The application of strip strain gages for measuring residual surface stresses in beryllium

Evaluation and application of tiny, 0.031-in. gage length, constantan foil-strip strain gages for measuring residual surface stresses in beryllium parts are discussed. The strain gages were applied to one side of the beryllium parts at locations known to have high residual surface stresses. Small coupons were machined from the parts and the opposite side of the coupon was chemically milled with a 10 to 20-percent hydrochloric-acid solution. The strain-gage installation was protected with micro-crystalline wax. The etching process required approximately 20 hr at 75°F. Because of their high stability, glass-fiber-reinforced epoxyphenolic strain gages were used rather than the more readily available polyimide-backed strain gages. Details of the strain-gage installation are presented.     

Measuring static strains at high temperatures

A recently introduced strain gage is described. It depends on the variation in electrical capacitance brought about by the deflection of an arched strip when the separation of its feet is altered. The gage has a wide range (1-percent strain) and good stability even in the 1100–1200°F range. With the design adopted, the output when a gage is heated but not stressed can be controlled and reduced. Using the techniques described, problems of connections for field work are not severe and remote readings can be made of the small capacitances involved, which are about 0.5 pF. Several trials are in progress. In one installation, automatic logging from scores of gages has proved successful. The gage has particular attractions for monitoring the formation of cracks in structures working at high temperatures.     

Stress and failure analysis of a glass-epoxy composite plate with a circular hole

Experimental and finite-element analyses are presented for the anisotropic states of stress, strain and fracture of a glass-epoxy plate containing a circular hole and subjected to uniaxial tension. Strains were experimentally measured using foil gages, moiré and birefringent coating. Stresses are computed in the linear range from the measured strains. While the hole reduces the plate strength by a factor of two, the maximum tensile strain at fracture is greater than the ultimate strain in a plate without a hole. Fracture consists of crack initiation at the hole boundary but off the horizontal axis. Away from the hole, failure is accompanied by considerable delamination. Discontinuous crack propagation is present.     

The evaluation of a strain gage having long-time stability characteristics

A standardizing strain gage that meets the requirements for long-time stability of strain measurements is described in this paper. Statistical analyses of the strain calibration results taken over a period of 13 months showed no significant differences between successive calibrations at a 5-percent significance level. From the results of the strain calibrations of 134 gages, the resistance-deformation characteristicdR/dW had a mean of 120.31 ohms/in. with a coefficient of variation of 1.43 percent. Evaluation tests using 6×12 in. concrete cylinders compared the strains measured by the standardizing gage to those measured by the Berry gage. Statistical analyses of these results showed equivalent accuracy of strain measured by the standardizing strain gage at comparable precision.     

Effects of pressure on small foil strain gages

This paper describes the behavior of small foil strain gages under high pressure. Effects of pressure were determined and calibration curves were established in prelininary experiments. The calibrations were then used for correcting measured strains in pressure vessels. Preliminary experiments at room temperature were conducted on small foil strain gages for pressures up to 35,000 psi. The effects of pressure on the gages bonded with a cynoacrylate contact cement, a room-temperature epoxy cement, a high-temperature epoxy cement and a filled epoxy resin were evaluated. Because the contact cement was least affected by pressure and was easiest to apply, it was chosen for use in successive experiments with different gage installations. Calibration curves were determined for strain gages of 0.031-, 0.062- and 0.125-in. gage lengths. The compensating gages were under atmospheric pressure. The calibrations included the pressure effects of gages bonded on both concave and convex surfaces, and the effect of tensile prestrains. Data could be duplicated for successive pressure tests and for several gage installations. The calibration curves proved to be an effective way for obtaining accurate readings from the foil strain gages bonded internally to a pressure vessel.     

复合材料面板夹杂粘弹性阻尼材料结构的弯曲实验分析

对单层板的轴向、横向拉伸强度及粘弹性材料的拉伸强度进行测定以确定其弹性模量及泊松比。为了同理论结果比较,制作了专用夹具,用于测量复合材料方板的实验数据。该实验装置模拟四边固支边界条件,用于测量复合材料夹杂粘弹性阻尼材料结构受均布载荷时的静态挠度和应变,并通过转换公式得到复合结构下表面中点的正应力。实验结果同理论分析基本吻合,其误差主要由单层板与粘弹性阻尼材料的粘结剂厚度引起。     

简支梁振台的理论解及位移测试

利用由机械振动理论推导出的等截面简支梁的理论解,推导出了带集中质量的简支梁振动台的理论解,并通过分析动态条件下简支梁的位移与应变之间的关系,证明了:在动态条件下梁的位移与应变之间存在很好的正比关系。利用梁的位移与应变之间正比关系,如果在梁上适当位置粘贴应变片,通过测试应变就能准确地测得梁的动态位移。本文通过实例证明了该方法确实可行,理论解与实测结果非常吻合。这为简支梁的动态测试提供了有效而简便的方法。     

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.     

