(Velcro) Hook and Loop strength calculations under G force

Aus Inam · Sep 8, 2020

Hi Everyone,

I am a trainee engineer working on a design to come up with a pouch(camouflaged) that is secured using a hook and loop method to another bag(green). Please refer to the attached pics.

I am trying to work out if the pouch would hold under severe G forces.
I have the total mass of the pouch, the dimensions, area and material specifications of the Velcro and would like to know at what force would the bag come undone in following situations:

1)8G vertical Force
2)4G lateral force

Would be greatly appreciated if I could have the right formulae to tackle this problem.

Inam

3DDave · Sep 8, 2020

Force is not measured in "G" so that's the first problem to be dealt with.

Aus Inam · Sep 8, 2020

G is merely referring to 9.81m/s2 [smile]

3DDave · Sep 8, 2020

That's great. And what are the units of force?

Sparweb · Sep 9, 2020

Test it!
If you were getting good guidance from your mentor, and remember your high school physics, you could be done in 10 minutes (most of it spent checking the calibration sticker on the pull-scale. Hint-hint)

You forgot the forward and rearward load cases.

Because velcro degrades with use, this pouch cannot move once stowed, or else you will commit your project to a long-term reliability test.

Because velcro is flammable, you also need a burn test. Reference Apollo 1. Velcro is the same product it was in the 1960's.

Because velcro can be mis-aligned but still seem fastened, you need to add a method to assure alignment of the two strips.

For this reason (and probably more that don't yet come to mind) engineers in aviation NEVER USE VELCRO to restrain items of mass in a cabin. NEVER.

There are alternatives and I would encourage you to examine your aircraft for preferable locations to stow this item.

http://www.sparweb.ca

Aus Inam · Sep 9, 2020

thanks SparWeb.

We did some good testing on it using a Pull scale. We attached the scale to the CoG of the pouch.
We tested in 3 different directions shear lateral, shear width wise and tension.
In all of these cases the pouch started "peeling" off rather than shear or sudden separation. Now I know what force it takes to peel if off.
Is it now as simple as F=ma to calculate the max acceleration the bag can sustain before failure? or need to dig deeper with bending moment and stress? Please advise

Taken your advise regarding risks of using Velcro. thanks for that. We have already considered this hence we could potentially be using a fire retardant Velcro of same spec as above, passed to 25.853(A) Mil-Spec.

Due to very confined space and lack of restraining options which utilise equivalent protrusion, Velcro seemed the one of small number of option I could pick for this task.

cheers

Inam

Sparweb · Sep 10, 2020

That's a good start. At this point I really have to clarify if you are working in the military world or on a civilian aircraft? Either way, you will need to address all of these issues with an authority to their satisfaction. I don't work on military aircraft, but I know they don't add stuff randomly, and in the civilian world I am referring to authorities such as ODA or DOA or ADO engineering units that report to the FAA or EASA or CASA.

There are ways to deal with the complicating issues that I have raised above, and the method you use must be made acceptable to that person. Some of the questions you write are implying that you are not in contact with that person yet, for some reason.

For instance, if you did your load test with no agreed-upon planning document, then your authority may declare your results are invalid and you are still at square one. It may be a pouch with trivial mass but the rule is the same for seats and cargo bulkheads. The approving authority will tell you what you need to do to make your test valid.

If you've invented something for aircraft use, you should contact a certification engineer like myself (but in the proper national jurisdiction with the appropriate domain of mil/civvy authorization) in the real world for proper guidance.

http://www.sparweb.ca

verymadmac · Sep 10, 2020

Depending were it is located the critical load case may be a practical one of will it remain attached if it catches on a person brushing past or being pull off by someone stashing a bag beside it, etc, etc.

3DDave · Sep 10, 2020

I did work on US military aircraft systems, mostly radar systems. Sections of MIL-STD-810 applied per the contract. Fatigue was mostly by analysis, while crash and essentially durability was done on impact, drop, or vibration tables in environmental chambers as required. The largest item I worked on was a 65,000 pound vehicle that needed to be tied down in a C-17; they certainly did not want that heading forward, but they also didn't have a deck that could withstand the typical crash retention requirement. (It's been a while - crash load of 15 Gs?)

Fire retardance for a 20+psia pure oxygen atmosphere for Apollo 1 is a bit different that what most aircraft operate with. It appears the original bag already uses Velcro as a closure. It's not my favorite material as it ages rapidly with use.

One thing that is not clear is why the pouch is detachable. I'd look at adding a pocket to the original bag with a flap closure to retain the pouch, allowing the use of snaps or clips or any of dozens of other closures to hold the flap shut.

Fun story in using Velcro. Adhere two opposite large-areas with Velcro to separate pieces of metal. Press them together. Have fun getting them apart. The joint failure mode for most Velcro use is line of peeling rather than areal separation. A buddy of mine thought that holding metal boxes in place with Velcro was a good idea. I think it was on the order of 200 pounds to get them apart instead of the few ounces that peeling normally requires. I see the example is about 6 psi vs a bit of 1 pound per inch. It would have been OK if there was any place on the box to apply 200 pounds of force without damaging it.

rb1957 · Sep 10, 2020

25.561 crash is 9g fwd

another day in paradise, or is paradise one day closer ?

rb1957 · Sep 10, 2020

my opinion of Velcro is much like friction (or fay seal sealant) … yes they exist, but can you rely 100% on them … (no).

Sure I'll use it to hold down "nuisance" items (small loads), but not much more.

another day in paradise, or is paradise one day closer ?

3DDave · Sep 10, 2020

Like I said, I just did military contracts; nothing to do with CFRs.

WKTaylor · Sep 11, 2020

FYI... ONLY... for-what it's worth... Yeah I've done this too, guys...

