Small-scale Strain Research Articles

Effects of coflow velocity on the lift-off characteristics of autoignited jet flame in hot air coflow

This paper aims to understand the effects of coflow velocity on the lift-off characteristics of autoignited jet flame and to develop a lift-off height model that considers such effects. A jet-in-coflow burner was used to conduct autoignition experiments. As the jet velocity increased, three distinctive flame types were observed: attached flame (AF), random spots (RS), and lifted flame (LF). The coflow velocity was found to have an interesting impact on the transition from one type to another. As the coflow velocity increases, the jet velocity needed to transition from AF to RS monotonically decreases, which aligns with our expectation. However, as the coflow velocity increases, the jet velocity required for RS to LF transition initially increases and then decreases. Such a shift implies that the dominant factor of the RS flame stabilization changes from the mixing to small-scale strain. The lift-off characteristics of RS under different coflow velocities were analyzed. With the increase of coflow velocity, the spots number increases non-monotonic, and the average lifetime decreases monotonically, further indicating that mixing and small-scale strain jointly determine the generation and extinction of RS. Moreover, the sensitivity of lift-off height to jet velocity is not only greater than that of coflow velocity but also affected by the coflow velocity. Therefore, considering the coflow velocity's effect, a modified mixing-strain model of RS was developed by improving the calculation of both strain rate and mixing time.

Synergistic competition mechanism of phonon scattering in uniaxial (100) strain diamond: A first-principles study

The miniaturization of high-frequency electronic devices has led to heat accumulation in their components. The ultrahigh thermal conductivity of diamond makes it an ideal material for heat dissipation, but factors such as temperature changes during the working process, special structures, thermal stress, and lattice mismatch can affect performance. This study investigated the effects of stress, temperature, isotope purity, and feature size on the thermal conductivity of diamond using a first-principles method. The sensitivity to size effects of the thermal conductivity of uniaxially strained diamond differed according to the scale. Under a low temperature (100 K), and with a feature size of 1 µm, the thermal conductivity of diamond with large-scale tensile strain exceeded that of a diamond with small-scale strain, which was an anomalous phenomenon. Isotope purification did not significantly affect the variation rule of the thermal conductivity of strained diamond.

Vial Breakage in Lyophilization- Case Studies from Commercial Manufacturing and Laboratory Studies

Vial breakage during lyophilization reduces yield and can lead to product contamination with glass particulates, personnel interventions during manufacturing and damage to equipment. We present case studies of full-scale commercial lyophilization operations and small-scale laboratory lyophilization studies to understand and mitigate the sources of vial breakage for sterile injectable products.In the first case study, changes to the lyophilization cycle caused the breakage of 11% of the vials. Breakage rates were higher on the top than the bottom shelves and higher for vials on the edges of the shelves than for center vials. Laboratory strain gauge and process parameter ranging studies confirmed the breakage mechanism to be thermal expansion of the frozen plug early in primary drying, the temperature of which was increased by the cycle changes. We postulate that residual heat from steam sterilization coupled with edge effects drove the breakage patterns.In another case study, we reduced breakage from 3.5% to 0.4% in commercial production by changing the freezing temperature from -45°C to -25°C. Laboratory strain gauge studies confirmed reduced incidence and severity of “break-free” / “plugging-off,” which occurs when the frozen plug abruptly detaches from the vial sidewalls as it is cooled well below Tg’.The final case study is a “breakage challenge” study in the lab using higher fill volumes and aggressive drying to challenge the strength of vials. For borosilicate vials, breakage rates were dramatically higher after washing and tunnel depyrogenation in commercial manufacturing compared to vials that were used as received. Corning Valor® vials remained unbreakable even after processing. Tin oxide external coating provided borosilicate vials significant protection against damage from vial processing.These case studies illuminate vial breakage mechanisms, show how small-scale strain gauge and breakage challenge studies can be used to nearly eliminate vial breakage during full-scale commercial lyophilization.

