Gauge Length Research Articles

Enhanced sensitivity distributed sensing of magnetic fields in optical fiber using random Bragg grating

We show that the use of random optical grating using UV exposure (ROGUE) can significantly reduce the noise floor of an optical frequency domain reflectometry (OFDR) measurement of Faraday rotation in the polarization. We compare it with unexposed spun fiber, which shows a S/P minimum ratio (signal noise floor) 20 dB higher than when using our ROGUE. High sensitivity magnetic field measurements are achieved by spatially filtering (setting a gage length) the derivative of the S/P ratio’s evolution. An example of a calibrated electromagnet spatially resolved B-field measurement is demonstrated, which can measure fields down to 10 mT with 10 cm spatial resolution. The potential for current sensing using the ROGUE apparatus is discussed and simulation shows a noise floor of ∼1 A with 40 probing loops spatial resolution.

Tensile failure mechanisms of historic fibrous plaster ceiling wads: Experiments at the UK Diamond Light Source

Fibrous plaster (FP) ceilings, prevalent in late 19th- and early 20th-century UK theatres, are suspended using ‘wads’. Wads are hangers made of Plaster of Paris, reinforced with twisted woven jute fabric. Several recent collapses in historic fibrous plaster ceilings have been attributed to tensile failures in wads. To understand the failure mechanisms involved, tensile tests were performed on laboratory-produced wad-like samples at the I12 beamline of the UK Diamond Light Source. The tested samples were designed with a dog bone shape and mounted with clevis-grips at each end, to ensure controlled failures along the gauge length. The beamline offered the opportunity to conduct simultaneous synchrotron X-ray computed tomography (sCT) and diffraction measurements during loading, enabling the monitoring of internal crack formation and strain propagation at the microstructural scale. Simultaneously, acoustic emission (AE) and digital image correlation (DIC) measurements were conducted. Preliminary results from these datasets are discussed in this paper. The datasets will provide useful information to validate the ongoing development of algorithms which can categorise the internal failure mechanisms and damage state of wads using only AE signals.

Calibration of tensile tests in drop-weight impact machine and implementation in simulation of Charpy impact tests

Material modeling and simulation of deformation under very high loading rate in the dynamic/impact range are applied in crash resistance, ballistic impact analysis, and other time-sensitive phenomena. Servo-hydraulic UTM tensile test data from medium to higher strain rate > (500 s-1) is reliable but expensive and complexity in experiment. Tensile testing at very high strain rate (upto 1500 s-1) can be done in a drop-weight impact testing machine with a special fixture and the impactor velocity controls strain rate. In INSTRON CEAST 9350 drop weight impact testing machine tensile tests are conducted at four velocities for amour steel. A 5MHz data acquisition system (DAQ) measures strain in specimen gauge length mounted with high-elongation strain gauge. An extensometer is used to calibrate the strain gauge in a quasi-static 10-4 s-1 tensile test. Tensile test data from impact machine is compared with stress-strain data from the INSTRON servo-hydraulic testing machine with DIC for different strain rates. Close matching in elastic modulus, yield strength, tensile strength, and elongation are observed. Material parameters of a strain rate-dependent material model (modified J-C model) are extracted from drop weight impact testing machine data clubbed with other tensile test data, used in FE simulation of very high strain rate tensile tests, and validated with experimental results. Later, the same material model and material parameters are used to simulate Charpy impact tests at various velocities and compared with experimental results which also show close matching.

The influence of pre-strain levels on the microstructure and performance of aircraft 7b04 aluminum alloy skin

