End Of Specimen Research Articles

A method of extracting dynamic fracture toughness from CT tests

Recent research on methods of determining the dynamic fracture resistance for rapidly propagating cracks has progressed to the point where standardized testing procedures are being proposed. Because direct measurements of dynamic fracture resistances cannot be made, the resistances must be inferred from other measurement. In the present paper, an analysis which is fundamental to a method whereby the dynamic fracture toughness as a function of crack speed can be inferred from measurements of the specimen's end displacement at initiation of crack growth and of the total crack extension is described. All of the reference curves required for making the inference are developed for a standardized CT specimen. The procedure for using these curves to establish the dynamic fracture toughness-crack speed relationship and the minimum fracture toughness is given.

Upper limit of heaving pressure obtained by observing pore water pressure under partially soil freezing

Experiments on heaving pressure were conducted with a saturated clay under partially freezing in a closed system.Measurement of drop in prestressed pore wate pressure Pw was continued under constant total pressure P until change of Pw ceased, maintaining the constant temperature at both ends of specimen above and below freezing respectively. The elapsed change of pore water pressure continued for a long time and it was observed that ice lens was growing in the vicinity of cooling surface. These facts reveal that the veins of unfrozen water are connected from freezing front, where only free water freezes, to ice lensing interface and unfrozen water migrates slowly in frozen soil. When the partially frozen soil can draw water no more under the effective stress calculated from P and Pw, we define this effective stress as upper limit of heaving pressure σu. σu obtained depends clealy upon the temperature at cooling surface θc and empirical formula is obtained as follows, σu= -11.1θc [kg/cm2] This formula agrees with modified Clausius-Clapeyron equation that gives the change in freezing point in the case where the pressure on the ice differs from that on the water. This agrees with the experimental results obtained by Radd and Oertle (1973) but differs from maximum heaving pressure calculated from “Capillary Force Theory” in which it is constant, not depending on temperature.

Deformation resistance and breaking strength of glass and glass-ceramics under biaxial compression

1. Regardless of the volume of the sample, the state of stress, the conditions at the specimen ends, the chemical composition of the material, and the state of the lateral surfaces of the specimens, the distribution of the probable strengths of glasses and glass-ceramics is satisfactorily represented by the normal law. 2. When assessing the limiting states of glass and glass-ceramic structures operating under compression, we must take account of the extent of the surface damage resulting from technological processes and the conditions under which the artifacts operate. 3. The maximum stress theory must be used for practical calculations of structural components operating under plane stress. 4. Unfavorable boundary conditions when glasses and glass-ceramics are subjected to load may lead to incorrect determination of the strength characteristics and the limiting fracture condition, and hence to their inefficient use.

有限要素法による鉄筋コンクリートの非線形解析第 2 報 : 異形鉄筋とコンクリートの付着-その 1

In the previous paper, Part 1, the discussion has been made about the applicability of the previous analytical models which represent the biaxial stress-strain relations for concrete. It is the purpose of this investigation to develop an analytical model to simulate the bond-slip behavior between the deformed bars and concrete, which is also one of the most fundamental mechanical characteristics of reinforced concrete. The model is designed as follows. The theory of orthotropic materials is applied to the internal crack zone of concrete around the deformed bar and the modulus of elasticity, E_1, nearly at right angles to the internal crack is reduced in company with the shear modulus of elasticity, G_12. Finite element calculations are compared with experimental results for the previous pull out specimens (26, 27). The results are as follows. 1) When the distribution patterns of the internal crack zone are decided by curve fitting the bond stress-slip relations to the experimental data, the analytical results of the load-slip and the load-steel strain distribution relations obtain good agreements with experimental results. 2) About concrete strain around the deformed bar, when the angle of the internal crack is 90°, the analysis in which the width of the internal crack zone is varied along the bar axis gives closer agreement with experimental data than the analysis in which the constant width is assumed. 3) When the angle of the internal crack is 45°, the deformation perpendicular to the bax axis has appeared. This deformation was also observed near the loaded end of the real specimen.