Lift-off height of autoignited jet flame in hot air coflow with different O2 contents

This paper aims to understand the effects of coflow oxygen content on the lift-off height of autoignited jet flame and develop a lift-off height prediction model. A jet-in-coflow burner was used to performed autoignition experiments in hot air coflow with different oxygen contents. At a fixed oxygen content, as the fuel jet velocity increases, two types of autoignited flames were observed: (1) nozzle-stabilized flame (NS); (2) lifted flame (LF). As the oxygen content increases, it takes a larger fuel jet velocity for the transition from NS to LF and the transition from LF to blow-out. For a fixed fuel jet velocity, the lift-off height decreases significantly with increasing oxygen content and is more sensitive to the oxygen content at oxygen-lean conditions. Based on the modified large-scale mixing model, a lift-off height model was developed that considers the combined effect of small-scale strain and large-scale mixing.

Vertical fluxes conditioned on vorticity and strain reveal submesoscale ventilation

AbstractIt has been hypothesized that submesoscale flows play an important role in the vertical transport of climatically important tracers, due to their strong associated vertical velocities. However, the multi-scale, non-linear, and Lagrangian nature of transport makes it challenging to attribute proportions of the tracer fluxes to certain processes, scales, regions, or features. Here we show that criteria based on the surface vorticity and strain joint probability distribution function (JPDF) effectively decomposes the surface velocity field into distinguishable flow regions, and different flow features, like fronts or eddies, are contained in different flow regions. The JPDF has a distinct shape and approximately parses the flow into different scales, as stronger velocity gradients are usually associated with smaller scales. Conditioning the vertical tracer transport on the vorticity-strain JPDF can therefore help to attribute the transport to different types of flows and scales. Applied to a set of idealized Antarctic Circumpolar Current simulations that vary only in horizontal resolution, this diagnostic approach demonstrates that small-scale strain dominated regions that are generally associated with submesoscale fronts, despite their minuscule spatial footprint, play an outsized role in exchanging tracers across the mixed layer base and are an important contributor to the large-scale tracer budgets. Resolving these flows not only adds extra flux at the small scales, but also enhances the flux due to the larger-scale flows.

Inkjet printing of multi-stripes based deflection monitoring sensor on flexible substrate

This work presents an interesting approach towards rapid manufacturing of multi-stripes based deflection sensors developed through printing technologies. The deflection sensor is made of multiple small-scale strain sensing stripes sandwiched between consecutive conducting stripes. The drop-on-demand inkjet printing is used to pattern silver (Ag) nanoparticles ink as conducting interconnects and metallic stripes, whereas the piezoresistive sensing layer is obtained by mixing powdered carbon black and MWNTs in PDMS base matrix, resulting into a conducting nanocomposite. Different physical, optical, electrical, and adhesion-loss tests were performed as evaluation of the printing experiments. Deflection sensing was evaluated by mounting the sensors on opening cover-head of a plastic box to monitor different opening angles. Four detection angles at 0°, 10°, 45° and 90° were evaluated, for which the sensor showed prominent response at each level. The plastic box was used to represent the fill levels of a waste collection bin. The sensors will be advanced further by making an array of sensors where other parameters such as temperature, humidity, and concentration of the VOCs can also be monitored. All these advancements will be headed towards smart waste collection that is an integral part of Smart Cities development as well as internet-of-things (IoT) applications.

Dynamic plastic behaviour of strain rate sensitive tubes under axial impact

In this paper, the axisymmetric buckling of circular strain rate sensitive tubes subjected to axial impact is investigated theoretically and experimentally. The nonlinear governing equations are first derived in an incremental form. Then, the finite difference method is employed to solve the obtained equations implicitly for the case when a stationary tube is impacted by a striking mass. Experimental tests are performed by using a gas gun. The effect of linear and multi-linear approximation for strain hardening and strain rate on the shortening, buckling shape and axial force of steel tubes are investigated in which the strain rate is considered as a function of time. By applying the strain rate effect and using the multi-linear approximation for strain hardening, we showed there is a significant association between theoretical and experimental results of steel tubes. Also, using the nonlinear dynamic equations and newly developed inexact finite similitude theory it is revealed that the response characteristics of full-scale strain rate sensitive tubes can be predicted using small-scale strain rate insensitive tubes.

