Perhaps of interest regarding the DeepFlight Challenger from way back in 2009:
https://www.ingentaconnect.com/content/mts/mtsj/2009/00000043/00000005/art00029?crawler=true
"There were two problem areas: First, and as Jerry Stachiw predicted, modern development of carbon fiber resins had not solved the “unexplained” loss in compressive strength of massive carbon fiber at extreme compressive load. This surprised both the knowledgeable carbon manufacturer as well as myself. In order to test to failure, we had several 1/3-scale pressure hulls built, each with variations from the intended design in order to cause failure within the available test range. Typically, the model hulls predicted to fail at 13,000 psi would fail at 11,000 psi—a significant loss."
Apparently Mr. Hawkes is an expert on submersible design, among other things.