Measuring the Uniaxial Compressive Strength of Ice

AbstractAn attempt was made to develop a simple but accurate method for making compressive strength tests on right circular cylinders. Compliant loading platens were designed to apply uniform normal stress without introducing significant interface radial shear stresses. The compliant platens gave reproducible results that agree well with results obtained by a precise conventional technique. Accurate results were obtained with simple specimen preparation, and with short specimens where the length-to-diameter ratio was less than unity. Platens were made from a rubber-like urethane which was molded in aluminum cylinders to provide lateral restraint. Uniaxial compression tests on cylindrical polycrystalline ice specimens were made to determine the characteristics of the platens. For 21 specimens with ends prepared on a lapping plate to obtain a mirror finish, the measured strength showed a variation of only 13% for length-to-diameter ratios from 0.74 to 2.5, with no systematic trend. Another 21 specimens with length-to-diameter ratios of about 2.35 were tested with various platens and various methods of specimen end preparation. The strength for specimens with saw-cut ends and for those with ends lapped showed very little difference when tested with the rubber platens.

Measuring the Uniaxial Compressive Strength of Ice

AbstractAn attempt was made to develop a simple but accurate method for making compressive strength tests on right circular cylinders. Compliant loading platens were designed to apply uniform normal stress without introducing significant interface radial shear stresses. The compliant platens gave reproducible results that agree well with results obtained by a precise conventional technique. Accurate results were obtained with simple specimen preparation, and with short specimens where the length-to-diameter ratio was less than unity. Platens were made from a rubber-like urethane which was molded in aluminum cylinders to provide lateral restraint. Uniaxial compression tests on cylindrical polycrystalline ice specimens were made to determine the characteristics of the platens. For 21 specimens with ends prepared on a lapping plate to obtain a mirror finish, the measured strength showed a variation of only 13% for length-to-diameter ratios from 0.74 to 2.5, with no systematic trend. Another 21 specimens with length-to-diameter ratios of about 2.35 were tested with various platens and various methods of specimen end preparation. The strength for specimens with saw-cut ends and for those with ends lapped showed very little difference when tested with the rubber platens.

Hot compression behaviour of thermo-mechanically processed alpha zirconium

Hot compression tests were performed on cylinders of alpha zirconium at a temperature of 625°C and a strain rate of 10-3S-1. In the first series of tests, the specimens had polished ends and were tested without lubrication. Additional tests were carried out with glass lubricant applied to the specimen ends. In further tests the specimens had grooved ends and these were tested both with and without lubricant. The results indicated that the presence of both grooves and lubricant reduced the slope of the initial loading line, but had only a slight effect on the level of the high-temperature yield stress. Three different grain structures (equiaxed, coarse Widmanstätten, andftne Widmanstätten) resulted from the prior thermomechanical processing. The effect of these three initial microstructures on the subsequent high-temperature flow behaviour was determined. The flow stress of the fine Widmanstätten structure rose to apeak of ∼100MN/m2, which was about 40% greater than the steady-state flow stress for this and the other initial microstructures. Only a slight peak inflow stress was observed for the coarse Widmanstätten structure while the flow stress of the equiaxed alpha rose gradually to the steady-state level of ∼73MN/m2. All specimens were quenched immediately after removal of the load in order to preserve the dislocation substructure. Observations of the substructural changes occurring during de formation were made by post-test optical examination and thin-foil electron microscopy.

An extensiometer for molten plastics

AbstractA new rheometer has been developed to measure the extensional viscosity of molten thermoplastics at typical extrusion temperatures. It is at the same time versatile and easy to operate. One end of the test specimen is held in a fixed position by an oil‐cooled chimp mounted on the end of a force transducer. The other end of the specimen is held by, a moving, oil‐cooled chimp. The moving clamp is suspended from a carriage mounted on a ball nut which travels on a long lead screw. The lead screw is driven by an electric motor operating through a gear box, and its speed can be controlled to produce either a constant stress (creep) or a constant strain‐rate (stress growth) experiment. Experimental results have verified that in the “constant strain rate” mode the strain rate does rise rapidly to a constant level. Measurements on several polyethylene resins indicated that the extensional viscosity rises with strain rate at low strain rates.

