Use Of Strain Gauges Research Articles

Diurnal changes in branch diameter as indicator of water status of Hinoki cypress Chamaecyparis obtusa

We investigated use of strain gauges for monitoring the water status of trees by measuring changes in the diameter of the largest spreading branch of a 27-year-old Chamaecyparis obtusa tree. The change in xylem diameter in the branch is more closely related than the change in phloem diameter to the change in leaf water potential. Since the diurnal changes in diameter match the diurnal changes in water balance (sap flow velocity – transpiration), measuring the change in xylem diameter using a strain gauge is useful in evaluating the water status of C. obtusa.

ROLLING CONTACT FORCE ENERGY RECONSTRUCTION

Knowledge of the forces at the wheel–rail contact is fundamental to estimate the consequences in terms of noise and vibration. The traditional use of strain gauges mounted on the wheel web and axle is not capable of determining the high-frequency content of the contact force. Measurements made on the rail are characterized by the spatial variability of input–output transfer functions which makes it difficult to estimate the contact force by simple inversion of the point frequency response function. In this study the problem of rolling contact force reconstruction has been approached through the following steps: (i) the track has been characterized precisely for a finite length by the analysis of the time series of several impacts supplied with an instrumented hammer by using an ARMAX model that proved to be capable of modelling the vertical dynamics of the rail up to 5 kHz; (ii) the response of the rail has been simulated with a random force acting on the system, and the variability of the transfer function has been taken into account by distributing the force on adjacent elements; (iii) the simulated response has been compared with the rail acceleration measured for the passage of several trains; (iv) the wheel–rail contact force has been estimated with a closed-loop algorithm. It has thus been possible to reconstruct the13octave power spectrum of contact forces with a simple and stable iterative procedure. Forces reconstructed from different sensors were found to be practically the same for a given wheel; forces from nominally similar wheels are statistically examined and partial results of comparisons made on different rolling stock are shown.

The use of strain gauges in the measurement of the B1/B2 phase transformation in shock-loaded potassium chloride

The shock induced phase transformation in potassium chloride (from the NaCl B1 structure to the CsCl B2 structure) has been investigated using constantan strain gauges. These gauges have been mounted in pressed, polycrystalline samples in an orientation that renders them sensitive to the uniaxial strain that characterizes the plate impact geometry. At the phase transformation stress (of ∼2 GPa), measured strains of ∼ −0.1 were observed, slightly higher than the values quoted in the literature. We suggested that residual porosity within the microstructure is a possible explanation. Over impact stresses of 2.5–4.0 GPa, total measured strains agree with the work of others at the lower end of our measured range, but are significantly lower at higher stresses. We suggest that this might be due to the nonequilibrium state of the material at the time of measurement.

Strain gauges used in the mechanical testing of bones Part I: Theoretical and technical aspects

The internal restructuring of bone as a reaction to physiological load described by Wolff's law is related to both stress and strain in the bone. Bone is a rather brittle material, with nearly equal properties in tension and in compression: Elastic modulus E = 20 Gpa, tensile strength sigma tens = 200 Mpa (cortical bone). In biomechanics, it is very easy to assess strain using strain gauges. The theoretical and technical aspects specific to the use of strain gauges on bones are presented in this paper including how the measurements are taken and how the data is analyzed.

In reply

We thank Drs Wolfe and Panjabi for their insightful comments. As documented in both the Materials and Methods and Discussion sections of our paper, instrumentation of the external fixator bar does not readily allow direct measurement of loads acting across the fracture site. While we could have calibrated each fixator bar to measure the axial force (as opposed to pure bending and torsion), this would be potentially misleading as measurements in situ would not be due only to the axial force across the fracture. The use of strain gauges to measure axial forces in the bar was not efficacious for the relatively small magnitudes of loads in this study, principally due to the sensitivity of the device. We thus chose to measure flexion-extension bending, radioulnar bending, and torsion, which occurred in the fixator bar, and to use these results to quantify the relative changes in the magnitudes of loads acting in the external fixation apparatus. To provide some inference with regard to axial loading, we described in the Discussion a calculation based on the premise that the axial force that acts across the wrist is the only contributor to coronal plane or radioulnar bending.

