1- Linear Elastic Fracture Mechanics (LEFM) is a

theory which was first introduced by Griffith in 1924. LEFM is a theory of

fracture which deals with sharp cracks in elastic bodies. It is based on the

assumption that the material is isotropic and linearly elastic. With this assumption

in mind, LEFM has been developed using the stress intensity factor (K) which is

determined by the stress analysis and expressed as a function of stress and

crack size. There are three modes of fracture. Mode 1 is where crack surfaces

move opposite and perpendicular to each other, while modes 2 and 3 involve

sliding and lateral tearing. It is important to note that LEFM only works when

inelastic deformation is small compared to the crack size.

2- The author predicts that the future will include

applications of fracture mechanics in tiny functional devices that need to have

structural integrity. While these applications are outside traditional

engineering problems, they can be solved using the current techniques.

3- Griffitth’s main discovery on

fracture mechanics was energy balance for cracked body. His motivation was to

understand the effect of scratches on fatigue. Previously it was thought that

it should be possible to estimate the fatigue limit of a scratched component by

using the maximum principal stress criterion or the maximum principal strain

criterion. Using the results of Inglis, Griffith showed that the scratches

could raise the maximum stress or strain by a factor from 2 to 6. However,

Griffith discovered that the maximum stress or strain would be the same on a

shaft of 1 in regardless of whether they were one ten thousand or one hundredth

of an inch deep which forced Griffith to reject the earlier belief. This led

Griffith to conclude that certain minimum work was needed to produce a fracture

and saw the fracture problem an extension of the elastic theory of minimum

potential energy. in addition, he was also able to realize the critical strength

depended on the crack length. Griffith performed his experiments on model glass

material. His experiment showed that the tensile strength of glass was lower

than the theoretical strength, thus making him believe that glass had internal

flaws.

4- Orowan’s work led to believe that

Griffith’s work was for less brittle materials. Irvin, using Orowan’s work

noted that the energy expended in the plastic straining could be estimated.

Finding that the fracture energy was two thousand times the surface energy,

Irvin theorized that Griffith’s theory could be used if the plastic work were

to be substituted for the surface energy thus modifying Griffith’s theory.