CFRP-Strengthening and Long-Term Performance of Fatigue Critical Welds of a Steel Box Girder

Roland Koller,Giovanni Terrasi,Sebastian Valet,Iwan Stoecklin

doi:10.3390/polym6020443

Abstract

Empa’s research efforts in the 1990s provided evidence that a considerable increase of the fatigue strength of welded aluminum beams can be achieved by externally bonding pultruded carbon fiber reinforced polymer (CFRP) laminates using rubber-toughened epoxies over the fatigue-weak welding zone on their tensile flange. The reinforcing effect obtained is determined by the stiffness of the unidirectional CFRP laminate which has twice the elastic modulus of aluminum. One can therefore easily follow that an unstressed CFRP laminate reinforcement of welded beams made of steel will not lead to a substantial increase in fatigue strength of the steel structure. This consideration led to the idea of prestressing an external reinforcement of the welded zone. The present investigation describes experimental studies to identify the adhesive system suitable for achieving high creep and fatigue strength of the prestressed CFRP patch. Experimental results (Wöhler-fields) of shear-lap-specimens and welded steel beams reinforced with prestressed CFRP laminates are presented. The paper concludes by presenting a field application, the reinforcement of a steel pendulum by adhesively bonded prestressed CFRP laminates to the tensile flanges of the welded box girder. Inspections carried out periodically on this structure revealed neither prestress losses nor crack initiation after nine years of service.

Highlights

Welding is a commonly used joining technique for mechanical and civil engineering metallic structures
The epoxy resin “Er 4” shows a shear strength only in the midfield of the static shear results given in Figure 5, its creep behavior (Figure 6) and fatigue strength are excellent (Figure 7)
The welded steel box girder of the artwork pendulum was dismantled, all welds renewed and the two tension/compression flanges strengthened by three prestressed carbon fiber reinforced plastics (CFRP) laminates in the vicinity of the maximum bending moment

Summary

Introduction

Welding is a commonly used joining technique for mechanical and civil engineering metallic structures. These structures sustain static and alternating loads with different amplitudes. In the heat affected zone structural changes of the metal may occur from the welding process which may result in reduced material strength [2]. Without heat treatment residual stresses remain in the weld having a maximum magnitude of approximately the yield stress of the material. Since these residual stresses act as static stresses their effects are the same as mean stresses from external loads. Empa demonstrated in the 1990s that applying carbon fiber reinforced plastics (CFRP)

Methods

Results

Conclusion