For laminated composites as well as pultruded thin-wall sections, final failure is often initiated by delarninations. For bonded repair, debonding of the repair material can occur over time. In this thesis, a systematic method for the detection of delamination and debonding is investigated. The detection method relies on measuring the integrated strain along a line on the structural member as a function of load position. By comparing the results for damaged and undamaged specimens, the location and extent of delamination or interfacial debonding can be deduced. To measure the small change in integrated strain induced by the delamination or debonding, an interferometry-based fiber optic sensor is employed....[ Read more ]

For laminated composites as well as pultruded thin-wall sections, final failure is often initiated by delarninations. For bonded repair, debonding of the repair material can occur over time. In this thesis, a systematic method for the detection of delamination and debonding is investigated. The detection method relies on measuring the integrated strain along a line on the structural member as a function of load position. By comparing the results for damaged and undamaged specimens, the location and extent of delamination or interfacial debonding can be deduced. To measure the small change in integrated strain induced by the delamination or debonding, an interferometry-based fiber optic sensor is employed.

The research work involves both experimental and theoretical investigations. An experimental set-up involving fiber optic interferometry has been developed. To reduce noise, a quasi-impulse loading is applied. Laminate composite members with different delamination locations and sizes, and bonded repair with interfacial debonding, have been prepared and tested. In the theoretical study, FEM is employed. 2D model and 3D model has been developed to study the effect of delamination on the integrated strain. Based on both the experimental and theoretical results, approaches for identifying the delamination are proposed.

The results show that for through-width delamination in laminate composites with middle-layer embedded fiber, the extent and the location of delamination can be identified from a sudden slope change of the normalized integral strain vs. load position curve. Preliminary investigation is also carried out for delamination patches in composites with fibers embedded along the mid-layer. The numerical and experimental data show that the delamination area could still be observed from the sudden slope change of the integral strain even when the fiber is located away from delaminated area. However, the sensitivity of delamination detection decreases with increasing transverse distance between the fiber and the edge of the patch.

For through-width debonding in bonded repair with surface-mounted fiber, if the plate is bonded on relatively thin substrates that act as beam members, debonding at the middle of the bonded plate can be observed from a sudden slope change in the integral strain vs. load position curve. Also, debonding at the edge of the plate can be reflected by a sudden change of the sign of the integral strain vs load position curve. If the plate is bonded on a stiff substrate that acts like an elastic foundation, the presence of debonding can be identified from the region where the integral strain suddenly increases to a significantly higher magnitude. In this thesis, the detection of delamination at the web/flange junction of GFRP I-beam is also presented. For high stiffness thin-walled composite beam, we found that it was difficult to identify the location of the delamination directly from the normalized integral strain vs. load position curve, obtained with quasi-impulse loading. An alternative approach, the ultrasonic pulse-echo method was hence utilized. When the delamination is approached, an additional frequency component can be identified from the frequency spectrum of the sensor output. The new frequency corresponds to the multiple reflected waves between the transmitter and the delamination. Based on this information, the delamination location can be obtained.

The present study has clearly demonstrated the simplicity and efficiency of the proposed approach, which is therefore believed to have high potential for practical applications. One of the major advantages of using this technique is its capability of in-situ damage detection while in service. The method is particularly useful as a first method to approximately locate delaminations in composite members. When the critical damage state is approached, a more sophisticated and costly technique can be employed to give more accurate information.