Grain-scale processes governing shear band initiation and evolution in sands

A variety of experimental techniques have been used to advance understanding of strain localization phenomena in sands. However, all of these methods have fallen short in characterizing the evolution of the grain-scale processes that necessarily control shear band formation and growth in sands. This paper presents results of application of the non-destructive displacement measurement technique of digital image correlation (DIC) to measure two- and three-dimensional surface displacements on plane strain and axisymmetric sand specimens over short time steps. The abundance of local displacement data, high level of accuracy, and nearly continuous (spatially and temporally) record of displacement evolution afforded by the DIC technique has finally enabled a means to quantify local displacements to particulate-scale intensity. The data have been used to evaluate the local displacement mechanisms leading to the triggering of the formation of persistent shear bands, the timing of shear band formation with regard to the achievement of peak stress, and the character of displacements within fully formed shear bands. Insights are offered regarding the relation between strain localization and global stress-strain behavior, and the ensuing interpretations of shear banding as a hardening or softening phenomenon. Comparison of behavior between plane strain and triaxial tests offer additional perspective on the influences of three-dimensional stresses and boundary conditions on shear banding. The results further shed light on the micro-deformation mechanisms (i.e. buckling columns) responsible for the observed local strain non-uniformities that characterize “steady-state” shear band evolution.     

Comparison of expander and Instant Controlled Pressure-Drop DIC technologies as thermomechanical pretreatments in enhancing solvent extraction of vegetal soybean oil

As an intensification process of the unit operation of vegetable oil extraction, the cooking pre-treatment systematically results in damaging thermal degradations. However, recently, two thermomechanical expansion processes have been suggested and applied at industrial scales to pre-treat oilseeds: expansion and instant controlled pressure drop DIC. In both cases, it is a flash transformation from high-temperature/high-pressure to 100 °C/atmospheric pressure and 33 °C/vacuum phases, respectively. The use of DIC resulted in cumulative impact yields of 247 mg oil/g db, compared to 210, 230, and 224 for cracking, cracking/blocking, and expanding, respectively. To achieve the same yields obtained in 160 min for cracked or cracked/flaked soybean, the expander took 120 min against 35 min for DIC. Besides, thanks to the short heat treatment and the induced instant cooling, DIC allowed the highest preservation of soybean oil quality, with fatty acid concentrations almost identical to that of untreated seeds.Practical impactsIn the sector of the vegetal oil industry, researchers and engineers mostly look for increasing the extraction yields while preserving or improving the sensorial and biochemical quality of the oil. Henceforth, the modification of the oleaginous seed texture may usually be the major factor affecting and normally enhancing the capacity of extraction. Based on the improvement of material structure through adequate expansion, the higher the volume of vapor issued from the autovaporization throughout the instant controlled pressure-drop (DIC) technology, the higher the oil yields. Moreover, using the instantaneous thermodynamic theory, the feasibility of integrating DIC technology as an HTST treatment in the industrial-scale soybean extraction process could be performed and optimized without any biochemical degradation.     

Accurate kinematic measurement at interfaces between dissimilar materials using conforming finite-element-based digital image correlation

Digital image correlation (DIC) is now an extensively applied full-field measurement technique with subpixel accuracy. A systematic drawback of this technique, however, is the smoothening of the kinematic field (e.g., displacement and strains) across interfaces between dissimilar materials, where the deformation gradient is known to be large. This can become an issue when a high level of accuracy is needed, for example, in the interfacial region of composites or joints. In this work, we described the application of global conforming finite-element-based DIC technique to obtain precise kinematic fields at interfaces between dissimilar materials. Speckle images from both numerical and actual experiments processed by the described global DIC technique better captured sharp strain gradient at the interface than local subset-based DIC.     

