Embedded Strain Gauges: Effect of the Stress Normal to the Grid

Abstract:  In general, a strain gauge embedded in a model is subjected to a stress normal to the grid, whereas a gauge on the external surface is free from such a stress. This paper concerns the effect of the stress normal to the grid on the output of the strain gauge; usually, the influence of such a stress has a negligible effect, however, in some cases a notable influence has been noted. Therefore, the output of the strain gauge is determined in function of the strains in the plane of the gauge, ɛ _l and ɛ _t, and of the stress, σ _n, normal to the grid. The analysis shows that the output of the strain gauge is influenced by the coupled effect of transverse sensitivity and pressure sensitivity of the strain gauge. In particular, the analysis shows that, for Constantan gauges compensated for steel, the influence of the transverse sensitivity is in general prevailing on that one due to the pressure sensitivity. The results reported in this paper explain the experimental data obtained by various researchers.     

Correcting for the effects of temperature on strain gauges —-use of computer–logger on Milford Haven Bridge project

The effects of temperature changes on strain gauge readings depend partly on the characteristics of the gauge and measuring equipment, and partly on the thermal properties of the structure being tested. The effect on the gauge can be numerically corrected, or compensated for by experimental techniques, and if required, the effect of temperature changes on the structure can be reduced by careful selection of the time of reading. Correction of Demec, vibrating wire and electrical resistance strain gauge outputs is discussed.
Experience with a computer controlled data logger used to correct for thermal effects on a 1/4-bridge electrical resistance strain gauge installation is described. The use of this system on the Milford Haven Bridge is estimated to have reduced potential temperature errors from between ±100 to 200 μstrain to about ±20 to 30 μstrain.     

The construction and stability of a surface mounted vibratingwire strain guage

The construction of a surface–mounted vibratingwire strain gauge for use in creep tests is described and some results on the stability of the gauge are presented. Novel features of the gauge construction include a pressed one–piece clamp for holding the wire and provision for a secondary strain measuring system. Relaxation of the vibrating wire caused relaxation of the gauge. There was no significant relaxation due to the clamping method. Cyclic straining of the gauge before use virtually eliminated zero displacement. The use of an unloaded control specimen, which is gauged at the same time and with the same wire tension as the creep specimen, is recommended. Alternatively, relaxation measurements on the wire can be used to correct measured strains. Either of these procedures will reduce the effect of gauge relaxation on creep strains to less than 3 μstrain.     

Use of displacement data to calculate strain gauge response in non-uniform strain fields

A simple and effective method is described for calculating the strain indicated by a practical strain gauge mounted in a nonuniform strain field. The method uses integration or summation of the displacement field along the perimeter of the strain gauge area. This procedure gives results which are equivalent to those obtained by much lengthier calculations involving double integration of the strain field within the strain gauge area. The displacement summation method can easily take into account significant strain gauge design details such as number, length, width and spacing of the grid lines. The method also demonstrates that a strain gauge can be used as a displacement sensor as well as a strain sensor.     

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.     

The measurement of thermal strain using self–temperature compensated strain gauges

The effect of a temperature change on a strain gauge installation may be to produce strain readings of a magnitude greater than the mechanical strain being recorded. This paper discusses some of the difficulties encountered in the measurement of strain. In particular, the selection of the most suitable type of self–temperature compensated strain gauge for a particular specimen and its associated wiring technique are considered.     