Temperature and rise-time effects on dynamic strain measurement

Strain pulses in a test specimen were measured over a temperature range of ?73 to +149°C with foil and semiconductor strain gages. These tests were performed to determine if the rise time and amplitude of the gage output change as a function of temperature. The existence of a constant that should be added to the theoretical rise times of resistance strain gages, as suggested by Koshiro Oi, was reexamined. ‘Long’ rise-time strain pulses were produced in the test specimen by an impacting steel ball. The rise times of these pulses were on the order of 7 μs and the amplitudes were approximately 65 μm/m. The results of these tests show that the rise time and amplitude of the gage do not change as a function of temperature. ‘Short’ rise-time strain pulses of approximately 500 μm/m with a rise time of 2 μs were produced in a test specimen by a short pendulum-type hammer apparatus. The results of these tests showed that the amplitude of the gage output was relatively independent of test temperatures but exhibited a slight hysteresis effect. The rise times for these tests remained constant up to a temperature of 93°C, then started to increase. The rise times at 149°C were approximately 100 percent longer than at room temperature. Under optimum conditions, a pulse with a measured rise time of 0.18 μs could be generated. The results of these tests indicated that the theoretical rise-time additive constant of resistance strain gages is 0.05 μs or less. This is one-half the value that Bickle arrived at by reevaluating Oi's data. However, since the real rise time of the pulse was unknown, this additive constant is not necessarily a property of the gage.     

An evaluation of weldable strain gages in nuclear-reactor environments

The paper presents the results of an experiment to evaluate the performance of weldable strain gages (Microdot) in nuclear-reactor environments. The technique used to study the behavior of strain gages in an experimental reactor is described. Integral-lead weldable strain gages attached to constant-strain cantilever beams were installed in the core of NRX reactor. Presurized bellows were used to induce in the cantilever beams strains which were measured with the strain gages. After more than 200 days under irradiation in air at 70 to 100° C (estimated fast neutron dose: 0.19×10²⁰ n/cm²) the strain gages were still in satisfactory operating condition. Strains up to 1000 μin./in. were measured successfully. The measurements were repeated with accuracy. Although the total gage-resistance variation during the entire experiment was approximately 7.5 percent, the strain-gage sensitivity was practically not affected by irradiation. It was demonstrated that the gage-resistance variation can be successfully compensated. The average drift rate for an active and a compensating gage coupled in a half-bridge arrangement was below 1.5 μin./in./h. There was no indication of insulation-resistance degradation due to the effects of irradiation. The lowest resistance measurement was above 100 MΩ at 80°C. The effects of nuclear radiation on other strain-gage characteristics such as linearity, hysteresis, creep and signal noise were also investigated. It is concluded that weldable strain gages are very promising for nuclear applications.     

A Vibration Estimation Method for Wind Turbine Blades

This paper reports the development of a vibration monitoring system for wind turbine blades. This system is used to estimate the deflection at the tip blade on a wind turbine tower. Technical accidents of wind turbine blades have become increasingly common. Thus, regular monitoring of the blades is very important to prevent breakdowns, especially in cases when the blades begin to vibrate excessively. The monitoring system developed in this study satisfies two main objectives for practicality. First, our system is easy to install on existing wind turbines. Second, blade deflection is measured in real time. Our system can be operated using a few strain gages attached at the blade root, and the deflection is calculated based on the monitored stress. Thus, direct measurement of deflection at the blade tip is unnecessary. An estimation algorithm for this purpose is adopted based on the experimental modal analysis. This paper focuses on the evaluation of the estimation algorithm to investigate the feasibility of our system. Basic experiments were conducted using a simple blade model of a 300 W scaled wind turbine under rotation. Signals from the strain gages were acquired by a sensor network and sent to a computer through a wireless connection. The results show that the estimation accuracy is acceptably high. Therefore, we conclude that our proposed system is practical.     

