Composite Materials Testing qualification? This is the “Bercella how-to”

A journey that began ten years ago and that, like the company, is in continuous and constant evolution. A growth project, therefore, which has led Bercella to be one of the few European companies to have its own internal Laboratory for testing and qualifying materials, much more than a simple in-house Quality Control.

The analyses that are carried out on the raw materials (prepreg) consent to gradually enrich an increasingly large database relating to the properties of these materials. The data collected in this way are fundamental for the design department, which designs a particular component in order to optimize their performance. The Laboratory also has an educational function since it forms an ever-wider company know-how. In fact, just as composites have different characteristics from metals, it is worth underlining that even the composite materials themselves are substantially different one from the other, making it necessary to test each new material to define its performances. The Data Sheet alone, that is the documentation that summarizes the characteristics of a material, can be useful for making rough estimates on the behavior of the components, but it is not enough to have a complete picture and guarantee a correct design. It is precisely in these situations that the ability to carry out tests quickly and effectively, in addition to an internal database, becomes crucial both for the knowledge of the raw material and for the excellent success of the project. The activities carried out in the Bercella Material Testing Lab can be mainly grouped into two categories: mechanical tests and physical tests, and into three goals: incoming inspections, characterization campaigns and production controls.

Incoming Inspections

The goal of the Incoming Inspection, or Acceptance Control, is to verify the compliance of a material or component with the specifications with which it was purchased. This verification is carried out on all critical materials, or where the customer indicates it as necessary also on standard materials. The raw material / component is studied using its sample, called coupon, which can be made from both untreated material (for example in the case of prepregs) and cured (in the latter case it has already been carried out the autoclave polymerization cycle, called cure cycle) and tests are carried out on this to verify that its actual mechanical and chemical-physical properties correspond to those expected.

Characterization Campaigns

They are launched when you deal with new materials or when it is necessary to determine the mechanical performance of the materials to be used for the design, in order to be able to lay solid foundations for the project to come. In this context, compression and bending tests are carried out to support the Design phase, both on the so-called solid laminates (consisting only of Carbon Fiber), and on sandwich panels(Carbon Fiber sheets with an internal core made accordingly to specific procedures). These tests, carried out in association with strain gauges, allow to determine lots of fundamental parameters such as Young’s modulus, tensile strength, Poisson’s modulus, Interlaminar Shear Strength and Stress, compression modulus, etc.

Production Control

In parallel to the realization of a component, typically in the Aerospace sector, witness samples are produced; they are examples of the production process and allow the validation of a particular phase (lamination, gluing, etc.): their performance will therefore be representative of the reliability of the “real” component. Production Control is always part of the Quality Control trend, but it is no longer a matter of tests on raw materials, but of tests aimed at validating internal production processes. This derives from the characteristics of our production processes, defined as special ones, which are almost all the time different from each other and therefore require continuous and accurate controls.

MECHANICAL TESTS

The mechanical tests described above are possible thanks to the presence of two main machines. The first, from 2017, is the Zwick Roell Z100 and features a very large full scale (100kN) and advanced management software. This machine allows a lot of flexibilityalso on the customer side, especially for its versatility: in fact, it is equipped with a series of accessories (equipment and software packages) that enrich its operation based on the tests to be carried out (traction, bending, compression) or on the materials studied. It therefore guarantees a multiplicity of applications and a wide range of improvements, a bit like an open-source software that can be reworked and enriched many times. The other machinery supplied to the Laboratory was purchased in 2011, when the Lab was born; it is called METROCOM and is a traction machine (up to 50kN).

CHEMICAL-PHYSICAL TESTS

Composite Materials are made up of two elements: the fibersand the matrix. While the mechanical tests seen above measure the properties of the fibers, the chemical-physical ones test the performance of the matrix; Bercella is specialized in the use of epoxy resin, by far the most widespread and performing in the world of Prepreg and Autoclave technology. This thermosetting resin, fluid and sticky at room temperature, hardens when it reaches a certain temperature, thus having the function of consolidating the fibers, giving the piece its shape and performing the load transfer function. It is important to test the characteristics of the matrix as, beyond a certain temperature, it loses its properties and once this happens, although the fibers may still be intact, they are no longer able to transfer load between them and therefore the material isn’t adapt anymore.

The most important machine in this field is used for DSC tests (Differential Scanning Calorimentry), that is the main thermal analysis technique that can be used to characterize many types of materials including composites. The basic principle of this technique consists in obtaining information on the material by heating or cooling it in a controlled manner and allows to characterize the temperature response of the resin, both cured and untreated, that is both before and after the autoclave cure process. Subsequently, by comparing the results obtained both on the sample of untreated material and on the treated sample, it is clear whether the results obtained are to be considered in line with those expected or not.

Another fundamental machine is used for DMA tests (Dynamic Mechanical Analysis), which applies a constant force to the witness sample, while increasing the temperature. In this case it can be seen how, within a certain temperature range, the sample almost completely loses its mechanical properties; this range is called the glass transition temperature (TG). This determines the maximum operating temperature of that component. For example, if I know that a BT120 resin has a glass transition temperature of 120 ° – 130°C, I can say that a component produced with that resin can be used in an environment that does not exceed that temperature.

Obviously, the scale is also one of the most useful tools in the Laboratory as the density of the material can be deduced from the weight variations: ours are extremely accurate, with a full scale of 220gr and a division of 0.1mg!

Finally, the muffle furnace is an oven that reaches temperatures up to 700°C and is used to measure the percentage of impregnation. In practice, the supplier provides pre-established percentages, based on which the properties of the material can change even significantly. It is therefore very important to measure the percentages of fibers and resin that make up the component, as they could explain variations in performance. The cured specimen, firstly measured at room temperature, it is then placed in a high temperature oven which, by degrading the resin, returns only the fibers. At this point, by weighting the fibers and making the delta with respect to the weight of the specimen, it is verified whether the percentage of impregnation was correct or not. In conclusion, thanks to the muffle oven, both the material supplied and the quality of the internal care process are verified.

Matteo Menoni, Material Testing Lab Manager, describes the advantages of having this technology in-house: “As previously mentioned, having an internal Material Testing Lab certainly allows the company to develop its own considerable know-how, which increases with the tests carried out and with the materials qualified each time. Furthermore, the Laboratory leads to the birth of an internal Research and Development department and vice versa, in a circle of positive influences. Secondly, the customer is also guaranteed a more effective cost control and a more efficient lead time management.”

To sum up, the tests that can be performed within the Bercella Material Testing Lab (and the related reference standards) are:

Solid Laminate tests