Assessing strain mapping by electron backscatter diffraction and confocal Raman microscopy using wedge-indented Si

The accuracy of electron backscatter diffraction (EBSD) and confocal Raman microscopy (CRM) for small-scale strain mapping are assessed using the multi-axial strain field surrounding a wedge indentation in Si as a test vehicle. The strain field is modeled using finite element analysis (FEA) that is adapted to the near-indentation surface profile measured by atomic force microscopy (AFM). The assessment consists of (1) direct experimental comparisons of strain and deformation and (2) comparisons in which the modeled strain field is used as an intermediate step. Direct experimental methods (1) consist of comparisons of surface elevation and gradient measured by AFM and EBSD and of Raman shifts measured and predicted by CRM and EBSD, respectively. Comparisons that utilize the combined FEA–AFM model (2) consist of predictions of distortion, strain, and rotation for comparison with EBSD measurements and predictions of Raman shift for comparison with CRM measurements. For both EBSD and CRM, convolution of measurements in depth-varying strain fields is considered. The interconnected comparisons suggest that EBSD was able to provide an accurate assessment of the wedge indentation deformation field to within the precision of the measurements, approximately 2×10−4 in strain. CRM was similarly precise, but was limited in accuracy to several times this value.

Full-field strain measurements at the micro-scale in fiber-reinforced composites using digital image correlation

Few studies have been conducted to monitor inter-fiber deformations in fiber-reinforced composites. In the present work, we demonstrate full-field strain measurements in the composites at the micro-scale, using digital image correlation (DIC). The study is performed on a unidirectional glass fiber reinforced composite loaded in transverse three-point bending inside an environmental scanning electron microscope. A nano-scale random speckle pattern of high quality is created. Validity of the measured fields is assessed against results of a finite element (FE) model with boundary conditions retrieved from the experiment. A good agreement is found between the DIC-measured and FE-predicted results. The precise recognition of very small-scale strain concentrations requires enhancement of the correlation process and removal of microscopy imperfections. The investigated methodology shows promise for real-time deformation measurements in composites at the micro-scale.

Adhesive bonding of measurement equipment on impact-driven offshore monopile foundations

To a certain extent, adhesive bonding of measurement equipment is very common in science and technology, e.g. adhesive bonding of small-scale strain gauges. Adhesive bonding of the entire equipment for a fully autonomous pile driving monitoring of an impact-driven large-scale foundation structure for an offshore wind farm is a completely new application method. Several offshore wind farms are currently under construction in the North and Baltic Seas. Impact pile driving of the large-scale foundations usually causes much louder noise than permitted by regulations, so methods for noise reduction are necessary. Geotechnical engineers of the TU Braunschweig are investigating combined methods for reducing that noise, and in 2014 they had the opportunity to install measurement equipment for the investigation of dynamic pile deflections during pile driving into three of in total eighty monopiles (length: 60 m, diameter: 6 m) of an offshore wind farm in the German North Sea. Due to certification issues conventional methods of fastening such as screwing or welding were not permitted. Instead, adhesive bonding of all parts (sensors, cables, shielding, recorder/computer) was successfully applied and withstood impact driving with several thousand blows of up to 1200 g (earth gravity). The authors would like to present the concept and preceding tests of the adhesive bonding applied within the research project ‘triad’.

