the blends was systematically investigated. For the deformed drop retraction experiments, different sandwich model systems (droplet/matrix), representing various scenarios of compatibilization, were prepared, aiming to probe the role of the epoxy-functionalized chains on the interface. The decrease of the interfacial tension in the modified/compatibilized PLA_PBAT and the formation of the PLA-Joncryl-PBAT copolymer were highlighted. A new relaxation peak relative to this copolymer was detected by the relaxation spectrum. Transient start-up shear and nonlinear stress relaxation experiments were carried out and confirmed the obtained results. In addition, the interface contribution was demonstrated using the Lee and Park model. The relaxation time increased with the amount of added Joncryl. Hence, the coexistence of chain extension/branching chains coupled to the PLA-Joncryl-PBAT copolymer formation had to be taken into account to explain the improved mechanical properties.
152 citations
TL;DR: In this article, a recent study on weld residual stress relief mechanisms associated with furnace-based uniform post-weld heat treatment (PWHT) is presented, where both finite element and analytical methods are used to quantitatively examine how plastic deformation and creep relaxation contribute to residual stress recovery process at different stages of PWHT process.
115 citations
TL;DR: In this article, the authors derived the detailed expression of stress relaxation time, which reveals an Arrhenius type dependency of material relaxation behavior on the applied temperature, and determined the energy barrier for the BERs in different networks.
Abstract: Thermally malleable polymers which undergo covalent bond exchange reactions (BERs) have been shown to be able to rearrange their network topology at high temperatures without impairing the network integrity. At low temperatures, the BERs are so sluggish that the materials behave like traditional thermosetting polymers. In this paper, we demonstrated that the temperature dependent BER rate could be tuned by adjusting the stoichiometry of monomers. As the ratio of hard segments in the epoxy thermoset network is increased, the material's glass transition temperature (Tg) is increased, with a corresponding increase in the temperature required to achieve a given stress relaxation rate. The material stress relaxation behavior was studied from both a theoretical and experimental point of view. Based on the kinetics of BERs, we derived the detailed expression of stress relaxation time, which reveals an Arrhenius type dependency of material relaxation behavior on the applied temperature. Subsequently, from the experimental stress relaxation curves, we determined the energy barrier for the BERs in different networks. With the Tg being elevated from 30.3 °C to 63.0 °C, the BER energy barrier is linearly increased from 68.2 kJ mol−1 to 97.3 kJ mol−1. Such a correlation between these two thermomechanical behaviors provides an additional design parameter (beyond catalyst choice) which can aid in achieving highly tunable service conditions for practical engineering applications of thermally malleable thermosets.
110 citations
TL;DR: In this article, a nonlinear visco-elasto-plastic creep model with creep threshold and long-term strength was proposed by connecting an instantaneous elastic Hooke body, a visco elastoplastic Schiffman body, and a non linear viscoplastic body in series mode.
Abstract: Triaxial creep tests were performed on diabase specimens from the dam foundation of the Dagangshan hydropower station, and the typical characteristics of creep curves were analyzed. Based on the test results under different stress levels, a new nonlinear visco-elasto-plastic creep model with creep threshold and long-term strength was proposed by connecting an instantaneous elastic Hooke body, a visco-elasto-plastic Schiffman body, and a nonlinear visco-plastic body in series mode. By introducing the nonlinear visco-plastic component, this creep model can describe the typical creep behavior, which includes the primary creep stage, the secondary creep stage, and the tertiary creep stage. Three-dimensional creep equations under constant stress conditions were deduced. The yield approach index (YAI) was used as the criterion for the piecewise creep function to resolve the difficulty in determining the creep threshold value and the long-term strength. The expression of the visco-plastic component was derived in detail and the three-dimensional central difference form was given. An example was used to verify the credibility of the model. The creep parameters were identified, and the calculated curves were in good agreement with the experimental curves, indicating that the model is capable of replicating the physical processes.
104 citations
TL;DR: In this article, the transient recovery characteristics of GaN-based HEMTs with a SiO2 gate dielectric induced by forward gate bias stress are systematically and comprehensively investigated for stress times from 100 ns to 10 ks, recovery times from 4 μs to 10ks, and stress biases from 1 to 7 V.
