Method for manufacturing a load-indicating connection part and corresponding load-indicating connection part

Abstract

The present invention relates to a method for manufacturing a load-indicating connection component, in which a connection element (10) and a piezoelectric ultrasonic transducer (20) are provided and interconnected. The method includes forming a layer structure on the surface (14) of the connection element (10), the layer structure including, from the surface (14) of the connection element (10), an optional first solder layer (16), a reactive layer (30), an optional second solder layer (22), and the ultrasonic transducer (20). The reactive layer (30) is designed for an exothermic reaction upon activation by heat, electromagnetic radiation, or electric current. The ultrasonic transducer (20) is then pressed against the connection element (10) to generate a specified contact pressure, activating the reactive layer (30). The present invention also relates to a load-indicating connection component of this type.

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a load indicating connection piece, in which a connecting element and a piezoelectric ultrasonic transducer are provided and interconnected.Another aspect of the invention relates to such a load indicating connection piece. [Background technology]

[0002] Mechanical connections such as screws and bolts can be checked for pretension in a simple way using ultrasonic measuring methods. For this purpose, the signal propagation time of ultrasonic waves, which are coupled into the connection part and recombined as an echo signal, is measured. The ultrasonic pulses used for excitation can be both fixed-frequency signals and signals composed of different frequency components. In the course of progress in materials science, a large number of composites, graded materials and special alloys have been introduced in recent years, from which connection parts such as bolts and screws are manufactured, which place high demands on the measurement methods for determining the pretension due to their material properties.

[0003] A reliable measurement of the ultrasonic signal propagation time in a component requires a good bond between the ultrasonic transducer and the connection part. For example, if the ultrasonic transducer and the connection part are glued together, the condition of the glue layer will affect the quality of the measurement. In particular, the condition of the glue layer can be highly dependent on the current temperature and may also change over time, especially due to aging of the glue layer. Furthermore, commonly used glue layers have limited resistance to chemical influences from the environment. Particularly when using load-indicating connections in offshore wind turbines, it is problematic that commonly used glues are often not resistant to salty sea air.

[0004] From EP 2010883 a connection part with a heat-resistant sensor element is known, in which a piezoelectric ultrasonic transducer is applied onto the connection element by a sputtering method, thereby avoiding an adhesive layer for connecting the connection part and the piezoelectric ultrasonic transducer.

[0005] However, such deposition methods have the disadvantage that they are expensive. Both the investments for acquiring the necessary vacuum components and the costs for operating and maintaining them are relatively high. In particular, due to the limited volume of deposition plants, the throughput in the production of load-indicating connections decreases exponentially as the size of the connection increases. As the size of the connection increases, the cost per part rises as well. In vacuum plants, load-indicating connections can generally only be coated at low cost up to a nominal diameter of M20. For larger threads, which are increasingly being used, for example in the wind power sector, the end customer has to accept increased costs for equipping the connection elements with ultrasonic sensors.

[0006] From WO 2010 / 085942 a microsystem is known which consists of two connecting substrates and a structure between the connecting substrates. The structure comprises a reacted reactive layer system. In one embodiment the microsystem is a sensor which comprises a piezoelectric component. A reactive multilayer system is used for connecting the connecting substrates. The reactive multilayer system comprises at least one surface layer with vertically oriented nanostructures arranged at a distance from one another and areas between the nanostructures which are filled with a material which is a reaction partner for the material of the nanostructures.

[0007] From EP 0 915 524 A1 a piezoelectric device is known with a vibratable element comprising at least two elements capable of performing bending vibrations. Each element carries a piezoelectric drive element and at least one of the elements carries a piezoelectric output element. The drive elements are excited by a voltage which causes a lateral or longitudinal deformation of the piezoelectric drive element. This vibration causes a bending vibration of the element. The output element likewise comprises a piezoelectric element and comprises electrodes for tapping off the voltage. The device can be used, for example, as a filter or transformer. A reactive metal thin film can be used to connect the piezoelectric elements to the vibration element.

[0008] From WO 2010 / 096439 A1, a resonant sensor is known which comprises a two-layer system of an active piezoelectric layer and a passive layer which are bonded to one another as an active element. To produce the two-layer system, a wafer made of piezoelectric material is metallized on both sides. As a passive layer, a wafer made of aluminum oxide is likewise metallized and bonded to the active layer by conventional methods or reactive soldering.

