Hypothesis Of Summation Research Articles

Estimation of the strength and lifetime of technical objects using the acoustic-emission method

Estimation of the lifetime of a construction consists of forecasting the time remaining before failure and is based on extrapolation of the time dependences of the state parameters of a technical object to their critical values. The use for this purpose of simplified damageability models (e.g., a hypothesis of linear summation of flaws) and the absolute values of acoustic-emission (AE) parameters (e.g., activity, counting rate, duration of pauses) leads to a dependence of the estimate results on a large number of destabilizing factors and, as a result, to low accuracy or laboriousness of these techniques. An earlier suggested micromechanical model of AE parameters makes it possible to find a trade-off between simplicity and accuracy.

Theoretical-experimental model for predicting crack growth rate in structural alloys under combined action of fatigue and creep

Holding periods of 300 and 3600 s in a trapezoidal load cycle are shown to increase the crack growth rate dozens of times for alloy ÉP962 and several-fold for alloy ÉP742 at a temperature of 973 K. It is demonstrated that in the presence of the first portion on the creep crack growth diagram, whereon the crack growth rate decreases, the crack growth kinetics for a trapezoidal load cycle can be predicted using the hypothesis of the linear summation of fatigue and creep crack growth rates provided that the peculiarities of the first portion of the creep crack growth diagram are taken into account. Empirical approaches are proposed for determining the mean crack velocity in the first portion of the creep crack growth diagram.

Method for the evaluation of the service life under random loading based on the energy criterion of fatigue fracture

We propose a method aimed at the assessment of the service life of structural elements under random loads based on the energy criterion of fatigue fracture and the model of cyclic deformation of materials after overloading. The possibility of determination of the maximum amplitude of stresses according to the number of loading cycles prior to fracture is substantiated. The results of evaluation of the service life under conditions of uniform stressed state are compared with the experimental data obtained by Swanson, Raikher spectral hypothesis of summation of defects, and linear hypothesis of summation. The dependence of the right-hand side of the linear hypothesis equation on the dispersion of the random loading process is established.

Probabilistic hypothesis of fatigue-damage summation

A probabilistic model is examined for evaluation of fatigue damages sustained by material in equipment of the chemical and petrochemical branches of industry. The basis of the expediency of special tests of materials coming under cyclic loads in a σa = const regime, and the work up of criteria, or “degrees of allowable risk,” especially for potentially dangerous fabrications, is given.

Theoretical-Experimental Model for Predicting Crack Growth Rate in Structural Alloys of Aircraft Gas Turbine Engine Disks with Regard to Fatigue and Creep

Hold times of 300 and 3600 s under trapezoidal cyclic loading are shown to result in an increase in crack growth rates by many factors of ten for EP962 alloy and several times for EP742 alloy at a temperature 973 K. It has been found that in case the creep crack growth diagram has a first segment where the crack growth rate decreases, the crack growth kinetics under trapezoidal cyclic loading can be predicted by means of the hypothesis of linear summation of fatigue and creep crack growth rates considering special features of the first segment of the creep crack growth diagram. The authors put forward empirical approaches to determining the mean rate on the first segment of the creep crack growth diagram.

Evaluating the Operations Underlying Multisensory Integration in the Cat Superior Colliculus

It is well established that superior colliculus (SC) multisensory neurons integrate cues from different senses; however, the mechanisms responsible for producing multisensory responses are poorly understood. Previous studies have shown that spatially congruent cues from different modalities (e.g., auditory and visual) yield enhanced responses and that the greatest relative enhancements occur for combinations of the least effective modality-specific stimuli. Although these phenomena are well documented, little is known about the mechanisms that underlie them, because no study has systematically examined the operation that multisensory neurons perform on their modality-specific inputs. The goal of this study was to evaluate the computations that multisensory neurons perform in combining the influences of stimuli from two modalities. The extracellular activities of single neurons in the SC of the cat were recorded in response to visual, auditory, and bimodal visual-auditory stimulation. Each neuron was tested across a range of stimulus intensities and multisensory responses evaluated against the null hypothesis of simple summation of unisensory influences. We found that the multisensory response could be superadditive, additive, or subadditive but that the computation was strongly dictated by the efficacies of the modality-specific stimulus components. Superadditivity was most common within a restricted range of near-threshold stimulus efficacies, whereas for the majority of stimuli, response magnitudes were consistent with the linear summation of modality-specific influences. In addition to providing a constraint for developing models of multisensory integration, the relationship between response mode and stimulus efficacy emphasizes the importance of considering stimulus parameters when inducing or interpreting multisensory phenomena.

