INSTALLATION

1. Field of the Invention This invention relates to strain gages and processes how to produce and how to bond the strain gages to various surfaces.

2. Background

Strain gages known from the prior art typically are made of a thin metal foil which may be structured by photoresist-etching methods on laminates wherein the metal foil is bonded to insulating films via a permanently cured adhesive. The structure of the metal foil, i.e. the "strain gage-grid", is forming a highly stable electric resistor. Typically strain gages are bonded to test surfaces which belong to mechanical parts as diverse as wings or the fuselage of airplanes or I-beams used in buildings or generally to parts which are subjected to stresses caused by the intended use of those parts. The bonding method is of high importance, since only a correct and very intimate bonding of the strain gage to a respective test surface assures that the stress under load of the parts investigated can be determined correctly via the measured strain on the surfaces of the stressed parts.

The term strain gage also includes strain gage-panels comprising many structured strain gage-grids which can be separated into smaller fully functional strain gages.

In another typical class of applications the strain gages are bonded to especially designed parts - mostly formed of metals - which are intended to be used as sensors to measure mechanical quantities like force, pressure or load, to only name some. Those sensors are universally used, frequently in scales or in hydraulic systems or in test rigs of all sorts, but also for overload detection e.g. in cranes or in elevators. Apart from the bonding process, the materials used in the process of strain gage production and for their installation are influencing the achievable accuracy

performance of those sensors or the correct measurement of surface strains.

One key component of prior art strain gages is to provide a highly stable electrical resistor, having a precise temperature coefficient of its resistance, which is formed as a structure, the strain gage-grid. Made of a thin metal foil, the strain gage-grid, is comprising a typical thickness between 1 and 10 micrometers and covering an area of typically between 0.1 mm ² up to several 100 mm ² . This prior art strain gage-grid is permanently bonded with its lower side to a second key component - the carrier - an insulating film, most often made of polymers like Polyimide or Polyetheretherketone, to name a few. Other insulators used are thin films of cured synthetic resins, sometimes reinforced with glass fibers. In order to improve the mechanical properties some strain gages use fillers in their carriers but are limited in the amount to levels typically below 30% since higher filling levels cause too high a brittleness for the strain gage- production- and the bonding process.

To protect the delicate structure formed of the thin metal foil the strain gage-grid often is encapsulated on its top free three sides with a cured synthetic resin or an adhesive which bonds thin insulating films over it, most often consisting of identical materials as the carrier. To electrically contact the strain gage-grid, pads are provided as part of its structure, which can be contacted electrically to measure the grid-resistance. This resistance changes its value which is proportional to its strain which is transferred from the strain of the test surface. These pads are left open to remain accessible when the strain gage-grid is encapsulated, as shown in FIG. 1 which represents a prior art strain gage. In the prior art the strain gages are bonded with its carrier side to test surfaces using cold curing adhesives like Cyanoacrylates or more often hot curing adhesives like epoxy resins or phenolic resins, like Bakelite, between the carrier and the test surface the strain gage is bonded to. In using all these adhesives it is common practice that the strain gage must be pressed onto the test surface typically by using very specially designed fixtures in order to provide an intimate bond and to achieve as thin a bond line as possible. These fixtures are not only costly but also labor intensive to apply. To minimize any errors in the transfer of the strain from the test surfaces into the strain gage-grid great care is applied to minimize the overall thickness of all materials used between the test surface and the strain gage-grid - the bonding adhesive, the carrier film and the laminating adhesive between the carrier film and the metal foil -. The temperature, humidity and time dependence of the mechanical properties of those materials vary with temperature, humidity and time and therefore are causing most of the errors in the transfer of the surface strain into the structure of the metal foil.

As one of the approaches to reduce those errors, in US 2,963,773 an "upside down" installation of a strain gage to a test surface is disclosed. This solution has severe disadvantages, inter alia difficulties to provide means to contact the grid via the carrier or to remove the carrier from the pads without destroying the delicate structure of the strain gage-grid. For metallic test surfaces an additional complication of the disclosed solution is the need for a thin glass fabric which must be used in the bonding layer to assure electrical insulation. Commercially available strain gages are not known where the installation is intended to be "upside down".