[0009] One object of the present invention is to provide a method for producing a reliable load indicating component which is simple to construct, offers high throughput and is resistant to temperature and chemical influences. DISCLOSURE OF THEINVENTION

[0010] A method for manufacturing a load indicating connection component is proposed, in which a connection element and a piezoelectric ultrasonic transducer are provided and interconnected. The method includes forming a layer structure on a surface of the connection element, which layer structure comprises, in this order from the surface of the connection element, an optionally first solder layer, a reactive layer, an optionally second solder layer, and an ultrasonic transducer. The reactive layer is adapted for an exothermic reaction by activation by heat, electromagnetic radiation, or electric current. After forming the layer structure, an ultrasonic transducer is pressed towards the connection element to generate a predetermined contact pressure. While the layer structure is pressed, the reactive layer is activated.

[0011] After the reaction layer has fully reacted, the ultrasonic transducer is firmly soldered to the connection element. Pressing is finished and the completed load-indicating connection part can be removed from the corresponding device that applies the pressing pressure.

[0012] When forming a layer structure, the first solder layer is always used if the surface of the connection part itself cannot perform the function of a solder layer, i.e. is not directly suitable for soldering. Similarly, the second solder layer is always used if the surface of the ultrasonic transducer is not directly suitable for soldering. For example, if a piezoelectric ultrasonic transducer is used, the second solder layer is used.

[0013] Surfaces of the connection elements that are particularly suitable for direct soldering may already be present if the surface of the connection element is already coated with a solderable material, for example in the form of a corrosion protection layer. Suitable directly solderable surface coatings include in particular pure zinc layers that are applied by hot-dip galvanizing or electroplating.

[0014] Preferably, both a first solder layer and a second solder layer are used in the layer structure to allow optimal soldering to the connection elements of the ultrasonic transducer.

[0015] Activating the reactive layer generates heat. If a corresponding solder layer is present, this heat at least partially causes the melting of the first and / or second solder layer, thereby providing a connection between the system of fully reacted reactive layers and the solder layer and thus between the connection element and the surface of the ultrasonic transducer. If the surface of the connection element itself is suitable as a solder layer, the first solder layer can be omitted, in which case the heat of the reactive layer at least partially melts the surface of the connection element. Similarly, if the surface of the ultrasonic transducer is suitable for direct soldering, the second solder layer can be omitted.

[0016] Activation of the reactive layer can be achieved, for example, by heating with a laser beam focused on the surface of the reactive layer. Alternatively, the reactive layer can be locally heated for a short period of time, for example by contacting the reactive layer with an electrode and introducing an electric current, where the connecting element serves as the second electrode.

[0017] Preferably, the connecting element is a screw, and preferably the surface of the connecting element is the surface of the screw head of the screw or the surface of the end of the screw opposite the screw head. The method is in principle suitable for screws of all sizes and shapes, and the advantages of the method are particularly evident for large screws, especially those of size M20 and above.

[0018] Preferably, the connecting elements are made of steel, stainless steel, high alloy steel, special steel, titanium and its alloys, TiAl6V4, aluminium and its alloys, nickel alloys (Inconel), non-magnetic steels such as A286, brass or copper and its alloys.

[0019] The piezoelectric ultrasonic transducer preferably comprises a piezoelectric element adapted to generate vibrations in ultrasonic waves when excited by a corresponding electrical signal. To input the electrical signal, the piezoelectric ultrasonic transducer preferably comprises an electrode on a surface, which can be designed, for example, as a printed electrode or an electrode coated by a deposition method. When the layer structure is formed, this surface provided with the electrode faces away from the surface of the connection element so that the electrode is accessible.

[0020] In particular, the electrodes can be thick-film electrodes produced by imprinting, for example with a thickness in the range of 5 μm to 10 μm, for example by imprinting a silver-containing paste by a screen printing method. Alternatively, the electrodes can be thin-film electrodes, for example with a typical thickness of about 1 μm, which can be obtained, for example, by sputtering nickel-chromium, gold or silver thereon.

[0021] In the manufactured load indicating connection part, the surface of the connection element preferably serves as the counter electrode. Advantageously, the fully reacted reaction layer and possibly the solder layer provide a good electrical connection to the side of the ultrasonic transducer facing the connection part.

[0022] The piezoelectric elements are preferably made from a piezoelectric ceramic such as lead zirconate titanate (PZT).

