Strain-induced Structural Changes Research Articles

Unintended consequences: Assessing thermo-mechanical changes in vinyl nitrile foam due to micro-computed X-ray tomographic imaging

Micro-computed X-ray tomography (μCT) is a volumetric imaging tool used to quantify the internal structure of materials. μCT imaging with mechanical testing (in situ μCT) helps visualize strain-induced structural changes and develop structure-property relationships. However, the effects on thermophysical properties of radiation exposure during in situ μCT imaging are seldom addressed, despite potential radiation sensitivity in elastomers. This work quantifies the radiation dosage effect on thermo-, chemical-, and mechanical-properties for a vinyl nitrile-based foam. Material properties were measured after (0, 1, 2, and 3)days at (8.1±0.9)kGy/d. Morphological characteristics were investigated via scanning electron microscopy. Thermal transitions were assessed using differential scanning calorimetry. Viscoelasticity was measured with dynamic mechanical analysis over a range from -30°C to 60°C. Higher dose lead to stiffening and increased dissipation. Chemical structure was assessed with Fourier transform infrared spectroscopy and energy-dispersive X-ray spectroscopy. Soxhlet extraction was used to measure gel content. In summary, substantial changes occur in thermophysical properties, which may confound structure-property measurements. However, this also provides a modification pathway. Quantitation and calibration of the properties changes informed a finite element user material for material designers to explore tunablity and design optimization for impact protection engineers.

High-pressure response of vibrational properties of b-As x P1–x : in situ Raman studies

The structural evolution of black arsenic-phosphorous (b-As x P1–x ) alloys with varying arsenic concentrations was investigated under hydrostatic pressure using in situ Raman spectroscopy. High-pressure experiments were conducted using a diamond anvil cell, which revealed pressure-induced shifts in vibrational modes associated with P–P bonds (A 1 g , A 2 g , ), As–As bonds (A 1 g , A 2 g , ), and As–P bonds in b-As x P1–x alloys. Two distinct pressure regimes were observed. In the first regime (region I), all vibrational modes exhibited a monotonic upshift, indicating phonon hardening due to hydrostatic pressure. In the second regime (region II), As0.4P0.6 and As0.6P0.4 alloys displayed a linear blueshift (or negligible change in some modes) at a reduced rate, suggesting local structural reorganization with less compression on the bonds. Notably, the alloy with the highest As concentration, As0.8P0.2, exhibited anomalous behavior in the second pressure regime, with a downward shift observed in all As–As and As–P Raman modes (and some P–P modes). Interestingly, the emergence of new peaks corresponding to the Eg mode and A 1g mode of the gray-As phase was observed in this pressure range, indicating compressive strain-induced structural changes. The anomalous change in region II confirms the formation of a new local structure, characterized by elongation of the P–P, As–As, and As–P bonds along the zigzag direction within the b-As x P1–x phase, possibly near the grain boundary. Additionally, a gray-As phase undergoes compressive structural changes. This study underscores the significance of pressure in inducing structural transformations and exploring novel phases in two-dimensional materials, including b-As x P1–x alloys.

Hydrogen Bonds under Stress: Strain-Induced Structural Changes in Polyurethane Revealed by Rheological Two-Dimensional Infrared Spectroscopy.

The remarkable elastic properties of polymers are ultimately due to their molecular structure, but the relation between the macroscopic and molecular properties is often difficult to establish, in particular for (bio)polymers that contain hydrogen bonds, which can easily rearrange upon mechanical deformation. Here we show that two-dimensional infrared spectroscopy on polymer films in a miniature stress tester sheds new light on how the hydrogen-bond structure of a polymer is related to its viscoelastic response. We study thermoplastic polyurethane, a block copolymer consisting of hard segments of hydrogen-bonded urethane groups embedded in a soft matrix of polyether chains. The conventional infrared spectrum shows that, upon deformation, the number of hydrogen bonds increases, a process that is largely reversible. However, the 2DIR spectrum reveals that the distribution of hydrogen-bond strengths becomes slightly narrower after a deformation cycle, due to the disruption of weak hydrogen bonds, an effect that could explain the strain-cycle induced softening (Mullins effect) of polyurethane. These results show how rheo-2DIR spectroscopy can bridge the gap between the molecular structure and the macroscopic elastic properties of (bio)polymers.

