To reliably numerically detect the material parameters of adhesives and seals, knowledge of the mechanical values and the layer thickness of the adhesives and seals is essential. This applies in particular when simulating the behaviour of adhesives and joints provided with liquid seals when using the finite element method (FEM).

Background Art An apparatus is generally known in which a test seal is placed between two annular model flanges, the model flanges are then damped against each other by means of a central screw and the distance between the model flanges is recorded by means of a distance-sensor system which operates with eddy-current displacement sensors. Such systems can also operate with strain gauges as displacement sensors. These known apparatus do not however succeed in aligning the joining surfaces of the model flanges exactly parallel and the distance-sensor systems used are not accurate enough. Nor are they suitable for measurements over several weeks as the zero point drifts if temperature fluctuations occur.

Disclosure of Invention Technical Problem The object of the invention is to create an apparatus and a method with which the layer thickness of a liquid adhesive or a liquid seal between two oonponents welded together can be measured at a preset pressure.

Technical Solution This object is achieved according to the invention by an apparatus according to daim 1 and a method according to daim 3.

The apparatus according to the invention has a base plate and a movable plate which can be moved relative to each other by means of linear guides. The linear guides are aligned normally relative to the two plates so that the parallelism of both plates is maintained during their relative movement. Means are provided on each plate for attaching a joining element of the two-part testpiece thereto. The two joining elements have facing joining surfaces and the distance between the joining surfaces is recorded by means of at least one displacement sensor.

[007] The displacement sensor is preferably an incremental displacement sensor, as com- mercially available from DR. JOHANNES HHDENHAIN GmbH, 83292 Traunreut, Germany. These displacement sensors have an adequate measurement accuracy of 0.2 Fm and do not display zero point drifts during longer measurements. Depending on the accuracy requirements and the measurement conditions, however, strain gauges can be used as displacement sensors or eddy-current displacement sensors.

[008] In the method according to the invention the two joining elements are firstly pressed against each other with a preset force so that the joining surfaces lie directly on top of each other. The preset force is the force required to achieve the pressure at which the layer thickness of the adhesive or seal is to be ascertained. The displacement sensors are set to zero in this state.

[009] The two joining elements are then separated again, and the adhesive or the sealing material is applied. The lower joining element of the testpiece need not be removed to apply the adhesive or the sealing material. The adhesive or the sealing material is applied manually by means of a computer-controlled application device or by means of a stencil, and excess material is scraped off by means of a doctor blade. Finally both joining elements are pushed together by means of the pressure frame of the apparatus according to the invention and subjected to the preset pressure. The layer thickness is recorded continuously or at specific time intervals by means of the displacement sensors so that a diagram is obtained which shows the time dependency of the layer thickness at the preset pressure.

[010] Tests are preferably carried out by means of a pressure-sensitive paper before the displacement sensors are set to zero to check whether the same pressure prevails everywhere between the two joining surfaces. Both joining elements are axially symmetrical and they are rotated relative to each other until the pressure-sensitive paper displays an even pressure distribution.

[011] The joining surfaces of the joining elements are preferably flat. In principle however it is also possible to measure the layer thickness at curved joining surfaces by means of the method according to the invention.

[012] The method according to the invention and the apparatus according to the invention are also suitable in particular for measuring the thickness of an adhesive layer or a sealing material for varying surface roughness of the joining surfaces.

[013] As the measurements are carried out over several days and weeks, it is advisable to record the temperature by means of temperature sensors and to compensate the measurement values displayed by the displacement sensors for the temperature fluctuations.

Brief Description of the Drawings [014] An embodiment of the invention will be explained below using the drawing. There are shown in: Fig. 1 the apparatus for determining the layer thickness, in vertical section; Fig. 2 the joining elements in vertical section lying on top of each other; Fig. 3 the joining elements in vertical section, separated; Fig. 4 the pressure plates with the damped-in joining elements and a strain gauge- displacement sensor in vertical section; Fig. 5 a section along 5-5 of Figure 4; Fig. 6 the pressure plates with the damped-in joining elements and an incremental dis- placement sensor, in vertical section and Fig. 7 a section along 7-7 of Fig. 6.