WHAT Combination, exactly are You using... lots of material options and important physical variety.

MIL-F-21840 CX FASTENER TAPES, HOOK AND LOOP, SYNTHETIC
A-A-55126 FASTENER TAPES, HOOK AND LOOP, SYNTHETIC
ASTM D5169 Standard Test Method for Shear Strength (Dynamic Method) of Hook and Loop Touch Fasteners
ASTM D123 Standard Terminology Relating to Textiles
ASTM D5646 Standard Terminology Relating to Seams and Stitches Used in Home Sewing
ASTM D6193 Standard Practice for Stitches and Seams

My company has design standards developed from tests and years of experience... perhaps Yours's does also... or an affiliate.

Or perhaps you need to run some tests of the materials that will be used [see listed specifications].

CAUTION: Constant use [O-C-O-C-O-C-etc], environment [sun, chemicals, sand, mud, water, etc] and laundering are all factors affecting long-term utility and reliability. In many cases [aviation] heat/cold/fire and environmental [ozone, etc] resistance are design factors. Also, installed tape-loads must be shear... with 'peel' only applied for opening/closing.

CAUTION: Selection/use of suitable/matched thread, cloth, webbing [textile tape], etc... and sewing practices ['the stitching']... are important factors. These affect available surface area and longevity of the Tape when in-position on the assembly... and must match the operational environment. Threads are available in natural plant fibers, glass-fibers, aramid or para-aramid fibers, Dacron or rayon or other mixed fibers, etc [choose wisely]. Choose stitching that is appropriate for the product... and does not blank-out [reduce] available hook/pile surface area.

Also... never hurts to evaluate/test similar products 'on-the-market'... especially sporting goods... that may have similar performance requirements.

Also, there may come a time when the product must be repaired in-the-field or back-shop to prevent further damage and restore utility. GIs hate when something can't be repaired in-time-of-need. A general maintenance, washing and repair document [printed or referenced on-line]… or fabric placard... could be useful. There is a joint Army/USAF manual for fabric items repair FM 10-16 / T.O. 00-25-120... 'General Fabric Repair'.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Sparweb · Sep 13, 2020

Fun story in using Velcro. Adhere two opposite large-areas with Velcro to separate pieces of metal...

IME, the glue gives up first.

http://www.sparweb.ca

drawoh · Sep 18, 2020

SparWeb,

One of the first big, complex things I ever designed was a laser optical instrument to be installed in the customer's DC[‑]3. I designed a frame out of square 4130 steel tube. The system was fabricated, assembled and tested. At this point, I was informed that we needed covers for it. The laser was Class[ ]IV, so it had to be enclosed.

I solved the problem by designing a set of flat .032"[ ]aluminium covers retained by Velcro. You removed the covers by peeling the thin aluminium panel off from a corner. Everybody was happy. The customer was concerned about crash safety, but their DC[‑]3 was classified as experimental. The Velcro was retaining lightweight panels. Perhaps you have heard of DC[‑]3s!

Lessons learned:
[ol]
[li]Anything with a high powered laser in it must be covered.[/li]
[li]Covers are difficult to design. The task must never be left for last. Cover design is an integral part of structural design, and DFMA.[/li]
[/ol]

--
JHG

rb1957 · Sep 18, 2020

lesson … the damned things can sneak out of the woodwork and scupper your work !

another day in paradise, or is paradise one day closer ?

Sparweb · Sep 19, 2020

drawoh,
Was that a SHOALS system?
In early 2001, I was "this close" to travelling to Wallops Island to support a... I don't remember what they called that one... "SHOALS-2" system. Then late 2001 happened, and my trip didn't happen.

I've done a few Dak mods myself. Consider 4 tanks the size of refrigerators on their sides, all full of diesel fuel to top up the generators in Antarctica & North polar science outposts.

http://www.sparweb.ca

drawoh · Sep 19, 2020

SparWeb,

It was around 1983, and it was a Larsen[ ]500 system, which eventually evolved into SHOALS. The devices were tested and used up in the arctic. Back in 1983, I got to visit the customer's DC[‑]3. Unfortunately, I did not have a camera. This is one of the reasons I got into photography.

Speaking of DC[‑]3s, Canadian Warplane Heritage just flew theirs, along with their Lancaster, over my house. Next time, I disable auto-focus.

I still have a baseball hat. If I lend it anybody, I make them memorise the acronym [—] Scanning Hydrographic Operational Airborne LiDAR Survey.

--
JHG

Sparweb · Sep 20, 2020

Load test
The SHOAL's data acquisition unit was enormous. Like, 600 pounds.

Stiffness of the aircraft floor simulated (not very well but at least approximated) by the I-beams across the table.
Mounted to the seat-tracks and some of that structure is simulated by the other bars, too.
Not seen in the photos are the other restraints to keep the table from flipping over, or the Jeep winch to do the pulling, or the chains to keep the Jeep from sliding across the floor... it was a bit ad-hoc but the load was put and the rack was fine.

http://www.sparweb.ca

drawoh · Sep 20, 2020

SparWeb,

An EDAK Rack! Those things drove me nuts. They are 19"[ ]rack shipping cases, not airworthy structures. That thing was operated without those covers, which provide quite a bit of strength and stability, especially side-to-side. Note how with the covers off, the anti-vibration mounts at the top of the cage are attached to the end of flexible cantilevers.

On later programs, I wrote specification controls that required the racks to survive 10G acceleration side to side without the covers and with our load, and the vendor agreed to it. I tried to explain to management that I could design a certifiably air-worthy 19"[ ]rack, and it would almost certainly be cheaper than that thing, but they never took me up on it.

--
JHG

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