Performance Implications of Imperfectly Plastic Material in Metallic Structures with Nonmetallic Reinforcement

The performance under load of flexural metallic structures with adhesively bonded nonmetallic—specifically fiber reinforced polymer (FRP)—reinforcement can be strongly influenced by the stresses developed in the adhesive connection. High shear stresses in the adhesive can lead to connection failure and so to significant loss of structural integrity. If the metal is steel or wrought iron, one source of high shear stress in the adhesive is yield of the metal. This is because the reduced flexural stiffnesses in yield zones lead to high curvatures and so to high axial stresses in the FRP, and the resulting increased axial stress gradients along the FRP dictate the presence of high concomitant shear stresses from the adhesive. The present study uses a verified finite element analysis of a FRP-plated steel member to show that these adhesive shear stresses can be sensitive to small deviations of the metal from elastic-perfectly plastic behavior. Two deviations from this idealized behavior, namely a rounded “knee” instead of the abrupt kink in the transition from elastic to inelastic behavior, and strain hardening in the inelastic regime, are considered. The effects of change in stiffness of the adhesive and of the FRP plate are also considered. The results show that yield zone shear stresses can be significantly sensitive to small changes in the steel constitutive model and to stiffness changes of the plate, and palpably sensitive to stiffness changes of the adhesive. Some of the results are counter-intuitive, in that increase in stiffness of the FRP plate results in lower, not higher, shear stresses in yield zones. It is concluded that robust prediction of performance requires accurate constitutive modeling of the metal.