Date of Completion
Dianyun Zhang, Jiong Tang, George Lykotrafitis
Field of Study
Master of Science
Structural health monitoring of aerospace structures is becoming vital as lightweight composite materials are increasingly used in aero-structures. Composites have various types of failure modes, which make it even more important to identify fault or damage compared to isotropic materials. Therefore, on-site health monitoring is one of the most preferred methodologies for detecting internal crack or damage at its initial stage to enhance operating safety and reduce repair cost. Recently, piezoelectric transducers are on high demand for on-site health monitoring because of its high bandwidth sensitivity. They simultaneously perform as actuators and sensors and embedded in the structure for real time response indicative of the condition of the structure. Their characteristics show direct coupling between the electrical impedance and structural impedance. The electrical impedance or admittance changes according to modification in mass, stiffness or damping properties of the host body to which the transducer is attached.
In this study, plain-weave textile composites are utilized to demonstrate the capability of piezoelectric transducer. The elastic properties of the composite are determined using the fiber tow properties that are derived via the Concentric Cylinder Assemblage micromechanics model. Finite element modeling is done in commercial software ANSYS APDL to observe electrical admittance variation around natural frequency of the entire structure to investigate structure condition. Delamination was studied at first but results show very small shift in frequency of the same mode number for healthy and defective structures along with same magnitude of admittance. This makes delamination detection via impedance approach not feasible. So, investigation was shifted to detection of intra-ply damage. Damage can be thought of as reduction of stiffness in the whole structure. Fair amount of shift in the modal frequency was observed with respect to damage level. Modal frequency of the structure decreased as well as admittance value lowered with damage increment within the body. Pre-preg manufacturing method is carried out to fabricate composite panel from which samples are cut for testing according to ASTM standards. Panel is fabricated in such a way that enabled 45-degree fiber orientation with respect to the longitudinal axis of specimen. This strategy is taken to easily incorporate damage into the specimen via tensile test. 45-degree fiber orientation will make it easier to create damage compared to the longitudinally aligned fiber sample. Piezo patch is attached to the body and sine sweep voltage is applied to actuate the transducer. It provided voltage drop across the resistor that denoted the coupling effect of the patch with the host structure. Electrical admittance plot expressed response to body deformation actuated by the piezoelectric transducer. Results from finite element modeling and experimentation both indicate shift of frequency value of specific mode for damaged beam from healthy status. Therefore, piezoelectric admittance technique is efficient in detecting damage or degradation within composite structure.
Ahmed, Sazid Irfan, "Damage Detection in Textile Composites via Piezoelectric Admittance Methods" (2018). Master's Theses. 1231.