Interest of a Tissular Strain Evaluation Method for the design of sports equipments: Application to mountaineering backpacks

The aim of this paper is to study the behaviour of three tooth configurations protected by mouthguards under simple dropweight impacts. Frontal and uppercut impact tests were undertaken with one front tooth strain gauged on each set. The results showed that the performance of the mouthguard can be assessed by the use of strain gauges on the teeth whilst the photoelastic analysis are effective for monitoring the reduction in process stresses for the material. Simple theoretical solutions were suggested in order to differentiate between frontal and uppercut impacts and the influence of adjacent teeth were examined for their role on the local strain distribution for uppercut tests. A more realistic approach appears to be that of monitoring of the load path from the impact indentor.

The effect of strain gage size on measurement errors in textile composite materials

The heterogeneous nature of textile composites on the microstructural scale leads to variation in the local strain field within the unit cell. This variation can result in inaccurate measurement of the average global strain when using strain gages of insufficient size. A statistical model for sensor error is presented which leads to expressions for sensor variance. The model includes the effects of unit cell size, gage length, gage width, microstructural defects and irregularities, and instrument error. The proposed model is verified experimentally by using a plain woven-graphite-epoxy composite material and strain gages of various lengths. The experimental results are compared with analytical predictions. A set of guidelines is given for the use of strain gages on textile composites.

Development of a combined-type tool dynamometer with a piezo-film accelerometer for an ultra-precision lathe

The cutting force is the most important variable by means of which to understand the mechanics of ultra-precision machining. However, most tool dynamometers monitor the static cutting force, although it is necessary to measure the dynamic cutting force to clarify the machinability of the material, the formation of the chip, chatter and the wear of the tool. In this research, a combined-type tool dynamometer which can measure the static cutting force and the dynamic cutting force together by use of strain gauges and a piezo-film accelerometer has been developed. An analysis of the dynamometer has also been carried out.

An investigation of the use of strain gages to measure welding-induced residual stresses

The measurement of weld-induced residual stress is important in structures that are subjected to cyclic loading during their service life. Depending on their magnitude, stresses can influence the rate of crack growth under cyclic loading and hence affect the life of the structure. Because the level of residual stress may change during service, measurement of these changes is necessary for accurate life prediction of the structures. The measurement of welding-induced residual stress using strain gages poses significant problems, the most important being the potential damage to the gages by high temperatures generated in the welding process. This laboratory study was undertaken to assess the suitability and signal stability of commercially available resistive strain gages for the measurement of postweld residual stresses in a submarine hull structure. Adhesively bonded and weldable-type strain gages were attached to the surface of a 35 mm thick steel plate, which was then subjected to thermal cycles similar to those encountered during welding construction of a submarine pressure hull. This paper describes the strain gage application procedure, changes in the strain gage output at end of each experimental stage and the history of changes in the residual stress.

Biomechanics of oral implants: a review of the literature

Considering biomechanics of oral implants, both loading on the implant itself and the transferred load to the bone need our attention. Mastication induces vertical and transverse forces, which induce axial forces and bending moments and exert stress gradients in the implant as well as in the bone. By the use of strain gauges or piezo-electric force transducers, one succeeds in precise intra-oral force measurements which make it possible to study a wide range of varying conditions in implant dentistry. A key determinant of the success or failure of an oral implant is the way mechanical stresses are transferred to the surrounding bone. The load transfer from implants to surrounding bone depends on the type of loading, the bone-implant interface, the length and diameter of the implants, the implant shape, structure of the implant surface, the superstructure and the quality and quality of the surrounding bone. Finite element analyses indicate maximum stress concentrations in the contact area of the implants with the cortical bone and around the apex of the implants in the trabecular bone. Although the precise mechanisms are not fully understood, it is clear that there is an adaptive remodelling response of the surrounding bone to the current situation.

