Carvelli / Lomov Fatigue of Textile Composites

1. Auflage 2015
ISBN: 978-1-78242-293-8
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 510 Seiten

Reihe: Woodhead Publishing Series in Composites Science and Engineering

Fatigue of Textile Composites provides a current, state-of-art review on recent investigations on the fatigue behavior of composite materials, mainly those reinforced with textiles. As this particular group of composite materials is extremely important for a wide variety of industrial applications, including automotive, aeronautical, and marine, etc., mainly due to their peculiarities and advantages with respect to unidirectional laminated composites, the text presents comprehensive information on the huge variety of interlacement geometric architectures that are suitable for a broad range of different applications, their excellent drapability and versatility, which is highly important for complex double-curvature shape components and three-dimensional woven fabrics without plane reinforcement, and their main mechanical characteristics which are currently in high demand from industry. - Presents the current state-of-the-art investigations on fatigue behavior of composite materials, mainly those reinforced with textiles - Contains invaluable information pertaining to a wide variety of industries, including automotive, aeronautical, and marine, amongst others - Provides comprehensive information on the huge variety of interlacement geometric architectures that are suitable for a broad range of different applications

Autoren/Hrsg.

Carvelli, Valter

Lomov, Stepan V

Leseproben

1 A conceptual framework for studies of durability in composite materials
R. Talreja     Texas A&M University, College Station, TX, USA Abstract
This chapter presents the fundamental considerations in treating durability of composite materials with fibrous reinforcement. General concepts underlying irreversibility in stress–strain behavior are addressed and the associated dissipative mechanisms are discussed. Particular focus is placed on fatigue, for which a conceptual framework for interpretation of the roles of fibers, matrix, and fiber/matrix interface, introduced by the author in a 1981 paper, is described. The extension and variation of the baseline framework, called the fatigue life diagram, to general cases of loading and fiber architecture are treated. Finally, guidelines are offered for modeling changes in material response and consequent loss of functionality under different fatigue conditions. Keywords
Composite fatigue limit; Composites durability; Failure mechanisms; Fatigue life diagrams; Fatigue life prediction 1.1. Introduction and background
Durability, understood as the ability to sustain mechanical or thermomechanical loads over a period of time, is in many structures a critical design consideration. The underlying processes responsible for loss of this ability can be rooted in time-dependent causes, such as viscoelasticity of polymers and stress corrosion cracking of fibers, or not explicitly in such causes but rather due to repeated (cyclic) application of loading over a period of time. In the latter case, the phenomenon is referred to as fatigue and is described with respect to some selected measure of load reapplication, such as the number of cycles of a sinusoidal load. This exposition will focus on durability in the context of fatigue in composite materials, but a brief treatment of a constant-in-time loading case will also be presented. The first basic consideration in fatigue is concerning mechanisms triggered by load reapplication, that is, what happens in the second application of load that did not occur when the load was applied the first time. Clearly, if no difference exists, then there is total reversibility, which in the context of mechanical loading would be called elasticity. To be sure, apparent elasticity may be indicated by stress–strain behavior such that no measurable irreversibility (difference in loading and unloading responses) is seen at the (macro) scale of measurements, but irreversible processes (e.g., crack formation and permanent morphological rearrangements in a polymer) can be occurring at a smaller scale that are not yet significant to affect the macro scale response. Often, addressing the basic issue noted above presents challenges that do not seem surmountable, resulting in a resort to less fundamental approaches to fatigue. A common type of approach to fatigue of materials—monolithic or composite—is to view it as a material property, the same way as elasticity, representing it by a number of material constants. These material constants are derived from the so-called S–N curve (or diagram) wherein the number of cycles of a constant-amplitude sinusoidal load needed to fail a material specimen is plotted against the applied stress amplitude (or stress maximum). The test data show significant scatter, unlike those for measuring elastic constants, and fitting a curve through average fatigue lives provides an empirical description of the fatigue properties. The entire fitted curve, or parts of it, are used to compare fatigue behavior of candidate materials for the purpose of material selection. For safety against fatigue in service environments, more empirical fitting of curves is needed, as the time variation of service-induced stresses differs from the standard sinusoidal stress variation used to obtain the S–N curve. For example, if