A multi-view digital image correlation for extracting mode shapes of a tire

Abstract

Modal analysis is used to extract dynamic characteristics of structures for correlation and validation purposes. These characteristics are usually extracted using conventional measurement tools. A known force is imparted to the structure using impact hammers or mechanical shakers and the response is measured using accelerometers. However, accelerometers may induce mass loading effects; thus, the results may not show the true dynamics of the structure. On the other hand, conventional measurement tools can only measure at few discrete locations. Digital Image Correlation and photogrammetry has provided test engineers with a new tool for measuring dynamics of structures. These techniques are non-contacting so they do not induce mass loading and they can provide full-field results. For a DIC measurement to extract mode shapes, operating data of a structure is measured while the cameras are in fixed positions. A stereo camera system has line of sight only on a section of the structure. If the entire dynamics of a structure is desirable, several stereo cameras needs to be used. Another approach is to move a single stereo camera measure dynamics of the structure from multiple field of views; however, the time domain results can only be stitched together if the entire structure is fixed or if the applied force is constant during the entire time of measurement. This assumption is not valid for many dynamic measurements. On the other hand if the DIC system is moved from a view to another view, because the response is measured at different instants of time, operating shapes from each camera view may have a different scaling factor. In the current work, we propose an approach to identify a uniform scaling factor that enables us to stitch these operating shapes extracted from different views of cameras. The new approach is proposed based on conventional modal analysis theory. In the proposed technique, a known force is applied to a structure using a mechanical shaker or an impact hammer. The measured response of the structure in the time domain is transferred to the frequency domain to extract the mode shapes of a section of the structure. A similar procedure is repeated for other sections of the structure to capture the entire area of interest. Mode shapes for all views of the structure are stitched together to extract the mode shape for the entire structure. The proposed technique suggests stitching the views in the frequency domain after being scaled rather than stitching them in the time domain. The proposed method was applied to measure mode shapes of a tire using measured time series from different sections of the tire.