Traditional Strain Gauges Research Articles

Optical strain gauge vs. traditional strain gauges for concrete elasticity modulus determination

In the field of Civil Engineering, the evaluation of concrete elasticity modulus relies heavily on traditional strain gauges like electric resistance strain gauge, demec mechanical strain and compressometer. These strain gauges work pretty well but the sample surface preparation and the strain gauge adhesion are tedious and time-consuming. On the other hand, convenient non-destructive testing methods like rebound hammer and ultrasonic pulse velocity have the disadvantage of large data scattering and low reliability. In this paper, we aim to develop a handy, reliable and cost-effective method for concrete property evaluation based on an optical technique called automatic digital image correlation (ADIC), which acts as an optical strain gauge. By incorporating an automatic searching approach, the proposed ADIC greatly expands the convergence range of normal digital image correlation (DIC) algorithms and is able to automatically determine concrete strain in real time without human interaction. After the optical strain gauge is set up, the axial concrete strain as well as the stress–strain curve is plotted in real time when the concrete sample under testing is loaded. The concrete elasticity modulus is then determined from the curve. A series of experiments have been conducted to evaluate the performance of the proposed optical strain gauge. Comparisons are also carried out between the proposed optical strain gauge and traditional strain gauges such as electronic strain gauge, compressometer and demec mechanical strain gauge, which further confirm the accuracy, convenience and reliability of the proposed technique.

Comparing polymer optical fiber, fiber Bragg grating, and traditional strain gauge for aircraft structural health monitoring

Systems for structural health monitoring in aeronautical structures use methods of measuring the elongation that normally require too heavy setups or difficult assembly jobs, such as those based on traditional strain gauges. Alternative methods based on fiber Bragg gratings tend to be very expensive. We analyze the possibility of improving the existing designs with the aid of low-cost plastic optical fiber sensors. For this purpose we test these sensors in a rudder flap subjected to different types of bending forces. The results show that they offer good stability and repeatability, and the measured values are very similar to those obtained with Bragg sensors.

Shear Strengthening of Concrete Structures with the Use of Mineral-Based Composites

Rehabilitation and strengthening of concrete structures have become more common during the last 10–15years, partly due to a large stock of old structures and partly due to concrete deterioration. Also factors such as lack of understanding and the consequences of chloride attack affect the need for rehabilitation. In addition, more traffic and heavier loads lead to the need for upgrading. Existing externally bonded strengthening systems using fiber-reinforced polymers (FRP) and epoxy as bonding agents have been proven to be a good approach to repair and strengthen concrete structures. However, the use of epoxy bonding agents has some disadvantages in the form of incompatibilities with the base concrete. It is, therefore, of interest to substitute epoxy with systems that have better compatibility properties with the base concrete, for example, cementitious bonding agents. This paper presents a study on reinforced concrete beams strengthened in shear with the use of cementitious bonding agents and carbon fiber grids, denoted as mineral-based composites (MBC). In this study it is shown that the MBC system has a strengthening effect corresponding to that of strengthening systems using epoxy bonding agents and carbon fiber sheets. Different designs and material properties of the MBC system have been tested. An extensive monitoring setup has been carried out using traditional strain gauges and photometric strain measurements to obtain strains in steel reinforcement, in FRP, and strain fields on the strengthened surface. It has been shown that the use of MBC reduces strains in the steel stirrups and surface cracks even for low load steps as compared to a nonstrengthened concrete beam.

Shape Control of a FRP Airfoil Structure Using SMA-Actuators and Optical Fiber Sensors

The aim of this work was to design a control system for the shape memory alloy (SMA) actuator operated airfoil (a cross section of wind turbine blade). Design of SMA control is focused on the reliable operation of the SMA actuators. The actuator should follow the targeted shape accurately and without too much delay. Another objective is to avoid overheating which is the most critical damage to the structure. SMA actuator shape control is in principle possible to do with any position control method, but the specific properties of the SMA actuators, like the hysteresis, the first cycle effect and the long term changes, need to be taken into account. In this work, a wing profile prototype was measured using optical fiber sensors and traditional strain gauges. Also, external laser sensors were used to measure displacements of upper/lower surface and trailing edge. Shape change was obtained by embedding SMA wire actuators into fiber reinforced polymer (FRP) composite structure. SMA actuators were laminated in such way that bending of trailing edge is always downwards. Actuators are activated with Joule heating and the temperature is measured with integrated thermocouples and optical fiber temperature sensors. As a result, this work gave information about the usability of optical fibers sensors in active FRP composite structures. Measurements also give information about the efficiency of SMA actuators in shape control of relatively stiff FRP structures.

