Distributed Strain Measurement Research Articles

Comparison between different Fiber Optical Strain Measurement Systems Based on the Example of Reinforcing Bars

Fiber optical measurement systems come to the fore after having been intensively investigated during the last two decades. One example is Fiber-Bragg-Gratings (FBG). This technology allows a sufficient strain measurement at predefined points. Another new technique deals with Rayleigh backscatter to realize a distributed strain measurement over the total length of an optical fiber. Such measurement is recognized to provide a high potential in civil engineering, especially in reinforced concrete.This paper deals with some preliminary comparisons between Rayleigh backscatter and FBG based on tension test with reinforcing bars. Therefor optical fibers were placed in notches, milled into the reinforcing bars. Analysis of the test data showed a strong correlation between the optical measurement systems and theoretical results up to the yield point of the steel. Furthermore, the advantages of distributed strain measurement were uncovered. In a last step, the paper offers a prospect concerning future applications for the Rayleigh backscatter system.

Surface Crack Detection in Prestressed Concrete Cylinder Pipes Using BOTDA Strain Sensors

Structural deterioration after a period of service can induce the failure of prestressed concrete cylinder pipes (PCCPs), with microcracks in the coating leading to the corrosion of the prestressed wires. In this paper, we propose the use of Brillouin optical time-domain analysis (BOTDA) strain sensors for detecting the onset of microcracking in PCCP coating: the BOTDA strain sensors are mounted on the surface of the PCCP, and distributed strain measurements are employed to assess the cracks in the mortar coating and the structural state of the pipe. To validate the feasibility of the proposed approach, experimental investigations were conducted on a prototype PCCP segment, wherein the inner pressure was gradually increased to 1.6 MPa. Two types of BOTDA strain sensors—the steel wire packaged fiber optic sensor and the polyelastic packaged fiber optic sensor—were employed in the experiments. The experimental distributed measurements agreed well with the finite element computations, evidencing that the investigated strain sensors are sensitive to localized deterioration behaviors such as PCCP microcracking.

Evaluation of Stiffness Degradation on CFRP Laminate by Measuring Strain Distribution Using Speckle Interferometry

Evaluation of Stiffness Degradation on CFRP Laminate by Measuring Strain Distribution Using Speckle Interferometry

経カテーテル大動脈弁留置術における大動脈弁モデルに生じるひずみ分布の計測に関する研究

経カテーテル大動脈弁留置術における大動脈弁モデルに生じるひずみ分布の計測に関する研究

Metallographic Analysis of Nakajima Tests for the Evaluation of the Failure Developments

The material characterisation in sheet metal forming is significantly improved with the introduction of optical measurement systems. The optical measurement is based on the digital image correlation (DIC) technique, which provides accurate strain measurement of the local strain distribution on specimens. The changes of the stochastic distribution due to the forming are therefore used in order to measure displacements and deformations. An acknowledged application of the DIC technique is the evaluation of the forming limit curve (FLC) for sheet metals. Typically, the FLC is evaluated by conducting Nakajima tests. The Nakajima test is a stretch forming-based test and allows the analysis of stretching at different stress conditions. A previous investigation based on pattern recognition on Nakajima tests has shown that for a conventional deep drawing steel DC04 the pattern evolution on the strain distribution during the test can be examined in order to predict the material failure. However, even if the investigation shows promising results, the correspondence between pattern on the surface and changes in the material structure is missing. The aim of the present work is an analysis of the surface structure of Nakajima tests at different stress states and drawing depths. The correlation between surface modifications and failure behaviour is conducted on the deep drawing steel DC04. The scanning electron microscope (SEM) (Merlin Zeiss GmbH) is employed in order to investigate the evolution of the surface at different strain conditions. The results of the surface and the metallographic investigation are finally compared with the forming limit prediction of the FLC and the failure behaviour at different stress conditions is discussed.

