Local Strain Gauge Research Articles

Experimental evaluation of the use of cruciform specimens for biaxial stability analysis

Novel research is proposed about the suitability of cruciform specimens for estimating the stress-strain response and buckling modes of flat laminates under equibiaxial compression. The experimental challenge lies in determining whether the specimen design and test procedure can be adequate to observe geometric instability at the coupon scale with small dimensions of the tested region. Although the literature on composite buckling is extensive, the number of studies that include experimental biaxial testing of flat laminates is limited, particularly when considering strong flexural-torsional coupling. In this work the region of interest of the cruciform specimen consists of a carbon-fibre reinforced [∓45]S laminate with shape and boundary conditions close to a clamped square plate. For the first time, the buckling mode of an anisotropic laminate is observed in the central region of a cross-shaped specimen using 3D deflection surface data recorded by Digital Image Correlation. The effect of flexural-torsional coupling at the bifurcation onset is experimentally corroborated through the orientation of the buckling mode, which is analytically utilized to quantify the proportion between bending and twist moments. Non-linearities exhibited in the stress-strain evolution of the biaxially loaded region are verified to be independent of the response of the sample's arms. Furthermore, no previous experimental investigation has shown the differences in local strain gauge measurements induced by the stress state at the ply level. Additionally, this work adapts the method for strength reduction to estimate the real critical stress of a bi-compressed laminate.

Preparatory Slip in Laboratory Faults: Effects of Roughness and Load Point Velocity

AbstractAseismic slip may occur during a long preparatory phase preceding earthquakes, and what controls it remains poorly understood. In this study, we explored the role of load point velocity and surface roughness on slow slip during the preparatory stage prior to stick‐slip events. To that end, we conducted displacement‐rate controlled friction experiments by imposing varying load point velocities on sawcut granite samples with different surface roughness at a confining pressure of 35 MPa. We measured the average slip along the fault with the recorded far‐field displacements and strain changes, while acoustic emission sensors and local strain gages were used to capture local slip variations. We found that the average amount of aseismic slip during the preparatory stage increases with roughness, whereas precursory slip duration decreases with increased load point velocity. These results reveal a complex slip pattern on rough faults which leads to dynamic ruptures at high load point velocities.

Fatigue monitoring and maneuver identification for vehicle fleets using a virtual sensing approach

Extensive monitoring comes at a prohibitive cost, limiting Predictive Maintenance strategies for vehicle fleets. This paper presents a measurement-based virtual sensing technique where local strain gauges are only required for few reference vehicles, while the remaining fleet relies exclusively on accelerometers. The scattering transform is used to perform feature extraction, while principal component analysis provides a reduced, low dimensional data representation. This enables direct fatigue damage regression, parameterized from unlabeled usage data. Identification measurements allow for a physical interpretation of the reduced representation. The approach is demonstrated using experimental data from a sensor equipped eBike, which is made publicly available.

Correlation between microstructure heterogeneity and multi-scale mechanical behavior of hybrid LPBF-DED Inconel 625

The two additive manufacturing processes Powder Bed Fusion (LPBF) and Directed Energy Deposition (DED) have different geometrical resolutions and production flexibilities, making their hybridization attractive. LPBF microstructure displays fine grains, with weak preferential crystal orientation. DED generates a highly textured and inhomogeneous microstructure with equivalent grains diameters ranging from a few micrometers to over a millimeter. The microstructure of the hybrid LPBF-DED sample is the addition of these two microstructures with an interface free from cracks or particular pores. The effect of this strong heterogeneity of the hybrid microstructure on mechanical behavior is analyzed by tensile tests instrumented with local strain gauges, others using digital image correlation method and finally on samples tested inside a scanning electron microscope. This multi-scale characterization showed that the difference in the elastic properties causes the localization of the strain field and generates a plastic incompatibility at the interface. An optimized heat treatment leads to isotropic and homogeneous hybrid microstructure, with a larger DED grain size. It leads to identical plasticity mechanisms during tensile tests and lowers the strain gradient around the interface.

