FR2460724A1 - METHOD OF SEALING AND MOUNTING AN EXTENSOMETRIC GAUGE TO A TEST BAR AND FORMULATION OF AN ADHESIVE FOR THIS SEALING - Google Patents

Abstract

THE INVENTION RELATED TO A SLIP-RESISTANT SEAL, A SUPPORT OF A BLADE EXTENSOMETRIC GAUGE AT THE SURFACE OF A TEST BAR. THE PROCESS FOR PREPARING THIS SEALING INCLUDES: THE PREPARATION OF A FIRST VOLUME OF EPOXIDE RESIN IN LIQUID FORM; THE PREPARATION 11 OF A SECOND VOLUME OF TRIMELLITIC ANHYDRIDE; - THE MIXTURE 12 OF THE EPOXIDE RESIN AND THE TRIMELLITIC ANHYDRIDE TO FORM A FLUID MIXTURE IN WHICH THE TRIMELLITIC ANHYDRIDE IS IN A WEIGHT RATIO OF APPROXIMATELY 35 TO 45 WITH THE EPOXIDE RESIN; -APPLICATION 13 OF THE SAID FLUID MIXTURE IN A THIN LAYER BETWEEN THE SURFACE OF THE BAR AND THE BOTTOM OF THE GAUGE HOLDER; AND THE CURING 14 AND 15 OF THE MIXTURE WHILE THE BRACKET AND BAR ARE MAINTAINED IN DETERMINED CLOSE CONTACT. THE INVENTION ALSO CONCERNS THE FORMULATION OF THE TWO-COMPONENT ADHESIVE FOR THIS SEALING AND THE MOUNTING OF THE SAID GAUGE ON THE CORRESPONDING TEST BAR.

Description

An extensometer gauge of the metal blade type, comprising

  a thin and non-porous insulating support carrying a thin layer of photo-etched metal in a diagram

  determined mesh, is sealed to the deformable surface by

  sion of a force-sensitive metal part by means of a crosslinked resin layer resulting from the mixing of a selected epoxy resin and a given volume of trimellitic anhydride used as hardener, of a solvent in sufficient quantity to facilitate the proper application of the two-component resinous composition, in the form of tetrahydrofuran; elevation to moderate curing temperature for a relatively short time causes sufficient crosslinking

  to allow the assembly to remain secured without any

  tion; postcuring at a substantially higher temperature

  then ensures that the installed gauge is very stable and

  in particular, it will exhibit only a very small slip.

  The present invention relates to improvements in the sealing of electrically resistive strain gages such as those with a metal blade on a non-porous support, providing stable bonds and showing low slip with their substrates, and in one particular aspect thereof

  relates to adhesive compositions and techniques

  both the use of long-lasting epoxy resin blends, hardening materials and solvents which can be easily applied and cured to accurately transmit to and through a metal blade strain gauge support the surface tensions exhibited by a bar of

loaded constraint.

  Precision measurements characterizing phenomena such as force, moment, weight and pressure, are commonly carried out using strain gauges equipped with grids made of

  elongated and fine conductive strands whose resistance varies

  the underlying surfaces of the bar on which the

  strands are fixed are subject to distortions

  so-called phenomena. Extensometric gauges, both wire-type and metal-blade type, have found there

  wide range of applications and are well suited to

  in highly miniaturized composite forms o

  thin strands of the gauge, their ends and the conductive tabs

  associated with them are first permanently attached to an insulating support plate, called a "support", which isolates them

  electrically and door, and allows easy handling.

  In some cases, wire-type strain gauges have been made on impregnated paper supports, which can in turn be fixed to the surface of a test bar with a simple cellulosic glue, while other cases

  gauge wire has been molded into a phenol resin thermoset

  cissable; the latter type of "Bakelite" gauges required the use of thermosetting phenolic resin-based glue

  to fix them on the bars.