The study focuses on the 7B04 alloy sheet. It investigates the effects of different pre-strain levels on the microstructure, mechanical properties, and fatigue behavior of the alloy through the use of optical microscopy, scanning electron microscopy, and DIC (Digital Image Correlation) system. The experimental results demonstrate that the yield strength and tensile strength of the alloy increase with the increase in pre-strain levels. At the same time, the elongation decreases with the increase in tensile deformation. In the non-uniform region, the plastic deformation capacity index of the material (denoted as ψ) follows the order ψ (non-uniform region (DIC value)) > ψ (original gauge length value) > ψ (uniform region (DIC value)). It is recommended to control the maximum local strain below ψ (uniform region (DIC value)) during production to avoid deformation instability or concentration. Furthermore, the thickness of the material decreases linearly with increasing tensile deformation, with each 4% increase in tensile deformation resulting in a 1.2% reduction in thickness. The fatigue life of the alloy increases gradually with the increase in pre-strain levels in the direction perpendicular to the fiber. At the same time, it decreases in the direction parallel to the fiber. Different deformation levels and loading directions exhibit similar fatigue fracture characteristics. Within the range of 12% pre-strain levels, the microstructure of the material remains continuous and intact without noticeable cracks at grain boundaries or phase boundaries. At the macroscopic level, when the pre-strain level exceeds 6%, distinct slip bands appear on the sheet, which is correlated with the micro-scale grain slip. This is accompanied by deformation bands and micro-cracks in the microstructure. The findings of this study are of significant importance for understanding the influence of pre-strain levels on the mechanical properties and fatigue behavior of the 7B04 alloy. The study provides valuable references for engineering practices in related fields, thereby ensuring the integrity and usability of manufactured components.

Fully Distributed Multi-Channel Fiber-Optic Sensor for Simultaneous Relative Humidity and Temperature Measurement with Finer Gauge Length based on Spatial-domain Time-delayed Multiplexing

Fully Distributed Multi-Channel Fiber-Optic Sensor for Simultaneous Relative Humidity and Temperature Measurement with Finer Gauge Length based on Spatial-domain Time-delayed Multiplexing

The Effect of Buckling Restraint on Axial Strain Life Fatigue Data

Abstract Focused on the common problem of easy specimen buckling in the axial strain–controlled fatigue testing of thin automobile steel sheets, a strain fatigue test method for thicknesses less than 2.5 mm is proposed. Because of the unrestrained grip end of specimens between the anti-buckling device and the testing machine fixture, specimens were prone to buckling in this area. In addition, if the designed specimen sizes were unreasonable, buckling may have occurred in the width and thickness directions of the gauge length of the specimen. Both situations can lead to invalid tests. This paper adopted a testing machine fixture and an anti-buckling plate as a mortise-and-tenon structure for the anti-buckling method; the upper and lower fixtures of the testing machine were cut in straight grooves, and the left and right anti-buckling plates were designed to be connected with a mortise and tenon. This is equivalent to narrowing the width of the unrestrained part of the specimen’s grip end to avoid buckling failure due to plane strain. The factors affecting the stiffness of fatigue specimens are also discussed herein. In order to avoid common instability in the thickness and width directions of the gauge length and unconstrained part of the specimen’s grip end at a strain ratio of Re = −1 during testing, six recommended specimen sizes are given on the premise of ensuring maximum specimen stiffness. When using the anti-buckling device, the recommended specimen sizes and test points of this study (i.e., the lower limits for yield strength and thickness of strain control testing) are 110 MPa and 0.5 mm, respectively, with which continuous and smooth stress–strain curves that result in accurate and reliable experimental data can be obtained.

Behavior of TRIP-aided medium Mn steels investigated by in situ synchrotron X-ray diffraction experiments and microstructure-based micromechanical modelling

Medium Mn steels belong to a new generation of advanced high-strength steels whose superior mechanical properties are explained by their ultrafine-grained ferrite/austenite/martensite microstructures and a possible transformation induced plasticity (TRIP) related to the stability of retained austenite. The mechanical behaviour of a set of model medium Mn steels is investigated during tensile testing using a combination of high-energy X-ray diffraction (HEXRD) and digital image correlation (DIC) measurements. HEXRD allows for the time-resolved determination of transformation kinetics and in situ stress partitioning among the different constituting phases. DIC provides precise spatiotemporal information on the strain evolution along the gauge length, particularly at the position of the diffracting volume. These experiments served to calibrate an innovative mean field micromechanical model which accounts for the local behaviour of each phase, as well as the strain-induced martensitic transformation (SIMT) of retained austenite. The work-hardening of both austenite and ferrite is modeled using a dislocation-based size-sensitive approach which includes kinematic hardening contributions. The behaviours of fresh and strain-induced martensite are predicted using a genuine model derived from the continuous composite approach. The model for SIMT is based on a thermodynamic assessment of the stability of retained austenite. The overall model is thus sensitive to the size of the microstructure components, their local chemistry, and their respective stability.

