7075-T651 Fatigue Failures 2

[mewgg]

We have a part fabricted from 7075-T652 aluminum and subjected to axial compression loading and bending in two orthogonal planes. We have problems with fatigue failures and have performed an FEA to address high-stress areas and reduce stress concntrations. Currently, the maximum applied stresses are ar or below 20 kpsi in most areas. The part has passed a 3 million cycle fatigue test in the unanodized condition. After a hard-coat anodize layer (0.0004" thick) is applied, the part fails at about 380,000 cycles. The failure is at the tangent point of a radius between section changes. The FEA shows the Max Principal Tensile Stress magnitude is approximately 20 - 26 kpsi in the area of the failure. (A fatigue limit of about 36 kpsi was assumed for the material). Any ideas concerning a possible reason for the huge change in performance is welcome.

 

[TVP]

The hardcoat anodizing is the cause of the huge reduction in fatigue performance. It is well-known that hard ceramic layers like anodizing on aluminum initiate cracks and reduce fatigue life.

 

[CoryPad]

Reduction in fatigue life of anodized aluminium is a well-recognized phenomenon with a plethora of information available.

For example, according to MIL-A-8625F ANODIC COATINGS FOR ALUMINUM AND ALUMINUM ALLOYS,

6.10.7 Effect on fatigue. The fatigue properties of aluminum alloys can be severly reduced by anodic coatings.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Goahead]

The alternative, if the hardness of the anodizing is not a design requirement, is to use chromate conversion coating, (Mil-C-5541E), one commercial version of it is called allodine.

http://www.welding-advisers.com/

 

[Kwan]

You can also shot peen prior to the anodize to gain a little fatigue resistance. Another improvement can be done by specifying a smoother surface.

What kt are you getting from the fem?