Fatigue

This study presents and illustrates a methodology to calculate the capacity of an existing reinforced concrete bridge under a non-conventional blast load due to low and intermediate pressures. ATBlast program is used to calculate the blast loads for known values of charge weight and stand off distance. An excel spreadsheet is generated to calculate ultimate resistance, equivalent elastic stiffness, equivalent elastic deflection, natural period of the beam, the maximum deflection, and the maximum rotation in the support for a simple span solid slab and T-Beam bridges. The allowable rotation could be taken as to two degrees. Naval Facility Engineering Command (NAVFAC) approach was adopted, where the inputs were material properties, span length, and area of reinforcement. The use of the Fiber Reinforced Polymer for increasing the capacity of an existing bridge is also presented in this study. Parametric studies were carried out to evaluate the performance of the solid slab and T-Beam bridges under the assumed blast load.

This study focused on the fracture and fatigue crack growth behavior in polyvinylchloride (PVC) and polyethersulfone (PES) foams. A new sandwich double cantilever beam (DCB) test specimen was implemented. Elastic foundation and finite element analysis and experimental testing confirmed that the DCB specimen is appropriate for static and cyclic crack propagation testing of soft polymer foams. A comprehensive experimental mechanical analysis was conducted on PVC foams of densities ranging from 45 to 100 kg/m3 and PES foams of densities ranging from 60 to 130 kg/m3. An in-situ scanning electron microscope study on miniature foam fracture specimens showed that crack propagation in the PVC foam was inter-cellular and in the PES foam, failure occurred predominately by extensional failure of vertical cell edges. Sandwich DCB specimens were loaded cyclically as well. For the PVC foams, the crack growth rates were substantially influenced by the density. For the PES foams, there was no clear indication about the influence of foam density on the crack growth rate.

The study presents a reliability-based fatigue life prediction model for the ocean current turbine rotor blades. The numerically simulated bending moment ranges based on the measured current velocities off the Southeast coast line of Florida over a one month period are used to reflect the short-term distribution of the bending moment ranges for an idealized marine current turbine rotor blade. The 2-parameter Weibull distribution is used to fit the short-term distribution and then used to obtain the long-term distribution over the design life. The long-term distribution is then used to determine the number of cycles for any given bending moment range. The published laboratory test data in the form of an ε-N curve is used in conjunction with the long-term distribution of the bending moment ranges in the prediction of the fatigue failure of the rotor blade using Miner's rule. The first-order reliability method is used in order to determine the reliability index for a given section modulus over a given design life. The results of reliability analysis are then used to calibrate the partial safety factors for load and resistance.

The success of harnessing energy from ocean current will require a reliable structural design of turbine blade that is used for energy extraction. In this study we are particularly focusing on the fatigue life of a 3m length ocean current turbine blade. The blade consists of sandwich construction having polymeric foam as core, and carbon/epoxy as face sheet. Repetitive loads (Fatigue) on the blade have been formulated from the randomness of the ocean current associated with turbulence and also from velocity shear. These varying forces will cause a cyclic variation of bending and shear stresses subjecting to the blade to fatigue. Rainflow Counting algorithm has been used to count the number of cycles within a specific mean and amplitude that will act on the blade from random loading data. Finite Element code ANSYS has been used to develop an S-N diagram with a frequency of 1 Hz and loading ratio 0.1 Number of specific load cycles from Rainflow Counting in conjunction with S-N diagram from ANSYS has been utilized to calculate fatigue damage up to 30 years by Palmgren-Miner's linear hypothesis.