Development and selected applications of a low hydraulic head laboratory permeameter : MCINNES, DB, PUBLIC WKS DEPT. W. A. AUS 4F, 1T, 13R. 2ND AUS-N. Z. CONF. GEOMECH, BRISBANE, 1975, INST. ENGRS, AUS. NAT. CONF. PUBL. NO. 75/4, P228–232

Liquid flow through soil developed under low hydraulic head conditions is discussed. Such conditions arise for example in studies of seepage from shallow ground evaporation tanks, and infiltration phenomena. A laboratory technique for the determination of soil permeability under these conditions and closely simulating those evident insitu is described. Features of the technique include means for accurately maintaining and measuring the vertically downwards flow from a shallow pool of water ponded on one of the flat ends of a cylindrical specimen. Special attention has been given to the problem of specimen boundary leakage often encountered in laboratory permeability testing of gravels. A bentonite gel jacket is used to surround the specimen and eliminate leakage. The use of the technique also allows for the direct testing, without re-compaction, of field tube samples and for a study of the effect of drying subsequent to compaction upon permeability. Certain applications of the use of the technique and associated results are discussed. These applications include a determination of: (a) The influence of compaction upon the water conductivity characteristics of road shoulder materials after long periods of wetting. (b) The influence of desiccation, following compaction and prior to wetting, upon the permeability of a clayey-sand. Comments are then made relating to the practical implications of these permeability measurements.

The problem of testing glass in compression

1. The bearing capacity of composite specimens is extremely heavily dependent on the geometric parameters of the rings and material, with the use of which the ends of the specimens are secured in their recesses. Maximum glass strength is ensured by rings with a wall thickness of not less than d and seating depths of 0.5d-d. 2. The strength distribution of compressed glass rods is satisfactorily described by the normal rule.

Contribution to Defining Pressure Drop/Zur Definition des Zugwiderstandes

AbstractWith constant volumetric flow rate F at the end of the test specimen the pressure drop is dependent on the absolute pressure under which the volumetric flow rate is measured. The correlation between pressure drop Dp at the test specimen and volumetric flow rate F can be adequately described byin which k is a constant for a certain object measured and p* designates the pressure under which the volumetric flow rate F is measured. p1 and p2 are the absolute pressures at the test specimen entrance and exit respectively. For the definition of draw resistance it results from this that the Coresta suggestion that p1 = 1 atm 760 Torr be determined as reference pressure is inexpedient for practical purposes. It would be more expedient to determine p* = p2 = 1 atm 760 Torr, i. e. that pressure is determined as reference pressure under which the volumetric flow rate F is measured at the end of the test specimen. The simple correction formulae resulting for this with fluctuating atmospheric pressure are quoted.

Measurement of dynamic and static strain in small specimens at elevated temperatures

A method of strain measurement in small specimens is described which involves measurement of strain in terms of the net displacement of reference points within the specimen ends rather than on the gauge length. The reference points are the bottoms of axial holes drilled into the specimen heads. The method has been developed for high-frequency dynamic-strain measurement using a novel electro-optical displacement transducer and for low-frequency creep-fatigue work using conventional displacement transducers.