应变传感器计量特性的校准方法

为了合理评价应变传感器（应变计）的计量特性,讨论了应变计的相关国家标准和技术规范中计量特性的不同之处,并以振弦式应变计为例,归纳出应变计的计量特性参数。对应变计校准现状进行分析,设计了一种基于激光干涉法的高精度校准装置,标距范围可以达到500mm,并提出了系列计量特性的高精度计算方法。通过试验对应变计的计量特性进行测试,并评估了综合误差的校准不确定度U=0.10%（包含因子k=2）。     

Fatigue crack propagation in tensile shear stainless steel spot welded specimens

Fatigue tests were carried out on 4 mm thick spot welded joints; the material was stainless steel AISI 301, quarter hard. Some specimens were instrumented with a strain gauge bonded in correspondence with one of the edges of the spot weld. Strain gauge output was demonstrated to be a reliable instrument to monitor the nucleation and propagation of fatigue cracks. A good correlation was found between strain gauge output and spent fatigue life. Some fatigue tests were suspended when the strain gauge output was equal to pre-fixed values, corresponding to fatigue life in the range from 15 to 85%. Subsequently, the specimens were dissected to observe fatigue cracks. The same correlation existed between crack depth and fatigue life. Small cracks were observed in specimens fatigue tested up to 15% of the mean fatigue life; fatigue cracks in the joints under examination would be nucleated between 5 and 10% of fatigue life.
Finite Element calculations were carried out, introducing in the models cracks similar to those observed in the fatigue tests. Calculated strain at the external surface compared well with the measured strain as a function of crack depth. Calculations demonstrated that small errors in strain gauge position can be tolerated without appreciable deterioration in crack dimension prediction.     

Selected mechanical properties of sisal aggregates (Agava sisalana)

The effects of gauge length and test speed on the tenacity and strain of sisal aggregates (Agava sisalana) were determined. Tenacity was generally higher with shorter gauge lengths and slower test speeds. Strain was higher with shorter gauge lengths. However, the effect of test speed on both tenacity and strain was not the same at every gauge length. The mode of fibre failure appeared to differ with the test conditions. Longer specimens and those tested at faster speeds were prone to higher levels of longitudinal splitting. The effects of different ageing regimes on tenacity and strain of sisal aggregates were also determined. Tenacity was affected by elevated temperature and UV; strain by UV and simulated natural ageing.     

Stress-impedance effects in sandwiched FeCuNbCrSiB/Cu/FeCuNbCrSiB films

Sandwiched FeCuNbCrSiB/Cu/FeCuNbCrSiB films with a meander structure have been realized on silicon cantilever by Microelectromechanical Systems (MEMS), and the stress-impedance (SI) effects have been studied in the frequency range of 1–40 MHz. Experimental results show that the values of SI ratio increase with the deflection, and a large SI ratio of − 24.5% at 5 MHz with the deflection of 2 mm is obtained in the sandwiched FeCuNbCrSiB/Cu/FeCuNbCrSiB films, and the strain gauge factor is 1255 at 5 MHz, and is larger than the conventional metal strain gauge and semiconductor strain gauge, which is attractive for the applications of strain sensors.     

Double bridge circuit for self‐validated structural health monitoring strain measurements

In structural health monitoring (SHM) applications, sensor faults and structural damage need to be assuredly discriminated. A self‐diagnosis strain sensor operating in a continuous online SHM scenario is considered. The strain sensor is based on full electric resistance strain gauge Wheatstone bridges. The state of the art shows that such a sensor has not yet been developed. The loop current step response (LCSR) is a well‐known method to detect strain gauge debonding. However, applying the LCSR method to a full strain gauge Wheatstone bridge has some limitations analysed in this paper. To enable the use of the LCSR method in an online SHM scenario, the double bridge circuit is proposed in this work. Two new strain gauge debonding fault detection methods and a new debonding fault isolation method—based on the double bridge circuit measurements—are proposed and evaluated. Two new sensor fusion weighting approaches are also proposed and evaluated—to achieve strain gauge debonding fault tolerance on the double bridge circuit. The experimental results show that the proposed methods can detect, isolate, and tolerate a strain gauge grid debonding fault and can be applied in an online SHM self‐diagnosis sensor scenario.     

