Strain gauge reinforcement of plastics

A major disadvantage in strain gauge work on plastics is the effect of strain gauge reinforcement, which induces signifcant errors in the quantitative evaluation of strain derived from the plastic. The parameters influencing reinforcement are well documented and procedures have been developed that permit the identification and correction of this effect in certain situations. The principal causes, effects and corrective procedures are reviewed and where appropriate compared with results obtained by the authors of this article.     

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.     

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.     

Advances in Strain Gauge Measurement on Composite Materials

Abstract: This article gives an overview on the application of strain gauge techniques to the analysis of the strains in composite materials. The orthotropic behaviour of the composite influences the performance of strain gauges that are calibrated for use on isotropic materials. The article considers therefore the typical topics of the strain gauge technology applied to composites with particular reference to the compensation of thermal output, the measurement of the coefficients of thermal expansion, the determination of the strain and stress state, the influence of the misalignment error, the reinforcement effect, the determination of the stress intensification factor, the analysis of residual stresses by the hole drilling method and the effect of transverse sensitivity on the measurement of strains along the fibres.     

Contact-free measurement and numerical and analytical evaluation of the strain distribution in a wood-FRP lap-joint

Wood specimens to each of which a laminate of carbon fibre reinforcement polymers (FRP) was glued (creating a lap joint in each case) were loaded to failure. A total of 15 specimens of three types differing in the glued length (anchorage length) of the FRP laminate (50, 150 and 250 mm respectively) were tested, their strength, stiffness and strain distribution being evaluated. Synchronized digital cameras (charge-coupled devices) used in testing enabled strain fields on surfaces they were directed at during the loading procedure to be measured. These results were also evaluated both analytically on the basis of generalized Volkersen theory and numerically by use of the finite element method. The lap joints showed a high level of stiffness as compared with mechanical joints. A high degree of accuracy in the evaluation of stiffness was achieved through the use of the contact-free evaluation system. The load-bearing capacity of joints of this type was found to be dependent upon the anchorage length in a non-linear fashion. The experimental, analytical and numerical results were shown to be in close agreement with respect to the strength and the strain distribution obtained.     

孔口缝合补强对含孔层板应变集中影响的实验研究

对复合材料开口缝合补强结构进行了实验研究。测试了不同缝合参数(针距、行距、边距、单重和双重缝合) 补强的含孔复合材料层板的拉伸强度, 研究了孔边应力集中、应变集中对强度、刚度等力学性能参数的影响, 分析讨论了孔边及邻近区域应变集中及应变分布的规律, 得到不同缝合参数、孔边不同位置以及不同载荷条件下的应变集中系数。结合实验结果和分析讨论, 给出了含孔复合材料层板缝合补强的缝合设计参数。      

Effects of migrating inhibitors on corrosion of reinforcing steel covered with repair mortar

The present study concerns the investigation of the protective effect of migrating corrosion inhibitors, against rebar corrosion, in specimens of repair fiber reinforcement mortars, in relation to these containing corrosion inhibitors. The strain gauge (SG) measurement technique was used for fast corrosion measurements, under anodic potential application in corrosive environment of 3.5% wt NaCl solution. The corrosion protection effectiveness was also tested by galvanic current measurements between different categories of specimens in order to estimate the galvanic corrosion between repaired and non-repaired areas. According to these investigations the positive effect of the migrating corrosion inhibitors in fiber-reinforced mortars against corrosion of the rebars is evident, providing a prevention of steel rebars' corrosion in the cases of repairing concrete structures. The presence of fiber reinforcement in repair mortar specimens prevents the microcracking effects, resulting in decrease of concrete cracking due to reinforcement corrosion.     

UTILIZATION OF STRAIN GAUGES OVER LONG PERIODS OF TIME AT ELEVATED TEMPERATURES

Tests are described for the evaluation of a particular strain gauge/cement combination under steady loading at elevated temperatures over long periods of time. Although the strain gauges showed a relaxation of their reading with time, due to creep of the gauge backing and cement, it proved possible to correct for this effect to within the scatter associated with the specimens and the instrumentation. It is concluded that application of such corrections to other strain gauge/cement combinations should be possible.     

Mechanism and behavior of fiber-reinforced asphalt mastic at high temperature

Abstract

The rheological behaviour and reinforcement mechanism of asphalt mastic mixed with fibres at high temperature were investigated in this study. Fibres, including basalt, polyester and glass, were added to asphalt mastic. Repeated creep and multi-stress creep tests were conducted to characterise the high-temperature properties of the mastic, and numerical simulation was performed with ABAQUS software to analyse the reinforcement effect of fibres. Test results indicate that the fibres have excellent reinforced performance; for example, the accumulated strain and its change rate decrease, and its creep stiffness modulus increases after the fibres are mixed into the mastic. The creep recovery rate increases, and its creep residual value decreases at a high stress level. The creep stiffness modulus under different loading cycles can be expressed by a power function. Numerical simulation shows that the fibres effectively absorb mastic stress; hence, creep strain in the mastics decreases. The Burgers model was utilised to present the rheological behaviours of mastics with fibres; the model parameters were estimated.     