Study of sliver irregularity caused by fiber acceleration motion during the drafting process

During the drafting process, the fiber acceleration motion contributes significantly to the sliver irregularity. In this study, the drafting simulation based on the improved model of fiber arrangement in the sliver was developed to analyze the sliver irregularity caused by the acceleration motion during the drafting process. The results indicated that the drafting simulation could provide an effective reference for the parameter optimization of the drafting process under the condition of saving raw material cost and time cost. The simulations and calculations denoted that a shorter gauge length and larger input sliver linear density by more concentrated accelerated motion could be conducive to more even slivers during the drafting process. In addition, in order to generate higher sliver evenness, in the double-zone drafting process, the draft ratio of the back drafting zone should be selected to be slightly greater than 1 so that the draft ratio of the front drafting zone would be relatively large when the total draft ratio is constant due to the trend of the additional irregularity caused by the acceleration motion increasing to a peak value firstly and then decreasing as the draft ratio grows.

A thermodynamic model for predicting the stress–strain relation of stochastic heterogeneous materials with experimental verification

Materials are intrinsic stochastic heterogeneity regardless of their natural or artificial origin. Traditional models for predicting mechanical properties of materials typically involve homogenization, which exhibits certain limitations in practical applications, such as investigating the size effect in stress–strain relationships. In this study, a random field thermodynamic theory for predicting the mechanical properties of stochastic heterogeneous materials was proposed, and its applicability and reliability were verified by testing hard-drawn 304 stainless-steel wires with gauge lengths of 10–500 mm. In these wires composed of the lamellar martensite with a thickness of ∼47.1 nm, it is found that failure strains decreased significantly with increasing gauge length, whereas fracture stresses show little dependence on the gauge length leading to a great increase of the effective modulus. Digital image correlation investigations revealed a significant strain heterogeneity during the tensile process, even at the elastic deformation stage. Nanoindentation tests show the non-uniformly distributed reduced modulus and hardness, which can be attributed to the intrinsic stochastic heterogeneity of materials. Taking Young's modulus as a one-dimensional stationary random field along the longitudinal direction, our random field model can well demonstrate the variation of failure strain and effective modulus with changing gauge lengths. The size effect on the macroscopic mechanical properties mainly originated from the microscopic mechanical property correlation of different points within the materials. These findings could provide new insights into the size effect on the mechanical properties of materials.

Extraction and characterization of a novel cellulosic fiber derived from the bark of Rosa hybrida plant

The current investigation aims to choose an alternate potential replacement for the nonbiodegradable synthetic fibers used in polymer composites. This goal motivated the thorough characterization of Rosa hybrida bark (RHB) fibers. The research explored fiber characterization such as morphological, mechanical, thermal, and physical properties. The suggested fiber features a percentage of cellulose, hemicellulose molecules, and lignin of 52.99 wt%, 18.49 wt%, and 17.34 wt%, respectively according to chemical composition studies, which improves its mechanical properties. It is suitable for lightweight applications due to its decreased density (1.194 gcm−3). The purpose of the Fourier transform infrared spectroscope was to observe and record how various chemical groups were distributed throughout the surface of the fiber. The presence of 1.41 nm-sized crystalline cellulose and further XRD analysis showed a crystallinity index of 75.48 %. Scanning electron microscope studies revealed that RHB fibers have a rough surface. According to a single fiber tensile test, for gauge length (GL) 40 mm, Young's modulus and tensile strength of RHB fibers were 6.57 GPa and 352.01 MPa, respectively, and for GL 50 mm, 9.02 GPa and 311 MPa, respectively. Furthermore, thermo-gravimetric examination revealed that the isolated fibers were thermally stable up to 290 °C and the kinetic activation energy was found to be 75.32 kJ/mol. The fibers taken from the Rosa hybrida flower plants' bark exhibit qualities similar to those of currently used natural fibers, making them a highly promising replacement for synthetic fibers in polymer matrix composites.