In US 4,050,976 a strain gage is disclosed comprising a carrier of so called B-stage epoxy resin having a glass-fiber layer as spacer to warrant proper insulation. Here the bonding to metallic surfaces is making use of the fact that the B-stage resin can serve as a bonding adhesive too, since it will liquefy again when heated and the glass-fiber layer will prevent an electrical contact between the strain gage-grid and the test surface. One improvement of disclosure of US 4,050,976, is that the electrical contacts of the grid are easily accessible, as with known strain gages. A common problem of US 2,963,773 and 4,050,976 is that both use layers of glass-fiber fabric in combination with the bonding adhesive and both need special fixtures to exert pressure onto the adhesive and onto the glass-fiber layer in order to achieve a proper curing and an intimate bond. Due to variations of the thickness of suitable glass-fiber layers of typically more than 25 micrometer, it is more difficult to achieve the reproducible thicknesses of the insulation between the strain gage-grid and the metallic test surface and, due to the inhomogeneous combination of a resin matrix with the glass-fiber layer, the mechanical properties after curing vary. The strain gage as disclosed in US 4,050,976 has additional disadvantages in that the resin matrix is limited to resins which are "B-stageable". - A -

It is the objective of the invention to provide improved strain gages which overcome the problems of the prior art and which can be installed in a simple and reproducible way "upside down" to various test surfaces.

3. Summary of the invention The objectives can be achieved by strain gages according to the independent claim 1 , by an installation process for strain gages according to claim 15, and a production process according to claim 25.

A strain gage comprises a strain gage-grid laminated to a carrier, wherein the strain gage-grid and the carrier are laminated by a bonding material and the carrier is removable from the strain gage-grid.

In a process for the installation of a strain gage according to the invention to a test surface the strain gage is mounted by an adhesive, in particular an electrically insulating bonding material, to the test surface with the carrier positioned outwardly and the carrier is removed from the strain gage-grid. A process for the production of a strain gage comprises the following steps: a) production of strain gage panels as laminates in laminating a removable carrier to a strain gage foil

b) structuring of the strain gage foil into special patterns as strain gage grids c) coating of the structured strain gage grid with a layer of an electrically insulating curable material

Exemplary embodiments of the invention are subject of the dependent claims.

The strain gages according to the invention continue to use as resistance material a metal foil as in the prior art with its superior electrical properties, but uses a releasable bonding material between strain gage-grid and carrier, so that the carrier can easily be removed. The new strain gage facilitates the "upside down" bonding to test surfaces and allows to choose from a wide variety of electrically insulating bonding materials between the test surface and the strain gage-grid, which - after curing - provide mechanical properties with low sensitivity to temperature, humidity and ageing and at the same time allow for minimal thicknesses, only given by the requirements to provide a sufficient insulation between the strain gage-grid and metallic surfaces. This eliminates the need for additional insulating material between strain gage-grid and test surface. An especially important effect is that the strain gage-grid can be fully embedded on three sides into the bonding adhesive thus providing an intimate bond to the test surface.

4. Brief Description of the Drawings FIG. 1 shows a perspective view onto an encapsulated prior art strain gage with the strain gage-grid 1 , the carrier 2, the encapsulation 3 and contact pads 4, with a permanently cured layer of the lamination adhesive 9.

FIG. 2 shows a perspective view onto a strain gage according to the invention without an encapsulation, with the strain gage-grid 1 , the carrier 2, the contact pads 4 and the releasable bonding material 10.

FIG. 3 shows a perspective view onto a strain gage with a frame 5 with the strain gage-grid 1 , the carrier 2, the frame 5 and the releasable bonding material 10.

FIG. 4 shows a perspective view onto a fully embedded strain gage-grid on a test surface 11 with the stencil 6, the cured electrically insulating bonding material 7 embedding the strain gage-grid 1 on three sides, and the test structure 8 with the test surface 11.