Rheological Properties of Plastic Clay Body

In order to study the rheological preperties of plastic clay body, cyclic tortion test method by Astbury was examined on a plastic mass for electrical insulators. Thin hollow cylindrical tubes, 10φ-20φ×100, were formed by extruding. One end of specimen was fixed, and the other free end was oscillated torsionally, while the rotating angle was changed in a sinusoidal way. By analyzing the hysteresis loops of the stress-strain curves, the following results were obtained. (1) By the sinusoidal torsional motion, S-S curve was constricted in a steady hysteresis loop, showing that the phenomenon of the “Memory” was lost. (2) The remanent sterin depended markedly on the maximum strain (torsional strain amplitude). (3) The coercive force and the maximum stress much decreased on the increase of the moisture content. (4) The ratio of the coercive force to the maximum stress was constant regardless of experimental conditions. (5) The energy absorbed during one cycle was directly proportional to the maximum strain, and was reduced with increasing moisture content. (6) If the torsional strain amplitude and the frequency of sinusoidal oscillation were taken as the same condition, the ratio of energy lost during one cycle to the one for the perfectly plastic body was equal to the ratio of the coercive force to the maximum stress, which was constant, irrespective of experimental conditions. (7) The critical value of the strainen ergy, which determines the ratio of the number of the elastic elements to the that of viscous elements was not constant.The writers, on the basis of the results, pointed out, (1) the plastic property should be added to the Astbury's model as one of the mechanical components, (2) the assumption that the elastic elements would be changed to the same number of the viscous elements due to the increase of strain energy, shoud be modified, (3) the term which gives the effect of moisture content on the behaviour of plastic clay body must be added in the fundamental rheological equation, (5) the model by Astbury should be revaluated after determining more precisely the model parameters.

Crack growth and faulting in cylindrical specimens of chelmsford granite

This study focuses on processes of fracture propagation and faulting in specimens of Chelmsford granite subjected to various end-boundary conditions and confining pressures. Chelmsford granite is a brittle material and contains numerous small defects which are preferentially oriented along three mutually perpendicular planes. The defects significantly affect the physical properties of the granite and the way the granite fails. Theoretical solutions for stresses within cylindrical elastic bodies indicate that stresses within a test specimen are markedly nonuniform. The stresses are not simply axial and equal to the axial load divided by the cross-sectional area of the specimen, as is assumed by many rock mechanicists. Regardless of end-boundary conditions of specimens, cracks in Chelmsford granite propagate parallel to the direction of axial loading. Cracks in a specimen of granite, with thin neoprene inserts or teflon inserts placed between its ends and the loading platens, grow most intensely near the longitudinal axis, at the ends of the specimen. A theoretical solution for stresses in specimens subjected to these end conditions indicates that compressive axial stresses and tensile radial stresses are maximum in those places. Thus, a combination of Griffith's theory of crack propagation and the theoretical solution for stresses seems to account for the observed crack pattern. Uniform loading was achieved for Chelmsford granite by inserting specially made steel discs between a specimen and the platens of the loading machine. Cracks in specimens subjected to uniform loading propagate relatively uniformly throughout the specimens, as predicted by theory. Specimens subjected to end-boundary conditions of uniform loading or of teflon inserts or of neoprene inserts, fail by longitudinal splitting, parallel to the direction of axial loading, and then by buckling of the split slabs. Specimens that were placed in direct contact with the relatively rigid platens of the loading machine also are stressed highly nonuniformly. The areas of most intense crack growth are two bands, extending from the corners of a specimen to a distance of about one-quarter of the specimen height from the specimen ends, as predicted theoretically. The specimens of granite subjected to direct contact end conditions or to triaxial loading, regardless of end condition, fail by faulting. The type of faulting depends on frictional properties of the platens. The fault surfaces consist of small steps, arranged as in a staircase. The surfaces of the steps are roughly perpendicular to the loading axis of a specimen and the vertical risers are surfaces of cracks that have propagated axially. Thus, immediately before faulting occurs, a specimen appears as though it consists of closely fitting, tiny beams, bounded by axial cracks. We have developed a theory of faulting of specimens of Chelmsford granite, based on beam-buckling theory and frictional contact among beams. The theory seems to describe relations between average ultimate compressive strengths and confining pressures, ranging from zero to 10,000 psi, for samples of Chelmsford granite. Neither the Coulomb criterion nor the modified Griffith criterion fit the experimental data.

