Final conclusion/description of the cause of the SpaceX Sept. 1, 2016 anomaly? What's a "buckle"?

Question

I've just read the phys.org new item which summarizes the conclusions by SpaceX about the cause of the September 1st 2016 anomaly.

I'm citing quotes in the news item which come from a SpaceX announcement which is further quoting investigators, so I am not sure I'm getting the correct picture yet:

"Investigators concluded that super chilled LOX can pool in these buckles under the overwrap. When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail."

"Investigators determined that the loading temperature of the helium was cold enough to create solid oxygen (SOX), which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition."

Question: Does the term "buckles" refer to existing static indentations of some kind in the as-fabricated aluminum liner, or to spaces that are formed by the aluminum liner actually buckling during the loading process?

Update: The 2018 Florida Today article After 2016 launch pad explosion, SpaceX updating Falcon 9 for astronauts suggests that it is the tank that was buckled (lower your volume to avoid advertisements):

Changes in helium loading operations prevented a repeat of the problem believed to have caused the explosion: buckles in helium tanks that trapped slushy liquid oxygen in gaps between the tanks and their composite wraps. Friction "or other mechanisms" ignited oxygen in the upper stage.

While Falcon launches resumed fairly quickly, a redesigned helium bottle has not yet completed testing nor won NASA’s approval for crew launches.

The next launch date has now been announced as January 8, 2017, just a few days away. Does the plan to avoid this happening in the immediate future consist primarily of ways to change the helium loading process to keep it from getting so cold that liquid oxygen in these trapped spaces simply can't freeze into solid oxygen (SOX), or are they also going to try to prevent these pocket of LOX from forming between the Aluminum liner and the carbon overwrap in the first place?

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Aluminum in contact with pure oxygen, carbon in contact with pure oxygen, huge stresses, huge vibrations, only a bit of friction necessary to start a reaction... no physical barrier?

He built gradually to a crying jag, during which he claimed to be deeply touched by the idea of an inhabited planet with an atmosphere that was eager to combine violently with almost everything the inhabitants held dear. He was speaking of Earth and the element oxygen.

“When you think about it boys”, he said brokenly, “that’s what holds us together more than anything else, except maybe gravity. We few, we happy few, we band of brothers - joined in the serious business of keeping our food, shelter, clothing and loved ones from combining with oxygen.”

--Kurt Vonnegut, God Bless You, Mr. Rosewater (1965)

Answer 1

In this context, Buckles are voids between carbon composite overwrap and aluminum liner.

compositesworld

Here is a quote from the NASA ASAP:

The SpaceX investigation did not find a single most probable cause of the initiating event, instead identifying several credible causes involving the COPV helium tanks. All credible causes were similar in that they involved LOX trapped between the overwrap and the liner with subsequent ignition through friction or other mechanisms.

The evidence recovered from the mishap showed indications of buckles in the COPV liner where LOX was likely trapped. Acting from the report findings, SpaceX was able to recreate a buckle event during a COPV test. Additional testing allowed SpaceX to identify specific conditions which would cause a buckle and trap oxygen in the gap between the liner and overwrap.

Using this data, SpaceX modified its helium loading configuration, process, and controls to ensure that the COPVs would not be exposed to these identified conditions and, accepting any residual risk, successfully resumed commercial launches with the existing COPV design. However, to further improve safety, SpaceX and NASA agreed that a redesign of the COPV was necessary to reduce the risk for missions with crew onboard. Using what they learned from the mishap investigation, SpaceX redesigned the COPV and NASA started a rigorous test program to characterize the behavior of the new COPV in the cryogenic oxygen environment.

ASAP Report

I found one source that says:

“Each COPV consists of an aluminum inner liner with a carbon overwrap.”

Given that the tensile strength of CF is much higher than AL, it's likely the AL yielded to some small extent.

COPV construction

Answer 2

My reading was that the buckles were in the carbon fibre overwrap, but on the underside, thus the "buckles under the overwrap".

Answer 3

I've found some buckles!

From Scott Manley's video The Dumbest Mistakes In Space Exploration

tl;dr: The metal tank buckles inward

Yes it was sort-of funny when SpaceX had a rocket explode on the pad, but the reason for that was down to some pretty complicated material science and physics.

The slides show a buckled wall of a gas-filled metal tank (with carbon overwrap):

If Exterior pressure is great wall buckles in slightly, creating a void.

Because helium is colder the liquid oxygen can freeze into solid oxygen.

That says clearly that yes, the metal tank buckles inward.

As helium is loaded, the rising pressure pushes out SOX, which can not flow out, adding extra stress to small regions on the composite cover.