I might be completely off on this, but… supposing you had a composite beam loaded in bending, to find the stress you would need to know the thickness, but you can’t know the thickness until you’ve determined the stresses, right? So I assume you just make educated guesses and then interpolate until the numbers agree?
You’re completely right, you won’t know the stress. This guess and check work applies to almost everything in engineering work. In shaft design you won’t know the stress, stress risers, or fatigue life without knowing the shaft diameter, you have to guess one and continue one. In fluid mechanics you won’t know the pipe losses without knowing Reynold’s number and and sometimes you have to guess a flow rate to figure and iterate until certain values match. You won’t know the speed of a plane without knowing the weight but you need heavier engines to go faster but that changes how much fuel you need, etc etc etc.
It’s a common challenge in engineering work that involves experience, good technique and sometimes a fair bit of luck.
heh, it’s funny how I’m almost getting done with my degree and that wasn’t already ingrained into my head (let me rephrase that… it’s not funny, it’s embarrassing). I guess that is also part of the difference between the class room and the real world. I suppose that means I just need to take it upon myself to do some more design work. Not that I have not done a lot of iterative work to find solutions in engineering, just that most of the “design” work I have done has been much more constrained.
I wouldn’t be embarrassed. I’m in the same boat as you. I’m almost done with my degree but I haven’t done any ground up designs. All my practice problems and examples have been focused on a very narrow aspect. The only reason I know what I said is because two of my professors have shoved that reality in our face, even putting iterative problems on our tests.
Many professional engineers have told me that classroom work is a very small part of our engineering careers, one said professional engineering is like sprinting and a bachelors degree only shows you how to crawl.
I can only second that. The relation between my study and my work, which SHOULD interconnect seamlessly, is far-fetched. Basicly what I learnt at school was 30-40 years old engineering. I guess I would be better off applying for a job at a museum…
One of my learning documents mentioned the recent development of the transistor… I know physics laws still apply, but still…
The Internet was still upcoming then. If I see what is possible now, and about the ways to spread knowledge now, I sometimes am jealous at the new students.
To find the stress, one must first know the load. Figure it out from an existing design, or find an expected maximum and incorporate a safety factor.
i am new here and i want to do a question if any know how i calculate the first ply failure load with damage mechanics because is easy with strength failure theory but with damage mechanics i dont understand
Damage mechanics is a tricky field, quantifying the effects of damage is usually done through empirical formulas. If you want specific advice post what formula’s and references you’re using 
First ply failure doesn’t require “damage mechanics” ( correctly konwn as “fracture mechanics”). It is sufficient to know the material allowable, the loads applied, the constraints, and safety factors to apply. The analysis starts from a still undamaged structure and the procedure is basically what described in many texts of structural analysis ; I (strongly) suggest you to take a look.
Fracture mechanics involves a different approach, where the damage is already present in form of cracks, flaws etc. and its aim is to determine how long the structure will be safe before an inspection or control will find them for a subsequent repair.