METHOD FOR TESTING AN AIRBAG COVER WITH A DEFINED BREAK LINE AND DEFINED TEAR RESISTANCE

Description

[0001] The invention relates to a method for the non-destructive testing of an airbag cover, namely the testing of whether a predetermined tear line incorporated into the airbag cover exhibits a specified tear resistance. It is particularly advantageous when the airbag cover is made of a non-transparent material.

[0002] To ensure that an airbag located behind an airbag cover can reliably deploy in an emergency, it is known to provide the airbag cover with a predetermined breaking line that tears open when the airbag deploys, allowing the airbag to pass through the resulting opening. The tear resistance must not be too high, so that the predetermined breaking line tears open reliably and completely when the airbag deploys. At the same time, it must not be too low, to prevent the predetermined breaking line from being damaged by unintentional mechanical impacts from vehicle occupants. A specified tear resistance must meet both of these conditions. This does not have to be an exact value, but can be a range of values. A specified constant tear resistance, as described below, is a range of values ​​within a tolerance limit.A tear resistance that varies along the length of a predetermined line will accordingly also have a different local range of values. The tear resistance can be constant along the length of the predetermined breaking line, but it can also be subject to a defined change, for example, by being lower locally where the tearing is to be initiated.

[0003] When testing such a predetermined breaking line, also called a weak line or tear line, it is important that the measurement taken for testing is sufficiently accurate and reproducible.

[0004] To test the tear resistance of the predetermined breaking line, it is important to determine or know both the remaining wall thickness (thickness of the remaining material below the resulting blind holes or grooves) and the density of the material.

[0005] For an airbag cover made of a homogeneous material with a homogeneous density and a constant thickness, determining the remaining wall thickness is sufficient to assess the tear resistance. In this case, a constant remaining wall thickness equates to a constant tear resistance.

[0006] Known non-destructive testing methods are used exclusively during the machining of the airbag cover.

[0007] All subsequent publications aim to test a material or the predetermined breaking point of an airbag cover during processing.

[0008] US Patent 5,103,676 A discloses a method for non-contact process monitoring using ultrasound. In this method, the surface of a material is illuminated with a pulsed laser, generating ultrasound at the surface of the illuminated material. This ultrasound propagates through the material and is reflected at another, opposite surface within the material. Upon returning to the surface where it originated, it causes deformations. These deformations can be measured by changing the frequency of a detection light, preferably a laser, using an interferometer. The transit time of the ultrasound waves through the material is determined. The method disclosed here makes it possible to measure the density and elastic properties of a material without contact and without irreversibly altering the material.

[0009] JP 2005 017 130 A discloses a method for measuring the residual wall thickness of an airbag cover using an ultrasound transducer. Ultrasound is applied along the predetermined breaking line, which is propagated and reflected by the airbag cover. The residual wall thickness can be determined by detecting the ultrasound. The method is said to be suitable for measuring predetermined breaking lines that are too close together to be characterized using a laser.

[0010] EP 1 459 943 B1 discloses a device and a method in which an ultrasonic sonotrode designed as a cutting blade is moved along a line, cutting a predetermined breaking line into the material of an airbag cover. During cutting, the distance between a distance sensor carried with the cutting blade and a counter-cutting plate holding the material to be cut is measured. The remaining wall thickness is calculated as the difference between the distance between the distance sensor and the counter-cutting plate, and the distance between the distance sensor and the base surface. The measuring method used here is dependent on the use of a mechanical tool whose penetration depth determines the depth of the cut into the material.

[0011] DE 10 2013 202 081 B3 discloses a device for making cuts in a planar material web and a method for controlling a cutting device. The device includes a cutting device for making cuts in the material web, a support surface for the planar support of the material web, a measuring device for determining a remaining residual wall thickness along the cutting path, and a control and regulating device that, based on the determined measured values ​​and depending on predetermined setpoints, provides corresponding control signals that are transmitted to a drive unit, which, based on the control signals, causes an adjustment of the relative position of the cutting device to the support surface.Determining the remaining wall thickness is possible here if the cutting device and the support surface are made of an electrically conductive material and a voltage source is connected to both. The measuring device measures the electrical current flowing between the cutting device and the support surface during cutting, which depends on the specific resistance of the material web between them. Unlike the aforementioned methods, the measurement result is then a value that does not change solely depending on the remaining wall thickness if the material is inhomogeneous. However, a disadvantage is that this measuring method can only be implemented with a mechanical tool and the material being processed must be conductive.

