Method for nondestructively evaluating junction structure

Abstract

To provide a method with which it is possible to easily and nondestructively evaluate cracks generated in the adhesive layer of sheet members.SOLUTION: The present invention is a method for nondestructively evaluating cracks generated in an adhesive layer (3) of a joined body (M) of a first sheet member (1) and a second sheet member (2). The method includes: a transmission step for transmitting an ultrasonic wave (W1) in a diagonal direction to the first sheet member; a reception step for receiving Lamb waves (L2, W2) of the ultrasonic wave having propagated via the adhesive layer in a diagonal direction from the second sheet member; and an evaluation step for evaluating a crack appearing in the adhesive layer on the basis of the amplitude of the Lamb waves. In the evaluation step, for example, the greater the reduction rate of the amplitude, the crack having generated in the adhesive layer is evaluated to be large. The sheet members can be of metal, resin, or composite material.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a non-destructive evaluation method for a bonded structure. [Background technology]

[0002] A method has been proposed for non-destructively evaluating the adhesive state of plate materials using plate waves (Lamb waves), which are a type of guided wave obtained by exciting (resonating) ultrasonic waves. Related descriptions can be found in, for example, the following patent documents. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 63-177055 [Patent Document 2] Patent Publication No. 63-175762 Summary of the Invention [Problem to be solved by the invention]

[0004] In Patent Document 1, ultrasonic waves are applied to a floor material in which a steel plate is bonded to a concrete core from a tire-shaped probe in contact with the steel plate, and the quality of the bond between the steel plate and the concrete core is determined based on the presence or absence of a plate wave reflection echo detected at the edge of the floor material. Specifically, the system detects whether plate waves are generated within the steel plate itself into which the ultrasonic waves are applied, and if no plate wave reflection echo is detected, the bond is determined to be good. This takes advantage of the fact that the concrete core is sufficiently large compared to the wavelength of the ultrasonic waves, so if the bond is good, ultrasonic waves are not excited within the steel plate and no plate waves are generated.

[0005] In Patent Document 2, ultrasonic waves are emitted from a probe abutting the flange to a honeycomb sandwich panel where the flange and core are bonded, and the transmitted waves that pass through the bonded joint between the flange and core multiple times are received from another flange, and the bonding condition (bonding length) of the bonded joint between the flange and core is inspected based on the echo height of the transmitted waves.This method cannot identify bonded joints with poor bonding or quantitatively evaluate the bonding condition of a specific bonded joint.

[0006] The present invention has been made in view of the above circumstances, and aims to provide a new evaluation method etc. that can evaluate the adhesive state between thin plate portions. [Means for solving the problem]

[0007] As a result of intensive research, the inventors have newly discovered that the initiation and propagation of cracks at the adhesive interface between thin plates can be evaluated based on the amplitude of leaky Lamb waves. By expanding on this finding, the present invention, which will be described below, has been completed.

[0008] <<Non-destructive evaluation method for joint structure>> The present invention is a non-destructive evaluation method for a bonded structure in which a first thin plate portion and a second thin plate portion are overlapped and bonded with an adhesive layer between the opposing surfaces to be bonded, and includes a transmitting step of transmitting ultrasonic waves obliquely to the first thin plate portion, a receiving step of receiving Lamb waves propagated by the ultrasonic waves through the adhesive layer obliquely from the second thin plate portion, and an evaluation step of evaluating a crack that has appeared in the adhesive layer based on the amplitude of the Lamb wave, wherein the evaluation step evaluates the crack as being larger the lower the amplitude is relative to a reference amplitude.

[0009] The nondestructive evaluation method for a bonded structure of the present invention (simply referred to as the "evaluation method") makes it possible to easily and accurately inspect and monitor the presence or absence and extent (crack length, etc.) of cracks that may occur in the adhesive layer (adhesion interface) between thin plate parts. For example, the evaluation method of the present invention makes it possible to monitor in real time the progress of cracks over time in the adhesive layer between thin plate parts.

[0010] Evaluation Device The present invention can be understood as not only an evaluation method but also an evaluation device. For example, the present invention may be a nondestructive evaluation device for a bonded structure formed by overlapping a first thin plate portion and a second thin plate portion and bonded with an adhesive layer between opposing bonded surfaces, the nondestructive evaluation device comprising: a transmitting means for transmitting ultrasonic waves obliquely to the first thin plate portion; a receiving means for receiving Lamb waves obliquely from the second thin plate portion, the Lamb waves propagating from the ultrasonic waves via the adhesive layer; and an evaluation means for evaluating a crack that has appeared in the adhesive layer based on the amplitude of the Lamb waves, wherein the evaluation step evaluates the crack as being larger as the amplitude decreases relative to a reference amplitude.

