Strain Gauge Placement Research Articles

Design and verification of a strain gauge based load sensor for medium-speed dynamic tests with a hydraulic test machine

System ringing in load measurement during dynamic tests is investigated with a second order system consisting of a mass, a spring and a damper. According to the analysis of the time-domain and harmonic performances, a procedure for designing load sensor to relieve the influence of system ringing is proposed. Subsequently, a practical design of load sensor and its application procedure are presented. The dynamic test results of two metal materials under strain-rates of 10, 100 and 200 s−1 show that the sensor is effective for reducing system ringing. Based on this design, system ringing as well as the influence of several design variables is investigated with the help of Finite Element simulations. Simulation results indicate that the loading velocity and the stress–strain relation of the tested material are the two important influential factors. In the end of the paper, recommendations are made for the design of load sensor and the position for placing strain gauges.

An Ideal strain gage placement plan for structural health monitoring under seismic loadings

An Ideal strain gage placement plan for structural health monitoring under seismic loadings

Dynamic Response and Identification of Critical Points in the Superstructure of a Vehicle Using a Combination of Numerical and Experimental Methods

In this work, a systematic methodology is applied, leading to an accurate prediction of the dynamic response of a complex vehicle, carrying a superstructure. This methodology helps in locating areas of the superstructure exhibiting a large stress concentration, for given loading conditions, without requiring any knowledge of the dynamic characteristics of the vehicle suspension subsystem. The basic idea is to first measure the acceleration time histories at the connection points of the vehicle superstructure with its suspension system and use them subsequently as a base excitation in a finite element model of the superstructure. More specifically, the vehicle superstructure is first discretized by appropriate finite elements. The resulting model is linear and is then updated through an experimental measurement of its dynamic response, yielding all the elements of the corresponding frequency response matrix of the superstructure. Then, a series of experimental trials is performed in real operating conditions, aimed at recording the acceleration time histories at the connection points of the superstructure with the chassis. These time histories are subsequently used as a ground excitation for the finite element model of the superstructure and the stresses developed under the specific loading conditions are evaluated. In this way, the critical points of the superstructure can be identified by numerical means. Finally, the reliability of the methodology applied was tested by two numerical examples as well as by placing strain gauges at the critical points of the superstructure and performing a new set of measurements for the vehicle under similar loading conditions. Direct comparison of the numerical and experimental data obtained in this manner verified the reliability and accuracy of the hybrid methodology applied.

Measurement of Local Deformations in Steel Monostrands Using Digital Image Correlation

The local deformation mechanisms in steel monostrands have a significant influence on their fatigue life and failure mode. However, the observation and quantification of deformations in monostrands experiencing axial and transverse deformations is challenging because of their complex geometry, difficulties with the placement of strain gauges in the vicinity of the anchorage, and, most importantly, the relatively small magnitude of deformation occurring in the monostrand. This paper focuses on the measurement of localized deformations in high-strength steel monostrands using the digital image correlation (DIC) technique. The presented technique enables the measurement of individual wire strains along the length of the monostrand and also provides quantitative information on the relative movement between individual wires, leading to a more in-depth understanding of the underlying fatigue mechanisms. To validate the proposed image-based measurement method, two different tests were performed, with the one correlation method showing good agreement. Data collected from the DIC technique creates a basis for the analysis of the fretting and localized bending behavior of the monostrand and provides relevant information on the internal state of displacement of the monostrand under bending load.

The Morlet and Daubechies Wavelet Transforms for Fatigue Strain Signal Analysis

This paper presents the convenient wavelet family for the fatigue strain signal analysis based on the wavelet coefficients. This study involves the Morlet and Daubechies wavelet coefficients using both the Continuous and Discrete Wavelet Transforms, respectively. The signals were collected from a front lower suspension arm of a passenger car by placing strain gauges at the highest stress locations. The car was driven over public road surfaces, i. e. pavé, highway and UKM roads. In conclusion, the Daubechies wavelet was the convenient wavelet family for the analysis. It was because the wavelet gave the higher wavelet coefficient values indicating that the resemblance between the wavelet and the signals was stronger, closer and more similar.

