Vassilopoulos Fatigue Life Prediction of Composites and Composite Structures

1. Auflage 2010
ISBN: 978-1-84569-979-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 576 Seiten

Reihe: Woodhead Publishing Series in Composites Science and Engineering

The use of composites is growing in structural applications in many industries including aerospace, marine, wind turbine and civil engineering. There are uncertainties about the long term performance of these composites and how they will perform under cyclic fatigue loading. Fatigue life prediction of composites and composite structures provides a comprehensive review of fatigue damage and fatigue life prediction methodologies for composites and how they can be used in practice.After an introductory chapter, Part one reviews developments in ways of modelling composite fatigue life. The second part of the book reviews developments in predicting composite fatigue life under different conditions including constant and variable amplitude loading as well as multiaxial and cyclic loading. Part three then describes applications such as fatigue life prediction of bonded joints and wind turbine rotor blades as well as health monitoring of composite structures.With its distinguished editor and international team of contributors, Fatigue life prediction of composites and composite structures is a standard reference for industry and researchers working with composites and those concerned with the long-term performance and fatigue life of composite components and structures. - Examines past, present and future trends associated with fatigue life prediction of composite materials and structures - Assesses novel computational methods for fatigue life modelling and prediction of composite materials under constant amplitude loading - Specific chapters investigate fatigue life prediction of wind turbine rotor blades and bonded joints in composite structures

Autoren/Hrsg.

Vassilopoulos, Anastasios P.