[0011] "system" The present invention can also be understood as a manufacturing system that reflects (feeds back) the results obtained by the evaluation method and evaluation device in the manufacturing process (adhesion conditions) of a bonded structure, etc. The present invention can also be understood as a test system in which the above-mentioned evaluation device is incorporated into a test device (tensile tester, fatigue tester, etc.).

[0012] "others" (1) The elements of a method, "steps," and the elements of a product, "means," are interchangeable. Steps or means executable by a computer are also elements of a program (including recording media, data structures, etc.) that can be loaded into a computer.

[0013] (2) The amplitude, the amount of decrease, the size of the crack, etc. referred to in this specification may be represented by index values ​​that are normalized, non-dimensionalized, etc. For example, the amplitude (V) of the leaky Lamb wave received from the second thin plate portion may be evaluated by using the amplitude of the leaky Lamb wave obtained when the adhesive layer is in a sound state (for example, an unused state or an initial state) as a reference amplitude (V0), and by using the difference from this amplitude (ΔV=V-V0) or the ratio (ΔV / V0) of this difference to the reference amplitude (V0).

[0014] (3) Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values ​​or numerical ranges described in this specification may be used as a new lower limit or upper limit value to create a new range such as "a to b." Unless otherwise specified, "x to y MHz" in this specification means x MHz to y MHz. The same applies to other unit systems (mm, etc.). [Brief explanation of the drawings]

[0015] [Figure 1] 1A and 1B are a front view and a plan view showing the shape of a test piece (bonded structure) and the arrangement of air probes. [Figure 2] FIG. 1 is a schematic diagram illustrating an overview of an evaluation device. [Figure 3] 1 is a scatter diagram showing the relationship between crack length and Lamb wave amplitude reduction rate, and a table showing the load amplitude and number of cycles applied to a test piece in a fatigue test. DETAILED DESCRIPTION OF THE INVENTION

[0016] One or more components selected from this specification may be added to the above-described components of the present invention. The contents described in this specification may also be applied to devices, systems, etc. as appropriate. Which embodiment is best depends on the target, required performance, etc.

[0017] 《Joint structure》 The joint structure to be evaluated is formed by joining a first thin plate portion and a second thin plate portion with an adhesive layer between their opposing joined surfaces. The joint structure may be, for example, a single lap joint or a flange. As long as it has a joint structure, the shape and use of the specific object (structure, member, etc.) are not important.

[0018] The "thin plate portion" may have any shape as long as it excites and propagates Lamb waves, and the specific plate thickness, material, shape, etc. are not important. The first thin plate portion and the second thin plate portion may have the same or different plate thickness, material, shape, etc.

[0019] The "thin plate (part)" referred to in this specification may be any plate that, when ultrasonic waves are incident at an oblique angle, allows guided waves (Lamb waves) excited (resonated) within the plate to propagate along the in-plane direction. Specific plate thickness, etc., is not important, but it is preferable that the plate thickness is smaller than the wavelength of longitudinal ultrasonic waves within the material. Specifically, the thickness of the thin plate part (plate thickness) is, for example, 0.5 to 10 mm or 1 to 5 mm. The plate thickness may be constant or may vary between the ultrasonic wave transmission position and the ultrasonic wave reception position.

[0020] The thin plate portion may be made of any material, such as metal, resin, or composite material, as long as it allows propagation of Lamb waves. Composite materials are made of, for example, fillers (reinforcing fibers, reinforcing particles, etc.) and matrices (resins, metals, etc.). Typical composite materials include carbon fiber reinforced plastics (CFRP) and glass fiber reinforced plastics (GFRP).

[0021] The thin plate portion is not limited to a flat plate shape, but may be curved, have a curved surface, etc. In the case of a non-flat thin plate portion, the incident angle and the outgoing angle of the ultrasonic wave are angles relative to a tangent plane or a tangent line.