Mode for the Invention [015] Figure 1 is an overall view of the apparatus for determining the layer thickness. The apparatus contains a pressure frame 10. The pressure frame 10 has a horizontal base plate 12 and a movable plate 14 which can be moved towards and away from each other by two vertical linear guides 16, the parallel alignment of both plates 12,14 being kept extremely exact. The base plate 12 and the movable plate 14 have ap- proximately the same longitudinal rectangular shape, the linear guides 16 being mounted in the middle of the narrower end areas. Each of these consists of a guide column 18, a linear ball bearing 20 and a guide sleeve 22. The guide columns 18 are mounted in the base plate 12 and the guide sleeves 22 in the movable plate 14. The guide columns 18 and the guide sleeves 22 are each aligned normally relative to the base plate 12 and the movable plate 14 respectively. The guide sleeves 22, and thus the movable plate 14, in which they are mounted, can be moved free from play by means of the linear ball bearings 20 along the guide columns 18. A first pressure plate 24 is secured to the base plate 12 between the linear guides 16. Mounted on the underside of the movable plate 14 between the linear guides 16 is an approximately cylindrical pressure cartridge 26, which holds a second pressure plate 28 on its underside. The ar- rangement is overall such that the two pressure plates 24,28 have mounting surfaces 30,32 which face each other and, upon a movement of the movable plate 14 along the linear guides 16, are always aligned parallel to and aligned with each other.

[016] Model flanges or joining elements 36,37 are secured to the mounting surfaces 30, 32 of the first and second pressure plates 24,28. The two joining elements 36,37 together form a two-part testpiece 35 (Figures 2 and 3). The joining elements 36, 37 are secured to the first or second pressure plate 24,28 by fitting pins 33 which are inserted at the periphery of the joining elements 36,37 into the mounting surfaces 30, 32, and by damping screws 34 which press the joining elements 36,37 against the fitting pins. The testpiece 35 is shown in Figures 2 and 3 in vertical section. The lower, first joining element 36, which lies on the first pressure plate 24, is a circular plate with an annular projection 38 on the top and a somewhat wider but flatter annular projection 40 on the bottom. The projections 38,40 are vertically aligned. The upper, second joining element 37 is designed correspondingly, the projection 42 being wider and flatter on the top than the projection 44 on the bottom. An axial opening 45 is hollowed out in the middle of the upper joining element 37. The facing horizontal surfaces of the projections 38, 44 are the joining surfaces 46,48 of the joining elements 36,37. The testpiece 35 overall and both joining elements 36,37 are axially symmetrical.

[017] The function of the pressure frame 10 is to raise and lower the movable plate 14, together with the pressure cartridge 26 secured to it, the second pressure plate 28 secured to the bottom of the pressure cartridge 26, and the second joining element 37 secured thereto, by means of the linear guides 16, the joining surfaces 46 and 48 being aligned exactly parallel to each other and being able to moved in vertical alignment away from or towards each other. The function of the pressure cartridge 26 is merely to transmit the force from the movable plate 14 to the upper, second pressure plate 28.

The liquid adhesive is applied between the joining surfaces 46, 48, its layer thickness then being measured as a function of the pressure acting between the joining surfaces 46,48.

[018] In the embodiment shown, the annular joining surfaces 46,48 have an internal diameter of approximately 40 mm and an external dimaeter of approximately 60 mm, their surface area is therefore approximately 1570 mm2 in each case. Up to a pressure of approximately 2.5 MPa, which with the given size of the joining surfaces 46,48 corresponds to a weight force of approximately 4000 N, the pressure can be produced by simple application of weights. To achieve higher pressures the pressure frame is inserted into a servohydraulic tensile testing machine, e. g. Instron 8504. In order to ensure that the action of force on the pressure frame 10 is exdusively vertical, an adapter 50 with spherical articulation surfaces 52 is placed on the movable plate, which transmits the force produced by the testing machine to the movable plate 14 (Figure 1).

[019] The joining surface 46 of the lower joining element 36 projects slightly inwards and outwards, e. g. by 0.4 mm, over the joining surface 48 of the upper joining element 37 so that the joining surface 48 of the upper joining element 37, even in the case of a slight misalignment, lies fully opposite the joining surface 46 of the lower joining element 36. The effective size of the joining surface within which the pressure is effective thus always corresponds to the smaller of the two joining surfaces 46,48, and in the case described of the joining surface 48 of the upper joining element 37. The conditions can also be reversed, and the joining surface 48 of the upper joining element 37 can project slightly over that of the lower joining element 36.

[020] The thickness of the adhesive layer is established by means of displacement sensors 60 which are mounted on the upper, second pressure plate 28, the measurement points on the edge surface of the lower, first joining element 36 lying at an angular distance of 120° and in the middle of the first joining element 36. In total, there are thus four displacement sensors 60. In order that the measurement points of the three dis- placement sensors 60 arranged on the periphery of the upper pressure plate 28 lie on this projecting edge of the lower joining element 36, the external diameter of the lower joining element 36 is greater than that of the upper joining element 37.

