US3797303A

US3797303A - Biaxial compression-testing machine

Info

Publication number: US3797303A
Application number: US00310094A
Authority: US
Inventors: J Moulis; A Bascoul; J Bourdes
Original assignee: Agence National de Valorisation de la Recherche ANVAR
Current assignee: Bpifrance Financement SA
Priority date: 1971-11-29
Filing date: 1972-11-28
Publication date: 1974-03-19
Anticipated expiration: 1991-03-19
Also published as: GB1357679A; IT971250B; FR2162717A5; DE2257887A1

Abstract

The machine comprises two compression assemblies, each of which is called upon to apply a monoaxial compression load to the test piece in a direction horizontal and perpendicular to the load applied by the other assembly and coplanar therewith; each assembly includes a jack, the body of which is integral with the frame, and a piston applying pressure to a vertical plate adapted to apply a load to one surface of the test piece; an opposing vertical plate, coaxial with the first plate, is connected to the frame for applying to the opposite parallel surface of the test piece a load of modulus equal to the load applied by the first plate and in opposite direction thereto. The machine is characterized in that each compression assembly is carried, independently of the other assembly, on guide means having little friction and making it possible for the assembly to move freely in a horizontal direction, parallel with the direction of application of the load by the assembly; the guide means associated with the other assembly similarly making it possible for the latter to move in a direction parallel with the direction of application of the load by the other assembly and at right angles to the direction of movement of the first assembly.

Description

United States Patent [191 Bascoul et al.

[ Mar. 19, 1974 [5 BIAXIAL COMPRESSION-TESTING MACHINE [75] Inventors: Alain Georges Bernard Bas coul,

Toulouse; Jean-Paul Bourdes, Albi; Jacques Moulis, Ram Onville-Saintagne, all of France [73] Assignee: Agence Nationale de Valerisation de la Recherche, Tour Aurore, France [22] Filed: Nov. 28, 1972 [21] Appl. No.: 310,094

[30] Foreign Application Priority Data Nov 29, 1971 France 71.42699 [52] U.S. Cl 73/94, 73/l03, 73/88 [51] Int. Cl. G01n 3/08 [58] Field of Search 73/88 C, 88 R, 89, 90, 73/94, 103

[56] References Cited FOREIGN PATENTS OR APPLICATIONS 39,988 7/1957 Poland 73/94 Primary Examiner-James J. Gill Assistant Examiner-Anthony V. Ciarlante 57 ABSTRACT The machine comprises two compression assemblies, each of which is called upon to apply a monoaxial compression load to the test piece in a direction horizontal and perpendicular to the load applied by the other assembly and coplanar therewith; each assembly includes a jack, the body of which is integral with the frame, and a piston applying pressure to a vertical plate adapted to apply a load to one surface of the test piece; an opposing vertical plate, coaxial with the first plate, is connected to the frame for applying to the opposite parallel surface of the test piece a load of modulus equal to the load applied by the first plate and in opposite direction thereto. The machine is character ized in that each compression assembly is carried, independently of the other assembly, on guide means having little friction and making it possible for the assembly to move freely in a horizontal direction, parallel with the direction of application of the load by the assembly; the guide means associated with the other assembly similarly making it possible for the latter to move in a direction parallel with the direction of application of the load by the other assembly and at right angles to the direction of movement of the first assembly. i

7 Claims, 7 Drawing Figures SHEET 1 OF 5 FIG. 1a

PATENIED m 1 9 I974 FlG.1b

FIG

PATENTED MR 1 9 1974 SHEET 3 0F 5 FIGS 1 BIAXIAL COMPRESSION-TESTING MACHINE The invention relates to a biaxial compression-testing machine making it possible to subject a test piece in the form of a right-angled parallelepiped to a biaxial field of compression stresses.

In designing concrete structures, use is at present made of the permissible working load of the concrete defined by breaking tests under simple tension and compression. In the case of thick structures subjected to multiaxial stresses, however, this safety definition is adequate, and it becomes necessary to understand the real behaviour of concrete subjected to such stresses. A case in point is that of structures subjected to biaxial stresses.

There are many types of test machines in existence which are capable of subjecting a concrete test piece to a biaxial field of compression stresses. These machines may be divided into two categories: one of these categories covers machines, known as flexible machines, consisting of elements capable of undergoing deformation as the test piece is loaded; these machines usually have the advantage of compactness and low cost but they cannot provide accurate measurements, since the deformation suffered by the machines themselves, and the energy of deformation released upon rupture, produce by no means negligeable distortions in the results obtained.