[0023] The piezoelectric ceramic can be provided, for example, in the form of a disk. The disk can have, for example, a diameter in the range of 1 mm to 10 mm, preferably in the range of 2 mm to 5 mm. A typical thickness is in the range of 0.1 mm to 0.75 mm, preferably in the range of 0.25 mm to 0.5 mm. In addition to having a disk shape, the piezoelectric ceramic can also have a square or rectangular base region with a side length in the range of 1 to 10 mm, preferably in the range of 2 mm to 5 mm. Again, a typical thickness is in the range of 0.1 mm to 0.75 mm, preferably in the range of 0.25 mm to 0.5 mm.

[0024] In one embodiment of the method, the layer structure includes applying a first solder layer on a surface of the connection element, applying a second solder layer on the ultrasonic transducer, cutting a reactive bonding foil constituting the reactive layer, placing the reactive bonding foil on the first solder layer of the connection element, and placing the ultrasonic transducer on the bonding foil, with the second solder layer facing the bonding foil.

[0025] In a variant of this method, a second solder layer is used next to the first solder layer on the surface of the connection element, this second solder layer being applied to the side of the piezoelectric ultrasonic transducer facing the bonding foil. The solder layer can be applied, for example, by applying molten solder or by a deposition method. The first and / or second solder layer can also be omitted if the corresponding surfaces of the connection element or ultrasonic transducer are suitable for direct soldering.

[0026] In one variant of the method, a bonding foil is used, which is already soldered both on the upper and lower sides next to the reactive layer structure, thus constituting the first and second solder layers. The application of the first solder layer on the connection element and the second solder layer on the ultrasonic transducer can therefore be omitted. In a variant of this method, it is necessary that the solder can be applied to each side of the connection element and the ultrasonic transducer facing the bonding foil. This means that in this case no additional layers, such as adhesion-promoting intermediate layers, are required to connect to the respective corresponding surfaces of the connection element and the ultrasonic transducer.

[0027] Preferably, the bonding foil is cut to dimensions such that the shape of the bonding foil corresponds to the shape of the ultrasonic transducer. In this case, activation of the reaction layer is preferably carried out by introducing the required activation energy from the side of the reaction layer.

[0028] Alternatively, it may be preferable to place the piezoelectric ultrasonic transducer on the ultrasonic transducer and then cut the bonding foil to a size such that no part of the bonding foil is covered by the ultrasonic transducer. For example, the shape of the bonding foil for this purpose can be chosen to be generally larger or to include the shape of a tab that is not covered by the ultrasonic transducer. In this case, activation of the reactive layer can be performed from the top side of the bonding foil.

[0029] Cutting the bonding foil to size can be done mechanically, for example by using a rolling knife. Particularly preferably, cutting the reactive bonding foil to size is done by laser cutting. To cut the bonding foil without activating the reactive system of the bonding foil, it is preferable to use a short pulse laser, in particular an ultrashort pulse laser (USP laser). The pulse duration of the laser pulse is in the picosecond (ps) or femtosecond (fs) range.

[0030] Instead of providing a reactive layer in the form of a joining foil, a layer structure can be formed by coating the surface of the ultrasonic transducer with a second solder layer and a reactive layer system. The coated ultrasonic transducer is then placed on the surface of the connecting element such that the reactive layer faces the surface of the connecting element. The surface of the connecting element is previously provided with a first solder layer. If the surface of the ultrasonic transducer is directly solderable, the second solder layer can be omitted. If a first solder layer is required, the same can be applied to the surface of the connecting element.

[0031] The first solder layer, if present, and / or the second solder layer, if present, is preferably made of a metal or metal alloy, such as gold, silver, tin and zinc. A suitable metal layer is, for example, a tin layer. In particular, the first solder layer and / or the second solder layer can be applied onto the corresponding surface as a film, a molded part, a powder or a paste, or can be applied by electroplating or deposition methods.

[0032] Alternatively, the first solder layer (if present) and / or the second solder layer (if present) preferably consist of so-called active solders. Active solders are metallic solders, which, due to their alloy composition, are also able to wet non-metallic inorganic materials other than metallic surfaces. As solder materials, such active solders contain the components silver, nickel, copper, gold and so-called active elements such as titanium, hafnium, zirconium or vanadium, which only provide the soldering process. Suitable active solders are available, for example, under the name Incusil®. For example, Incusil® ABA is an alloy of silver, copper, indium and titanium, which is suitable for direct application to ceramic surfaces. In particular, active solders can be applied onto the corresponding surfaces as films, molded parts, powders or pastes, or they can also be applied by electroplating or deposition methods.

[0033] Alternatively, it is preferred that the first solder layer (if present) and / or the second solder layer (if present) consist of a layer system comprising at least one adhesion-improving intermediate layer and a layer of solder material as main layer.