Seismic Dispersion and Attenuation in Fractured Fluid‐Saturated Porous Rocks: An Experimental Study with an Analytic and Computational Comparison

Although different fluid pressure diffusion mechanisms caused by fractures have been extensively studied using analytical and numerical methods, there is little to no experimental work completed on them in laboratory conditions. In this paper, hydrostatic-stress oscillations (frequency − 0.04 to 1 Hz) are used on an intact and saw cut sample, in dry and water saturated conditions, in a triaxial cell at different effective pressures in undrained conditions. The objective is to study the fracture’s effect on the elastic properties of the sample and validate some computational fracture models, that have been explored in the literature. Experimental results highlight dispersion and attenuation in saturated conditions due to the fracture, which diminishes in amplitude as the effective pressure is increased, i.e. as the fracture is closed. From local strain gauge measurements, it is found that there is a local negative phase shift between stress and strain in water saturated conditions for the fractured sample, due to the location of the strain measurements. No attenuation observed in dry conditions. A simple 1D model using mass balance and mechanical equilibrium equations for a linear isotropic poroelastic homogeneous medium give prediction in very good agreement with the experimental results. A 3D model was also developed to allow a comparison between analytic, numerical and experimental results.

Effects of loading regimes on the structural behavior of RC beam-column sub-assemblages against disproportionate collapse

The majority of previous quasi-static tests on disproportionate collapse simulated the column removal through applying concentrated load/displacement on the top of the removed column until failure. However, uniformly distributed service load always exists on the frames. Therefore, to reflect the actual load condition more accurately, uniformly distributed load should be applied along the beams first. Then, the temporary support is gradually removed to simulate the process of column removal. By this way, the beams may undergo only a small deflection as the dynamic effect is neglected. Thus, a subsequent concentrated loading process is employed to evaluate the behavior of the beams at the ultimate stage, which may be reached if the dynamic effect is considered. Such a loading process is named sequential loading regime. To evaluate the effects of loading regimes on the behavior of reinforced concrete (RC) frames under a middle column removal scenario, two series of half-scale RC beam-column sub-assemblages were tested in this study. It is found that the conventional concentrated loading regime could accurately estimate the yield strength and the compressive arch action capacity of the RC beam-column sub-assemblages, but it may over-estimate the catenary action (CA) capacity and the deformation capacity. Moreover, although the concentrated loading regime is convenient and able to demonstrate the load transfer mechanisms of the sub-assemblages against disproportionate collapse, it may mistakenly identify the locations of critical sections. Furthermore, based on the failure modes and local strain gauge results, analytical models were proposed for predicting the CA capacity of the tested specimens under two loading regimes. Results suggest that the analytical models could predict the CA capacity well.

Stress-Strain behaviour of Cement-Stabilized Hong Kong marine deposits

The deep cement mixing (DCM) technique is an in-situ ground improvement method to stabilize and solidify soft clay ground. To facilitate the practical design of DCM, it is necessary to establish the relationship between the strength and stiffness of cement treated soil with governing factors first. In this study, the influence of different seawater and cement contents on the strength and stiffness of cement stabilized Hong Kong marine deposits (HKMD) was investigated by a series of unconfined/confined compression tests. According to the experimental results, an attempt was made to predict the unconfined compressive strength (UCS), qu, by using a simple empirical equation based on water/cement ratio (w/c). The correlation between the strength and secant modulus of improved HKMD was obtained. Importantly, a linear relationship between small-strain (ε < 0.1%) stiffness and qu was formulated based on the measurement results from local linear variable differential transformers (LVDTs) and strain gauges. Besides, the effect of w/c on the failure mode of the specimens was revealed. In addition, the consolidated undrained (CU) triaxial tests indicated that specimens gained higher peak strength with increase of confining pressure. All the findings are of practical significance for the local ground improvement industry as well as for other coastal cities around the world.

Local strain gauge based on the nanowires ring resonator embedded in a flexible substrate

The authors proposed a flexible microresonator based on nanowires composed of Ⅱ–Ⅵ compounds to detect the small strain caused by external motion. The nanowire ring resonator is embedded in polydimethylsiloxane (PDMS) flexibility to improve the coupling efficiency. In this work, CdS nanowire is fabricated onto a PDMS flexible substrate. With the help of a fibre tip, the single nanowire is manipulated under a microscope, allowing the curved line to be a ring and making the litter overlapping. This overlap increases coupling efficiency and sensor performance. The ring cavity has the parameters of diameter 1 µm, length 75 µm and radius ∼10 µm. Experiments demonstrated the process of fabricating a strain sensor and detected peak shifts. This resonant wavelength appeared red-shift and linear tuned when stretching the flexible substrate. The quality factor was about 2000 and the gauge factor was about 80 nm per stretching unit. Being a small structure and high sensitivity, the sensor can be integrated into the chip. This promotes the development of miniaturisation to some extent. As a result, this work is beneficial to optical manipulation, further being extended to the tunable light source.

