Page 279 - Boiler_Operators_Handbook,_Second_Edition

264 Boiler Operator’s Handbook

where the stress doesn’t go any higher. That’s where
the coupon is deforming so much that its cross sectional
area is reducing so, even though the stress in the coupon
increases, the force it can withstand decreases because
the area is decreasing. Shortly after the ultimate strength
is reached the material ruptures. If the coupon is not too
deformed we can measure the cross sectional area at the
rupture to determine the actual stress when it ruptured.
That’s how metal is tested and although you may never
see it done this explanation should give you a better un-
derstanding of material strength and what us engineers
are talking about.
Cast iron and similar materials, including concrete,
that are not extremely strong in tension but very strong
in compression are tested differently. The test method
helps describe what compressive stress is all about. A
Figure 9-1. Stress - strain diagram metal sample is machined to prescribed dimensions
over its entire length to form a test coupon. All those
pulls on the material the force or pull on the material is short round chunks of concrete you’ve seen laying
recorded. That value is converted to stress by dividing around any construction site are test coupons that were
by the cross sectional area of the specimen. poured. The coupon is placed in a machine with a firm
Modern machines allow the operator to enter the bottom plate and pressure is applied to the top of the
area on a keyboard so the machine also calculates the coupon. (Figure 9-2) The force applied by the machine
stress (pounds pull divided by the area in square inches) is divided by the cross-sectional area of the coupon
to imprint it on the diagram. The machine measures the to determine the stress. Some materials, like cast iron
change in distance between the two center punch marks and concrete, withstand considerable stress until they
to determine the strain. fail and they fail quickly when their yield strength is
The stress strain diagram shows what is normally reached. They produce a failure that is closer to shear
called the proportional range where, from zero stress,
the stress and strain are proportional. If the machine
were stopped while the metal was in the proportional
range and the force removed the metal would return to
its original length. Metal in that range acts the same as
the rubber band, always returning to its original shape.
At the end of that straight line is the proportional limit
where the metal’s properties change and it will not re-
turn to its original size when the force is removed. It’s
the same situation when we were pulling on the wire.
Application of a little more force creates a stress
where the metal simply stretches out without adding
resistance (the slope of the line is horizontal). The point
where that starts is called the yield point. When metal
reaches its yield point it deforms. That action is similar
to “cold working” the metal which hardens most steels
making them stronger. I’m sure you’ve heard that cold
worked metal is stronger than hot worked metal. The
sudden cold working of the metal increases its strength
and, despite the cross sectional area being reduced a tiny
bit, it can handle more stress.
The metal continues to resist force but it stretches
dramatically until the ultimate strength is reached, Figure 9-2. Compression stress coupon in machine