Effect of moistureproof coatings on the sensitivity of strain gauges to deformations

Conclusions The rise in the sensitivity factor of strain gauges with a direct application of the moistureproof epoxy-resin compound coating is due mainly to the change in the stiffness of the beam as a result of the firm adhesion of the 2–3 mm thick resin layer and the strain gauges to the beam and the formation of a single monolithic structure. The amount of change in the sensitivity of the strain gauges depends in each specific case on the relationship of the stiffness (strength) of the beam (structure) to that of the applied coating, i.e., to its thickness, strength characteristics, and hardening time.Therefore, in order to avoid obtaining results which are known to be distorted, it is necessary to limit the application of moistureproof epoxy-resin compounds or other materials which have relatively large strength characteristics when hardened in cases when the coating together with the strain gauges changes considerably the stiffness of the structure and it is impossible to calibrate the glued gauges. As an example of this, it is possible to cite the measurement of stresses in thin-walled articles.The method of moistureproofing glued strain gauges described in [2] does not lead to a change in their sensitivity factor, this can be explained in the light of the above considerations by the fact that in this case the epoxyresin layer applied to the PKhL film does not contact the beam directly; i.e., it does not change the beam stiffness on the section of the glued strain gauges and, therefore, their sensitivity to deformations remains constant.Translated from Izmeritel'naya Tekhnika, No. 7, pp. 28–29, July, 1971.     

Local stiffening effect of 'double deck' bending strain gauges: part 1

In stress measurement where it is difficult or impossible to access the inside surface of the structure, "double deck" (DD) or "sandwich" bending strain gauges can be used to measure both the membrane and bending strains. A DD gauge consists of a pair of constituent foil gauges which are bonded on the upper and lower surfaces of a plastic base or filling. It is mounted on the external surface of the test specimen to measure the external surface strain, and by extrapolation, the internal surface strain of the specimen. The sensitivity of the DD bending gauge increases when the separation between the pair of constituent foil gauges increases. However, this also increases the stiffness of the whole gauge. If the stiffness of the DD gauge is significant compared with that of the plate to which it is attached, the presence of the gauge may cause significant changes in local stiffness and result in errors of strain interpretation. This study is presented in two papers. Part I presents an approximate, analytical solution for the local stiffening effect of a DD gauge and quantifies the errors in the strain measurement. Part 2, to appear subsequently in Strain, will present the verification of the solution using a three dimensional finite element calculation, a parametric study and a correction method that can be applied easily. The extrapolation errors due to the stiffening effect and random reading noises are also explored.     

Local stiffening effect of 'double deck1 bending strain gauges: part 2

In stress measurement where it is difficult or impossible to access the inside surface of the structure, "double deck" (DD) or "sandwich" bending strain gauges can be used to measure both the membrane and bending strains. A DD gauge consists of a pair of constituent foil gauges which are bonded on the upper and lower surfaces of a plastic base or filling. It is mounted on the external surface of the test specimen to measure the external surface strain, and by extrapolation, the internal surface strain of the. specimen. The. sensitivity of the DD bending gauge increases when the separation between the pair of constituent foil gauges increases. However, this also increases the stiffness of the whole gauge. If the stiffness of the DD gauge is significant compared with that of the plate to which it is attached, the presence of the gauge may cause significant changes in local stiffness and result in errors of strain interpretation. This study is presented in two papers. Part I, appeared in the Feburary 1996 issue of Strain, presented an approximate analytical solution for the local stiffening effect of a DD gauge, and quantifies the errors in the strain measurement. Part 2 (this paper) presents the verification of the. solution using a three-dimensional finite element calculation, a parametric study and a correction method that can be applied easily. The extrapolation errors due to the stiffening effect and random reading noises are also explored. The figure and equation numbers in this part are continuedfrom Part 1, but the reference numbering is specific to Part 2.     

Nine–gauges device for strain measurements inside concrete

Surface strain measurements of bulk concrete structures such as dams provide insufficient information and embedded strain measuring devices are necessary. For complete analysis six strain measurements are required. The device described indicates nine independent readings from which not only is a complete analysis possible by solution of the strain matrix constructed from these readings, but gross errors in individual readings may be identified and these readings discarded. An evaluation may be made of the errors in the results determined from the remaining gauges.     

The effect of trimming on strain gauge accuracy

During a programme of fatigue testing welded tubular joints, similar to those in use in North Sea offshore structures, it was necessary to measure the strain as close to the weld toe as possible and also to detect crack initiation. To this end general purpose miniature foil strain gauges were used. The backings of these gauges were trimmed prior to installation to enable placing the measuring grid as close as possible to the weld toe. The investigation described was carried out to assess the accuracy of these strain gauge installations.     