A method suitable for in vivo measurement of bone strain in humans

Strain gages are the gold-standard for measurement of bone strains in vivo. The use of strain gages in humans, however, is limited by the need for surgery to implant them and by the use of cyanoacrylate adhesives to bond them to bone. Cyanoacrylate adhesives are not FDA approved for implantation in humans, making it difficult to justify their use in experimental procedures. To surmount this difficulty, a method was developed to bond strain gages to bone using an approved substance: polymethylmethacrylate (PMMA). The technique and the validating experiments are presented. The PMMA bonding method gave strain gage readings within an average of 0.25% (range 0–5%) of those found using cyanoacrylate bonding in a side by side comparison on cast acrylic. On bone, the PMMA bonding method produced results comparable to extensometer readings. This method of strain gage application is accurate and straightforward. It is currently being successfully used for in vivo strain measurements in both humans and animals for up to several days following gage application.

In vitro measured strains in the loaded femur: Quantification of experimental error

The application of strain gauges to bone surfaces has been extensively employed as a method of determining, strain fields in response to implanted devices in orthopaedics. The aim of this study was to determine some of the experimental errors associated with the use of strain gauges in in vitro experimental investigations of the loaded femur. An experimental protocol was devised to obtain strain data at 20 strain gauged locations on the proximal femur. These data were interpolated using a parametric model. The parametric model was then used to estimate the errors associated with mispositioning of the gauges and deviations in their direction of application to the bone. This sensitivity analysis was also supported by a finite element analysis for the purposes of comparison and cross-validation. The results indicated that the nature of the loading normally employed in the literature can contribute to making the readings for some of the gauges (anterior and posterior) unreliable and redundant, even for small positioning errors. The greatest predicted errors for the lateral and medial gauges were due to misalignment of the gauge as opposed to mispositioning. The size of the gauge had a negligible effect on the errors predicted relative to those caused by misalignment.

The use of strain gauges to measure low velocity fluid flow

A device was designed, built, calibrated, and installed in a small shallow lake for the detection and measurement of water movement at low velocities in natural environments. The device used sets of typical strain gauges in a full-bridge configuration positioned orthogonally with respect to each other and in an array to receive multiple signals. Standard analog to digital detection and recording, via simple computing techniques, was used. This article also reports the first directly measured slow flow directionality of shallow water movement in the three coordinate system and its conversion to cylindrical and spherical systems.

Experimental strain analysis of Clarens Sandstone colonised by endolithic lichens

Endolithic lichens occur commonly on Clarens Sandstone in South Africa, where they significantly contribute to the weathering of sandstone by means of mechanical and chemical weathering processes. This preliminary investigation reports on the success- ful use of strain gauges in detecting strain differences between sandstone without epilithic lichens and sandstone colonised by the euendolithic lichen Lecidea aff. sarcogynoides Korb. Mechanical weathering, expressed as strain changes, in Clarens Sandstone was studied during the transition from relatively dry winter to wet summer conditions. Daily weathering of sandstone due to thermal expansion and contraction of colonised and uncolonised sandstone could be shown. Our results show that liquid water in sandstone enhances the mechanical weathering of uncolonised Clarens Sandstone while water in the gaseous phase enhances mechanical weathering of sandstone by euendolithic lichens.

On the use of strain gages in dynamic fracture mechanics

An experimental study is conducted to show the application of strain gages in studies of dynamic fracture. In particular, strain gages have been used to evaluate the dynamic stress intensity factors in different specimen geometries in a variety of materials. A direct comparison of the results obtained from strain gages is made with those obtained using dynamic photoelasticity. A detailed evaluation of the effect of the position and orientation of the strain gage on the determination of the fracture parameters is also presented.

Monitoring fracture healing with strain gauges: the effect on strain values of strain gauge location on the pin circumference

In an attempt to establish some principles for the use of strain gauges in monitoring fracture healing, four strain gauges were fastened onto the most proximal pin of a Shearer external fixator applied to a hardwood fracture model. Compression and bending loads were applied and the strain data were obtained. The results show that the location of the strain gauge on the circumference of the pin has a significant effect on strain values.

