QUANTITATIVE FRACTOGRAPHIC ANALYSIS OF FATIGUE FRACTURE SURFACES

                        N. Ranganathan

                      Université de Tours


When failure of a metallic part occurs in service, it is very
important to determine the main reasons for failure.In the case
of a fatigue failure, certain frctographic details, such as
striations, indicate the crack growth rate and thus to a certain
extent, the corresponding stress intensity factor amplitude.
However, in the case of variable amplitude loading, the situation
can be quite different as the fracture surface appearence is more
difficult to analyse.

In this presentation, a method developed by the author to analyse
such facrure surfaces is presented. The method consists of
determining the surface coverage by all the fracture modes, such
as, quasi-clivage facets, striations and dimples. It is shown
that this spatial distribution is quite represenative of fracture
surfaces observed under constant amplitude test conditions and
allows the determination of the loading conditions, such as, the
stress intensity factors and the load ratio. In the case of
variable amplitude test conditions, representative of aircraft
loading, as long as the crack growth mechanisms are the same as
those under constant amplitude conditions, the same fractographic
features permit the determination of the maximum stress intensity
factor and an equivalent constant amplitude loading. Finally,
guidelines for further development of such methods are presented. 

This talk adresses material scientists, Aircraft Engineers and
Mathematicians, open to new ideas!