The other category covers machines, known as rigid machines, in which the elements are of a size such that they undergo only minor deformation during loading; The major difficulty faced by the designers of machines of this type is the necessity of applying to the surfaces of the test piece loads which remain accurately centered in all stages of deformation of the test piece since, if meaningful results are to be obtained, it is desirable to avoid introducing any poorly known parasitic stresses, so that only easily measurable biaxial compression stresses are created within the test piece. Indeed, during the deformation of the test piece, the directions in which the loads are applied can be longer merged with the axes of thetest piece, since eccentricity of the load has two consequences, one being the appearance of compound bending which destroys the uniformity of distribution of normal stresses, and the other being the birth of dissymmetrical tangential stresses on the surfaces of the test piece.

In order to eliminate these centering defects, some machines apply the load through diaphragms subjected to specific pressures, but the maximal loads applicable by these machines are very limited, and the dimensions of the test pieces used must be carefully adapted to those of the machine. The dimensions of the test piece cannot be altered without completely modifying the machine. 7

Other machines apply the loads through jacks. One of the most advanced machines of this type consists of a fixed frame and a mobile frame suspended within the fixed frame by means of a plurality of rods. The fixed frame is equipped with a jack, the piston of which, integral with a plate, may exert pressure on one surface of the test piece, while an opposing plate, attached to this frame, exerts a pressure in the opposite direction on the opposite surface of the test piece. The mobile frame comprises similar elements arranged at right angles to the first elements and adapted to exert pressure on two other surfaces of the test piece. This type of machine ensures centering of the stresses applied to the surfaces of the test piece as long as the deformation thereof is small, the mobile frame being adapted to follow the deformation of the test piece and thus to centre the directions in which the loads are applied (in relation to the surfaces of the test piece). However, as soon as the deformation of the test piece andtherefore the movement of the mobile frame becomes appreciable, the restoring forces exerted by the suspension rods depart from the vertical and introduce parasitic stresses which increase as the movement of the mobile frame and the angle between the rods and the vertical increase. Moreover, in this type of machine, the direction of the load applied by the mobile frame is not suitably defined, since this frame, suspended within the fixed frame, may pivot about a vertical axis which produces a poorly known distribution of stresses.

It is the purpose of this present invention to provide a machine adapted to produce automatic centering of stresses at all stages of deformation of the test piece, with a view to creating a biaxial field of compression stresses in the test piece and to preventing the apparatus of parasitic eccentric stresses, especially tangential stresses.

Still another purpose of the invention is to provide a machine of simple and compact design, low weight, and relatively low production cost.

To this end, a biaxial compression machine comprises two compression assemblies, each of which is called upon to apply a monoaxial load to the test piece in a direction known as the loading direction horizontal and perpendicular to the direction of the load applied by the other assembly; each of the assemblies comprises a jack, the body of which is integral with a frame, while the piston thereof applies pressure to a vertical plate adapted to apply a load to one surface of the test piece; an opposing vertical plate, connected to the frame, is arranged coaxially with and opposite to the plate, for the purpose of applying, to the opposite parallel surface of the test piece, a load of modulus equal to the load applied by the first plate and in the opposite direction thereto. According to the invention, each compression assembly is carried, independently of the other assembly, on guided means of displacement, in contact with which the frame of the assembly may move with little friction; these guide means make it possible for the relevant assembly to move freely in a horizontal direction parallel with the direction of application of the load by the said assembly. In a similar manner, the guide means associated with the other as sembly make it possible for the latter to move in a direction parallel with the direction of application of the load by this assembly, and therefore at right angles to the direction of movement of the fiirst assembly.

As will be substantiated hereinafter, a machine of this kind can produce (apart from the friction forces acting upon the guide means) a biaxial field of compression stresses, centered on the two axes, in all stages of defor- ,mation of the test piece; for thepurpose of substantiating this and making it more readily understandable, one of the compression assemblies of the machine according to the invention is illustrated schematically in FIG. 1a, in which 1 indicates the frame of the assembly, 2 and 3 indicate the body and piston of the jack fitted to this assembly, 4 indicates the plate associated with

p iston

3, and 5 indicates the opposing plate attached to frame 1.

.By examining this diagram, it will be grasped intuitively that, since each compression assembly is free to move in a horizontal direction (OH in the example illustrated), the axes of test piece 6 remain, while the test piece is being deformed, constantly merged with the directions of loading, as a result of the forces of action and reaction occurring between the plate, the opposing plate, and the test piece.