[0034] Suitable adhesion-improving intermediate layers include, in particular, layers made of titanium, chromium or nickel-chromium. The main layer of the layer system comprises the actual solder material, for example gold, silver, tin, zinc.

[0035] Preferably, at least one adhesion-improving intermediate layer is obtained by coating the surface of the connection element, the surface of the reactive layer or the surface of the piezoelectric ultrasonic transducer. Further layers containing solder can be correspondingly obtained by coating the respective free surfaces of the intermediate layers with solder material.

[0036] When the first solder layer and / or the second solder layer is present, their total thickness is preferably in the range of 10 μm to 200 μm, particularly preferably in the range of 20 μm to 100 μm, for example 30 μm.

[0037] For example, the solder layer may be designed as a tin layer having a thickness of 30 μm.

[0038] If the first and / or second solder layer is designed as a layer system, the thickness of the adhesion-improving intermediate layer is preferably selected in the range of 10 nm to 1 μm and the thickness of the main layer comprising the solder material is preferably selected in the range of 20 μm to 100 μm.

[0039] In order to generate a stable connection between the ultrasonic transducer and the connection element, the ultrasonic transducer is pressed against the connection element with a specified contact pressure at least during the connection operation, thereby achieving a full-surface and clear connection when the solder layer melts. The specified contact pressure is preferably specified in the range of 0.3 MPa to 3 MPa, for example 100 psi (about 0.69 MPa).

[0040] Furthermore, it is also possible to carry out the connecting operation in a protective gas atmosphere or in a vacuum. To carry out the connecting operation in a vacuum, the connecting element with the layer structure can be introduced into a receptacle, the air can be evacuated and the connecting operation can then be started by activating the reaction layer. During this operation, a specified contact pressure is applied, for example by means of a pressure device also integrated into the receptacle. After cooling, the connecting operation is completed and the resulting load-indicating connecting part can be removed.

[0041] If the joining operation is carried out in a protective gas atmosphere, this can be carried out in a corresponding chamber, as well as in a vacuum, or the ambient air can simply be replaced locally by flowing it over the formed layer system. A suitable protective gas is, for example, argon.

[0042] By carrying out the joining operation in a protective gas atmosphere or in a vacuum, the formation of oxides, in particular with atmospheric oxygen, is suppressed, thereby improving the quality of the soldered joints obtained.

[0043] Preferably, the reactive layer or reactive layer system comprises a system of alternating layers, the alternating layers being selected from Ni / Al, Al / Pd, Al / Ti and combinations of these alternating layers.

[0044] Preferably, the coating is carried out by using a vapor deposition method, in particular a physical vapor deposition (PVD) method or a chemical vapor deposition (CVD) method, to produce the solder layer, the reaction layer and / or the at least one intermediate layer.

[0045] Sputtering is the preferred PVD method, in which atoms are dislodged from a sputtering target by ion bombardment in a vacuum chamber and then deposited on the substrate to be coated.

[0046] The reactive layer can be produced, for example, by providing a substrate holder and arranging a number of piezoelectric ceramic sensors on the substrate holder. A reactive layer comprising a number of individual layers is then deposited on the piezoelectric material. The individual layers form a reactive layer system.

[0047] Preferably, the reactive layer or reactive layer system has a total thickness in the range of 10 μm to 100 μm. Depending on the properties of the solder layer and preferably on the thickness of the piezoelectric ultrasonic transducer used and the set contact pressure, the thickness of the reactive layer is selected such that, on the one hand, sufficient energy is available for the activation of the reactive layer to achieve a reliable connection, and, on the other hand, such that the piezoelectric ultrasonic transducer is not mechanically damaged by the thermal energy released in a short time.

[0048] Similarly, the thickness of the reactive layer or reactive layer system is selected according to the amount of heat released after activation, which is preferably such that the first solder layer and, if present, the second solder layer, are at least partially melted.

[0049] Suitable reactive layers are available in the form of bonded foils under the name NanoFoil® from Indium Corporation. These reactive layer systems consist of alternating layers of nickel and aluminum and are available with total thicknesses ranging from 40 μm to 80 μm. The maximum temperature during the reaction is in the range of 1350 °C to 1500 °C and the amount of heat released is in the range of 1050 to 1250 J / g. After activation of the reactive layer system, the composition of the fully reacted system is Ni 50 Al 50 become.

[0050] Preferably, the surface of the connection element is galvanized, in particular hot-dip galvanized, in which case the galvanization serves as the first solder layer.

