Unloading Data Research Articles

Interparticle Contact Behavior and Wave Propagation

The low-strain stiffness and energy dissipation in particulate materials is strongly determined by the behavior of contacts. This paper presents results of a test program designed to study the effect of contact response on the propagation of waves. Wave velocity and attenuation were measured during isotropic loading using a resonant column device at shear strains varying from γ= 10 −5 to γ= 10 −6 . Elastic, viscoplastic and brittle contact behaviors were studied with steel spheres, lead shot, and silica-kaolinite pellets. All measured velocity-stress exponents were b /2 >≈ 1/6, which is the theoretical value for spherical contacts. High-tolerance steel spheres approximated this value. Contact crushing showed the highest exponent. Theoretical analyses confirmed that several phenomena conduce to a velocity-stress exponent b /2 = 0.25: buckling of particle chains and increase in coordination number, elastoplastic behavior, and cone-plane contacts. Load and unload data for viscoplastic lead shot showed that contact deformation is the governing parameter for low-strain stiffness, regardless whether the causing mechanism was elastic deformation, creep, or yield. All measured damping-stress exponents were between κ=−0.45 for steel and κ=−0.11 for the brittle pellets, while the theoretical value for frictional Mindlin contacts is κ=−2/3. Damping showed higher sensitivity than velocity to stress and time.

Surface mechanical property testing by depth sensing indentation

In this paper, procedures for load and displacement calibration for depth sensing indentation instruments are described. Different methods for instrument compliance and indenter shape calibration are compared and the results evaluated using bulk materials with distinct elastic/plastic properties. After traceable calibration of load, displacement, frame compliance and indenter shape, mechanical properties measured with a depth sensing indentation instrument agreed with literature bulk values. The pronounced load dependence observed for the metal's moduli might be explained in terms of systematic errors in the frame compliance and in the procedures used to analyse the unloading data.

The Effect of Chemical Functionality on Adhesion Hysteresis: A Study using the JKR Method

ABSTRACTThe adhesion of crosslinked PDMS surfaces to self-assembled monolayers with different chemical functionality was investigated using the JKR method, the contact mechanics of solids spreading their interfacial area under load. Interfacial H-bonding was shown to be an important chemical interaction causing significant adhesion hysteresis. The number of H-bonds between PDMS and silanol groups on SiO2/Si surfaces increased with time of the contact under a constant load, indicating pressure-induced reorganization of the PDMS network near the interface. The interaction between PDMS and carboxylic acid groups showed somewhat smaller hysteresis which suggests weaker H-bonding strength. The interaction between PDMS and functionalized biphenyl groups exhibited small hysteresis which is believed to be caused by dipolar interaction. whereas that between PDMS and nonpolar perfluorocarbon groups showed negligible hysteresis. The distinction in the behavior of the unloading data between H-bonding related interaction and dipolar interaction seems to indicate the difference in the nature between non-specific (van der Waals, dipolar) and specific (donor-acceptor, H-bond, acid-base) interactions.

Large-strain undrained pressuremeter interpretation based on loading and unloading data : R.S. Ferreira & P.K. Robertson, Canadian Geotechnical Journal, 31(1), 1994, pp 71–78

Large-strain undrained pressuremeter interpretation based on loading and unloading data : R.S. Ferreira & P.K. Robertson, Canadian Geotechnical Journal, 31(1), 1994, pp 71–78

Large-strain undrained pressuremeter interpretation based on loading and unloading data

Traditionally pressuremeter tests results are interpreted using only the loading pressure–expansion data. An interpretation method has been developed that makes use of both the loading and unloading portions of a pressuremeter test and incorporates nonlinear, undrained soil behaviour. This method has been modified to handle large trains. The proposed interpretation accepts that some level of soil disturbance exists during the early stages of a pressuremeter test, and hence more emphasis is placed on the unloading portion of the test. A methodology is described to allow this interpretation procedure to be applied to self-bored, prebored, and full-displacement undrained pressuremeter tests. The proposed method is evaluated using self-boring and full-displacement pressuremeter test results from various published field projects. The interpretation is accomplished using commercially available microcomputer software that can perform the curve-matching process for both loading and unloading portions of the test. Key words : pressuremeter, loading data, unloading data.