Abstract: The transient recovery characteristics of the threshold voltage drift (ΔVth) of GaN-based HEMTs with a SiO2 gate dielectric induced by forward gate bias stress are systematically and comprehensively investigated for stress times from 100 ns to 10 ks, recovery times from 4 μs to 10 ks, and stress biases from 1 to 7 V. The measured recovery data are analyzed using the concept of capture emission time maps. It is shown that the observed data cannot be explained by simple first-order defect kinetics. It is revealed that the recovery curves for constant stress times scale with the stress bias. Furthermore, the shape of the recovery curves changes from concave to convex with increasing stress time, independent of the stress bias. For short stress times and low stress bias, a dominant rate limiting effect of the III/N barrier layer is proposed. Defect-related physical processes with a broad distribution of characteristic time constants are discussed to explain the logarithmic time dependency of ΔVth stress and recovery, at which the role of the Coulomb feedback effect, complex defects, and spatially distributed defects are considered.
103 citations
TL;DR: Aydan et al. as discussed by the authors used the ISRM suggested method for the case of creep, which is particularly relevant for cases where the applied load or stress is kept constant.
Abstract: It is important to note that creep is only one aspect of the time-dependent behavior of rocks. In Fig. 1, three cases are illustrated with respect to the complete stress–strain curve: creep, i.e., increasing strain when the stress is held constant; stress relaxation, i.e., decreasing stress when the strain is held constant; and a combination of both, when the rock unloads along a chosen unloading path. This ISRM suggested method deals only with the case of creep, which is particularly relevant for cases where the applied load or stress is kept constant. Creep tests have also been carried out on soft rocks such as tuff, shale, lignite, and sandstone, medium-hard rocks such as marble, limestone, and rock salt, and hard rocks such as granite and andesite (i.e., Akagi 1976; Akai et al. 1979, 1984; Ito and Akagi 2001; Berest et al. 2005; Doktan 1983; Passaris 1979; Serata et al. 1968; Wawersik 1983; Okubo et al. 1991, 1993; Masuda et al. 1987, 1988; Ishizuka et al. 1993; Lockner and Byerlee 1977; Boukharov et al. 1995; Fabre and Pellet 2006; Aydan et al. 1995; Chan 1997; Cristescu and Hunsche 1998; Hunsche 1992; Hunsche and Hampel 1999; Ito et al. 1999; Mottahed and Szeki 1982; Perzyna 1966; Slizowski and Lankof 2003; Yang et al. 1999). These experiments were mostly carried out under compressive loading conditions. There are few studies on rocks using creep tests under a tensile loading regime (Ito and Sasajima 1980, 1987; Ito et al. 2008; Aydan et al. 2011). In particular, shallow underground openings may be subjected to a sustained tensile stress regime, which requires the creep behavior of rocks under such conditions. Please send any written comments on this ISRM suggested method to Prof. Resat Ulusay, President of the ISRM Commission on Testing Methods, Hacettepe University, Department of Geological Engineering, 06800 Beytepe, Ankara, Turkey.
102 citations
TL;DR: In this paper, a series of natural rubber and nitrile rubber (NR/NBR) nanocomposite vulcanazite, reinforced with two different organically modified clay (OMt) were prepared.
100 citations
TL;DR: In this article, the authors developed a unifying framework to quantify rate-dependent deformation in the brittle field and establish links between the microscale time-dependent crack growth processes and the macroscopically observed rate dependency.
Abstract: We develop a unifying framework to quantify rate-dependent deformation in the brittle field and establish links between the microscale time-dependent crack growth processes and the macroscopically observed rate dependency Triaxial deformation experiments have been performed under both constant strain rate and constant stress (creep) conditions on three types of sandstone The measured relative evolution of P wave speeds as a function of inelastic axial strain is similar for both types of test, despite differences in strain rate of up to 3 orders of magnitude This similarity indicates that there exists a direct, time-independent link between the microstructural state (as reflected by the variations in P wave speed) and the inelastic axial strain Comparison of applied stresses between constant strain rate and creep experiments as a function of inelastic strain indicates that creep deformation requires less mechanical work to bring the sample to failure This energy deficit corresponds to a stress deficit, which can be related to a deficit in energy release rate of the microcracks We establish empirically that the creep strain rate is given by e∝exp(ΔQ/σ∗), where ΔQ is the stress deficit (negative) and σ∗ is an activation stress This empirical exponential relation between creep strain rate and stress deficit is analogous to rate-and-state friction law We develop a micromechanical approach based on fracture mechanics to determine the evolution of an effective stress intensity factor at crack tips during creep deformation and estimate the activation volume of the stress corrosion reaction responsible for brittle creep
88 citations
TL;DR: In this article, synchrotron-based X-ray microdiffraction was used to characterize changes in the local orientation, morphology and strain distribution inside individual martensitic plates, as well as the effect of parent orientation on variant selection in bulk polycrystalline 304 stainless steel (SS) during in situ uniaxial tensile loading at the low temperature of 210 K.