Strain and illumination triggered regulations of up-conversion luminescence in Er-doped Bi0.5Na0.5TiO3[sbnd]BaTiO3/Mica flexible multifunctional thin films

External stimuli induced effective regulations of luminescent material are of significant interest in the development of smart optical devices. Here, by simply doping with Er3+ in the 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (BNTBT) ferroelectric host, using the bendable mica substrate, and exerting mechanical strain (bending) or light illumination (via photochromic reaction), the all-inorganic, highly-transparent and flexible Er-doped BNTBT/Mica luminescent-ferroelectric thin films were designed and fabricated, displaying strain-induced dramatically elevation of up-conversion photoluminescence (PL) intensity, suppression of PL concentration quenching, outstanding endurance and durability, convenient illumination-mediated PL quenching. And the strain-induced structural changes and local lattice distortions of the thin films were further explored through theoretical calculations and Raman measurement. Our results can supply the guidance of designing other luminescent-ferroelectric materials with controlled PL properties via easy mechanical/photo stimuli for expanding the application of multifunctional wearable memory devices.

Strain-induced structural change and mechanical properties of 1,3,5-triamino-2,4,6-trinitrobenzene probed by neutron diffraction

TATB (1,3,5-triamino-2,4,6-trinitrobenzene) is a widely used insensitive high-energy explosive. It is significant to study its structural evolution in order to get a credible understand of its properties and performances. Direct mechanical tests such as hardness tests are useful approaches to get information about its mechanical properties and shed light upon the microstructures. However, due to the poor solubility of TATB to most solvents, it is hard to produce large crystals for such tests. Fortunately, microcosmic approaches such as powder diffraction can reinforce it and has low requirement on samples. In this study, series of in-situ neutron diffraction experiments under different pressures are performed to investigate the structural change and mechanical properties of TATB. The recently reported phase transition at 4 GPa is studied. The microstrains in normal TATB (12–17 µm) are analysed to reveal the information of slip system and transitional process. The lattice parameters and bulk moduli of TATB under different conditions are given. These neutron diffraction results are significant supplement to correlative studies and will help to understand the performances of TATB, such as deformation and phase transition.

Stress Effects on Vibrational Spectra of a Cubic Hybrid Perovskite: A Probe of Local Strain

Inhomogeneous strain may develop in hybrid organic metal-halide perovskite thin films due to thermal expansion mismatch with a fabrication substrate, polycrystallinity, or even light soaking. Measuring these spatially varying strains is difficult but of prime importance for understanding the effects on carrier mobility, non-radiative recombination, degradation, and other optoelectronic properties. Local strain can be mapped using the shifts in vibrational frequencies using Raman or infrared microscopy. We use density functional theory to investigate the effect of uniaxial strain on the vibrations of pseudo-cubic methylammonium lead iodide (CH3NH3PbI3)and identify the vibrational modes most favorable for local strain mapping (86, 97, 1457, and 1537 cm–1) and provide calibration curves. We explain the origin of the frequency changes with strain using dynamical matrix and mode eigenvector analysis and study strain-induced structural changes. We also calculate mode Grüneisen parameters, giving information about anharmonicity and anisotropic negative thermal expansion as recently reported for other phases. Our results provide a basis for strain mapping in hybrid perovskites to further the understanding and control of strain and improve stability and photovoltaic performance.

Molecular anisotropy and rearrangement as mechanisms of toughness and extensibility in entangled physical gels

Dynamic networks formed by physically crosslinked, entangled polymers have emerged as self-healing, stretchable, and functional materials. Entangled associative gels with remarkable toughness and extensibility have been produced by several distinct chemical approaches, suggesting that these enhanced mechanical properties result from molecular-scale topology. Previously, artificially engineered associative proteins were designed to provide an ideal model system to investigate the role of entanglement on gel mechanics via well-defined entangled or unentangled physical gels. Herein, uniaxial strain-induced structural changes in these model gels were observed using in situ small-angle x-ray scattering (SAXS) and in situ polarized optical microscopy (POM) up to 2000% engineering strain. Anisotropic optical responses to uniaxial strain at the nano-, micro-, and macroscales suggest that stress dissipation mechanisms enable high extensibility and toughness. Nano- and microscopic anisotropy observed by SAXS indicate stretching and alignment of flexible polymer strands along the straining axis, and nonmonotonic macroscopic anisotropy observed by POM suggests relaxation within the hydrogel due to rearrangement of associative network junctions. Unentangled hydrogels exhibit low toughness and a strain-rate-dependent transition from ductile to brittle tensile behavior, which is typical for associative polymer solutions. These findings indicate that topological entanglements and the freedom of individual chains to align at the nanoscale due to junction relaxation are both critical to achieving high toughness and elongation in entangled physical gels.

