Showing papers on "Vibration fatigue published in 1978"

A Study on Higher Mode Vibration of Ships (1st report)

Abstract: A method for the calculation of the coupled vibration has been derived using the concept of modal synthesis. The modal parameters such as frequency, modal mass and modal damping and the vibration mode of the coupled vibration can be calculated by the present method using the modal parameters, vibration mode and mass distribution of uncoupled vibration. The applicability of the method is studied to the higher mode vibration of ship structure which is the complicated coupled vibration of the substructures and the main hull girder. The isolated calculation of individual local structure, such as superstructure, double bottom in engine room etc can be combined to yield the final results coupled with main hull vibration. By the present method, it is easy to distinguish what is the main part of coupled vibration. This is very useful from the point of view of the structural design. The calculation and the experiment should be compared in order to confirm the validity of the method in the future works.

Reliability of machinery using fatigue damage accumulation due to random vibrations

Abstract: A new approach to machinery maintenance using fatigue damage accumulation theory is presented. The vibration generated by industrial equipment is related to the stress history experienced by the internal machine elements at the corresponding measurement points assuming a linear elastic and isotropic behavior. The resultant stress history is modelled as a piecewise stationary, Gaussian wide-band process. Employing Miner's linear damage hypothesis in conjunction with available constant-amplitude fatigue data, expression for the expected accumulated fatigue damage and its variance are developed. A machinery maintenance programme based on the accumulated damage parameters calculated directly from the properties of the exhibited vibration history is proposed. The main advantage of this method is that it provides equipment reliability in terms of known and measureable system properties.

Some Consideration on Fatigue Design Diagram for Ship Hull Structure

Abstract: Fatigue tests on ship hull structures have been conducted by R. Weck and J. J. W. Nibbering and also in Japan by Mitsubishi Heavy Industries, Ltd. and other shipbuilders. On the basis of the results of these tests, this paper deals with a tentative plan of a fatigue design diagram centering on consideration of fatigue crack initiation life.Structural members under consideration are transverse strength members with joints, longitudinal strength members with openings and longitudinals with scallops.In calculating damage (D), the hot spot stress is used which is obtained by multiplying nominal stress by given stress concentration ratio Kt in cracked part of each structural member and stress increase ratio Ke due to decrease in effective width of face bars.The diagram shows the lower limit of the relation between the hot spot stress and crack initiation life Nc. By using the diagram, frequency distribution of external force, and Miner's law, the possibility of fatigue failure of each structural member can be examined by whether damage (D) is greater than 1.0 or not.The author has verified the appropriateness of the design diagram by comparing it with analysis results of fatigue failures in actual ship and the diagram has been compared with the fatigue test results of basic welded joints of simple shapes and the fatigue test results of smooth, simple shape specimens by constant strain amplitude.

Fatigue Crack Propagation Analysis of Aircraft Structures.

Abstract: : This thesis is a comparative study of aircraft fatigue life calculations based upon crack propagation and upon cumulative damage. The stress concentration factor, which supplies sufficient geometric information for Miner's Law of cumulative damage, is found to not completely specify the geometry for the crack propagation approach. Effects on fatigue life of variations in initial crack length, plate width, hole size, and hole geometry for the same stress concentration factor have been investigated; also both ordered and random load histories were used to compare the two approaches. Complete FORTRAN computer program input documentation for the IBM 360/67 system has been included as an appendix to enable this thesis to serve as a user's manual for CRACK'S II, an Air Force crack propagation program for aircraft fatigue damage. (Author)

Fatigue Crack Growth under Random Loading (2nd Report)

Abstract: Analysis on fatigue lives under random service loading conditions has been extensively attemped to apply the design of offshore structures, ship hulls, pressure vessels and other various structures for the evaluation of the safety in service of their components.At present, Miner's cumulative law is used in most cases for the prediction of fatigue lives in spite of its large deviations and inaccuracies. The principal obstacles in the accurate prediction of fatigue lives are the interaction effects between sequential loading cycles which may cause retardation or acceleration of crack growth.The authors previously analyzed the fatigue crack growth retardation following single, multiple and intermittent overloads and proposed a semi-empirical model for the evaluation of these crack growth retardation processes using a concept of effective stress intensity factor range.In the present report, experimental studies were conducted on the fatigue crack growth under block programed loading, as the second stage of whole the present studies on random loading. The results of experiment showed that the fatigue crack growth behavior under block programed loading was much influenced by the block size and number of cycles in maximum stress range. In case where the block size is small, the interaction effects is relatively small. But the degree of retardation becomes larger as the block size increases.A semi-empirical model which was deduced from the retardation model was proposed for the prediction of these crack growth behavior. The life estimation by this model agree well with the experimental results.

Fatigue Life Estimation of a Local Element in Ship Hull Structures

Abstract: Fracture mechanics approaches have extensively been applied in the estimation of safety against fatigue fracture in ship hull structures. Fatigue failure often depends on the stress concentration, discontinuities in geometrical shapes and stiffness of local elements in the structure.This paper presents a series of numerical calculation on fatigue crack growth arround a man-hole in a double hull structure. Stress analyses were performed both by an ordinary finite element method and by a special one which used a substructure with a singular element in the crack tip region so as to calculate the stress intensity factors.Crack growth was analyzed by using Paris' formula. The direction of crack growth was determined by σθ criterion. A simplified analysis was tried for the calculation of the stress intensity factor using a semi-infinite plate model and the numerical results of the ordinary finite element analysis without crack. The influence of the parameter m in Paris' formula to fatigue life was as well discussed.

Relationship of the fatigue of metals

Abstract: The idea of using the characteristics of inelasticity for rating fatigue damage and predicting fatigue streng~l consists of the following. It is assumed that in the area of stresses not exceeding the fatigue limit, the mechanism of dissipation of energy differs from that for stresses exceeding the fatigue limit. With an increase in stresses the transition from one mechanism of dissipation of energy to another causes a change in the rules in the course of the amplitude relationships of the characteristics of inelasticity [1]. If the coordinates are transformed in such a manner that the amplitude relationships of the dissipation of energy up to the fatigue limit are a straight line, then it is not difficult to extend this rule into the area of stresses exceeding the fatigue limit and to determine the difference in energies caused by the inclusion of the new mechanism. This difference is the energy responsible for fatigue failure, which is called critical. Based on this, as the fatigue limit we will take the maximum stress for which the critical portion of the energy of dissipation will be equal to zero. As experience shows, for the majority of metals the amplitude relationships of the characteristics of inelasticity up to the fatigue limit to logarithmic coordinates are a straight line [2-4]. This means that these relationships are exponential. In this case the analytical expression for the critical portion of the energy of dissipation at a definite fixed test frequency may be presented in the form [5]