A striking materials science lab demonstration

On November 26, 2019 I blogged about Avoid falling on your face during a new product demonstration. That post got me thinking about a great metallurgy lab demonstration I had seen almost five decades ago at Carnegie Mellon University. We were shown a Charpy impact test machine and how much the energy to break a V-notched steel specimen varied with temperature. As shown above, the machine has a weighted pendulum which can be released to hit the 1 cm (0.394”) square specimen opposite the notch and measure absorbed energy via rebound height.

At a low temperature there just was a quiet click, the sample broke in two in a brittle fashion, and pieces flew across the room. The pendulum rose to almost the same height as it had started at (less than 10 J). At a high temperature the sample instead bent in a ductile manner. It did not even break completely and it absorbed lots of energy (over 100 J). Examples of intact and broken specimens are shown above.   

Let’s look at an example set of data for a conventional pressure vessel steel plate in the (L-T) orientation (from the ASM Metals Handbook Volume 8, Mechanical Testing, Ninth Edition, 1985 - Figure 3 on page 262). At – 78 C (-108 F), using dry ice and ethanol for cooling, it took just 5 J to break. At ice water temperature, 0 C (32 F), it took 97 J. At room temperature, 20 C (68 F), it took 143 J. At boiling water temperature 100 C (212  F) it took 200 Joules (J). The S-shaped curve has a transition from ductile behavior and high absorbed energy at high temperatures to brittle behavior and low absorbed energy at low temperatures. It is typical for steels.  

Back during World War II this transition was not well understood, and some spectacular brittle failures occurred in welded structures. As shown above, The T2 tanker SS Schenectady broke almost in two on a cold January 1943 day while it just was sitting at a dock in Portland, Oregon. Then in October 1944 a cylindrical tank in Cleveland, Ohio used for storing liquified natural gas broke and the resulting explosion killed 130 people and destroyed a square mile area.  

You can read more about the test in a web page from TWI (The Welding Institute) titled What is Charpy testing? You also can watch a five-minute YouTube video from Materials Science 2000.

Images of the Charpy test (from Laurens vanLieshout), test specimens (from Dumontierc), and the SS Schenectady all came from Wikimedia Commons.