5. Detailed Description

Strain gages according to this invention can be produced, as known from the prior art, in structuring a thin metal foil which has been laminated to a carrier 2, a flat sheet of reasonable thin material having a thickness in the range between 10 micrometers and 1 mm, preferably between 25 micrometers and 100 micrometers. In the inventive concept, the carrier 2 is easily removable to facilitate a simplified process of "upside down" bonding to the test surfaces 11 (see FIG. 4), e.g. by using as lamination bonding material 10 a releasable adhesive as is known from some adhesive-tapes which are used as temporary protection of surfaces. For these tapes the releasable adhesive is especially selected e.g. for coating applications where certain areas must be exempted from being coated. Another area where release coated tapes are used is in anodizing Aluminum surfaces where again certain areas need to be exempted from forming an anodized layer and where the releasable adhesive must withstand heat and etching liquids. Some of the releasable adhesives used are known as pressure sensitive adhesives, another type of releasable adhesive is known as hot melt adhesive and still another type of releasable adhesives intended for only a temporary adherence are materials which may be dissolved with solvents or microwaves. Still another application of films with releasable adhesive is used on so called transfer tapes needed for applications in graphical arts.

Known materials used as releasable adhesives are Polyacrylates or versions of natural rubbers or silicon rubbers.

In the context of this patent application the "bonding material" or "releasable bonding material" referred to by reference numeral 10 may comprise a "releasable adhesive".

Depending on the production method of the strain gage-grid 1 and the adhesives being used to bond the finished strain gage "upside down", the releasable adhesive may be selected appropriately. For example if known photoresist-etching methods are used for structuring the laminated metal foil to form the strain gage-grid 1 , an adhesive is recommended as is used in tapes for the anodizing process, since this adhesive - a silicon rubber type - is suitable to withstand heat and aggressive liquids, having pH- values below or above seven. If the structuring is done using a laser, which is advantageous with very thin metal foils of thicknesses below one to two micrometers, other than a sufficient adherence of the structured strain gage-grid 1 during the resistance trimming process is not needed.

Regardless of the structuring method, a strain gage of this invention comprises a the strain gage-grid 1 having a specified shape and resistance value and is bonded via a releasable bonding material 10 to a carrier 2 which is just a little bit larger than the dimensions of the strain gage-grid 1 and which is not encapsulated and which is not permanently bonded to its carrier 2, as shown in FIG. 2.

To provide a sufficient insulation layer between the top of the strain gage-grid 1 and the test surface 11 the following innovative processes are described. The first process is using a small frame 5 surrounding the strain gage-grid 1 and which is also bonded to the carrier 2 as part of the strain gage and which is thicker than the metal foil the strain gage-grid 1 is structured from. The thickness difference may be in the range from 5 micrometers to 100 micrometers, preferably in the range of 10 micrometers to 20 micrometers. For this process the embodiment of the strain gage is shown in FIG. 3.

Using this version of the strain gage, the frame 5 will be bonded together with the strain gage-grid 1 upside down to the test surface 1 1. The bonding is done either in filling the cavity within the frame 5 completely with an adhesive comprising an electrically insulating bonding material 7 of high viscosity. This way the electrically insulating bonding material 7 remains in place during the mounting of the strain gage onto the test surface 11 and during the curing time. Alternatively a sufficiently thick layer of the electrically insulating bonding material 7 is applied onto the test surface 11 so that when the strain gage is positioned upside down the strain gage-grid 1 and the frame 5 can sink into the electrically insulating bonding material 7. Using slight pressure any excessive amount of electrically insulating bonding material 7 will be squeezed out assuring that the frame 5 is touching the test surface 1 1 and the strain gage-grid 1 is embedded completely on its three sides.

In another, second process a stencil 6 with a cavity, just a little bit larger than the strain gage-grid 1 , is fixed to the bonding surface 11 e.g. using adhesive tape as describe above. The thickness of the stencil 6 is dimensioned identical to the frame 5 as described above, which also applies to the dimensions of its cavity. In the bonding process the cavity is filled with the electrically insulating bonding material 7 and the strain gage-grid 1 is carefully pressed into the electrically insulating bonding material 7 until the carrier of the strain gage will touch the surface of the stencil 6 leaving a predetermined gap between the strain gage-grid 1 and the test surface 1 1. After the electrically insulating bonding material 7 is cured and tightly bonding the strain gage- grid 1 on three sides, the carrier 2 is carefully removed without destroying the strain gage-grid 1. See FIG. 4 for the installed strain gage-grid 1 on a test surface 11.

