A low cost calibration conditioner for dynamic strain gauge measurements

A conditioner for dynamic strain measurements that allows for a series of calibration signals with periodic and predefined time occurrence, during the whole period in which the measurements have to be carried out, is presented in this work. The calibration signals, having suitable size ratios between them, allow one to evaluate the linearity of the entire measurement chain.     

Analysis of strain averaged data from finite length gauges and predictions of peak strain for planar notch and fillet problems

From a previously derived modal (i.e., series) representation of the stress and strain fields in stress concentration regions, exact analytical expressions are obtained for the averaging effects of finite length gauges located on the boundary of a notch fillet. A simple method of analysing such data is described. It is then applied to two problems to show i) how easily poor or faulty data can be detected and ii) how a very reliable estimate can be made of the magnitude and location of the peak stress (or strain), even when using data from gauges beyond the very highly stressed (strained) region.     

A temperature calibration of a temperature/strain transducer based on two strain gauges

The operation of a temperature/strain transducer, called “duplex gauge”, based on two electrical resistance strain gauges, that are temperature compensated for use on materials with different coefficients of linear expansion, was briefly recalled. Three steel specimens were equipped with “duplex gauges” and were subjected to temperature variations and no external loads were applied. The chosen experimental setup (i) to measure the outputs of the gauge temperature compensated for steel and titanium silicate and (ii) to impose known temperature, assured good accuracy and precision in evaluation of the temperature calibration curve. The obtained results showed that the strain differences could be considered, at least for the experimental arrangement examined, a linear function of temperature over the range of 20 to over 140°C. The prepared calibration chart, that allows the correction of temperature induced apparent strain, will be used in future research when generic temperature and strain fields are applied to specimens.     

A Novel Strain Sensor for Reinforced Concrete Structures

Abstract:  Reinforced concrete (RC) is the most commonly used structural material in civil engineering applications. RC structures have long-term service lives under normal loading conditions; however, overload caused by misuse or statistically remote events such as earthquakes may create damages that, if not detected in time, may eventually cause failure. Hence, it is important to monitor RC structures to take necessary precautions and save human lives. A long-gauge strain (LGS) sensor has been developed to monitor these structures. While it has been developed mainly with concrete applications in mind, the new sensor can also be used in a variety of applications, including measuring strains in pipelines, steel structures, and the like. The proposed sensor system has a very low cost compared with the commercially available competing systems. Prototypes of the proposed strain sensors have been built and calibrated. Test results prove the accuracy, repeatability and reliability of the proposed strain sensor. When the LGS sensor was incorporated into a concrete beam there was very good agreement between the experimental measurement of strain using the LGS sensor when compared with two strain-gauged parallel steel rebars in the same concrete beam.     

The use of vibrating wire strain gauges for long term measurements on buried gas mains

A trial agreement concerned with the effect of adjacent deep excavations on the risk of failure of buried cast iron gas and water mains was implemented by British Gas and National Water Council on 1 January 1983. To complement this agreement, field experiments are being carried out at selected sites throughout the UK. The British Gas Engineering Research Station (ERS) is working in cooperation with the Water Research Centre to install instrumentation to measure pipe strains, joint movement, ground temperature and ground movement.
Field measurements impose environmental and operating conditions of much greater severity than those encountered in a laboratory. This note outlines the site techniques developed by ERS which allow reasonably quick and reliable installation of equipment capable of withstanding the adverse conditions typical of backfilled trenches. The principle of operation of the VW gauge, its method of fixing, the factors affecting its output and the interpretation of the readings themselves are described.     

Limits of agreement method for comparing TSD and FWD measurements

This article uses the limits of agreement (LOA) method to compare the falling weight deflectometer (FWD) and the traffic speed deflectometer (TSD), two pavement structural evaluation devices. The TSD measures deflection slope, whereas the FWD measures deflection. For this reason, measurements were converted to the surface curvature index (SCI) and the base damage index (BDI), which can be obtained from each device. The SCI and BDI agreement between the two devices was then evaluated. Although the relationship between the calculated SCI and BDI using both equipments is reasonably close to the line of equality, there is a significant variation and a bias in this relationship. For example, for an average SCI or BDI value of 300 μm, the bias was 30 μm (FWD values lower than TSD values), and the LOA was 380 μm.     

A simple technique for installing strain gauges on cylindrical surfaces

In this paper a simple technique for the installation of strain gauges on circular and conical cylindrical surfaces has been described. This technique may be adopted in an experimental stress analysis laboratory where sophisticated equipment is not available.     

