Research on failure of equipment when subject to vibration

Abstract

This report is in the nature of a progress report; it sets forth in some detail the status of the research on the several tasks as of the date of the report. In general, the research was incomplete on that date and no conclusions are presented. The report is written primarily to keep parsonnel of the sponsoring agency apprised of work that has been accomplished. It is not intended for outside distribution or publication. This is the first annual report under a continuing research project whose objective is to gain a better understanding of the failure of equipment when subjected to vibration. This is essential to (1) the attainment of improved practices in design of equipment that is required to withstand vibration and (Z) to the application of more rational procedures for conducting vibration tests in the laboratory. In principle, the laboratory test creates a vibration condition that causes such failure of the equipment as would occur during actual service use. However, a laboratory test cannot reproduce the service condition in all of its details; thus, a good knowledge of the mechanics of failure is necessary to relate laboratory and service conditions in a constructive manner. The research program includes two general aspects that are being pursued concurrently, at least in the initial phase of the research program: 1. Actual electronic and mechanical components of equipment with known susceptibility to failure; e.g., a vacuum tube in the initial task, are subjected to vibration of various defined types. The components are connected in circuits designed to simulate a typical application and the effect of vibration on the operation of the circuits is monitored. An objective of the experiment is to infer from the circuit operation and response of the component an analog that may be used to describe the characteristics of the component that are significant with respect to vibration. Such analogs are then used in further analyses of the component. 2. Concurrent with the above experiments using actual components, a group of analogs having apparent application to known components is selected initially. Additional analogs are added to this group as experimental evaluation of actual components proceeds. The response of each selected analog to various idealized forms of vibration is investigated. Then from the relation between the response of the analog to such vibration and observed performance of the component to which the analog is related. the response of the component and its operational capability under more general types of vibration can be predicted. A representative vacuum tube has been mounted upon a vibration exciter and subjected to both sinusoidal and random vibration. Its performance was Monitored by noting the voltage output (microphonic noise) that could be ascribed to vibration. This report includes records of output voltage as a function of vibration frequency, for further use in attempting to infer an appropriate mechanical analog. Five idealized forms of excitation have been selected for initial investigation: (1) sinusoid, (2) combined sinusoids, (3) scanning or sweeping sinusoid, (4) broad band random with Gaussian distribution and (5) broad band random with magnitude limitation. For each type of excitation, the response of the most common mechanical analog, a damped, single degree of freedom system, has been analyzed. By relating the characteristics of the analog to an actual component and applying appropriate criteria of failure, these responses are of use in predicting the performance capability of the component. For example, consider a structure vulnerable to failure by fatigue; then the distribution of response cycles included in the analysis of the analog response when taken with hypotheses of cumulative damage is applicable to predict the fatigue life. Some experiments on fatigue have been carried out, with the objective of determining whether existing hypotheses of cumulative damage are applicable to the problem being discussed here. A simplified structure having certain characteristics of a typical equipment has been subjected to fatigue tests. Because of certain unanticipated characteristics of the response of the structure, it has become evident that additional experiments are required to interpret the results obtained to date. This phase of the program is continuing. Section 1 of the report includes a detailed discussion of research program, its hypotheses and objectives. A brief summary of the results also is included in Section 1, with references to other sections of the report and to the appendices when complete details are set forth.

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