An Explanation for the Shape of Nanoindentation Unloading Curves based on Finite Element Simulation

AbstractCurrent methods for measuring hardness and modulus from nanoindentation load-displacement data are based on Sneddon's equations for the indentation of an elastic half-space by an axially symmetric rigid punch. Recent experiments have shown that nanoindentation unloading data are distinctly curved in a manner which is not consistent with either the flat punch or the conical indenter geometries frequently used in modeling, but are more closely approximated by a parabola of revolution. Finite element simulations for conical indentation of an elastic-plastic material are presented which corroborate the experimental observations, and from which a simple explanation for the shape of the unloading curve is derived. The explanation is based on the concept of an effective indenter shape whose geometry is determined by the shape of the plastic hardness impression formed during indentation.

Performance modeling of multiprocessor systems for different data loading schemes

When evaluating the performance of an algorithm in a multiprocessor system, most authors assume that the data are already loaded in the system. This assumption gives rise to a large error between the predicted and actual performances, specially for fine-grained tasks like image processing. Different data loading and interprocessor communication techniques can result in a significant difference in the performance of a machine. To quantify the amount of time spent for data loading and unloading as compared to the total processing time, different data loading and unloading schemes have been developed in this paper. A comparison of performance of a machine under the different schemes has also been presented. The comparison will give an insight into the considerable amount of time data loading, unloading, and interprocessor communications may require depending on the task, the architecture of the machine, and the method used to load and unload data to and from the machine respectively. It has been shown that concurrent data loading, unloading, communication, and processing using state-of-the-art processors helps in significantly reducing the overhead. An expression to determine the optimum number of processors in a machine has been developed.

An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments

The indentation load-displacement behavior of six materials tested with a Berkovich indenter has been carefully documented to establish an improved method for determining hardness and elastic modulus from indentation load-displacement data. The materials included fused silica, soda–lime glass, and single crystals of aluminum, tungsten, quartz, and sapphire. It is shown that the load–displacement curves during unloading in these materials are not linear, even in the initial stages, thereby suggesting that the flat punch approximation used so often in the analysis of unloading data is not entirely adequate. An analysis technique is presented that accounts for the curvature in the unloading data and provides a physically justifiable procedure for determining the depth which should be used in conjunction with the indenter shape function to establish the contact area at peak load. The hardnesses and elastic moduli of the six materials are computed using the analysis procedure and compared with values determined by independent means to assess the accuracy of the method. The results show that with good technique, moduli can be measured to within 5%.

Analysis of internal stress and anelasticity in the shock-compressed state from unloading wave data

Time-resolved shock-wave measurements have often been used to infer microstructural behavior in crystalline solids. We apply this approach to an interpretation of the release-wave response of an aluminum alloy (6061-T6) as it is dynamically unloaded from a shock-compressed state of 20.7 GPa. The anelastic behavior in the initial portion of the unloading wave is attributed to the accumulation of internal stresses created by the shock process. Specific internal-stress models which are investigated are (i) the double pile-up, (ii) the single pile-up, and (iii) single dislocation loops between pinning points. It is found that the essential characteristics of double and single pile-ups can be represented by a single dislocation between two pinned dislocations of like sign. Calculations of anelastic wave speeds at constant unloading strain rate are then compared with experimental data. The results suggest that the residual internal stress is due to pinned loops of density 10 15m −2, and that the viscous drag coefficient in the shock-compressed state is on the order of 10 −7 MPa s (approximately two orders of magnitude greater than expected under ambient conditions).

ANELASTIC RELEASE FROM THE SHOCK-COMPRESSED STATE

Anelastic release from the shock-compressed state is analyzed in terms of internal self stresses acting on dislocation pile-ups and pinned dislocation loops. Reverse anelastic deformation occurs immediately upon release due to these internal stresses and is responsible for departure from ideal elastic-plastic unloading wave behavior: anelastic release is commonly observed in all shock-loaded metals. The rate at which this readjustment of internal stress occurs is determined by the viscous drag coefficient in the shock-compressed state and has important consequences regarding determination of elastic moduli from unloading wave data. 6 refs., 4 figs.

