Hi Monel,

I can tell you the following about current engineering practices @ Boeing.

Boeing has spent a *lot* of money developing methods for predicting the fatigue life of aircraft structure - and even have a group of engineers specifically appointed to developing new tools and methods for fatigue life prediction on all of their fleet.  

Boeing provides a set of 4 internal books which cover a lot of the fatigue (and corrosion) problems  with aircraft (including "lessons learned.")  Since I am a stress engineer with Boeing (and have access to these books), I am constantly checking for any type of fatigue problems that might exist on any primary  structural design (either with repairs or with a new design or modification), because even if a design is good for ultimate loads, it isn't any good if it breaks after two flights because of poor fatigue design. :)

The major steps to reduce metal fatigue at Boeing are as follows:

We (stress engineers at Boeing) review all primary load bearing structure to ensure that there is adequate fatigue life in any new design, repair or modification.

1.  Ensure "gentle" loading of joints - we don't what very high load transfers from one "plate" to the next "plate" through any fastener. The "gentle" loading has to do with the number, type and size of fasteners in a joint, as well as the thickness of the materials across the joint.

2.  Ensure no hard points exist on structure, i.e. if a thick part is attached to a thin part, there is a possibility of fatigue cracking around the periphery of the attached thick structure.

3.  The type of fastener, the size of the holes, the finish of the part, the eccentricity of the joint, the amount of material in the joint also play important roles in reducing the metal fatigue.

In sum, you can think of it this way:

If you have a good joint design, which has good load transfer across and adequate number of fasteners, *and* you don't have poor stress details (like nicks, gouges, scratches, tight radii), and the material finish is acceptable, you are on your way to a good fatigue design.

I am writing a conclution on fatigued materials and want to know if you can help.