Fastener Testing

By Alan Thomas, ZwickRoell Ltd

In the world of fastener testing, some standards stipulate the need to perform impact tests, often at sub zero temperatures, to verify fasteners can maintain good impact resistance even in low temperature service conditions. But what is the history of the pendulum impact test?

The origin of some standard mechanical tests conducted in 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 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 range of steel products including ships’ steel hull plate, pressure vessels in nuclear plants, as well as forgings for electric power plant components.

The typical Charpy test is performed using precisely machined specimens – usually 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 testing. If available records are correct, all Charpy’s tests were conducted at room temperature or above. Had the ductile to brittle transition been well known in the early 20^th century, the steel plates manufactured in 1910 by the steelworks of David Colville & Sons in Scotland, UK, 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 14^th 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.

The Charpy impact test is important as it provides a comparative result for the impact strengths of various materials. When a material is tested across a range of temperatures, the Ductile to Brittle Transition Temperature (DBTT) can be plotted on a graph giving engineers an understanding of the temperature at which a normally ductile material will become brittle.  

Impact testing has become firmly established in materials and component testing and the characteristics determined are part of basic material characterisation.