Composite Structures

Effects of Defects

Rani Elhajjar, Peter N. Grant, Cindy Ashforth

• Publisher: Wiley
• Year: 2018
• Print ISBN: 9781118997703
• eBook ISBN: 9781118997727
• Edition: 1
• Topic: Technologie et ingénierie
• Subtopic: Génie mécanique

Presents the latest strategies in the development and use of composite materials for large structures and the effects of defects

Practical Design and Validation of Composites Structures: Effects of Defects offers an important guide to the use of fiber-reinforced composites and how they affect the durability and safety of engineering structures such as aircraft, ships, bridges, wind turbines as well as sporting equipment. The text draws on the authors' direct experience in industry and academia to cover the most recent strategies in the development of composite structures and uniquely integrates the assessment of the effects of defects introduced during production.

This comprehensive resource builds on an essential introduction to the characteristics of composites and the most common types of defects encountered in production. The authors review the recent manufacturing methods and technologies used for inspecting composite structures and the design issues related to an analysis of their failure and strength incorporating the variability of processing. The text also contains information on the latest regulatory requirements and the relevant standards associated with the testing and design within a robust design philosophy and approach. This important resource:

• Offers a comprehensive review of the most current regulatory developments in the use of composites for the construction of complex composite structures
• Presents information on the basic characteristics of composites
• Includes testing strategies for determining the impacts of production defects
• Reviews the most current manufacturing methods and inspection technologies in the field
• Contains methods for statistical analysis and processing of experimental effects of defects test data

Written for professional engineers in mechanical engineering, automotive engineering, aerospace engineering, civil engineering, and energy engineering as well as industry and academic researchers, Practical Design and Validation of Composites Structures: Effects of Defects is the hands-on text that covers the essential information needed to understand the use of composites and how they affect complex engineering projects using composites.

1
Characteristics of Composites

The purpose of this introduction to composites is to review the essential differences of their structural behavior from the more traditional metallic materials. Since the use of composites has become much more commonplace in the past 20 years, and many of the issues are now well known, this review will not be prolonged; instead, it will reference sources in the literature rather than repeat previously documented information.

1.1. Introduction to Behavior

Advanced polymeric composite material systems consist of fibers of high specific strength and stiffness embedded in a resin matrix of significantly lower stiffness and strength. (Note this description is not applicable to ceramic matrix composite materials.) The finished material can be of a laminar or woven structure, with or without core materials. Both the inhomogeneity of the finished structure and its orthotropic structural response are two major characteristics that are not seen in metallic structures to anywhere near the same degree. These structural characteristics lead to the following behavioral differences:

• Composites are notch sensitive, but they shouldn't be considered “brittle.” Cracks, holes, and other notch geometries that cause a stress concentration will reduce tension, compression, and shear strength. Matrix‐dominated damage types, such as delamination, can grow after development of a “sharp crack,” assuming out‐of‐plane loads can sustain growth.
• Composites are dimensionally stable in‐plane, but out‐of‐plane/through‐thickness loads are important because composites are generally anisotropic with the lowest strength coming in out‐of‐plane tension and shear. The very low thermal expansion can lead to significant residual stresses when composites are combined with metals in hybrid structure. Residual stresses can also occur in the composite in service or during the cure process due the different coefficients of expansion between the fiber and matrix.
• Composite structures will generally have competing failure modes that will be a function of specific part geometry and may vary with environment.
• Composites are more sensitive to environmental effects and overheating. Most composite matrices absorb moisture in a diffusion process that adds weight. Tension, compression, and shear strength can all be affected by temperature and moisture content.
• Composites typically exhibit flat S‐N curves if defects are not present.
• Composite structure must be characterized as a function of laminate layup, materials, thickness, process variables, and individual part geometry.

An unfortunate consequence of these behaviors is that composite characterization is non‐standardized and typically proprietary for each composite manufacturer. Composites have high non‐recurring costs for product development and certification; therefore, most companies are reluctant to share the composite technology from their investment. In addition, existing composites standards organizations, such as Composite Materials Handbook‐17 (CMH‐17) and American Society for Testing and Materials (ASTM) Committee D30, are stymied in their standardization efforts because they are primarily based on volunteer support. In contrast, metal standardization has benefited from governmental actions to accelerate development and ensure a trained workforce during times of need, such as the Second World War. There are no similar driving functions to push composite standardization at this time.

To further describe composite behavior, Table 1.1 provides a general overview of the differences between metallic and composite static strength.

Table 1.1 Differences between metallic and composite structural static strength.

Metal structure	Composite structure
Tensile residual strength is affected by cracks (compression response is typically not).	Tensile and compressive strength are affected by stress concentrations.
Yielding to minimize the effects of small holes and design details that cause stress concentration.	Localized bearing failure in reaction to small damage, holes, and design details that cause stress concentrations.
Net section analysis may be used to size static strength for the presence of holes.	Semi‐empirical methods or advanced analyses needed to size composite structure with stress concentrations.
Not sized for static strength with cracks or “manufacturing defects.”	Static strength sizing with acceptable damage and manufacturing defects.

1.2. Introduction to Composite Analysis

In general, the discrepancy between the fiber and matrix stiffnesses is very significant for most fiber‐reinforced composite materials. Typically, the graphite (or carbon fiber) longitudinal Young's modulus is 20 times that of the matrix. For glass fibers, this discrepancy is not as great, and the ratio between the two moduli is typically 3.5. It is well known that these fiber‐matrix stiffness discrepancies are difficult to model for an individual ply, and result in increased analytical difficulties of composite structures compared to the analysis of metallic structures. In addition, most practical composite structures consist of multidirectional laminates. The development of laminate structural properties and ply‐level stresses further increases the analytical effort compared to that required for homogeneous, isotropic, and metallic structures. Practical (semi‐empirical) structural failure prediction methods have been well documented in the literature; examples being presented by Halpin [1], Jones [2], and Jayne and Suddarth [3]. For the purposes of this book, the relationships required to develop multidirectional laminate stiffnesses and lamina (ply)‐level stresses and strains for a laminate subjected to in‐plane and flexural loading are documented as follows.

The Hooke's law relationships for orthotropic laminae in a plane stress state are:

The components of the lamina stiffness matrix (Q) are:

In industrial applications, the load‐strain and curvature relationships for the complete multidirectional laminate under applied in‐plane loading, are developed using Classical Laminated Plate Theory (CLPT), resulting in the following:

(1.1)

where the superscript “0” denotes mid‐plane strains and curvatures.

The matrices A and B are the transformed stiffness matrices (from the lamina axes to the laminate principal axes, “x” and “y”) and summated (through the laminate thickness) lamina stiffness matrices (Q). The derivations can be found in Refs. [2, 4]. For the general multidirectional laminate both the A and B matrices are fully populated, which would not be the case for an isotropic material.

In the case of a multidirectional laminate with through the thickness symmetry about the mid‐plane (i.e., in thermal equil...

Table of contents

• Cover
• Table of Contents
• Preface
• Chapter 1: Characteristics of Composites
• Chapter 2: Design Methodology and Regulatory Requirements
• Chapter 3: Material, Manufacturing, and Service Defects
• Chapter 4: Inspection Methods
• Chapter 5: Effects of Defects – Design Values and Statistical Considerations
• Chapter 6: Selected Case Studies in Effects of Defects
• Glossary
• Index
• End User License Agreement