An Improved Unsaturated Triaxial Device to Characterize the Pore-Water Pressure and Local Volume Change Behavior of a Compacted Marl

Abstract Compacted fills in earthworks are in an unsaturated state, with suctions (negative pore-water pressures) usually ranging from several kPa to several MPa. Characterizing the variation of pore-water pressure is a major concern in the estimation of embankment stability and, at the same time, a big challenge when its value falls below –80 kPa. This technical note presents an improved triaxial system based on the standard triaxial cell, equipped with a tensiometer, a thermocouple psychrometer, and local strain gauges. The main objective of these improvements is to measure the pore-water pressure and local volume changes during isotropic loading. In this study, three compacted specimens were tested under undrained conditions. Pore-water pressure was continuously measured by means of the psychrometric probe in the range of –3,000 to –100 kPa and by the tensiometer in the range of –80 to 1,100 kPa. Volume change was measured by the local strain gauges. Analysis and development of the results from both this study and the literature lead to the conclusions that i) specimens with occluded air bubbles (e.g., at the standard Proctor optimum) behave like a saturated soil, as Bw is close to unity when pore-water pressure becomes positive, and ii) a pore-water pressure coefficient Bw close to 1 is not necessarily a sign of saturation, as soil in an unsaturated state may also have a pore-pressure coefficient close to unity, especially at the beginning of the mechanical loading.

A frequency-tunable nanomembrane mechanical oscillator with embedded quantum dots

Hybrid systems consisting of a quantum emitter coupled to a mechanical oscillator are receiving increasing attention for fundamental science and potential applications in quantum technologies. In contrast to most of the presented works in this field, in which the oscillator eigenfrequencies are irreversibly determined by the fabrication process, we present here a simple approach to obtain frequency-tunable mechanical resonators based on suspended nanomembranes. The method relies on a micromachined piezoelectric actuator, which we use both to drive resonant oscillations of a suspended Ga(Al)As membrane with embedded quantum dots and to fine-tune their mechanical eigenfrequencies. Specifically, we excite oscillations with frequencies of at least 60 MHz by applying an AC voltage to the actuator and tune the eigenfrequencies by at least 25 times their linewidth by continuously varying the elastic stress state in the membranes through a DC voltage. The light emitted by optically excited quantum dots is used as a sensitive local strain gauge to monitor the oscillation frequency and amplitude. We expect that our method has the potential to be applicable to other optomechanical systems based on dielectric and semiconductor membranes possibly operating in the quantum regime.

Cantilever-based freestanding InGaP/InGaAlP quantum wells microring lasers

Cantilever-based freestanding microring lasers were demonstrated based on InGaP/InGaAlP quantum wells emitting in the red region. The rotational symmetry of the microring cavity is broken by introducing the connected cantilevers, which induce degenerated mode splitting and single mode lasing emission. The connected cantilevers can not only provide mechanical support for freestanding microring lasers but also function as waveguides for efficient light extraction. In addition, the application of cantilever-based microring lasers had been studied for a refractive index sensor with a measured sensitivity of ∼7.3 nm per refractive index unit and flexible local strain gauges with the measured gauge factor of ∼6.33 nm per stretching unit.

Fatigue Testing of Large-Scale Steel Structures in Resonance with Directional Loading Control

Offshore structures such as jacket and monopile foundations for wind towers and platforms for oil and gas are large steel structures that are subjected to severe fatigue loading conditions. When developing constructions using novel welding techniques, high-strength steel grades or specific bolted assemblies there is a need for validation testing of large-scale components. Conventionally, such fatigue tests are carried out on servo-hydraulic test which are time-consuming.Alternatively, fatigue tests can be performed in resonant bending by a cyclic excitation force with frequency close to the first eigenfrequency of the test specimen. In this paper a new test setup is presented suitable for testing large-scale steel components in resonance with control of the loading direction by two counter-rotating excentric masses.A wide range of loading conditions can be applied with a frequency from 20 to 40 Hz. The test specimen, that can weigh up to 25 ton, is supported in the nodes of its natural wave-form, so that no dynamic forces are transmitted to the setup.The working principles of the test setup are illustrated based on test results of a 711 mm diameter x 25.4 mm wall thickness steel pipe, a welded X-node representative for a welding detail of an offshore jacket foundation structure and a large-scale HE-beam. Crack initiation is detected using acoustic emission while crack growth is monitored by local strain gauge measurements as well as the global stiffness reduction of the test specimen. Finally, the beach marking method is used to visualize crack fronts for post-mortem analysis.

