Εναπομένουσα αντοχή και πρόβλεψη ζωής σε σύνθετα υλικά μετά από κόπωση

Abstract

Subject of this dissertation is the investigation of the static strength degradation phenomenon caused by fatigue in FRP composite materials and its integration to structural design procedures. In the first part of the study, several phenomenological residual strength models from literature, some of them modified to enhance their performance, along with a newly proposed methodology for reliability based residual strength prediction, are implemented to both the experimental data produced as well as to published data referring to a wider range of materials and lay-ups. The implementation procedures proposed are oriented towards simplicity and minimization of the required experimental effort. Models predictions, regarding both deterministic strength degradation behavior and statistical characteristics of residual strength, are assessed in order to clarify the predictive ability of each method and propose specific engineering solutions for the prediction of residual strength after fatigue. Once concluded on a number of efficient engineering models, in the second part of this work, residual strength is integrated in life and residual strength prediction methodologies. As a first step, fatigue life prediction of macroscopically studied composite laminates under variable amplitude (VA) loading is attempted. The effect of each module of the state-of-the-art life prediction schemes, i.e. the counting method and constant life diagram (CLD) is investigated along with the possible benefits from incorporating residual strength as damage accumulation metric instead of the commonly used Palmgren-Miner rule. Predictions are evaluated through tests performed on [04]T and [±45]S laminates of the reference material, under three different loading spectra extracted either from processing strain measurements on operating Wind Turbine Rotor Blades (WISPER and NEW WISPER) or from aero-elastic simulations (MWIND). As a subsequent step, the FADAS (FAtigue DAmage Simulator) life prediction methodology is developed and implemented in computer code. The algorithm, takes into account the plane stress conditions developing into each ply during fatigue by means of classical lamination theory, models the ply-by-ply degradation of strength and stiffness and implements progressive damage principles, based on Puck failure criterion, to predict failure of a laminate after arbitrary cyclic loading. Predictions of the FADAS algorithm, once its parameters are tuned accordingly for the reference UD material, are compared with constant amplitude (CA) fatigue tests performed on three types of specimens: The first consist of a multidirectional (MD) laminate of [(±45/0)4,±45]T lay-up under R=0.1 and R=-1 cyclic loads and the other two are 10° and 60° off-axis coupons cut from the same laminate under R=-1, in an effort to validate the algorithm under various combinations of imposed stresses and induced damage modes.