Computational intelligence methods for the fatigue life modeling of composite materials

Novel computational methods such as artificial neural networks, adaptive neuro-fuzzy inference systems and genetic programming are used in this chapter for the modeling of the nonlinear behavior of composite laminates subjected to constant amplitude loading. The examined computational methods are stochastic nonlinear regression tools, and can therefore be used to model the fatigue behavior of any material, provided that sufficient data are available for training. They are material independent methods that simply follow the trend of the available data, in each case giving the best estimate of their behavior. Application on a wide range of experimental data gathered after fatigue testing glass/epoxy and glass/polyester laminates proved that their modeling ability compares favorably with, and is to some extent superior to, other modeling techniques.

Artificial neural networks in spectrum fatigue life prediction of composite materials

Anastasios Vassilopoulos

An artificial neural network has been proved to be a sufficient tool for modelling fatigue life of multidirectional composite laminates made of GFRP composite materials and tested under constant amplitude loading patterns. Modelling efficiency of the network was satisfactory for both on- and off-axis coupons’ life, irrespective of the test conditions, i.e., R-ratio that defines the developed stress state on the coupon. Tension–Tension, Compression–Compression and even Tension–Compression loading patterns were investigated and modelling accuracy of the proposed ANN model was validated. The main benefit of this new modelling tool is that only a small portion, in the range of 40–50%, of the experimental data is needed for the whole analysis. Thus, expensive and time consuming tests required by the conventional way for the establishment of S–N curves could be significantly reduced without significant loss of accuracy. We applied the neural network method using experimental data from two different (in nature) material systems and proved that constant life diagrams (CLDs), which are very useful for the design of structures loaded under variable amplitude loading spectra, can be efficiently modelled using a much smaller set of experimental data compared to that needed for the development of CLDs by the conventional way.

Elsevier2007

Load history effects in fibre-reinforced polymer composite materials

Abdolvahid Movahedirad

Today, it is widely accepted that fatigue is one of the most common failure mechanisms in structural components, and pure static failure is rarely observed. Engineering structures are subjected to complex mechanical fatigue loading histories, which cause damage that eventually leads to functional and structural deterioration. One reason for this complexity is that the cyclic loading is accompanied by time-dependent deformations including creep and recovery. Fiber-reinforced polymer (FRP) composite materials are susceptible to time-dependent deformation, due to the viscoelastic nature of their matrix. Therefore, fatigue-creep-recovery interactions can play a role in the overall deformation behavior of FRP composite materials. The limited experimental investigations of FRP composite materials showed considerable effects of time-dependent phenomena on the fatigue behavior. However, no systematic analysis to clearly determine the effect of creep and recovery on developed damage mechanisms of FRP composites has yet been conducted. The aim of this research is to understand the fatigue behavior of glass fiber-reinforced composite (GFRP) materials and establish a reliable methodology to analytically/numerically model the fatigue behavior of FRP composite materials. The fatigue behavior of (±45)2S angle-ply glass/epoxy laminated composite at different stress levels has been investigated in this thesis. It was observed that cyclic-dependent parameters (cyclic creep, fatigue stiffness, and hysteresis loop area) as well as damage development were a function of applied stress levels. In tension-tension continuous fatigue, by decreasing the stress ratio, the damage became more severe, fatigue stiffness exhibited greater changes, the hysteresis loop area became larger and a higher and more uneven self-generated temperature was recorded. The effect of loading interruption on fatigue behavior was studied by subjecting the specimens to tension-tension constant amplitude fatigue loading interrupted at ¿max by creep intervals. At high stress levels, the fatigue life was improved when hold time was short as a result of the fiber realignment caused by the creep strain, while longer hold time caused considerable creep damage and therefore premature fatigue failure of the specimens. The cyclic loading was also interrupted at zero stress level. The stiffness degradation rates and hysteresis loop area decreased during the cyclic loading phase, while specimen recovery occurred after each stress removal. It was observed that specimens loaded under interrupted fatigue exhibited longer fatigue lives than those continuously loaded at high stress levels as a result of partial fatigue stiffness restoration and crack blunting. It was concluded that fatigue design allowables based on continuous fatigue could be conservative. A model based on the theory of viscoelasticity was developed to simulate specimen recovery in viscoelastic materials and predict their cyclic-dependent mechanical properties. The developed model predicted the cyclic-dependent parameters well - the fatigue stiffness, hysteresis loop area, cyclic creep, storage and loss moduli as well as tan(¿).

EPFL2019

The history of fiber-reinforced polymer composite laminate fatigue

Anastasios Vassilopoulos

Investigations of the fatigue performance of composite materials have accompanied their introduction in several engineering domains since the 1950s. An abundance of publications have emerged dealing with the experimental investigation of the fatigue performance of composites under different loading and environmental conditions, as well as the development of theories for the modeling of the fatigue behavior and/or prediction of the fatigue life of the materials systems under consideration. This work aims to briefly review and present the history of fiber-reinforced polymer composite laminate fatigue investigations, dividing the last 70 years into three periods. The early 1950-1975 period, when the "new" materials and their behavior under (simple) fatigue loading patterns were discovered. The mature, 1975-2000 period, when more loading and material parameters were investigated and the basic theoretical background was established. And finally, the later period, in the new millennium, when more detailed experimental campaigns were performed (assisted by developments in a multitude of engineering and scientific fields) and parameters that had previously been overlooked by researchers were taken into account.

ELSEVIER SCI LTD2020

Load history effects in fibre-reinforced polymer composite materials

Abdolvahid Movahedirad

EPFL2019