  The metal blade gauges are nowadays manufactured on supports made of materials such as polyimide, a phenolic composition reinforced with glass fibers, paper impregnated with nitrocellulose, and a phenolic composition.

  epoxy. Whatever the support, its sealing, in a unique way

  shape, narrow, and stable, the bar under load, is absolutely essential for the accuracy of the device, because the performance of the gauge is simply limited by the

  qualities that this sealing allows.

  In addition to cellulosic glues and phenolic resin

  mentioned above, the use of a two-component adhesive includes

  a separately presented resin and an activator that produces

  a self-polymerising mixture when they are put into

  The room temperature, which will be increased by heat-hardening, is also known. (Ref. "The Strain Gage Primer", by C. C. Perry and H. R. Lissner, McGraw Hill Book Company, Inc., N.Y., 1955, pp. 29-38). The epoxy resin has also been described as being used in a non-tacky adhesive layer at room temperature which can be rendered flowing by heating for the purpose of sealing. (US Patent No.

3609624).

  Among the serious problems that accompany the implementation of

  this blade-type gauges by gluing or sealing amounts to

  to make the "glue line" as thin as possible,

  the specific advantages due to the very low

  The thickness of the metal sheet and the thinness of the support are not lost due to the fact that there is too much separation space between the sheet and its support and the surfaces of the test bar due to the adhesive. In addition, this thin adhesive

  must contain neither lumps nor bubbles that would vary the space-

  or distort the gauge, or leave it unattended

  portions of the support; it must both extend uniformly

  easily and "soak" the surfaces of the support and the

  gauge with which he must be in contact. It is very important

  as long as this adhesive is not responsible for a "slip"

  important of the gauge. This term "slip" refers to a displacement

  the strain gauge is measured in relation to the surface of the bar to which it is to be joined; softening and / or any other undesired collapse involving the adhesive material and causing such slippage and resulting in measurement errors should therefore be minimized as much as possible. The best way to remove slip is to use an adhesive that hardens completely, rather than

  retaining significant residual thermoplastic characteristics

  tants, and that does not require high crosslinking temperatures that could damage the metal blade, the support, the connection of the conductors, etc ... The sealing materials that require to be frankly mixed and used quickly before they harden, can, obviously, be enough

inconvenient to use.

  In a brief description of the technique of the invention

  in one aspect, a two-part sealant is used

  components, for strain gauges of metal-blade type

  and the like, comprising an epoxy resin portion and a trimellitic anhydride hardener portion of

  t, 35 to 45% of the weight of the resin part; Tetrahy-

  drofurane as solvent, in amounts equal to both parts, and sufficiently to give the resulting mixture fluidity and moisture qualities that will allow it to spread finely on the surface of a bar and on the free face of a support , by means of a brush or any other applicator. After mixing its parts, the sealing material has a long shelf life of up to four weeks, and during which it can be used if it remains unchamined,

  and if one avoids too much evaporating solvent and expo-

  be at high temperatures. Once the sealing composition is applied to the bar and the support, they are pressed against each other, preferably with the aid of a protective buffer equalizing the pressure, then a first

  curing is carried out at a temperature of about 121 ° C.

  approximately two hours, after which the gauge will remain

  ex officio, well in place. Definitive hardening of the thermal type

  moderation and total cross-linking ensuring that the

  thin adhesive layer will be both hard and weak and

  so elastic modulus to prevent slipping, are ensured

  post-hardening lasting four hours at a time

  at least 280C higher than the first.

  Accordingly, one of the objects of the present invention is

  the production of improved sealing for strain gauges

  blade tacks carried by non-porous supports, or for similar gauges, which can be easily made and showing a

low slip.

  Another object of the invention is the production of new

  advantageous two-component adhesives for strain gauges

  which lend themselves to total cure at relatively low curing temperatures, offering improvements

  reducing slippage, and which have a

  prolonged storage and a low viscosity favoring their

  preparation and application in a simple and effective way.

  Another object of the invention is the production of epoxy adhesives for strain gauges where the advantages provided by the strength of the hardening and the ease and convenience of handling are provided by a trimellitic anhydride hardener whose dose weight depends on the weight of resin, and in which tetrahydrofuran is used as a solvent to provide the viscosity required for application or

  to make the hardener liquid for this purpose.