温度应变同时测量的光纤光栅系统的研究

为了同时传感温度和应变,在同一根光纤的两个不同地方写入相同的Bragg光栅,设计制成一种特殊机构的传感系统。由于测量结构特殊,两处光纤光栅的反射峰具有成比例的应变响应系数和不同的温度响应,测出温度后,应变也同时算出。实际测量表明,该系统结构简单,测量精度高,能进行实时测量。     

Precision equipment for symmetrical bending— Part 1. The loading system

The design and performance of precision bending equipment for testing straight beams with constant rectangular section under a pure bending moment is described. Part 1 deals with the loading arrangements and Part 2 with the rig for measuring beam deflection. The methods for recording deflection of strain gauges applied on the beam have been described elsewhere.^1–4
The reactions to the deadweight gravity loads are taken via ball bearings. The total bending moment inaccuracy, i.e. the sum of estimated systematic errors and random errors, the latter expressed as the 2Ga confidence interval about the mean (2G_cim), is about ± 25 x. 10^-4%, for 23 tests with a steel beam with 15 mm × 75 mm cross sectional dimensions, loaded to 1-1.5 mm/m strain.
The measuring rig stands on the beam via hardened and polished spherical segment feet and uses a capacitive transducer system with less than 5nm resolution. Various application tasks are described. The total inaccuracy of the measured deflection, i.e. apart from bending moment inaccuracy, is about ± 35 times 10^4% for about 10 reproduced, not repeated, tests with the same steel beam. The deflection for such a beam is of the order of 2 to 4 mm.     

A nonconstraining templated powder grid for measurement of strain

A method was developed and validated for mounting grid patterns with pitches on the order of tens to hundreds of micrometres onto specimens for the purposes of mechanical characterisation and fracture analysis. The method uses a low stiffness, low‐toughness photopolymer to create a textured pattern of pillars on the sample, and the spaces between these pillars are then filled in with an opaque powder to provide optical contrast. This type of grid is desirable both because of its relatively quick preparation time and because of its minimal influence on the mechanical properties of the sample. The method for mounting this powder‐based grid onto samples is discussed. Then, the accuracy of this new type of grid is validated against current state‐of‐the‐art epoxy‐mounted lithography grids as well as digitally fabricated sinusoidal grids using synthetic digital manipulation. Finally, the experimental performance of the powder‐based grid is assessed in both uniaxial tension on a poly(methyl methacrylate) specimen with a stress concentrator and fracture of stereolithographically printed photopolymer specimens. Synthetic testing shows that the powder grids are of comparable accuracy to lithography grids, but additional processing variability leads to strain errors of about 3% in powder grids compared with the 2.1% seen in lithography grids. In experiments, the powder‐based grids produce deformation patterns comparable with those expected from analytical results, and fracture toughness values for homogeneous stereolithographically printed specimens are uniform and consistent with expected values.     

Optimum location of a three strain gauge rosette for measuring mixed mode stress intensity factors

This paper analyzes the errors inherent to the determination of mixed mode stress intensity factors from data obtained by using a three strain gauge rosette. The analysis shows that the errors are mainly due the third characteristic value (3/2) and its corresponding coefficients. It is also shown that the errors do not depend on the orientation angle of the rosette, the angle between the strain gauges and the material properties. The error mainly depends on its location (radius, angle), being linear in the radius. For pure mode I, an angle of 90° will completely eliminate the error due to the angle, while for pure mode II, a 0° angle will minimize it. The normalized variation of the errors with the angle at any radius is shown for different ratios of the corresponding coefficients of the third characteristic value. The analytical results are applied to a numerical example of an edge crack subjected to mixed mode loading. From the numerical example, it is recommended to use two strain gauge rosettes at the same angle, and linearly extrapolate their results, if errors less than 15% for a mixed mode field are desired.     

An assessment of a frictional strain gauge probe for dynamic strain measurement

A probe for the measurement of surface strains on dynamically loaded structures has recently been tested at the National Engineering Laboratory (NEL). The procedure for estimating the amplitude of the conditioned signals from the frictional strain gauge attached to the probe is described and the usefulness of the technique is discussed. Calibration data are presented from comparative tests of the performance of the frictional gauge and bonded gauges on an axially loaded testpiece subjected to sinusoidal and random loading conditions.     

Experimental evaluation of a method for separating axial and bending stresses from strain gauge measurements taken on one surface by three-element rosettes

A method is described and results presented for the separation of axial and bending components of stress taken on one surface of a structure with a two-tier, three-element rosette assembly.     