Measurement of strain and temperature distributions on an aluminum plate by using fiber-optic sensors

Measurement of strain and temperature distributions on an aluminum plate by using fiber-optic sensors

Determination of the formability limits for Grade 1 titanium sheet by means of ALSAD method

Stamping is one of the most widely used sheet metal forming processes. Esthetics and durability of the deformed products are limited by defects in the form of strain localization, grooves or cracks. Forming limits, e. g. in form of forming limit curve (FLC), help to avoid above-mentioned defects in the sheet metal during forming. Besides the common steel sheets for deep drawing processes, e.g. DC04, components can also be made of metal with specified strength and chemical resistance properties. One of such materials is titanium sheet. In this paper, FLC for GR1 titanium sheet will be presented. It was obtained by means of a new author’s method named as ALSAD (Analysis of Laser Speckle Activity Differences). So far, applied methods of FLC determination have been mainly based on the measurement of the strain distribution in the area of the created groove, deformation gradient and deformation speed. For this reason, detection of strain localization has been dependent on the strain measurement method. The ALSAD, based on laser speckle phenomenon allows detection of defects initiation on the sample surface independently of the strain distribution measurement and it allows, for example, to relate a moment of defect initiation directly to the sample height or forces occurring in the process. In this paper strain values for strain localization, grooves and cracks for GR1 titanium sheet will be shown. Obtained results show good formability of this material, which was verified by means of 3D digital microscope. Such results allow tracing precisely the formation of the defects starting from strain localization up to the crack appearance, which would be helpful for advanced numerical simulations.

Novel Non-Contact Evaluation of Strain Distribution Using Digital Image Correlation with Laser Speckle Pattern of Low Carbon Steel Sheet

An uni-axial tensile test is one of the general methods to obtain plastic properties for estimating deformation behavior of materials in the simulation of practical metal forming process. Non-contact measurement is generally used because of determination of accurate plastic properties excepting for contact effects. In the conventional method, strain distribution of specimen sprayed with random pattern is measured by digital image correlation (DIC). However, sprayed treatments on the specimen surface is unsuitable under various testing conditions, for example an elevated temperature, micro scale, and large deformation. Thus, non-sprayed specimen is desirable under such test conditions. In this study, we propose perfectly non-contact strain distribution measurement by using DIC with laser speckle pattern generated on specimen surface by irradiation of laser source. True strain in the un-axial tensile test is measured by following methods (1) laser speckle pattern by DIC, and (2) conventional measurement method with tracking gage marks. As a result, strain level measured by DIC with laser speckle pattern has good agreement with that of conventional extensometer. Furthermore, the strain distribution can be measured successfully under uniform and non-uniform deformation during necking by proposed method. Thus, we can verify the validity of proposed measurement method for determination of plastic properties.

Stretchable strain sensor for distributed strain measurement and design of measurement circuit

Stretchable strain sensor for distributed strain measurement and design of measurement circuit

Dynamic measurement of surface strain distribution on the foot during walking

To clarify the mechanism underlying the development of foot disorders such as diabetic ulcers and deformities, it is important to understand how the foot surface elongates and contracts during gait. Such information is also helpful for improving the prevention and treatment of foot disorders. We therefore measured temporal changes in the strain distribution on the foot surface during human walking. Five adult male participants walked across a glass platform placed over an angled mirror set in a wooden walkway at a self-selected speed and the dorsolateral and plantar surfaces of the foot were filmed using two pairs of synchronized high-speed cameras. Three-dimensional (3D) digital image correlation was used to quantify the spatial strain distribution on the foot surface with respect to that during quiet standing. Using the proposed method, we observed the 3D patterns of foot surface strain distribution during walking. Large strain was generated around the ball on the plantar surface of the foot throughout the entire stance phase, due to the windlass mechanism. The dorsal surface around the cuboid was stretched in the late stance phase, possibly due to lateral protruding movement of the cuboid. It may be possible to use this technique to non-invasively estimate movements of the foot bones under the skin using the surface strain distribution. The proposed technique may be an effective tool with which to analyze foot deformation in the fields of diabetology, clinical orthopedics, and ergonomics.