[0022] The "adhesive layer" is interposed between the joined surface of the first thin plate portion and the joined surface of the second thin plate portion, and bonds the joined surfaces. The adhesive layer may be any layer that allows ultrasonic waves (Lamb waves) to propagate from the first thin plate portion to the second thin plate portion. Typically, the adhesive layer is made of a material (e.g., adhesive, brazing material, etc.) different from the first thin plate portion and the second thin plate portion. When a general adhesive is used, the adhesive layer is a resin layer. The thickness of the adhesive layer is usually much thinner than the thin plate portions. There are no specific restrictions on the thickness, but it is, for example, about 10 to 1000 μm.

[0023] Cracks in adhesive layers usually occur when repeated or excessive loads (fatigue load, tensile load, thermal load, water absorption degradation, etc.) are applied to the bonded structure. The distribution and extent of cracks in the adhesive layer are not important. Cracks are not limited to those that occur after the fact or over time, but also include those that occur due to initial defects.

[0024] Ultrasonic / Lamb Wave Ultrasonic waves incident on the thin plate portion at an oblique angle propagate in the in-plane direction of the thin plate portion as Lamb waves excited under certain conditions. The frequency of the ultrasonic waves is, for example, 0.1 to 5 MHz, 0.3 to 3 MHz, or 0.5 to 1.5 MHz. The angle at which the Lamb waves are received by the second thin plate portion (emission angle) should be approximately equal to the angle at which the ultrasonic waves are transmitted by the first thin plate portion (incident angle). This angle should be, for example, within ±3° or even ±1.5° of the optimal angle (the calculation method will be described later).

[0025] The mode order of the ultrasonic waves (leaky Lamb waves) associated with cracks in the adhesive layer does not matter, but usually the zeroth mode (A0 mode), which has a large amplitude, should be used.

[0026] If ultrasonic waves can be transmitted and received without contact with the thin plate, the degree of freedom in the evaluation object and evaluation situation increases, and the evaluation of the adhesive condition becomes easier. Furthermore, if ultrasonic waves can be transmitted and received from one side of the bonded structure, the configuration, arrangement, installation, etc. of the evaluation device can be simplified. [Example]

[0027] The cracks occurring in the adhesive layer of the bonded body (bonded structure) were evaluated non-destructively using ultrasonic waves. The present invention will be described in more detail with reference to such specific examples.

[0028] The details of the bonded body M used in the test are shown in Figure 1, and an overview of the evaluation device D is shown in Figure 2. Unless otherwise specified, the directions indicated by the arrows in Figure 2 are up / down or left / right.

[0029] Test piece The joint M is made by bonding together the ends of an Al alloy plate 1 (first thin plate portion) and a steel plate 2 (second thin plate portion). The Al alloy plate 1 (JIS A5052 wrought material / plate thickness: 3 mm) and the galvanized steel plate 2 (JIS SGC400 / plate thickness: 3.2 mm) are made by cutting flat plates (materials) into the shape shown in Figure 1.

[0030] The flat surfaces (bonding surfaces) of the ends (10 mm wide) were overlapped with a thermosetting epoxy adhesive. At this time, both outer sides of the ends were clamped and pressed, and the adhesive was cured by heating at 170°C for 30 minutes. In this way, as shown in Figure 1, a bonded body M (test piece) was obtained in which an Al alloy plate 1 and a steel plate 2 were bonded via an adhesive layer 3 (width in the left-right direction: 10 mm). Several such bonded bodies M were produced and subjected to the fatigue test and evaluation test described below.

[0031] <Bonding evaluation device> The evaluation device D includes an ultrasonic wave transmitting probe 61, an ultrasonic wave (Lamb wave) receiving probe 62, an ultrasonic wave transmitting / receiving source 63, and an evaluation unit 64 for the bonded structure.

[0032] The transmitting probe 61 and the receiving probe 62 are air probes arranged on the same side (upper side) of the bonded body M to be inspected. By using an air probe used in the airborne ultrasonic method, a contact medium is not required, and non-contact inspection can be easily performed. In this example, an air probe (0.8K10×10N) manufactured by Japan Probe Co., Ltd. was used. In addition, a pulser receiver (JPR-600C-2CH) manufactured by Japan Probe Co., Ltd. was used as the ultrasonic transmission / reception source 63.

