| Book Review: Damage Tolerance in Advanced Composites | |
As the use of advanced composites increases, these materials are
exposed to ever harsher environments. Despite their high strength and
high stiffness, composites are surprisingly fragile. Damage can come
from a number of sources, both during initial processing and in
service. Even seemingly minor impact events can have a large effect on
thin-walled structures.
Since damage can never be entirely avoided, composite structures
should be designed to function safely despite the presence of flaws. This
concept is called damage tolerance, and is explored by Robert
Sierakowski and Golam Newaz in the very readable book Damage
Tolerance in Advanced Composites.
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The authors divide Damage Tolerance into three main sections: damage tolerance concepts, analytical methodology and evaluation.
Most work in the damage tolerance field—both theoretical and experimental—has been targeted at aerospace applications. This is because the aerospace industry was one of the earliest users of composite materials, and because margins in aerospace tend to be lower than in other industries.
A general definition of damage tolerance is difficult, so the authors provide ten different definitions from the literature. They do provide one basic rule for damage tolerance design: composite structures should be at least as damage tolerant as the metal structures they replace.
Designing for damage tolerance includes selecting materials that are inherently damage resistant, identifying sources and types of damage, understanding damage propagation mechanisms, and designing structures to operate with some degree of damage. Discussion of these topics varies from general, as in the case of damage tolerance approaches, to specific, as in the case of damage resistant materials.
The section on analytical methodology takes a detailed look at seven different damage models. In all cases, these are theoretical models that predict the effect of foreign body impact on composite structures. The key variables in the models are striker mass and velocity, and panel stiffness and thickness.
The models have all appeared in the technical literature. Each section gives a detailed theoretical description, but reference would need to be made to the original sources before attempting an analysis.
The description of each model ends with a brief summary and design rules of thumb based on the model predictions. These rules are very useful, even without a detailed application of the models. For example, Model I predicts that fiber damage resistance increases as fiber strength increases and fiber modulus decreases.
The book concludes with a section on evaluation. There are two steps to this process: 1) identifying damage; and 2) evaluating the severity of the damage. Identification is done through one of many nondestructive evaluation (NDE) techniques. Ultrasonic scans (A-, B- and C-) and X-ray radiography are described in detail.
Severity can only be determined by reference to a standard. Coupons with known defects are prepared, then tested to failure. Field damage can then be compared to the standards to determine if a flaw is severe enough to cause failure. Test methods include compression after impact (CAI), tensile edge delamination, and open hole tension and compression.
Damage Tolerance is mainly a review and overview of damage in composites. Primary sources would have to be consulted before dealing with actual field damage, but Damage Tolerance is a good starting point before beginning any work. One of its most important contributions is the description of the general damage tolerance approach. The authors provide a guide that can be used to set up damage tolerance criteria for any program.
Details: Damage Tolerance in Advanced Composites, by
R.L. Sierakowski and G.M. Newaz, published by
Technomic Publishing Company, 1995, ISBN 1-56676-261-8.
1. Damage Tolerance of Composites; 2. Analytical Methodology;
3. Damage Tolerance Evaluation