5,184 research outputs found
Time dependency of strainrange partitioning life relationships
The effect of exposure time (or creep rate) on the CP life relationship is established by conducting isothermal CP tests at varying exposure times on 316 Ss at 1300 and 1500 F. A reduction in the CP cycle life is observed with an increase in the exposure time of the CP test at a given inelastic strain-range. This phenomenon is characterized by modifying the Manson-Coffin type of CP relationship. Two new life relationships: (1) the Steady State Creep Rate (SSRC) Modified CP life relationship, and (2) the Failure Time (FT) Modified CP life relationship, are developed in this report. They account for the effect of creep rate and exposure time within the CP type of waveform. The reduction in CP cyclic life in the long exposure time tests is attributed to oxidation and the precipitation of carbides along the grain boundaries
A Single-Expression Formula for Inverting Strain-Life and Stress-Strain Relationships
Starting with the basic fatigue lift formula, an inversion formula is derived. The inversion formula is valid over the entire life range of engineering interest for all materials examined. Conformity between the two equations is extremely close, suitable for all engineering problems. The approach used to invert the life relation is also suitable for the inversion of other formulas involving the sum of two power-law terms
Fatigue life prediction in bending from axial fatigue information
Bending fatigue in the low cyclic life range differs from axial fatigue due to the plastic flow which alters the linear stress-strain relation normally used to determine the nominal stresses. An approach is presented to take into account the plastic flow in calculating nominal bending stress (S sub bending) based on true surface stress. These functions are derived in closed form for rectangular and circular cross sections. The nominal bending stress and the axial fatigue stress are plotted as a function of life (N sub S) and these curves are shown for several materials of engineering interest
Treatment of low strains and long hold times in high temperature metal fatigue by strainrange partitioning
A procedure for treating creep-fatigue for low strainranges and long hold times is outlined. A semi-experimental approach, wherein several cycles of the imposed loading is actually applied to a specimen in order to determine the stable hysteresis loop, can be very useful in the analysis. Because such tests require only a small fraction of the total failure time, they are not inherently prohibitive if experimental equipment is available. The need for accurate constitutive equations is bypassed because the material itself acts to translate the imposed loading into the responsive hysteresis loops. When strainrange partitioning has been applied in such cases very good results have been obtained
Interpolation and Extrapolation of Creep Rupture Data by the Minimum Commitment Method. Part 3: Analysis of Multiheats
The Minimum Commitment Method was applied to two sets of data for which multiple heat information was available. For one alloy, a 304 stainless steel studied in Japan, data on nine well characterized heats were used, while for a proprietary low alloy carbon steel studied in the United Kingdom data were available on seven heats - in many cases to very long rupture times. For this preliminary study no instability factors were used. It was discovered that heat-to-heat variations would be accounted for by introducing heat identifiers in the form A + B log sigma where sigma is the stress and the constants A and B depend only on the heat. With these identifiers all the data could be collapsed onto a single master curve, even though there was considerable scatter among heats. Using these identifiers together with the average behavior of all heats made possible the determination of an accurate constitutive equation for each individual heat. Two basic approaches are discussed for applying the results of the analysis
A new formulation of mean stress effects in fatigue
A common method of treating the mean stress effect on fatigue life is to displace the elastic line on a Manson-Coffin-Basquin diagram while retaining the position of the plastic line. Manson and Halford pointed out that this procedure implies that mean stress significantly affects the cyclic stress-strain curve. Actually, however, they showed experimentally and by more general reasoning, that mean stress has little, if any, effect on the cyclic stress-strain curve. Thus, they concluded that it is necessary to displace the plastic line as well as the elastic line in order to keep the cyclic stress-strain curve unaltered. Another way to express the common displacement of the two lines is to keep the lines in place and change the horizontal coordinate to include a term relating to the displacement. Thus, instead of life, 2N sub f, as the horizontal coordinate, a new coordinate can become 2N sub f (1-sigma sub m/sigma sub f) superscript 1/b, thereby displacing both the elastic and plastic lines by an amount (1-sigma sub m/sigma sub f) superscript 1/b where sigma sub m is the mean stress and sigma sub f is the intercept of the elastic line at N sub f = 1/2 cycles and b is the slope of the elastic line
Re-examination of cumulative fatigue damage analysis: An engineering perspective
A method which has evolved in our laboratories for the past 20 yr is re-examined with the intent of improving its accuracy and simplicity of application to engineering problems. Several modifications are introduced both to the analytical formulation of the Damage Curve Approach, and to the procedure for modifying this approach to achieve a Double Linear Damage Rule formulation which immensely simplifies the calculation. Improvements are also introduced in the treatment of mean stress for determining fatigue life of the individual events that enter into a complex loading history. While the procedure is completely consistent with the results of numerous two level tests that have been conducted on many materials, it is still necessary to verify applicability to complex loading histories. Caution is expressed that certain phenomena can also influence the applicability - for example, unusual deformation and fracture modes inherent in complex loading - especially if stresses are multiaxial. Residual stresses at crack tips, and metallurgical factors are also important in creating departures from the cumulative damage theories; examples of departures are provided
Life prediction of thermal-mechanical fatigue using strain-range partitioning
The applicability is described of the method of Strainrange Partitioning to the life prediction of thermal-mechanical strain-cycling fatigue. An in-phase test on 316 stainless steel is analyzed as an illustrative example. The observed life is in excellent agreement with the life predicted by the method using the recently proposed Step-Stress Method of experimental partitioning, the Interation Damage Rule, and the life relationships determined at an isothermal temperature of 705 C. Implications of the study are discussed relative to the general thermal fatigue problem
Metal cooldown, flow instability, and heat transfer in two-phase hydrogen flow
Studies of the properties of five metals with varying tube-wall thickness, with or without and internal coating of trifluorochloroethylene polymer, show that wall characteristics influence flow stability, affect heat transfer coefficients, and influence the transition point from dry- to wet-wall flow
Practical implementation of the double linear damage rule and damage curve approach for treating cumulative fatigue damage
Simple procedures are presented for treating cumulative fatigue damage under complex loading history using either the damage curve concept or the double linear damage rule. A single equation is provided for use with the damage curve approach; each loading event providing a fraction of damage until failure is presumed to occur when the damage sum becomes unity. For the double linear damage rule, analytical expressions are provided for determining the two phases of life. The procedure involves two steps, each similar to the conventional application of the commonly used linear damage rule. When the sum of cycle ratios based on phase 1 lives reaches unity, phase 1 is presumed complete, and further loadings are summed as cycle ratios on phase 2 lives. When the phase 2 sum reaches unity, failure is presumed to occur. No other physical properties or material constants than those normally used in a conventional linear damage rule analysis are required for application of either of the two cumulative damage methods described. Illustrations and comparisons of both methods are discussed
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