Experimental comparison between a portable DSPI device with diffractive optical element and a hole drilling strain gage combined system

This paper presents a portable device to measure residual stress fields outside the optical bench. This system combines the hole drilling technique with digital speckle pattern interferometry. The novel feature of this device is the application of a binary diffractive optical element (DOE) in order to generate double illumination and radial in-plane sensitivity. The use of the DOE ensures constant sensitivity since it only depends on the grating period, having lack of dependence on the wavelength of the illumination source. Descriptions of the DOE and the portable device are followed by the presentation of experimental results showing that the combined system can measure residual stress fields with an uncertainty of 7%. On the other hand, these fields were measured with the traditional strain gage hole drilling technique. A set of results obtained for a residual stress level of 45% of the yield stress of the material, showed a good agreement between both evaluated systems.

Tunneling effect in a polymer/carbon nanotube nanocomposite strain sensor

A strain sensor has been fabricated from a polymer nanocomposite with multiwalled carbon nanotube (MWNT) fillers. The piezoresistivity of this nanocomposite strain sensor has been investigated based on an improved three-dimensional (3D) statistical resistor network model incorporating the tunneling effect between the neighboring carbon nanotubes (CNTs), and a fiber reorientation model. The numerical results agree very well with the experimental measurements. As compared with traditional strain gauges, much higher sensitivity can be obtained in the nanocomposite sensors when the volume fraction of CNT is close to the percolation threshold. For a small CNT volume fraction, weak nonlinear piezoresistivity is observed both experimentally and from numerical simulation. The tunneling effect is considered to be the principal mechanism of the sensor under small strains.

Residual stress and thermal expansion of graphite epoxy laminates subjected to cryogenic temperatures

In composite structures understanding and quantifying the existence of residual stresses is a very important issue. The presence of these residual stresses can lead to unexpected failure of structures as seen in the fuel tank of the X-33 reusable launch vehicle. A study into the development of residual stresses as a function of temperature was performed to help provide insight into this situation. A recently developed optical technique, the cure reference method, and a modification of traditional strain gage techniques were used in this pursuit. Diffraction gratings and strain gages were applied to IM7/977-2 composite panels of differing stacking sequences. Strains were measured under no load while temperature was varied. Consequently stresses developed between the plies and a method to calculate them was proposed. Residual stress levels were dependent on stacking sequence and quasi-isotropic laminates perform poorly when exposed to cryogenic temperatures. Laminates should be optimized to account for both mechanical and thermal loading.

Design and testing of the Minotaur advanced grid-stiffened fairing

A composite grid-stiffened structure concept was selected for the payload fairing of the Minotaur launch vehicle. Compared to sandwich structures, this concept has an advantage of smaller manufacturing costs and lighter weight. To reduce weight the skin pockets are allowed to buckle visibly up to about 0.5 cm peak displacement. Various failure modes were examined for the composite grid-stiffened structure. The controlling criterion for this design was a joint failure in tension between the ribs and skin of the structure. The identification of this failure mechanism and the assessment of bounding strains required to control it required extensive test and analysis effort. Increasing skin thickness to control skin buckling resulted in reduced strains between the skin and ribs. Following the identification of the relevant failure criteria, a final design for the fairing was generated. The resulting 6 m tall fairing was constructed of a tow-placed carbon fiber composite grid structure that was over-wrapped to create a laminated skin. Upon completion of curing and machining, the fairing was cut in half to create the classic “clam-shell” fairing. Static qualification testing demonstrated the structural integrity of the fairing, thereby proving the design and manufacturing process. Loads were applied incrementally in a static loading scenario. The applied load envelope exceeded worst-case dynamic flight conditions with an added safety factor of 25%. At peak load the fairing maintained structural integrity while remaining within the required displacement envelope for payload safety. Data were collected during the test from a variety of sensors including traditional displacement transducers and strain gages. In addition, full field displacement was monitored at critically loaded fairing sections by means of digital photogrammetry. This paper summarizes the test results, presents the overall performance of the fairing under the test loads, correlates test response and analysis, and identifies lessons learned. Work continues at the Air Force Research Laboratory (AFRL) and Boeing to identify means of further controlling tensile failure of the un-reinforced polymer bonded joint between the ribs and skin. Stiffening of skin adjacent to the joints and introduction of lightweight foam jackets at the interior of the fairing both show promise of delaying joint failure to higher loads.

