Showing papers on "Vibration fatigue published in 1987"

Fatigue life prediction in bending from axial fatigue information

Abstract: 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.

Rain flow cycle distributions for fatigue life prediction under Gaussian load processes

Abstract: We present a new and simple definition of the Rain Flow Cycle count method for the analysis of a random load process. It is combined with the Palmgren-Miner damage rule, and a stochastic model for the fatigue life and fatigue limit variability. Algorithms are presented which make it possible to calculate the RFC-amplitude distribution, based on a Markov Chain approximation of local maxima and minima. The method derived would apply to structures subjected to random fatigue loads such as acoustic noise, random vibration or sea waves, etc.

Reliability and Durability of Marine Structures

Abstract: A study of the dynamic response of offshore towers to wind‐generated random ocean waves is presented. The analysis is performed in the frequency domain and the equations of motion are solved using equivalent linearization techniques. Two methods of reliability analysis are considered in this study: a statistical fatigue damage analysis using the American Welding Society model (high‐cycle fatigue analysis) to demonstrate the influence of the design stress level for fatigue‐critical members on the fatigue reliability and a first‐passage failure probability with periodic inspections. For the latter, a crack growth analysis is performed using the fracture mechanics method to estimate propagated crack size and corresponding residual strength under random service loading conditions. The effect of periodic inspections is very important if a fatigue crack already exists. It is also shown that there is always a limit in the number of inspections beyond which no significant improvement can be achieved.

Non-Gaussian response of offshore platforms: fatigue.

Abstract: A fatigue analysis procedure is proposed for offshore platforms where the response is non‐Gaussian due to nonlinear wave kinematics and free‐surface fluctuations. The non‐Gaussian distribution of the stress process is considered to be a mixture of Gaussian and shifted exponential distributions. The level crossings of this process are estimated considering it a translation process, and the probability density function of the peaks is estimated numerically. Using Palmgren‐Miner's rule for fatigue damage accumulation, the total cumulative damage over the long‐term wave climate is estimated. It is shown that at higher sea states, where the effect of intermittent loading is significant, the probability density function of the peaks of the stress deviates significantly from the normally used Rayleigh density. The assumption of Rayleigh density for peaks at higher sea states is shown to be unconservative, and the unconservatism increases with an increase in significant wave height. Since field observations confi...

Response of stiffened panels for applications to acoustic fatigue

Abstract: The surface protection systems of aerospace and aircraft structures are often constructed from discretely stiffened panels. This paper presents an analytical study on dynamic stress response of these structures to random surface flow and acoustic loads. To account for aerodynamic heating of high speed flow, thermal effects are included in the structural model. A generalized transfer matrix procedure is developed to obtain the required dynamic response solutions. Numerical results include spectral densities of stress, response root mean square values and fatigue damage for a variety of loading and thermal conditions.

Structural Modification To Minimize Response Of Electro-Optical Systems To Random Excitation

Abstract: Future electro-optical space systems require low ambient vibration response levels in the presence of numerous broad and narrow-band disturbances. A structural design methodology which addresses this vibration suppression problem is devised. Dynamic response is described in terms of random vibration theory. Input power spectral density is represented compactly in terms of its poles and zeroes. Output power spectral density is integrated in closed form using residue theory to obtain root mean square re-sponse. An optical performance index based on this response is minimized subject to a constraint on structural weight. That structural design which provides the best combination of stiffening, tuning and mode shape changes is thus employed. Large gains in optical performance with little or no weight penalty are demonstrated.

The LIFE computer code: Fatigue life prediction for vertical axis wind turbine components

Abstract: The LIFE computer code was originally written by Veers to analyze the fatigue life of a vertical axis wind turbine (VAWT) blade. The basic assumptions built into this analysis tool are: the fatigue life of a blade component is independent of the mean stress; the frequency distribution of the vibratory stresses may be described adequately by a Rayleigh probability density function; and damage accumulates linearly (Miner's Rule). Further, the yearly distribution of wind is assumed to follow a Rayleigh distribution. The original program has been updated to run in an interactive mode on a personal computer with a BASIC interpreter and 256K RAM. Additional capabilities included in this update include: the generalization of the Rayleigh function for the wind speed distribution to a Weibull function; the addition of two constitutive rules for the evaluation of the effects of mean stress on fatigue life; interactive data input; and the inclusion of a stress concentration factor into the analysis.

Computer-Aided Fatigue Design of Power Transmission Shafts Using Three-Dimensional Finite Shaft Element and Updated Mean Stress Diagram

Abstract: A three-dimensional flexural shaft element is developed for elastic analyses of power transmission shafts and geared systems. It is used for the computer-aided fatigue design of three-dimensionally loaded power transmission shafts on many rigid or elastic bearings. Bagci’s mean stress diagram—which removes the flaw related to the load line slope of zero value in the presently used mean stress theories—is used to determine the failure value of the reduced fatigue strength of the shaft at the critical sections considered. Basic stresses are defined using maximum energy of distortion theory. An interactive computer program is prepared for fatigue design of power transmission shafts, in which some of the strength reducing factors for each notch location are determined using generalized expressions. The program initially locates the weakest section on the shaft, then it redefines the dimensions for the desired factor of safety, reliability and the fatigue life at that section. An application example is included.

Fatigue crack propagation under service loading, the third report: estimating method for fatigue crack growth under narrow band random loading

Abstract: It is essential to be able to estimate the fatigue strength of structural members under random in-service loading on the basis of conventional fatigue test data. In a previous paper by the authors, fatigue crack propagation under narrow-band random loading was studied and the relationship between fatigue behaviors under random loading and constant amplitude loading was discussed. The consideration of crack closure was assessed as indispensable to the examination of fatigue behavior under random loading. This paper deals with additional fatigue tests that were conducted to obtain data under different stress conditions. Crack closure in the specimens was automatically recorded over an extended period of fatigue cycles, using a strain gage method aided by a microcomputer system. The significance of crack closure in understanding crack propagation under variable-amplitude loading is emphasized and a method for estimating fatigue crack behavior under random loading is proposed.

Fatigue of gas-turbine blades in random vibrations

Abstract: The critical parts of modern machines are subjected to a complex combination of loads. With improvement in machines the degree of stress of the parts and assemblies increases. Therefore, to evaluate the reliability it is necessary to study in more detail the structure and amount of the effective variable mechanical stresses and the resistance to these stresses of machine parts. Many critical aircraft parts and assemblies (the rotating blades of helicopters, aircraft airfoils and skins, and the blades and other parts of aircraft gas-turbine engines) are subjected to random loads which may be characterized as narrow-band random oscillatory processes [i].

Estimating Method for Fatigue Crack Growth under Narrow Band Random Loading

Abstract: It is essential for the structural engineer to be able to estimate fatigue strength of structural members under the effect of randomness in service loading on the basis of the conventional fatigue test data. In the previous paper, fatigue crack propagation tests under narrow band random loading were carried out and the relationship between fatigue behaviors under random loading and constant amplitude loading was discussed. It was concluded that consideration of crack closure was indispensable for examination of fatigue behavior under random loading.In this study, additional fatigue tests are conducted in order to obtain the data under different stress conditions. Crack closure phenomena of the specimens are automatically recorded over a long period of fatigue cycles using strain gage method aided by microcomputer system.The significance of crack closure for understanding crack growth under variable-amplitude loading is emphasized and an estimating method for fatigue crack behavior under random loading is proposed.