Metals used in car manufacturing
must be tested accurately
In every industry, metal components must be strong and safe if they are to be reliable. This requires the metals to be rigorously tested. If the tests are not accurate, safety may be compromised. If test results cannot be easily compared between laboratories, disputes can occur.
One metal property, the impact toughness, can be tested using a simple procedure called the Charpy test. The project team tested the method together, with a view to developing a standard European test procedure. They also developed four reference materials for laboratories to use to verify their accuracy.
Today, our highly automated lifestyles require many goods made from metal components. Cars, trains, buildings, machinery .... the list goes on and on. It is important that these metals are strong enough to be safely used in these varied applications. The European Commission's Standards, Measurements and Testing programme has long been involved in developing common ways of testing the properties of metals to ensure that they have particular properties, such as a strength or toughness. Projects have been initiated to verify the accuracy and reliability of various methods and improve the quality of metal property measurements.
Producing Certified Reference Materials (CRMs) to be used by industry to calibrate the equipment used to make the measurements has also been a priority. These materials provide a means to test the accuracy of measurement techniques as well as the individuals performing the tests. They can be used by any organisation needing to perform metal testing.
The Charpy test for impact toughness of metals
One property of metals that must be tested is the impact toughness, which is defined as the ability of a metal to resist fracture under the effect of shock loading. It is defined by the energy required to break a piece of metal of standardised shape and with a cross-sectional area of 1 cm2. A test called the Charpy test is used to measure a metal's impact toughness.
The test is very simple; the metal to be tested is formed into a rectangular bar, with a V-shaped notch taken out of it. This is carefully placed on the apparatus' anvils with precision tongs. Then the bar is struck, behind the notch, by a striker mounted on a pendulum. The energy absorbed in the fracture is measured by the height to which the pendulum rises.
Precise details of the type of specimen and the procedure for the test is described in written standards. One complication is that the that there are several different standards - the ISO standard, the ASTM (American Standards, Testing and Measurement) standard as well as several national European standards. The metrological parameters of the striker, machine and test pieces are slightly different so the results from these tests are subtly different and therefore difficult to compare.
The need for
a European standard
In order to harmonise the testing technique across Europe, a project team of metal measurement experts from different European laboratories came together. Their work will facilitate the development of a European standard for impact testing of metallic materials.
The group of thirteen laboratories worked collaboratively to examine the causes of error in the procedure. They carefully assessed all aspects of the experiment through interlaboratory studies. As well as performing the measurements, they also analysed their measurements together in many meetings and discussions.
The partners also developed four reference materials to provide metal testing laboratories with samples to be used to calibrate their Charpy test equipment. They are metal bars which have been formed in the same way as the usual test specimens. The only difference is that these bars are known to require a specific energy for fracture. These reference materials are now available to laboratories everywhere.
The steel bars required careful preparation. They had to be made from a specified steel (SAE 4340) and they had to be cut to 60 mm before undergoing heat treatment. Batches of 750 bars were made at a time and more than 2000 samples were made in all. All bars of the same fracture energy had to be as identical as possible and the group had to test the bars to make sure that the impact toughness did not vary with time or temperature. Of course, the temperatures considered were not vastly different to room temperature.
In order to be able to guarantee the energies (30 J, 60 J, 80 J and 120 J) of the four different reference materials, the partners had to test them rigorously. They worked together again to perform the tests and analyse the results in depth. They tested the materials according to both ISO and CEN standards (CEN is the European Committee for Standardisation). The bars were packaged in oil to prevent corrosion, which would alter the impact toughness measurements.
Reference materials are
important for quality control
The use of the Certified Reference Materials can help laboratories achieve accreditation if they use them correctly as part of their quality control schemes. They can also help laboratories' test machines. If a machine is constantly performing inaccurate measurements, it can be fixed before the effects of inaccuracy reach the consumer.
The partners drafted a written standard to submit to CEN. They are using the Certified Reference Materials to test the reproducibility and repeatability of the proposed standard method. By introducing a standard method, the results between laboratories will be easier to compare and the quality of the measurements should improve in the many industries using the method. These include the nuclear, automotive and manufacturing industries who fabricate industrial components from both steel and other metal materials.