Still another, third process can be used by first coating the test surface 11 with a sufficiently thick layer of insulating cured material and then in a second step by putting fresh electrically insulating bonding material 7 on top of this already cured layer and position the strain gage-grid 1 onto the fresh electrically insulating bonding material 7 using slight pressure to assure that the strain gage-grid 1 is sinking in being completely embedded into the electrically insulating bonding material 7 and touching the surface of the already cured insulation layer. Still another, fourth process of bonding the strain gage-grid 1 to the test surface 11 is by using adhesives or electrically insulating bonding materials 7, respectively, which are designed for protecting surface mounted electronic components in filling gaps between those components and the mounting surface due to capillary action. If these electrically insulating bonding materials 7 are used, either the frame 5 on the strain gage or the stencil 6 must have gaps through which the electrically insulating bonding material 7 can flow between the strain gage and the test surface 1 1.

Prime selection criteria for the bonding materials are to have low sensitivity of its mechanical properties to humidity and temperature and high fracture toughness.

Surprisingly it is found that light curable adhesives are especially suitable as electrically insulating bonding materials, e.g. cycloaliphatic epoxy resins using photo initiators, are meeting those requirements. In addition the innovative bonding process is very favorable since it allows using those resins with a high level of filling, up to 95% with inorganic powders or fibers, thus largely reducing the sensitivity to humidity.

In strain gages of the prior art it is not possible to use carriers filled to such high a level since such the carrier will be very brittle and is easily cracked during its production process or when bonded under pressure.

Examples of those resins and photo-initiators are provided from the company IGM Resins B.V. in the Netherlands e.g. as resin Omnilane OC1005 and as Photoinitiator Omnicat 432. Fillers are e.g. available from the company Evonik in Germany under the name Aerosil. Another very suitable bonding material is a mixture of Siloxane and Oxirane again using a photo-initiator similar to the Omnicat 432. As filler again a quartz powder like Aerosil may be used, but any other suitable inorganic material, like AI ₂ O3, may be used as filler as well. Beyond those adhesives there are many other adhesives available which may be used as electrically insulating bonding materials since it is one of the advantages of the innovative bonding processes as described here, that it is allowing to use adhesive or electrically insulating bonding material, respectively, of many different materials in a wide range of viscosities and which may become very brittle after being cured or which can withstand high temperatures up to 500°C. Using fillers in the bonding material has an additional advantage avoiding the need for any spacers since fillers are inherently generating a sufficient insulation layer determined by the dimensions and the shape of the filling materials.

In summary the advantages of the "upside down" bonding process are that first no other material is used between the strain gage-grid 1 and the test surface 11 , second that the gap between the strain gage-grid 1 and the test surface 1 1 can be minimized, only limited by insulation requirements and third that the strain gage-grid 1 is embedded on three sides into the electrically insulating bonding material 7.

An additional advantage of any light curable adhesive is that the curing process does not need any heat and that the complete curing is done in seconds or only in a few minutes.

In contrast, strain gages as known from the prior art are not designed to use light curable adhesives as a bonding material and the bonding adhesives recommended use pressure during the curing time especially for sensor applications to achieve a sufficient intimate bond between the carrier and the test surface and to minimize the bond line between the carrier and the test surface. To apply the pressure, special mechanical fixtures and pressure pads are needed which will absorb any light and prevent the use of light curing adhesives or at least make it very difficult to achieve a complete curing. For stress analysis though, the advantages of light curing electrically insulating bonding materials may favorably be used also for strain gages of the prior art if the carrier and the encapsulation are specially chosen for sufficient transparency. For stress analysis it is often sufficient to only press the strain gage into the bonding adhesive for a short time onto the test bonding surface to assure its correct position. Light curing electrically insulating bonding materials can accelerate the bonding process advantageously which is highly recommendable for the investigation of large structures where often hundreds, sometimes thousands of strain gages must be installed. On the other hand, the "upside down" process allows the use of almost any carrier, especially those with good light penetration from visible light down to ultraviolet light of wavelength below 200 nanometers. And since during the cure time no pressure pads are needed, the light can pass freely through the carrier. The strain gage-grid is not a problem, since light curing electrically insulating bonding materials will also cure well with stray light. They even cure when resins are used with light absorbing fillers like Aerosil up to extreme filling levels of 95% of weight.