试样的形状公差对应变测定的影响

就形状公差对拉伸应变测定的影响做了理论分析。引入等效标距的概念，建立与等效标距、名义标距、应变硬化指数和形状公差相关的方程式。利用这些公式可以定量地描述形状公差的试样对拉伸应变测定的影响。以国家标准和国际标准中各自规定的形状公差为例，说明定量评定形状公差对应变测定误差的影响。根据应变误差的计算结果，可知国际标准允许的形状公差比国家标准宽得多。     

深基坑坑底加固体的刚度效应研究

基于平面数值模拟方法对坑底加固体的刚度效应进行了探讨.研究发现,随着坑底加固体刚度的增加,整个基坑的地面沉降、坑底隆起、支护桩水平向位移等减小,支护桩正负弯矩趋向对称,且弯矩绝对值基本无增加,整个基坑的整体稳定性安全系数提高,说明坑底加固体刚度的提高能增加基坑的稳定性;当只加固坑底宽度范围的一半时,另一半未加固区的坑底...     

The electrical resistance response of continuous carbon fibre composite laminates to mechanical strain

Measurements have been made of the effect of mechanical strain on potential distributions and resistance of unidirectional and multidirectional carbon fibre epoxy laminates. The effects of current flow direction and technique for current introduction on piezo-resistance have been studied. It was found that uniform current introduction at sample edges produced by sputtered Au–Cr contacts across the entire cross-section produced consistently low values of gauge factor of 1.75 for current flow parallel to the fibres and 2.7 for transverse current flow. Non-uniform current introduction, produced variously by local point introduction of current, or use of viscous adhesives producing intermittent contact, resulted in a wide range of apparent gauge factors ranging from 20.6 to −89. These anomalous values may be explained by a model in which the high anisotropy of resistance in unidirectional CFRP maintains initial non-uniform current throughout the sample. Under mechanical strain points of fibre contact will change, altering the distribution of current carrying fibres and leading to local changes in current. Thus changes in potential difference between two points produced by mechanical strain will not be exclusively caused by changes in local resistance. The presence of transverse plies in multidirectional laminates ensures that in plane non-uniform current distributions are largely eliminated, and the effect on piezo-resistance of non-uniform current introduction is minimised.     

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.     

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.     

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.     

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.     

Stiffness of particulate reinforced metal matrix composites with damaged reinforcements

Abstract

During tensile plastic deformation particulate reinforced metal matrix composites (MMCs) undergo reinforcement damage and a parallel reduction in stiffness. An analytical model is developed to calculate this stiffness reduction using the equivalent inclusion technique proposed by Eshelby. The model considers both damaged and undamaged reinforcement particles as ellipsoidal inclusions but with different stiffness tensors. The effect of the aspect ratio of the reinforcing particles has been accounted for in the model. The model is very flexible and can meet different specific damage situations by designing a suitable stiffness tensor for the damaged reinforcements. Finite element analysis is used to modify a numerical stiffness tensor for cracked reinforcement particles. The model is compared with an earlier model of modulus reduction in MMC materials and with a few experimental measurements made on a 15 vol.-%SiC particulate reinforced aluminium alloy 2618 MMC.     

铝合金梁受低速撞击动态响应的试验研究

梁受横向撞击时,其撞击力、局部变形和整体变形等动态响应受到撞击体质量、撞击速度、撞击位置以及梁刚度特性等因素的影响,研究这些参数对动力响应以及对撞击局部效应和整体效应的影响规律,是合理提出抗撞击防护手段的基础。该文利用低速落锤冲击试验装置进行了铝合金梁横向撞击试验,着重研究不同撞击速度、撞击位置和试件刚度等因素对铝合金梁所受撞击力、整体与局部变形、应变与能量等的影响规律。实验结果表明:随着撞击速度的增加,试件吸收能量的速率增大,撞击力幅值不断增加,撞击荷载主峰值段的持续时间减小;试件的整体变形和局部变形都明显增大,但局部效应相对增加更为显著;随着试件刚度（厚度）的增加,试件受相同速度撞击的吸收能量速率增大,撞击力荷载峰值显著增加;试件整体变形减小,而局部变形增加,说明试件刚度越大,其撞击整体效应减弱,而局部效应增强;试件受撞击后整体变形与撞击力相比明显具有滞后性,表明整体效应滞后于局部效应。     

The reinforcement role of carbon nanotubes in epoxy composites with different matrix stiffness

This paper systemically evaluates the different reinforcement roles of carbon nanotubes in those nanocomposites with different matrix stiffness while the curing process is controlled. Both mechanical test and microscope observation indicated that such reinforcement would gradually reduce while increasing the stiffness of matrix. However in the soft and ductile composites, carbon nanotubes show a significant reinforcement without fracture strain decreasing. The interface interaction is poor between carbon nanotubes and matrix in the stiff composite, and therefore, they have little contribution to the mechanical properties of composite. This research may help to propose a further positive solution for designing and fabricating carbon nanotube–epoxy nanocomposites.