Showing papers on "Vibration fatigue published in 1982"

Fatigue Reliability: Variable Amplitude Loading

Abstract: Models which designers use to predict fatigue under variable amplitude and random loading are summarized. Basic terminology associated with random process theory is presented. Models for computing fatigue damage associated with various statistical distributrions of stress ranges are reviewed. The Palmgren-Miner rule is described and statistical summaries which provide a description of the performance of Miner's rule is presented. A closed form expression for fatigue damage under wide band stress spectra is given. Finally, methods of predicting fatigue crack growth under variable amplitude stresses are reviewed.

Fatigue damage of turbine-generator shafts due to fast reclosing

Abstract: Breaking in a large electric power system causes torsional vibrations on the turbogenerator shafts. Damping of such vibrations is very low compared with the damping of the electrical system. Therefore, during a strong electric transient, the shafts connecting the turbine and the generator rotors may undergo heavy loading and considerable deformations, both elastic and plastic, for prolonged periods of time. As a result, the fatigue life of the shafts may be substantially reduced. The paper introduces an improved mechanical dynamic model of the system for torsional response, which can account for the plastic material model, to yield the cumulative damage during the transients due to high and low cycle fatigue. Moreover, a procedure for the calculation of fatigue life expenditure is developed based on the `equivalent strain range? derived from a known twist angle history of the shaft. A detailed discrete model for both the mechanical and the electrical system was used in conjunction with modelling of the fast reclosing mechanism. In this manner, fatigue life reduction due to a certain fast reclosing strategy was evaluated. The system thus developed can lead to a better understanding of the mechanism of fatigue life reduction due to the electrical transients.

Use of Low-Cycle Fatigue Data for Pressure Vessel Design

Abstract: Pressure vessels are usually designed for only a few thousand stress cycles and seldom for more than 10' cycles. For fatigue analysis to be practical for most of the ASME Code applications, a concept has been developed different from the practice used in machine design for high-cycle fatigue in rotating machinery. The concept uses fatigue design curves developed for the different materials. The curves account for most of the factors that reduce fatigue life as shown by a comparison with fatigue data generated with laboratory specimens. Because most pressure vessels are of welded fabrication, the fatigue design curves have full corrections for tensile mean stress in the applicable region to account for tensile residual stresses. For corrosive conditions, the fatigue design curves may require further corrections. A modified concept of strain range partitioning has been developed for the analysis of the creep and fatigue interaction for high-temperature design. The concept uses the strain difference range to calculate creep and fatigue damage, which is similar to the shear stress failure used in lower temperature design. The use of the fatigue design curves is demonstrated with the evaluation for fatigue life of two fossil-fuel boiler components. One of these components illustrates the difference in use of the fatigue design curves when creep is a further consideration.

An improved methodology for predicting random spectrum load interaction effects on fatigue crack growth

Abstract: Fatigue crack-growth data for center-through cracks contained in 2219-T851 aluminum plates subjected to random spectrum loadings were generated. A crack-growth prediction methodology which accounts for overload retardation and compressive load acceleration effects on fatigue crack-growth was developed using the generalized Willenborg retardation model as the base. Fatigue crack-growth analyses were performed employing the proposed methodology. Analytical predictions were correlated to the test data. Good correlations were shown. KEY WORDS Fatigue crack-growth, random spectrum loading, retardation, acceleration, 2219-T851 aluminum, fracture control plan.

The significance of dynamic response in the estimation of fatigue life

Abstract: The fatigue life of offshore structures is investigated under the conditions that dynamic response to waves is assumed to play a significant role. Under these conditions, the study emphasizes the significance of the placement of natural frequencies and the amount of modal damping. The results may be used to assess the confidence bounds on fatigue life estimates that result from uncertainties in design stage estimates of structural natural frequencies, and damping.