Charpy V-notch Impact Testing, History & Process
The origin of some common mechanical testing that we perform today can be traced back to research and discoveries from the 1800s. In that era there was an International Association for Testing Materials that would meet every 2-3 years in various cities around the world. By 1913 it had 2,682 members including such famous names as Brinell (hardness test), Martens (martensite), Heyn (grain size), Bauschinger, Le Chatelier and Charpy (impact test).
Georges Augustin Albert Charpy, a Frenchman, was born in 1865 and graduated from the École Polytechnique in 1887 with an engineering degree majoring in marine artillery. He went on to become a metallurgical engineer and later a professor. Charpy became interested in measuring the impact properties of steel because of the many premature failures of armament, steam boilers and steam engines in that era. He presented a paper to the Association in 1901 on the results of a test for impact resistance of steel using the aid of a pendulum. Charpy also found that the use of a notch in the test specimen was crucial in increasing the sensitivity and reproducibility of the measurement. His name became associated with the Charpy Impact Test for notch toughness because he spent years thoroughly investigating the parameters of the test.
This impact test shows the relationship of ductile to brittle transition in absorbed energy at a series of temperatures. Since iron and all other body-centered cubic metals undergo a transition from ductile behavior at higher temperatures to brittle behavior at lower temperatures, this test is required today for a number of important steel products including steel hull plate for ships, nuclear plant pressure vessels, forgings for electric power plant generator rotors, etc.
The test is performed using several machined bar specimens 1cm x 1cm x 5.5cm with a 2mm deep notch at the middle of a specified flat surface – usually a “V” notch. The specimens are tested at a series of specified temperatures (e.g. -20ºC, -10ºC, 0ºC, +10ºC, +20ºC). Once a specimen reaches the precise temperature, it is quickly placed into a special holder with the notch oriented vertically and toward the origin of impact. The specimen is struck by a “tup” attached to a swinging pendulum of specific design and weight. The specimen breaks at its notched cross-section upon impact, and the upward swing of the pendulum is used to determine the amount of energy absorbed (notch toughness) in the process.
It is still a question if Charpy and others knew about the ductile to brittle transition that occurs with temperature in steel during these early years of impact tests. As far as it is known, all of Charpy’s tests were conducted at room temperature or above. If the ductile to brittle transition was well known in the early 20th century, the steel plates manufactured in 1910 by the steelworks of David Colville in Scotland and used for the RMS Titanic could have been tested at sub-zero temperatures to reveal the brittle behavior that resulted upon impact of the hull plate with an immense iceberg at the icy North Atlantic ocean temperature of -2ºC that fateful night of April 14, 1912. Ignorance of the ductile to brittle transition in steel was again evident in the numerous Liberty ships that literally fractured in half during WW II. The over-stressed steel welds (no post-weld heat treatment) became brittle at icy water temperatures and catastrophic crack propagation took place even when the ships were at dock.
It is interesting to note that the Charpy Impact Test was not issued as a standard test method by ASTM until 1933. It is described in ASTM E23 “Notched Bar Impact Testing of Metallic Materials”.