The developments and rationalisation that have affected the advanced ceramic industry over the last decade have produced – by economy of scale – a production line split up into highly specialised and consecutively located stations; this industry configuration was determined by the need the maximise the efficiency of each station with special reference to energy consumption and to that of raw materials. This has caused the virtual disappearance of “closed cycle” ceramic manufacturers, where the clay went in at the one end and finished products came out of the other. Their place was taken by:
a) companies specialised in preparing raw materials;
b) companies specialised in forming, decorating and sintering the main semi-finished product (consisting in a single tile with maximum dimensions as permitted by the kiln – e.g., 100x300 cm);
c) companies specialised in finishing off the main semi-finished product in the shapes and sizes demanded by the market.
The latter have therefore had to bear the environmental burden
determined by main semi-finished product cutting and finishing
processes as indicated above. It should be pointed out that what
causes such environmental load – now inert and insensitive
to further processing operations – are special wastes and
are now stocked in dumps: due to the development described in
the introduction, the quantity of such wastes is growing.
The project intends making use, in the best possible way, of recent developments in the knowledge of fragile fracture mechanics to influence how the fracture extends within sheets of ceramic or vitrous material. Such knowledge, only partially and roughly implemented in modern cracking/cutting systems, will find practical translation in the new ultrasonic assisted cutting system. The use of the fracture to separate two sections of material is in fact a very convenient method in terms of energy because it exploits the release of elastic energy to convey to the material the energy needed to create two new surfaces. This principle, which is at the bottom of Griffith’s theory, permits identifying the critical state conditions overcome whereby the fracture will be extended. This does not however fully explain the way in which the fracture extends in the material. Yet it is this very aspect which, in the case of the application that interests us, makes the difference between a successful cut and a wrong cut. A fracture that proceeds in a straight line and which, as regards the thickness of the piece, does not produce excessive waves (the pieces are installed side by side) is what we are in fact looking for, while an irregular or even branched fracture is absolutely unacceptable and some pieces can only be recovered by grinding.
The problem of the extension of the fracture is an old one; only in the past 10-15 years have physicists and material scientists taken an interest in it. By observing a fracture in a fragile material, the presence of dynamic behaviour is easily recognisable, meaning that after a relatively short transitory period, the speed of expansion of the material fracture reaches a limiting average velocity, equal to a fraction (between ¼ and 2/5) of the speed of sound. Furthermore, when the speed of expansion of the fracture is still low, the resulting fracture surface is very smooth, while above the critical value it becomes gradually rougher and the main fracture line often starts to produce lateral microfractures which help make the fracture itself even more irregular [Unsteady Crack Motion and Branching in a Phase-Field Model of Brittle Fracture, A. Karma and A. E. Lobkovsky]. Moreover, a recent experiment showed that by stressing the material in two directions, it is possible to induce fracture extension with coherent “snake” oscillations [J. Fineberg and M. Marder, Instability in dynamic fracture, Physics Reports, vol. 313, pages 1-108, 1999]. In fact, by modulating the action of the applied load, and therefore extending and interrupting the extension of the fracture, the best conditions can be maintained for achieving straight fracture lines and smooth fracture surfaces.
The translation of these phenomena in industrial reality is anything but commonplace, because the fracture extension conditions, besides by the material and load conditions, are affected by the presence of faults, and especially by the extension and shape of the fracture. Furthermore, the speed of action of such phenomena, makes it impossible, as things stand at present and on an industrial scale, to imagine a control system that is successful in offsetting changes in extension conditions. For this reason, the proposer intends implementing a cutting system assisted by ultrasounds, in which a pulsating load, with frequency established beforehand depending on the characteristics of the material to be cut, permits establishing the best conditions for fracture extension in the required way (straight, without branching and with smooth fracture surfaces).
From a system engineering viewpoint, the heart of the new system will consist of knives which typically make up a load system of the “3-point bending” type. In this system, two knives act as supports for a slab-shaped material surface, while the third, in intermediate position between the two, and positioned on the opposite material surface, applies the load, creating a maximum bending moment precisely where the third knife is. On the material section, this conveys a traction force, which is maximum on the surface and which, after passing a critical point, leads to the rapid fracture of the material (it should be remembered that ceramic materials have a resistance to tensile stress ten times lower than resistance to compression). Because critical conditions are affected by the presence of faults, such as cracks or cuts, to provide an initial fracture guideline, the material to be cut is first of all scored by means of a hard and abrasion-resistant tool, which scratches the surface to be cut according to a controlled depth.
The third knife, positioned along this cut, will be in condition to apply the minimum load needed to fracture the material. To make sure the fracture spreads as required, this knife will be connected integrally to an ultrasonic transducer, which will cause it to vibrate at very high frequencies, creating a loading/unloading cycle that will be repeated numerous times in a second. The frequency and acting load will be modulated so that, in the time necessary for the fracture to spread, there is at least an “n” number of vibrations affecting the material, each of which corresponding to the application of a gradually decreasing load (considering the dimension of the fracture is increasing and the critical conditions require the application of a lower force) alternated with a moment of “unloading” or load lower than the critical conditions. As “n” grows, the effect of guiding the fracture and maintaining the surfaces smooth will tend to increase and optimise the quality of the cut.
The project will lead to the achievement of a complete pilot line.