Punch test?

the punch test on fabric... are the results a go/no-go decision, or
can an A&P make a determination as to how long the covering will last
based on the test? - Mike

I argue that the punch test is go-nogo, but there are those who argue that a
lab test may be required for marginal fabric. My answer is that "marginal"
is like being a little pregnant. IT passes or it doesn't.

It says NOTHING about how long the fabric will last. That is a function of
environment -- hangared or outside, humid or dry, hot or cold...

Jim



"Michael Horowitz" wrote in message
...
the punch test on fabric... are the results a go/no-go decision, or
can an A&P make a determination as to how long the covering will last
based on the test? - Mike

Jim - I'm looking at a fabric a/c with 25 year old fabric. A recovery
will run into 5 figures. Assume we don't know the entire history of
this coating; no log entries stating "under cover from X to Y" so I
have to guess at how long we have remaining on the fabric.

Any suggestions on how that can be done? - Mike




"RST Engineering" wrote:

I argue that the punch test is go-nogo, but there are those who argue that a
lab test may be required for marginal fabric. My answer is that "marginal"
is like being a little pregnant. IT passes or it doesn't.

It says NOTHING about how long the fabric will last. That is a function of
environment -- hangared or outside, humid or dry, hot or cold...

Jim



"Michael Horowitz" wrote in message
.. .
the punch test on fabric... are the results a go/no-go decision, or
can an A&P make a determination as to how long the covering will last
based on the test? - Mike

No way in hell other than extrapolating 100% strength to where the fabric
punches right now and drawing a straight line through 25 years to the
ultimate fabric strength limit. What kind of fabric is it?

Jim

Dacron/Ceconite - Mike

"RST Engineering" wrote:

No way in hell other than extrapolating 100% strength to where the fabric
punches right now and drawing a straight line through 25 years to the
ultimate fabric strength limit. What kind of fabric is it?

Jim

The punch-test gives an approximation of the tensile strength of the
fabric. It has nothing to do with how long the fabric will last.

New Grade 'A' cotton ('standard' weight; about 6oz per square yard)
must have a minimum tensile strength of 80 pounds, equal for both warp
& weft. When applied to a wing or control surface, once the strength
degrades by 30% the fabric should be replaced. (The test is normally
done to the upper surface; that which has the most exposure to
sunlight.)

For comparison, new 2.7oz/yd polyester (ie, Dacron) has a tensile
strength of about 85 pounds, 3.7 oz about 130, although those numbers
may vary slightly from one manufacturer to another. When warp & weft
are not equal you must use the lower strength for your calculations.

Other than fiberglas, all fabrics used for aircraft covering are
subject to degradation. And even WITH fibergals (ie, 'Razorback') the
airframe UNDER the fabric requires periodic maintenance, making claims
of 'Lifetime' coverings a bit iffy :-)

The fact a 20 year old covering measures high-green on a strength test
is no guarantee the thing won't fall apart next week. There could be
corrosion, dry-rot or rampant rusting going on under that slick,
always-hangared fabric skin.

-R.S.Hoover

No!

"Michael Horowitz" wrote in message
...
the punch test on fabric... are the results a go/no-go decision, or
can an A&P make a determination as to how long the covering will last
based on the test? - Mike

Bob...cut me some slack...


wrote in message
oups.com...

The punch-test gives an approximation of the tensile strength of the
fabric. It has nothing to do with how long the fabric will last.

That is the absolute truth. The condom is only so strong from manufacture;
how many times can you use it before it breaks catastrophically?




The fact a 20 year old covering measures high-green on a strength test
is no guarantee the thing won't fall apart next week. There could be
corrosion, dry-rot or rampant rusting going on under that slick,
always-hangared fabric skin.


Which was not the question. The question was the fabric itself. The
remainder of the airframe is quite another matter.

Jim

RST Engineering wrote:
Bob...cut me some slack...

----------------------------------------

Okay. But I wasn't aware you needed any.

My msg was in response to Mike's. The fact it appeared after yours is
an artifact of my ISP, which has taken as long as 24 hours to upload a
message to a Usenet newsgroup.

My closing para was to make sure Mike understood that the condition of
the covering is not the only criteria for re-covering.

-R.S.Hoover

RST Engineering wrote:
Bob...cut me some slack...

----------------------------------------

Okay. But I wasn't aware you needed any.

Moi? {;-)



My msg was in response to Mike's. The fact it appeared after yours is
an artifact of my ISP, which has taken as long as 24 hours to upload a
message to a Usenet newsgroup.

Got it.



My closing para was to make sure Mike understood that the condition of
the covering is not the only criteria for re-covering.

Absolutely true.



-R.S.Hoover


Jim