A question of tolerance
Material compatibility tests for plastics and metals
Modern passenger aircraft consist of millions of individual parts made of many different materials. Particularly frequent use is made of metals such as aluminium, magnesium, titanium, steel, copper, and nickel-based alloys, along with plastics and composite materials. During flight operations and also in the process of maintenance work, these materials come into contact with various lubricants and cleansing agents. In order to ensure that these substances are not at all detrimental for the various components or the high safety standards, they must undergo precise material compatibility testing.
As long ago as the 1960s, Lufthansa Technik established its own central laboratory in Hamburg for conducting such wide-ranging material compatibility tests on plastics and metals. Today, the Laboratory Services Division has its own chemistry laboratory and a further laboratory for materials technology and materialography. Customers include, alongside MRO services and airlines, external manufacturers of cooling lubricants for use in workshops, cleansing additives for use on aircraft outer surfaces, and cleansing and disinfectant products for aircraft cabin surfaces, who commission tests on the compatibility of their substances with the plastics and metals used in aircraft.
Depending on the product and the planned area of deployment, up to nine different test procedures are carried out in the laboratories. Some fundamental processes are applied for all substances. All products, for example, are subject to an initial test which determines the pH value and the concentration, before material abrasion is tested. This is done by applying the substances to various polished and artificially aged media (metal plates), both pure and in a variable level of concentration, maintaining them at a constant 37 degrees Celsius, and examining the plates both visually and with the aid of measuring equipment to determine abrasion after 24 hours and again after a week.
Two other test procedures which are applied to almost all substances are the so-called hydrogen embrittlement test and the sandwich corrosion test. The hydrogen embrittlement test is used to assess the suitability of cleansing agents for use in the undercarriage area of an aircraft. Various standardized metal rods are fixed into a mounting jig and then immersed into the substance being tested for 150 hours. If no hydrogen permeates the metal rods, meaning that no hydrogen-induced cracks can form, the metal rods will not break and the substance will pass the test.
In the sandwich corrosion test, a sheet of filter paper is soaked in the solution and then placed between to aluminium-alloy plates for exactly one week. The plates are maintained at a temperature of 37 degrees Celsius in alternately humid and dry environments, and then inspected for corrosion. If the substance being tested achieves a better result than water, the manufacturer will be issued with the appropriate test certificate from the laboratory. Further test procedures are dependent on the products and where and how they will be deployed. In individual cases some of the test procedures - up to nine in total - may become unnecessary because the product has already failed one of the initial tests.
The customer also receives a detailed test report when the result is negative, so that the product can be improved. Customers know that the specialists in the Laboratory Services Division work quickly and reliably, which is why many of them have become regular customers of the Hamburg laboratory over the years. Furthermore, the laboratories are capable of carrying out all analyses and material compatibility tests specified by Lufthansa, Airbus, Boeing and ASTM (American Society for Testing and Materials).
Manufacturers test their modern products before market launch to avoid the situation where the use of cleansing agents, disinfectants, and lubricants results in costly or safety-relevant damage to materials. The demand for reliable laboratory services specifically tailored to the requirements of the aviation industry remains very high. The materialography and chemistry laboratory facilities were centralized at a single location in 2014, making it possible to carry out tests in parallel and even more quickly than before.