  Although the features and aspects of the invention

  considered to be new are listed in the

  annexed, additional details concerning

  preferences for other objects and advantages of the invention.

  tion will be better understood by referring to the description which

  following in conjunction with the accompanying drawings, in which Figure 1 is a plan view of a metal blade strain gauge on its support and sealed through

  from it to the underlying surface of a deformed test bar

  mable. Figure 2 is a cross-sectional view along the line

2-2 of Figure 1.

  Figure 3 represents, in a schematic diagram, some

  preferred sealing methods and the materials concerned.

  Referring to the drawings - on which like reference numerals denote corresponding parts in the different views - and more particularly to Figs. 1 and 2, an electric resistance strain gauge of the metal blade type is shown, provided with an ordinary grid-like grid 4 consisting of strands or parallel metal conductive filaments joined in series by terminal tabs 4 'and associated to enlarged terminals to which can be welded

  connection cables. The support 6, on which are

  tograined strands by use of processes and substances

  already well known to professionals, is preferably a film

  an extremely thin material of known non-porous material such as polyimide, a phenolic composition, or an epoxide; the thin strands are closely united in permanent abutments with the top of the support in the state where they were formed. The strands of the gauge are naturally intended to undergo effective resistance variations as their cross-sections change with the lengthening and retraction of the strands. Indeed,

  these follow the elastic distortions of a surface 7 provo-

  the effects of compression and tension due to

  test bar 8; for example, the latter can take the form of a steel column, a beam, a barrel

or a membrane.

  The best measurements are obtained when the gauge is as uniformly close and perfectly sealed as possible to the surface 7 of the test bar. It is well known that an adhesive layer such as layer 9, through which a gauge support is attached to the surface of a test bar, is a decisive link in the chain of factors that can seriously affect the accuracy, long-term reliability and stability of measurements. More specifically, errors

  slippage will occur if the 9-layer

  amplify a relative sliding between the surface of the bar and the gauge, sliding due to the lack of hardness of this sealing layer. At the same time, it must be possible to easily apply this adhesive on metal bars and non-porous supports, with good wetting and without bubbles or lumps, as well as to prepare easily and without excessive cost, with extended shelf life to ensure ease, convenience and economy of use when attaching strain gauges. In addition, mounting, gauges must involve relatively short times and

  reasonable curing temperatures that will set the seal

  permanently and will release enough of a posterior slipping, while nevertheless being able to withstand very high temperatures (260 - 290 ° C for example) without causing a thermoplastic softening or any

other damage.

  Adhesives meeting these requirements and in accordance with the present teachings have had to be partially determined

  empirically, because technical certainties and predictions

  dependence on what is appropriate or inappropriate, tend to be very evasive in this area. However, these improved adhesives necessarily include, as a first component, a selected epoxy resin or a perfect blend of such

  epoxy resin with a specific solvent, tetrahydrofuran.

  The second component is a specific curing agent or

  trimellitic anhydride in a ratio of

  with the resin, diluted or not in the same solvent, the tetra-

  hydrofurane. These two components are prepared in advance and

  packaged separately. In Figure 3, the stage of preparation

  The composition of the epoxy resin component, referenced 10, preferably comprises the removal of a predetermined volume of a liquid resin such as "EPON-828" from the Shell Chemical Company or a melted resin (tetrahydrofuran) such as the resin "Cresol Novolac ECN-1235" from the firm Ciba-Geigy. The stage of the preparation of the second component, trimellitic anhydride, referenced 11, requires either the spraying of trimellitic anhydride flakes in the form of dry powder (of a size of 37 microns or less) or the dissolution of these flakes. in the solvent tetrahydrofuran. When the adhesive

  must be applied, the two components are mixed carefully

  (referenced step 12), the weight of trimellitic anhydride in the mixture to be kept equal to at least 35% and 45%

  plus the weight of resin. Subsequently, during the storage period

  mixture that can vary from 24 hours to 4 weeks, depending on the

  resin-based preparation and the use of the tetrahydro-

  furan, the mixture is applied to an extensometer

  and the cleaned surface of the test bar (referenced step).