Study on mechanical behavior of adhesively bonded laminated envelope composite joints

The airship exhibits great potential as an aerostat; however, the issue of connection in envelope composites, which serve as the primary structural material, significantly hampers its progress. Here, for the first time, we investigate the potential application of adhesive bonding techniques for connecting envelope composites. Several adhesively bonded joints based on Vectran-based envelope composite are prepared using the hot-pressing method. The obtained adhesively bonded joints exhibit a failure strength of 3.57 MPa, which is more than twice that of the welded envelope composite joints. Then, the mechanical response behaviors of adhesively bonded joints are investigated using mechanical testing, failure morphologies analysis, infrared thermal imaging, digital image correlation technique, and numerical simulation. Experimental results show that the structural parameters (adhesive content, bonding area, and the gauge length of the tested strips) of adhesively bonded joints have a significant impact on mechanical performance. Simulation results show that the stiffness mismatch of the component materials has a negative effect on the failure strength of adhesively bonded joints, which explains why the failure occurs at the fabric interface rather than the adhesive region.

Three-dimensional distributed acoustic sensing at the Sanford Underground Research Facility

Distributed acoustic sensing (DAS) is a valuable tool for monitoring seismic signals as it provides high spatial and temporal resolution strain sensing along the length of a fiber-optic cable. DAS records the seismic wavefields essentially synchronously at each sensing location (i.e., sensing channel with gauge lengths) because the interrogator senses the distributed strain at the speed of light in the fiber. Unlike traditional seismic sensors, DAS has an intrinsic directional sensitivity to the axial strain change along the fiber, leading to difficulties when using standard seismic analysis and interpretations that rely on 3D particle velocity sensing. In addition, cable deployments on the surface can be dominated by high-amplitude wind or urban noise, impeding the detection of low-amplitude distant seismic sources. Here we investigate the capabilities of a unique 3D array with spiral-like portions in the Sanford Underground Research Facility (SURF), the former Homestake Mine, between 1250 m (4100 ft) and 1488 m (4850 ft) in depth for detecting local, regional, and teleseismic sources of ground vibrations. Our pilot array finds that DAS records high frequency (above 5 Hz) vibration sources well, such as mine activities and local and regional blasting events. Furthermore, our deployment method (fiber resting on the surface with rocks placed every meter or so) may contribute to low-frequency noise that contaminates the interpretation of teleseismic waves, particularly lower frequency S-wave arrivals. Nevertheless, this 3D DAS array provides significant data for future analysis as well as the basis for improving and expanding the array in SURF.

Tensile properties and morphological insights into chemically modified fibres of Pseudoxytenanthera bamboo species as sustainable reinforcements in composites

This study evaluated the tensile properties of fibres from Pseudoxytenanthera ritcheyi and P. stocksii, with reference to the more commonly used Bambusa balcooa, in order to determine their viability as reinforcements in structural composite materials. Tensile tests were conducted on individual fibres under three distinct moisture conditions: ambient conditions, desiccated (fully dry) conditions, and saturated conditions for both untreated and chemically treated fibres. The investigation focused on comparing the axial tensile modulus and ultimate strength of bamboo fibres with varying gauge lengths. Notably, fracture morphology analysis using scanning electron microscopy (SEM) allowed a comprehensive correlation between natural fibre characteristics and their mechanical properties. A water absorption study was also conducted on these fibres. Additionally, thermogravimetric analysis was performed to assess the thermal stability of the fibres. The results indicate that Pseudoxytenanthera species exhibit promising potential for the development of high-performance, sustainable materials within the realm of composite materials. This research contributes to expanding the understanding of bamboo fibres and their suitability as reinforcements in structural composites, thus promoting advances in the field of composite material engineering.

