Mike wrote:
<<...In using the Marc program I see that 4130 has a yield strength of
115000 psi for .250 wall thickness. What is the strength for .120 wall
thickness? Is it exactly half? >>
Hello Mike,
The "yield stress", measured in pounds per square inch, is a property of the
material the mast is made from, regardless of its physical shape. The
material strength (yield stress) *AND* the size and shape (geometry) of the
part (in this case, a mast) are used together to determine the strength of
the *part* itself.
Once you tell the MARC program both the material, *and* the size (ID, OD,
length, etc), it can characterize that particular mast and its ability to
withstand forces for its intended function.
So, to answer your question, the yield strength for your example is still
the same for the smaller size, since you're checking out a mast made from
the exact same material. The change in size will result in an overal change
in the strength of the mast as a part.
When you talk about the "strength" of a mast, or any part, for that matter,
in termas of being able to hold a particular load, is founded on:
1) The basic strength of the material (yield stress) and
2) The geometry of the part,
which are both completely independant variables.
There is a hidden assumption, here, that metals of the same type always have
the same yield strength, which is not always true. Metals, even of the same
composition, such as 4130, can vary greatly in yield strength depending on
their *treatment*. Heat treating, cold working, forging, etc, can all
greatly effect the properties.
That is why it is so risky to use a mast made from an unknown material,
without testing its yield stress. Just identifying the metal type is only a
portion of the battle.
There is a recap of this from earlier posts in the "MAST AND BOOM MATERIAL
SELECTION" section of:
http://www.qsl.net/mparc/towers.txt
Mike, good luck with your project and hope this helps,
--...MARK_N1LO...--
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