Showing papers on "Vibration fatigue published in 1984"

Stochastic Fatigue Damage Accumulation

Abstract: A study is made of several aspects of the problem of predicting fatigue failure by using the Palmgren‐Miner (PM) approximation, with cycles defined by the rainflow procedure. The stress time history is considered to be a stationary stochastic process, and the term fatigue is considered to refer to crack initiation or crack propagation, or both. It is first shown that PM analysis is consistent with a fairly broad class of theoretical models that give nonlinear growth of damage. Next, simulation results are presented for a bandwidth correction factor, relating damage predicted from rainflow cycles to that predicted from narrowband analysis. It appears that there may be a need to use some bandwidth parameter other than the irregularity factor in order to obtain consistent results for this correction factor. Results are also presented regarding the effect of nonnormality of the stress process. Both analytical and simulation studies show that this effect may be quite significant.

Continuous measurement of material damping during fatigue tests

Abstract: An experimental procedure for continuously measuring strain level, temperature, and energy-dissipation rate during resonant fatigue tests is described. The technique is based on a previous method for measuring loss factor (Ref. 3) using base-excited cantilever-beam specimens vibrating at resonance. The amplitude and frequency dependence of loss factor is therefore included directly in the measurement. For beams vibrating in vacuum, energy-dissipation rate and temperature measurements provide a basis for irreversible thermodynamic analysis of fatigue. This procedure provides a continuous measurement of energy-dissipation rate during fatigue-crack nucleation, and is the basis for experimental study of the hypothesis that the entropy gained during fatigue failure is a material constant.

Assessment of random vertical vibration on human body fatigue using a physiological approach

Abstract: Most of the studies of the effects of mechanical vibration on man are based on investigations into the effects of harmonic vibration, although there is evidence that the human body is more sensitive to the effects of stochastic ones. To create optimum oscillation parameters in transport vehicles, it is necessary to consider the reaction of the human body to stochastic vibration. This paper analyses this problem from a physiological point of view. It concurs with the hypothesis adopted in the literature that vibration exposure in transport vehicles must not exceed that associated with normal walking. Vertical, lateral and longitudinal acceleration of the head was recorded during normal walking, and its frequencies were analysed. The results are used to reach conclusions on the permissible vibration strain to which the human head may be exposed in transport vehicles. For the covering abstract of the conference see TRIS 451834. (TRRL)

On fatigue-creep-environment interaction and the feasibility of fatigue maps

Abstract: The degradation of fatigue properties with increasing temperature and decreasing frequency is generally attributed to the fatigue-creep-environment interaction. A recently developed model which can be used to quantitatively predict the critical frequency below which intrinsic creep damage, i.e., intergranular cavitation, will occur, has been introduced in this paper. Based on this model, it is argued that for most laboratory tests the reported frequency dependence of fatigue life was mainly due to environmental degradation rather than creep damage. The vital role of the environment in determining the fatigue fracture mode is emphasized. Examples of fatigue cavity nucleation maps and fatigue crack propagation maps are included to explicitly display the various mechanisms involved in fatigue crack initiation and propagation.

The application of cumulative damage fatigue theory to highway bridge fatigue design

Abstract: The influence of small stress cycles caused by the dynamic response of a bridge upon the fatigue life of welded components was studied. Various loading waveforms were used to load a cantilever welded tee specimen. The fatigue life was measured and means of transforming the loading waveform to constant amplitude waveform producing the same damage were investigated. The waveforms investigated included actual stress histories measured on an in-service bridge loaded with both a single test vehicle and under normal traffic. The results of the study indicate that the small stress cycles cause considerable fatigue damage and cannot be ignored in the design and evaluation of steel bridges for fatigue. Based on the results of the experimental study and an evaluation of the stress histories of three bridges, a simple means for estimating the damage done by these small cycles was developed using a fatigue factor. The design stress range including the normal AASHTO impact fraction for a single vehicle passage should be multiplied by a fatigue factor of 1.15 to include the fatigue damage done by these minor cycles. The factor of 1.15 is the best estimate for medium span girder bridges. Other type and span bridges may produce different values according to their dynamic behavior. The most accurate means of obtaining this value is through field stress measurements of the actual bridge.

Evaluation of Spectrum Fatigue Data Under Conditions Applicable to Welded Steel of Offshore Structures

Abstract: In order to most effectively interpret variable amplitude fatigue data it is necessary to establish a parameter which characterizes the particular stress history and which permits test results to be correlated with constant amplitude data. In the present research a statistical stress analysis theory is reviewed which permits calculation of an equivalent stress for a realistic (variable amplitude) fatigue loading spectrum, and this has been applied toward interpretation of research data where welded steel specimens were fatigued in flowing natural sea water. Analysis of various alternative equivalent stress parameters has permitted determination of how variable amplitude corrosion fatigue data may best be correlated with constant amplitude results. In addition, the equivalent stress concept has been extended to fracture mechanics analysis in terms of an effective stress intensity; and fatigue data has also been evaluated in terms of this. The resultant correlations indicate good agreement between calculated and experimental results.

On the fatigue crack propagation behavior under repeated impacts using a new type of impact fatigue testing machine.

Abstract: Machines and structures often encounter repeated impacts in practice. So, it is very important to study fatigue fracture of steel under repeated impacts. Usually a rotating disk type impact fatigue testing machine is used for such study, but the analysis of the data obtained by using this machine is considerably difficult, because the impact phenomenon is transient and has complex characters. So, a new type of machine was made by modifying a rotating disk type impact fatigue testing machine, and fatigue tests under repeated impacts were carried out. Then, the influences of velocity of repeated impacts and stress ratio on the fatigue crack growth rate of stainless steel were investigated by means of fracture mechanics and fractography. The difference in crack growth rate between repeated impacts and more gradually-applied cyclic load was large. This difference was explained by the effective stress intensity factor based on the equivalent stress ratio which includes the repeated impact loading effect.

Stochastic Estimation on the Fatigue Strength under Variable Stress Amplitude

Abstract: In this study authors proposed the stochastic model for the fatigue crack initiation and discussed on the estimation method of fatigue life under variable stress amplitude. The outline of the model is as follows. In fatigue test, although great many micro fatigue damages are occurred in the material, the number of cracks which can grow up to visual length is limited, because many micro cracks can not penetrate the grain boundary. Actual fatigue crack in itiates from the position where the number of cycles for visual crack initiation takes the smallest value in the material.In the model, the crack initiation life, Nc, is expressed as the sum of ND and NW, where ND is a number of cycles consumed by the crack propagation of transgranulars, NW is a number of cycles consumed by the crack penetration of grain boundaries. Since ND and NW are different with the positions in the material, Nc takes several values. To estimate the smallest number of cycles of Nc, smallest value distribution is introduced in each variable stress amplitude.In this paper, it is considered with the conditions of constant stress amplitude and variable stress amplitude with the exponential and the uniform stress frequency distribution. Estimated results are a good agreement with the experimental results qualitatively. It is clarified that the fatigue strength under variable stress amplitude may be stochastically estimated by the consideration of fatigue damage induced by the stress below fatigue limit.