Deformation analysis of ferrite/pearlite banded structure under uniaxial tension using digital image correlation

The ferrite/pearlite banded structure causes the anisotropic behavior of steel. In this paper, digital image correlation (DIC) was used to analyze the micro deformation of this microstructure under uniaxial tension. The reliability of DIC for this application was verified by a zero-deformation experiment. The results show that the performance of DIC can satisfy the requirements of the tensile deformation measurement. Then, two uniaxial tensile tests in different directions (longitudinal direction and transverse direction) were carried out and DIC was used to measure the micro deformation of the ferrite/pearlite banded structure. The measured results show that the ferrite bands undergo the main deformation in the transverse tension, which results in the relatively weaker tensile properties in the transverse direction than in the longitudinal direction. This work is useful to guide the modification of the bands morphology and extend the application scope of DIC.     

Tensile mechanical properties of basalt fiber reinforced polymer composite under varying strain rates and temperatures

High-strength woven fabrics and polymers are ideal materials for use in structural and aerospace systems. It is very important to characterize their mechanical properties under extreme conditions such as varying temperatures, impact and ballistic loadings. In this present work, the effects of strain rate and temperature on the tensile properties of basalt fiber reinforced polymer (BFRP) were investigated. These composites were fabricated using vacuum assisted resin infusion (VARI). Dynamic tensile tests of BFRP coupons were conducted at strain rates ranging from 19 to 133 s⁻¹ using a servo-hydraulic high-rate testing system. Additionally, effect of temperature ranging from −25 to 100 °C was studied at the strain rate of 19 s⁻¹. The failure behaviors of BFRP were recorded by a Phantom v7.3 high speed camera and analyzed using digital image correlation (DIC). The results showed that tensile strength, toughness and maximum strain increased 45.5%, 17.3% and 12.9%, respectively, as strain rate increased from 19 to 133 s⁻¹. Moreover, tensile strength was independent of varying temperature up to 50 °C but decreased at 100 °C, which may be caused by the softening of epoxy matrix and weakening of interfaces between fibers and matrix when the glass transition temperature was exceeded.     

Modified slotted shear test for a thin sheet of solid polymer under large deformations

A modified slotted shear test based on ASTM B831 is proposed and used for determining the mechanical behavior of a solid polymer at large deformations. The shear stress-strain response of a thin sheet of polytetrafluoroethylene was experimentally investigated and a V-notched rail shear test was carried out to validate the experimental results obtained with the proposed approach. Displacement and strain fields of the test specimens were estimated by the Digital Image Correlation method. The results indicate agreement between both tests for small deformations, while for large deformations significant discrepancies in shear stress-strain responses are observed. These discrepancies can be attributed to unwanted distortions of the gage section observed in the V-notched specimens. The modified slotted shear test provided a uniform shear strain distribution across the gauge section of specimens. In addition, the principal advantages of the proposed approach over V-notched rail shear test are a wide range of shear strains and absence of unwanted distortions.     

Reusable sensor based on functionalized carbon dots for the detection of silver nanoparticles in cosmetics via inner filter effect

We report a low cost selective analytical method based on inner filter effect (IFE) for citrate-silver nanoparticle (cit-AgNP) detection, in which fluorescent amine-derivatized carbon dots (a-CDs) act as the donor and aggregated cit-AgNPs as the energy receptor. Carbon dots (CDs) were chemically modified with ethylenediamine (EDA) moieties via amidic linkage displaying an emission band at 440 nm. The presence of cit-AgNPs produces a remarkably quenching of a-CD fluorescence via IFE, since the free amine groups at CD surface induce the aggregation of cit-AgNPs accompany by a red-shifting of their characteristic plasmon absorption wavelength, which resulted in “turn-on” of the IFE-decreased in CD fluorescence. The proposed method, which involves the use of chelating agents for removal of metal ions interferences, exhibits a good linear correlation for detection of cit-AgNPs from 1.23 × 10⁻⁵ to 6.19 × 10⁻⁵ mol L⁻¹, with limits of detection (LOD) and quantification (LOQ) of 5.17 × 10⁻⁶ and 1.72 × 10⁻⁵ mol L^−1, respectively. This method demonstrates to be efficient and selective for the determination of cit-AgNPs in complex matrices such as cosmetic creams and reveals many advantages such as low cost, reusability, high sensitivity and non time-consuming compared with other traditional methods.