Showing papers on "Vibration fatigue published in 1972"

Fatigue damage factor in structural design

Abstract: When the structure is subjected to repeated stresses with cyclic constant magnitudes, the resulting fatigue life is known to be random In reality, structures are subjected to various levels of repeated stresses To facilitate the design of a structure subjected to a stress history with given load repetitions at each of several stress levels, a fatigue damage factor and a damage index are introduced as design parameters The problem is formulated with the use of the Palmgren-Miner's cumulative damage theorem, by treating the denominators in the theorem as random variables rather than constants The design diagrams suggested can be constructed from single stress-level fatigue test data The design approach needs only to draw parallel lines in the design diagrams in the case of structural design against multi-stress-level fatigue To illustrate the relationship of the fatigue damage factor to cyclic stresses, number of cycles and failure probabilities, two numerical examples are also given

Bridges: synthesis of load histories and analysis of fatigue part iii

Abstract: THE PURPOSE WAS TO RECONSTITUTE OR SYNTHESIZE THE LOAD HISTORY OF CERTAIN BRIDGES AND TO DEVELOP A LOGICAL METHOD OF ESTIMATING THEIR REMAINING SERVICE LIFE. FIRST FATIGUE TESTING IS BRIEFLY EXAMINED AS REVIEW MATERIAL FOR UNDERSTANDING THE BASIC CONSIDERATION - FATIGUE EQUATION, VEHICLE LOADING DISTRIBUTIONS, TRANSFORMATION OF LOAD DISTRIBUTIONS INTO STRESS DISTRIBUTIONS, AND TRANSFORMATION OF STRESS DISTRIBUTIONS INTO FATIGUE HISTORY. COMPUTATION OF TOTAL FATIGUE INVOLVES (1) DETERMINING THE PROBABILITY OF THE NUMBER OF REPETITIONS OF EACH VEHICLE LOADING CONFIGURATION; (2) TRANSFORMING THE VEHICLE LOADING DISTRIBUTION GENERATED IN (1) INTO A CORRESPONDING DISTRIBUTION OF STRESSES IN STRUCTURAL MEMBERS; (3) DETERMINING THE APPROPRIATE FATIGUE CURVE FOR THE MEMBER BASED ON AVAILABLE DESIGN CRITERIA; (4) DETERMINING THE EQUIVALENT BRIDGE LOADING CONTRIBUTION RESULTING FROM THE APPLICATION OF ONE. STRESS IN EACH STRESS INTERVAL; (5) MULTIPLYING THE NUMBER OF STRESS APPLICATIONS IN EACH STRESS INTERVAL BY THE CORRESPONDING EQUIVALENT BRIDGE LOADING FACTOR (EBLF); AND (6) ADDING EBLF'S OVER ALL STRESS GROUPS AND COMPARING THE TOTAL TO THE MAXIMUM SAFE VALUE, (5) AND (6) ARE COMBINED INTO AN EQUATION. IT MUST BE NOTED THAT THE EXTENSION OF FATIGUE LIFE BY REINFORCING THE MEMBER WAS EXCLUDED FROM THIS STUDY.

Fatigue Tests with Random Flight-Simulation Loading

Abstract: Crack propagation was studied in a full-scale wing structure under different simulated flight conditions. Omission of low-amplitude gust cycles had a small effect on the crack rate. Truncation of the infrequently occurring high-amplitude gust cycles to a lower level had a noticeably accelerating effect on crack growth. The application of fail-safe load (100 percent limit load) effectively stopped subsequent crack growth under resumed flight-simulation loading. In another flight-simulation test series on sheet specimens, the variables studied are the design stress level and the cyclic frequency of the random gust loading. Inflight mean stresses vary from 5.5 to 10.0 kg/sq mm. The effect of the stress level is larger for the 2024 alloy than for the 7075 alloy. Three frequencies were employed: namely, 10 cps, 1 cps, and 0.1 cps. The frequency effect was small. The advantages and limitations of flight-simulation tests are compared with those of alternative test procedures such as constant-amplitude tests, program tests, and random-load tests. Various testing purposes are considered. The variables of flight-simulation tests are listed and their effects are discussed. A proposal is made for performing systematic flight-simulation tests in such a way that the compiled data may be used as a source of reference.

An Investigation of Fatigue Failure of Materials under Broad Band Random Vibrations

Abstract: I the present paper the fatigue life of material under the multifactor influence of broad band random excitations was investigated. Parameters which affect the fatigue life were postulated to be peak stress, variance of stress, and the natural frequency of the system. Experimental data were processed by the hybrid computer. Based on the experimental results and the regression analysis, a best predicting model has been found. All values of the experimental fatigue lives are within the 95% confidence intervals of the predicting equation. Research results in narrow band random fatigue have been reported.'

Summary of Test Results for Aluminum Alloy Box Beam Fatigue Program, Test Phases I-IV

Abstract: : The results of a four-phase fatigue program for bending tests of 7075-T6 aluminum alloy box beams for positive loads only and for positive and negative loads for both constant-and variable-load amplitudes are presented The relative damaging effect of four airplane flight-maneuver-loads spectra was determined, and the effects on fatigue life for variations in spectrum block size, stress level, stress direction, and load sequence were established and the results are discussed When compared to full-scale aircraft structures of like material under constant-amplitude unidirectional loading on a percent of ultimate strength basis the beams represented the upper bound of those data for full-scale structures and exhibited similar fatigue characteristics The beams were thus established as a suitable idealized structure for the investigation of those parameters which affect the structural fatigue life