In order to study the behaviour of each compression assembly and to provide strict substantiation of the characteristic mentioned above, the following symbols will be used:

C the undeformable system consisting of frame 1,

jack body 2, and opposing plate 5;

D, the displacement of system C within the reference H;V and V and y respectively the velocity and acceleration of this system;

P the undeformable system consisting of plate 4 and piston 3 of the jack;

D the displacement of this system within the same reference as before, and V, and respectively its velocity and acceleration.

In a reference l-l V associated with system C, the axis of the jack moves from D at velocity v with an acceleration 'y; it is to be understood that the test piece is loaded slowly andprogressively (between 5 and 10 minutes, for example, for a movement of a few tenths of a millimetre), and the value of -y is extremely low.

The breakdown of compression forces acting upon the compression assembly and upon test piece 6, shown diagrammatically in FIGS. la and 1b, is as follows:

F the force applied by opposing plate 5 to the surface of the test piece in contact therewith;

F p the force applied by plate 4 to the surface of the test piece in contact therewith;

F v the force applied by the other compression assembly (not shown) to the two other vertical surfaces of the test piece, F being regarded as a constant compressive force.

If we study in detail the possible movements of the test piece and those of systems C and P of the first compression assembly (FIG. 1a), there are two possible a priori cases. These will be studied consecutively:

either the centre of gravity of the test piece moves along H;

or the centre of gravity of the test piece does not move in that direction.

In the first case, the plate and opposing plate of the second compression assembly exert friction forcesf KF, wherein K is the coefficient of friction between the material of which the plate and opposing plate are made (more particularly steel) and the material of which the test piece is made (concrete).

The resultant Re of the forces acting upon the test piece in the direction of axis H may be written:

It should be noted that Re 0 in the reference H,V selected. It is easy to imagine that the location and shape of the test piece may be considered as the result of the sum of the two following operations:

a displacement of the supposedly incompressible test piece subjected to a force F F F 2KF;

a compression due to two equal and opposite forces of value F The displacement of the centre of gravity of the test piece is equal to (D c)/2 and its acceleration y may be written:

If m is the mass of the sample, then of course:

e m ya Moreover, the resultants R and R,, of the forces acting respectively on systems C and P may be written:

F F 32 M'y +m,;y

Furthermore, by applying the value of Re (equation 1) and y (equation 2) to equation (3), we obtain:

Fp Fc=F=2KF=m ('yc+7 y may be removed from equation (7) and applied to equation (8), which gives the following value for -y:

It has been seen that, if the test piece is to move, force F producing this movement must be greater than 2KF or than the limit equal to this value. F may therefore be made equal to a-ZKF, wherein a is equal or superior to one.

Acceleration 7 may then be written:

M m 2 M Since the loading velocity varies between zero and a certain operational value v, this value v is negative in the reference HV selected, and in this reference it is therefore usual for the sign of 'y to be negative.

In FIG. lie the curve of y is shown as a function of a; this curve is a straight line A almost parallel with the axis of 7; actually, the slope of this straight line is equal in absolute value to:

Coefficient of friction K is of the order of 0.2, whereas mass m of the test piece is of the order to 3 kg and never exceeds kg; therefore: p 0.2 F/lO 0.02 F

The maximal compression exerted by the compression assemblies is of the order of some tens of tons; it may therefore be assumed that the second compression assembly exerts a compression of more than 1 ton, which corresponds to a force F 10,000 Newtons. This hypothesis may be realized by subjecting the test piece to prior monoaxial compression greater than or equal to this value.

This produces p 200 which corresponds well, as already indicated, to a straight line almost parallel with the axis of 3 Moreover y is zero for:

Straight line A therefore has the configuration shown in FIG. 1c, and for small values of y (betweenly maxl and ly maxl), which are the only ones corresponding to reality (as already indicated), the value of a is less than one.

Thus the hypothesis of the displacement of the centre of gravity of the test piece wherein a a 1 must be rejected.

Reality is therefore the second case cited above, in

which the centre of gravity of the test piece does not 7 When loading begins, the increase in velocity v is not strictly zero and there is a slight amount of dissymmetry in the loading. This dissymmetry disappears when a stable condition is reached in which v constant and y 0; according to equation (9), this gives f 0, and, under this condition, the centering of stresses F 7 and F,

(applied by the first assembly), in relation to stresses F (applied by the second) is maintained.

Thus the previously mentioned characteristic of the invention is substantiated. It should be noted, however, that this substantiation overlooks the effect of frictional forces on the means for guiding the frames. These guide means preferably consist of horizontal rails with their axes parallel with the loading direction of the relevant assembly; the frame of the assembly will be supported by these rails through rolling means reducing the friction to a very low value.

