Let’s say I weigh 200 lbs. C’mon, it’s the Internet. If I wanna only weigh 200 lbs, this is the place where I can do it, ok? Good.
I weigh 200. I wear custom cut Orthotic inserts in my shoes. They’re made of a single bar of fiberglas that was cut by computer to match a mold made of my feet.
Let’s say I wore these orthotics every single day for 15 years. This is the truth. That’s 5, 475 days. AT 200 lbs a day, can’t I say that the orthotics have borne 1, 095,000 lbs of weight?
If they have, why haven’t they split in half yet? How does one calculate the load-bearing ability of a device that one has just invented, if one has NOT waited the 15 years to see how things pan out?
If I rest a million pounds on these babies, ( which is significantly closer to my actual size …), they would shatter. However, I have placed MORE than a million pounds ON them, and they’re doing just fine.
Explain the difference, and how one can test for 15 years of repeated stressors without waiting the 15 years, please?
I’m not sure I can take the question seriously as it makes no sense to me.
There’s a brick in the foundation of my building which has been supporting a weight of about a ton for something like 70 years now and it told me it isn’t getting tired and it feels like the first day. What’s time got to do with anything?
You’re serious? You’re asking why a million lbs of force at once is different than myriad smaller forces applied across time, the sum of which over time might equal or exceed the million lbs? It’s apples and oranges. You know that.
Lets just answer the last question.
1: There are machines which can mimic years of use and exposure in a few days or weeks. Engineer to or slightly beyond the projected time and load failure specs. derived from the test results in manufacturing.
2: History of working with these materials has given engineers a good working envelope of the materials probable failure characteristics under stress over time. Engineer to or slighly beyond that spec. in manufacturing.
Well, let’s take the case of an airplane fuselage. Every time the plane takes off and climbs to altitude and then lands, the fuselage is pressurized and then the pressure is relaxed. The material is stressed in tension and no-tension through one cycle. Assume the plane is a short haul jet that performes this cycle 5 times/day. A fuselage in a test setup can do this cycle probably 5 times/hour. So our test process can accelerate the aging of the fuselage by repeated loading and unloading by a factor of 24 and in 6 months can accumulate 12 years of fatigue on the fuselage.
There is no absolute guarantee that pressurizing the fuselage and then relaxing the pressure right away is precisely equivalent to pressurizing it, holding the pressure for 2 hours and then relaxing, but it’s the best we can do and should reveal any fatigue failures even if not a perfect simulation.
As to your question about the shoe inserts, Dr_Paprika called it when he pointed out that for a valid comparison your quantities have to be in the same units. You are putting on a force of 200 pounds (plus impact load when you walk) while your computed number has the units of pound-days. Different animals, or as the teacher used to say in grade school, “You shouldn’t compare apples and oranges.”
a material subject to load reversals which includes max to zero will suffer from fatique.
when these reversals are in the millions per year.
your inserts also have hysterisis qualities so that the microstructure is altering to suit the loading.
only static microstructures suffer from fatique
straight dopers suffer from creep which is linked to high temperature microstructure alterations.
like in the bath tub.
Another wrinkle is that you are not putting just 200 pounds of force on the shoes when you walk. When you stand perfectly still, you may be. But when you walk, run, trot, jump, or whatever, your shoes end up transmitting that accelerating and decelerating force through them - between your feet and the ground. In truth, you typically are putting much, much more than 200 pounds of force on the shoes in total just by a decent walk.
I’ve been accused of suffering from Creep :rolleyes:, but I’m workin’ on it.
Thank you for the answers, I suppose I was really asking how the things held up and how one would test them. No, I know I don’t weigh a million pounds- even when I’m retaining. It was the cumulative effects over time that interested me.
Dr_Paprika, thank you for the new phrase of measure. Nothing better than learning a new word or phrase. Time-Load. Hmmmm…gotta work on that.
Urban yeah, good point. It’d be pretty difficult to calculate how much force those orthotics REALLY bear in a day. As Anth pointed out, there are variables. Not only in walking, running, etc- but in terms of carrying cargo.