Experimental and finite element analysis of strains induced by axial tibial compression in young-adult and old female C57Bl/6 mice

Axial compression of the mouse tibia is used to study strain-adaptive bone (re)modeling. In some studies, comparisons between mice of different ages are of interest. We characterized the tibial deformation and force–strain relationships in female C57Bl/6 mice at 5-, 12- and 22-months age. A three-gauge experimental method was used to determine the strain distribution at the mid-diaphysis, while specimen-specific finite element analysis was used to examine strain distribution along the tibial length. The peak strains in the tibial mid-diaphyseal cross-section are compressive and occur at the postero-lateral apex. The magnitudes of these peak compressive strains are 1.5 to 2 times those on the opposite, antero-medial face (a site often used for strain gauge placement). For example, −10N force applied to a 5-months old mouse engenders a peak compressive strain of −2800µε and a tensile strain on the antero-medial face of +1450µε. The orientation of the neutral axis at the mid-diaphysis did not differ with age (p=0.46), indicating a similar deformation mode in young and old tibiae. On the other hand, from 5- to 22-months there is a 25% reduction in cortical thickness and moment of inertia (p<0.05), resulting in significantly greater tibial strain magnitudes in older mice for equivalent applied force (p<0.05). We conclude that comparisons of tibial loading responses in young-adult and old C57Bl/6 tibiae are facilitated by similar deformation pattern across ages, but that modest adjustment of force levels is required to engender matching peak strains.

Using the Own Flexibility of a Climbing Robot as a Double Force Sensor

Force sensors are used when interaction tasks are carried out by robots in general, and by climbing robots in particular. If the mechanics and electronics systems are contained inside the own robot, the robot becomes portable without external control. Commercial force sensors cannot be used due to limited space and weight. By selecting the links material with appropriate stiffness and placing strain gauges on the structure, the own robot flexibility can be used such as force sensor. Thus, forces applied on the robot tip can be measured without additional external devices. Only gauges and small internal electronic converters are necessary. This paper illustrates the proposed algorithm to achieve these measurements. Additionally, experimental results are presented.

Sensitivity and ex vivo validation of finite element models of the domestic pig cranium

A finite element (FE) validation and sensitivity study was undertaken on a modern domestic pig cranium. Bone strain data were collected ex vivo from strain gauges, and compared with results from specimen-specific FE models. An isotropic, homogeneous model was created, then input parameters were altered to investigate model sensitivity. Heterogeneous, isotropic models investigated the effects of a constant-thickness, stiffer outer layer (representing cortical bone) atop a more compliant interior (representing cancellous bone). Loading direction and placement of strain gauges were also varied, and the use of 2D membrane elements at strain gauge locations as a method of projecting 3D model strains into the plane of the gauge was investigated. The models correctly estimate the loading conditions of the experiment, yet at some locations fail to reproduce correct principal strain magnitudes, and hence strain ratios. Principal strain orientations are predicted well. The initial model was too stiff by approximately an order of magnitude. Introducing a compliant interior reported strain magnitudes more similar to the ex vivo results without notably affecting strain orientations, ratios or contour patterns, suggesting that this simple heterogeneity was the equivalent of reducing the overall stiffness of the model. Models were generally insensitive to moderate changes in loading direction or strain gauge placement, except in the squamosal portion of the zygomatic arch. The use of membrane elements made negligible differences to the reported strains. The models therefore seem most sensitive to changes in material properties, and suggest that failure to model local heterogeneity in material properties and structure of the bone may be responsible for discrepancies between the experimental and model results. This is partially attributable to a lack of resolution in the CT scans from which the model was built, and partially due to an absence of detailed material properties data for pig cranial bone. Thus, caution is advised when using FE models to estimate absolute numerical values of breaking stress and bite force unless detailed input parameters are available. However, if the objective is to compare relative differences between models, the fact that the strain environment is replicated well means that such investigations can be robust.