Crack-growth monitoring: Optimisation of the electrical potential technique using an analogue method

1) Electrical potential field distributions have been obtained for SEN and CTS fracture testpieces and used to derive the optimum conditions for current input and probe positions. 2) The most suitable position for the potential measurement probes is on the top face of the specimen as close to the open end of the notch as possible, since attachment near the crack tip leads to much larger errors from any slight variations in positioning the probes. 3) For SEN specimens, the greatest sensitivity is obtained by introducing the current at points close to the notch. However, the uniform current configuration (input of current at specimen ends) is prefered since it is not liable to errors from slight variations in current input position. 4) Introduction of the current by area contact on the top surface in CTS specimens is preferred because very low sensitivity is achieved for the initial stages of crack growth when the current is applied from the side flanks. 5) The mode of current lead attachment on CTS testpieces is dependent on the testpiece materials. In steels, reproducible area contact mayy be achieved by the use of brazed or soldered copper terminals. Howwever, in aluminium and its alloys, the samde degree of reproducibility cannot be achieved for any practical application by area contact. Point application of current through a single screw is therefore advocated.

Electromigration and associated void formation in silver

Electromigration in both 99.999 per cent pure silver with about 1 ppm oxygen and 99.99 per cent pure silver with about 40 ppm oxygen (presumably in the form of impurity oxides) has been measured using the vacancy flux method. The method consists of determining local volume changes in specimens subjected to a large d.c. current density ( ∼ 10 4 A cm 2 ) and a parabolic temperature distribution, which results from water-cooling the specimen ends. As a result of improved temperature determination, the effective charge for electromigration was found to be about −6.5 e in contrast to the earlier reported value of about −25 e. Microscopic void formation on the cathode side and consequent sample elongation were observed only in the less pure silver and could not be induced in the purer material even when charged individually with oxygen or hydrogen. Some evidence indicated oxygen electromigration to the anode in silver and also an enhancement of the self-electromigration of the latter in the presence of dissolved hydrogen.

The Influence of End Friction and Poisson's Ratio on Stresses in Compressed Specimens

The present paper is concerned with the influences of end friction and Poisson's ratio on stress in compressed rectangular and cylindrical specimens. In numerical calculations, the finite element method was employed. The following conclusions have been made on stress in the compressed specimens.(1) The more the friction between the end of the specimen and the platten is reduced and the smaller Poisson's ratio becomes, the more uniform stress is developed in the specimen.(2) For sufficiently small coefficients of the end friction, some portions of the end of the specimen slide and the shear stress on the end becomes terrace-like, and the axial stress becomes more or less uniform.(3) The stress is similar in both the rectangular and the cylindrical specimens. The difference is that the deviations of the axial stresses from the average for various coefficients of friction and Poisson's ratios are larger in the cylindrical specimens than those in the rectangular ones.(4) For a fixed Poisson's ratio with the coefficient of the end friction larger than a certain value, no lubrication can be practically expected, in other terms the stress in specimens with such coefficients of end friction are practically the same as the stress in the specimens completely restrained at the ends. This limit value of the coefficient of the friction varies with Poisson's ratio. For Poisson's ratios 1/3, 1/4, 1/5 and 1/6 in cylindrical specimen, for instance, the limit values of the coefficient of end friction become 0.52, 0.38, 0.31 and 0.26, respectively.(5) The load required for the unit axial displacement of the ends of specimen is widely influenced by the Poisson's ratio, the coefficient of end friction and slenderness of the specimen. For the specimen with height-diameter ratio 0.5, for example, the loads required for the completely restrained specimen are larger than those of fully lubricated one by 16.5% and 3.3% with Poisson's ratios 1/3 and 1/6, respectively.

Uniaxial compressive creep of polytetrafluoroethylene

AbstractA machine to measure the creep deformation of plastics under uniaxial compressive loads is described. The problems associated with accurate creep testing in compression, primarily the application of a uniform stress to the specimen and the measurement of the resultant strain, receive particular attention. For the specimen geometries used, the effect on the measured strain of frictional restraints at the specimen ends is negligible provided the strain measurement is made with an extensometer attached to the specimen.The effect of fabrication techniques on the deformation behavior of polytetrafluoroethylene (PTFE) has been examined. Sintering time and temperature are found to be the most significant variables in the processing of PTFE. A comparison of uniaxial tensile and compressive creep data has shown that the non‐linear viscoelastic behavior of the material extends into the low strain region.

