Factor of Safety

This mechanics of materials tutorial goes over the factor of safety, an important topic in mechanics of materials and structural analysis / design.

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[georgemorley]

When the o ring seals on the booster rockets for the challenger space shuttle eroded by 30% when under hot gas pressure load, they were deemed by the management (not the engineers) to have a safety factor of 3. The way they figured it, because the o rings had only burned a third of the way through, the o rings were three times stronger or thicker than they needed to be to prevent the blow through of hot gasses into the booster rocket’s fuel system. This is of course absolute horse shit. The o rings had a safety factor of zero, as by eroding up to 30% under normal load, they were reaching ultimate load immediately and were therefore immediately and irreparably damaged beyond their intended operating strength. If they had been able to withstand three times the normal gas pressure of rocket operation and NOT erode at all, then they could have claimed a safety factor of 3 or more. They completely misunderstood the principle.

Now, the method of operation on challenger was to throttle up once, get the throttles back as the shuttle and rockets punched through the weather, then rotate and go maximum thrust a second time to accelerate hard once clear of the turbulence the weather caused. That means that the 30% erosion took place over two distinct burn periods where hot gases were pushing at these o-rings for all they were worth. Only problem was, the shuttle had never been operated under such cold conditions as it was being told to by management on the day of the challenger launch.

Therefore, when the rockets were operated under abnormally low ambient temperatures on the morning of the challenger launch and had CONTRACTED significantly already due to the cold, the subsequent erosion of the contracted, never before used, completely “safe by a factor of three”(!) o-rings under the hot gas pressure meant that the hot gases eroded the o-rings to such a degree on the first burn, that when they initiated the second max thrust burn, the hot gases escaped past the o-ring seals, contacted the fuel system of the booster rocket and blew the space shuttle up with the loss of everyone onboard.

Safety factors! Important stuff!

[is-igzh]

Enjoy your presentation, sir! thank you

[progaming-uhrj]

Thanks alot mate this helped me out with my mechanical principles assignment

[LT]

you probably already caught this, as this video is 6 years old, but its not 44 N/mm^2. When you multiplied by a 1000, you converted it to meters, you just forgot to change the unit is all : )

[philipkarlsson]

You could mention something about what normal FS value is under some real world problems. Just a thought. Preciate the videos🤜🤛

[famiyo]

Thanks for the video, isn't it ultimate stress 'To' is same as Tensile strength of the material, say 250MPa to 350MPa for mild steel?

[MM-npmd]

@2:20 Dc-10 cargo door and engine pylon has entered the chat 😅

[sasankdhara]

Why is P(all)=5kN? arent there two loads of 5kN acting on the system?

[lukes]

I have a problem that gives me the ultimate tensile strength. And the saftey factor. It was me to find the largest allowable tension. Is ultimate tensile strength=ultimate stress? Is largest allowable tension=allowable stress? Are they the same thing?

[aizawa]

hi, Is it possible that factor of safety is yield stress / working stress, not ulitimate tensile stress / working stress?

[fionaragan]

Do you know why a safety factor is not a true margin of spare capacity?

[jcnotnot]

_how do you do, fellow kids?_

But in all seriousness, thanks for the videos man.

[lightningkhrizz]

May I ask why multiplying pi by 6mm turn out to be 113. 10?
what I'm missing here?