טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentPenias Shahar
SubjectMonitoring Natural Frequencies of Composite Beams as a Means
to Detect Defects
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Haim Abramovich


Abstract

The aim of this research was to evaluate the effect of composite materials inter-laminated delaminations on the natural frequencies of laminated beams. The ability to predict such changes in the natural frequencies and to measure it, can be used to detect delaminations in a composite beam.

The use of composite material structures is becoming very common in the industry, especially the aerospace industry. Composite structures are very significant in the un-manned aircraft industry' in which the weight of the product has a large impact on its commercial (or military) success. Inter-laminar delaminations can be the product of faulty manufacturing, which propagates itself in insufficient adhesion between the resin and the graphite fabric. Such fault might also be the result of time-dependent delamination as a result of load cycles or environmental effects. In the present research a numeric analysis was performed, in which various types of delaminations in a composite beam were modeled and their effect on the natural frequencies were evaluated.

The beams and their respective delaminations were modeled using the ANSYS code. For each beam, its first ten natural frequencies were calculated. At the first stage of the research, the effect of a symmetric delamination on the natural frequencies, in comparison to a non-delaminated beam was investigated. As expected, the delamination caused the stiffness of the beam to decrease, which yielded a reduction in the respective natural frequencies.

The second stage of the present study included modeling of delaminations, with various assumed parameters. Then, the delamination's effect on the beam's natural frequencies was evaluated. The chosen parameters were:

         Symmetric delaminations of different sizes.

         Delaminations with constant size in various locations.

         Non-symmetric delaminations.

         Delamination between layers at various location across the beam's thickness.

In addition to different types of delaminations, the influence of some other parameters of the beam was also evaluated, such as the material of the beam (isotropic versus composite), the physical dimensions and the boundary conditions of the beam (simple supports, clamping) and different layer orientation.

Analyzing the numeric results, shows that it is possible to evaluate, with a good certainty, the presence of a delamination and its size, given that there is a reference specimen (can also be a previous measurement of the same beam).

The numeric results show that by combining the measured natural frequencies, the location of the delamination in the X direction (by combining few bending frequencies) and in the Y direction (by measuring bending and torque frequencies) of the beam can be evaluated. These results can be the base to the development of a new NDI (Non Destructive Inspection) for composite beams, while still on the aircraft.

In addition, an experimental study was conducted, which qualitatively shows the change in natural frequencies of a beam with a prescribed delamination. The study shows that this variation in the natural frequency of the beam can be easily measured with simple acceleration sensors. The results show that a vibration based inspection for delamination detection is doable.