Fracture Mechanics basic concepts

The discussion of fracture mechanics will be divided in two parts. First, basic principles of fracture mechanics will be described. Second, the application of fracture mechanics concepts to composite materials will be discussed. In both parts, the basic approach is that of Wu [6-12],... 

Fracture mechanic The fracture mechanics theory developed for metals is also adaptable for use with plastics. The basic concepts remain the same, but since metals and plastics are different they require different techniques to describe their fatigue-failure behaviors. Some of the comments made about crack and fracture influences on fatigue performance relate to the theory of fracture mechanics. The fracture mechanics theory method, along with readily... 

Our discussion thus far has focused in a rather superficial way on the general evolution of the important area of fracture mechanics. The basic objective of fracture mechanics is to provide a useful parameter that is characteristic of the given material and independent of test specimen geometry. We wUl now consider how such a parameter, such as G (, is derived for polymers. In doing so we confine our discussion to the concepts of linear elastic fracture mechanics (LEFM). As the name suggests, LEFM apphes to materials that exhibit Hookean behavior. 

The equivalence of K and (7, which strictly holds for elastic materials with linear load-deflection characteristics, is referred to as linear-elastic fracture mechanics (LEFM). Subsequently this basic concept has been modified to describe also the behavior of ductile materials. For instance. Wells [3] considered the plastic strain at the crack tip as the crack... 

Background and Overaii Considerations. Many aspects of FCP including both experimental approach and theory evolve from concepts well established in the area of fracture mechanics. Many treatments dealing with both the theory (59-63) as well as associated treatments to pol5mieric materials (2,5,64,65) have been published. The basic tenent of fracture mechanics is that the strength of most real solids is governed by the presence of flaws. Many theories have been... 

The fracture mechanics approach to evaluating joint durability basically relates to the early work of Griffith(i 72) in 1920 with brittle materials like glass. IrwinO S) in i960 used the concept to measure a materials resistance to fast crack extension in the presence of a flaw. 

The main emphasis of this chapter will be on the basic fracture mechanics concepts for cohesive and adhesive fracture with some extension to crack branching and crack nucleation from bimaterial comers. Most of the current fracture mechanics practice in testing adhesives and designing of adhesively bonded joints is limited to linear elastic fracture mechanics concepts. The development of the background material presented here will therefore be similarly constrained, except for the last section. Historically, fracture mechanics developed from energy balance concepts and examinations of stresses around crack tips. The adhesive fracture community has tended to favor the former, but both have useful features and will be carried forward in the discussions that follow. 

Much of what is summarized here is presented in more detail in the excellent article on mixed-mode cracking in layered materials by Hutchinson and Suo [55]. In that reference, the authors consider not only interfacial fracture, but also cracking within coatings and cracking in substrates below coatings. Only interfacial cracking (delamination) will be considered in detail here. Before proceeding however, we will first discuss some basic concepts of fracture mechanics. 

One of the purposes of this paper is to differentiate between surface free energy and fracture surface energy. Thus, several basic principles of fracture mechanics are discussed with reference to the Griffith s energy-b J nce concept and the Irwin-Orowan s plastic-zone concept. We also define several frequently misused terms, e.g., effective fracture surface energy, fracture energy and fracture toughness. Actual values of related parameters are presented to illustrate the applicability of fracture mechanics to the selection of polymers for structural materials. 

In summary In this section we introduced some of the fundamental concepts starting from basic chemistry, intra- and inter-molecular interactions, polymer physics, and the nanomechanics and nanofracture mechanics of polymers. These theories can help scientists, engineers, technologists as well as other professionals understand why there is such a wide variety of properties available to polymers and even for a give polymer, different authors can give very different physical and mechanical parameters, for example, fracture stress of PVC reported in literature varies from 10 to 65 MPa. In fact, there are many factors that can lead to polymer property variation as discussed in Sections 1.1.1—1.1.7. The conclusion is that the variation of polymer properties is a combined effect of aU these factors. 

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