Quantitative 3D strain analysis in analogue experiments simulating tectonic deformation: Integration of X-ray computed tomography and digital volume correlation techniques

The combination of scaled analogue experiments, material mechanics, X-ray computed tomography (XRCT) and Digital Volume Correlation techniques (DVC) is a powerful new tool not only to examine the 3 dimensional structure and kinematic evolution of complex deformation structures in scaled analogue experiments, but also to fully quantify their spatial strain distribution and complete strain history.Digital image correlation (DIC) is an important advance in quantitative physical modelling and helps to understand non-linear deformation processes. Optical non-intrusive (DIC) techniques enable the quantification of localised and distributed deformation in analogue experiments based either on images taken through transparent sidewalls (2D DIC) or on surface views (3D DIC). X-ray computed tomography (XRCT) analysis permits the non-destructive visualisation of the internal structure and kinematic evolution of scaled analogue experiments simulating tectonic evolution of complex geological structures. The combination of XRCT sectional image data of analogue experiments with 2D DIC only allows quantification of 2D displacement and strain components in section direction. This completely omits the potential of CT experiments for full 3D strain analysis of complex, non-cylindrical deformation structures.In this study, we apply digital volume correlation (DVC) techniques on XRCT scan data of “solid” analogue experiments to fully quantify the internal displacement and strain in 3 dimensions over time. Our first results indicate that the application of DVC techniques on XRCT volume data can successfully be used to quantify the 3D spatial and temporal strain patterns inside analogue experiments. We demonstrate the potential of combining DVC techniques and XRCT volume imaging for 3D strain analysis of a contractional experiment simulating the development of a non-cylindrical pop-up structure. Furthermore, we discuss various options for optimisation of granular materials, pattern generation, and data acquisition for increased resolution and accuracy of the strain results.Three-dimensional strain analysis of analogue models is of particular interest for geological and seismic interpretations of complex, non-cylindrical geological structures. The volume strain data enable the analysis of the large-scale and small-scale strain history of geological structures.

Homogenisation of slender periodic composite structures

A homogenisation technique is introduced to obtain the equivalent 1-D stiffness properties of complex slender periodic composite structures, that is, without the usual assumption of constant cross sections. The problem is posed on a unit cell with periodic boundary conditions such that the small-scale strain state averages to the large-scale conditions and the deformation energy is conserved between scales. The method can be implemented in standard finite-element packages and allows for local stress recovery and also for local (periodic) nonlinear effects such as skin wrinkling to be propagated to the large scale. Numerical examples are used to obtain the homogenised properties for several isotropic and composite beams, with and without transverse reinforcements or thickness variation, and for both linear and geometrically-nonlinear deformations. The periodicity in the local post-buckling response disappears in the presence of localisation in the solution and this is also illustrated by a numerical example.

Comment on “Deformation of compliant fault zones induced by nearby earthquakes: Theoretical investigations in two dimensions” by Benchun Duan et al.

[1] Duan et al. [2011] recently presented analysis of deformation caused by dynamic and static stress changes on fault zones that have reduced strength and effective elastic moduli relative to the ambient rocks. Low-rigidity fault zones were previously proposed to explain anomalous geodetic signals around major crustal faults in the vicinity of large earthquakes [e.g., Fialko et al., 2002; Fialko, 2004; Hamiel and Fialko, 2007; Barbot et al., 2009]. Duan et al. [2011] claimed that “some deficiencies may exist” in the previously published results (in particular, those presented by Fialko et al. [2002]), and called for a reexamination of existing observations. Unfortunately, claims made by Duan et al. [2011] appear to stem from their misunderstanding of the published data and theory. Here I refute claims made by Duan et al. [2011], and explain why their arguments are flawed. [2] Inferences of low-rigidity zones surrounding major crustal faults have been made based on a number of seismic [e.g., Li et al., 1994; Ben-Zion et al., 2003; Spudich and Olsen, 2001; Cochran et al., 2009], and geodetic [e.g., Lisowski et al., 1991; Chen and Freymueller, 2002; Fialko et al., 2002] observations. Such zones most likely result from extensive damage associated with slip on faults, in agreement with geologic evidence [Faulkner et al., 2003; Oskin and Iriondo, 2004; Chester et al., 2005; Dor et al., 2006]. [3] Compliant fault zones are of considerable interest as they encapsulate a long-term record of time-dependent damage and healing associated with the earthquake cycle, likely affect the earthquake rupture dynamics, bear on the issue of strain localization in the brittle upper crust, and may even hold clues about the magnitude of stress at seismogenic depths [e.g., Hearn and Fialko, 2009]. Thus more studies of compliant fault zones are certainly warranted, from both the observational and theoretical perspectives. [4] Duan et al. [2011] (hereafter, DKL11) explored the effects of plastic yielding of fault zones due to dynamic stress changes from passing seismic waves, and compared predictions of their elastoplastic simulations to those of the elastic inhomogeneity model like that previously invoked to explain InSAR observations of small-scale strain anomalies on a number of faults in the Eastern California Shear Zone [Fialko et al., 2002; Fialko, 2004]. In the framework of the elastic inhomogeneity model, the observed strain anomalies represent elastic response of massive compliant fault zones to static stress changes from a nearby earthquake.