Determination of stiffness properties of braided composites for the design of a hip prosthesis

This paper describes an experimental procedure used to determine the stiffness properties of two different composites made of braided glass and hybrid carbon–glass fibre reinforced epoxy resin. Tubular specimens manufactured by reinforcing an epoxy resin system with commercial braided preforms were used to determine the elastic constants. All specimens were manufactured using compression moulding technique assisted with internal pressure. The stiffness properties were determined from axial and circumferential strains recorded from strain gauges using internal water pressure tests. Identical types of composite laminates were used to manufacture two prototypes of a composite femoral prosthesis with controlled stiffness.     

Piezo-resistive semi-conductor gauges

Though the piezo–resistive semi–conductor strain gauge has a very high gauge factor compared with wire or foil resistance strain gauges, some precautions ate necessary in its use if a reasonable degree of stability and accuracy are to be attained. The introduction of thermistors in the bridge circuit can compensate for errors caused by temperature changes.     

Monitoring quasi-static and cyclic fatigue damage in fibre-reinforced plastics by Poisson’s ratio evolution

Even if the extent of fatigue damage in fibre-reinforced plastics is limited, it can already affect the elastic properties. Therefore, the damage initiation and propagation in composite structures is monitored very carefully. Beside the use of nondestructive testing methods (ultrasonic inspection, optical fibre sensing), the follow-up of the degradation of engineering properties such as the stiffness is a common approach.In this paper, it is proved that the Poisson’s ratio can be used as a sensitive indicator of fatigue damage in fibre-reinforced plastics. Static tests, quasi-static cyclic tests and fatigue tests were performed on [0°/90°]_2s glass/epoxy laminates, and longitudinal and transverse strain were measured continuously. The evolution of the Poisson’s ratio ν_xy versus time and longitudinal strain ε_xx is studied. As the transverse strain measurement is crucial to monitor the degradation of the Poisson’s ratio, three techniques were applied to measure the transverse strain (strain gauges, mechanical extensometer and external optical fibre sensor).Finally, the technique has been applied to a totally different material: a carbon fabric thermoplastic composite. The results show a very similar degradation of the Poisson’s ratio, although no stiffness degradation can be observed during fatigue loading of this material.It is concluded that the degradation of the Poisson’s ratio can be a valuable indicator of fatigue damage, in combination with the stiffness degradation.     

Sensor for Stiffness Measurements Within the Adult Rat Hippocampus

This paper presents a sensitive compliance measurement system for determining the stiffness of the adult rat hippocampus. The device has been successfully fabricated which consists of two strain gauges incorporated in the SU-8 based cantilever. The cantilever-sensor has been fully characterized and its strain sensitivity was found to be approximately 2.5. in vitro testing on the brain tissue has successfully demonstrated that the system is sufficiently sensitive to provide distinctive direct stiffness measurements on various hippocampal regions.      

Effect of temperature on vibrating wire strain gauges

Vibrating wire strain gauges attached to buried pipe can exhibit changes in strain readings when subjected to changes in ground temperature. It is essential that the correct interpretation is placed upon the readings. An analysis is given which, it is suggested, will give true pipe strain values from the indicated strain readings. Two solutions are outlined, a graphical analysis which tends to be intuitive and a theoretical analysis which derives a solution from first principles.
True strain in the pipe is influenced by thermal effects due to changes in ground temperature, and mechanical effects due to the constraint placed upon the pipe by the surrounding soil. Both analyses consider the thermal and mechanical components of true strain and the theoretical analysis enables these individual components to be determined from the indicated strain readings.     

Some aspects of clip gauge design

Measurements show that simple beam theory accurately models the performance of clip gauge transducers. Assuming typical dimensions for the single and twin cantilever gauge it is found that the single cantilever gauge possesses about four times the sensitivity of the twin cantilever gauge, for the same stiffness. Design rules are specified by which gauges of high sensitivity and low stiffness may be constructed. A simple and reliable technique for fastening clip gauges to a test piece is introduced.     

Stiffness and Reinforcement Effect of Electrical Resistance Strain Gauges

Abstract:  The increasing use of low-modulus materials, on which the reinforcement effect of the electrical resistance strain gauge is not negligible, has re-opened the research interest into this issue. This study deals with the evaluation of stiffness, and of the strain gauge Young's modulus involved in the estimation of both the global and the local reinforcement effect; the relationship between the strain gauge stiffness and the local reinforcement effect is also analysed. In particular, the experimental technique used to determine the stiffness of some commercial strain gauges is described. The results show that the strain gauge stiffness alone does not permit an accurate evaluation of the local reinforcement effect.     