Probabilistic service-life assessment for safety analysis for preparing a power-generating unit of a nuclear power plant for service-life extension

Information about the actual history of cyclic loading in the actual state of the metal in equipment components must be used for analyzing the safety of power-generating units in operating nuclear power plants, in preparation for service-life extension, and to determine the moment of onset of rapid aging of the metal and the end of the period of stable operation. The properties of the exponential distribution function and probability functions which are constructed for various loading scenarios using the hypothesis of probabilistic summation of fatigue damage for estimating the γ-percentage residual service life of equipment components are examined. Probabilistic estimates of the service life for operating nuclear power plants make it possible to control effectively the residual service life of the components of a nuclear power plant on the basis of information provided by the diagnostics systems and by the systems monitoring the state of the metal and the data on loading parameters from the control systems.

Fatigue life of welded tee-joints subjected to two-frequency pulsations of internal pressure

A method is proposed for experimentally determining the fatigue life of welded tee-joints of steel 20 loaded by internal-pressure pulsations of low (∼0.15 Hz) and high (10.5 Hz) frequency. The three parts of the joint had the same cross section. The beginning of yielding was taken as the criterion of endurance. The results are interpreted on the basis of the proposition that the stage of crack propagation plays the main role in the overall process of fracture. A modified variant of the hypothesis of linear damage summation is used to compare the results of the two-frequency tests with data obtained earlier from one-frequency tests. Satisfactory agreement is obtained between the experimental and theoretical data.

Effect of prior plastic deformation and creep strain on fatigue damage for a high-temperature alloy

A procedure is developed, an experimental study is provided, and a comparative evaluation is given for the effect of prior plastic deformation and creep strain on fatigue damage for high-temperature metallic materials on the example of alloy eI827. A different effect is demonstrated for prior plastic deformation and creep strain on fatigue damage of this alloy at 1073 K. In the case of prior creep a marked difference is established from the hypothesis of linear summation for damage in the direction of weakening during evaluation of the overall fatigue and static damage both in the form of relative endurance and relative strain.

Visual contrast detection by a single channel versus probability summation among channels.

It is now generally accepted that the human visual system consists of subsystems ("channels") that may be activated in parallel. According to some models of detection, detection is by probability summation among channels, while in other models it is assumed that detection is by a single channel that may even be tuned specifically to the stimulus pattern (detection by a matched filter). So far, arguments in particular for the hypothesis of probability summation are based on plausibility considerations and on demonstrations that the data from certain detection experiments are compatible with this hypothesis. In this paper it is shown that linear contrast interrelationship functions together with a property of a large class of distribution functions (strict log-concavity or log-convexity on the relevant set of contrasts/intensities) uniquely point to detection by a single channel. In particular, models of detection by probability summation based on Quick's Model are incompatible with linear contrast interrelationship functions. Sufficient (and observable) conditions for the strict log-concavity/log-convexity of distribution functions are presented.

Predicting endurance in programmed cyclic loading taking into account scattering of the properties

1. A method of constructing the fatigue curves for individual specimens by measuring the inelasticity characteristics on the reference level of the stress amplitude is proposed. 2. If the scatter of the fatigue strength characteristics of the individual specimens is taken into account using the proposed method, the agreement between the experimental data and the linear hypothesis of damage summation is improved. 3. In the given steels and loading regimes, the main error in applying the Palmgren-Mainer hypothesis is caused by the fact that the scatter of the properties of the individual specimens is not taken into account in calculating endurance. 4. The best agreement between the experimental and calculated values of the number of cycles to fracture is recorded in the range of short endurances.

