Damage Detection by Experimental Modal Analysis in Fiber-Reinforced Composites

Uncertainties during the manufacturing process or due to unexpected load variations in the course of operation can cause damages or faults in structures. These deviations from the original or intended properties can impact the durability and the application for the component. Destructive testing methods are often impractical since the investigated component becomes unfeasible. Instead, a non-destructive testing method, which is based on the change of the dynamic behaviour caused by the faults in the structures, allows examining the produced or built-in component. Experimental investigation of the changes can be employed to detect and quantify the damages to some extent, if the relationship between the properties and the damage is not known. The non-destructive vibrational testing methods, in particular, are based on the relation between the material properties of a structure and its measured vibrational behaviour, e.g. natural frequencies and mode shapes. The structural responses of several FRC structures are investigated in this paper by using the experimental modal analysis. To measure the structural responses without changing the dynamical behaviour, a laser-scanning vibrometer is used. Two different sets of measurements are performed. In the lower frequency range a shaker is used for excitation. In the higher frequency range a loudspeaker is used as excitation with a microphone as phase reference. In order to extract the eigenfrequencies and modal data from the measurement, a modal analysis is performed afterwards. For identifying the changes in the natural frequencies and mode shapes, specimens without and with several different kinds of artificial faults are tested. The experimental results are compared with a finite element simulation of the structures. The advantages and merits of this method as well as its limits and drawbacks are discussed in this study.