Showing papers on "Vibration fatigue published in 1968"

Random load fatigue testing: a state of the art survey

Abstract: NEW TECHNIQUES FOR THE EVALUATION OF FATIGUE LIFE FOR METAL STRUCTURES ARE ANALYZED. THE TECHNIQUE FOR CONTINUOUS LOAD ANALYSIS, REFERRED TO AS PSD ANALYSIS (AFTER THE POWER SPECTRAL DENSITY PLOT WHICH RESULTS) IS BECOMING RECOGNIZED AS A VALID APPROACH TO THE DELINEATION OF FATIGUE LOADS. THE PSD PROFILE IS THE RESPONSE IN STRESS OF A STUCTURE TO UNIT ENERGY SINEWAVE EXCITATION AT EACH FREQUENCY, OVER THE FREQUENCY RANGE OF INTEREST IN SERVICE LOADING. ONCE THE SERVICE LOADS HAVE BEEN ANALYZED, THE FATIGUE ENGINEER USES ONE OF THE FOLLOWING FOUR DIFFERENT METHODS TO FIND A STATISTICALLY-DEFINED LOAD SPECTRUM FOR THE VEHICLE IN QUESTION: (1) CUMULATIVE DAMAGE THEORY WITH CONSTANT AMPLITUDE S/N CURVES FROM TESTS, OR FROM THE LITERATURE, (2) REPEATED BLOCKS OF A PROGRAM OF CONSTANT AMPLITUDE CYCLES AT STRATIFIED STRESS LEVELS IN ARBITRARY SEQUENCE IN APPROXIMATE CONFORMITY TO THE RELATIVE FREQUENCY OF SERVICE LOADS FROM THE LOAD SPECTRUM, (3) A TEST LOADING INVOLVING GROUPS OF CONSTANT AMPLITUDE CYCLES OR INDIVIDUAL CYCLES USUALLY CALLED UP IN A COMPUTER-RANDOMIZED SEQUENCE OF VARYING LOAD LEVEL, BUT WHOSE NUMBERS AT A GIVEN LOAD LEVEL ARE PRESELECTED TO RESULT IN THE FINAL RELATVIE FREQUENCY DICTATED BY THE SERVICE LOAD SPECTRUM, AND (4) USING EITHER THE ACTUAL RANDOM PROCESS, AS WITH ACOUSTIC FATIGUE, OR WITH SAMPLES FROM SERVICE, OR AN ANALOGOUS RANDOM PROCESS WITH SIMILAR GENERAL CYCLE CORRELATION CHARACTERISTICS TO THE PROCESS IN SERVICE, EITHER AS A STATIONARY PROCESS, OR WITH THE RMS PROGRAMMED TO RESULT IN THE SAME LOAD FREQUENCY AS GIVEN BY THE LOAD SPECTRUM. THE GROWTH OF TEST DATA IN THE LAST TWO CATEGORIES IS SPECIFICALLY DESCRIBED. LITERATURE AVAILABLE IN TWO OTHER RANDOM LOAD TEST AREAS ARE DESCRIBED: ACOUSTIC FATIGUE AND RANDOM LOAD CRACK PROPAGATION. A FEW EXAMPLES ARE PRESENTED OF TESTING PRACTICAL STRUCTURES WITH BOTH STATIONARY AND PROGRAMMED RMS STRESS LEVELS. THE UNIVERSALITY OF ENDURANCES OBTAINED FOR A GIVEN MATERIAL TESTED WITH A STANDARD FREQUENCY DENSITY PROFILE (PRESENTED IN THE FORM OF AN RMS-STRESS LIFE CURVE) IS NO LESS THAN THE CORRESPONDING S-N DATA FOR THE SAME CONDITIONS. TABLES ARE INCLUDED CONCISELY, LISTING MUCH OF THE AVAILABLE RANDOM FATIGUE ENDURANCE DATA, WITH SOURCES, FOR SIMPLE SPECIMENS. NINETY PAPERS WERE FOUND TO CONTAIN RELEVANT TEST DATA, AND THE RESULTS OF APPROXIMATELY 3,000 TESTS OF VARIOUS TYPES ARE TABULATED WITH CROSS REFERENCES.

