Page 258 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 6

INTERFACE MECHANICS AND FRACTURE
TOUGHNESS THEORIES

6.1. Interface-related fracture toughness theories

6.1.1. Introduction

Fundamental considerations of design efficiency for improved mechanical
performance and structural reliability of composite materials require a basic
understanding of how the fracture process initiates and progresses to final failure.
There are ever-increasing concerns pertaining to the ability of a composite to sustain
both static and dynamic loads without the danger of sudden catastrophic failure.
The local response of the fiber-matrix interface within the composite during fracture
is particularly important. If the interface in a composite is to be designed to resist
fracture and thus to enhance the damage tolerance prior to failure, it is necessary to
understand the basic failure mechanisms or origin of fracture toughness.
The term ‘fracture toughness’ or ‘toughness’ with a symbol, R or G,, used
throughout this chapter refers to the work dissipated in creating new fracture
surfaces of a unit nominal cross-sectional area, or the critical potential energy
release rate, of a composite specimen with a unit kJ/m2. Fracture toughness is also
often measured in terms of the critical stress intensity factor, K,, with a unit
MPaJm, based on linear elastic fracture mechanics (LEFM) principle. The various
micro-failure mechanisms that make up the total specific work of fracture or
fracture toughness are discussed in this section.
Theories for both elastic modulus and strength of composites have been well
developed, and the factors governing these fundamental mechanical properties are
relatively well understood. The rule of mixtures (RoM) concept has been most
widely used for strength and modulus predictions although it is not completely
adequate for composites containing short, randomly oriented fibers. The fracture
process in fiber composites is seldom straightforward because of their microstruc-
tural inhomogeneity and macroscopic anisotropy. Because the presence of interfaces
that form the boundaries between dissimilar media makes the fracture behavior of
composites even more complicated, the simple RoM cannot be employed to
quantify their fracture resistance. It is well known that the fracture toughness of a

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