The behaviour of a recycled road base aggregate and quartz sand with bender/extender element tests under variable stress states

The paper focuses on laboratory testing investigating the small strain dynamic properties of a recycled road base material of a single fraction. A widely examined quartz sand was also used in the study for comparison purposes. The shear wave velocities (Vs) and primary wave velocities (Vp) were measured in both isotropic and anisotropic stress states with bender/extender element inserts implemented in a stress path triaxial apparatus. Subsequently, the small strain constrained modulus (Mmax), Young’s modulus (Emax) and Poisson’s ratio (ν) were derived and further analysed. For the measurement of sample volumetric strains, particularly under the application of stress anisotropy, local strain gauges were used to capture the changes of their radius while the deviatoric compressive load was gradually increased. The responses of the two materials during isotropic compression and swelling as well as under the application of stress anisotropy were thoroughly stated and quantified.

Impact of two columns missing on dynamic response of RC flat slab structures

Sudden removal of columns caused by unexpected extreme loading may increase the bending moment and shear force at surrounding column-slab connections significantly, which may trigger punching shear failure at these connections and lead to progressive collapse of entire reinforced concrete (RC) flat slab structures. To quantify the dynamic load redistribution of flat slab structures subjected to different extents of initial local damage (one-column or two-column removal), two multi-panel RC flat slab substructures were tested subjected to simulated sudden column removal scenarios. These two specimens have identical dimensions and reinforcement details. One of the substructures suffered a loss of an interior column scenario while another one was subjected to a two columns (one interior column and one edge column) missing scenario. The dynamic response, deformation shape, failure mode, and local strain gauge results are presented. It is found that although both specimens had exceeded their yield load capacity, no collapse occurred as considerable compressive membrane action developed in the RC slab to help redistribute the loads. With the drop panels, no punching shear failure was observed in the slab-column connections after removal of the column. To further study the progressive collapse robustness of flat slab structures, finite element (FE) models were validated and parametric studies were carried out. Numerical analysis indicated that the axial force initial carried by the lost columns may amplify up by 1.25 times before redistribution into surrounding columns. Moreover, the dynamic ultimate load capacity of Specimens S1 and S2 are as large as 1.92 and 1.18 times of their design service load, respectively.

Cantilever-based microring lasers embedded in a deformable substrate for local strain gauges

A cantilever-based microring laser structure was proposed for easily integrating III-V active layer into mechanically stretchable substrates. Local strain gauges were demonstrated by embedding cantilever-based microring lasers in a deformable polymer substrate. The characterizations of microscale local strain gauges had been studied from both simulated and experimental results. The lasing wavelength of strain gauges was blue-shift and linear tuned by stretching the flexible substrate. Gauge factor being ∼11.5 nm per stretching unit was obtained for a cantilever-based microring laser with structural parameters R=1.25 μm, W1=450 nm and W2=240 nm. Such microring lasers embedded in a flexible substrate are supposed to function not only as strain gauges for monitoring the micro- or nano-structured deformation, but also tunable light sources for photonic integrated circuits.

Microscale local strain gauges based on visible micro-disk lasers embedded in a flexible substrate.

Microscale local strain gauges with low-power consumption and large strain range were demonstrated by integrating microdisk lasers in a deformable and flexible polymer substrate. The lasing spectra of microdisk lasers were sensitive to substrate deformation and can be modulated by strains. The measured relative wavelength tuning under strains of the novel strain sensors illustrated a linear behavior with the gauge factor being ~4.0 nm and ~6.7 nm per stretching unit for microdisk lasers with the diameter of 1.2 μm and 1.5 μm, which corresponding to a smooth wavelength tuning of 1.5 nm and 2.6 nm under 36% strain, respectively. In addition, to being used as microscale strain gauges, the visible lasers on the deformable substrate can also function as a tunable light source for the photonic integrated circuits and flexible laser projection displays.

