The process of preparation of the test specimens has been mainly carried out by Chausal company. The materials are supplied in large blocks that require an initial pre-cutting. Besides, the requested blocks were asked to be as homogeneous as possible, avoiding the presence of possible streaks or defects that could cause the prompt failure of the test. After pre-cutting, the new blocks are cut again in sheets with a slightly higher thickness than the final specimens (30 + 2 mm). These sheets, in turn, are cut once again in order to obtain the desired length (180 mm in our case).
The cuts are automatically made by abrasive disks, which allows precisions of the order of millimeters as well as parallel cuts. It is necessary to ensure that opposite faces of the specimens are flat and parallel, since the goodness of the results depends on the correct support of the rollers of the bending testing device. However, the tilting of the rollers and the spherical joint of the upper head of the machine offer greater freedom of movement and enables correct support even in the case that perfectly parallel faces are not achieved. In addition, the load application axis remains centered at any time (See bending testing machine).
Once the sheets are finished with the mentioned thickness and length, a new cut is made with a thickness slightly higher than desired (30 + 2 mm). The cutting system consists of semi-automatic abrasive discs in this occasion, in which a stop limit is used in one of the sides as the rock sheet moves forward. In this way, slightly oversized specimens are obtained and those in which some type of defect is visually observed are discarded.
The next step is to polish the different faces of the test specimens, a process by which the thicknesses are reduced to the desired 30 mm. Thanks to the polishing it is possible to improve the finish and equalize the faces making them as parallel as possible. A group of metal strips have been used as a guide and limit, which allows precisions of the order of half a millimeter despite being a manual process.
This process is repeated for each of the analysed materials, and once the specimens without notches are finished , the part corresponding to the notches with a diameter smaller than 4 mm is sent to Portugal, to the Frontwave company where they have the means to perform notches with radii up to 0.15 mm. The latter are made by means of diamond wire, whereas abrasive discs are used for notches between 1 mm and 4 mm. All of them are cut in the center of the specimens, up to a depth of 15 mm, that is, to the center of the thickness.
On the other hand, notches with radii larger than 4 mm are made by Chausal company , the same in charge of the preparation of the test specimens. Here, notches of up to 15 mm radius are prepared, since this is the maximum possible radius limited by the thickness of 30 mm. The procedure uses once again abrasive discs similar to those used by Frontwave company. Notches are to be executed in such a way that the tip draws half circumference, being the inner walls parallel to each other. That is, the notches must be U-shaped.
With all this, specimens are ready for 4-point bending tests. More than 250 units have been made for each of the 4 chosen materials, apart from the prismatic pieces from which cylinders will be extracted for simple compression and Brazilian tests.
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