[0051] Preferably, the surface of the connecting element is designed so that there are no embossments or marks in the area of ​​the screw head where the ultrasonic transducer is to be attached. More preferably, the corresponding surface is flat, which allows the ultrasonic transducer to be directly attached without prior mechanical processing of the screw head.

[0052] The invention further relates to a load-indicating connecting part obtained by one of the methods described herein, comprising a connecting element and a layer structure including a piezoelectric ultrasonic transducer arranged on a surface of the connecting element, the layer structure comprising, on the surface of the connecting element, in this order from the surface of the connecting element, a reacted reactive layer and an ultrasonic transducer.

[0053] A portion of the first solder layer may be disposed between the surface of the connection element and the reacted reactive layer, the portion being bonded to the surface of the connection element by the heat of reaction of the reactive layer and partially fused with the reactive layer. If a second solder layer is used, a portion of the second solder layer, possibly partially fused with the reacted reactive layer, may be disposed between the reacted reactive layer and the ultrasonic transducer.

[0054] Advantages of the Invention Conventional adhesive bonding of piezoelectric ceramic sensors, e.g. lead zirconate titanate disks (PZT disks), has various disadvantages. In addition to the strong temperature dependence and ageing of the adhesive layer and its lack of resistance to the influences of the chemical environment, there are also increasing requirements regarding the roughness and flatness of the surface of the connection element to which the piezoelectric element is adhesively bonded. In general, before applying the piezoelectric element, the connection element needs to be partially mechanically processed, preferably milled. Furthermore, adhesive connections require that drying or curing times be observed until the glued joint is stable and loadable.

[0055] According to the invention, these drawbacks are overcome by using a soldering process, in which the solder is able to completely fill the space between the components, thus compensating for surface defects of the components to be connected. Soldering provides a permanent and stable connection between the components. Moreover, in terms of the ultrasonic transmission parameters, the solder has similar properties to the metallic material of the connecting element.

[0056] In a typical soldering process, the entire connection element must be heated prior to the actual soldering operation to ensure a secure connection, and the heat introduced during the soldering operation must be sufficient to melt the solder between the connection element and the piezoelectric ceramic sensor.

[0057] However, fully heating the connection elements before soldering is often not permitted due to the very high costs and high temperatures required. For the reasons mentioned above, conventional soldering of the piezoelectric ceramic sensor to the connection elements is generally not possible.

[0058] The proposed method provides the heat required for soldering via a reactive layer arranged between the load-indicating components and the components to be connected, the energy released for a short time upon activation of this reactive layer is sufficient to generate such temperatures on a locally limited scale, even in connection elements with large thermal masses, such as, for example, wind turbine screws, that provide a reliable connection between the connection element and the piezoelectric ultrasonic transducer.

[0059] The advantage is that the heat is released suddenly over a short period of time, so that heating is substantially limited to the reactive system itself, the adjacent solder layer, and a small area on the surface of the connecting or piezoelectric element, thereby avoiding damage to the components being connected and allowing for a reliable solder connection. [Brief description of the drawings]

[0060] Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

[0061] [Figure 1] FIG. 13 illustrates the connection of an ultrasonic transducer and a connection element using a reactive layer system before the reactive layer system reacts.

[0062] [Diagram 2] FIG. 2 shows a load indicating connection part to which an ultrasonic transducer is connected.

[0063] [Diagram 3] FIG. 2 shows a schematic diagram of a pressure device for generating a specified contact pressure.

[0064] [Figure 4] FIG. 2 shows a comparative measurement between a load indicating connection piece according to the invention and two comparative examples.

[0065] In the following description of exemplary embodiments of the invention, similar parts and elements are designated with the same reference numerals and repeated description of these parts or elements is omitted in individual cases. In the drawings, the subject matter of the invention is represented only diagrammatically.

[0066] 1 shows diagrammatically the manufacture of a load indicating connection part 1. An ultrasonic transducer 20 and a connection element 10 are interconnected by using a reactive layer system 30.

[0067] In the situation shown in Fig. 1, the connection element 10 is a screw. An ultrasonic transducer 20 is intended to be connected to a surface 14 of the connection element 10 in the region of the screw head 12 of the screw. The connection is made by a soldering process. For this purpose, the surface 14 is provided with a first solder layer 16 and the surface of the ultrasonic transducer 20 is provided with a second solder layer 22.

[0068] For example, the surface 14 of the connection element 10 and the surface of the ultrasonic transducer 20 can be provided with a tin coating as the first solder layer 16 and the second solder layer 22 in a simple manner by the PVD method of sputtering.

