恶劣环境中应变测量的国内外发展综述

机器部件、构件和微电子元件,很多是在恶劣环境(包括高温、高压、低温、磁场、辐射、等离子体、腐蚀环境、高速旋转、液下等)中工作的。它们的力学破坏,对人们的安全和生产有非常不利的影响。为防止这些破坏,需要通过分析、实验和计算来改进设计、制造和检测,以提供减少破坏所必要的数据。实验力学,特别是在恶劣环境中进行力学测量,在此起了关键的作用。它提供了基本数据和检测手段,用于计算和建立破坏理论模型。本文介绍在恶劣环境中,主要在高温中,进行应变测量的国内外一些发展情况。     

Measurement of surface strain rates in glaciers using embedded wire strain gages

Natural scientists and engineers are continuing to seek an understanding of the mechanism of flow and deformation of glaciers. A necessary component of this exploration is the accurate determination of strain rates in glacier ice. The purpose of this investigation was to develop a strain-measuring method which is dependable and precise under difficult field conditions. The measuring technique which was developed uses unbonded electrical-resistance strain gages which consist of single strands of 5-mil Constantan wire 10-ft long. Six gages are embedded in the glacier-ice surface in the form of two delta rosettes in order to obtain strain at a point with some redundancy of data in this two-dimensional problem. The rigid-body rotation of the gage anchor posts was measured by sensitive inclinometers in order to assess the effect of pressure melting on the strain data. The data are interpreted using cross-correlation and best-fit programs to yield maximum shear-strain rate and average normal-strain rate. Strain readings were conducted over a period of eight days on the Ptarmigan Glacier near Juneau, Alaska. The maximum shear-strain rate at the surface ranged from 0.25 to 1.2×10^?6/hr., which agrees with estimates derived from known flow rates. The wire gages were found to adhere to the ice well enough to make the gage anchor posts unnecessary—pressure melting is therefore insignificant. A tolerance of ±6.0 microstrain was determined for the strain gages.     

Evaluation of high-elongation foil strain gages for measuring cyclic plastic strains

The behavior of a certain type of high-elongation foil strain gages (l/32-in. gage length) was checked against the indications of a clip-on extensometer under conditions of cyclic plastic strain (strain range 0.5 percent to 2.8 percent). The gages exhibited limited capability of measuring cyclic plastic strains. Transverse and axial strain measurements by means of the gages enabled determination of Poisson's ratio for elastic and plastic conditions. Results are tabulated and discussed.     

Use of strain gages for uquipment-reliability improvement in the electric-utility industry

This applications paper describes the utilization of strain gages by the Detroit Edison Company in defining and solving major power-plant-equipment problems that had an adverse effect on the availability of the Company's large coal-fired generating units. Case histories of the use of strain gages for troublesome problems involving boiler tubes, air preheaters, stack liners and fans are presented. The role of the strain gage in helping to define the source of the problem and devleop a timely and cost-effective solution is highlighted.     

A highly sensitive noncontacting electromagnetic device for detecting dynamic stress in structures

A highly sensitive, noncontacting electromagnetic device has been developed to detect stress waves in structures. It is shown that for detecting an induced strain this device is over 500 times more sensitive than conventional bonded strain gages. The principle of detecting the strain by this device is based on the fact that dynamic stresses in a structure induce similar stresses in a bonded piezoelectric material. This, in turn, creates a magnetic field which extends beyond the material itself. An electromagnetic device has been built to detect this magnetic field and thus monitor the dynamic stresses. This method provides a noncontacting means of measuring strain in structures with improved sensitivity.     

Strain gages in cryogenic environment

The increasing use of cryogenics in all fields of endeavor necessitates the use of strain gages for stress analysis of structures under extreme temperatures. This has been a continuing program at the McDonnell Douglas Astronautics Co.'s Strain Gage Engineering Laboratory (Huntington Beach facility). Many characteristics of strain gages were investigated, and a majority of the commercial strain gage types were evaluated. Tests results are presented, as well as composite curves, showing the comparison between various types of strain gages.     

Experimental stress analysis in massive offshore structures

Immense steel and concrete platforms are being used for offshore oil production. Loads in these fixed-rigs, in floating-rig components and in production pipelines are being measured with strain gages and transducers. Certain of these offshore structures and typical testing which has been performed are discussed, including some using environmentally protected weldable strain gages.     

Evaluation of fatigue strenght of welded structures by local strain measurements

The object of the paper is the experimental evaluation of the local strain conditions near the fatigue-crack starting zones in welded structures, at the toe of weld joints. These zones are characterized by steep gradients for strain and stress. The evaluation is performed using strain gages and finite-element techniques on real cruciform weld joints. The results illustrate the possibilities and the limitations posed by the use of local strain measurements to assess the fatigue strength of welded structures. These factors are usually estimated on the basis of a nominal stress approach.Paper was presented at 1983 SESA Spring Meeting held in Cleveland, OH on May 15–19, 1983.     

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.     

On the measurement of strain in the hostile environment of high-pressure water (80 MPa)

The measurement of strain on a structural member in the presence of high-pressure water environment requires waterproofing of the gages as well as understanding of the pressure effect on the foil gages. Two new waterproofing methods are developed to protect the gage under these extremely high pressures. This study has resulted in a new compensating method which eliminates the effects of temperature and pressure simultaneously. The waterproofing materials studied during this investigation are superior to other earlier techniques reported in literature since they provide protection up to 11,500 psi (80 MPa) while previously existing techniques failed before this pressure. Also, these studies on the pressure effect deal with the realistic situation of coated strain gages in a water environment, as compared to previous studies which dealt with uncoated strain gages in an oil environment.     