Irreversible phase transitions and wave propagation in silicate geologic materials

Shock and unloading experiments on quartz and silicate rocks indicate that the release adiabats lie below the Hugoniot. The hysteresis and energy dissipation inherent in this situation have important wave propagation implications. On loading, there is a pressure-induced transition to the stishovite phase which does not occur under conditions of thermodynamic equilibrium, in that the Hugoniot passes through a metastable mixed-phase region for several tens of GPa. One interpretation of the unloading data is that the transition is not reversible, and the phase mixture remains frozen on unloading. However, material strength may also play a role. A complete thermodynamically consistent equation of state which includes phase transitions and strength effects has been developed and used to examine shock and release data on quartz and silicate rocks in order to quantify the kinetics of the reverse transition and to separate the hysteretic effects due to reverse phase transition kinetics from those due to material strength. The model allows quantitative determination of the effect of reverse transition kinetics on ground shock propagation in silicate materials.

Age-related effects of fatigue and recovery from fatigue in rat medial gastrocnemius muscle.

Force-velocity, power-velocity and unloaded shortening data were obtained from in situ medial gastrocnemius muscle-tendon complexes (stimulated at 60 Hz) with intact circulation of mature male rats (approximately 125 days old). Measurements were carried out at the end of a long (15 s) contraction (fatigued muscles) or with a short (1 s) contraction either in the fresh state (fresh muscles) or in muscles which had recovered for 15 min after a long contraction. Compared to the fresh state fatigue reduced isometric force by 57%, maximal shortening velocity by approximately 40% and maximal power output by 81%. These reductions were similar to data previously obtained with younger rats (40 days old). However, the velocity data of the muscles which had recovered for 15 min after a long contraction showed a greater reduction in the mature rats. This difference between the two age groups together with a difference in the changes in the initial parts of the isometric force time curves suggest an age-dependent response of the fast-fatigable fibre population of these mixed muscles. In a separate series of experiments the underlying mechanism of the recovery from fatigue was studied in a group of young rats. Fatigue was induced with five long (15 s) contractions (each at 5 min intervals). The recovery of isometric force and power output was monitored with short contractions which indicated a plateau of recovery but the absolute values were still reduced after 60 min (85 and 71% of prefatigue values, respectively). Phosphocreatine concentration recovered rapidly, whereas the ATP concentration was still markedly reduced after 1 h of recovery. The time courses of recovery of inosine-5'-monophosphate (IMP) and lactate concentrations resembled those of force and power output. Thus it is possible that age-dependent differences in IMP and/or lactate production may play a role in fatigue and recovery from fatigue.

Dynamic unloading behavior of soda lime glass

An experimental study was conducted to see if the reverberation plate technique, using commercial manganin stress gauges, could be used to determine the dynamic unloading behavior of brittle materials. Using a 50-mm powder gun, plate impact experiments were conducted on soda lime glass, providing unloading data from stress states from below the Hugoniot elastic limit to 17.0 GPa. The slopes of the unloading paths were found to increase as the peak compressive stress was increased. Such unloading behavior is similar to that of quartz, suggesting that the technique is viable.

The application of real-time monitoring and rating to HPOF pipe cable systems

The Boston Edison Company has installed real-time monitoring and rating equipment on two independent pairs of 115 kV HPOF (high-pressure oil-filled) pipe-type cables. The real-time systems are remotely located at cable terminal locations so that critical data inputs can be hard-wired to the computing processor. Real-time information is made available via analog SCADA (supervisory control and data acquisition) connections and also by IBM PCs equipped with modems, which can call the remote systems to view real-time conditions or unload historical data. A PC software package complements the real-time system and gives planners, system operators, and cable engineers the ability to simulate cable system operation in an offline environment. In general, the identification of effective earth parameters in real-time is shown to result in increased emergency and normal ratings by replacing the conservative assumptions that must be made at the design stage with accurate real-time data. Lightly loaded circuits have exhibited large increases in short-time emergency ratings, as they normally operate at relatively low temperatures. >

The Application of Real-Time Monitoring and Rating Equipment to HPOF Pipe Cable Systems

The Boston Edison Company has installed realtime monitoring and rating systems on two independent pairs of 115 KV HPOF pipe-type cables. This paper describes these installations and gives some results of their operation. These circuits exhibited large increases in short time emergency ratings because they normally operate at relatively low temperatures. The identification of effective earth parameters in real-time generally results in increased long term and normal ratings because of the conservative assumptions that must be made at the design stage. The real-time systems are remotely located at cable terminal locations so that critical data inputs can be hard-wired to the computing processor. Real-time information is made available via analog SCADA connections and also by IBM-PC's equipped with MODEMs which can call the remote systems to view real-time conditions or unload historical data. A PC Software package compliments the real-time system and gives Planners, System Operators, and Cable Engineers the ability to simulate cable system operation in an off-line environment.