Kinetics of accumulation of scattered damage in polycrystalline materials with various grain sizes under small-scale strain conditions

The paper addresses damage accumulation processes that occur during static deformation in steels 45 and 09G2S and in brass L63 in the initial state and upon “grain annealing.” The authors discuss new experimental evidence on the damage accumulation kinetics during deformation of polycrystalline materials that differ in nature and grain size, including materials in the unloaded state and those under stresses, with small and very small strains (0.05–0.15%). The results obtained have been the necessary physically consistent explanations. Analytical approximations of the correlation between the main mechanical properties of materials (hardness, yield stress, and ultimate strength) and their structure parameters (grain size) are given.

Geotechnical Properties of Cemented Sands in Steep Slopes

An investigation into the geotechnical properties specific to assessing the stability of weakly and moderately cemented sand cliffs is presented. A case study from eroding coastal cliffs located in central California provides both the data and impetus for this study. Herein, weakly cemented sand is defined as having an unconfined compressive strength (UCS) of less than 100 kPa, and moderately cemented sand is defined as having UCS between 100 and 400 kPa. Testing shows that both materials fail in a brittle fashion and can be modeled effectively using linear Mohr-Coulomb strength parameters, although for weakly cemented sands, curvature of the failure envelope is more evident with decreasing friction and increasing cohesion at higher confinement. Triaxial tests performed to simulate the evolving stress state of an eroding cliff, using a reduction in confinement-type stress path, result in an order of magnitude decrease in strain at failure and a more brittle response. Tests aimed at examining the influence of wetting on steep slopes show that a 60% decrease in UCS, a 50% drop in cohesion, and 80% decrease in the tensile strength occurs in moderately cemented sand upon introduction to water. In weakly cemented sands, all compressive, cohesive, and tensile strength is lost upon wetting and saturation. The results indicate that particular attention must be given to the relative level of cementation, the effects of groundwater or surficial seepage, and the small-scale strain response when performing geotechnical slope stability analyses on these materials.

Modelling lifted hydrogen jet fires using the boundary layer equations

The focus of this paper is the simulation of lifted hydrogen jet fires. The computational modelling of rim-stabilised fires is mature. This is not the case for lifted jets where computational studies have focussed on understanding the mechanism for the location of the combusting flame base and good agreement between predicted and measured flow properties is not universal. The simulation of hydrogen jet fires is an interesting and current area of research due to the sensitivity to global warming and the potential to address this problem with “The Hydrogen Economy” concept. To utilise hydrogen successfully requires the development of robust and accurate models to investigate new techniques for assessing the safety of operations involving hydrogen to ensure the inherent hazards of using hydrogen do not negate the benefit of reduced CO 2 emissions. The paper presents preliminary findings of an investigation into modelling lifted hydrogen jet fires using the boundary layer equations. The use of the boundary layer equations means that any model calibration is rigorous confident that the numerical error is negligible. A number of lift-off models based on the flamelet quenching concept and the turbulence time-scale are implemented and evaluated using available experimental data taken from the open literature. The lift-off models based on flamelet quenching use multi-flamelet libraries for the state relationships used to calculate the mean flow properties. Initial results suggest that a lift-off model based on the flamelet quenching concept incorporating the small-scale strain rate gives reasonable agreement with the measured lift-off compared to the other modelling approaches considered. This is an interesting result as it contrasts with earlier studies of methane and propane lifted jets, where the large-scale strain rate gave better agreement between observation and theory. This would suggest the appropriate strain rate model for a particular fuel and jet could be related to the residence time of the jet or the bulk strain rate.