Use of electrical-resistance strain elements in three-dimensional stress analysis

A method for constructing resistance-wire strain gages which may be imbedded in a plastic model without materially altering the stress pattern in the model is presented. The methods used to calibrate the gages and the photoelastic tests made to investigate the effect of the gages on the stress pattern are described. The application of this new three-dimensional technique to evaluate the stress distribution in a thick-walled cylindrical pressure vessel is discussed. Correlation between experimental data and calculated values is given.     

Experimental investigation of the fatigue behavior of thread-type projections

In this paper the effect of flank angle, root radius and pitch on the fatigue life of thread-type projections is determined. Projections simulating threads with flank angles of 7, 20 and 30 deg, root radii ranging from 0.030 to 0.078 in. and pitches of 1/4, 3/8 and 1/2 in. were tested. Predictions of maximum fillet stresses were made by Heywood's method for correlation with measurements taken with miniature strain gages in the specimen thread fillets. A graph of the percent increase in load vs. percent decrease in life for the 3/8 in. pitch specimens is included.     

Enhanced measurement of strain distributions

A theoretical approach is presented that uses multiple strain gages to accurately measure complicated strain distributions. The technique is based on the method of weighted residuals in conjunction with measured strain data and is applicable for arbitrary in-plane strain distributions. Conventional measurements using strain gages are shown to represent a particular case of the approach presented. The experimental characterization of unidimensional strain fields is discussed in detail. Two approaches are presented; these are based on linear and quadratic approximations of the strain field. The strain distribution for two important practical problems is evaluated assuming ideal conditions to assess the performance of the proposed approach. In both cases, the simulated results demonstrate that measurement error resulting from the finite size of a strain gage may be reduced. That is, a larger strain gage may be used for a given maximum admissible error. The method also allows a minimal error of measured nonlinear strains.     

An electromagnetic shear-stress gage for large-amplitude shear waves

In many materials, especially plastics, ceramics and rocks, large-amplitude shear-wave propagation studies could provide valuable information for the development of constitutive equations. A newly developed electromagnetic-gage configuration provides an output voltage which is directly related to the dynamic shear stress in the material. The electromagnetic shear-stress gage has been used to make direct measurements of shear-wave stresses in PMMA and Solenhofen limestone. Large-amplitude shear waves were obtained with a new plate-impact technique which generates shear waves by a controlled-reflection process. The configuration of the stress gage permits it to be used simultaneously with more conventional electromagnetic velocity gages, thus providing both types of data in one experiment.     

Progress in high-temperature and radiation-resistant strain-gage development

This paper covers the performance characteristics of bonded resistance strain gages in high-temperature and nuclear-radiation environments. Typical electrical properties of ceramic-bonded strain gages are given for various environmental conditions. The methods used to minimize errors induced by the extreme environments are fully discussed.     

A polymeric split Hopkinson pressure bar instrumented with velocity gages

Polymeric split Hopkinson pressure bars are often used to test low-impedance materials at elevated strain rates. However, they tend to be viscoelastic, and a viscoelastic wave propagation model is required to analyze the data. This considerably complicates the analysis over the more common linear elastic split Hopkinson bar. In this research, a polymeric split Hopkinson bar is instrumented with electromagnetic velocity gages. The gages are placed at the interfaces between the bars and the specimen. By using this arrangement, viscoelastic effects in the bars are negligible and the need for a viscoelastic correction is eliminated. The method is applied by testing low-density foams.     

Measurement of static strain at 2,000° F

This paper describes the work being done at Hughes Aircraft Company using capacitance strain gages to measure static strains at temperatures up to and including 2,000° F. A new configuration of the capacitance gage is described with a gage length of 0.50 in. Methods of attaching the gage have been developed for welding, bonding and flame-spray installations. Instrumentation procedures and methods are defined. Statistical accuracy and test results are discussed on the results obtained from a lot of 60 gages. Results discussed were obtained on Rene 41, titanium and L605 stainless steel. Data analysis on the gage includes gage factor, gage factor vs. temperature, apparent strain, linearity, hysteresis, temperature effects, drift, and zero shift.     

High-temperature strain gages for use in sodium environments

High-temperature long-term static strain measurements have, for many years, been a problem to the experimental stress analyst. Strain gages operating in environments from 600–1000° F over extended periods of time have varying characteristics as to temperature compensation, gage-factor shifts and, also, drift. The weldable-type high-temperature gage offers a great deal of promise, considering the ease of application to nonlaboratory-type structures. In addition, the Microdot-type weldable gage offers additional protection against most environmental conditions. In the high-temperature range, the Microdot half bridge containing a platinum-tungsten alloy encased in magnesium oxide and protected by an outer shell of Inconel-X was evaluated. One of two major difficulties encountered with this gage is the silver-gold alloy used for brazing all of the connections that provide a sealed gage element. This alloy was a difficulty because the gages were to be used in a sodium environment which easily dissolves the silver-gold elements. The other difficulty is the highly individual characteristics of the gages. This paper describes two significant developments made in adapting this gage for use in sodium environments at high temperatures. These are:

1. 1.
   A calibrating technique whereby the bonded-gage characteristics of each gage can be obtained prior to actually bonding the gage to the structure under test     
   
   

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.