Use of Composite Materials to Repair Steel Structures Vulnerable to Fatigue Damage

Tez Türü: Yüksek Lisans

Tezin Yürütüldüğü Kurum: University of Kansas, yüksek lisans fakültesi, Amerika Birleşik Devletleri

Tez Danışmanı: Adolfo B. Matamoros

Tezin Onay Tarihi: 2010

Tezin Dili: Türkçe

Özet:

A study was conducted to investigate the use of Carbon Fiber Reinforced Polymers (CFRP’s) to prevent and repair fatigue damage to bridge structures, which is a very common problem afflicting older steel bridges with welded connections. The presence of fatigue cracks in older steel bridges depends on factors such as the frequency of loading and the intensity of loading. For example, the intensity of loading (defined in terms of the stress range) is inversely proportional to the rule of the fatigue life of the structure, with a relationship that is linear when plotted using a logarithmic scale. Another important factor affecting fatigue life is the geometry. For example, regions of a structure that have abrupt changes in geometry connection are most sensitive to fatigue damage. This study investigates mainly two different topics related to the use of CFRP’s for strengthening and repair of steel bridges: the use of CFRP’s to prevent damage in fatigue vulnerable welded connections prior to fatigue-crack initiation, and the use of CFRP’s to stall the propagation of fatigue cracks in welded connections and in steel plates subjected to tension. The findings are discussed in three different papers presented in the following chapter. The use of CFRP’s to prevent fatigue damage was investigated experimentally and analytically through the analysis of subassemblies representing a segment of the tension flange of a steel bridge with a welded coverplates. This type of welded connection is defined as a fatigue category E’ in accordance to the AASHTO bridge design code if the weld tying the flange to the coverplate is smaller than 102 mm (4 in.), and perpendicular to the direction of the flange. The subassembly used to simulate the flange-coverplate connection was loaded under three point bending, and consequently it is referred to as a bending specimen. Bending specimens were repaired with three different types of preventive treatments to minimize fatigue damage in the welded connections: CFRP overlays made with continuous fibers that were prefabricated under laboratory conditions, chopped-fiber sprayed overlays which were simpler to fabricate, and smoothing of the surface of the welds. The findings related to the repair methods for the bending specimens are presented in the first article. There were several important findings from three point bending specimen study. The method of reinforcing the welded connections with the continuous fiber overlays was very effective in reducing the stress demand on the welds, driving the fatigue-crack initiation life into the infinite fatigue life range of the AASHTO design curves. The chopped fiber technique evaluated in this study is fairly new. The study showed that this technique was still limited by the bond strength of the overlay under fatigue loading. Future studies will focus on improved fabrication techniques to address this problem. Crack propagation studies were performed both on bending specimens and is tension specimens consisting of a steel plate with a center hole. The latter type of specimen was chosen to investigate the possible use of CFRP’s to repair fatigue cracks propagating through webs and flanges of plate girders.

The second study using a tension plate with a hole studied two different methods of continuous and chopped fiber. The details of the crack propagation study are presented in the second article. Specimens reinforced with continuous fiber overlays had an excellent response, extending the fatigue-crack propagation life of the specimens by more than 30 times.

The third study that was carried out was intended to provide a better understanding of the factors that affect the bond between the CFRP overlays and the steel. This study was carried out using pull-out tests of various bond layer configurations under monotonic loading. During the development of the CFRP overlays, tests showed that the limiting factor was maintaining the bond between the composite and the steel during cyclic loading. Several improvements were made to the configuration of the bond layer to improve the strength under fatigue loading. One of the most important ones was the embedment of a layer of breather cloth within the bonding layer, which led to large improvements in fatigue life. The pull-out tests were carried out to gain a better understanding of the reasons that made the breather cloth improve the performance of the bond layer under cyclic loading. Another important finding of this study is related to the importance of using breather cloth in resin layers of different thickness. It was found that if the bond layer was thin, the bond strength was not sensitive to the embedment of the breather cloth. However, if the bond layer was thick enough, specimens with breather cloth developed bond strengths 26% higher than those without breather cloth.