Mechanical Properties and Fracture Resistance of 3D-Printed Polylactic Acid

Abstract 3D printing is a layer-by-layer deposition process, which results in highly anisotropic structures and contains interfaces. Complex shapes manufactured by 3D printing carry defects. Complete elimination of these defects and interfaces is not possible, and these defects degrade the mechanical properties. In the present study, mechanical properties of printed dog bone samples are quantified as a function of building parameters, in particular, filling patterns, raster angle, and orientation of build direction with respect to that of loading, in polylactic acid (PLA). The tensile strength of 3D-printed PLA is the same for hexagonal and linear pattern filling when the build direction is along thickness and width, and failure was initiated at the defects in the structure, while better overall toughness is offered by hexagonal pattern filling. Build direction along specimen gauge length gives very low tensile strength and toughness, and failure happens between the printing layers. To minimize the defects especially near the grip section, cuboid samples were first deposited and micro-machined by laser into dog bone shape to perform tension test. Tensile strength and elastic modulus of micro-machined samples are surprisingly lower, while failure strain is highest among line filling printed samples. Damage resistance was quantified in terms of work of fracture, and hexagonal filling provided better damage resistance than line filling patterns for conditions of 0 deg raster angle with respect to the crack, whereas line filling with 45 deg and 90 deg raster angle tolerated damage better than hexagonal filling.

Construction of Formability Limit Curves for Low-Carbon Steels Used in the Manufacture of Pressure Cylinders

Context: The aim of this study is to determine the formability of SG295 (2,2 mm thick) and SG325 (2,3 mm thick) steel sheets, as well as their relationship with the sheets’ behavior in deep drawing and stretching operations. To this effect, chemical, metallographic, and mechanical analyses of the sheets were carried out. Method: The chemical analysis was carried out via optical emission spectrometry, and the metallographic structure was analyzed using the ASTM E3 standard. The intrinsic properties related to the formability of materials such as the elongation to fracture for a 50 mm gauge length, the conventional yield limit at 0,2% elongation, the ultimate strength, the strain hardening exponent, and the anisotropy coefficient at 15% elongation were determined through tensile tests according to ASTM E8M, ASTM E646, and ASTM 517. Forming limit curves were determined under ASTM E2218, for which a device was designed, built, and attached to a universal testing machine. Results: The results for the SG295 and SG325 steel sheets were as follows: tensile strength; 450 and 520 MPa; elongation at fracture: 24,9 and 17,2%; strain hardening exponent: 0,24 and 0,19; normal anisotropy: 1,64 and 1,29; planar anisotropy: 0,23 and -0,02. The FLD0 determined from the formality limit curves (FLCs) for the two steel sheets showed ε1 values of 0,281 and 0,336, respectively. Conclusions: Although the intrinsic properties (such as A50, n, and rm) of the SG295 steel sheet show values related to a greater formability, the FLCs show that SG325 steel performs slightly better due to its greater thickness.

Research on strain aging behavior of ultra-high strength dual-phase steel

The bake hardening value is one of the vital strength indexes of dual-phase steel, representing the strengthening ability of materials after pre-strain and baking, playing an important role in vehicle safety and lightweight design. Studying and improving the strain aging mechanism of dual-phase steel helps one to understand the material characteristics and enhances its utilization value. However, the ultra-high strength dual-phase steel is often prone to fracture outside the gauge length of a tensile specimen of the bake hardening value test. No suitable theory explains the fundamental law of dislocation pinning during the saturation stage at present. This paper used FEA, DIC, SEM, TEM, internal friction, and metallographic methods to study the strain aging behavior of dual-phase steels under different pre-strain, bake time, and bake temperature conditions. The results show that the fracture outside the gauge length is related to factors such as the uneven distribution of pre-strain and the ultra-high upper yield strength. The rolling pin shape tensile specimen testing has successfully solved this testing problem. The measured results at the saturation stage of dislocation pinning are in good agreement with the fitting results of the dislocation pinning strengthen mechanism based on the probability event quantization assumption.

Directional sources and distributed acoustic sensing deployments for near-surface characterization

Active near-surface surveys using distributed acoustic sensing (DAS) have been practically limited to 2D inline acquisitions followed by Rayleigh-wave dispersion analysis. In this study, we analyze different acquisition setups and the subsurface properties that can be estimated from them. First, we find a surface survey dedicated to Love-wave acquisition. We use a horizontal source and common-receiver sorting of the DAS data for a pure Love-wave velocity dispersion analysis yielding an S-wave velocity profile. Common-receiver sorting has the added benefit of practically eliminating the effect of the gauge length. Next, downhole DAS is used in a vertical seismic profile-type survey to recover a 1D P-wave velocity through checkshot analysis. Finally, we find that the horizontal portion of a fiber deployed in a deviated borehole records high-frequency data from surface sources. We use first arrivals for diving-wave tomography, yielding a 2D P-wave velocity model above and below the fiber. We validate the inversion results of all acquisitions through a comparison with geophone-derived velocity models. We conclude that source orientation plays a crucial role and should be considered during survey planning in conjunction with DAS directivity. As anticipated, fiber coupling can strongly influence the signal-to-noise ratio and needs to be adequately planned. In vertical boreholes, infilling offers a viable solution for deployment in existing wells. For deviated boreholes, installation inside the casing yields suboptimal coupling, but more adequate deployment protocols may yield high-frequency data useful for velocity model building and imaging. From a practical point of view, the acquisitions that we describe are difficult to justify due to their operational costs and limitations. Nonetheless, they highlight the potential of DAS in going beyond traditional analysis.

