The origin of some standard mechanical tests that we conduct in our laboratories today can be traced back to research and discoveries from the 1800s. At that time there was an International Association for Testing Materials that would meet every few years in various locations around the world. By the early 1900s it had over 2,500 members including famous names such as Brinell (hardness test), Martens (martensite), Heyn (grain size), Bauschinger, Le Chatelier and Charpy (impact test).
Georges Charpy, was born in France in 1865, and graduated from the École Polytechnique in 1887 with an engineering degree. He later became a metallurgical engineer and subsequently a professor. Charpy became interested in measuring the impact properties of steel because of the significant premature failures exhibited in armaments, steam boilers and steam engines at that time. He presented a technical paper to the Association in 1901 on the results of a test for impact resistance of steel using the aid of a pendulum mechanism. Charpy also established that the use of a notch in the test specimen was important in increasing the accuracy and reproducibility of the measurement. His name became linked with the Charpy impact test for notch toughness and many more years were spent investigating the parameters of the test.
The Charpy impact test establishes the relationship of ductile to brittle transition in absorbed energy at a series of test temperatures. Since a variety of metals undergo a transition from ductile behaviour at higher temperatures to brittle behaviour at lower temperatures, the Charpy test is now specified for a number of steel products including ships’ steel hull plate, pressure vessels in nuclear plant, forgings for electric power plant components, etc.
The Charpy test is performed using precisely machined specimens typically measuring 10mm x 10mm x 55mm with a 2mm deep V-notch in the middle of one of the specimen faces.
Specimens are usually tested over a range of sub-ambient and elevated temperatures. Once a specimen reaches the specified test temperature it is quickly located onto a special fixture in the pendulum impact tester with the notch oriented vertically.
Following the release of the test machine pendulum the specimen is struck by the tup attached to the swinging pendulum of appropriate design and mass. The specimen breaks on impact, at its notched cross-section, and the upward swing of the pendulum is used to determine the amount of energy absorbed in the process.
It may never be established if Charpy and others knew about the ductile to brittle transition that occurs with temperature in steel, during those early days of impact tests. If available records are correct, all of Charpy’s tests were conducted at room temperature or above. Had the ductile to brittle transition been well known in the early 20th century, the steel plates manufactured in 1910 by the steelworks of David Colville & Sons in Scotland, U.K., and used in the construction of RMS Titanic, could have been tested at sub-zero temperatures. This would have revealed the brittle behaviour that resulted on impact, of the hull plate with an enormous iceberg in the icy waters of the North Atlantic ocean on the night of April 14th, 1912. Lack of understanding of the ductile to brittle transition in steel was again evident in the numerous Liberty ships that literally fractured in half during World War II. The over-stressed steel welds became brittle in icy water temperatures and catastrophic crack propagation took place even when the ships were moored.
Impact testing has become firmly established in materials and component testing. The characteristics determined are part of basic material characterisation. ZwickRoell has been delivering quality testing systems for impact testing for many years. Its product line includes pendulum impact testers for metals and plastics, drop weight testers with an energy range of only a few joules to 100,000 Joules, as well as high-speed testing machines.
All ZwickRoell pendulum impact testing machines can be fully or partially automated to provide automatic temperature conditioning, machine loading and testing of Charpy specimens at a range of sub-ambient and elevated temperatures as specified in the requirements of EN 148 & ASTM 23.