Ultra-flat pressure plate material tester and method for its manufacture

The ultra-flat pressure plate material tester for sounding concrete, comprises two facing side plates in the form of circular segments, of which the circular edges are turned back inwards by more than 90.degree. and assembled against one another by a flexible weld. The rectilinear edges of the side plates are assembled by using a U shaped obturator. Two elements for coupling to the hydraulic circuit are provided.

BACKGROUND OF THE INVENTION 
The invention relates to an ultra-flat pressure plate material tester and 
more particularly to ultra-flat material testers for measuring stresses. 
In concrete construction, the direct measurement of absolute internal 
stresses is an essential element for evaluating its residual strength 
capacity. In the absence of any other means, a direct stress-release 
measuring method has been adopted. It consists of forming a kerf in the 
concrete with a circular saw, then re-establishing the strain-field by 
means of an ultra-flat material tester adapted to this kerf. The 
compensation pressure indicates the stress sought. This is a method used 
in geological survey but of which the crude transposition to concrete had, 
in the past, left failures. In fact, the differences in scale and in 
nature between the two materials require the taking into account of an 
increased number of parameters. 
Through the documents U.S. Pat. No. 4,326,420 and DE-A No. 2,904,844, 
material testers are already known for measuring forces in a kerf formed 
in concrete or another medium. 
However, testers, designed in a circular shape, have within a kerf formed 
by a circular saw, a surface which is always different from the surface 
freed by the kerf, the difference being further increased by the fact that 
the effective surface of the tester is reduced by reason of its convex 
deformation. 
Through this fact, the testers cannot be used simply and reliably for 
measuring absolute internal stresses, and still less to achieve a stress 
gradient through a range of kerfs of standardized sizes. 
GENERAL DESCRIPTION OF THE INVENTION 
It is an object of the invention to provide a novel tester of flat 
construction, as well as a particular technology for its manufacture, free 
of the aforesaid drawbacks and reconciling better the following 
essentials: 
a miniaturization imposed by the need to reduce the destructive character 
of the measurement; 
minimization of natural stiffness, rendered difficult by miniaturisation 
and however indispensable in order that the transmission of the pressure 
to the walls of the kerf should be faithful and uniform; 
strength enabling the pressure to reach the level of the stresses 
encountered in the constructions being sounded, 
application of the tester to substantially the whole of the walls of the 
kerf. 
This purpose is achieved, according to the invention, by providing a 
material tester composed essentially of a chamber bounded by two facing 
side-plates shaped as circular segments, an obturator of the rectilinear 
edge of the chamber, the circular edges of the side plates being turned 
over inwardly by more than 90.degree. and assembled against one another, 
advantageously by a flexible weld. 
This particular structure of the edges enables the flanges of the tester to 
be moved, under the influence of the pressure, substantially parallel to 
themselves, the welded circular edges spreading out in somewhat accordion 
fashion. 
The rectilinear edges of the side plates are advantageously assembled by a 
U shaped obturator which is brazed to them. This assembly, less flexible 
than that of the circular edges, is better suited to the rectilinear edges 
since they occur above the measuring kerf. In fact, the unbalanced, excess 
pressure of the U shaped portion, neutralizes the rigidity of the 
obturator under flexion; in addition, the U shape withstands better than 
the W shape, the outward thrust; finally this shape has less technological 
difficulties of construction. 
The circular segment shape of the tester according to the invention enables 
the latter to correspond exactly to the shape of the circular kerfs, and 
hence to measure stresses reliably. By successive kerfs, it is possible to 
achieve a direct knowledge of the stress gradient. 
The thickness of the testers of the invention is advantageously of the 
order of 4 mm.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The ultra-flat tester 1 comprises two side plates 2 and 3 in circular 
segments of radius R=150 mm for example, formed in sheet metal of 0.4 mm 
thickness. It is possible to provide, for example, a range of testers 
manufactured with respective penetration depths h of 20, 30, 40, 50, 60 mm 
etc. for a same thickness e of 4 mm, the total height H being slightly 
greater. 
The upper rectilinear edges of the side plates 2 and 3 are assembled by 
means of a longitudinal obturator 4, of U shape, fastened by welds 5 to 
the edges of the side plates which enclose the sides of the obturator 4 
(FIG. 3). 
The circular edges 6 and 7 of the side plates 2 and 3 are turned over 
inwards by almost 180.degree. and are assembled to one another, on their 
convex surface (FIG. 4), by a flexible weld 8. Two coupling members 9 and 
10 enable connection to the hydraulic circuit. It is seen in the section 
of FIG. 6, that they comprise an elongated shoulder 11 of which the base 
12 is U shaped to adapt itself to the obturator 4 on which they are 
brazed. 
The manufacture of the ultra-flat testers of the invention is as follows. 
The side plates are cut out with a radius R+5 mm, the burrs are removed and 
they are annealed. 
The two side plates 2 and 3 are then enclosed with good symmetry between a 
disc 12 and a tapered former 13 leaving a regular overlap of 5 mm (FIG. 
5). The assembly is mounted on a lathe 14. 
The overlapping portions of the metal sheets are then rounded by means of a 
suitable tool, to an overlap of 3 mm. 
The edges 6 and 7 are pressed down by means of a milling wheel 15, until 
they mate the tapered former by clamping them with the milling wheel on 
the tapered edged 13, the rotary speed of the lathe being minimal. The 
side plates are dismounted and the folded back edge is machined to obtain 
the width dimension of 2 mm, namely half the thickness of the tester. 
To make the obturator 4, a strip of sheet metal is cut out to a width of 10 
mm and of length equal to that of the rectilinear edges of the side 
plates, and the strip of sheet metal is given its U shape by press-forming 
in a die 16 by means of a punch bar 17, (FIG. 7). 
By means of internal shims, the two side plates are fixed along their 
circular edges 6, 7, with a flexible weld, then the obturator 4 is welded. 
The coupling elements 9 and 10 are brazed to the obtruator, then a hole of 
2 mm is pierced in the obturator 4 in the axis of the threaded hole 18 
provided in each coupling element. 
In operation, it is possible to use any one of the coupling units; the 
other can be plugged after filling with oil, with a screw-plug. 
A general burr removal terminates the fabrication. 
When they are placed in the cut formed in the medium being sounded (FIG. 
2), the testers are supported on the edge of the cut by the shoulders 19 
of the coupling elements 9 and 10. 
Testers manufactured by this method have been checked to test their 
capacity to re-establish the stress and strain field, disturbed by the 
presence of a cut in a medium under stress. 
For this purpose, several trials were carried out on Plexiglas models by 
two complementary optical methods (photo-elastic test and moire pattern 
test). 
These tests have confirmed the quality and reliability of the measurements 
carried out by the testers of the invention and their capacity to ensure 
the reliable and uniform transmission of a pressure of 0 to 9 MPa.