Strain-induced structural and dynamic changes in segmented polyurethane elastomers

Polyurethane elastomers have been widely used in the industry and daily life due to their versatile physical and chemical properties. Therefore, a fundamental understanding of the structures and dynamics at a molecular level will provide piercing insights into the precise design and application of new polyurethane materials. In this study, we mainly focused on investigating the strain-induced structural and dynamic changes in a typical polyurethane elastomer composed of poly(ε-caprolactone) (PCL) and 4,4′-diphenylmethylene diisocyanate (MDI) as the soft and hard segments, respectively. Obvious strain-hardening phenomenon was observed during the mechanical tensile test, and a systematic comparison was performed on the fractured and pristine samples. DSC results revealed that the crystallization of PCL chains was still going on after the sample was fractured, and the crystallite structures became stable after physical aging at 25 °C for two days. 1H solid-state nuclear magnetic resonance (NMR) experiment was further employed to determine the fraction of mobile PCL chains that were converted to crystallites during stretching. Besides, the microphase separation was also significantly enhanced in the fractured sample. The mobility of amorphous PCL chains was largely reduced due to the strain-induced crystallization of nearby PCL segments, as revealed by the 1H magic-sandwich echo (MSE) NMR experiments. 1H multiple quantum (MQ) NMR experiments also quantitatively revealed the strain-induced orientation of amorphous PCL chains in the fractured sample, indicating that the PCL crystallites were acting as the physical cross-linkages to prevent the contraction of the elongated PCL chains when the sample is fractured.

Nonlinear Mechano-Optical Behavior and Strain-Induced Structural Changes of l-Valine-Based Poly(ester urea)s

The uniaxial mechano-optical behavior of a series of amorphous l-valine-based poly(ester urea) (VAL-PEU) with varying diol lengths was studied to elucidate the molecular mechanism associated with their thermal shape memory properties. A custom, real-time measurement system was used to capture the true stress, true strain, and birefringence during the temporary shape programming at stretching temperatures above the glass transition temperature (Tg). The mechano-optical response of VAL-PEUs exhibits an initial photoelastic behavior related to enhanced segmental correlation at low temperatures above the Tg. A characteristic temperature, defined as the liquid–liquid (Tll) transition (rubbery–viscous transition), was found at about 1.05 Tg (K) (at Tg + 15 °C) at strain rate of 0.017 s–1, above which the segmental contacts largely “melt” and the initial slope of the stress-optical curves becomes temperature independent. This temperature corresponds to the temperature where mean relaxation time for the polymer i...

Structural origin of fast yielding-strain hardening transition in fluoroelastomer F2314

The mechanical response of fluoroelastomer F2314 under uniaxial extension and its relation with strain-induced structural change are investigated by mechanical analysis, in-situ small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). Yielding of samples followed by strain hardening is found without necking. Meanwhile, the strain range of strain softening is short, which is 0.77 at 25 °C and 0.3 at 60 °C. Microscopically, solution-cast F2314 shows microphase separation and low crystallinity. Results of WAXS and SAXS indicate strain-induced formation of fibrillar crystal during strain hardening. Based on these findings, it is supposed that the fast relay between destruction of microphase separated domains and formation of fibrillar crystal is the origin of the unique mechanical response.

Strain-induced stiffness-dependent structural changes and the associated failure mechanism in TBCs

Dynamic structural evolution of thermal barrier coatings (TBCs) during thermal exposure is highly important to account for the failure mechanism of TBCs. In this study, to begin with, the dynamic structural changes were investigated assisted with series of TBCs with different Young’s modulus in their top-coat. Results show that the strain-induced structural changes varied from dispersive microscopic inter-tearing to concentrated macroscopic vertical cracks, owing to the gradually stiffening top-coat. Subsequently, the associated failure mechanism of TBCs was revealed based on the stiffness-dependent structural changes. In a gradient thermal cyclic test, gradient stiffening degrees occurred across the top-coat. After certain thermal cycles, some macroscopic vertical cracks were generated in the much stiffer top zone of the top-coat. Consequently, partial delamination occurred when the large vertical cracks are connected with some interfacial cracks. This can be responsible for the failure mechanism of TBCs.

