STRESS CONCENTRATION FACTORS & STATIC STRESSES

[StressMan2506]

Ladies & Gents:

The conventional approach in the office in which I work is that we calculate net static stresses as gross stress times the ratio of gross area to net, i.e. no stress concentration at the edges of holes. I do not believe this approach to be correct and I cite section 17.1 of Roark, 7th edition. This section is entitled "Static Stress & Strain Concentration Factors".

Comments etc are invited.


Regards,
Louis

 

[corus]

Stressman, I think you made a typo there as you quote Kt twice.
As I understand it, you quote an expression for the stress concentration taking into account non-linear stresses. The design SN curves I've seen, however, are all based upon linear elastic values and thus you should use the expression for a stress at the hole based upon the linear stress range for fatigue. For a static assessment, however, fatigue doesn't come into it and the recommendation from the design codes I've seen is to classify stresses into primary/secondary/peak components and not to use the peak component at the hole for static loads. The same argument would also apply to secondary stresses (strain dependent) where the stress range is limited to twice yield to prevent incremental collapse or ratchetting. As there is no range of stress for static loads, secondary stresses would be also excluded from an assessment. I think the general opinion in your office is therefore correct for static loads. I'd refer to ASME or British standard codes for pressure vessels which give good descriptions of stress classification and which appear generally applicable to all structures.

corus

 

[crackman]

I have in the past seen static strenght analysis take into account Kt effects however this is rare and in isolated cases only. I can say this much, if the structure being analyzed is so critical that a Kt must be investigated then it is only a stop-gap prior to a full scale or component test. In fact, a couple of years ago, I reviewed a static strenght analysis of a lower wing cover where the +0.0 Margin of Safety for the entire wing was at a fastener hole near a cutout in the skin. A detailed non-linear FEM was used to develop the MS and of course it included the Kt effect. HOWEVER, the entire analysis was later substantiated by full scale ultimate testing. I guess in summary, if you think you have a design for which a local Kt may impact your MS, then you should probably think about testing.

Just a thought, not sure it helps any though.

James Burd
Avenger Aircraft & Services

 

[StressMan2506]

Hi Corus:

I deliberately wrote Kt twice. Kt in fatigue, being greater than Ktf, is conservative.

A stress simply calculated using Kt is not non-linear. Where the nominal stress is high, Kt will give a fictitious result, which the Neuber method corrects. Stresses in fatigue analysis are lower, therefore application of Kt to them will not usually incur an excursion into the non-linear range.

Louis