In Situ Determination Of The Complex Modulus From Strain Measurements On An Impacted Structure

A method is proposed for in situ determination of the complex modulus of a member of an impact-loaded structure. The modulus was obtained in terms of strains measured at three different cross-sections of a uniform part of the member, by using one-dimensional theory. The potential of the method was examined for a cylindrical polypropylene bar mounted as a member in each of two simple frame structures, and the modulus was obtained in the frequency range of 500 Hz to 3 kHz. For comparison, the modulus was also determined from tests on the same member, dismounted from the structures, by using a different method based on measurement of end-point accelerations. The results obtained from the in situ tests on the two frame structures and from the tests on the dismounted member are in fair agreement. The larger scatter in the results of the in situ tests is probably explained by a lower signal-to-noise ratio due to the use of strain gauges rather than accelerometers and to differences in excitation.

Cover cracking as a function of bar corrosion: Part I-Experimental test

The appraisal of concrete structures suffering rebar corrosion is one of the most urgent needs regarding the selection of the technical and economical optimum time for repair. Up to now this appraisal has been mainly based on empirical and subjective considerations. Among the different distressing consequences of rebar corrosion the best known is the cracking of concrete cover. However, very few data have been reported in the literature on the amount of corrosion needed to induce this cracking. In the present paper, some preliminary experiments are reported in which small reinforced beams are artificially corroded by an impressed current, and the amount of current (and loss of bar cross-section) needed to induce the crack at the surface are monitored, together with the evolution of crack width, by the use of strain gauges applied to the surface of the specimens. In a companion paper, a numerical model to relate the decrease in rebar cross-section to the cover cracking will be developed. That model is based on the orderly imposition of corrosion to finite elements of the rebar by a fictitious temperature increment that produces analogous effects, while concrete cracking is introduced by a standard smeared-crack model. The experimental results indicate that only a few micrometres of loss in rebar cross-section are needed to induce visible cover cracks (0.1 mm width) in the conditions of the test.

Crack detection in adhesive joints: use of strain gages in aggressive environments

A large three-dimensional finite element model was used to predict the response of miniature resistance strain gages to the initiation and growth of center and corner cracks in the fillet of an asymmetric single lap-shear joint. Experiments were conducted to assess the feasibility of the technique under conditions of accelerated durability testing. Drift in the strain-gage output complicated the interpretation of the data and placed restrictions on the slowest rate of crack growth which could be detected. The finite element model was also used to predict the strain-gage response to two-dimensional cracking under conditions of cyclic loading. This was seen to eliminate the experimental problem of signal drift and to create a more robust technique for the detection of crack initiation and growth. Using this method, it is predicted that cracks as small as 0.2 mm can be detected by comparing the best-fit slopes of the strain-gage output vs. the applied load in subsequent loading cycles.

Evaluation of Sharpness for Dental Scalers

As a plaque control method, the removal of calculus deposited on the teeth surface is indispensable in the prevention and treatment of periodontal disease. The hand scalar, which is used for removing calculus, must be sharpened just after used it in order to keep it original sharpness. But the qualitative checking method is not established yet. The authors have designed and made an apparatus as an experiment for the evaluation of a model scalar' s sharpeness. The model scalar is straight and large as compared with a real hand scalar. This report deals with the apparatus and it's chracteristics.The ranges of rake angles of the scalers covers from-20deg to 0 deg, and speeds of the bed on which the model teeth(test piece)is fixed from0.5mm/ sec to3.0mm/ sec. Vertical pushing force of the scalar to the test piece and cutting resistance are measured by the use of strain gages. Metals such as steel and aluminum, glass are used as the material for the model teeth.Plastics cement which were dripped on the surface of model teeth are used as the material of calculus. After the cement became solid and set up the parameters such as speed of the bed and pushing force to the scaler, it is ready to scrub away the cement. In the case of the dripped cement were removed and impulsive force data were observed, then it may conclude that the scaler is sharp.