Force transmission and its effect on structural changes in plasma-sprayed lamellar ceramic coatings

In this study, thermal mismatch strain-induced structural changes in plasma-sprayed lamellar ceramic coatings upon heating were investigated. Experimental results showed that the main structural change is the propagation of inter- and intra-splat cracks along their tips. Correspondingly, significant changes in properties were observed. A lamellar model with connected inter- and intra-splat cracks was developed. The modeling results suggested that shear stress would be primarily concentrated at the interface between two layers, accounting for the propagation of the inter-splat cracks. Subsequently, the stress was transmitted through the residual bonding areas into the upper layer, and thus a tensile effect was generated inside the splat segment. This can be responsible for the propagation of the intra-splat cracks. In addition, dependence of the crack propagation behavior on the structural parameters is discussed. These microscopic structural changes may provide fundamental understanding on the global structural evolution and failure mechanism of coating/substrate systems during service conditions.

Strain-Induced High Coercivity in La<sub>0.7</sub>Sr<sub>0.3</sub>CoO<sub>3</sub> Films

Here, we report strain-induced high coercivity in La0.7Sr0.3CoO3 (LSCO) films, which suffer in-plane tensile strains due to the positive lattice mismatch between the substrate and the LSCO bulk. The films on (011)-0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 exhibit large uniaxial anisotropy, large coercivity, and high saturation magnetization at low temperature in contrast to the well soft magnetic behaviors in LSCO bulk. It is found that the coercivity of the 40 nm (001)-LSCO/SrTiO3 film can be as high as 1.45 T at 10 K and the observed coercivity decreases rapidly as the thickness increases, though the Curie temperature is below room temperature. The large coercivity and anisotropy should be closely related to the strain-induced structural changes and the different orbital ordering of Co3+ and Co4+ ions. Meanwhile, the enhanced domain wall pinning by the tensile strain may also contribute to the observed high coercivity.

Effect of the simultaneous biaxial stretching on the structural and mechanical properties of PLA, PBAT and their blends at rubbery state

This works deals with the study of the strain-induced structural changes of PLA, PBAT and their blends with/without multi-functional styrene-acrylic oligomers (Joncryl ADR®-4368). The films were prepared through a single-screw extrusion. Uni and bi-axially simultaneous stretching, at a temperature above the glass transition temperature, were performed. The experimental conditions were chosen so as not to generate significant thermal crystallization during tests duration. Stretching induced semi-crystalline films with a supposed in-plane mechanical isotropy. Their structural, morphological and thermo-mechanical properties were characterized using dynamic mechanical and thermal analysis (DMTA), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and transmission electron microscopy (TEM). The strain-induced crystals of PLA combined mesophase and α′-form crystals. Besides, the stretched PBAT was found to be composed of mixed-crystal structure of BT and BA units in a PBT-like crystal form. The Joncryl addition promoted significant changes on the crystallization kinetics of PLA without affecting the crystallization kinetic of PBAT. In fact, Joncryl acted as a nucleating agent making thus faster the thermal crystallisation of PLA. In parallel, the strain induced crystallisation in PLA was accelerated and stresses increased. The same effect was still observed in the PLA_PBAT blends.

架橋密度の異なるガラス状エポキシネットワークの大変形過程における過渡的粘弾性

The single relaxation time of glassy epoxy networks having different crosslink densities was evaluated during uniaxial compression processes by using a simple nonlinear single relaxation model to study the effect of crosslinked molecular structures on the transient nonlinear viscoelastic behavior of glassy epoxy networks. The model consisted of two elastic springs expressing linear viscoelastic behavior and a dashpot with variable viscosity as a single parameter representing strain-induced structural change. We calculated the strain-dependent relaxation time τSS during the compression by fitting the model to the experimental stress-strain curves obtained at various strain rates and at temperatures T 18℃ lower than the glass transition temperature Tg of the material being compressed. With increasing strain εn, τSS obtained for each epoxy network steeply decreased in the pre-yield region of strain and then reached to low steady values appearing in strain ranges larger than the yield strain, reproducing previously reported typical behavior observed for glassy polymers under large deformation. When compressed at an identical strain rate, τSS-εn relations for all epoxy networks almost coincided each other despite the difference in the crosslink density. This result indicates that, if the deformation is performed at the same condition i.e., strain rate and T-Tg, strain-induced variation of relaxation time in the epoxy glasses is almost not influenced by crosslinked molecular structures.