[021] In the embodiment of Figures 1,4 and 5, the displacement sensors 60 operate with strain gauges. The probe 62 of the strain gauge rests on the horizontal edge surface of the lower joining element 36. Three such displacement sensors 60 are arranged on the periphery of the testpiece 35. In the middle of the upper pressure plate 28 is arranged a displacement sensor 60 which is designed as an incremental displacement sensor and the measuring pin 64 of which rests via the opening 45 of the upper joining element 37 in the centre of the lower joining element 36.

[022] In the embodiment of Figures 6 and 7, the displacement sensors 60 are incremental displacement sensors which operate essentially such that the lines etched into a glass rod are read by means of a light diode. These displacement sensors are commercially available, e. g. from DR. JOHANNES HHDENHAIN GmbH, 83292 Traunreut, Germany, type ND 221B. Their measurement accuracy is 0.2 Zm. The displacement sensors are likewise mounted in the upper pressure plate 28 and their measuring pin rests on the horizontal edge area and in the middle of the lower joining element 36.

[023] The determination of the layer thickness of liquid adhesive or sealing material which is applied between the joining surfaces 46 and 48 takes place in the following steps: [024] Step 1: The joining elements 36 and 37 are arranged on the lower pressure plate 24 and on the upper pressure plate 28, where they are held between the fitting pins 33 and the damping screws 34 which are provided on each of the pressure plates 24,28. The fitting pins 33 and the damping screws 34 engage with the periphery of the joining elements 36,37.

[025] Step 2: Pressure-sensitive paper is inserted between the joining elements 36,37 and the joining elements 36,37 are pressed against each other with force, e. g. with a pressure of approximately 2 MPa. This pressure can be lower than that at which the layer thickness is later measured. The pressure-sensitive paper changes colour under pressure so that the pressure distribution between the joining elements 36,37 can be established by means of the colour of the pressure-sensitive paper. If the pressure- sensitive paper shows that there is an uneven pressure distribution, the lower joining element 36 is manually rotated by an angle of e. g. 30° after loosening of the damping screw 34, and the pressure distribution is retested by means of a pressure-sensitive paper. The lower joining element 36 is rotated again if necessary and this measure repeated until the position of the lower joining element 36 in which the pressure dis- tribution is as even as possible is found. This rotation position of the lower joining element 36 is fixed by means of the damping screw 34.

[026] Step 3: When the rotation position of the joining elements 36,37 in which there is an even pressure distribution is found, the two joining elements 36,37 are subjected in the pressure frame 10, by application of weights or by insertion of the pressure frame 10 into the servohydraulic tensile testing machine, to the pressure at which the thickness of the adhesive layer is then to be established. If the desired pressure obtains, the displacement sensors 60 are set to zero, i. e. the values displayed by them are chosen as zero point.

[027] Step 4: A precisely dosed quantity of a liquid adhesive, e. g. an anaerobic adhesive, is applied circularly by means of a stencil in the shape of a bead along the centre-line of the joining surface 46.

[028] Step 5: The pressure frame 10 is subjected to the force which is required to achieve the desired pressure. The signals produced by the displacement sensors 60 are recorded and plotted at specific time intervals, e. g. 1 second at first and later 1 minute, or con- tinuously.

[029] In order in the fourth process step to apply a precisely preset quantity of the liquid adhesive to the lower joining element 36, a two-part stencil can be used which is placed onto the joining surface 46 of the lower joining element 36, a gap of e. g. 5 mm remaining free on the joining surface 46 between the two stencil parts. The liquid adhesive is applied to this gap and the excess adhesive can be stripped off by means of a doctor blade which is placed onto a central peg of the inner part of the stencil and is rotated about this peg.

[030] The measurements are carried out in an air-mnditioned room, the temperature fluctuations being less than 2°C. Temperature sensors 66 are also attached to the joining elements 36,37, and the measurement values are subjected to a temperature compensation.

[031] The layer thickness which the adhesive or the sealing material has at a particular pressure value can be established from the measurement results. The incremental dis- placement sensors 60 display no zero point drift, with the result that the layer thickness can be measured over a period of several days or weeks and the creep behaviour of the adhesive or sealant can thus be established. The layer thickness can also be measured for different surface roughnesses of the joining surfaces.

[032] List of Reference numbers 10 pressure frame 12 base plate 14 movable plate 16 linear guides 18 guide columns 20 linear ball bearings 22 guide sleeves 24 pressure plate 26 pressure cartridge 28 second pressure plate 30 mounting surface 32 mounting surface 33 fitting pins 34 damping screw 35 testpiece 36 joining element 37 joining element 38 projection 40 projection 42 projection 44 projection 45 opening 46 joining surface 48 joining surface 50 adapter 52 spherical articulation surfaces 54 edge surface 60 displacement sensors 62 probe 64 measuring pin 66 temperature sensors