Assessment of compressor blade fatigue life by accelerated method

1. The method of statistical processing of results obtained in testing compressor blades developed by us produces statistically founded equation of empirical regression. 2. Analytical expressions were obtained for determining the fatigue limit by accelerated Locati method based on the linear hypothesis of damage summation and the corrected hypothesis of damage summation. 3. The suggested analytical methods are free of human errors typical of conventional graphoanalytical methods and make possible data processing in a computer. In addition, the corrected damage summation hypothesis makes it possible for the effect of overloading and underloading in the loading programs to be taken into account. 4. For determining fatigue limits by the accelerated test method, computer programs in FORTRAN IV language for use in the ES-1033 computer are available; they make use of the analytical equations suggested in this paper.

Damage summation in nonstationary cyclic loading of fiberglass-plastic

1. Damage accumulation in stationary or nonstationary isothermal cyclic loading of fiberglass-plastic is described by the very same relationships and is determined by the instantaneous values of damage and stress. On the basis of data on the kinetics of damage buildup in a stationary regime, it is possible to predict lifetime for a number of nonstationary isothermal regimes. 2. More damage builds up in the stationary regimes at small loads. These differences lead to deviations from the linear hypothesis of summation of relative lifetimes in isothermal experiments. In a stepwise diminishing load, lifetime is larger, and in a rising and programmed load it is less than that calculated by the linear hypothesis. The linear hypothesis may be used to calculate lifetime in isothermal programmed experiments having a predominance of high loads. Comparison of the stationary and nonstationary nonisothermal regimes on the basis of the linear hypothesis may lead to appreciable errors in estimating lifetime.

Strength of the materials used for pistons of internal-combustion engines on subjection to fatigue and thermal fatigue loading

1. The strength characteristics of the piston alloys under consideration, measured under conditions of isothermal and thermal fatigue approaching those encountered in actual practice, are considerably lower than the static strength characteristics. This indicates the importance of allowing for the factors in question when calculating the strength of interual-combustion-engine pistons. 2. Of the alloy studied in the present investigation, the lowest thermal-fatigue strength characteristics corresponding to approximately normal working conditions are those of aluminum alloy AL25. For temperature cycles of 55⇌330°C the breaking stress on a base of 2·104 cycles of heat exchangers falls to 1 kgf/mm2, as compared with the isothermal fatigue limit (6.1 kgf/mm2) and the static tensile strength (12 kgf/mm2) at the same temperature. 3. Under conditions of combined loading, the fatigue life of alloy AL25 falls more rapidly than would follow from the linear hypothesis of damage summation, allowing for both mechanical and thermal fatigue. A vital factor in this case is the damage associated with interaction between the fatigue and thermal fatigue processes due to multiple-factor effects. 4. Criteria for the failure of piston materials under multiple-factor conditions call for urgent and more extensive investigation.

Programmed fatigue testing of welded specimens

1. When the loading process is schematized by the method of enlarged amplitudes the stress amplitude distributions can be taken as homologous. This greatly simplifies the methods of evolving fatigue testing programmes. 2. When programmed testing of welded specimens is conducted under symmetrical cyclic stresses higher than the endurance limit the accumulation of defects follows the hypothesis of linear summation of defects. It can be assumed, with sufficient accuracy, that the average limiting value of cumulative defectsa is equal to unity. (This assumption is valid for the stages of crack development and fracture.) The distribution fora follows the logarithmic-normal law quite closely. 3. The considerable concentration of stresses in welded specimens causes the early development of fatigue cracks. (Cracks 2–3 mm long have been noticed after an average period corresponding to 30% of the life before fracture.) This fact increases the destructive effect of stresses lower than the initial endurance limit. When fracture is the criterion of fatigue life the lower stresses should be taken into account during calculations. This is done by extending the summation of defects to a stress of 0.7 σ−1 after extrapolating the fatigue curve. When crack development is the criterion the effect of stresses lower than σ−1 should be neglected.