Full scale strain monitoring of a suspension bridge using high performance distributed fiber optic sensors

This paper investigated field monitoring of a 1108 m suspension bridge during an assessment load test, using integrated distributed fibre-optic sensors (DFOSs). In addition to the conventional Brillouin time domain analysis system, a high spatial resolution Brillouin system using the differential pulse-width pair (DPP) technique was adopted. Temperature compensation was achieved using a Raman distributed temperature sensing system. This is the first full scale field application of DFOSs using the Brillouin time domain analysis technique in a thousand-meter-scale suspension bridge. Measured strain distributions along the whole length of the bridge were presented. The interaction between the main cables and the steel-box-girder was highlighted. The Brillouin fibre-optic monitoring systems exhibited great facility for the purposes of long distance distributed strain monitoring, with up to 0.05 m spatial resolution, and 0.01 m/point sampling interval. The performance of the Brillouin system using DPP technique was discussed. The measured data was also employed for assessing bridge design and for the assessment of structural condition. The results show that the symmetrical design assumptions were consistent with the actual bridge, and that the strain values along the whole bridge were within the safety range. This trial field study serves as an example, demonstrating the feasibility of highly dense strain and temperature measurement for large scale civil infrastructures using integrated DFOSs.

Evaluating the early-age behaviour of full-scale prestressed concrete beams using distributed and discrete fibre optic sensors

This paper evaluates the results of a monitoring study that captures the early age behaviour of four 11.9m prestressed concrete bridge beams utilising both distributed and discrete fibre optic sensor (FOS) arrays. The performance of the beams is evaluated before they are placed in-service as part of new concrete railway bridges in the Midlands in the UK. Two types of prestressed beams were monitored, two TY7 internal beams and two TYE7 edge beams. The beams incorporated high strength (up to 90.7MPa) self-consolidating concrete. The entire manufacturing process which included early-age curing and the detensioning process was captured in great detail using the installed FOS system. An analysis of the curing strains within the beams revealed the significant effect that ambient temperature, curing duration, and formwork restraint has on the development of prestress losses prior to detensioning. Based on the distributed FOS readings, it was observed that the strain remained uniform along the length of the beams during the various beam monitoring stages. The measured strain data was then used to calculate prestress losses in the first six months after casting (prior to casting of the in-situ concrete bridge deck). The TY7 and TYE7 beams experienced losses that were 79% and 72% of the ultimate losses predicted using Eurocode 2 equations, respectively. Distributed strain measurements were used to provide estimates of the change in beam camber with time. The pre-camber values calculated using the recorded FOS strain data at the time of detensioning closely match the theoretically calculated values. However, camber values increased by up to 1.7 times in the first six months compared with the post-detensioning values and deviated significantly from the theoretically calculated values. The future aim of this research is to establish integrated FOS systems as viable tools for monitoring strain evolution in concrete bridges in order to establish comprehensive baselines to facilitate long term data-driven bridge monitoring programmes.

Dynamic Optical Fiber Sensing With Brillouin Optical Time Domain Reflectometry: Application to Pipeline Vibration Monitoring

Brillouin distributed optical fiber strain sensors are often limited to static events because of necessary long acquisition time due to the frequency sweep technique. Dynamic distributed strain monitoring is necessary to ensure the integrity of linear structures submitted to fatigue loading or vibrations, such as subsea risers or flowlines, overhead pipelines, long bridges, railways, or high-rise towers. In this paper, a novel dynamic Brillouin optical time domain reflectometer for single ended truly distributed strain measurement based on the slope-assisted method is presented. The optical local oscillator frequency is adjusted to measure Brillouin backscattered power at the maximum slope of the Brillouin gain spectrum along the fiber length. Any strain variation inducing Brillouin frequency shift will be detected by amplitude variation. To demonstrate the performance of our acquisition system, three optical fibers have been implemented along a 10 m steel pipe submitted to vibration. Through this mockup testing, we were able to measure the oscillation damping time and the geometrical displacement of the pipe in real time. A 7.6 Hz acquisition rate was achieved with a strain error of ± 40 μ strain and with a spatial resolution of 1 m. From integration of longitudinal strains, pipe displacement is calculated with an error of ± 12 mm. A performance comparison between frequency sweep and slope assisted method is made with a 2 km fiber sensor.

