Page 278 - Boiler_Operators_Handbook,_Second_Edition

Chapter 9

Strength of Materials

and you call it a rubber band. It’s a little hard to imagine
An understanding of the strength of the materials yourself stretching a metal but you can do it; just clamp
used in construction of the boiler plant is essential. No one end of a piece of lightweight wire in a vise, lead it
element of a plant is designed to operate anywhere close out about twenty feet and grip it with a pair of pliers. Set
to its breaking point for reasons of safety and mainte- a stepladder or something else next to the wire to get a
nance of that margin of safety protects the life of the reference point then pull on the wire and ease off. You’ll
operator and others. discover that the wire is just like a rubber band, you
can stretch it and it will shorten when you ease off your
pulling on it. That’s the elastic action where you apply a
STRENGTH OF MATERIALS stress and the material resists it. You’ll also notice if you
pull a little too much that the wire suddenly gives and
Lots of boiler operators are not like me. They’ve will not shorten to its original length when you ease off
never broken anything. Are you one of them? I’ve bro- the pull; you just over-stressed it.
ken everything from the standard lumber 2 by 4 to some That operation is a simple form of the tensile tests
rather expensive fiberglass piping and witnessed some that are performed on materials to determine their
serious destruction of everything from a bag of rags to strength. In the case of the wire you pulled with a vari-
pressure vessels and boilers. It’s not uncommon to break able force that could be measured in pounds and you
things and there’s nothing I can tell you that will ensure applied that load to the cross-sectional area of the wire
you never do. which is the area of a circle with a diameter equal to the
I tend to argue that those operators that don’t diameter of the wire. Since any wire we could stretch
break anything manage to do so by not doing anything. would be very thin the area is very small. The more cross
If you are doing something it’s important to understand sectional area of the material the more force you need to
a little bit about the strength of materials in order to stretch it. You can easily stretch a rubber band but a rub-
make sound operating and maintenance decisions. ber hose is another story.
That way you may break less than I did. I’ll try to do Tensile tests on material use a sample a little larger
that without all the technical engineering but still give than a piece of wire to get an average value. The typical
you an adequate understanding of what’s involved and tensile test specimen consists of a piece of metal about
what some of the buzzwords mean. six inches long with the center three inches machined
uniformly to a thickness of about one quarter of an inch
and a width of three quarters of an inch to produce a
STRESS cross sectional area of three sixteenths of an inch (1/4
times 3/4 equals 3/16) so the cross sectional area is
Stress in materials is very much like pressure. We 0.1875 square inches. The two ends of the specimen are
measure it in pounds per square inch and it’s basically clamped in a machine that pulls them apart. For stan-
force (in pounds) applied over a surface area measured dard metal testing the sample is also marked with a cen-
in square inches. We can determine the stress we apply ter punch about one inch from the center on each side so
to a material and, by testing, know how much it takes. the machine can sense the location of the punch marks
Since most of the materials involved in a boiler plant are and measure very precisely the distance between them.
metal I’m going to use it to explain the application of The stress-strain diagram (Figure 9-1) shows a
stress and the strength of the material. common graph produced by the machine as the mate-
We’ll start with tensile stress because it’s the most rial is tested. The stress, which is the applied force per
common. A material is subjected to tension when you square inch of material, is indicated on the left of the di-
try to pull it apart. You expose a material to tensile stress agram and the strain, which is the amount the material
on a regular basis, the material isn’t steel, it’s rubber is stretched is indicated on the bottom. As the machine
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