Effects Of Temperature Variations On The Interfacial Behavior Of FRP Plated Beams

The use of adhesively bonded fiber-reinforced polymer(FRP)composites as a strengthening procedure has received much attention.The FRP composites are usually used to strengthen structural components such as steel or concrete members within the civil engineering practice.Such structural members are likely subjected to significant temperature variations(e.g.,due to seasonal ambient temperature changes or fire exposure),which may have detrimental effects on the strengthening efficiency of the adhesively bonded FRP plate.That is because the FRPplated structural members could be compromised by the additional thermal-induced interfacial stresses,which could be developed due to the different thermal-expansion coefficients of the substrate members and the FRP composite material.In addition,when the FRP-plated members are exposed to elevated temperatures,the bond between the FRP and the substrate member may degrade which can trigger the debonding failure of the adhesively bonded FRP plate.Within this study,the cohesive zone modeling procedure was implemented to examine the effects of temperature variations on the performance of an FRP-plated steel beam.This approach allowed for a closed-form theoretical solution to be formulated and analytical results to be obtained,then comparisons made against the finite element predictions.This made provisions for the accuracy and reliability of the proposed theoretical analysis to be determined.In most of the existing research studies,the shear stress has been the focus point while neglecting the contribution caused by the normal stresses.This thesis comprises of two main studies related to the thermal-stress and bond-degradation effects on the debonding behavior of an FRP-plated steel beam as it is exposed to varying temperatures.The first study in this research,for comparison purposes and to have a clearer understanding of situations that could appear in civil engineering applications,considered both normal and shear stresses at the FRPto-steel interface.As a result,a coupled-mixed cohesive zone model was created with the temperature effects being considered.For the second study,the Mode-II shear bond behavior for the Sika 30 adhesive which considered the degradations in mechanical properties with temperature,was applied to the analytical model.Since the interfacial data was only available for the mode II behavior,only a single mode of cohesive behavior was examined.For the first study,the temperature effects were taken into consideration by implementing the additional thermal-induced strains due to the temperature variations,into the proposed analytical model.This assumption then led to the creation of analytical solutions for the interface which includes the temperature effects.This led to the inclusion of the thermal stresses for the interfacial stresses(mode I and mode II)and their respective slip.The analytical model was validated by using finite element models built within the commercial software called ABAQUS,where a good agreement is observed between the models with satisfactory accuracy.Lastly,a parametric study was performed to examine how the changes in the FRP plate properties in addition to the temperature variations,can affect the interface.These parametric studies revealed that the FRP properties can change the interfacial behavior of the FRP-to-steel interface.The studied showed that based on the individual properties,the behavior could change for each property.Very thin plates were more susceptible to the temperature variations.A similar observation was made as the FRP plate lengths were extended.However,when it came to the stiffness of the FRP plate,the stiffer the plate caused significant reductions in strength and therefore could aid in the causing more damage to the interface.However,when difference plates(with varying properties)were studied,the thinner,longer and less stiff plate had the better performance as the temperature variations were experienced.The second study considered the bond degradation as well as the thermal stresses as they act on the interface.However,due to the preliminary test done on the Sika 30 adhesive at the time,the interfacial data was only available for the mode II behavior.The inclusion of the bond degradation model allowed several bond-slip relations to be applied for different temperature variations,unlike the first study which neglected this behavior.Therefore,a similar approach as discussed within the previous study was conducted,and the interfacial stress and slip were developed including the bond degradations.The finite element models for this study were changed so as to reflect the several bond slip relations and when plotted against the analytical model,a good agreement was achieved with satisfactory accuracy.Finally,a parametric study was conducted with differences being observed as compared to the previous study.For the softening loads,the reaction of the interface was quite similar to observed in the first study.As the positive and negative temperature variations were observed,it resulted in the softening loads to decrease and increase,respectively.However,this ultimate load behavior of the interface changed as the ultimate load cases were examined.When the bond degradations were considered,the thickness of the FRP plate had negligible differences to the interfacial performance.This response was the same for both the softening and ultimate load cases.For the change in FRP plate length,at low positive variations,the longer lengths provided higher ultimate load carrying capabilities.However,as higher temperature variations were observed,it caused a reduction in the ultimate load carrying capacity of the interface and in the most extreme case,the shortest member had the best performance.This proved that the extended length was only beneficial up to a particular temperature variation as further increases in temperature variations caused a negative response by the interface.The ultimate load performance of the stiffness was slightly better than the extension in length.But,the main result proved that the less stiff plate provided a better performance with respect to the temperature variations.In both studies,the positive and negative temperature variations caused the softening loads to decrease and increased,respectively.In the first study,the consideration of the thermal stresses solely,showed that the ultimate load behavior was similar to that as seen at the softening load stage.That is,positive(or increasing)temperature variations caused premature debonding.In the second study,and due to the bond-slip relations,positive temperature variations caused higher ultimate loads with longer lengths of softening.In addition,the results presented how the mixed mode effects could cause the softening criteria of the adhesive could occur sooner than if the shear stress is solely considered.Further results and findings are presented,and their importance to civil engineering applications subsequently highlighted.In conclusion,it is hoped that this study would have highlighted the importance of considering the durability aspects of FRP plated beams.The primary focus of this research would be on the temperature variations and their implications on FRP plated beams,as these beams are exposed to both mechanical and thermal loadings.From this,this research seeks to develop useful analytical solutions and finite element models which consists of both mechanical and thermal considerations,which could be further used,developed and applied to other engineering cases.