Fatigue in aluminium thread

Question

Fatigue in aluminium thread

SteffenDK (Mechanical)

(OP)

21 Aug 03 04:16

I would like to ensure that some threads in a hydraulic block (subjected to pulsating pressure) made in aluminium
do not fail in fatigue. I see two options.

1) Do a lot of advanced FEA, that probably result in a situation where the results are extremely sensitive to the way I model the thread and I further more do not have sufficient material data to judge the result.

2) Use a rule of thumb that says that if I have a 20 mm bore and a 200 bar pulsating pressure I at least need a 40 mm wall thickness in the block (or something like that).

Anybody out there with a good rule of thumb, or a third option?

Steffen

CoryPad (Materials) · Answer 1 · 2003-08-21 08:33:03

Is the aluminium block internally threaded? If so, then the stresses on the threads are shear, and therefore, not likely to initiate and propagate fatigue cracks. If they are external threads, then you can design the proper preload to keep the cyclic stresses below the Al alloy's fatigue limit.

Regards,

Cory

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SteffenDK (Mechanical) · Answer 2 · 2003-08-21 08:41:19

It is internally threaded, which in my opinion can be compared to a Mode I crack, so I am pretty certain that it can fail in fatigue.

CoryPad (Materials) · Answer 3 · 2003-08-21 08:48:33

No, internal threads have Mode II loading through the shear area. An externally threaded fastener has Mode I loading through the tensile stress area.

Regards,

Cory

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

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Fatigue in aluminium thread