Freeze-Etching Simplified

Freeze-etching, as a tool in the preparation of biological specimens for electron microscopy, has seen relatively little use since it was introduced in 1957. The involved procedure, originally described, and, until recently, the cost of special equipment, must certainly have played a part in the failure of freeze-etching to become a commonly used technique. Efforts to solve some of the problems have resulted in the development of a modified freeze-etching module that can fit on nearly any vacuum evaporator and will permit the production of good specimens by an unskilled technician in less than one-half hour.The freeze-etching module (Fig. 1) consists of a collar (A) with two opposing hollow tubes supported by small collars so that the specimen end of the specimen tube (D) can rotate within the rotatable cold shroud (F).

Three-Phase Relative Permeability Measurement Using a Nuclear Magnetic Resonance Technique for Estimating Fluid Saturation

Abstract A method is described for measuring two- and three-phase relative permeabilities in sandstones or sand packs using a nuclear magnetic resonance (NMR) technique to determine fluid saturations Two- and three-phase relative permeabilities have been determined on Boise sandstone using the NMR technique of saturation measurement. Three- phase relative permeability to water was found to depend only on the water saturation, whereas three-phase permeability to oil depended on both the water and oil saturations. Relative permeability to gas in three-phase flow was found to depend only on the total liquid saturation. Introduction Three-phase relative permeabilities are extremely useful in calculating field performance for reservoirs being produced by simultaneous water and gas drives. Three-phase relative permeability data are also needed for analyzing solution gas-drive reservoirs which are partially depleted and are being produced by water drive. Some thermal recovery processes involve three-phase flow which require three-phase relative permeability data for predicting reservoir-behavior. Unfortunately three-phase relative permeability measurements have rarely been made. Also, because of the scarcity of three-phase data, it has not been possible to date to relate other measured rock characteristics to the relative permeabilities with a great certainty. Leverett and Lewis, Reid and Snells have reported three-phase relative permeability data on unconsolidated sands. Leverett and Lewis used ring electrodes spaced along the length of the sand sample to -measure the resistivity of the sample which was assumed to be monotonically related to brine saturation. Gas saturation was determined from pressure-volume measurements. Oil saturation was obtained by material balance on the cell containing the sand sample. This method is involved and time consuming. Another difficulty arises from the fact that the resistivity of the sand is a function not only of saturation of brine but also of the distribution and saturation history of the brine in the pore spaces. Reid used a gamma ray absorption technique for measuring liquid saturation. This method has the disadvantage that total liquid saturation rather than oil or brine saturation is all that can be measured and still another method is required to determine the saturations of individual components. Snells used a neutron bombardment method which also required a separate determination of the individual component saturations. Caudle et al. measured three-phase relative permeability on consolidated sandstones using vacuum distillation for determining fluid saturations. Distillation after each reading makes this technique very lengthy and time consuming. Corey et al and Naar and Wygal measured three-phase relative permeability on sandstones by the capillary- pressure method. Semipermeable diaphragm assemblies were used at each end of the core specimen to keep the water base in the core. Gravimetric methods were used to determine fluid saturations. Sarem recently repeated an unsteady-state method for measuring three-phase relative permeability on sandstones. This method is an extension of Weige's methods for measuring two-phase relative permeability. Although Sarem's method is simple and comparatively fast, the assumptions involved may oversimplify the problem. Sarem's assumption, that in all rocks relative permeability to each fluid will depend only on the saturation of that fluid, seems to be rather unrealistic. Neglecting capillary effects at the end of the core is also a weak assumption Donaldson and Deans measured three-phase relative permeability using a method similar to Sarem's.