FR2629549A1 - Method for increasing the mechanical resistance to the formation of microcracks in a structural adhesive joint between two components or in a multi-material mass - Google Patents

Abstract

The present invention relates to a method for increasing the mechanical resistance to the formation of microcracks in a structural adhesive joint between two components or in a multi-material mass consisting of an agglomerate or aggregate of solid particles joined together by means of a binder, such as in particular micromortar, mortar and concrete. This method is characterized in that the adhesive joint 4 is subjected to a load which varies periodically according to a sinusoidal or substantially sinusoidal cycle and whose maximum strength is less than the threshold for starting microcracks; that is to say always remaining in the overall elastic field of the assembly.

Description

 The present invention relates to a process for .au menting the mechanical resistance to the formation of secure microcracks in a structural adhesive joint between two parts or in a mass of multimaterial consisting of an agglomerate or an aggregate of solid particles assembled by a binder.

such as in particular micro mortar. mortar and concrete.

Most bonded assemblies are the seat of stress concentrations in the vicinity of their ends (maximum of the shear stress) as well as at their ends themselves (maximum of the normal detachment stress) or at the location of their edges. stresses considerably reduce the mechanical resistance of the joint or its resistance to the aggression of the environment. It is therefore of the utmost importance. to allow the development of the use of bonded assemblies in the industrial field to be able to reduce as much as possible these stresses and in particular the shearing in the vicinity of the ends and the detachment at the ends
In addition, agglomerates or aggregates of the mortar type. concrete etc comprise. in their structure of the angular parts which are the seat of variable shear stresses. with significant local variation gradient constraints which contribute to the earlier initiation of microcracks when the structure is stressed
The present invention therefore relates to a method for increasing the mechanical resistance to the formation of microcracks in a structural adhesive joint between two parts or in a mass of multimaterial consisting of an agglomerate or an aggregate of sclid particles assembled by a binder such as in particular micro mortar mortar and concrete. characterized in that the adhesive joint is subjected to a force varying periodically and of maximum intensity below the threshold for absorbing micro-cracks, that is to say always remaining in the overall elastic range of 1 assembly
A non-limiting example will be described below.
an embodiment of the present invention with reference
to the attached drawing in which
Figure 1 is an elevational view of a body
test consisting of a glued assembly provided with gauges
electric and subjected to a periodic tensile force.

Figure 2 is a diagram illustrating the variation
of the tractive effort.

FIG. 3 is a diagram illustrating the displacement
ment of the peak of the shear stress as a function of the number of cycles applied to the test body
Figures 4 and 5 are diagrams illustrating
the variation of the surface deformation as a function of
the effort exerted. for different numbers of fati cycles
lies respectively in 1 place of the two most
near the end of the joint.

To accurately determine 1 effect of the process
according to 1 invention on the starting threshold of microfis
safe in a structural adhesive joint and on their propaga
slow (stable) or fast (unstable)
until the rupture of a test body we used; of a
share the extensometry method with electric gauges which
allows to measure the surface deformations of the ace
semblage and, on the other hand. 1 acoustic emission of the joint
adhesive during its progressive damage This
last method constitutes a method of control not
tructive to confirm the initiation and propagation of micro
cracks revealed by the extensometric method
The analysis is based on the comparison of the results obtained
at different stages of the process i.e. after 1 app
cation of numbers of different fatigue cycles
For the tests we used test bodies 1
such as that shown in FIG. 1, the joint of which is
applied to preponderant shear all along the
joint This test body 1 is made up of two substrates
2 and 3 assembled by means of an adhesive bevel 4.

The substrates 2.3 consist of two parallel bars
piped, with a square section of 10 mm side, made of ferritic steel of grade XC 18 These substrates have been previously machined so as to present. at their glued ends, two front faces 2a.3a. inclined at the same angle with respect to 1 longitudinal axis of the test body.

therefore parallel to each other. These faces or "substrates" are separated from each other by the thickness ej of the adhesive joint which in this case is 0.5mm and this joint extends over a length 1 of 18mm in the plane of this joint in bevel The adhesive used is an epoxy adhesive known as "EPONAL 317". monocomponent and polymerized at 20 C. Substrates. that is to say the faces 2a .3a of the substrates 2.3 which are in contact with the adhesive of the joint 4 have undergone. before being glued. adequate mechanical and chemical treatments so that their surface conditions are optimal. In particular, each substrate preferably has a roughness Rt of 7 micrometers of the order of magnitude of the mineral fillers contained in the adhesive.

 For the tests, the coplanar side faces 2b.3b were equipped with substrates into which the adhesive seal 4 opens. With electrical strain gauges # 1.J2, # 3 JII These gauges are of the SCHENK type. HBM. 4 120 KYlî temperature-compensated for XC 18 steel with film weft. Each body of evidence was then subjected to a study on an AMSLER traction machine.

 The various strain gauges Jl, J2.J3..JIl in the form of a chain are distributed at a distance from each other on the lateral faces 2b 3b of the substrates 2.3.

the first gauge J1 being located at a distance x = 2.5mm from the transverse plane P passing through the origin 0 or first end of the adhesive joint 4. taken as being the connection edge of the two faces 3a, 3h of the substrate 3 The gauges
D2 D3. - .J7 were applied successively in this order to the face 3b of the substrate 3. with a constant axis entry of 4mm. The gauge J7 which is the farthest from the origin 0 of the seal is located beyond the transverse plane P1 passing through the second end of the adhesive seal 4 The other gauges J8.J9, JIO.JI1 are they located on 1R face 2b of the substrate 2. The distance between the gauges J8 and Jl is 4mm as well as the spacing between the gauges J8 39 and 39 J10
The center distance between the last 510-511 gauges is 8, Smm
All Jl-Jll extensometry gauges were connected to a VISHAY 4000 extensometry device equipped with software for acquisition and processing of results and more particularly for the acquisition of relative deformations and the plotting of curves F = ff (t being the deformation and F the tensile force.