小尺寸混凝土试件双K断裂参数试验研究

采用最大尺寸为680mm×160mm×40mm的标准三点弯曲梁试件,利用在初始裂缝两侧粘贴电阻应变片并利用混凝土裂缝扩展到此处时其应变回缩的方法测得了起裂荷载Pini,在此基础上根据Pini及初始缝长a0得到了起裂断裂韧度KIiCni;根据在试验中测得的最大荷载Pmax及对应的裂缝口张开位移CMODC计算了混凝土等效裂缝长度aC,据此计算了失稳断裂韧度KIuCn。结果表明:采用电阻应变片法可准确测定混凝土的起裂荷载Pini,且方法简单。试验结果还表明:在本试验范围内,三点弯曲梁法测得的混凝土双K断裂参数KIiCni、KIuCn与试件高度无关,进一步说明了混凝土双K断裂参数可以作为描述混凝土裂缝扩展的断裂参数。     

The effect of a sliding load on strain gauges embedded under the contact surface

Strain gauges embedded in plastic models can be used in three dimensional stress analysis. When gauges are located immediately below a surface subjected to the tangential traction of a sliding load, unexpected values are recorded. This phenomenon was investigated by suitably loading a semi–infinite solid with normal and shear stresses and comparing theoretical strain values with the experimental results. The comparison showed that strain gauges embedded in epoxy can be successfully used under sliding loads provided that the gauge depth below the contact surface is greater than a critical value.     

Some factors affecting long-term stability of strain measurements using metal foil gauges

The stability of long-term strain measurements is very much dependent on the correct selection of techniques, materials and equipment for the conditions under which the measurements are to be taken.
Temperature effects are also a major factor. Temperature-induced changes in resistance of the Wheat-stone bridge completion and balancing components can result in an output signal indistinguishable from that produced by change of gauge resistance due to surface strain. The signal conditioning and readout equipment can also be affected by temperature and become an additional source of instability.
The term 'instability', in strain gauging, generally refers to 'zero drift' and random deviations of readings when the loading conditions are stationary. In particular it refers to those situations where a measurement system has been balanced and measurements have been taken with a particular load condition which cannot be repeated to re-check the original zero.
In this paper the major sources of instability are identified and recommendations are made as to how best to obtain long-term stability of strain measurements using metal foil gauges.
Information on principal error sources in strain measurement systems using metal foil gauges is included for completeness and because an appreciation of these sources is also vitally important.     

The Reinforcement Effect of Strain Gauges Embedded in Low Modulus Materials

The reinforcement effect of electrical resistance strain gauges is well‐described in the literature, especially for strain gauges installed on surface. This paper considers the local reinforcement effect of strain gauges embedded within low Young modulus materials. In particular, by using a simple theoretical model, already used for strain gauges installed on the surface, it proposes a simple formula that allows the user to evaluate the local reinforcement effect of a generic strain gauge embedded on plastics, polymer composites, etc. The theoretical analysis has been integrated by numerical and experimental analyses, which confirmed the reliability of the proposed model.     

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Highest precision in short cycle time – highly sensitive thin fiim strain gauges produced in an industrial high‐rate‐sputtering system Commonly, sputtered strain gauges are already used for highly precise pressure sensors. By using special highly sensitive piezoresistive nanocomposites consisting of metal nanoparticles embedded in an insulating matrix made from diamond‐like carbon (DLC) the sensitivity to strain can be significantly increased. Essential parameters for the characterization of the thin films used as sensor layer are the strain sensitivity (described by the gauge factor) and the temperature coefficient of resistance (TCR). Conventionally used NiCr alloys have a gauge factor of approximately 2. By using metal‐DLC nanocomposite films a factor 5 to 10 higher strain sensitivity in combination with a TCR close to zero was reached on laboratory scale. At first, the highly sensitive films were produced by static deposition using a box coater which led to quite long process times. By using a dynamic deposition process in the same machine the throughput of samples was slightly increased. But for using these highly sensitive films on industrial scale much higher cost and process efficiency is necessary. Hence, the process was transferred to a highrate sputtering system. A 20 times higher throughput of samples was reached in combination with a higher strain sensitivity compared to the dynamic process in the box coater. The used high‐rate sputtering system is also commercially available with enlarged process chambers which enables for a further up‐scaling for efficiently industrial production.     

Measurements of internal strains by nine-gauge devices

An assembly of nine vibrating-wire strain gauges designed for the measurement of internal strain is described. Tests have been carried out on concrete specimens containing nine-gauge devices of different size and stiffness. The processing of the gauge readings is outlined and the results of the test series are critically discussed.     

Effect of deformations in the bending of the shaft on the accuracy of torque measurements

Summary The above experimental and theoretical investigations have shown that in order to compensate for the error produced by the bending of the shaft in measurements of torque by means of strain gauges it is necessary to fix the transducers in positions which have the smallest bending strains and place them in strict conformity with the relations deduced above.     

THE EFFECT OF CROSS-SENSITIVITY ON THE DETERMINATION OF STRESS BY STRAIN GAUGES

Some free-filament wire strain gauges used at high temperatures have a higher than normal cross-sensitivity so that errors in the principal stress calculated by standard formulae may be significant. The magnitude of the error is shown and the formulae are modified to obtain true principal stresses. As the modifications are simple, all results obtained from electrical resistance strain gauges can be corrected similarly.