Engineering Mechanics Query - Axial stress loads/plastic collapse
Engineering Mechanics Query - Axial stress loads/plastic collapse
(OP)
Hi,
I am currently trying to work through some example questions and have become stuck on one word based answer. Firstly, I wasn't sure where to post this so if anyone knows of a more appropriate area please let me know and I'll move it.
I would just like to let you know the question and then the lines that I have gone along in order to answer. I am not looking to be spoonfed, more just to know whether I'm in the right area or not and if not, maybe where I could look to help.
Here is the question:
"By considering the axial stress distribution in a double-l section beam at the instant of it's plastic collapse due to bending(in the absence of any axial loads), explain why the neutral axis then no longer passes through the centroid of the section".
I am currently trying to work through some example questions and have become stuck on one word based answer. Firstly, I wasn't sure where to post this so if anyone knows of a more appropriate area please let me know and I'll move it.
I would just like to let you know the question and then the lines that I have gone along in order to answer. I am not looking to be spoonfed, more just to know whether I'm in the right area or not and if not, maybe where I could look to help.
Here is the question:
"By considering the axial stress distribution in a double-l section beam at the instant of it's plastic collapse due to bending(in the absence of any axial loads), explain why the neutral axis then no longer passes through the centroid of the section".





RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
Is this along the right sort of lines?
Thanks in advance,
Mike
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
elastically, yes, plastically ??
in plastic bending, the material is stressed beyond the elastic limit, but the nett axial load on the section is still zero.
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
Mike
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
So there's no easy answer i think.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
"at the point of plastic collapse, axial stress begins to act on the section and this cause the neutral axis to change; to a new position other than that of the centroid" > no. the neutral axis will shift as soon as some yielding starts, unless the tension and compression stress-strain curves are identical and there is a pure moment on the section and the section is symmetric. Plastic collapse does not occur until sufficient yielding of the section occurs such that additional load cannot be carried; this can be well beyond the load at which yielding of the outer fiber occurs.
do you not have a strength of materials text that explains this?
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
Answering your question 'SWComposites'..all the material is delivered in a very maths based way will little wording within any of my lecture material. It has been up to me to do additional reading into the topic as well as trying to answer the questions using the maths to back it up. I have been reading a textbook on the topic but it can be quite hard to narrow an answer down.
Thanks for everyone's help
Mike
RE: Engineering Mechanics Query - Axial stress loads/plastic collapse
What kind of engineer or technician are you, what engineering or technical experience have you had on these kinds of problems? This is not really intended to be a student forum. You would do well to open your Strength of Materials, Theory of Elasticity and Structural Stability text books and study these subjects a bit more. Then discuss them in class with your instructor, you are paying them for an education after all. A 'more appropriate area' for this discussion would be the classroom or your Prof's. office, and you won't be the only student with this question. You will just be the bravest for asking for a better explanation. Eng-Tips is really not the first place to come for this kind of elementary structural mechanics discussion and education. This is not the student study room or student lounge. You will be given the bum's rush, outa here, fairly quickly.
You really do need a good understanding of basic Strength of Materials and Structures, or it is a certainty that the mathematical presentation will boggling your mind. Can you give us a proper definition of the section centroid and the neutral axis? How are these different in doubly symmetrical sections and in sections which are symmetrical about only one axis, or not really symmetrical at all? Is your question actually talking about axial stress distribution or normal stress distribution due to bending moment through the depth of the section? You said there is no axial load or stress, right? Can you cut a section through an idealized bending member and draw the normal bending stress distribution as the max. stress progresses to yield and beyond? Cut this same section on a beam which has both bending and axial loading, and again draw us the 'combined' normal stresses across the depth of the beam, as the stress level increases. What is a plastic hinge? Can a member go fully plastic if it is not properly braced, that's the stability aspect? What is likely to happen if the beam or structure is not properly braced? Tell us about this. You have two similar questions on this forum at the moment and neither of them are well enough defined to elicit an intelligent discussion. It sorta seems you've combined a bunch of structural engineering phrases without any understanding of their interrelation.
SWComposite's explanation is very good, but read it carefully, there is a lot said btwn. the lines, for a full understanding. I think the question in your OP is kinda poorly worded, and your Prof. should be faulted for that. At least in my world (and by Theory of Elasticity definitions), axial stress and normal stress due to bending are two different things, but they could act in the same vectorial direction on your idealized beam, parallel to the length of the beam, but may be of different signs. I think it is kind of a trick question. Look back at SWC's explanation about neutral axis, read it carefully, for full meaning. When the beam goes fully plastic where is the N.A. (neutral axis)? Explain this to us. In the idealized beam, first course teaching world, the tension and compression material properties are the same (the stress/strain curves, etc.), the N.A. doesn't move (with pure bending and no axial load), look back at the cross sectional stress diagrams I asked you to draw. The real world situation can be quite different and much more complex. There's still a trick question here, and it lies in the definition of the N.A., when the section goes fully plastic. Tell me about this.