Fiberglass and fatigue

[Bruce Tuncks]

I reckon this topic is worth its own thread so here goes...

It is asserted again that fiberglass does not fatigue.

There is a type of overload failure where the resin powders and eventually falls out leaving a "greenstick " failure.

There is another type where the crack can avoid glass fibers and the fitting suffers delamination failure.

Apparently this has actually happened on Jabiru u/c legs, where the leg gets soft as a result and ground handling becomes difficult.

But this is not an example of fiberglass fatigue, it is an example of poor design and /or construction.

I would really like to hear of an example of a properly-designed and constructed fiberglass part which has failed in fatigue. After all, the glass fibers are crack-stoppers, and if there are sufficient of these in the crack pathway then the crack has to stop and fatigue is halted in its tracks. The glass fibers themselves do not propagate cracks.

I would like the example to include numbers and other design details. Personally, I doubt that there are any real examples, but I have respect for turbs and onetrack and their knowledge .

Here's my understanding of "fatigue" and gliders...

When Blaniks ( metal) gliders arrived, they looked great and nobody had ever thought about fatigue. The wooden gliders we flew had unlimited life.

Then there was an example of a Blanik in Europe falling to bits in the sky. When the dust settled, Blaniks were given a 3000 hour life. ( This was later extended by the heroic efforts of Daffyd LLewellyn )

Well the Blanik importer got angry, and he discovered that FRP ( fiberglass reinforced plastic)  gliders ( all the rest in those days ) did not have a special exemption, and he prevailed on CASA to enforce this ridiculous limitation.

There never has been an example of a FRP glider failing in fatigue.

The german manufacturers responded by submitting paperwork and eventually the "life" of a glider can be up to 18,000 hours. Unfortunately, Grobs went out of the glider business and so Grob gliders are limited to 12,000 hours. Several airworthy gliders were scrapped by this nonsense. At the Adelaide Soaring Club, we investigated paying for engineering paperwork to extend this life, but really the membership wanted new stuff anyway, so we practically gave away 2 pretty good gliders.

About this time, a great engineer ( Patching ) got the RMIT to put a Janus wing onto a steel loading machine. I think GFA helped with the funding.

Not unexpectedly, there was no fatigue discovered at all. Well the machine fatigued, but the Janus wing did not. When I saw the machine in action, it was bending the wing alarmingly, with the tip almost at 90 degrees to the root. In 3000 hours gliding , I never saw such a load applied in the air.

On enquiring, I was told that the project was running out of time and money and people were getting impatient with no results. Then they measured the wing thickness, and found it to be a few mm over so they increased the loadings . They also did some deliberately substandard "repairs". Eventually, some of the metal fittings surroundings started to show signs of greenstick failure.  Then the project was concluded. It was the only proper scientific fatigue test done on a real wing that I have ever heard of, and it proved to me that the whole FRP fatigue business was  full of  bureaucratic fraud.

[turboplanner]

We're back to calling a material a US name from the 1950s and 60s - fiberglass, and have dropped all the designations which clarify what the material reallyu is and talking about greenstick and gliders, so I'll drop this one.  Most of the answers are in the existing thread, and there's no point in confusing people to the point where they spend money to make a failure.

[Bruce Tuncks]

Fair point turbs, specifically I am talking about epoxy resin ( shell epikote MGS with L20/epikure MGS ) or any epoxy systems approved by the LBA.

Glass would be Interglas 210 gm/m2 for example.

Now that I have been specific, perhaps you could be too. Please no vague references to boats for example.

 

[Bruce Tuncks]

Jabirus provided Araldite 3600 with my kit 20 years ago. I think the glass was cheaper than the interglas but it was ok. Interglas has an oval cross-section to each strand and this makes it drape better apparently.

Mark Morgan ( Waikerie ) sold me Scheufler L285 resin and hardener for my last big job years ago. It was good stuff, blue resin going greenish when mixed. Be very careful about the exact type as not all epoxy resins are compatible. There is a table published by the GFA, there would be more up to date stuff from RAAus I hope.