Design of a three-component capacitive force transducer

A three-component capacitive force/moment transducer was designed and developed. The moments Mx, My, and the normal force Fz are sensed. The capacitor electrode conformation and disposition was designed to allow a greatly simplified circuit topology and limit the influence of stray capacitances. The capacitive displacement sensor exhibits an intrinsic sensitivity of about 20 (V/V)/rad for the tangential components and 6(V/V)/mm for the normal one. With proper amplifying circuitry a minimum detectable signal of 0.3×10−4 rad/Hz−1/2 and 0.1 μm/Hz−1/2 was achieved. Different elastic elements were tested in order to vary the nominal load of the force transducer and to achieve different sensitivities. These values are roughly 102 times the values of traditional strain gages units. The proposed device is mainly intended for the mechanical characterization of nonstandard specimens, and is presented for the purpose of performing microtraction tests on biological samples while monitoring off axis components.

Optical strain sensor using position-sensitive detector and diffraction grating: error analysis

A new optical strain sensor is developed to rival the traditional electrical strain gauge. It directly measures in-plane strains using a high frequency grating and two position-sensitive detectors (PSDs). The strain measurement is independent of the rigid body motion of the grating. A high frequency diffraction grating attached to the surface of a specimen is illuminated by a focused or collimated laser beam. The centroids of the two symmetric diffracted beam spots from the grating are determined by the PSD sensors connected to a personal computer. The shift of diffracted beam spots due to the specimen deformation is transformed to strain components. Several measures are taken to improve strain sensitivity including residual strain error due to the misalignment of laser and grating. The measured strain is directly displayed on the monitor in both digital and graphical forms in real time. Strain sensitivity of 1 x strain and a spatial resolution of 0.1 mm can be achieved.

Strain Gauge Measurements of Interfacial Cracks in Bimaterial

An improved method for interpreting strain gauge data is proposed to reduce the effects of the measurement errors. The results obtained by this modified method are compared with both a traditional strain gauge solution using two data points and a theoretical solution derived from complex potentials. The comparison shows that the modified solution is in better agreement with the theoretical solution as compared with the traditional solution.

Extended dynamic range detection system for in-fibre Bragg grating strain sensors based on two cascaded interferometric wavelength scanners

A novel interferometric wavelength-shift detection system based on the use of two cascaded two-beam interferometers with different optical path differences (OPDs) is developed for static and quasi-static strain measurement with in-fibre Bragg grating (FBG) sensors. These two interferometers are frequency-division-multiplexed and used to provide two sets of interferometric fringes induced by strain. The optical phase output from interferometer 1 with the larger OPD gives a high-resolution measurement whilst the phase output from interferometer 2 with the shorter OPD is used to determine the number of the fringes obtained with interferometer 1. Compared with the detection scheme using a single interferometric scanner, the unambiguous measurement range has been considerably improved. An experimental system with enhancement factors between 11 and 40 in the unambiguous measurement range has been demonstrated for absolute static strain measurement. A large range to resolution ratio of has been achieved. Due to the adjustability of the OPDs in these interferometers, the range to resolution required is also variable, making this system suitable for different applications. On the other hand, a range to resolution of makes FBG strain sensors potentially competitive with traditional strain gauges with the well known benefits of fibre-optic sensors. Therefore, it can be anticipated that after further engineering such a system will find important applications in the field of fibre-optic smart structures.

Enhancement of magnetorestrictive properties of amorphous ribbons for a biomedical application

A novel method of monitoring bone fracture healing has been developed. The sensor is based on the stress dependence of permeability of the highly magnetostrictive amorphous ribbons. The magnetoelastic sensor is bonded to a stainless steel plate. The detection is provided by an outside coil surrounding the limb. Stresses introduced by the adhesives degrade the sensor's performance. These are overcome by prestressing and annealing the material bonded with a high temperature adhesive. Using these techniques, it is possible to design a tensile stress or compressive stress sensor to be used in strain measurement of the plates. Such a sensor has considerably higher sensitivity, lower power consumption and is less invasive than a traditional strain gauge. The characteristics of these enhanced sensors are studied and optimum conditions on sensors' performance are reported.