[0012] A method for the non-destructive testing of the properties of a solid before or after processing is described in US Patent 8,210,045 B2. This laser-based method generates ultrasound by scanning a solid with a continuous wave (CW) high-power laser. This causes thermoelastic expansion within the solid with specific rise and fall times, generating and propagating longitudinal, transverse, or surface waves. By measuring these various wave components, a non-destructive analysis of properties such as wall thickness, porosity, or the presence of defects in the solid is possible. Local changes in the physical properties of the solid cause changes in the amplitude of the detected signal. Unlike pulsed lasers, the scan rate in this method is not limited by the repetition rate of the laser pulses.For the use of this method to test a predetermined breaking point in an airbag cover, it is particularly disadvantageous that the measurement of the wall thickness with a resolution of approximately 0.1 cm is too inaccurate.

[0013] It is known to experts that acoustic emission measurements can be used to test materials such as metals, ceramics, glass, rocks, concrete, brittle polymers, wood, and composite materials, detecting damage to components such as cracking, crack propagation, delamination, dislocation movement, phase transformation, and corrosion processes. The respective damage leads to a significant change in the sound wave emitted by the component.

[0014] US Patent 2002 / 0152813 A1 discloses a method for measuring a physical parameter in which a measurement signal is generated, its frequency information is extracted, and then converted into the physical parameter. The physical parameter can be the thickness of a thin film, and the measurement signal can be a laser. The method is limited to the characterization of thin films.

[0015] US Patent 8,301,029 B2 discloses an acousto-optic converter comprising a laser, an optical detector, and a beam splitter. The beam splitter divides the laser beam into a first and a second beam. The first beam travels a path length exposed to an acoustic field. The second beam travels an acoustically isolated path length. The propagation speed of the first beam changes due to density variations induced by the acoustic field. This alters the phase relationship between the two partial beams, and the optical detector generates an electrical signal directly proportional to the phase shift of the first beam.

[0016] In an advantageous embodiment, the first and second beams are each reflected several times by two plane-parallel mirrors, the space between one pair of mirrors being exposed to an acoustic field, while the space between another pair of mirrors is acoustically isolated.

[0017] From US patent 2015 / 0139451 A1, a device is known which includes a measuring interferometer and at least one reference interferometer. The signal-to-noise ratio can be improved by using multiple reference interferometers. Light from the same source is coupled into the interferometers. The measuring interferometer is affected by changes in a physical parameter, while the at least one reference interferometer is isolated from these influences. Changes in one or more physical parameters alter the intensity of the light coupled out of the measuring interferometer. These changes are measured by a photodetector, thus detecting the changes in the physical parameter(s) by measuring the change in the intensity of the light emitted by the interferometer.The device also includes a signal processor which generates a differential signal from the respective detected signals of the measuring and reference interferometers.

[0018] The aforementioned US patent 2015 / 0139451 A1 also discloses an implementation of the device as an optical microphone. For this purpose, the measuring interferometer is acoustically coupled to the environment, and the physical parameter is atmospheric pressure. Local changes in atmospheric pressure or density, caused by a sound field, result in local changes in the refractive index of the interferometer. This modulates the amplitude of the laser radiation coupled out of the interferometer. The intensity of the coupled-out light depends on the incoming sound wave.

[0019] This optical microphone enables the measurement of sound waves with a wide bandwidth (up to 1 MHz in air, up to 25 MHz in liquids). It can also measure relatively high frequencies (>100 kHz). Conventional methods are typically limited to a bandwidth of approximately 100 kHz. The measurement method is contactless, and the measuring device is diaphragm-free and very compact. Therefore, it is not susceptible to interference from mechanical vibrations. Another advantage over optical measurement methods is that electromagnetic radiation from the environment does not affect the detected signal.