TL;DR: In this paper, the electrical resistive response of an elastomeric composite material, composed of poly(dimethylsiloxane) (PDMS) and multi-walled carbon nanotubes (MWCNTs), to large mechanical deformations was investigated.
TL;DR: The good fit of the relaxation modulus to decrease exponential functions suggests that an intrinsic viscoelastic mechanism dominates the transients.
TL;DR: In this article, a new method for estimating stress magnitudes as a function of depth in sedimentary formations based on a laboratory constrained viscous rheology and steady tectonic loading is proposed.
TL;DR: In this article, a mathematical model for cyclic creep in compression is proposed, which is based on the fatigue growth of pre-existing microcracks in hydrated cement and the resulting macroscopic strain is calculated by applying fracture mechanics to the micro-cracks.
Abstract: Recent investigations prompted by a disaster in Palau revealed that worldwide there are 69 long-span segmental prestressed-concrete box-girder bridges that suffered excessive multi-decade deflections, while many more surely exist. Although the excessive deflections were shown to be caused mainly by obsolescence of design recommendations or codes for static creep, some engineers suspect that cyclic creep might have been a significant additional cause. Many investigators explored the cyclic creep of concrete experimentally, but a rational mathematical model that would be anchored in the microstructure and would allow extrapolation to a 100-year lifetime is lacking. Here it is assumed that the cause of cyclic creep is the fatigue growth of pre-existing microcracks in hydrated cement. The resulting macroscopic strain is calculated by applying fracture mechanics to the microcracks considered as either tensile or, in the form of a crushing band, as compressive. This leads to a mathematical model for cyclic creep in compression, which is verified and calibrated by laboratory test data from the literature. The cyclic creep is shown to be proportional to the time average of stress and to the 4th power of the ratio of the stress amplitude to material strength. The power of 4 is supported by the recent finding that, on the atomistic scale, the Paris law should have the exponent of 2 and that the exponent must increase due to scale bridging. Exponent 4 implies that cyclic creep deflections are enormously sensitive to the relative amplitude of the applied cyclic stress. Calculations of the effects of cyclic creep in six segmental prestressed concrete box girders indicate that, because of self-weight dominance, the effect on deflections absolutely negligible for large spans ( > 150 m ) . For small spans ( 40 m ) the cyclic creep deflections are not negligible but do not matter since the static creep causes in such bridges upward deflections. However, the cyclic creep is shown to cause in bridges with medium and small spans ( 80 m ) a significant residual tensile strain which can produce deleterious tensile cracking at top or bottom face of the girder.
TL;DR: In this paper, the stress relaxation behavior of age-forming for an Al-Zn-Mg-Cu alloy was studied using a designed device that can simulate the age forming process.
TL;DR: It is shown that the equilibrium uptake of moisture leads to the decrease in gauge factor of the fiber decrease, i.e., the reduction in the sensitivity of fiber resistivity to loading, as well as the origin of piezoresistivity is discussed.
Abstract: The complex structure of the macroscopic assemblies of carbon nanotubes and variable intrinsic piezoresistivity of nanotubes themselves lead to highly interesting piezoresistive performance of this new type of conductive material. Here, we present an in-depth study of the piezoresistive effect in carbon nanotube fibers, i.e., yarnlike assemblies made purely of aligned carbon nanotubes, which are expected to find applications as electrical and electronic materials. The resistivity changes of carbon nanotube fibers were measured on initial loading, through the elastic/plastic transition, on cyclic loading and on stress relaxation. The various regimes of stress/strain behavior were modeled using a standard linear solid model, which was modified with an additional element in series to account for the observed creep behavior. On the basis of the experimental and modeling results, the origin of piezoresistivity is discussed. An additional effect on the resistivity was found as the fiber was held under load whic...
TL;DR: In this article, the authors evaluated the restrained shrinkage behavior of ultra high performance fiber reinforced concrete (UHPFRC) and found that the degree of restraint provided a linear relationship with the ratio of steel and concrete wall thickness, whereas those factors were rarely affected by the diameter of the inner steel ring.
Abstract: In order to evaluate the restrained shrinkage behavior of ultra high performance fiber reinforced concrete (UHPFRC), ring-tests with three different wall thicknesses and two different diameters of inner steel ring were performed. Partially exposed free shrinkage and tensile tests were carried out simultaneously to assess the theoretical elastic stress, stress relaxation, degree of restraint and potential for cracking in the concrete. Test results indicated that the UHPFRC ring specimen with a thicker steel ring demonstrated a faster theoretical cracking time, higher stress relaxation and degree of restraint than that of a thinner steel ring, whereas those factors were rarely affected by the diameter of the inner steel ring. About 39–65 % of the theoretical elastic stress was relaxed by the sustained interface pressure. Since the actual residual tensile stress of all specimens was less than the tensile strength, the computed cracking potential varied from 0.43 to 0.7, and thus no shrinkage crack was observed. Finally, the degree of restraint provided a linear relationship with the ratio of steel and concrete wall thickness.