Modelling of waterfront cantilever sheet pile walls subjected to earthquake loading

The seismic performance of waterfront cantilever sheet pile retaining walls is of continuing interest to geotechnical engineers as these structures suffer severe damage and even complete failure during earthquakes. This is often precipitated by liquefaction of the surrounding soil, either in the backfill or in front of the wall. This paper presents results from a series of small-scale plane strain models that were tested on a 1-g shaking table and recorded using a high-speed, high-resolution digital camera. The technique of Particle Image Velocimetry (PIV) was applied in order to allow the failure mechanisms to be visualised. It is shown that using PIV analyses it is possible to obtain failure mechanisms for a cantilever wall in liquefiable soil. These failure mechanisms are compared with those obtained for a cantilever wall in dry soil, previously carried out at a similar scale. It was observed that seismic liquefaction causes significant displacement in much larger zones of soil near the retaining wall compared to an equivalent dry case. The failure mechanism for a cantilever wall with liquefiable backfill, but with a remediated zone designed not to liquefy, is also presented and compared to the unremediated case.

Small-scale aspects of flows in proximity of the turbulent/nonturbulent interface

The work reported below is the first of its kind to study the properties of turbulent flow without strong mean shear in a Newtonian fluid in proximity of the turbulent/nonturbulent interface, with emphasis on the small-scale aspects. The main tools used are a three-dimensional particle tracking system allowing one to measure and follow in a Lagrangian manner the field of velocity derivatives and direct numerical simulations. The comparison of flow properties in the turbulent (A), intermediate (B), and nonturbulent (C) regions in the proximity of the interface allows for direct observation of the key physical processes underlying the entrainment phenomenon. The differences between small-scale strain and enstrophy are striking and point to the definite scenario of turbulent entrainment via the viscous forces originating in strain.

Physical Mechanism of the Two-Dimensional Inverse Energy Cascade

We study the physical mechanisms of the two-dimensional inverse energy cascade using theory, numerics, and experiment. Kraichnan's prediction of a -5/3 spectrum with constant, negative energy flux is verified in our simulations of 2D Navier-Stokes equations. We observe a similar but shorter range of inverse cascade in laboratory experiments. Our theory predicts, and the data confirm, that inverse cascade results mainly from turbulent stress proportional to small-scale strain rotated by 45 degrees. This "skew-Newtonian" stress is explained by the elongation and thinning of small-scale vortices by large-scale strain which weakens their velocity and transfers their energy upscale.

An investigation of the failure mechanism of a cantilever retaining wall under earthquake loading

The seismic behaviour of retaining walls is of continuing interest to geotechnical engineers as many of these structures were found to fail following recent earthquakes. In this study an investigation of the development of the failure of a cantilever retaining wall subjected to dynamic loading in dry sand was performed. The small-scale plane strain models produced for this purpose were tested using a new shaking table facility at Cambridge University. Images of the cross-section of the model for each one of these events were recorded by means of a high-speed and high-resolution image-capturing device, which allowed for the complete information on the displacement field to be recorded. The principles of Particle Image Velocimetry, as applied by the GeoPIV software package developed at Cambridge, were then used to process this data by removing the base displacement of the model and calibrating the images from pixel to real space. In this way it was possible to produce displacement and strain plots for any time interval, allowing for the real-time development of the failure mechanism for such a structure to be investigated for the first time. In particular, it was possible to isolate time intervals as small as 10 ms and hence obtain a visualisation of a nearly instantaneous failure mechanism for the structure.