[0020] The article "Acoustic Monitoring for Laser Material Processing," published in the journal "Laser," issue 2 / 2019, explains that capturing airborne acoustical emissions in the frequency range of 10 Hz to 1 MHz, now possible with an optical microphone, opens up new avenues for testing and characterizing machine processes. Acoustic emissions in air are measured, and the resulting data curves are used to determine material parameters. The application examples presented include the detection of irregularities or defects on surfaces, the monitoring of surface conditions (such as zinc coatings), and the real-time detection of crack formation in laser welding processes, as well as the identification of spatter formation during welding. These applications serve the purpose of process control.

[0021] The object of the invention is to find a method for testing an airbag cover with a predetermined breaking line, in which the tear resistance can be tested non-destructively after completion of the predetermined breaking line, regardless of the transparency of the airbag cover.

[0022] This problem is solved for a method for testing an airbag cover with a predetermined breaking line. An initiator is guided along the breaking line relative to the airbag cover, applying energy at the test points along the breaking line, thereby generating sound waves. An optical test signal is generated as an interference signal with a sound signature of the sound waves, which is compared with a reference signal to which a predetermined tear resistance of the breaking line is spatially assigned.

[0023] Advantageously, the interference signal is generated from a measuring laser beam, at least a portion of which is guided along a path through a medium that is influenced by the sound waves caused by the initiator at the respective location to be tested, so that a phase shift required to generate the interference signal is induced.

[0024] It is advantageous if the route is arranged in a fixed relative position to the respective location to be tested.

[0025] A mechanical initiator is advantageous.

[0026] From a measurement perspective, the path is more advantageous along a side of the airbag cover facing away from the initiator.

[0027] Alternatively, the initiator is advantageously a laser beam with laser parameters different from the measuring laser beam, wherein the measuring laser beam is advantageously a beam component coupled out of the laser beam.

[0028] A quality pass for the tested airbag cover can be created from the instantaneous values ​​of the test signal assigned to the test locations, which can be stored.

[0029] The method according to the invention is explained in more detail below using various exemplary embodiments with reference to drawings.

[0030] Show this: Fig. 1 shows a device setup for a first embodiment of the method with a laser beam as an initiator for generating the sound at the test location; Fig. 2 shows a device setup for a second embodiment of the method with a laser beam as an initiator for generating the sound at the test location; Fig. 3 shows a device setup for a third embodiment of the method with a laser beam as an initiator for generating the sound at the test location and a measuring laser beam coupled from it; and Fig. 4 shows a device setup for a fourth embodiment of the method with a milling cutter as an initiator for generating the sound at the test location.

[0031] In all embodiments of the method, an initiator is guided along a predetermined breaking line 3 relative to an airbag cover 2. The initiator can be either an optical initiator, e.g., a laser beam LS, or a mechanical initiator, e.g., a blunt knife 9. With an optical initiator, the energy required to generate the sound is supplied by the absorption of photons in the material. With a mechanical initiator 9, energy is supplied by mechanical friction. In both cases, the characteristics of the generated sound depend on the properties of the remaining wall below the initiator, such as its density or thickness. These properties determine the tear resistance of the airbag cover 2 along the predetermined breaking line 3.The tear resistance is a parameter that depends on the remaining wall thickness, as well as the density and chemical composition of the material being processed, and can vary locally along the predetermined breaking line 3 of an airbag cover 2. For an airbag cover 2 made of a material with a homogeneous density distribution, a constant tear resistance is achieved if the remaining wall thickness is constant. If the density distribution of the material varies along line L, then the remaining wall thickness is generally dependent on the density distribution in order to guarantee a defined tear resistance along the predetermined breaking line 3. For quality assurance, it is necessary to be able to reproducibly quantify the tear resistance of an airbag cover along the predetermined breaking line.

[0032] To test the predetermined breaking line 3, an optical test signal is generated and compared with a reference signal for evaluation. The test signal can be understood as a sequence of instantaneous values, each assigned to a location to be tested on a surface. An instantaneous value, as described here, represents a segment of the test signal that is assigned to a location over the time the initiator spends at that location. In the case of a slot or multiple slots with interruptions, the locations to be tested can be directly adjacent to one another. In the case of blind holes, for example, they are separated by preferably regular intervals.

