RBX:
It seems to me that the OP suggests some serious lack of understanding of the entire engineering design process. There are some good points made in some of the above posts, but you must be smart enough to read btwn. the lines a bit to get them. You must have had that mechanics of materials course and several other related courses (maybe a college degree) to really be proficient at what you say you are doing. And, then you would know that we have many uncertainties in our materials, loadings, manufacturing methods and our analysis methods. The F.S. (factor of safety), M.S. (margin of safety), R.F. (resistance factors) and L.F. (load factors) are all means of adjusting our analysis and design to account for these uncertainties, and these are not only dependant on yield or ultimate stress. You shouldn’t be using a F.S. to covering you’re a$$ for lack of knowledge of what you are doing (ignorance?). That’s not good design or engineering, and you shouldn’t be calling it engineering if that’s what you are doing.
The yield stress and the ultimate stress and their relationship for a given mat’l., are important mat’l. properties in the design and analysis process. Mild steel has a well defined yield point, a linear stress/strain curve to that point, and allows much strain beyond that point, plastic range and strain hardening, before it approaches its ultimate strength. Many engineering mat’ls. do not exhibit well defined yield points, for some high strength steels we use a .2% strain offset to define a yield strength, and some strain remains, but not a significant additional increase in stress before ultimate. Many brittle or hard materials don’t have much of a linear region prior to yield, or a definable yield strength, they just go to ultimate strength, in a gradual curve, but without much warning before failure. There are other interrelated material properties which must be considered in a good design, such as elastic and shear modulus, Poisson’s ratio, etc. Then you have the actual mat’l. specs. which call a for min. yield and ultimate and % elongation (ductility), which are usually exceeded in the mat’ls. delivered. These are mechanical properties of the material. Should you design for the min. or the actual values? The above covers only a few of the many mat’ls. we might use in a design.
Then you have the deflection, elongation, shortening, crushing, etc. etc. within the elastic range and beyond; until failure, which you define as part of your design process. Can your design tolerate these? Designing to ultimate usually means a sudden failure, can you tolerate that and what are the consequences? Fatigue and fracture have many fathers other than yield or ultimate strength. And, dealing with them involves considerable design and engineering finesse. Some buckling problems give some forewarning, others are sudden and catastrophic, and don’t forget secondary effects here. While the above items (and others) are a function of the material properties, they are as much, or more, a function of your particular design (widget). And, thus the many questions about ‘your mode of loading and failure; and does the machine just slowing down or does it hurting someone.’ You didn’t respond very well to these questions. Who does set your F.S.? Our design factors of safety or margins of safety are very important part of our work, and Fy or Fu, however they are actually set, are only a small part of determining what the F.S. or M.S. should be. Then, don’t forget such things as residual stresses due to your manufacturing processes, remember toughness, resilience, ductility, temperature effects, strain rate, mat’l. flaws, etc. Which materials are isotropic and which are anisotropic, you better know that. How are they affected by various processes, or their environment? Fact is, most codes give you a F.S. and tell you it is wrt to Fy or Fu, you don’t get to pick. That’s a short course in strength of materials, where each of my sentences equals a few chapters in the books you should be studying.
Don’t kid yourself, many engineers with the latest FEA package on their computers, aren’t very precise either. They are just kidding themselves if they tell you that they are. And, in many instances it allows them to pretend they know what they are doing, without having a very good intuitive understanding of the part or structure they are dealing with or how it really reacts to forces and constraints. They don’t need common sense or a feel for how the structure really acts, they have a computer program which actually inhibits this understanding. And, in some instances this yields (I mean results in) crap, and it’s still crap even when printed out to eight decimal places.
Ignorance wouldn’t have been my choice of words, but drawoh’s use of it may not have been out of line either in his context. It isn’t a dirty word the way it’s been used here, we might just as well admit something less than absolute intelligence, knowledge or certainty. As long as we understand that is our meaning, in this discussion and in this context. But, because of the potential for misinterpretation by judges and juries (the gen. public) and the intentional misinterpretation by lawyers, I would not use that word outside of this discussion. On the witness stand I might substitute the term ‘some degree of uncertainty,’ and explain some of the reasons for our inability to be exact and certain.
Many industries dictate design stresses, factors of safety, load factors or resistance factors and the like, and these are not always only related to Fy or Fu, nor do we get a constant F.S. for various failure modes or design considerations. Examples: ASME for pressure vessels and other products; AISC, ACI, NDS, and IBC for buildings and the like. In many instances we have to come up with our own design criterial, guided by some of these other codes and design guidelines, and we need to set our own F.S. or M.S. and you better do this in an intelligent way. In the machine design business, maybe you want to do some reverse engineering; what has worked in similar situations, for what expected part life, what types of failure and the reasons for same; maybe you should increase or can decrease your F.S.
