40207_35 - DURABILITY AND DAMAGE TOLERANCE OF FIBROUS...

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DURABILITY AND DAMAGE TOLERANCE 35 OF FIBROUS COMPOSITE SYSTEMS Ken Reifsnider 35.1 DEFINITIONS AND ISSUES Durability and damage tolerance are critical to the design of composite structures. Damage tolerance is the approach often required for the certification of safety-rated structures such as aircraft components; dura- bility has been identified as one of the most important technical drivers for the design of major composite structures such as the High Speed Civil Transport. Recent reports from the National Materials Advisory Board and a great volume of other literature focus on these Of course, there are many nuances in the definitions of durability and damage toler- ance. However, the basic concepts are quite simple, and are illustrated in Fig. 35.1. Damage tolerance is the remaining strength after some period of service, and durability, in general, has to do with how long the compo- nent will last, i.e. with the life of the structure. In this context, durability is often discussed in terms of the resistance or susceptibility to damage initiation. Both concepts imply that the subject component is being exposed to applied conditions such as mechanical loading and environments such as temperature and chemical agents over long periods of time that are typical of the projected service life of the component. There are several technical concepts that form a foundation for our discussion of these closely related topics. The first of these is the 1 Normalized stress level Time / Cycles Damage Tolerance (Remaining strength) Life Locus Durability (Life) 4 Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54.020 7 Fig. 35.1 Basic definitions ’durability’ and ‘dam- age tolerance’.
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Definitions and issues 795 question of the relationship of material strength to structural strength. In general, the strength of (fiber reinforced) composite mate- rials is represented by an array of values that reflect the anisotropic nature of the materials (Fig. 35.2). For planar materials, at least the tensile strength and compressive strength in the fiber direction and transverse to the fibers and the in-plane shear strength are required for a complete answer to the question of ’how strong is this material’. However, as an array, those values do not directly show ’how strong is a composite structure’. Several possible answers to that question are typically given. One may use a ’failure criterion’ that compares all of the point stress components with all of the material strength components (such as the Tsai-Hill or Tsai-Wu riter ria)^ in some collec- tive form based on concepts such as critical energy, critical shear resistance, etc. The salient point to be made is that the complexity of (inhomogeneous) composite materials and their array of anisotropic material strengths give rise to the development of a correspond- ing array of damage and failure modes in these materials that must be understood and correctly modeled to answer the question of
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This note was uploaded on 03/16/2010 for the course MECHANICAL ME765401 taught by Professor Prof.sulis during the Spring '10 term at Institut Teknologi Bandung.

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40207_35 - DURABILITY AND DAMAGE TOLERANCE OF FIBROUS...

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