Experimental measurements and analysis of the loading and unloading response of longitudinal and lateral manganin gauges shocked to 90 kbar

Experimental results and analysis describing the response of manganin gauges oriented parallel and perpendicular to the shock front and embedded in an aluminum-oxide matrix are presented. Both loading and unloading data to 90 kbar were obtained. An analytic model for piezoresistance response coupled with the electromechanical constants, determined from independent quasistatic experiments, was used to calculate the gauge response. Good agreement was obtained between the experimental results and the analytic model calculations. The analytic model was also used to quantitatively analyze the residual resistance measurements. The results presented here can be used to analyze gauge data during unloading and multiple wave loading.

Pressure sensitivity of low permeability sandstones

Detailed core analysis has been carried out on 32 tight sandstones with permeabilities ranging over four orders of magnitude (0.0002 to 4.8 mD at 5000 psi confining pressure). Relationships between gas permeability and net confining pressure were measured for cycles of loading and unloading. For some samples, permeabilities were measured both along and across bedding planes. Large variations in stress sensitivity of permeability were observed from one sample to another. The ratio of permeability at a nominal confining pressure of 500 psi to that at 5000 psi was used to define a stress sensitivity ratio. For a given sample, confining pressure vs permeability followed a linear log-log relationship, the slope of which provided an index of pressure sensitivity. This index, as obtained for first unloading data, was used in testing relationships between stress sensitivity and other measured rock properties. Pressure sensitivity tended to increase with increase in carbonate content and depth, and with decrease in porosity, permeability and sodium feldspar. However, scatter in these relationships increased as permeability decreased. Tests for correlations between pressure sensitivity and various linear combinations of variables are reported. Details of pore structure related to diagenetic changes appears to be of much greater significance to pressure sensitivity than mineral composition.

A self-consistent technique for estimating the dynamic yield strength of a shock-loaded material

A technique is described for estimating the dynamic yield stress in a shocked material. This method employs reloading and unloading data from a shocked state along with a general assumption of yield and hardening behavior to estimate the yield stress in the precompressed state. No other data are necessary for this evaluation, and, therefore, the method has general applicability at high shock pressures and in materials undergoing phase transitions. In some special cases, it is also possible to estimate the complete state of stress in a shocked state. Using this method, the dynamic yield strength of aluminum at 2.06 GPa has been estimated to be 0.26 GPa. This value agrees reasonably well with previous estimates.

The existence of a friction stress for high-temperature creep

AbstractRecent experimental work indicating the existence of a friction stress, σ0 during high temperature creep is reviewed. The friction stress concept has been used to alter the normal power law creep equation to the relationship e=B(σ−σ0)4. It is contended that linear treatment of the decremental unloading data used to determine ao is inadequate in view of the inherent nonlinearity of the stress/strain rate relationship. A modified analysis which accounts for the power law nature of the creep process is proposed in which these data are treated in a manner similar to stress relaxation data. On this basis, the evidence for a friction stress in a pure metal and in a precipitation-hardened alloy is shown to be inconclusive. However, the existence of a threshold is confirmed for a dispersion-strengthened alloy. In addition, further experimental results are presented for pure lead in which delay times of zero creep are not observed for small unloadings. Since these delay times are a necessary part of the de...

Dynamic unloading in shock compressed feldspar

Unloading data for a perthitic feldspar from shock compressed states between 250 and 500 kilobars are reported. Continuous stress wave profiles were measured with manganin transducers and the appropriate Riemann invariant integrated to determine the stress‐volume states during unloading. The results are in agreement with data obtained on other silicate minerals. A physical mechanism based on thermal heterogeneity and partial transformation to the high coordination phase (hollandite structure) incurred during dynamic compression provides a consistent explanation for the observed stress‐volume behavior.