Detection of Delamination Damages in Thin Composite Plates using Noncontact Measurement of Structural Dynamic Behavior

Plate-shaped composite structures are typically implemented in a variety of applications related to the aeronautics or automotive industry. Subjected to real environmental conditions, those structures may be burdened by impact loads. Low-velocity impacts in particular are a high safety issue since they can cause barely visible or invisible damages inside the structure. The caused defects vary from notches and cracks to delaminations underneath the surface. Their common consequence is a significant reduction of compression and fatigue strength. To determine the integrity of composite structures, different principles, mainly ultrasonic testing, can be used. Because these methods are typically time consuming, a faster and more easily applied integral test method might offer advantages. Such a test method can be gained by analyzing the vibrational behavior of composite structures. Using the vibrational analysis approach, not only the response frequencies are considered, but also other modal properties, especially the modal damping. The single measurement of the response frequencies might prove an insufficient damage indication because their frequency shift is subtle and is strongly dependent on its present bearing. For the experimental tests, plate-shaped specimens were chosen due to the simple geometry and relevance to applications. By using structural dynamic measurements and analysis, intact as well as impact damaged plates were measured. For the contact-less measurements of the vibrating specimens, a Laser Scanning Vibrometer is used. The plate-shaped specimens are oscillated by acoustic excitation. The necessary bearing is realized in the form of a thread suspension or, more generalized, a free boundary condition. This minimizes any hampering of the evolving modes of vibration. The obtained results of the conducted measurements are compared and discussed.