[0069] To form a stable solder connection between the connecting element 10 and the ultrasonic transducer 20, the two solder layers 16 and 22 must be melted at least on their surfaces so that a materially bonded connection can be formed. For this purpose, a reactive layer 30 is provided which comprises a reactive layer system of alternating layers, e.g. nickel layers alternating with aluminum layers. The reactive layer system is configured such that after activation, e.g. by the introduction of heat, the thermal energy required for the creation of the solder connection is released in an exothermic reaction.

[0070] In the layer system formed on the surface 14 of the connection element 10, the reactive layer 30 is arranged between the first solder layer 16 and the second solder layer 22. In the example shown in FIG. 1, the surface of the reactive layer 30 is selected to correspond to the surface of the ultrasonic transducer 20, which completely covers the reactive layer 30. The total thickness of the reactive layer 30 is selected so that the reaction of the reactive layer releases enough heat to at least melt the first and second solder layers 16, 22 and materially connect them to the reactive layer 30. After activation, the reactive layer 30 releases heat in a short time, so that the ultrasonic transducer 20 and the connection element 10 are hardly heated.

[0071] To activate the reactive layer 30, activation energy must be provided. In the example outlined in FIG. 1, a laser beam 42 is focused by the laser 40 from the side onto the reactive layer 30, thus introducing the necessary activation energy at the edge of the reactive layer 30. When the layer 30 is cut to size such that its surface area is larger than that of the ultrasonic transducer 20, activation energy can also be introduced at the top side of the reactive layer 30. After activation of the reactive layer 30, the reactive layer 30 is heated via an exothermic reaction between the alternating layers contained therein, which reaction spreads over the entire volume of the reactive layer 30, melting the reactive layer 30 and parts of the adjacent solder layers 16, 22.

[0072] In order for the molten layer to form a tight connection and for any possible surface irregularities to be filled and compensated for by the molten material, the ultrasonic transducer 20 must be pressed against the connecting element 10 with a defined contact pressure during the soldering operation. In the example shown in FIG. 1, the pressing force required to generate the defined contact pressure is transferred to the ultrasonic transducer 20 via a punch 50.

[0073] After cooling, the solder connection between the connecting element 10 and the ultrasonic transducer 20 is formed and the punch 50 can be removed. The reaction that occurs within the reactive layer 30 is fast so that the bond typically takes less than a second.

[0074] 2 shows a load-indicating connection part 1 obtained according to the method outlined in FIG. 1. The connection part 1 comprises a connection element 10, in this example designed as a screw, and an ultrasonic transducer 20, which are materially connected by a solder connection. A part of the first solder layer 16 is arranged between the surface 14 of the connection element 10 and the reacted reaction layer 32, and is bonded to the surface 14 of the connection element 10 by the reaction heat of the reaction layer 30 and is partially fused with the reacted reaction layer 32. A part of the second solder layer 22 is located between the reacted reaction layer 32 and the ultrasonic transducer 22, and a part of the second solder layer 22 is fused with the reacted reaction layer 32.

[0075] FIG. 3 shows a schematic diagram of a pressure device 60 for generating a defined contact pressure for pressing the ultrasonic transducer 20 against the connection element 10 during a soldering operation.

[0076] The pressure application device 60 comprises a base 61, on which the connection element 10 is arranged with the layer system formed on its surface 14 and comprising the reactive layer 30 and the ultrasonic transducer 20. The pressure application device 60 further comprises a linear unit 62 for height adjustment. A linear guideway 63 is attached to the linear unit 62, which comprises a movable carriage 64 and a stop 65. A spring pin receiver 69 with a platform 66 is attached to the carriage 64 of the linear guideway. The movable carriage 64 initially rests on the stop 65.

[0077] The platform 66 serves as a support for a movable weight 67. In the example of FIG. 3, a single weight 67 is shown, but it is also possible to use multiple weights 67 to specify the desired pressing force. The spring pin receiver 69 serves as a holder for the spring pin 68, whose function is to transmit the pressing force to the ultrasonic transducer 20 as a predetermined contact pressure. The spring pin 68 is therefore pre-tensioned by a spring into a fully extended position. To transmit the contact pressure, the spring pin 68 includes a punch 50 at the end facing the ultrasonic transducer 20, which serves as a contact surface.

[0078] After positioning the connection element 10 together with the layer system comprising at least the reaction layer 30 and the ultrasonic transducer 20, the stop 65 is lowered using the linear unit 62 and the parts connected to the movable carriage 64 are received in the linear guideway 63, i.e. the spring pin 68, the spring pin receiver 69, the platform 66 and the weight 67 are likewise lowered following the stop 65.