  13), using a spatula, brush or other suitable applicator. At the step referenced 14, the

  tightening of the dipstick against the bar followed by a first hardening

  period of two hours at approximately 120% C. Then cooling, removing the clamping pressure and postcuring for about 4 hours at a temperature at least 28% C higher than the initial temperature, step referenced 15, give a uniform, hard and well sealed thinness layer, shown in 9 (fig.2), which will keep the gauge in a stable and reliable relationship with the test bar, allowing

precision measurements.

EXAMPLE A

  A two-component adhesive is manufactured for sealing

  strain gauges with moisture and resistance

  excellent chemical performance and which functions satisfactorily

  in a temperature range of -195.6 to 260% C, using as components a clear, viscous liquid epoxy resin and a white powder of trimellitic anhydride. The liquid epoxy resin has a viscosity of between 100 and 160 poises and an epoxide equivalent of between 185 and 192, as

  for example the "EPON-828" resin marketed by the Shell Che-

  mical Company or Miller Stephenson Co. Inc .; it should preferably be packaged and sealed in small containers filled to a specified resin weight. The hardener is

  in the form of a dry powder obtained by spraying

  flakes of trimellitic anhydride into particles of

  about 37 microns in size or less; this powder is

  in dry form in small containers each of which

  holds a weight of trimellitic anhydride between 35 and 45%

  about the weight of liquid resin with which it will be mixed.

  If the weight ratio becomes significantly less than 35%, the resulting mixture has an undesirably long curing time and, moreover, incomplete curing or partial curing will likely occur, so that the resulting sealant layer will exhibit thermoplasticity or softening that would allow a slip. On the other hand, if there is a weight ratio greater than 45%, the

  sealing will show reduced resistance to shear forces

  and an untimely predisposition to breaking.

  In an implementation quite characteristic of the

82460724

  Assigned, small plastic bags of liquid resin containing about 7 grams of epoxide and bags of powder to be mixed containing about 3 grams of trimellitic anhydride are used. Larger bags, containing about 35 grams of resin, are mixed with bags of powder containing about grams. During the packaging period, neither the liquid nor the powder shall be exposed to an environment with a relative humidity exceeding 50% and, even with lower humidity,

  exposure time must not exceed one hour; the absorption

  humidity is detrimental to this epoxy mixture and must not be

not exceed 0,01%.

  The two components are thoroughly mixed, for example by kneading, just before using the adhesive to seal a dipstick on a test bar, and within 24 hours (which is the recommended storage time, sometimes if the mixture is maintained at 4.41C), the adhesive is applied in a light layer, flattened to a uniform thickness on the back of the gauge support and on the surface to be equipped. The gauge is then applied to this surface, the two layers being brought into close contact, and a pad of silicone or identical material is placed on the gauge, with a thin sheet of Teflon interposed to prevent the pad and the adhesive layer from stick to each other. A pressure of 1.41 to 2.11 Kg / cm 2 is applied for clamping and the assembly is cured at a temperature of 12 ° C. for about two hours. When cooling to room temperature, the pad and the Teflon sheet are removed and it will be seen that the gauge is physically in place. Postcure occurs for about 4 hours at a higher temperature

  approximately 280C is at the maximum temperature of use desired.

  of the installation, ie at the hardening temperature

  preceding (whichever is the higher) or during

  4 hours at - 2320C. As a result, the installed gauge can be considered ready for use for

  Stable and reliable measurements as announced above.

