JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 46

fatigue behaviour

Towards structural fatigue evaluation in composite structures
This paper focuses on the fatigue phenomena and mechanisms in composite structures with application to both aeronautical structures (CFRP) and wind turbine blades (GFRP). It explains how fatigue should be understood in such structural materials and how mechanistic models can be developed to describe the fatigue damage growth sequence from first initiation to final failure.

presented indicate how people have sought to apply the data to structural design (Goodman, Haigh, Smith diagrams, etc.). The way the understanding of fatigue has developed over the years is of importance, because it explains the limitations to the current understanding, especially for composites. Due to the fact that damage resulting from cyclic loading was first discovered and investigated in metallic structures, fatigue is often considered a metallic phenomenon. The Dutch word ‘metaalmoeheid’ (metal fatigue) and assertions that composites do not suffer from fatigue clearly illustrate this. Even these days, there are still limitations to the understanding; courses on fatigue commonly start to explain fatigue phenomena with initiation of micro-cracks along slip bands and subsequent propagation through the grains. And in comparison with metals, the advantages of composites are often attributed to their high fatigue limit and resistance to corrosion. Fortunately, we have reached the point where we are starting to acknowledge that not only metallic structures suffer from cyclic loading. Although the process to come to this understanding has been very long [2], especially in educating the industry, a lot of research on the fatigue of composites has been performed within academia.

By
René Alderliesten, Associate Professor Delft University of Technology

F

atigue is a major factor when assessing the durability and structural integrity of aeronautical structures. This is part of the damage tolerance design philosophy. The evaluation of fatigue-related phenomena has evolved over the past decades predominantly based on metal fatigue failures [1]. The development of design philosophies from safe-life design principles, via fail-safe concepts towards damage tolerance design philosophies has been driven by increasing understanding of fatigue. Noteworthy here is that the knowledge on fatigue in metallic materials and structures has been gained over the past centuries rather than over the past decades. After fatigue failures were reported in train axles, various researchers studied the phenomena by cyclically loading structures and materials. The main observation by August Wohler is that the amplitude of this cyclic loading has more influence on the failure life than the mean stress of the load cycles. As a consequence, materials were subjected to constant-amplitude loading and failure lives were recorded and documented with so-called S-N curves. The many graphical variants in which these data have been

S-N fatigue life approach
Today, fatigue tests are being performed until failure, and Goodman diagrams are composed based on the resulting S-N curves (see Fig. 1). These diagrams are subsequently used to determine design lives [3]. Because there are quite a few uncertainties and unknowns related to the data, severe

Fig. 1: Goodman diagram illustrating constant life curves [2]

46 jec composites magazine / No74 July 2012



JEC COMPOSITES MAGAZINE - Issue #74 - July 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #74 - July 2012

Cover
Edito
Point of view: Closer to the shop floor
Contents
Agenda of Events
COMPANY & BUSINESS
- In Brief
- Glass fibre
APPLICATIONS
- Pipes & tanks
- Building
MARKET
- Human resources
- Glass fibre
- Turkey
FEATURE AUTOMOTIVE
- Braiding
- Lightening
- D-LFT
- Sustainability
- RTM
- RIM
RESEARCH & DEVELOPMENT
- Design
- Fatigue behaviour
TECHNOLOGY & INNOVATIONS
- Wind
- Reinforcement
Index
In the world
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Cover
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 2
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Edito
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Point of view: Closer to the shop floor
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 5
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Contents
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 7
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Agenda of Events
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - In Brief
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Glass fibre
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 11
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 12
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Pipes & tanks
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Building
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 15
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 16
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 17
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Human resources
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Glass fibre
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 20
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 21
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Turkey
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 23
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Braiding
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 25
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Lightening
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 27
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 28
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 29
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - D-LFT
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 31
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 32
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 33
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Sustainability
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 35
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 36
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - RTM
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 38
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 39
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - RIM
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Design
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 42
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 43
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 44
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 45
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Fatigue behaviour
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 47
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 48
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 49
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 50
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Wind
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 52
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 53
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - - Reinforcement
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 55
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 56
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - Index
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - In the world
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 59
JEC COMPOSITES MAGAZINE - Issue #74 - July 2012 - 60
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