3 ms·

> The hull on the other hand is ... is impossible to model accurately. Why is the hull difficult to accurately model? (assuming the exact materials and precise

by nomilk 3y ago

> The hull on the other hand is ... is impossible to model accurately. Why is the hull difficult to accurately model? (assuming the exact materials and precise dimensions were known).

akiselev 3y ago
Carbon fiber can't be inspected like metals can using radiographic, liquid penetrant, or magnetic particle testing, so even if we did know the exact materials and precise dimensions [1], it's very hard to characterize them and how they responds to stresses. [1] composite materials are not as homogeneous as metals are, so most of the time we actually don't know the exact materials and precise dimensions. Only manufacturers that make enough parts to afford to do destructive testing can characterize the process enough to make reliable guarantees about part strength.
- sottol 3y ago
  There are a variety of non-destructive inspection and testing methods for composite, like ultrasonic c-scans and pulsed thermography that I know of. And you can produce reliable composite parts (you don't want homogenous, but them to come out exactly as spec'ed) under strict control, but it doesn't look like they necessarily did that here. They seem to have eg foregone autoclave curing of the tube section probably due to cost.
  - _yb2s 3y ago
    They could have identified defects with non-destructive testing, but they almost certainly would have found a huge number of them, and had no way to reliably model the importance/potential impact of them.
_yb2s 3y ago
We're talking about a bunch of layers of fibers, and woven cloth impregnated in epoxy. It's a pretty messy, irregular thing with voids, gaps, broken fibers, etc. even if you lay it up robotically (as they did). The modeling they did assumes a perfect material, and then adds some arbitrary factor of safety that will hopefully account for imperfections. It probably would have been fine if only used for one dive, but not after a substantial number of fibers had snapped as the hull flexed across numerous dives.

by nomilk 3y ago

> The hull on the other hand is ... is impossible to model accurately. Why is the hull difficult to accurately model? (assuming the exact materials and precise dimensions were known).

akiselev 3y ago

Carbon fiber can't be inspected like metals can using radiographic, liquid penetrant, or magnetic particle testing, so even if we did know the exact materials and precise dimensions [1], it's very hard to characterize them and how they responds to stresses. [1] composite materials are not as homogeneous as metals are, so most of the time we actually don't know the exact materials and precise dimensions. Only manufacturers that make enough parts to afford to do destructive testing can characterize the process enough to make reliable guarantees about part strength.

sottol 3y ago
There are a variety of non-destructive inspection and testing methods for composite, like ultrasonic c-scans and pulsed thermography that I know of. And you can produce reliable composite parts (you don't want homogenous, but them to come out exactly as spec'ed) under strict control, but it doesn't look like they necessarily did that here. They seem to have eg foregone autoclave curing of the tube section probably due to cost.
- _yb2s 3y ago
  They could have identified defects with non-destructive testing, but they almost certainly would have found a huge number of them, and had no way to reliably model the importance/potential impact of them.

_yb2s 3y ago

We're talking about a bunch of layers of fibers, and woven cloth impregnated in epoxy. It's a pretty messy, irregular thing with voids, gaps, broken fibers, etc. even if you lay it up robotically (as they did). The modeling they did assumes a perfect material, and then adds some arbitrary factor of safety that will hopefully account for imperfections. It probably would have been fine if only used for one dive, but not after a substantial number of fibers had snapped as the hull flexed across numerous dives.