the reference testing is done with zero mean value of the sinusoidal load, then the fatigue properties at different nonzero mean stresses are related to the reference values by empirical means. This empirical approach originated in the field of metal fatigue in the nineteenth century and is known today as the “classical” approach. Modifications and additions to this approach came in the 1970s from the field of fracture mechanics, where stress analysis of cracks combined with energy considerations provided a means for addressing crack growth under cyclic loading. The metal fatigue approaches have inevitably affected the studies of fatigue in composites. Simple reflection suggests that there is little justification for doing this. While the irreversibility underlying metal fatigue is rooted in crystal plasticity, this is hardly the case for composites, in particular for polymer-based composites. It would make more sense, therefore, to view composite fatigue on its own, divorced from metal-based ideas, and search for sources of irreversibility that are the characteristic of composite materials. This author attempted this approach and reported the results (Talreja, 1981). Since that early work, refinements have been reported in several subsequent works (Akshantala & Talreja, 1998, 2000; Gamstedt & Talreja, 1999; Quaresimin, Susmel, & Talreja, 2010; Talreja, 1982, 1985a, 1987, 1989, 1990, 1993, chap. 13, 1995, 1999, 2000, 2003, 2008; Talreja & Singh, 2012). Appropriately called a conceptual framework for interpretation of fatigue in composites, this approach will be described in some detail in this chapter. The plan of what is to follow in this chapter is as follows. First, a discussion of material irreversibility will be presented where recoverable versus irrecoverable deformation will be clarified. Definitions of viscoelasticity and viscoplasticity will be put forth to facilitate reference to the sources of irreversibility. This will set the scene for describing accumulation of damage with load reapplication and the consequent fatigue failure. Fatigue life diagrams, introduced in Talreja (1981) as a conceptual framework for describing and interpreting fatigue behavior of composites, will follow from this discussion. With these diagrams as a backdrop, a mechanisms-based approach for predicting fatigue failure under general (multiaxial) time-varying loads will be outlined. 1.2. Fundamentals of material durability
As stated above, materials with full reversibility (same loading and unloading behavior) will remain durable as long as the reversibility holds. If time-dependent processes in the material behavior exist, then the aspect of recoverability must be considered. To clarify this, the following discussion is separated in loading cases that are described as constant-in-time and time-varying loads. 1.2.1. Constant-in-time loading
Consider a schematic stress–strain curve in Figure 1.1. It indicates three cases separated by their unloading response and labeled as A, B, and C. Case A is the reversible case with coincident loading and unloading paths. For simplicity, the stress–strain path shown for this case is linear, but in general it could be nonlinear, and if reversible, it would still represent elasticity of the material. Case B and Case C illustrate two cases in the inelastic (irreversible) regime of the time-dependent material behavior. In Case B the unloading path differs from the loading path, but on unloading to zero stress, the inelastic (residual) strain is fully recovered in time. In Case C, the residual strain is only partly recovered, leaving a permanent strain that indicates the nonrecoverable part of the internal material changes induced by loading. The differences in the material behavior between Case B and Case C are further illustrated in the strain–time response shown in Figure 1.1. In Case B, if unloading is not done, but instead the maximum stress reached is held constant, then the strain increases in time, as illustrated in the strain–time plot in Figure 1.1. This strain reaches a plateau, showing no further increase. In Case C, on the other hand, the strain at constant maximum stress increases in stages, characterized in increasing order by strain rates that are exponentially decreasing, constant, and exponentially increasing. The last stage leads to failure at time t = T, as indicated in the figure.
Figure 1.1 (a) Schematic stress–strain curves illustrating different loading–unloading behavior. (b) Time-variation of strain for cases B and C. A: reversible (elastic); B: irreversible with fully recoverable residual strain at unloading; and C: irreversible with partly recoverable residual strain at unloading. Case B and Case C illustrate the classes of material behavior described as viscoelasticity and viscoplasticity, respectively. Many polymers display these two types of behavior. At relatively low stresses, the induced molecular rearrangements increase in time until a limiting state is reached, and these rearrangements can be recovered by unloading. At higher stresses, the time-dependent behavior transitions to viscoplasticity, illustrated by Case C. Here, the molecular rearrangements get states of entanglements that cannot be fully recovered in time. Additionally, as stress increases, other permanent changes such as voids and cracks form. These changes can be self-intensifying, resulting in the exponentially increasing strain, as illustrated for Case C in...