Composite materials such as CFRP are used as a vital material especially in space structures and next-generation aircrafts because of their low weight and high strength. As the use of composites in aircraft increases, their weakness to foreign impacts becomes a crucial problem. The transverse stiffness of composite materials is smaller than the in-plane stiffness, and the mechanical properties of composite structures degrade severely by external impact events. Therefore, it is necessary to monitor structural conditions and to detect impact damages in real-time. The information of applied force is also very important and useful in assessment of integrity of structures. As a part of health-monitoring studies of composite structures, an identification method of impact force is presented in this study.
In the first part of the present paper, the relation between force histories and strain responses in composite laminates was formulated based on the finite element method for a low-velocity impact event. The strain responses are described as a product of transpose matrix and force histories. An output charge of thin piezoelectric materials embedded in CFRP laminated plates is used as strain sensors in dynamic response measurements. The fundamental properties of composite structures with PZT sensors are examined by impact tests.
Using the optimization method and the relation between force histories and strain responses, two methods are proposed to identify the location of the acting force. First method makes use of the relation between experimental strain responses and force location, and second uses disagreements between measured and calculated strain responses directly. Another problem in impact force identification is a determination of force histories. This is achieved by solving an inverse problem determining force histories from strain responses based on the least-squares method. In an actual structural system, however, the identified force history based on the least-squares method has a large oscillation because of infection of noise. The present study proposes more reliable force identification method by imposing the penalty of the derivative of the force history.
The present method is applied to CFRP laminated square rectangular plates to confirm its availability. First the influence of the measurement noise on the identification results of force location and histories is examined. Then the identification system consisting of a measurement part and identification codes is tested for the two kinds of laminated plates. A measurement part consists of the sensor network with SMART Layer and several signal conditioners. The influence of the modeling error involved in the finite element model is also discussed.
Finally the results from numerical or experimental investigations performed in this study are summarized. According to those results of force location and history identification, the validity of the present identification system is demonstrated successfully.
edited by Masanori TAJIMA