Fiber optic sensors in concrete structures: a review

Fiber optic sensors (FOSs) are ideally suited for monitoring strain in concrete structures due to their small size, low cost, ability to be embedded internally, and multiplexing capabilities. Several types of fiber optic strain sensor have been developed, including those based on intensity, polarization, interferometry, and fiber Bragg gratings (FBGs). The FBGs have the advantage of being absolute, linear in response, interrupt immune, and most readily multiplexed. The ability of embedded and surface mounted FOSs to detect strain in reinforced concrete has been demonstrated in several laboratory tests. Comparisons between various FOSs and traditional foil strain gages show good agreement. Sensors have been surface mounted on existing bridges to evaluate observed cracking and to monitor vibration due to traffic loading. FOSs have also been embedded in several newly constructed civil structures, including bridges, buildings, and dams, yielding information about static and dynamic strain, temperature, wind or water pressure, and structural health. Practical and logistical challenges to routine implementation of FOSs in civil structures are being addressed with each additional application.

In-plane shear testing of graphite-woven fabric composites

The losipescu shear test method was used to determine the in-plane shear response of AS4 and Celion carbon fiber/epoxy fabric composite materials. Several weave architectures were studied: AS4 uniweave, AS4 and Celion plain weaves, Celion 5-harness and 8-harness satin weaves. Specimens were tested using traditional strain gage techniques and full-field moire interferometry. A full-node localized hybrid analysis is introduced to perform efficient reduction of moire data, producing whole-field strain distributions in the specimen test section. It was found that the fabric yarn size greatly influenced the uniformity of the shear field in the specimen test section. However, consistent shear moduli still can be obtained using the modified losipescu specimen and Wyoming fixture except for fabrics with large fiber yarns.

Measurements of Two-Dimensional Strain Distribution Using the Electronic Speckle Pattern Interferometry.

Electronic speckle pattern interferometry (ESPI) is a very useful method for measuring in-plane surface displacement. Using the cw-laser and image-processing system, it is possible to measure displacement and strain. Unlike traditional strain gauges or the moire method, the ESPI method requires no special surface preparation or attachments and can measure in-plane displacement with no contact and in real time. In this experiment, a specimen was loaded in parallel with a load cell, which provided loading step. The specimen was a flat plate or a plate with a 12 mm-diameter hole in its center. Strain gauges were attached to the specimen in x and y directions. This study provides an example of how ESPI has been used to measure two-dimensional displacement and strain distribution in this specimen. The results measured by ESPI are comparable with the data which were obtained by the strain-gauge method in tensile testing in the 1-ton range

A precision six-load-component transducer: A design incorporating finite-length measurement paths

The design of an instrument is described that measures three resultant force components and three resultant moment components acting on a surface. Within the framework of linear elastostatics of an isotropic homogeneous material the device separates to a given precision the six resultant load components. Sensor paths of finite length are employed. Moreover if fiber-optic differential displacement sensors are used rather than traditional electrical resistance strain gages, the range and sensitivity of the instrument can in principle be improved without sacrificing the device stiffness. The primary reason for these improvements is that a complete solution to the equations of elasticity allows certain displacements to be measured over large distances and be combined to yield all of the resultant load components. These displacement measurements over a long distance accommodates the use of fiber-optic interferometric sensors. The use of optical sensors in contrast with electrical-resistance gages, has the potential to allow the measurement precision and range to scale with the geometry of the device rather than the maximum strain in the instrument. It becomes possible by virtue of these features to produce a better instrument.

ダブルアパーチャを用いた歯車のかみあい振動の測定

This paper describes the non-contacting measuring method of the circumferential vibration of gears by using the double-aperture method, in which a coherent laser beam is directed perpendicular to the side face of the tested gear, and the scattered reflected light is passed through two small apertures and the resultant interference stripes are directly detected with a photo-multiplier to measure the vibration in real time. It was found that the circumferential vibration due to the engagement of gear teeth can be measured by this method with the same accuracy as that by the traditional strain gage method.