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.     

Development and design of new methods of measurement of state of strain of structures

The present paper deals with development and design of new methods utilizing Wiedemann's effect for determination of state of strain in building structures. Wiedemann's effect and some features of torsional strain of magnetic field are the basis of new experimental method. Especially the point electromagnetic strain gages using the effect of pure torsion of electromagnetic field to enable universal examination. For strain-gage measurements, almost all physical quantities are used which can be related to the variation in length of the structures. From the electric strain measurements, the most commonly used methods are the measurements by resonance-wire strain gages or by electric-resistance strain gages. In this paper, electromagnetic strain gages are discussed using the Wiedemann effect, and the author describes some new measuring equipment and his own suggestions and methods based on an application of this effect.     

Strain-gage techniques in biomechanics

In order to successfully use strain gages to monitor the mechanical response of biological systems to mechanical inputs, new techniques have been developed. Special procedures for wiring, bonding and waterpooofing strain gages are illustrated. These may change with the type of biological specimen being studied—living or dead—the length of time the gages must function, and the depth of the desired point of application below the surface of the body. Strain gages have been successfully applied deep into the thoracic cavity of a cadaver and have remained operable for a month which is well beyond the required time. Gages applied to the rib cage and the facial bones have been used over a two-month period with no loss in function. For animal tests, applications of up to three weeks are possible with longer periods available with more involved techniques. Strain gages have a distinct advantage over any other type of measuring device which might be employed in that a correlation between static and dynamic results can be determined; this makes it a most useful and valuable tool for the investigator in biomechanics.     

A ten-inch extensometer measuring small low-frequency strains

A strain gage is described for the measurement of small low-frequency dynamic strains in concrete structures. It is in the form of a demountable extensometer, and its sensitivity is attained by a combination of mechanical amplification, electronic amplification, the use of semiconductor gages and the adoption of suitable filters. The gage has been successfully used for measuring strain in the range 0.1–5.0 microstrain at about 3 Hz on a 10-in, gage length.     

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.     

MONITORING OF STRAINS IN SMALL DIAMETER PIPE

New equipment has been developed for the surface preparation and installation of strain gages to the interior walls of small-diameter pipe. This procedure can be used for many types of pipe to obtain valuable strain information not attainable by other methods. A more complete understanding of exterior and interior strain behavior can be used to answer many of the unknowns concerning plastic pipe performance and to improve the design procedures for this type of pipe.     

Doubly Reinforcing Cementitious Beams with Instrumented Hollow Carbon Fiber Tendons

Surface mounted strain gages are used to characterize the behavior of polymer-enhanced cementitious beams designed to withstand reverse loadings. These unique composite structures are doubly reinforced with hollow carbon fiber (graphite) tendons equipped with strain gages and the study includes section design, materials considerations, structural testing, and finite element analysis. The primary purpose of strain gage integration is to insure that the stress in the materials remains within the elastic range so that damage does not occur. A finite element model is developed to characterize the structural response in the elastic range and a hybrid approach is suggested in which displacement, strain, and stress can be obtained with a single strain gage. The ability to characterize structural performance beyond the elastic range is also demonstrated by analyzing data obtained from displacement-controlled tests.     

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.     

Low-cycle fatigue-crack indications by strain gages operating in elastic strain fields

This paper describes the response of bonded electrical-resistance strain gages to low-cycle fatigue cracking of four, cyclically pressurized, boxlike structures. The strain-pressure “signatures” of gages installed on flexural parts of the structures were recorded semicontinuously. By observing the changes in signatures of certain gages, it was possible to follow indirectly the initiation and propagation of internal low-cycle fatigue cracks. The described technique appears feasible as a crack-detection system, or as an adjunct to some other type of damage-warning system. It is suggested that the sensitivity and accuracy of this technique make it possible to substitute “crack initiation” for “through cracking” as a criterion for fatigue failure in design.     

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.     

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.     

混凝土层裂强度测量的新方法

提出了利用Hopkionson压杆测量混凝土层裂强度的新实验方法:用高聚物材料取代传统的金属材料透射杆,混凝土试件为细长杆,由于高聚物波阻抗比混凝土小,试件中压缩波在试件/吸收杆界面反射后形成拉伸波使试件产生层裂破坏,通过吸收杆上透射波形可以确定混凝土层裂强度。由于波在粘弹性材料中的弥散效应,吸收杆中透射波形会发生变化,但三维有限元分析表明,在利用吸收杆上透射波确定混凝土层裂强度时弥散产生的影响可以忽略。按照一维特征线理论,可以由吸收杆上的应变波形确定出混凝土材料的层裂强度。     

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.     

On the use of electrical-resistance metallic foil strain gages for measuring large dynamic plastic deformation

The use of electrical-resistance metallic foil strain gages for measuring large plastic strain in dynamic experiments in studied. The maximum nominal strains obtained in this investigation are 35 percent in compression, 25 percent in tension. A linear variation of gage factor with strain is found in this range. The corrected maximum strains are in excellent agreement with permanent strains measured after the tests. Thus foil strain gages can be effectively used to measure the large dynamic plastic strains.     