EXAMPLE B

  A second two-component adhesive, which will even withstand higher temperatures (up to 2880C) and offer a very advantageous storage time (up to approximately 4 weeks after mixing) and which includes only components in liquid form, is prepared by using, as a first component, a semisolid transparent resin melted and dissolved in tetrahydrofuran. One of the semisolid resin materials usable for this purpose is a Cresol Novolac resin, having a molecular weight of 540 (the epoxide portion having a molecular weight of 200), having an epoxide functionality of 1, 7 and melting point equal to 350C. The ECN-1235 resin having satisfactory grades is marketed by Ciba Geigy Corp., N.Y., and melted at 107% C until complete liquefaction and then cooled to room temperature. Then, a solvent, tetrahydrofuran (Fisher Certified) is added in a weight ratio of 1.4 parts per part of resin and stirred until complete dissolution of the resin. Hardener

  is prepared from white flakes of trimellitic anhydride

  (e.g. those commercially available from Amoco Chemicals Corporation, Ill.), by adding tetrahydrofuran as a solvent in a weight ratio of 3.5 parts per part of flakes and stirring the i-mixture until complete dissolution of the flakes. The two components are kept separately, for example in small bottles containing 12 grams of the resin-solvent mixture and, in the case of the hardener, 9 grams of the anhydride-solvent mixture.When these pre-packaged products are mixed in order to used to seal strain gauges of the metal blade type on

  test bars, the weight ratio of trimellitic anhydride

  that the resin is 40%, which is in the range of -45% to which reference is made in Example A above,

this for the same reasons.

  The two freshly mixed components provide a useful shelf life of about 4 weeks, when kept away from air and contamination. The support of the gauge and the surfaces of the bar receive a light layer, for example with a brush; these applications are allowed to dry until they are ready to stick for about 5 to 10 minutes before being contacted. Then, the tightening,

  hardening, cooling and loosening, and post-

  hardening are carried out as in example A above,

  with the exception that a stronger clamping pressure of approx.

  3.52 to 4.92 Kg / cm 2 is preferred.

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Claims (18)

