Jerry is quite right about variability being one of the factors. Further
points on the comparison of metal and composite structural testing and
limits:
If a metal structure is tested to a particular equivalent 'g' loading and
it returns to its original shape with no deformation, then this can be used
as a maximum in-service load. This is because the breaking stress for metals
is normally at least 50% greater than the stress at which permanent
deformation starts to occur. If you test an all-metal structure to, say, +6
/-4 g and it doesn't suffer any permanent set, then it may be cleared to fly
to +6 /-4 g.
If a composite structure is tested to a given load, whether or not it
returns to its original shape, there is no guarantee that it will take any
higher load at all before failure. Hence aircraft composite primary
structure needs to be tested to the ultimate (max in-service load x safety
factor 1.5 x composite super-factor 1.2 to 1.5) in order to be cleared for
the in-service load. The CAA break down the elements of the super-factor
into factors that allow for each of the following: sample variability, build
variability and the effects of temperature, moisture & ageing. They can
provide graphs to show the rise in skin temperature for different colours,
leaving you in no doubt about the benefits of choosing a mainly white scheme
for any low-temp-cured composite aircraft.
You may ask: what happens when you have to test a composite structure with
metal bolts holding it together?
The answer is that if the bolts have been sized for the loads borne by an
equivalent metal airframe, they will fail when taken to the ultimate for a
composite structure. At least 2 American kit manufacturers found that the
wing attachment bolts failed first on their structural tests.
Sorry to go on at length, but as the Club Safety Officer I don't want anyone
thinking that these safety factors are just conjured up by the regulators
for no good reason: it's because they DO understand composites, not because
they don't!
Be safe.
Mike
On 25 January 2000 13:07 Jerry Davis sent
I also believe that generally composites are considered more varaible than
materials like steel and aluminium etc.. Statistically speaking there will
be greater variation in the possible ultimate strengths of materials ( that
is the standard deviation is greater ). The greater material safety factors
used therefor only reflect the inherant variations of the material.
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