High-temperature strain-gage behavior on carbon materials

Advantages of applying high-temperature strain gages to carbon materials such as conglomerates of aluminum-reduction-cell cathodes and graphite for models of the cathodes or for nuclear reactors are shown. Tests on graphite and carbon conglomerate specimens, to which strain gages have been applied, are related. From these tests it is possible to obtain apparent strain, thermal hysteresis, drift, creep and mechanical hysteresis of strain gages. Test results for carbon specimens are compared with those of similar tests on stain-less-steel specimens with gages from the same lot.     

Errors in high-temperature strain measurements

The results of a technical evaluation of the accuracy of two types of commercially available high-temperature electric-resistance strain gages, and a special high-temperature gage under development at the Liquid Metal Engineering Center (LMEC) are presented. These gages, the BLH type HT 1212-5A, Microdot type SG420, and the LMEC gage, were selected for evaluation because of the need for reliable electric-resistance strain gages for high-temperature stress or strain measurements and process instrumentation in the temperature range of 900 to 1200° F. The BLH gage is rated by the manufacturer as a 1000 to 1200° F gage; the Microdot gage is rated as a 900° F gage. The special LMEC gage was made for use up to 1200° F. Instrumentation of this type is needed to determine or ensure component structural integrity and over-all system reliability of fast breeder reactors.     

混凝土率型内时损伤本构模型

混凝土是一种典型的率敏感材料,为了更好地描述混凝土结构在动力、冲击荷载作用下的强度和变形特征,本文结合内时理论和损伤理论建立了一种考虑混凝土率效应的内时损伤本构模型。该模型的特点:将混凝土材料的受力软化效应分解为密实状态的塑性效应和由微裂缝扩展引起的刚度退化效应。前者由内时理论来描述,这使该模型摆脱了一般弹塑性模型中屈服面的概念,从而更符合混凝土的变形特性,并且简化了非线性计算过程;后者由损伤理论来描述,根据混凝土的动力试验结果建立了增量型的损伤演变方程,从而使该模型能够较好地反映混凝土的动力特性。最后,应用本文建议的模型对一钢筋混凝土简支梁进行了非线性分析,结果表明:当结构承受快速荷载作用时,应变率对结构的受力性能影响较大,在进行结构分析时必须予以考虑。     

Poisson's ratio variations in concrete

This paper presents the results of an investigation relating to the variations in the Poisson's ratio in compression of concrete. An embeddable strain-gage unit has been developed for use in this investigation, and the readings from this gage unit proved quite reliable. With embeddable gage units, and surface-bonded strain gages, concrete test specimens were prepared and tested under uniform load in compression. The test results from this investigation prove that the Poisson's ratio of concrete is minimum at the faces of the test specimens, and it increases as the point moves towards the specimens' axes, where it reaches a value about 56 percent larger than that at the faces of these specimens.     

Experimental analysis of concrete behavior under high confinement: Effect of the saturation ratio

This study focuses on the identification of concrete behavior under severe triaxial loading in order to better evaluate the vulnerability of sensitive infrastructure to near-field detonations or ballistic impacts. For the purpose of reproducing high stress levels with well-controlled loading paths, static tests have been conducted on concrete samples using a triaxial press offering very high capacities (stress levels of around 1 GPa). It is a well-known fact that the concrete drying process is a slow phenomenon. Massive concrete structures, such as bridge piers, dams and nuclear reactors, could retain a quasi-saturated core throughout most of their lifetime, even though their facing dries very quickly. The objective of this article is to evaluate the effect of the saturation ratio on concrete behavior under high confinement; this article will present triaxial test results on concrete samples over a saturation ratio range extending from dried to quasi-saturated concretes. The subsequent analysis of results will show that the saturation ratio exerts a major influence on concrete behavior, particularly on both the concrete strength capacity and shape of the limit state curve for saturation ratios above 50%. This analysis also highlights that while the strength of dried concrete strongly increases with confining pressure, it remains constant over a given confining pressure range for either wet or saturated samples.     

An equiangular rosette-type extensometer

For complete determination of the state of stress at a surface point of an elastic body, it is in general necessary to measure the strain in three directions. This is usually made by means of strain rosettes of the bonded resistance-wire type. In this paper, a new type of extensometer is described which is capable of measuring the strains in three directions simultaneously.The extensometer consists of three parallel and equidistant legs which are flexibly connected to a central part and supplied with sharp indentors. The change in distances between equivalent points of these legs is proportional to the surface strains. Unbonded resistance wires or reluctance gages have been used for converting the displacements into electrical signals. Attachment is achieved by means of permament magnets or rubber cups.Restoring forces from flexible connections and strained wires are analyzed in detail. For an extensometer with 12 mm gage length and supplied with strained wires, these forces were found to be 21 g for a principal strain sum of 0.001 and 19 g for a principal strain difference of 0.001. When using reluctance gages, these figures are reduced to 5.8 and 12 g, respectively.Paper was presented by title at 1958 SESA Spring Meeting held in Cleveland, Ohio, on May 14–16.