[0033] The essential aspect of the invention is that the optical test signal is an interference signal which is influenced by an acoustic signal (sound waves or a sound field) which is generated at the location to be tested by the action of the initiator on the airbag cover 2, which is why it is referred to as an interference signal with a sound signature.

[0034] To generate the interference signal, a measuring laser beam ML is used to create interfering partial beams. These partial beams are either influenced by the sound waves over distances of varying lengths, or only one portion of the measuring laser beam ML, i.e., one of the two partial beams T1 and T2, is influenced by the sound field, while the other partial beam is acoustically isolated. Crucially, the distance S over which at least one of the partial beams is influenced by the sound field must have the same relative position and length with respect to the locations being tested, ensuring that the influence occurs under identical conditions regardless of the specific location being tested.A part of a device setup used to carry out the method, responsible for measuring this measurement signal, in particular a measuring laser beam source 4 and an optical detector 5, must be moved synchronously depending on whether the initiator or the airbag cover 2 performs the movement required for the test. The path of influence can be located either on the side facing the tool or, preferably, on the side of the airbag cover 2 facing away from the laser beam LS or the mechanical initiator 9.

[0035] A first embodiment of a method according to the invention is described below with reference to Fig. 1The diagram describes a suitable device setup, illustrated in a schematic diagram. A laser beam source 1 is shown, which, as the initiator, directs a laser beam LS along a predetermined breaking line 3, perpendicular to the plane of the drawing, relative to an airbag cover 2. Upon impact of the laser beam LS, sound waves SW are generated at the respective point being tested. The properties of these sound waves are characteristic of the parameters of the remaining wall material below the predetermined breaking line 3 and propagate, forming a sound field. This sound field extends in all directions from its source, the point of impact of the initiator on the airbag cover. The propagation of the sound waves is indicated in the figures within a region relevant to its effect. The energy input from the laser beam LS is so low that no material is removed from the airbag cover at the respective point of impact.

[0036] To obtain an optical signal influenced by the generated sound waves, the part of the device setup relevant for measuring the test signal comprises a measuring laser beam source 4, an optical detector 5, and four mirrors 6.1-6.4. A measuring laser beam ML emitted by the measuring laser beam source 4 is split into a first partial beam T1 and a second partial beam T2 by a partially transparent first mirror 6.1. During its propagation over a distance S, i.e., a path of defined length, the first partial beam T1 is exposed to the sound waves SW generated at the respective location under test. These sound waves cause pressure changes in the medium along the path over which the first partial beam T1 propagates in a fixed position relative to the location under test. These pressure changes lead to a change in the refractive index and thus to a change in the propagation speed of the first partial beam T1.The phase of the first partial beam T1 is thus shifted. The second partial beam T2 is guided through a vacuum, completely unaffected by the sound waves SW, before being deflected via two mirrors 6.2 and 6.3 and then via a partially reflective mirror 6.4, where it is brought into interference with the first partial beam T1. The resulting interference signal is detected by the optical detector 5. Assuming that the temperature and wavelength of the measuring laser beam are constant, the phase of the first partial beam T1 shifts depending on the pressure changes caused by the sound waves, and the detected interference signal allows conclusions to be drawn about the pressure changes and thus the characteristics of the sound waves. The refractive index of the medium located along the path S is modulated by the sound waves.The generation of the detection signal by interference between a first partial beam T1, influenced by the sound waves, and an uninfluenced second partial beam T2 has the advantage that fluctuations in the parameters of the measuring laser beam ML do not affect the measurement result. For an objective and reproducible measurement, it is important that the geometric path traversed by the first partial beam T1 has a constant length and the same relative position to the object being tested throughout the entire procedure.

[0037] A second embodiment of a method according to the invention is described using the example in Fig. 2 The schematically depicted device setup is explained.

[0038] In this embodiment, unlike the previously described embodiment, an interference signal is generated by coupling a measuring laser beam ML into a resonator. The medium between the resonator mirrors 7.1, 7.2 is exposed to the sound waves generated by the energy input, resulting in a phase shift in the waves circulating in the resonator. This phase shift causes the signal coupled out of the resonator 7 to be modulated, due to the interference of the measuring laser beam ML with itself within the resonator, depending on the pressure differences generated by the sound waves SW.