After having cured the electrically insulating bonding material 7, the carrier 2 can be carefully removed thus releasing the contact pads 4 for any process of electrical contacting. The removal is especially convenient if a releasable electrically insulating bonding material 7 is used in just peeling of the carrier 2.

Since the electrically insulating bonding material 7 due to the "upside down" process allows a much wider choice of materials, e.g. materials from very low viscosities like oils or up to very high viscosities like pastes, or materials having a high resistance towards solvents or etching liquids, the carrier 2 may also be removed if made of plastic which can be easily dissolved with suitable solvents like acetone or benzene or ethylacetate or the like. Even a metal foil, e.g. Aluminum, may be used as carrier and being removed using a base, for example NaOH, as etchant.

In another embodiment of the invention curable materials can be used to create a removable carrier e.g. in coating the metal foil with so called masking resins as provided by Dymax, USA or Denka, Japan. For coating processes many methods are used today i.a. Spin Coating, Roll Coating or Screen printing. The curing in some of the curing materials favorably can be done using light curable materials.

Another useful embodiment of the invention is a process for the installation of the strain gage as shown in FIG. 2, to a test surface, wherein the strain gage is mounted to any test surface by an electrically insulating bonding material with the carrier 2 positioned outward. Preferably the strain gage-grid 1 is embedded on three sides into the electrically insulating bonding material. The bonding material is preferably a curable resin, which is most preferably curable by light in the wavelength range from 100 nm to 1000 nm. The electrically insulating bonding material can be a cycloaliphatic epoxy resin which optionally might comprise a photo initiator, or chemical compounds selected from the group of oxiranes and siloxanes.

In one preferred embodiment of this invention the strain gage is produced in panels as laminates consisting of a dissolvable carrier bonded to the strain gage foil into which after the lamination process special patterns, the strain gage-grids 1 , are structured and sometimes also trimmed in their resistance. The panels are then coated via spin or roll coating or screen printing, to name a few of the many possible coating methods, onto the strain gage grids 1 a suitable curable material which produces a defined thickness of up to 50 μm. If instead of etching as structuring method, in another embodiment of the invention, laser structuring is used, a metal foil as removable carrier may favorably be used, most preferably an Aluminum foil.

After the coating layer is cured the carrier can be removed from the panel leaving the strain gage grids 1 completely embedded in cured curable materials - on top just the releasable lamination adhesive 10 remains and on the vertical sides and on the bottom of the grids the coating material. This coating material is favorably chosen to be the same as the bonding material which is used to bond the strain gage onto the various test surfaces. Favorably the coating material and bonding material can be chosen to be curable with light, thus avoiding not only costly heat curing in the coating process but also eliminating bonding methods needing heat curing under pressure for strain gages of the prior art. In using a metal foil as removable carrier to be laminated onto the strain gage metal foil typically heat curing in a lamination press is used. But different to the permanent carriers in strain gages produced in prior art, there is no rolling of the laminates when removed from the press since the difference in the thermal expansion of the carrier and the strain gage metal foil is small. In addition this is also reducing residual stresses to be built into the strain gage foil during the lamination process, which often cause long term drifts of the zero signals of strain gages.

In another embodiment of the invention the structuring of the strain gage-Grid is done with a laser.

In another embodiment of the invention the strain gage comprises many structured strain gage-Grids. In this case the carrier is removed after the coating is cured and the strain gage is then separated into smaller fully functional strain gages. For the separation of the smaller strain gage any suitable technique can be used, preferred is the use of a laser.

list of reference numerals strain gage-grid

carrier

encapsulation

contact pads

frame

stencil

electrically insulating bonding material to bond the strain gage to a test surface test structure

bonding material layer to permanently bond the strain gage-grid to the carrier releasable adhesive or bonding material to releasably bond the strain gage-grid to the carrier

test surface