Modelling the drained/undrained transition: effect of the measuring method and the boundary conditions

ABSTRACTThe dependence of fluid‐saturated rocks' elastic properties to the measuring frequency is related to fluid‐flow phenomena at different scales. In the frequency range of Hz, for fully saturated rocks, two phenomena have been experimentally documented: (i) the drained/undrained transition (i.e., global flow), and (ii) the relaxed/unrelaxed transition (i.e., local flow). When investigating experimentally those effects or comparing different measurements in rocks, one needs to account for both the boundary conditions involved and the method of measurement used. A one‐dimensional poroelastic model is presented, which aims at calculating the expected poroelastic response during an experiment. The model is used to test different sets of boundary conditions, as well as the role of the measuring setup, i.e., local (strain gauges) or global (linear variable differential transformer) strain measurement. Four properties are predicted and compared with the measurements, i.e., bulk modulus, bulk attenuation, pseudo‐Skempton coefficient, and pore pressure phase shift. For the drained/undrained transition, because fluid pressure may not be homogeneous in the sample, local and global measurements are predicted to differ. Furthermore, the existence of a dead volume at both sample's ends is shown to be important. Due to the existence of the dead volume, an interplay between sample's and dead volumes' storage capacity determines both the magnitudes and the frequency dependence of the dispersion/attenuation measurements. The predicted behaviours are shown to be consistent with the measurements recently reported on very compressible and porous sandstone samples.

A 360-deg Digital Image Correlation system for materials testing

The increasing research interest toward natural and advanced engineered materials demands new experimental protocols capable of retrieving highly dense sets of experimental data on the full-surface of samples under multiple loading conditions. Such information, in fact, would allow to capture the possible heterogeneity and anisotropy of the material by using up-to-date inverse characterization methods. Although the development of object-specific test protocols could represent the optimal choice to address this need, it is unquestionable that universal testing machines (UTM) remain the most widespread and versatile option to test materials and components in both academic and industrial contexts. A major limitation of performing standard material tests with UTM, however, consists in the scarce information obtainable with the commonly associated sensors since they provide only global (LVDTs, extensometers, 2D-video analyzers) or local (strain gages) measures of displacement and strain.This paper presents a 3D Digital Image Correlation (DIC) system developed to perform highly accurate full-surface 360-deg measurements on either standard or custom-shaped samples under complex loading within universal testing machines. To this aim, a low cost and easy to setup video rig was specifically designed to overcome the practical limitations entailed with the integration of a multi-camera system within an already existing loading frame. In particular, the proposed system features a single SLR digital camera moved through multiple positions around the specimen by means of a large rotation stage. A proper calibration and data-processing procedure allows to automatically merge the experimental data obtained from the multiple views with an accuracy of 10−2mm.The results of a full benchmarking of the metrological performances of the system are here reported and discussed together with illustrative examples of full-360-deg shape and deformation measurements on a Grade X65 steel sample under tensile loading. The methodology demonstrated to be robust and accurate thus representing a viable practical solution to afford the modern challenges in the field of materials and components testing.

Reversible Control of In‐Plane Elastic Stress Tensor in Nanomembranes

A new class of strain actuators is presented, which allows the three components of the in-plane stress tensor in a nanomembrane to be independently controlled. The functionality of the actuators is demonstrated using a semiconductor nanomembrane, whose light emission acts as a local and sensitive strain gauge. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Basic set of experiments for determination of mechanical properties of sand

Abstract A basic set of experiments for the determination of mechanical properties of sands is described. This includes the determination of basic physical and mechanical properties, as conventionally applied in soil mechanics, as well as some additional experiments, which provide further information on mechanical properties of granular soils. These additional experiments allow for determination of steady state and instability lines, stress-strain relations for isotropic loading and pure shearing, and simple cyclic shearing tests. Unconventional oedometric experiments are also presented. Necessary laboratory equipment is described, which includes a triaxial apparatus equipped with local strain gauges, an oedometer capable of measuring lateral stresses and a simple cyclic shearing apparatus. The above experiments provide additional information on soil’s properties, which is useful in studying the following phenomena: pre-failure deformations of sand including cyclic loading compaction, pore-pressure generation and liquefaction, both static and caused by cyclic loadings, the effect of sand initial anisotropy and various instabilities. An important feature of the experiments described is that they make it possible to determine the initial state of sand, defined as either contractive or dilative. Experimental results for the “Gdynia” model sand are shown.