Influence of PMMA 3D Printing Geometries on the Mechanical Response

This work presents a study regarding the mechanical characterization of polymethyl methacrylate (PMMA) patterned samples manufactured via material-extruded additive manufacturing. In recent years, literature about mechanical analysis in additive manufacturing has been growing increasingly, especially for material extrusion-based techniques. However, this trend surpasses the speed of information released by standard councils, existing no clear specifications for polymer characterization apart from conventional techniques. This issue has led to premature breakage as well as fracture not located in the constant cross-section region of samples. The main purpose of this present research is focused on the analysis of diverse modifications of the standard injection geometries to tackle the mentioned problems. Several printing methodologies were compared, changing slicing and geometrical parameters such as number of walls, and fillet radius. Then, the manufacturing of PMMA samples with a material extrusion printer took place to characterize both the material and the effective properties of the structures. With the information post-processed from tensile and compression tests, disparities were found between different geometrical designs for both elastic modulus and ultimate stress. Moreover, diverse location of fractures were observed for the studied geometries. The data obtained from the analysis was valuable to establish a proper protocol for further studies. The experiments suggest that for tensile tests the golden standard is selecting rectangular specimens since they do not induce premature breakage nor fracture outside of gauge length.

Assessment of long-range distributed acoustic sensing (DAS) capabilities in controlled environments

Distributed Acoustic Sensing (DAS) is an emerging technology enabling the recording of disturbances along fiber-optic (FO) cables with a wide range of applications, such as monitoring marine fauna, anthropogenic activities, and ocean climate. Despite the extensive research on DAS technology, its long-range capabilities have not been structurally studied yet. We present results of a sequence of purposefully designed experiments of DAS capabilities in (1) a laboratory, (2) a pool, and (3) a marine facility. To simulate long cables, we use combinations of FO coils 10–200 km in length. In the laboratory, realistic strain signals are simulated by an FO stretcher. In the pool, realistic acoustic signals are transmitted by an underwater speaker. At the marine facility, a combination of playback and real sound sources is used. Simultaneously deployed hydrophones allow DAS calibration and performance assessment. In this series of experiments, we measure internal noise, compare different DAS equipment, and successively optimize acquisition parameters for each interrogator performing at different, long sensing ranges in increasingly realistic environments. We show that long-range capabilities are strongly dependent on DAS settings (optical power, pulse repetition frequency, gauge length, pulse width) and vary between different interrogators.

Effect of Friction Stir Welding on Short-Term Creep Response of Pure Titanium

Friction Stir Welding (FSW) is a recent joining technique that has received considerable attention. FSW causes significant variations in the material microstructure commonly associated with changes in the mechanical properties. The present study deals with the creep response of pure titanium (CP-Ti grade 2) after FSW. Dog-bone creep samples, obtained by machining, which show the longitudinal axis of each sample being perpendicular to the welding direction, were tested in constant load machines at 550 and 600 °C. The creep response of the FSW samples was analyzed and compared with that of the unwelded material. The shape of the creep curves was conventional, although the FSW samples went to rupture for strains lower than the base metal. The minimum creep rates for FSW samples were, in general, lower than for the unwelded metal tested in equivalent conditions. In addition, when the applied stress was high, deformation concentrated in the parent metal. The creep strain became more and more homogeneous along the gauge length as testing stress decreased. A constitutive model, recently developed for describing the creep response of the base metal, was then used to rationalize the observed reduction in the minimum strain rate in FSW samples.