By way of example, the weight of a compression assembly adapted to apply a maximal load of 60 tons is of the order of 1.3 tons; the frictional forces are of the order of 5 to 6 kg. It will be seen, therefore, that the ratio of these forces to the load (I e/ 10,000) makes it possible to regard them as completely negligeable.

Moreover, the rails may be placed on three ball-andsocket joints arranged in the form of a triangle, each mounted on a means for adjusting the height of the rails carried on a substantially horizontal base. These arrangements make it possible to set the rails completely horizontally, so that the force of gravity has no effect whatever on the movements of the compression assemblies.

Furthermore, according to one preferred example of embodiment, the frame of each compression assembly consists of rigid horizontal columns arranged in parallel with the loadingdirection of the assembly and connecting together two rigid plates located in close proximity at the ends of the columns, the body of the jack being attached to one of these plates and the opposing plate to the other. According to one characteristic of the invention, each plate is carried on these columns on lowfriction sliding or rolling means, which allow them to move freely along the columns which acts as guides therefor. The movements of this plate are therefore guided completely parallel with the loading direction, and if there is any defect in the alignment of the jack axis, the transverse component of the force applied is absorbed by the columns and is not transmitted to the test piece;

Moreover, each plate is connected, with advantage, to the corresponding columns by means of arms of variable length; these arms, fitted with means of adjusting their lengths, make it possible to centre each of the plates.

In a structure of this kind, it is also possible to interpose between each plate, and the end of the corresponding jack piston, a load sensor which is attached to the plate, to which it transmits the loads applied by the piston, and which measures these loads directly. It is obvious that this measurement of the forces actually applied is much more accurate than traditional measurements consisting of measuring, by means of a dynamometer, the pressure of the fluid acting in the jacks.

Moreover, tests carried out on concrete test pieces have shown that the rupture range of the concrete could be completely covered by using a loading constantly inferior to half the maximum. of the other loading. For tests on concrete test pieces it is therefore useless to provide two compression assemblies of equal power, since the total power of one will never be used. According to another characteristic of the invention capable of being applied to concrete-testing machines, the elements of one of the compression assemblies are made heavy enough to apply a predetermined maximal load, for example 60 tons, while the elements of the other assembly are made to be able to apply a maximal load half as great, for example, 30 tons.

The invention having been set forth in general, an example of embodiment thereof is illustrated by way of example in the drawings attached hereto, wherein:

FIGS. 1a, 1b and have already been dealt with;

FIG. 2 is a simplified view in perspective designed to facilitate the understanding of the general structure of a machine according to the invention;

FIG. 3 is a view of the machine from below;

FIG. 4 is a front elevation of this machine along A-A;

FIG. 5 is a section along the line B-B.

The biaxial compression testing machine, illustrated by way of example, consists of two independent compression assemblies l and 2 with no contact between them. Assembly 1 is designed to be able to apply to the test piece a maximal load of 60 tons along one axis, while assembly 2 is designed to be able to apply a maximal load of 30 tons along an axis perpendicular to, and coplanar with, the first assembly.

Each assembly comprises a rigid frame consisting of four columns 3 connected, in the vicinity of their ends,

by means of locking screws 4, to steel plates 5. Each plate 5 is carried by a shoe 6 on a guide rail 7 by means of rollers 8 adapted to roll in

slides

9 and 10 having lateral tracks; the fixed slide is attached to rail 7, while the mobile slide is attached to the shoe. Rollers 8 are enclosed in a cage and are arranged at 45 alternately in one direction and the other, in order to provide accurate guidance for shoe 6 in relation to the rail and to apply, under the action of the weight of the relevant compression assembly, substantially balanced lateral loads to

slides

9 and 10.

Furthermore, each rail rests upon three ball-andsocket joints 11 located at the apices of a triangle and carried on micrometer screws 12 on a base 13. Screws 12 make it possible to adjust the height of each joint 11 and therefore to locate each rail 7 in a perfectly horizontal position.

A hydraulic jack body 14 is attached to one of plates 5 of the frame of each assembly, the jack body being appropriately centered by means ofa centering pin. Attached to the other plate of this frame is an opposing plate 15. Piston 16 of the aforesaid hydraulic jack carries at its end a pressure ball 17 protected by a cap 18.