Application of aluminum foil for “strain sensing” at fatigue damage evaluation of carbon fiber composite

Surface layer of a loaded solid is an individual structural level of deformation that was shown numerously within concept of physical mesomechanics. This gives rise to advance in its deformation development under loading as well as allows using this phenomenon to sense the strain induced structure changes. It is of specific importance for composite materials since they are highly heterogeneous while estimating their mechanical state is a topical applied problem. Fatigue tests of carbon fiber composite specimens were carried out for cyclic deformation estimation with the use of strain sensors made of thin (80 μm) aluminum foil glued to the specimen’s surface. The surface images were captured by DSLR camera mounted onto an optical microscope. Strain relief to form during cyclic loading was numerically estimated using different parameters: dispersion, mean square error, universal image quality index, fractal dimension and energy of Fourier spectrum. The results are discussed in view of deformation mismatch in thin foil and bulk specimen and are offered to be applied for the development of Structural Health Monitoring (SHM) approach.

Similar nonlinear mechanical responses in hard and soft materials

A comparison of the mechanical responses of (bio)polymer networks and shape-memory alloys to cyclic loading reveals strong phenomenological similarities resulting from strain-induced structural changes.

エポキシガラスの非線形応力－ひずみ挙動に及ぼす架橋密度の影響

Effects of crosslinked molecular structures in glassy epoxy networks on the nonlinear viscoelastic behavior have been studied in terms of strain-induced structural change. We prepared four epoxy glasses with different crosslink densities from the same epoxy precursor by controlling fractions of chain-extender and crosslinker in curing agents. The epoxy glasses were subjected to uniaxial compression at a temperature 18K below the glass transition temperature of each sample. With increasing crosslink density, initial elastic moduli and yield stresses decreased, whereas flow stresses appearing after the strain softening almost unchanged, except for the most tightly crosslinked sample showing lower flow stresses. Results of density measurement and DSC showed that an increase in the crosslink density made the glassy structures more unstable. These results show that unstable glassy structures due to constraints on segmental mobility introduced by crosslinked molecular structures lessen the initial elastic modulus and the yield stress. On the other hand, the variation of crosslink density had almost no effect on the flow stresses of the glasses. This observation presumably indicates that molecular crosslinks have little effect on the strain-induced structural change. When the crosslink density comes to quite high, however, the crosslinks are likely to start affecting the flow stress, i.e. strain-induced unstable glassy structures.

The role of strain-induced structural changes on the mechanical behavior of PA6/PE multilayer films under uniaxial drawing

The uniaxial drawing of PA6/tie/PE multilayer blown films is investigated in relation to the behavior of the pure components. The tensile stress–strain behavior of the multilayers obeys a simple additive mixture law of the one of the components which is consistent with the parallel mechanical coupling of the layer structure of the films. No significant difference is observed in the crystallographic evolution of each layer in the multilayer films as compared with the parent monolayer films. The rupture of the multilayer films yet appears to be directly governed by the PA6 layer, as it triggers an early rupture of the PE layer without decohesion. This contrasts with the rupture behavior upon biaxial drawing that was previously shown to strongly depend on the layer thickness ratio: the thicker the PE layer, the higher the strain at break of both the PE and the PA6 layers. Besides, the strain at break of the PA6 layer in the multilayers under biaxial drawing was greater than that of neat PA6. However, a common feature to uniaxial and biaxial drawing is the similar β–α strain-induced phase transformation and the same limiting value of the α-crystal content at rupture in the PA6 layer, for all multilayer films and the PA6 monolayer film as well. This finding supports the previously proposed rupture criterion based on flaw propagation through the crack-prone α-crystalline phase when this latter phase reaches a critical content for mechanical percolation. The major differences and similarities between the two drawing modes are discussed in consideration of the different state of stress that modifies the pathway of the β–α phase transformation.

Strain-Induced Surface Dynamics of Aluminum Studied by In Situ Photoelectron Emission

In situ photoelectron emission in high-purity aluminum deformed under uniaxial tension while exposed to pulsed excimer laser radiation (248 nm) is reported in this paper. The effect of various surface treatments, including polishing, air oxidization, carbon coating, and gold coating, on photoelectron emission was investigated. The photoelectron emission signals are sensitive to strain-induced surface structure changes, such as slip lines and slip bands. In the case of aluminum with oxide of 4.5 nm thickness, the photoelectron emission increases gradually in a nonlinear fashion at the early stage of deformation (strain ≤0.05). Then the photoemission grows linearly until the accumulated strain reaches about 0.17. In the final stage, the photoelectron intensity increases parabolically until failure. The transition of photoelectron emission from linear to parabolic growth correlates with the onset of strain localization. The strain-induced microstructural change was also analyzed by scanning electron microscopy (SEM). The photoelectron emission signals from aluminum during tension were interpreted in the light of surface treatment, surface electron work function, and surface microstructural change.