FATIGUE ANALYSIS OF CONCRETE MEMBERS UNDER VARIABLE AMPLITUDE CYCLIC LOADING

Abstract

Concrete structures such as bridges, pavements, offshore structures etc. are exposed to fatigue loading of varying amplitude during their service life. Exposure to repeated load-ing can cause progressive, permanent internal microstructural changes in the material called fatigue. Since, failure due fatigue can occur at a much lower stress than the ulti-mate strength of the material consideration of fatigue phenomenon is important for the safe design of engineering structures. Further, in concrete structures the consideration of the sequence of variation in the load amplitude is important due to the acceleration effect in the crack growth rate due to overloads. This effect can be related to the sensitivity of the fracture process zone to fatigue loading. Fracture process zone is more sensitive to fatigue loading than the un-cracked ligament, hence damage due to the effects of normal loads are confined inside it, and when an overload is inserted there is a sudden growth in the fictitious crack tip causing an increase in the crack growth rate. Apart from the application of different types of loading, the material behavior of concrete is highly heterogeneous and complex in nature due to the occurrence of extensive micro-cracking and toughening mechanisms. A considerable size of fracture process zone exists ahead of crack tip which is responsible for the exhibition of size effect behavior in concrete. Furthermore, existence of substantial nonlinearity prior to peak load necessitates the use of nonlinear fracture mechanics theory in concrete. In this dissertation, an analytical model has been proposed to obtain fatigue life and an-alyze the crack propagation behavior under the action of fluctuating loading of varying amplitude. The model would be developed by including the effect of fracture process zone in the formulation. Apart from conventional parameters, loading history and frequency cffects have been taken into account. The mathematical formulation for the fatigue crack 11 growth law is made on the basis of theoretical arguments using the concepts of dimen-sional analysis and self-similarity. The model predictions have been compared with the experimental results and are found to capture the size effect behavior and overload effect. A sensitivity analysis was carried out to study the influence of important parameters on the crack growth rate/fatigue life. It is inferred that energy release rate is the most sensitive parameter followed by the structural size. Frequency and fracture energy due to over load cycle is found to have least influence on crack propagation behavior.