TL;DR: In this paper, the authors investigated the stress relaxation behavior of soft soils and proposed a practically useful coefficient with a formulation based on the behaviour of stress relaxation under one-dimensional conditions.
Abstract: Current studies rarely investigate the stress relaxation behaviour of soft soils. This paper proposes a practically useful coefficient with a formulation based on the behaviour of stress relaxation under one-dimensional conditions. Firstly, the stress relaxation coefficient is proposed after summarising stress relaxation test results according to the linear relationship between the vertical stress and time in a double logarithmic plot. Secondly, from the newly developed rate-dependency based elasto-viscoplastic formulations, an analytical solution for stress relaxation is derived. A unique relationship connecting the stress relaxation coefficient, the secondary compression coefficient and the rate-dependency coefficient is then obtained. The applicability of the stress relaxation formulation with its key coefficient to determine time-dependent parameters is finally validated with published experimental results on reconstituted illite and Berthierville clay.
TL;DR: In this article, a three-dimensional coupled chemo-mechanical model for intercalation-induced stresses is proposed, which is based on the concept of Isogeometric Analysis.
TL;DR: In this article, a series of multiaxial fatigue tests on a high-strength steel was performed to determine the multial fatigue strength under proportional and non-proportional loading conditions.
06 Oct 2014-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the authors assessed the creep rupture life of 9Cr-1Mo steel under multiaxial state of stress by incorporating circumferential U-notches of different notch root radii.
Abstract: Creep rupture life of 9Cr–1Mo steel under multiaxial state of stress has been assessed. Stress multiaxiality in cylindrical specimens during creep tests has been introduced by incorporating circumferential U-notches of different notch root radii. Creep tests were carried out on both smooth and notched specimens of the steel at 873 K over the net applied stresses in the range of 110–210 MPa. The creep rupture life of the steel was found to be higher in the presence of notch than that of smooth specimen indicating ‘notch strengthening’ behavior under multiaxial state of stress. The extent of strengthening tends to saturate for relatively sharper notches. Finite element analysis of stress and strain distributions across the notch was carried out to assess the notch strengthening behavior observed in the steel. The reduction in the von-Mises stress, extent of which increased and tends towards saturation with increase in notch root radii, resulted in increase in rupture life under multiaxial state of stress. Estimation of the creep rupture life under multiaxial state of stress has been assessed based on the different models, invoking the concept of skeletal point for calculating the representative stress. It has been observed that the creep rupture behavior of the steel under multiaxial state of stress is predominantly governed by the von-Mises stress. The creep rupture life has been predicted using finite element analysis coupled with continuum damage mechanics.
TL;DR: It is revealed that the elastic strains in the γ matrix originate from interactions among the ε-martensite phase, extended dislocations, and/or thinε-layers.
Abstract: Locally developed strains caused by athermal γ face-centered cubic (fcc)→e hexagonal close-packed (hcp) martensitic transformation were investigated for the γ matrix of Ni-free Co–29Cr–6Mo (wt%) alloys prepared with or without added nitrogen. Electron-backscatter-diffraction-(EBSD)-based strain analysis revealed that in addition to e-martensite interiors, the N-free alloy that had a duplex microstructure consisting of the γ matrix and athermal e-martensite plates showed larger magnitudes of both elastic and plastic strains in the γ phase matrix than the N-doped counterpart that did not have a e-martensite phase. Transmission electron microscopy (TEM) results indicated that the e-martensite microplates were aggregates of thin e-layers, which were formed by three different {111}γ〈11 2 ¯ 〉γ Shockley partial dislocations in accordance with a previously proposed mechanism ( Putaux and Chevalier, 1996 ) that canceled the shear strains of the individual variants. The plastic strains are believed to have originated from the martensitic transformation itself, and the activity of dislocations is believed to be the origin of the transformation. We have revealed that the elastic strains in the γ matrix originate from interactions among the e-martensite phase, extended dislocations, and/or thin e-layers. The dislocations highly dissociated into stacking faults, making stress relaxation at intersections difficult and further introducing local strain evolution.
TL;DR: In this article, the residual stress induced by laser shock processing and the thermal relaxation behaviors of residual stress in Ni-based alloy GH4169 were investigated by means of three-dimensional nonlinear finite element analysis.
