material yield strength

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hi there-i'm new to welding and fabrication and i'm have a great time building stuff for my truck.  i have a question about the yield strength for different types of material.  for example, i was able to find what the ASTM says the yield strength ought to be for A36 mild steel but what does that mean when the steel is shaped in a tube and its thickness.  for example, what would be the yield strength for 2"x4"x1/8th?  is there some sort of book or chart?  thanks!andy
Reply:Well, there isn't an easy way to answer your question - short of handing you a "Strengths of Materials" text book.  Yield strength as quoted in the specs you'll find is the average amount of stress you can give the material before it yields.  This is given in the form of PSI, or KPSI (1000 PSI) which is a force per square inch of material (or may be in an equivalent metric unit).  To equate this to the strength of your tube you need to know a LOT ov variables.  What will be the primary mode of failure - bending, shearing, torsional, etc.  If it's bending you'll then need to know how the load is distributed across the tube - and draw a shear/moment diagram.  Once you have that you work out the cross sectional area and moment of inertia for the tube and chug away with some equations...  That is just one example - each mode of failure has a different calculation.I hope I'm illustrating how complicated your question is - there just isn't an easy way to answer your question.  In fact, your question boils down to a full 2nd or 3rd year engineering course rather than a quick answer...Jason
Reply:Originally Posted by racerwadhi there-i'm new to welding and fabrication and i'm have a great time building stuff for my truck.  i have a question about the yield strength for different types of material.  for example, i was able to find what the ASTM says the yield strength ought to be for A36 mild steel but what does that mean when the steel is shaped in a tube and its thickness.  for example, what would be the yield strength for 2"x4"x1/8th?  is there some sort of book or chart?  thanks!andy
Reply:Originally Posted by The ApprenticeYield strength is not a function of the shape of the material. The yield strength of a tube is the same as for a solid rod piece. Yield strength, or the yield point, is the stress at which a material begins to plastically deform.
Reply:a round tube is stronger than a square one,(quote)says who?...zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:A 2" square tube has a greater cross sectional surface area than a 2 inch round tube OF IDENTICAL WALL THICKNESS.  Therefore, a 2 inch square tube will be stronger but the yield strength will be the same.Last edited by 76GMC1500; 09-19-2006 at 07:03 PM.
Reply:thank you.. but it all depends on the wall thickness also......zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:maybe i'm using the wrong terms or something.  how could two types of tubing (round vs square) have the same yield strength but one be stronger than another?also, i don't have the math in front of me right now about round vs square tubing but i thought that for a given wall thickness (and therefore, strength) round tubes were stronger than square?  isn't that why roll cages and frames are made from round tubes vs square?  again, i know that this discussion of strength also involves strength to weight and whatnot.anyway, the reason i'm wondering these things is that when i build stuff for my truck, i'd like to use the lightest possible material that maintains the strength i need.  is there anywhere i could go to find out this information?thanks!andy
Reply:Originally Posted by racerwad>snip<  i have a question about the yield strength for different types of material.  for example, i was able to find what the ASTM says the yield strength ought to be for A36 mild steel but what does that mean when the steel is shaped in a tube and its thickness.  for example, what would be the yield strength for 2"x4"x1/8th?  is there some sort of book or chart?  thanks!andy
Reply:lotech-thanks for the help.  i will look those books up ASAP.a
Reply:Yield strength is measured in pounds of force per square inch of cross secional area.  You have to multiply the cross sectional area by the yield strength to get the actual load the part will carry.
Reply:Yeild strength by cross sectional area will give tensile strength of the material.  Tensile strength is not beam loading capacity.  Round and square tubes used as beams are very complicated subjects.  A 2 inch round tube with 1/4 inch wall will have a different cross sectional area than a 2 inch square tube with 1/4 inch wall and will behave differently as a beam or in tension.
Reply:Like I said in the first response to this question - the answer is a 2nd or 3rd year engineering course...  No simple way to tell you how to calculate it, and no handbook is going to give a solution, either...
Reply:There are sources available where you can find the facts.Of course its more fun to read through several layers of thread where we are all winging it, trying to remember what we learned in school.http://en.wikipedia.org/wiki/Tensile_strength
Reply:Man, I'm lovin' this one! I posted a thread (Newbie 3) about the five types of stress and the three ways to apply them.  It is a very complicated issue, that's why most eng. take 5 years to get a 4 year degree..... The Math!The first question is always,  "What is (it) the part for?" That's where to start.Weldtek
Reply:The cross section on 2x2x1/4 square tubing is 1.75 square inches, and for 2"x1/4 wall round the cross section is 1.37 square inches so the square tube has more tensile strength if you are just pulling it apart.  But then you probable won't ever see a square torsion bar like on a car suspension.
Reply:just like "slapper" traction bars are made from square tubing rather than round..stronger in this application......zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:yield strength is a measure of the material strength (steel, iron, aluminum)  not the shape or size of it.  Shape or cross section is what determines the structural rigidity of the memeber.Last edited by structweld1; 09-27-2006 at 11:45 AM.
Reply:I think the person who started this topic got yield strength mixed up with rigidity.  But, yield strength is usually given in psi and the area of the cross section must be known in order to calculate the strength of the part in lbs force.