 The method according to the invention consists in applying to the test body 1 a force F varying periodically as illustrated in FIG. 2 in a sinusoidal or substantially sinusoidal manner. In the example of particular application described this effort is a longitudinal tensile force but other types of stress could be used, in particular in torsion. bending. etc. The method was applied with a frequency of 30 hertz but a higher frequency could be used to shorten the duration of the tests. The variation in the tensile force F can be of the ripple type; that is to say, this effort always remains positive or oriented in the same direction.

or alternatively it can be of alternating variation with part of the cycle exerted in compression. During the same period, the value of the force F passes twice through the same value. At points al and bl we have equal forces Fal = Fbl and similarly Fa2 = Fb2 at points a2 and b2.

 In the non-limiting embodiment illustrated by the diagram in FIG. 2, the tensile force F varies between a maximum value Fmax of 150 daN and a minimum value Fmin close to zero. The maximum value Fmax of the tensile force is chosen always lower than the value FD corresponding to the threshold of appearance of the first primer of microcrack. In other words, the method must be implemented while always remaining in the overall elastic range of the assembly.

 Various tests have been carried out with glued test bodies I of the aforementioned type, by varying on the one hand the maximum value Fmax of the periodic tensile force and on the other hand the number of fatigue cycles applied to a test body having a starting threshold for FD microcracks ranging from 150 to 200 daN. Such a body of evidence. not treated by the fatigue process according to the invention, exhibited a tensile resistance in monotonic linear traction, of 4100 N. The same test body, on the other hand, exhibited a tensile strength FR of 4300N after having been subjected at 7.105 cycles with a maximum value of the force Fmax of 100daN and a rupture resistance of 4700N. after the same number of cycles. for a value of Fmax of l20daN. A substantial increase in breaking strength is therefore obtained. after the same number of fatigue cycles. this breaking strength increasing with the value of Fmax.

The breaking strength also increases to a certain extent if the number of fatigue cycles applied to the test body is increased 1. Indeed, this breaking strength FR which, for a value Fmax of 100daN. is 4300N after 7.105 cycles. happens to be brought to 4700N after 2.106 cycles, for the same value
Fmax of lOOdaN.

 This property is observed for an adhesive of the bicomponent type of fragile behavior and with weak co-elastic screw behavior.

FIG. 3 illustrates the influence of the method according to 1 invention on the shear stress in the vicinity of the end of the joint 4. If the curve giving the variation of the shear stress # is plotted as a function of the distance x with respect to 1 end O of the joint as shown in FIG. 3 we see that this shear stress has a maximum value = m at a point A close to the end 0 of the joint. The method according to the invention, consisting in subjecting the joint to an effort of intensity varying periodically. allows the end parts of the joint to be transformed locally, which become less fragile by plasticizing, while remaining elastic. It is indeed observed that the more the number of fatigue cycles increases. the more the peak corresponding to the maximum shear <Fm moves towards the edge and the more it decreases in value. This is illustrated by the points BC which correspond to numbers of respective fatigue cycles nl and n2 different n2 being greater than nl. after which the respective maximum shear values ml and
m2 decreasing.

 In other words, we see. by implementing the method according to the invention, the displacement of the maximum deformation towards the end of the adhesive joint 4, as well as a reduction in the value of the maximum shear.

Overall the strains are weaker b equal external stresses. All the internal stresses are better distributed in the mass of the adhesive joint 4 An improvement in tensile strength is therefore observed if the tensile tests are carried out after treatment.
This sliding of the peak of the shear stress towards the end of the joint is also highlighted by the diagrams of FIGS. 4 and 5. These diagrams represent the variation of the surface deformation represented on the abscissa expressed in micrometers per meter in function of the force exerted F carried on the ordinate. expressed in decanewtons. The curves illustrated in the diagram in FIG. 4 are those which have been noted by the first gauge J1 which is closest to the end 0 of the adhesive seal 4 (distance between the end or origin O and the axis of the gauge: 2.5mm). The curves jl-l, jl-2.jl 3, jl 4.jl 5 and jl 6 correspond respectively to the results obtained after numbers of fatigue cycles equal to O in other words in the raw state after bonding. 27000.540OO.lO8ODO.l62000 and 216000. The curves j2-l, j2-2, j2-3, j2-.4, j2-5 and j2-6 correspond to these same measurements, performed after the same numbers of fatigue cycles at the location of the next gauge 32 located 4 millimeters away, that is to say at a distance x of 6.5 millimeters from the end 0 of the joint. We can see from these diagrams. that the slip DU displacement of the peak of the shear stress becomes smaller and smaller, when one moves away from the free edge.

 that is to say from the end O of the joint and consequently the stress becomes more and more homogeneous along the overlap.

Claims (3)

 1.-Method for increasing the mechanical resistance to the formation of microcracks in a structural adhesive joint between two parts or in a mass of multimaterial consisting of an agglomerate or an aggregate of solid particles assembled by a binder. such as, in particular. micro mortar, mortar and concrete. characterized in that the adhesive seal (4) is subjected to a force varying periodically and of maximum intensity lower than the initiation threshold of the microcracks. that is to say, always remaining in the overall elastic range of the assembly.  2.- Method according to claim 1 characterized in that the effort is varied periodically at a frequency of the order of 30Hz or higher.  3.- Method according to any one of the preceding claims, characterized in that one varies the force according to a sinusoldal or substantially sinusoldal cycle.