Relating Histopathology and Mechanical Strain in Experimental Contusion Spinal Cord Injury in a Rat Model

During traumatic spinal cord injury (SCI), the spinal cord is subject to external displacements that result in damage of neural tissues. These displacements produce complex internal deformations, or strains, of the spinal cord parenchyma. The aim of this study is to determine a relationship between these internal strains during SCI and primary damage to spinal cord gray matter (GM) in an in vivo rat contusion model. Using magnetic resonance imaging and novel image registration methods, we measured three-dimensional (3D) mechanical strain in in vivo rat cervical spinal cord (n = 12) during an imposed contusion injury. We then assessed expression of the neuronal transcription factor, neuronal nuclei (NeuN), in ventral horns of GM (at the epicenter of injury as well as at intervals cranially and caudally), immediately post-injury. We found that minimum principal strain was most strongly correlated with loss of NeuN stain across all animals (R2 = 0.19), but varied in strength between individual animals (R2 = 0.06–0.52). Craniocaudal distribution of anatomical damage was similar to measured strain distribution. A Monte Carlo simulation was used to assess strain field error, and minimum principal strain (which ranged from 8% to 36% in GM ventral horns) exhibited a standard deviation of 2.6% attributed to the simulated error. This study is the first to measure 3D deformation of the spinal cord and relate it to patterns of ensuing tissue damage in an in vivo model. It provides a platform on which to build future studies addressing the tolerance of spinal cord tissue to mechanical deformation.

Elastic Full‐Field Strain Analysis and Microdamage Progression in the Vertebral Body from Digital Volume Correlation

AbstractThe strain distribution in vertebral body has been measured in vitro in the elastic regime but only on the bone surface by means of strain gauges and digital image correlation. Digital volume correlation (DVC) based on micro‐computed tomography (micro‐CT) images allowed measurements of the internal strain distribution in bone at both tissue (trabecular and cortical bone) and organ (vertebra) levels. However, DVC has been mainly used to investigate failure of the vertebral body but has not yet been deployed to investigate the internal strain distribution in the elastic regime. The aim of this study was to investigate strain in the elastic regime and up to failure inside the vertebral body, including analysis of strain in all directions. Three porcine thoracic vertebrae were loaded in a step‐wise fashion at increasing steps of compression (5, 10 and 15%). Micro‐CT images were acquired at each step of compression. DVC successfully provided the internal strain distribution both in the elastic regime and up to failure. Micro‐CT images successfully identified regions of failure initiation and progression, which were well quantified by DVC‐computed strains. Interestingly, the same regions where failure eventually occurred experienced the largest strain magnitude also for the lowest degrees of compression (yet in the elastic regime).

Assessment of anisotropic hardening models for conventional deep drawing processes

Assessing the predictive capabilities of recent advanced constitutive modelling approaches for processes with industrial complexity is a challenging task. Real process conditions such as blankholder pressure distribution, friction and tool elasticity sensitively affect experimental observations, making the isolation of constitutive effects difficult. A systematic approach is proposed in this work to assess the performance of anisotropic hardening models with the least possible disturbance from process conditions. Two deep drawing examples were used for these purpose (“cross die” and “lackfrosch”) in conjunction with a mild steel (DC05). Optically measured strain distributions have been compared to corresponding simulations, which have been calibrated to accurately match the measured blank draw-in. The effect of initial yield locus shape as well as anisotropic hardening effects have been discussed.

Prediction of thermal strains in fibre reinforced plastic matrix by discretisation of the temperature exposure history

Prediction of environmental effects on fibre reinforced plastics habitually is made difficult due to the complex variability of the natural service environment. This paper suggests a method to predict thermal strain distribution over the material lifetime by discretisation of the exposure history. Laboratory results show a high correlation between predicted and experimentally measured strain distribution

Ultrahigh-speed distributed Brillouin reflectometry.