Leseproben

1 Introduction to the fatigue life prediction of composite materials and structures: past, present and future prospects
A.P. Vassilopoulos,     Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland Abstract:
This chapter aims to provide an overview of the fatigue life prediction methods for composite materials and structures, recalling methods used in the past, discovering the present status and attempting to foresee future trends. Key words fatigue composites life prediction methods constant amplitude loading variable amplitude loading S–N curves residual strength residual stiffness 1.1 Introduction
One of the first fundamental facts of which human beings become aware is that nothing lasts forever. Life may come to a sudden end or last longer, but still for only a finite period. This latter case is normally supplemented by a reduction in efficiency, known as aging. This human life experience is directly reflected in materials science; under a high load a structure or a component can fail at once, whereas it can effectually sustain lower loads. On the other hand, the same structure or component can also fail under lower loads if they are applied over longer time frames in a constant (creep) or circular (fatigue) way. The phenomenon of the degradation of properties of a material due to the application of loads that fluctuate over time is called fatigue and the resulting failure is called fatigue failure. Fatigue was identified as a critical loading pattern a long time ago by the scientific community. Already in 1829 the German mining engineer W. A. S. Albert was the first to carry out fatigue tests on metallic conveyor chains [1] and later to report his observations. Subsequently, numerous failures that could not be explained on the basis of known theory were attributed to fatigue loading. With the development of the railways, in the mid-nineteenth century, the failure of wagon axles was such a frequent occurrence that it attracted the attention of engineers. Between 1852 and 1870 a German engineer, August Wohler, realized the first extended experimental program on the fatigue of metallic materials [1]. The program comprised full-scale fatigue tests on wagon axles but also specimen tests under cyclic loading patterns of tensile, bending and torsional loads. Wöhler constructed a test rig on which he could test wagon axles under bending moments that were developed by loads suspended from the ends of the axles. The developed stresses were recorded together with the number of rotations up to failure. The results were drawn on the s–N plane to formulate the first S–N curve, which, however, was restricted to the representation of experimental data, without proposing any mathematical formulation to describe this behavior. These first attempts to analyze the fatigue behavior of materials and structures were based on experience with constructions operating under real loading conditions. Failures that could not be explained by existing theories were designated fatigue failures. As from 1850, engineers recognized fatigue as a critical loading pattern that could be the reason for a significant percentage of structural failures and it was thereafter widely accepted that fatigue should not be neglected. However, as mentioned in the work of Schütz [2], knowledge concerning certain methods was very advanced in one location, while a few kilometers away it was nonexistent. It was not until 1946, when the term fatigue was incorporated in the dictionary of the American Society for Testing and Materials (ASTM), when the E9 committee was founded to promote the development of fatigue test methods [3]. Today, it is documented that the majority of structural failures occur through a fatigue mechanism and, as mentioned in [4], after extensive study by the US National Institute of Standards and Technology, approximately 60% of 230 examined failures were associated with fatigue. This percentage was higher (between 80% and 90%) in another study carried out by the Battelle Institution [5]. During the following years, numerous experimental programs were conducted for the characterization of the fatigue behavior of several structural materials of that time. As technology developed and new test frames and measuring devices were invented, it became more and more straightforward to conduct complex fatigue experiments and measure properties and characteristics, something which some years earlier would not have been possible. As a result, almost all failure modes were identified and many theoretical models were established for modeling and eventually predicting the fatigue life of several different material systems. Although composite materials are designated as fatigue insensitive, especially when compared to metallic ones, they suffer from fatigue loads as well. The introduction of composite materials in a wide range of applications obliged researchers to consider fatigue when investigating a composite material and obliged engineers to realize that fatigue is an important parameter that must be considered in calculations during design processes. Initially composites were used as replacements for previous ‘conventional’ materials such as steel, aluminum or wood, and later as ‘advanced’ materials that allow engineers to adopt a different approach to design problems, propose alternative design concepts (based on the free formability and light weight characteristics of composites) and redesign structures. Unfortunately the situation regarding the fatigue behavior of composite materials is different from that of metallic ones. Therefore, the already developed, and validated, methods for the fatigue life modeling and prediction of ‘conventional’ materials cannot be directly applied to composite materials. Moreover, the large number of different material configurations resulting from the multitude of fibers, matrices, manufacturing methods, lamination stacking sequences, etc., makes the development of a commonly accepted method to cover all these variances difficult. As mentioned in [6], ‘obviously, it is difficult to get a general approach of the fatigue behavior of composite materials including polymer matrix, metal matrix, ceramic matrix composites, elastomeric composites, glare, short fiber reinforced polymers and nano-composites’. One way of dealing with the fatigue of composite materials is to undertake extended experimental programs and then develop analytical, mathematical, expressions in order to model fatigue life and be able to reproduce experimental results. Numerous experimental programs have been realized over the last three to four decades and very comprehensive databases have been constructed. Some of these are limited, refer to specific materials, and have been determined mainly in order to assist the development of a theoretical model, e.g. [7–12], but others, like [13] and [14], are more extensive and cover a wide range of materials for specific applications. Along with the aforementioned experimental work, a considerable number of theoretical models have been developed to model the fatigue behavior of the examined composites and consequently predict their behavior under unknown loading conditions, e.g. [7,9,15–19]. A literature search (www.scopus.com) with keywords ‘fatigue’ and ‘composites’ in the disciplines ‘Engineering’, ‘Materials science’, ‘Energy’, and ‘Multidisciplinary’ produces over 9500 research articles in the field, with more than 85% of them published after 1980, and around 400 articles per year after 1995. Despite this explosive production of scientific publications in the field, countless unresolved topics exist in the domain of composite fatigue. Typical areas requiring further investigation concern the S–N and constant life diagram formulations for the interpretation of existing fatigue data, nonlinear damage accumulation rules that can take load sequence effects into account, cycle-counting methods that do not scramble the load sequence of the applied load time series, consideration of non-proportional stress components in a biaxial/multiaxial loading case, the exploitation of material behavior at very high cycle regimes, the development of methods that take into account the stochastic nature of the phenomenon, etc. Researchers have attempted to address these topics in order to model or predict the fatigue behavior of composite materials of interest. However, the terms ‘modeling’ and ‘prediction’ have often been misused, usually by adopting the term ‘prediction’ when a ‘modeling’ is performed with the aim of interpolating between known fatigue data. The term ‘prediction’ must be used when extrapolation is performed outside the existing database in terms of prediction of the behavior of the same material under new loading conditions, e.g. spectrum loading based on constant amplitude fatigue data, or extension of the modeling to low or high cycle fatigue regimes, when data exist in the range between 103 and 106 cycles, or even prediction of the behavior of other material systems based on models derived for a specific...