The four columns 3 of each assembly carry sleeves 19 with ball races and protective bellows 25, an arm 20 being attached to each sleeve; the four arms 20 thus guided on these columns carry a mobile element consisting essentially of a plate 21 and a load sensor 22 of conventional type, against which pressure ball 17 of the jack piston rests. Each arm 20 is moreover fitted with a micrometer screw 23 having right and left-hand threads and a knurled wheel 24; accu rate centering of the assemblies. Furthermore, the columns of assembly 2 (which are smaller, this being the low-power assembly) pass between the columns of theother assembly; these columns may be chrome-plated and straightened to ensure perfect guidance.

A machine of this kind makes it possible to reduce almost to zero any parasitic stresses which might be introduced as a result of eccentricity of the load during the deformation of the test piece; as already indicated, the only possible source of such stresses in this machine is friction on rails 7, this frictional force being of the order of 5 kg in

assembly

1 and 2 kg in assembly 2. These forces, of the order of l/ 12,000 of the maximal load applicable by each assembly, are completely negligeable.

The invention having now been set forth and its interest justified, an exclusive right thereto is reserved for the entire duration of the patent, with no limitation other than that in the following claims.

We claim:

1. A biaxial compression-testing machine which makes it possible to subject a test piece in the form of a right-angled parallelepiped to a biaxial field of compression stresses, said machine comprising two compression assemblies, each of which is called upon to apply a monoaxial compression load to the test piece in a direction horizontal and perpendicular to the load applied by the other assembly and coplanar therewith, each of said assemblies comprising a jack having a body integral with a frame and a piston to apply pressure to a vertical plate adapted to apply a load to one surface of the test piece; an opposing vertical plate coaxial with said first plate and connected to said frame for applying to the opposite parallel surface of the test piece a load of modulus equal to the load applied by said first plate and in the opposite direction thereto, said test machine being characterized in that each compression assembly is carried, independently of the other assembly, on guide means having little friction, said guide means making it possible for the assembly to move freely in a horizontal direction, parallel with the direction of application of the load by said assembly; the guide means associated with the other assembly similarly making it possible for the latter to move in a direction parallel with the direction of application of the load by said other assembly, and at right angles to the direction of movement of said first assembly.

2. A testing machine according to claim 1, characterized in that said guide means associated with each assembly consist of horizontal rails, the axes of which are parallel with the loading direction of the assembly, the frame of said assembly being supported on said rails by rolling means.

3. A testing machine according to claim 2, characterized in that the said rails are located on three ball-joints arranged in the form of a triangle and each mounted on height adjusting means supported on a substantially horizontal base.

4. A testing machine according to claim 1, wherein the frame of each compression assembly consists of rigid horizontal columns arranged in parallel with the loading direction of said assembly and jointing together two rigid plates located in the vicinity of each end of said columns, the jack body being attached to one of said plates and the opposing plate to the other, said machine being characterized in that each plate is carried on said columns on low-friction sliding means enabling it to move freely in a direction parallel with said columns thereby acting as guides therefor, the end of the jack piston being adapted to apply pressure to the central area of said plate.

5. A testing machine according to claim 4, characterized in that the plate in each assembly is connected to said columns by means of variable-length arms, said arms being equipped with means for adjusting the length thereof for centering said plate.

6. A testing machine according to claim 4, characterized in that a load sensor, attached to the plate in each assembly, is interposed between said plate and the end of the jack piston, said sensor serving to transfer to the plate the forces exerted by the piston and to measure said forces.

7. A testing machine according to claim 1, designed for testing concrete test pieces, said machine being characterized in that the elements of one of the compression assemblies are of dimensions enabling it to apply a maximal predetermined load to the test piece, while the elements of the other assembly are of dimensions enabling it to apply a maximal load of the order of half of that applied by the first assembly.

Claims

1. A biaxial compression-testing machine which makes it possible to subject a test piece in the form of a right-angled parallelepiped to a biaxial field of compression stresses, said machine comprising two compression assemblies, each of which is called upon to apply a monoaxial compression load to the test piece in a direction horizontal and perpendicular to the load applied by the other assembly and coplanar therewith, each of said assemblies comprising a jack having a body integral with a frame and a piston to apply pressure to a vertical plate adapted to apply a load to one surface of the test piece; an opposing vertical plate coaxial with said first plate and connected to said frame for applying to the opposite parallel surface of the test piece a load of modulus equal to the load applied by said first plate and in the opposite direction thereto, said test machine being characterized in that each compression assembly is carried, independently of the other assembly, on guide means having little friction, said guide means making it possible for the assembly to move freely in a horizontal direction, parallel with the direction of application of the load by said assembly, the guide means associated with the other assembly similarly making it possible for the latter to move in a direction parallel with the direction of application of the load by said other assembly, and at right angles to the direction of movement of said first assembly.