טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentGecht Michael
SubjectHealth Monitoring of Composite Structures Using Dynamic
End Effects
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Emeritus David Durban
Dr. Raruch Karp
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Structural components made of composites carry potential benefits for the main structure in improving its performance, primarily on account of excellent specific strength and stiffness properties and fatigue characteristics. However, their actual application is relatively limited, mainly due to the nature of their failure modes, high sensitivity to stress concentrations, along with the well-known difficulty to estimate stress fields within a structural component due to reduced validity of Saint-Venant’s principle. A promising direction to optimal design with structural composites, promoted during recent years, is by employing structural health monitoring. In particular, early detection of edge damage, typical of composites and critical to their proper functioning, is sought.

Analytical results on decay of static and dynamic end effects in beam-like isotropic structures predict, that incipient damage at its ends can be measured and detected by strain gauges located at various distances from the monitored end. The decay distance of end effects in isotropic materials is about one thickness of a beam, implying that for efficient monitoring, one has to locate measuring sensors within the distance of one thickness of the beam from the monitored end. If a beam-like structure is relatively thin, say a few millimeters, the task of sampling the perturbed field parameters close to the end is quite challenging. On the other hand, analytical results for composite structures predict very slow decay rate of end effects, especially in highly anisotropic materials. For typical composites, the predicted decay distances of end effects is four to eight times larger than in isotropic materials.

The main purpose of the present study is to verify experimentally the ability to detect modifications in end conditions and possible damage by monitoring dynamic and quasi-static end effects in composite beam-like plates. Two methods were used to simulate damage; a) variation in beam clamping at the examined end to simulate joint damage, and b) overstress generating actual damage. Static loads and dynamic excitations were imposed to examine the ability of strain gauges, located near and far from the damaged area, to detect small variations in these control parameters.

It has been found that measurements of static and dynamic end effects can serve as a reliable marker for joint condition and for crack detection. That finding can extend the scope of structural health monitoring systems to include damage at suspected locations and of critical joints, issues that currently prohibit further usage of structural component made of composites.