R E V E N D I C A T IO N S   1. Process for preparing a slip-resistant seal   a non-porous part such as the support of a   extensometer with a blade on the surface of a test bar, CARAC-   TERISE INCLUDING: - the preparation of a first volume of epoxy resin in liquid form,   - the preparation of a second volume of trimellitic anhydride than,   the mixture of the epoxy resin and the trimeric anhydride   to form a fluid mixture in which the trimellitic anhydride is in a weight ratio of about 35 to 45% with the epoxy resin, - the application of said fluid mixture in a thin layer between the surface of the bar and the bottom of the support gauge, - and the hardening of the mixture while the support and the   bar are maintained in a determined close contact.   2. Method according to claim 1, CHARACTERIZED IN THAT the preparation of the first volume of epoxy resin comprises the conditioning of a liquid epoxy resin, viscosity   between 100 and 160 poise, and WHEREAS the preparation of   ration of the second volume includes glitter spraying   trimellitic anhydride to a fine powder and its conditioning   is lying. 3.Procédé according to claim 2, CHARACTERIZED IN THAT the liquid epoxy resin has the characteristics of EPON-828 resin, and IN THAT the fluid mixture is applied in 24   hours after mixing the first and second volumes.   4. A process according to claim 1, CHARACTERIZED IN THAT the preparation of a first volume of epoxy resin comprises the complete melting of a Cresol Novolac semi-solid resin, and that the preparation of the second volume comprises the dissolution   dry trimellitic anhydride in tetrahydrofuran.   The method of claim 4, CHARACTERIZED IN THAT the preparation of the first volume further comprises dissolving Cresol Novolac resin fused in tetrahydrofuran after   cooling at room temperature.   6. A process according to claim 5, CHARACTERIZED IN THAT the weight ratio of tetrahydrofuran to molten resin is about 1.4 and that the weight ratio of tetrahydrofuran to 11 2460724   trimellitic anhydride is about 3.5, and in that the durability   curing is carried out at temperatures which disperse the solvent   tetrahydrofuran. 7. A method according to claim 5, CHARACTERIZED IN THAT the Cresol Novolac resin has the characteristics of the ECN-1235 resin, and wherein the liquid mixture is applied within 4 weeks after mixing the first and second volumes. 8. A method according to claim 1, CHARACTERIZED IN THAT the application of the fluid mixture comprises the spreading of a light layer thereof on both the surface of the bar and on the back of the support of the blade gauge. metal, and IN THAT the hardening comprises the pressure clamping of the gauge carried by the support against the bar and the maintenance of the combination at a high temperature.   9. A method according to claim 3, CHARACTERIZED IN THAT it comprises clamping the gauge carried by the support against the bar at a pressure of about 1.41 to 2.11 Kg / cm 2 for about two hours at a time. temperature of about 121 ° C, then the   cooling and loosening of the gauge, then a post-dur-   curing for about 4 hours at a temperature above about 281C at the desired use temperature or at the cure temperature, whichever is higher, or about 232% C.   The method of claim 7, CHARACTERIZED IN THAT the curing comprises clamping the gauge carried by the support against the bar with a clamping pressure of about 3.52 to 4.92 Kg / cm 2 for about two hours at a temperature of about 1210C, then cooling to room temperature and loosening the gauge, and then post-curing for about four hours at a temperature of about 280C above the desired temperature of use or the temperature of the   hardening, whichever is the highest, or at about 232% C.   11.Formulation of a two-component adhesive for sealing   without sliding of a non-porous module carrying an extensometric gauge, on the surface of a bar, CHARACTERIZED IN THAT it comprises two separately dosed volumes acting in common,   epoxy resin in liquid form and a hardener, the anhy-   trimellitic anhydride, the amount of said hardener having a weight ratio of about 35 to 45% with the epoxy resin, at the 12 2460724   following which the mechanical mixing of said volumes will give a fluid mixture which can be applied in a thin layer between the   gauge module and the surface of the bar for a relatively long time   long after mixing, and which will seal them stably after hardening.   12. Formulation of a two-component adhesive according to the   claim 11, CHARACTERIZED IN THAT said resin in liquid form has the characteristics of the resin EPON-828, and that said hardener comprises trimellitic anhydride sprayed into a powder whose particles do not exceed size of 37 microns.   The formulation of a two-component adhesive according to claim 11, CHARACTERIZED IN THAT said resin in liquid form comprises an epoxy resin which is the cooled product of the fusion of a semi-solid Cresol Novolac resin dissolved in tetrahydrofuran, and in that said hardener is the liquid product of the dissolution of dry trimellitic anhydride in tetrahydrofuran.   Formulation of a two-component adhesive according to claim 13, CHARACTERIZED IN THAT said resin in liquid form has the characteristics of the resin ECN-1235 and EN   WHEREAS the same amount by weight of tetrahydrofuran is present in each of the dosed volumes.   15. Mounting of an extensometer gage resistant to slipping   CHARACTERIZED IN THAT it comprises a metal blade gauge carried by a non-porous support and IN THAT a thin and hard layer of a fully crosslinked epoxy resin tightly seals   the underside of the support to the metal surface of a sensi-   deformable elastic band, said layer being formed of a mixture of a liquid epoxy resin with trimellitic anhydride in a weight ratio of between 35 and 45%, this mixture   having been first cured at a high temperature with   cation of a pressure on the support and the bar, then post-   hardened to an even higher temperature without pressure so to cause its crosslinking.   16. Assembly of a sliding-proof strain gauge.   CHARACTERIZED IN THAT said layer is formed by the melange   of a liquid epoxy resin having the characteristics of EPON-828 resin, with trimellitic anhydride in the form of dry powder. 13 2460724   17. Mounting of an extensometer gage resistant to sliding   CHARACTERIZED IN THAT said layer is formed by mixing a liquid form epoxy resin which comprises the cooled product of a molten Novolac Cresol resin melted and then dissolved in tetrahydrofuran, said semi-solid resin   having the characteristics of the ECN-1235 resin and the tetra-   hydrofuran having been removed during curing.   18. Mounting of an extensometer gage resistant to slipping   according to claim 15, CHARACTERIZED IN THAT the first   hardening under a minimum pressure of 1.41 Kg / cm2 is effected   killed at a temperature of about 121 C for two hours and that the postcure is performed for about four hours at a   temperature of about 28 ° C above the operating temperature   desired, or hardening, whatever the   higher, or around 232 C.   BOFORS AMERICA, Inc. By Proxy Cabinet SCOPI