Separate and joint effects of variables in work and rest in clerical work

In order to retest the hypothesis advanced by Kimble & Bilodeau and supported by others that the joint effects of work and rest in cyclical motor learning are the simple summation of the two independent effects, 4 groups which represent each combination of two levels of work (15 or 30sec) and two levels of rest (5 or 30sec) variables practiced a task of printing over 16 min period.There was no significant interaction between the two variables, but a tendency toward interaction was observable in the second half period of practice. That is, the joint effect of simultaneous variation of both work and rest variables was a simple summation of the separate effects in the first half period of practice, but the work variable had a significantly higher effect than the rest variable on the second half.Of the two variables, the length of the rest period was more important. The relative importance, however, will depend on the kinds of task and the length of the two variables.The score differences produced by a variation in the amount of rest between trials are an increasing function of the amount of previous practice. The score differences due to the variation in the length of the practice trial became apparent at the 4 min practice although the amount was small, and stayed at a fairly constant level from that time on. The effect of changing the magnitude of one dose not depend upon the value of the other on the first half, but will be dependent upon each other on the second half.The hypothesis of independency and simple summation of work and rest variables was confirmed over shorter period of practice. With regard to the longer practice period, more data are needed to confirm the hypothesis. Thus it would be necessary to retest this hypothesis by extending the period of practice and the length of rest and work intervals.

Monaural Loudness Function at 1000 cps and Interaural Summation

A monaural loudness function is determined for 1000 cps and compared to a previously obtained binaural loudness function. Although the monaural function appears to have a somewhat smaller exponent than the binaural one, the empirical evidence does not permit rejection of the hypothesis of a simple interaural loudness summation. The psychophysical methods of magnitude estimation with and without a designated standard and of magnitude production without standards are used. A combination of magnitude estimation and production without standards is accepted as a method with minimum bias and classified under a more general methodological category called “psychological magnitude balance.”

Gallop rhythm and the physiological third heart sound: I. Characteristics of the sounds, classification, comparative incidence of the various types and differential diagnosis

1. 1. The only available criterion for distinguishing between physiological third heart sounds and gallop rhythm is the status of cardiac function. Both types of sound are accentuated by the same procedures. They have similar time relations, quality and intensity. Their positions of maximum audibility are identical except that right-sided physiological third heart sounds are unknown. 2. 2. Gallop and physiological third heart sounds invariably fall in either the protodiastolic or the auriculosystolic time zone. 3. 3. When these two time zones become superimposed, either as a result of tachycardia, minor grade heart block, sinus arrhythmia or auricular extrasystoles, gallop sounds may either appear for the first time, or, if already present, become markedly accentuated. This effect is so pronounced as to constitute a summation phenomenon. 4. 4. The hypothesis of summation is in accord with all known facts regarding gallop rhythm. Moreover, it offers a reasonable explanation for some of the variations in gallop sounds hitherto unaccounted for. 5. 5. A classification of physiological third heart sounds and gallop rhythm, based on Potain's classification of gallop rhythm, but altered to correspond with objective phenomena, is proposed. The erroneous concept of mesodiastolic gallop is discarded, and summation gallop is recognized. The classification abolishes the practical difficulties, heretofore encountered so frequently, in placing cases into proper categories. 6. 6. There appears to be no great disparity in the incidence of presystolic or protodiastolic gallop. Summation gallop is little if any less frequent. In can be produced in cases with either of the other types of gallop, or in cases with a predisposition to gallop, by the simple expedient of accelerating the cardiac rate enough to bring about superimposition of protodiastolic and presystolic events. 7. 7. Potain's conception of right-sided gallop rhythm, disputed by certain observers, has been confirmed by our investigations. It tends to occur in cases exhibiting predominantly right-sided heart failure. 8. 8. In the differential diagnosis of gallop rhythm and the physiological third heart sound, errors have been encountered involving one or another of the following: (1) the opening snap of mitral stenosis; (2) mid-diastolic murmur; (3) presystolic murmur; (4) reduplicated first heart sound; (5) reduplicated second heart sound. Other possible sources of error are (a) pericardial friction sound; (b) mid-systolic click; and (c) the so-called systolic gallop rhythm. In the great majority of cases differentiation is possible by clinical methods alone. In occasional cases, however, accurate timing requiring graphic methods is necessary.