用应变片法确定混凝土动态起裂时间的研究

在混凝土等准脆性材料的动态起裂韧度K_(1d)测试中,准确确定试件裂尖的起裂时间是测试工作的关键。采用分离式霍普金森压杆系统,对圆孔裂纹平台巴西圆盘混凝土试件进行动态径向冲击试验,通过在裂尖粘贴应变片的方法来确定起裂时间。讨论了应变片在裂纹尖端的粘贴位置、粘贴方向等因素对起裂时间测试值的影响,结果表明裂尖应变片的最佳粘贴方式是:在裂纹延长线上或在裂尖并与裂纹垂直的线上,都距离裂尖3 mm左右,且粘贴方向与裂纹延长线垂直。给出了考虑贴片位置和试件厚度的起裂时间计算公式。     

Verification of the duplex gauge using variable strains and temperatures

Four steel specimens were equipped with "duplex nances", a recently proposed transducer based on two different electrical resistance strain gauges, capable of correcting the effects induced by temperature. The examined innovative transducer, in comparison with traditional strain measuring systems, has the advantage of utilising two strain gauge channels of the same type, which significantly simplifies automatic measuring systems. The specimens were subjected to temperature and strain field variations in the ranges of=30–150°Cand=100–700μm/m respectively The temperature and strain values obtained from the duplex gauges were compared with those measured by means of traditional methodology, i.e. a strain gauge and a thermocouple.     

Self-compensated high plastic resistance strain gauges for use up to 300°C

The development of a high plastic strain gauge for use up to 300C is presented. This kind of gauge can measure strain up to 8% and possesses good self compensation on carbon steel. Basic analysis, gauge wire, adhesive material, gauge structure and examples of application are also described.     

Strain gauge measurements within the school laboratory practice

A simple technical device has been proposed suitable for exercising strain gauge measurements in introductory courses in tensometry. Its conception has followed from a theoretical strain analysis of the circular thin steel plate loaded by a concentrated central force. The analysis has taken into account a non linearity of the studied strain field and has used a strain tensor for original expression of Poisson's ratio and Young's modulus of the material of the plate. The derivation of analytic expressions of both these constants enabled one to estimate their values by means of the strain gauge measurements. In this way the proposed device demonstrates further application possibilities of the strain gauge method.     

标距和应变率对Kevlar49单束拉伸力学性能的影响

为探究Kevlar 49单束的尺寸效应及应变率敏感性, 首先, 利用MTS万能试验机对不同标距(25、50、100、150、200和300 mm)的Kevlar 49单束进行了准静态(应变率为1/600 s-1)拉伸测试; 然后, 利用Instron落锤冲击系统对标距为25 mm的试样进行了动态(应变率为40~160 s-1)拉伸测试; 最后, 利用Weibull模型进行统计分析, 量化了不同标距和应变率下Kevlar 49单束拉伸强度的随机变化程度。结果表明: Kevlar 49单束的拉伸力学性能与标距和应变率有相关性; 拉伸强度随标距的增加而减小, 但随应变率的增加而增大; 峰值应变和韧性均随标距和应变率的增加而减小; 提取的Weibull参数可用于数值模拟及工程应用。      

The laser strain gauge: micro machining of diffraction gratings using an Excimer laser

The laser strain gauge is a recently developed instrument for the single point measurement of surface strain to a resolution of 1 μm. It measures the deformation of a diffraction grating adhered to or etched into the surface of various substrates. In comparison with a conventional electrical resistance strain gauge the new gauge offers greater thermal stability and avoids the inconvenience of electrical connections. When gratings are etched directly into the surface of a component absolute strain measurements over its lifetime are possible.
High frequency (1000 lines/mm) diffraction gratings can be etched into the surface of various substrates including carbon steels, polymers and ceramics using an Excimer laser. This paper describes the optical beam delivery system necessary for this application and demonstrates the feasibility of this exciting new technique.     

Computer aided analysis of strain rosette data

In this paper a computer program has been developed for analysing the strain gauge rosettes by considering the transverse sensitivity of all gauges. The program is capable of calculating principal strains, maximum shear strain, principal stresses, maximum shear stress and principal directions from the data obtained by using strain gauge rosettes of various configurations. The gauge factor and transverse sensitivity of all gauges in the rosette must be the same.