一种以AOTF为核心的农药荧光检测系统

提出了一种将 AOTF 技术用于农药含量检测的荧光光谱系统。该系统分别采用以α-Quartz,TeO2 晶体作为声光介质的激发和发射色散元件 AOTF,以高压短弧氙灯为激发光源,利用光纤探测并传输荧光,由数据采集卡进行数据接收和 A/D 转换。该系统通过声光调制产生单色光,由超声射频的变化实现光谱扫描,没有机械调谐部件,故波长切换快、结构简单。西维因农药的荧光特性测量结果表明,系统的荧光波长范围为 500-750nm, 光谱分辨率为 10nm。当激发光波长为320nm,荧光波长为 647nm,西维因溶液浓度为 0.0-120.0μg/L 时,系统具有较好的线性关系,线性相关系数 r 为 0.9991。它可用于对粮食作物中的农药残留的检测和荧光光谱分析。     

A constitutive model for metals over a large range of strain rates Identification for mild-steel and aluminium sheets

A phenomenological model is presented to describe the mechanical behaviour for metals and alloys over a large range of strain rates. It is an elasto-plastic model with a strain rate and temperature-sensitive yield stress. This model partially relies on physical considerations and is specially developed for an easy application in explicit finite element method (FEM) codes. The process for identifying the constants from experimental data is presented, taking into account the exact testing conditions such as temperature increase and strain rate variations during the loading. Applying the model to data obtained for mild steel and commercial aluminium sheets yields satisfactory results.     

高速旋转物体的频闪结构光三维面形测量系统

提出了一个用于测量高速旋转物体三维面形的频闪结构光照明系统。系统自动跟踪检测旋转物体的转动信息,再用该信息同步控制光源发光和摄像系统工作,记录下旋转物体在闪光时刻的瞬间“静止”图像。使用该系统产生持续时间为424s的同步频闪结构光,对转速为每分钟1080转的家用电风扇旋转叶片三维面形进行测量,证明该系统能够准确地获得旋转叶片的瞬间静止图像,便于重建旋转叶片每个瞬间时刻的三维面形。同时,该系统还具备人工设定光源发光频率的功能,可以拓宽应用到无重复特征信号的高速运动物体三维面形测量。     

A Hybrid High-Order method for finite elastoplastic deformations within a logarithmic strain framework

We devise and evaluate numerically a Hybrid High-Order (HHO) method for finite plasticity within a logarithmic strain framework. The HHO method uses as discrete unknowns piecewise polynomials of order k ≥ 1 on the mesh skeleton, together with cell-based polynomials that can be eliminated locally by static condensation. The HHO method leads to a primal formulation, supports polyhedral meshes with nonmatching interfaces, and is free of volumetric locking. In addition, the integration of the behavior law is performed only at cell-based quadrature nodes, and the tangent matrix in Newton's method is symmetric. Moreover, the principle of virtual work is satisfied locally with equilibrated tractions. Various two- and three-dimensional benchmarks are presented, as well as comparisons against known solutions obtained with an industrial software using conforming and mixed finite elements.     

A new method for measurement of three dimensional residual stresses in a multi-pass butt welded joint

In this paper, a new method for the measurement of three dimensional residual stresses in a multi-pass butt welded joint is presented. The method involves the measurement of strain changes at a through hole surface as a coupon of material with the hole is separated from the body of interest and split. The finite element calculations are used to relate the strain changes at the measurenent points to the initial residual stresses at the hole location. The accuracy of the method is demonstrated by using it to measure a known stress field in a bent beam. The method has been used to measure the through thickness distribution of three dimensional residual stresses in a multi-pass butt welded joint. It could also be applied to the measurement of residual stresses in other welded joints.     

利用机械结构的振动加速度信号反算应变的新方法

针对结构紧凑的机械结构难以采用布置应变计的方法来测量应变的问题,从动力学角度出发,采用虚拟力技术建立了实际作用力的等效力。通过进行虚拟测点的计算,以解决实际测点不足的难题。提出了加速度响应信号与输入载荷的协调方程,利用运行状态下机械结构的加速度响应信号来反算结构应变的方法。并通过工程实例得以验证。研究结果对结构紧凑、不利于进行应变测试的部位,可采用该方法进行应变数据的分析,为进行疲劳寿命分析奠定基础。     

A finite element modelling study of strain localization in the vicinity of near-surface cavities as a cause of subsurface fatigue crack initiation

Recently, several observations of subsurface fatigue crack initiation in the vicinity of cavities in cast and in powder metallurgical (PM) alloys have been reported. In an attempt to explain these rather unexpected observations, an FE (finite element) modelling study of the stress and strain fields around spherical cavities of radius r lying at different distances below the surface of a tensile-stressed material has been performed. The results show that, due to the local stress enhancement in the `bridge' of material between the cavity and the surface, an increasingly severe strain localization develops, as the distance d between the centre of the cavity and the free surface decreases and as the ratio r/(r + d) approaches 0.5 for a cavity located just underneath the surface (r = d). It is concluded that, under conditions of cyclic stressing, the strain localization in the immediate near-surface vicinity of a cavity lying just beneath the surface can be severe enough to cause the initiation of subsurface fatigue cracks. Depending on the detailed situation, these fatigue cracks can be initiated at the pore or at the adjacent outer surface.