Using Impact Testing for Production Quality Control

Abstract

It is well known that when any physical property of a machine or structure changes, its modal parameters will also change. Modal frequency is the most sensitive parameter to changes in stiffness. If the stiffness of a test article is less than the stiffness of a properly assembled structure, its modal frequencies will be less than those of the properly assembled structure. If its stiffness is greater than the stiffness of a properly assembled structure, its modal frequencies will be greater than those of the properly assembled structure.

In this paper it is shown how a simple, low-cost three-step process can be used for pass-fail testing in a production environment, or for qualification testing of any assembled mechanical system. In the first step, the structure is impacted and Frequency Response Functions (FRFs) are calculated from the acquired force and response signals. In the second step, the FRFs are curve fit to obtain the modal frequencies of several modes of the structure. In the third step, the modal frequencies are numerically compared with the frequencies of a properly assembled structure.

A metric called the Shape Difference Indicator (SDI) is used to numerically compare two “shapes”. In this application, each “shape” contains the modal frequencies of several modes of the test article. A test article passes the qualification test when its SDI value with the shape of a correctly assembled article is “close to 1”.

In the example used for this paper, SDI is also used to search a database of modal frequency shapes. The search results are displayed in a bar chart of SDI values between the current & archived shapes of modal frequencies. Each shape in the database is associated with a known amount of torque applied to several cap screws that attach two aluminum plates together. When the modal frequencies of the test article fail to closely match those of a correctly assembled structure, the bar chart will also indicate how much torque must be applied to each cap screw to pass the test.