Design of a compact six-component force and moment sensor for aerodynamic testing

The measurement of steady and fluctuating forces acting on a body in a flow is one of the main tasks in wind-tunnel experiments. Usually, a multi-component strain gauge force and moment sensor (also known as balance) is used to generate signals which are processed by means of an adequate instrumentation. To design a wind-tunnel balance, the specifications of the load ranges and the available space (for the placement of the balance inside or outside the model) are required. The main challenge is to conceive the elastic element of the sensor as a monolithic part with a relative simple geometry and to identify the adequate placement of strain gauges to maximize the measuring sensitivities and to diminish the inter-influence of the components. This paper describes the design of a six-component force/moment sensor which is compact, has high measuring sensitivities, and can be used either as internal or as external balance in the aerodynamic testing.

Análisis de Tensiones mediante Elementos Finitos de una Válvula Esférica durante la Prueba Hidrostática, y su Verificación Experimental

The stress and deformation analysis in the body of a ball valve of diameter 8, class 600, during the hydrostatic test, using the finite elements method is done. The results are verified through experimental stress analysis method, using strain gauges. For the finite elements method the software Cosmos/M and information obtained during the experimental analysis were used. The experimental tests were carried out in a press type test machine, placing strain gauges in the body of the valve at certain points of interest. The values of deformation obtained allowed validating the application of the finite elements method.

Toward Characterization of Craniofacial Biomechanics

Surgical reconstruction of craniofacial deformities has advanced significantly in recent years. However, unlike orthopedic surgery of the appendicular skeleton, the biomechanical characterization of the human craniofacial skeleton (CFS) has yet to be elucidated. Attempts to simplify facial skeletal structure into straightforward mechanical device analogies have been insufficient in delineating craniofacial biomechanics. Advanced computational engineering analysis methods offer the potential to accurately and completely define the internal mechanical environment of the CFS. This study developed a finite element (FE) model in the I-deas 10 FEM software package of a preserved cadaveric human CFS and compared the predictions of this model against in vitro strain measurement of simulated occlusal loading forces from a single masseter muscle. The FE model applied shell element modeling to capture the behavior of the thin cortical bone that may play an important role in stabilizing the facial structures against functional loads. In vitro testing included strain measurements at 12 locations for a total of 16 independent channels with less than 150 N of tensile force applied through the masseter muscle into the zygomatic arch origin at 4 different orientations, with 3 trials of 500 recorded data points for each loading orientation. Linear regression analysis yielded a moderate prediction (r = 0.57) between the model and experimentally measured strains. Exclusion of strain comparisons in regions that required greater modeling assumptions greatly improved the correlation (r = 0.70). Future validation studies will benefit from improved placement of strain gauges as guided by FE model predicted strain patterns.

Deformable trailing edge flaps for modern megawatt wind turbine controllers using strain gauge sensors

AbstractThe present work contains a deformable trailing edge flap controller integrated in a numerically simulated modern, variablespeed, pitch‐regulated megawatt (MW)‐size wind turbine. The aeroservoelastic multi‐body code HAWC2 acts as a component in the control loop design. At the core of the proposed controller, all unsteady loads are divided by frequency content. Blade pitching and generator moment react to low‐frequency excitations, whereas flaps deal with high‐frequency excitations. The present work should be regarded as an investigation into the fatigue load reduction potential when applying trailing edge flaps on a wind turbine blade rather than a conclusive control design with traditional issues like stability and robustness fully investigated. Recent works have shown that the fatigue load reduction by use of trailing edge flaps may be greater than for traditional pitch control methods. By enabling the trailing edge to move independently and quickly along the spanwise position of the blade, local small flutuations in the aerodynamic forces can be alleviated by deformation of the airfoil flap. Strain gauges are used as input for the flap controller, and the effect of placing strain gauges at various radial positions on the blade is investigated. An optimization routine minimizes blade root fatigue loads. Calculations are based on the 5 MW reference wind turbine part of the UpWind project primarily with a mean turbulent wind speed close to rated power. A fatigue load reduction of 25% in the blade root moment was obtained for a continuous 6.3 m long flap. Copyright © 2009 John Wiley &amp; Sons, Ltd.