[0079] During descent, the ultrasonic transducer 20 comes into contact via the punch 50 of the spring pin 68. During further descent, the force on the ultrasonic transducer 20 increases and the spring pin 68 retracts more and more. For example, the spring action of the spring pin 68 can be selected such that a pressing force in the range of 1-3 N is achieved until the spring pin 68 is fully retracted.

[0080] When the stop 65 is further lowered by the linear unit 62 after the spring pin 68 has fully retracted, the spring pin receiver 69 no longer rests on the stop 65 and moves within the linear guideway 63 relative to the stop 65. As a result, the force of the entire weight of the weight 67, the platform 66, the spring pin receiver 69 and the spring pin 68 is transmitted to the ultrasonic transducer 20.

[0081] The corresponding pressing force is specified by selecting the mass of the weight 67. The pressing force is transmitted to the ultrasonic transducer through the contact surface of the spring pin 68, and the contact pressure is determined by the force of the weight and the surface area of ​​the ultrasonic transducer 20.

[0082] The size of the punch 50 for the spring pin 68 should be selected so that its contact surface corresponds as closely as possible to the surface area of ​​the ultrasonic transducer 20 facing the spring pin 68. The pressure device 60 is designed so that the spring pin receiver 69 and the spring pin 68 can be easily changed to ensure rapid adaptation to changes in ultrasonic transducer size.

[0083] Figure 4 shows a comparative measurement of the ultrasonic propagation time of a load-indicating connection part according to the invention, in which the ultrasonic transducer is soldered to the connection element, in comparison with two load-indicating connection parts as comparison examples. The ultrasonic transducers according to the prior art were attached by adhesive. The diagram in Figure 4 shows the propagation time t of sound measured by time-of-flight measurements as a function of the temperature T of the respective connection part.

[0084] In time-of-flight measurements using ultrasonic transducers, an ultrasonic signal is emitted in the form of a time-limited pulse. The ultrasonic pulse passes through a connecting element and is reflected at the end of the connecting element. The reflected ultrasonic signal is measured again as an ultrasonic echo. The propagation time of sound, t, is the time between transmitting the ultrasonic pulse and receiving the ultrasonic echo.

[0085] The load-indicating connections used each contain identical M12x50 screws made of steel as connection elements, in the head of which an ultrasonic transducer is attached.

[0086] The ultrasonic transducer used was a piezoelectric ceramic with a diameter of 5 mm and a thickness of 0.5 mm.

[0087] In the load-indicating connection component according to the invention, the ultrasonic transducer is soldered to the screw using a bonding foil. For this purpose, both the screw head and the piezoelectric ceramic are provided with a tin solder layer. For this purpose, a 30 μm thick tin layer is deposited on the surface by the sputtering method.

[0088] To connect the ultrasonic vibrator and the screw, a 40 μm thick Al / Ni bonding foil NanoFoil (registered trademark) manufactured by Indium Corporation was used, which was cut with a short pulse laser to match the diameter of the ultrasonic transducer.

[0089] In both comparative examples, the ultrasonic transducer was glued to the screw: in the first comparative example, an anaerobic acrylate adhesive was used, and in the second comparative example, an epoxy adhesive was used.

[0090] Figure 4 shows the measured sound propagation times as a function of temperature for three M12x50 screws. A first curve 101 shows the sound propagation times measured for a load indicating connection part according to the invention. A second curve 102 shows the sound propagation time measurements for a first comparative example and a third curve 103 shows the sound propagation time measurements for a second comparative example. It can be seen that for all three curves 101, 102, 103 the measured sound propagation times increase with increasing temperature.

[0091] Starting from -20 ° C, the propagation time of the screw increases gradually with each temperature level. The increase in temperature is performed in 20 K steps. Up to a temperature level of 100 ° C, the curves 101, 102, 103 extend similarly. At temperature levels of 120 ° C and 140 ° C, both ultrasonic transducers bonded with adhesive show signs of failure, which can be recognized by a sudden upward deflection according to the comparative example. A similar phenomenon is caused by a temporary loss of contact of the electrode on the side of the ultrasonic transducer facing the head of the screw. If the user or the automatic measuring system does not recognize these problems, erroneous measurements will occur. Equally problematic, since they are not always immediately recognizable, are the changes in the echo structure of the ultrasonic signal, caused by thermal and environmental effects on the adhesive layer and its aging. These problems also lead to erroneous measurement results. The use of ultrasonic transducers applied with bonding foils allows the above problems to be solved.