[0039] To eliminate fluctuations in the laser parameters from the measurement result, it is advantageous to decouple a small part of the laser beam LS and detect it through a monitoring detector 8, which serves to monitor the laser parameters during the testing process.

[0040] A third embodiment of a method according to the invention differs in that the measuring laser beam ML is a beam component coupled out of the laser beam LS. For this purpose, a device setup is used, as shown in Fig. 3 , which in its part relevant for measurement is set up according to the second embodiment.

[0041] According to a fourth embodiment, in which the measuring laser beam is also coupled out of the laser beam, the phase shift required for interference is realized according to the second embodiment.

[0042] Other embodiments differ from those mentioned above by using a different initiator. Instead of a laser beam, as in Fig. 4As shown, a mechanical initiator 9, e.g. in the form of a blunt knife, a peeling knife or a milling cutter, is used. The test signal is acquired analogously to the previously described embodiments.

[0043] The route S is according to the Figs. 1 and 2 on the side facing away from the laser beam LS or the mechanical initiator 9 and according to the Fig. 3 and 4 on the side facing the laser beam LS or the mechanical initiator 9. The arrangement of the path S is interchangeable for the embodiments and thus for the figures.

[0044] When the path S is arranged on the side of the airbag cover facing away from the mechanical initiator 9, it is advantageous that the tool is unable to influence the sound waves, and the generated sound waves only pass through the remaining wall. This means that the material adjacent to the remaining wall, which defines the target line 3, has at least a lesser influence on the propagation of the sound waves and thus on their parameters. The parameters of the sound waves are more clearly influenced by the parameters of the remaining wall than the sound waves on the side facing the tool. Arranging the path S on the side facing away from the initiator is particularly advantageous if the airbag cover 2 consists of a layered material and the remaining wall is formed by only a portion of one layer.

[0045] In all embodiments, to test the tear resistance at the respective locations to be tested, the detected interference signal is compared with a reference signal that is characteristic of the defined tear resistance of the airbag cover 2 along the predetermined breaking line 3.

[0046] The reference signal was previously obtained by testing one or more airbag covers, which demonstrably exhibit the expected tear resistance along their predetermined breaking point, using the method described above. The instantaneous values ​​of the test signal detected at each location under test are then stored as a reference signal and assigned to a specific time period. List of reference symbols

[0047] 1 Laser beam source 2 Airbag cover 3 Break line 4 Measuring laser beam source 5 Optical detector 6.1-6.4 Mirrors 7 Resonator 7.1, 7.2 Resonator mirrors 8 Monitoring detector 9 Mechanical initiator SW sound waves S distance ML measuring laser beam T1 first partial beam T2 second partial beam LS laser beam

Claims

1. A method for testing an airbag cover (2) comprising a predetermined breaking line (3), wherein an initiator is guided along the predetermined breaking line (3) relative to the airbag cover (2), while the initiator introduces energy at locations to be tested along the predetermined breaking line (3), thereby causing sound waves (SW), characterized in that an optical test signal is generated as an interference signal with a sound signature of the sound waves (SW), which is compared with a reference signal to which a predetermined tear resistance of the predetermined breaking line (3) is assigned in relation to the location.

2. The method according to claim 1, characterized in that the interference signal is generated from a measuring laser beam (ML), at least a portion of which is guided along a path (S) through a medium which is influenced by the sound waves (SW) caused by the initiator at the respective location to be tested, so that a phase shift required to generate the interference signal is induced.

3. The method according to claim 2, characterized in that the path (S) is arranged in a fixed position relative to the respective location to be tested.

4. The method according to claim 1 or 2, characterized in that the initiator is a mechanical initiator (9).

5. The method according to claim 4, characterized in that the path (S) is arranged along a side of the airbag cover (2) facing away from the initiator.

6. The method according to claim 2, characterized in that the initiator is a laser beam (LS) with laser parameters differing from those of the measuring laser beam (ML).

7. The method according to claim 2, characterized in that the initiator is a laser beam (LS) and the measuring laser beam (MS) is a beam portion decoupled from the laser beam (LS).

8. The method according to any one of the preceding claims, characterized in that instantaneous values of the test signal assigned to the locations to be tested are stored and a quality pass for the tested airbag cover (2) is created from the stored instantaneous values.