A Test Method for Assessment of Shear Properties of Thick Composites

Transverse stress—strain constitutive relations are required for structural analysis of thick-section composites. A standard technique for assessment of transverse shear properties is the V-notched beam method. Such technique is based on strain gage measurements, which require a large specimen thickness for strain gage placement and impose tight geometry tolerances to minimize variations of strain at the gage location. A full-field strain measurement capability could enable simpler test specimen designs. A method for assessment of shear stress—strain relations using a short-beam shear (SBS) test and a digital image correlation (DIC) technique is presented in this work. The DIC technique is based on quantifying locations of a random texture on a surface to measure surface shape and deformation. V-notched-beam and SBS test results are compared for a glass/epoxy tape composite. To illustrate accuracy of SBS stress calculations, finite element results are obtained. Highly nonlinear interlaminar stress—strain relations are documented.

High Frequency Quantitative Photoelasticity Applied to Jet Engine Components

For sometime, reflection photoelasticity has been used in a qualitative manner as an aid to the placement of strain gauges in vibration tests on turbine and compressor blades. Often, the motivation for such tests is the validation of numerical models used in life-time predictions. Strain gauges supply data at a small number of discrete points, whereas the photoelastic fringe patterns provide information over the whole field of view. Digital photoelasticity based on phase-shifting allows these fringe patterns to be processed into maps of surface strain data that can be used to verify the computational results. Digital photoelasticity has been developed over the last two decades utilising spectral or phase approaches sometimes combined with Fourier analysis; and in most reported applications only idealised case studies are considered. Recently, the application of digital photoelasticity in high frequency blade tests has been developed as a robust methodology that can be applied using standard equipment or using solid-state polariscopes. The paper includes descriptions of the methodology and exemplar results to demonstrate the efficacy of this advanced application of photoelasticity for full-field validations of numerical modelling.

Strain gauge analysis of laser forming

Laser forming has become a viable process for the shaping of metallic components, as a means of rapid prototyping and of adjusting and aligning. The laser forming process is of significant value to industries that previously relied on expensive stamping dies and presses for prototype evaluations. This investigation aims to complement the considerable amount of work already completed on two-dimensional laser forming, offering an insight into the mechanical behavior of a part during the process using a strain gauge analysis technique. The investigation was performed on mild steel CR4 sheet using a CO2 laser source. It includes empirical investigations to determine optimum processing parameters using the temperature gradient mechanism, thermocouple analysis to locate ideal strain gauge placement for temperature compensation, and strain gauge analysis of the transverse localized strains at a number of locations on the surface of the sheet during single and multipass laser forming. The results of the investigation demonstrate the relative complexity of the process even during a simple straight-line bend and that a residual strain component remains in the sheet after processing.

Assessment of forearm vasodilator responses to acetylcholine and albuterol by strain gauge plethysmography: reproducibility and influence of strain gauge placement.

To determine the within-subject reproducibility of the forearm blood flow response to acetylcholine and the beta2-adrenoceptor agonist albuterol as measured by strain gauge plethysmography. To examine the influence of strain gauge placement on these responses. Vasodilator response to brachial artery infusion of drugs was assessed by strain gauge plethysmography in six healthy men on each of three occasions separated by 1 week. Strain gauges were placed on both arms at the point of maximum diameter. On the infused arm two further gauges were positioned approximately 4 cm proximal and distal to the middle gauge. Within-subject coefficients of variation (WCV) of absolute blood flow responses for each dose of acetylcholine (7.5, 15, 30 micrograms min(-1)) ranged from 24% to 27%, as compared with WCV values of 41% to 62% for the percentage changes in blood flow ratio (infused : noninfused arm). For albuterol (0.3, 1, 3 micrograms min(-1)) the corresponding WCV values were 16% to 19% and 30% to 55% for absolute blood flow and percentage change in blood flow ratio, respectively. WCV for the area under dose-response curve (AUC) for absolute blood flow was 18% and 13% for acetylcholine and albuterol, respectively. Vasodilator responses were similar whether recorded proximal to or at the point of maximal forearm circumference. Distal strain gauge misplacement underestimated responses and the difference was greater for acetylcholine than for albuterol. In healthy men, the WCV for responses expressed as absolute blood flow, to acetylcholine and albuterol ranges from 16% to 27%.

