This chapter discusses the failure mechanisms associated with fiber-reinforced composites. It begins with a review of fiber-matrix systems and the stress-strain response of unidirectional lamina and both notched and unnotched composite laminate specimens. It then explains how cyclic loading can lead to delamination, the primary failure mode of most composites, and describes some of the methods that have been developed to improve delamination resistance, assess damage tolerance, determine residual strength, and predict failure modes.

Fracture control can be defined as a concerted effort to maintain operating safety without catastrophic failure by fracture. It requires an understanding of how cracks affect structural integrity and strength and the time that a crack can grow before it exceeds permissible size. The chapter describes some of methods used to determine maximum permissible crack size and predict growth rates. It explains how the information can then be used to control fractures through periodic inspection, fail-safe features, mandated retirement, and proof testing. It presents a number of fracture control plans optimized for different circumstances, examines the damage tolerance requirements used by different industries, and discusses various approaches for fatigue design.

This chapter discusses various types of material fracture toughness and the methods by which they are determined. It begins with a review of the basic principles of linear elastic fracture mechanics, covering the Griffith-Irwin theory of fracture, the concept of strain energy release rate, the use of fracture indices and failure criteria, and the ramifications of crack-tip plasticity in ductile and brittle fractures. It goes on to describe the different types of plain-strain and plane-stress fracture toughness, explaining how they are measured and how they are influenced by metallurgical and environmental variables and loading conditions. It also examines the crack growth resistance curves of several aluminum alloys and describes the characteristics of fracture when all or some of the applied load is in the plane of the crack.

Unlike metals, in which fatigue failures are due to a single crack that grows to a critical length, the effects of fatigue in composites are much more distributed and varied. As the chapter explains, there are five major damage mechanisms that contribute to the progression of composite fatigue, those being matrix cracking, fiber breaking, crack coupling, delamination initiation, and delamination growth. The chapter describes each mechanism in detail along with related factors. It also discusses the primary differences between composites and metals, the effect of manufacturing defects, damage tolerance, and testing and certification.

This appendix is a compilation of terms, definitions, abbreviations, and symbols associated with the mechanics and mechanisms of fracture.

This chapter presents a fracture-mechanics-based approach to damage tolerance, accounting for mechanical, metallurgical, and environmental factors that drive crack development and growth. It begins with a review of stress-intensity factors corresponding to a wide range of crack geometries, specimen configurations, and loading conditions. The discussion covers two- and three-dimensional cracks as well as the use of correction factors and problem-simplification techniques for dealing with nonstandard configurations. The chapter goes on to describe how fatigue loading affects crack growth rates in each of the three stages of progression. Using images, diagrams, and data plots, it reveals how cracks advance in step with successive stress cycles and explains how fatigue crack growth rates can be determined by examining striations on fracture specimens and correlating their widths with stress profiles. It also describes how material-related factors, load history, corrosion, and temperature affect crack growth rates, and discusses the steps involved in life assessment.

This chapter is largely a compendium of best practices and procedures for minimizing the effects of fatigue. It explains how to make products more resistant to fatigue by choosing the right materials and manufacturing processes, avoiding geometries and features that concentrate strains, preventing or removing surface damage, and by inducing compressive mean stresses that prolong fatigue life. It also discusses the use of property conditioning and restoration treatments, the benefits of interference fits and processes such as coaxing, the effects of assembly damage and operating overload, the importance of surface cleanliness and finish, and the role of inspection, testing, replacement, and repair in safe-life and fail-safe designs. Examples highlighting the benefits and potential pitfalls of proof loading tests are included as well.

This chapter covers the basic aspects of composite materials. It describes the arrangement, form, and function of their constituent materials and explains how they perform better in combination than on their own. It discusses the directional nature of isotropic, anisotropic, and orthotropic materials, the orientation of plies in unidirectional (lamina) and quasi-isotropic (laminate) lay-ups, and the dominant role of fibers in determining strength, stiffness, and other lamina properties. The chapter also compares the engineering attributes of composites with those of metals and includes application examples.