TL;DR: In this article, the structure and linear viscoelastic behavior of four different model star polymer melts were investigated experimentally, and the star polymers were prepared via different synthetic routes based on atom transfer radical polymerization.
Abstract: The structure and linear viscoelastic behavior of four different model star polymer melts were investigated experimentally. The star polymers were prepared via different synthetic routes based on atom transfer radical polymerization (ATRP). Stars with small elongated (linear backbone) cores exhibited slight differences in the asymmetry of the core, which however did not affect the rheological properties significantly. The relaxation behavior of these stars with an asymmetric core was well-described by available tube models. On the other hand, stars with large cross-linked cores exhibited a core–shell morphology and their stress relaxation was dominated by a power-law decay over about 8 decades, akin to gel-like soft systems. This behavior reflected their liquid-like ordering and small intercore distances, and bares analogueies to that of interpenetrating soft colloids and microgels.
TL;DR: In this paper, a numerical thermo-elastic-plastic model has been developed to simulate the welding of 9Cr-1Mo (Grade 91) steel in the form of plates welded together using a high intensity electron beam.
TL;DR: In this article, the authors investigated the effect of specimen constraint on the growth rate of crack-tip stress states and stress-dependent creep ductility of the steel in a ductility exhaustion-based damage model, and the results showed that the constraint effects on the creep crack growth behavior for a wide range of C∗-integral mainly arise from interaction of crack tip stress states.
TL;DR: In this article, the authors proposed a new approach where the BER induced macromolecular network alteration is modeled as a viscoplastic deformation process, based on the observation that stress relaxation due to light irradiation is a time-dependent process similar to that in viscoelastic solids with an irrecoverable deformation after light radiation.
Abstract: Light activated polymers (LAPs) are a class of contemporary materials that when irradiated with light respond with mechanical deformation. Among the different molecular mechanisms of photoactuation, here we study radical induced bond exchange reactions (BERs) that alter macromolecular chains through an addition-fragmentation process where a free chain whose active end group attaches then breaks a network chain. Thus the BER yields a polymer with a covalently adaptable network. When a LAP sample is loaded, the macroscopic consequence of BERs is stress relaxation and plastic deformation. Furthermore, if light penetration through the sample is nonuniform, resulting in nonuniform stress relaxation, the sample will deform after unloading in order to achieve equilibrium. In the past, this light activation mechanism was modeled as a phase evolution process where chain addition-fragmentation process was considered as a phase transformation between stressed phases and newly-born phases that are undeformed and stress free at birth. Such a modeling scheme describes the underlying physics with reasonable fidelity but is computationally expensive. In this paper, we propose a new approach where the BER induced macromolecular network alteration is modeled as a viscoplastic deformation process, based on the observation that stress relaxation due to light irradiation is a time-dependent process similar to that in viscoelastic solids with an irrecoverable deformation after light irradiation. This modeling concept is further translated into a finite deformation photomechanical constitutive model. The rheological representation of this model is a photoviscoplastic element placed in series with a standard linear solid model in viscoelasticity. A two-step iterative implicit scheme is developed for time integration of the two time-dependent elements. We carry out a series of experiments to determine material parameters in our model as well as to validate the performance of the model in complex geometrical and loading configurations. The comparison between the finite element simulations and experiments shows that the model can accurately capture the response of the LAP under a wide range of coupled photo-mechanical loading conditions, such as light induced stress relaxation, creep in tension, and bending. Furthermore, we demonstrate the versatility of the model by simulating a series of examples that exhibit complex three-dimensional, time-dependent photodeformation, including photoorigami, photoforming, and photobulge tests.
TL;DR: In this paper, an 80 nm thick single crystalline Ni film was deposited on a sapphire substrate and a sub-nanometer-high hexagonal topography pattern was observed by atomic force microscopy.
TL;DR: It is shown that the gradient surfaces with regular structures can be used in combinatorial studies related to pattern directed dewetting and for creating a gradient topography on nonplanar surfaces as well.
Abstract: We report a simple method for creating a nanopatterned surface with continuous variation in feature height on an elastomeric thin film. The technique is based on imprinting the surface of a film of thermo-curable elastomer (Sylgard 184), which has continuous variation in cross-linking density introduced by means of differential heating. This results in variation of viscoelasticity across the length of the surface and the film exhibits differential partial relaxation after imprinting with a flexible stamp and subjecting it to an externally applied stress for a transient duration. An intrinsic perfect negative replica of the stamp pattern is initially created over the entire film surface as long as the external force remains active. After the external force is withdrawn, there is partial relaxation of the applied stresses, which is manifested as reduction in amplitude of the imprinted features. Due to the spatial viscoelasticity gradient, the extent of stress relaxation induced feature height reduction vari...