Force-transducer-design Optimization for the Measurement of Die-wall Stress in a Compaction Simulator

A novel slotted-web transducer design concept for the measurement of die-wall stress in a compaction simulator has been developed and optimized by use of computer-aided design procedures. The optimized design was then built and tested. Geometric optimization of the novel design configuration and determination of strain-gauge placement were performed so that the transducer could withstand the high die-wall pressures encountered in a compaction simulator. Stress analysis was performed with Algor, a finite-element analysis software package. A model prototype was constructed with 4140 alloy steel and heat treated to a hardness of 43 Rockwell C. This prototype had a linear calibration curve (r2 = 0.99) at three different punch settings with no hysteresis. Radial stress versus axial stress transmission curves, and results from measurement of residual die-wall stress for dibasic calcium phosphate dihydrate, mannitol, sodium chloride, paracetamol and starch were consistent with literature data, which indicates satisfactory transducer performance.

A novel structural design for force/torque fingertip sensors

This work presents a novel structural design of force/torque fingertip sensors for sensing planar forces. The resolution can be increased by reducing the condition number of the calibration matrix and the sensitivity can be enhanced by placing strain gauges at specified positions. Moreover, the interaction from off-axis loads is intrinsically negligible in our design. A relatively simple and accurate calibration method for performance evaluation is introduced. Some experimental results are presented to demonstrate the capability of the proposed design.

Theory of force transducer design optimization for die wall stress measurement during tablet compaction: optimization and validation of split-web die using finite element analysis.

1) To illustrate how computer aided engineering stress analysis can be used to improve the transducer design process for tablet press instrumentation; 2) to use these optimal design procedures for the geometric optimization of a cylindrical, segmented, and a novel split-web die design. Discussion includes the selection of optimal die wall thickness, segment cutting angle, strain gage placement, Wheatstone bridge configuration, and the influence of tablet height and position within the die on signal output. Stress analysis was done with a finite element analysis (FEA) software package running on a personal computer. For the segmented die, the admissible range of die wall thicknesses depends upon cutting angle; the signal output is non-linear because the stress distribution in the die wall is influenced by tablet height and position within the die. For the split-web die, the optimal configuration consists of a 1/8 in. sensing web with a strain gage located at the peak of the sensing-web arch. This prototype had a linear calibration curve (r2 = 0.999) with no hysteresis. Radial versus axial stress transmission curves for: starch and sodium chloride were consistent with literature data. Finite element analysis (FEA) is a useful numerical tool for the systematic optimization of tablet press instrumentation. By enclosing the sensing web of a three layered die design in a cylinder, the split-web design can be directly mounted without modification of the die table.

A device for the measurement of pedicle screw moments by means of internal strain gauges.

Pedicle screws are commonly used in spinal reconstruction, and failure of pedicle screws due to bending is a significant clinical problem. To measure the moments typically placed on pedicle screws in situ we instrumented 7 mm Cotrel-Dubousset (CD) pedicle screws with internally mounted strain gauges. The screws were designed to measure flexion-extension moments at a single cross-section as dictated by strain gauge placement. It is possible to measure moments of up to 12 Nm at any location along the length of the screw by constructing transducers with varying strain gauge placements. These transducers are capable of measuring moments at points located within the vertebra including the pedicle, which is where failure usually occurs clinically. Transducer output was both linear and reproducible. These transducers are being used to investigate the load transfer characteristics between the pedicle screw and the vertebra. This technique could be applied to investigations of load sharing in reconstruction plates, lag-screws, and cross-locked intramedullary nails.