Model analysis of box culverts subjected to highway loading

The strain distribution and the deflections of reinforced-concrete box culverts associated with highway loading are determined by testing scale models. Two types of scale models were constructured: 1/6-size concrete models and a 1/24-size photoelastic model. The concrete models were instrumented with electric-resistance strain gages, and the deflections were measured with dial indicators. Strain and deflection data due to live loading are compared with values from testing of prototypes. The results of testing indicate that box-culvert sections conforming to ASTM C 850 are overdesigned structurally. Testing of models of a redesigned box culvert indicates that they perform satisfactorily. Paper was presented in the Proceedings of the 1988 SEM Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

The accuracy of residual stress measurement by the hole-drilling method

This paper describes the verification of the accuracy of residual stress measurement by the hole-drilling method. The strain measurement is simulated by the use of the indirect fictitious-boundary integral method. As an example, a finite rectangular plate subjected to initial stress is treated, and a simulated measurement of the residual stress is made using the strain relieved during hole drilling. The accuracy of residual stress measurement is estimated by comparing the simulated measured residual stress with the actual residual stress, i.e., the given initial stress. The results are shown for various distances and angles of strain gages. Also, the influences of the eccentricity of the hole from the center of the strain gages and the effect of a boundary near the hole are examined.     

Dynamic response of strain gages up to 300 kHz

Dynamic response of foil strain gages is investigated in the ranges of up to approximately 300 kHz in frequency, 2000 in strain and 750 s^–1 in strain rate. Impulsive input is applied to the gages using elastic wave pulses in a circular bar. The input is determined by measuring the velocity of the bar end with a laser interferometer. It was confirmed that deviations from the static gage factors do not exceed–5 percent within the frequency range up to 45 kHz, 110 kHz and more than 290 kHz for 20-mm, 10-mm and 3-mm gages, respectively, and that the gage length is the dominant factor in the gages' frequency characteristics. This paper also mentions self-generated voltages.     

Anticlastic behavior of flat plates

The purpose of this paper is to show that the readings from strain gages can be used effectively to compute small transverse deflections in a rectangular plate and, further, show that the theory developed by Lamb for the rectangular-plate problem agrees with experiment. A numerical procedure is developed, based on the trapezoidal rule, which determines the transverse deflections from the readings of strain gages mounted to the top and bottom surface of a rectangular plate subjected to large longitudinal curvatures. It is shown using the strain-gage technique that experiment agrees with Lamb's theory forb ² /Rt ratios up to 50.     

Investigation of strain gages for use at cryogenic temperatures

Strain sensitivity and resistance changes in Advance, Karma, Budd Alloy, Nichrome V, and stabilized Armour D foil gages, and Nichrome V-platinum temperature-compensated gages were evaluated at cryogenic temperatures. The more promising gage types, as determined from these studies, were tested to strain levels up to 11,000 μin./in. with tensile specimens. The other performance characteristics that were investigated included zero drift, creep, hysteresis, linearity and gage element size effects. A mounting method developed for foil gages to be used at cryogenic temperatures is described.     

用能量法研究双模量大挠度圆板的轴对称弯曲

双模量圆板在外载荷作用下发生轴对称弯曲变形时,会形成各向同性的拉伸区和压缩区.此种情况下,可把双模量圆板看成两种各向同性材料组成的层合板,采用弹性理论建立了双模量圆板在外载荷作用下的静力平衡方程,利用静力平衡方程确定了双模量圆板的中性面位置.在此基础上,采用能量法研究了双模量大挠度圆板轴对称弯曲变形问题,将该方法计算结...     

大理岩的高应变率动态劈裂实验

首次把平台巴西圆盘试样引入动态劈裂试验。利用直径100mm的分离式Hopkinson杆对大理岩巴西圆盘和平台巴西圆盘试样进行了动态劈裂实验。结合有限元分析,得到了大理岩的动态劈裂破坏的拉伸强度。分析了巴西圆盘和平台巴西圆盘的典型破坏方式。结果表明,大理岩的动态拉伸强度随着应变率的提高而增加。利用圆盘中心粘贴的应变片来测大理岩等脆性材料的动态拉伸强度,是一种简便高效的试验方法。和巴西圆盘相比,平台巴西圆盘具有更大的优越性和更好的测量效果。     

Elastoplastic analysis of a bent circular plate with a central hole

A circular aluminum plate with a small concentric hole (1/10 the plate thickness) and supported on its outer edge by a ring was subjected to a concentrated load at its center, applied through a rigid ball of radius equal to the plate thickness. Strains were determined using grids, moiré, and electrical strain gages on the top and bottom surfaces of the plate for loads up to and including the one associated with the appearance of the first crack in the plate. The investigation is related to the development of specimens to be used to determine fracture characteristics of materials used in lightweight construction.