[0092] Thus, in comparison, the measurements shown in FIG. 4 clearly demonstrate that an exemplary embodiment of the present invention with a soldered ultrasonic transducer can reliably measure ultrasonic propagation time even at high temperatures, in excess of 100° C.

[0093] The present invention is not limited to the exemplary embodiments and aspects highlighted herein described, but rather is susceptible to numerous modifications within the scope of the invention as defined by the claims and within the scope of the activities of those skilled in the art. [Explanation of symbols]

[0094] 1: Load display connection parts 10: Connection elements 12: Screw head 14: Surface 16: 1st solder layer 20: Ultrasonic transducer 22: Second solder layer 30: Reaction layer 32: Reacted reaction layer 40: Laser 42: Laser beam 50: Punch 60: Pressure device 61: Bass 62: Height adjustment linear unit 63: Linear guideway 64: Movable carriage of linear guideway 65: Stop 66: Platform 67: Weight 68: Spring pin 69: Spring pin receiving part 101: First Curve 102: The second curve 103: The third curve

Claims

1. A method for manufacturing a load indicating connection part (1), in which a connection element (10) and a piezoelectric ultrasonic transducer (20) are provided and interconnected, said method comprising the steps of: a) forming a layer structure on the surface (14) of the connection element (10), comprising, in this order from the surface (14) of the connection element (10), a first solder layer (16), a reaction layer (30), a second solder layer (22) and an ultrasonic transducer (20), the reaction layer (30) having a thickness in the range of 10 μm to 100 μm and adapted for an exothermic reaction by activation by heat, electromagnetic radiation or electric current; b) pressing said ultrasonic transducer (20) towards said connecting element (10) with a specified contact pressure; c) activating said reactive layer (30).

2. 2. The method of claim 1, wherein the layer structure is formed by applying the first solder layer (16) on the surface (14) of the connection element (10), applying the second solder layer (22) on the ultrasonic transducer (20), cutting a reactive bonding foil constituting the reactive layer (30) to a predetermined size, placing the reactive bonding foil on the first solder layer (16) on the connection element (10), and placing the ultrasonic transducer (20) on the bonding foil, with the second solder layer facing the bonding foil.

3. 3. The method of claim 2, characterized in that the bonding foil is cut to size such that the shape of the bonding foil corresponds to the shape of the ultrasonic transducer (20).

4. 3. The method according to claim 2, characterized in that after placing the ultrasonic transducer (20) on the bonding foil, the bonding foil is cut to a size such that no part of the bonding foil is covered by the ultrasonic transducer (20).

5. 5. The method according to claim 2, wherein the cutting to size of the bonding foil is performed by laser cutting.

6. 2. The method of claim 1, wherein the layer structure is formed by coating a surface of the ultrasonic transducer (20) with the second solder layer (22) and the reaction layer (30) and placing the coated ultrasonic transducer (20) on the surface (14) of the connection element (10) coated with the first solder layer (16), the reaction layer (30) facing the surface (14) of the connection element (10).

7. 7. The method according to claim 1, characterized in that as the first solder layer (16) and / or the second solder layer (22) a solder layer is used which consists of a layer system comprising at least one adhesion improving intermediate layer and a main layer comprising a solder material.

8. The method according to any one of claims 1 to 7, characterized in that the specified contact pressure is specified in the range of 0.3 MPa to 3 MPa.

9. 9. The method according to claim 1, characterized in that as the reaction layer (30) a reaction layer is used which comprises an alternating layer system selected from Ni / Al, Al / Pd, Al / Ti or Ti / Ni.

10. Method according to any one of the preceding claims, characterized in that the surface (14) of the connecting element (10) is galvanized.

11. 11. Method according to any one of the preceding claims, characterized in that the connecting element (10) is manufactured from the group of materials: steel, stainless steel, high alloy steel, special steel, titanium and its alloys, TiAl6V4, aluminium and its alloys, nickel alloys (Inconel), non-magnetic steels such as A286, brass or copper and its alloys.

12. 12. The method according to claim 1, wherein the connecting element (10) is a screw and the surface (14) of the connecting element (10) is a surface on the screw or on the end of the screw opposite the screw head.

13. A load indicating connection part (1) obtained by the method according to any one of claims 1 to 12, comprising a connection element (10) and a layer structure arranged on a surface (14) of the connection element (10), the layer structure comprising, in this order, a reaction layer (32) reacted from the surface (14) of the connection element (10) and an ultrasonic transducer (20).

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