Optical fibre sensors based on Brillouin scattering have been vigorously studied in the context of structural health monitoring on account of their capacity for distributed strain and temperature measurements. However, real-time distributed strain measurement has been achieved only for two-end-access systems; such systems reduce the degree of freedom in embedding the sensors into structures, and furthermore render the measurement no longer feasible when extremely high loss or breakage occurs at a point along the sensing fibre. Here, we demonstrate real-time distributed measurement with an intrinsically one-end-access reflectometry configuration by using a correlation-domain technique. In this method, the Brillouin gain spectrum is obtained at high speed using a voltage-controlled oscillator, and the Brillouin frequency shift is converted into a phase delay of a synchronous sinusoidal waveform; the phase delay is subsequently converted into a voltage, which can be directly measured. When a single-point measurement is performed at an arbitrary position, a strain sampling rate of up to 100 kHz is experimentally verified by detecting locally applied dynamic strain at 1 kHz. When distributed measurements are performed at 100 points with 10 times averaging, a repetition rate of 100 Hz is verified by tracking a mechanical wave propagating along the fibre. Some drawbacks of this ultrahigh-speed configuration, including the reduced measurement accuracy, lowered spatial resolution and limited strain dynamic range, are also discussed.

Measurement of strain distribution in cortical bone around miniscrew implants used for orthodontic anchorage using digital speckle pattern interferometry

An application of digital speckle pattern interferometry (DSPI) for the measurement of deformations and strain-field distributions developed in cortical bone around orthodontic miniscrew implants inserted into the human maxilla is presented. The purpose of this study is to measure and compare the strain distribution in cortical bone/miniscrew interface of human maxilla around miniscrew implants of different diameters, different implant lengths, and implants of different commercially available companies. The technique is also used to measure tilt/rotation of canine caused due to the application of retraction springs. The proposed technique has high sensitivity and enables the observation of deformation/strain distribution. In DSPI, two specklegrams are recorded corresponding to pre- and postloading of the retraction spring. The DSPI fringe pattern is observed by subtracting these two specklegrams. Optical phase was extracted using Riesz transform and the monogenic signal from a single DSPI fringe pattern. The obtained phase is used to calculate the parameters of interest such as displacement/deformation and strain/stress. The experiment was conducted on a dry human skull fulfilling the criteria of intact dental arches and all teeth present. Eight different miniscrew implants were loaded with an insertion angulation of 45 deg in the inter-radicular region of the maxillary second premolar and molar region. The loading of miniscrew implants was done with force level (150 gf) by nickel–titanium closed-coil springs (9 mm). The obtained results from DSPI reveal that implant diameter and implant length affect the displacement and strain distribution in cortical bone layer surrounding the miniscrew implant.

Feasibility study of strain based stability evaluation of locally loaded slopes: Insights from physical and numerical modeling

Distributed strain monitoring of geotechnical structures has gained increasing attention in the past decade. The distributed fiber optic strain sensing (DFOSS) technology enables the measurement of strain distribution in soil slopes. This paper aims to investigate the feasibility of strain based slope stability evaluation for locally loaded slopes. The measurements of horizontal strains at different elevations in a two-dimensional (2D) model slope subjected to a vertical surcharge load were analyzed. Empirical relationships between different types of strain parameters and factors of safety calculated by the conventional method were established. To verify the above findings, a 2D finite element model of a homogeneous soil slope was built. By applying a gradually increasing local load on the numerical model, the strains in the soil mass and eventually slope failure were induced. The strain distributions of several virtual monitoring lines under different loading levels were captured and analyzed in detail. At the same time, the strength reduction method (SRM) was used to perform slope stability analysis. The results show that the strain distribution characteristics are closely related to the propagation of plastic zones and the formation of the critical slip surface. Taking into consideration the convenience of field instrumentation and monitoring sensitivity, the maximum strains at different elevations can be used as characteristic parameters for estimating the slope stability condition. Compared with conventional displacement based slope stability evaluation method, the proposed methodology is more efficient and sensitive, which makes full use of the benefits of the DFOSS technology.