Finite-Element Analysis of Complex Panel Response to Random Loads.

Abstract: : The report describes and demonstrates a computerized analysis method for development of sonic-fatigue-resistant structure. Finite-element matrix methods are used to calculate elastic response of complex structure subjected to random pressure loads with random dependence on both space and time. A cross-power spectral density loading function is developed for laboratory noise sources. The methods are demonstrated by comparing computed random response and fatigue failure location of skin-stringer and honeycomb sandwich panels with multipanel measurements taken in a sonic test laboratory. The comparisons include natural frequencies, normal modes, rms and PSD deflection and stress, and prediction of fatigue failure location. An extension is required to account for nonlinear response in order to realize the full potential of finite element methods when applied to development of sonic fatigue resistant structure. (Author)

Effect of the Superimposed Stress of High Frequency on Fatigue Strength

Abstract: The fatigue property of low carbon steel under the composite stress which was made by superposing the ordinary rotating bending stress and the vibrating stress of higher frequency was examined in this study. The experimental results were discussed by focusing on the effect of the ratios of the magnitudes and the frequencies of component stresses. It was ascertained that in some ranges of these parameters the fatigue life under the composite stress could be estimated by the criterion that the composite stress was equivalently replaced by the stress of constant amplitude whose magnitude was the sum of those of the component stresses. For the values of the parameters outside the ranges, the criterion did not hold and the fatigue life was reduced appreciably from the value given by the criterion. Another method for estimating the fatigue life by calculation was also presented, which was available for the latter case.

Influence of Details on Fatigue Behavior of Structures

Abstract: The behavior of a structure subjected to cyclic loads is dependent upon a number of details, either intentional or unintentional, which could be potential initiation sites. Since the over-all effect of such details on fatigue strength cannot be calculated accurately, design values have usually been based on laboratory data generated in tests of specimens that simulate the characteristic stress-raiser of the actual detail. The data from tests of butt- and fillet-welded joints, beam details, riveted and bolted joints in base materials of the common classes of structural steels are presented and analyzed. Details have a major influence on the fatigue behavior of structures because they result in: (1) localized increases in stress (above the nominal stress); and (2) residual stresses. The test data generated in the laboratory have provided a basis for establishment of allowable stresses and the development of proper design techniques to prevent fatigue failures in structures. In addition, methods have been developed to improve the fatigue strength of structures containing critical details by minimizing the stress concentration or favorably altering residual stresses, or both.

On the Dynamical Stress Distribution of Stiffened Plate at Vibration (1 st report)

Abstract: One of the causes for the crack damage on the stiffened bulkhead in the neighbourhood of the joint with stiffeners is the fatigue fracture due to the vibration of the plate accompanied by the torsional and bending vibration of the stiffeners. The basic equation representing the stiffened plate which is freely vibrating are introduced by combining the plane stress theory and the equation of lateral vibration of plate, and the dynamical stress distribution of stiffened plate is analysed in the state of its free vibration to make this cause clear. The eigen-values and dynamical stress distributions of the simply supported rectangular plate stiffened by a inverted angle at its centre line are investigated theoretically and experimentally. The experimental results agree fairly well with theoretical ones. The dynamical bending stress in the plate produced in the neighbourhood of the intersection of stiffener and plate are considerably large. Assuming the fatigue limit as σF= 22 kg/mm2, at the time when the maximum dynamical bending stress of the vibrating stiffened plate fall on that fatigue limit, the ratio of the maximum deflection of the plate to the stiffener space is about 2× 10-21 × 10-3 for the 1 st3rd mode. But the assumed fatigue limit is the value under an ideal condition, and therefore, under an actual condition, this value will decrease to 1/2 1/3, provided that the shape factor as well as the change in quality of materials due to welding and the influence of corrosion are considered. Thus it is conceivable that the fatigue fracture of the stiffened plate may be caused by local vibration of ship.