Relative Conservatism and Drop Table and Shaker Shock Tests

摘要

Judging the conservation of a mechanical shock test is a crucial concern of the engineer responsible for interpreting the outcome of the test. Either an overtest or an undertest can have devastating implications if the former results in a costly redesign of the structure being tested, or if the latter fails to properly qualify the structure for use in its design environment. The objective of this paper is to quantitatively compare the relative conservatism of haversine and decaying sinusoid shock test input pulses which are generated on drop tables and electro-dynamic shakers. A complete evaluation of test conservatism between the laboratory and field shock environment requires characterization of shock severity and consideration of this characterization's statistical variation. A shock intensity spectrum (SIS) is introduced as a new shock characterization which provides information on the severity of the shock as a function of frequency. Test conservatism is quantfied in terms of an Index of Conservatism (IOC) and a novel criterion called an Overtest Factor (OTF). Accelerometer measurements were made on the fixed end of a cantilever beam structure. Data were gathered from a feld shock envronment and three laboratory test environments. The laboratory tests included two drop table test series and one decaying sinusoid test series. An analysis of test conservatism was carried out on the data using these new techniques. Results are presented for the case where multiple tests provide a complete statistical basis for IOC and OTF calculations and the case where only a single measurement is made from the field and laboratory environments. This research provides specific recommendations regarding the relative conservatism of two widely used shock test techniques. It also describes a general methodology which can be applied in the comparison of any two shock environments. (ERA citation 11:003502)