[0033] The ultrasonic waves (signals) emitted from the transmitting / receiving source 63 were 10 burst waves at 800 kHz (0.8 MHz). The mounting angle θ1 (ultrasonic wave incident angle) of the transmitting probe 61 relative to the Al alloy plate 1 (normal) and the mounting angle θ2 (ultrasonic wave emission angle) of the receiving probe 62 relative to the steel plate 2 (normal) were both set to 7.4°. This angle is the optimal angle for exciting A0 mode Lamb waves in the Al alloy plate 1 and the steel plate 2, and was calculated using the following equation based on the law of refraction. θ=sin -1 (Cair / CLamb) Cair: longitudinal wave speed of the surrounding medium CLamb: Phase velocity of Lamb waves determined from the mode and frequency to be excited

[0034] In this example, the evaluation test was performed in the atmosphere (approximately 15°C), so C was set to 340 m / s. C Lamb was set to 2656 m / s with the intention of exciting 800 kHz, A0 mode Lamb waves in Al alloy plate 1 (plate thickness 3 mm). The phase velocity of this Lamb wave was calculated using Plate Dispersion ver. 1.0 (manufactured by Nagoya Institute of Technology).

[0035] Incidentally, the phase velocity of Lamb waves varies depending on the thickness and acoustic properties of the plate material (such as the longitudinal wave sound velocity propagating through the plate material), as well as the mode and frequency of the Lamb waves to be excited. In the case of A0 mode Lamb waves, the phase velocity tends to increase as the plate thickness, longitudinal wave sound velocity, and frequency increase. Since the longitudinal wave sound velocity differs depending on the plate material, for example, resin has a lower longitudinal wave sound velocity than metal, and the phase velocity is also lower. For this reason, the aforementioned θ (optimum incident angle) is generally smaller for resin than for metal.

[0036] Furthermore, the amplitude of the excited Lamb waves is sensitive to the incident angle (θ1) of the ultrasonic waves with respect to the plate material, so it is advisable to set the incident angle (θ1) within ±3° of the optimal angle (θ), for example.

[0037] The evaluation unit 64 identifies the amplitude of the ultrasonic wave W2 obtained through the receiving probe 62 and the transmitting / receiving source 63. It also compares the amplitude or its index value (for example, the amplitude reduction rate described below) with a database prepared in advance to evaluate the length (crack length c) of the crack 31 that has occurred in the adhesive layer 3. These processes are performed by executing a predetermined program (algorithm) on a dedicated computer or a personal computer.

[0038] Evaluation flow The evaluation of the bonded body M using the evaluation device D is performed as follows. When ultrasonic waves W1 are incident on the Al alloy plate 1 by the transmitting probe 61 (transmitting means) (transmitting step), the ultrasonic waves W1 are excited in the Al alloy plate 1 and propagate as Lamb waves L1 in the in-plane direction of the Al alloy plate 1. The Lamb waves L1 propagate through the adhesive layer 3 in the in-plane direction of the steel plate 2 as Lamb waves L2. The Lamb waves L2 leaking from the steel plate 2 become ultrasonic waves W2 and are received by the receiving probe 62 (receiving means) (receiving step). The evaluation unit 64 (evaluation means) analyzes the ultrasonic waves W2 to identify their amplitude, and estimates the crack length c in the adhesive layer 3 based on the amplitude (index value) (evaluation step).

[0039] Testing and Measurement (1) Fatigue test A tensile load was repeatedly applied in a pulsating manner in the longitudinal direction (left and right direction) to six test pieces made of joint M. The load amplitude [(maximum load - minimum load) / 2] and the number of repetitions (number of cycles) for each test piece are summarized in Table 1 (Fig. 3). The tensile load was varied along a sine curve (frequency: 20 Hz).

[0040] (2) Amplitude index value Using evaluation device D, the amplitude of ultrasonic waves W2 (leaky Lamb waves L2) was measured for the test piece before the fatigue test (sound bonded state) and the test piece after the fatigue test, and the rate of change (amplitude reduction rate) was calculated. Amplitude reduction rate = 100 × (V0 - V1) / V0 (%), where V0 is the amplitude before the fatigue test and V1 is the amplitude after the fatigue test.

[0041] (3) Measurement of cracks After the fatigue test, the test specimens were measured using evaluation device D and then fractured (peeled off) at adhesive layer 3. Cracks that had initiated and propagated in adhesive layer 3 were clearly observed on the fracture surface of each test specimen. The maximum lengths of the cracks on the fracture surfaces of Al alloy plate 1 and steel plate 2 were measured, and their sum was taken as crack length c. The maximum length of the crack was the maximum length of the interfacial peel surface along the longitudinal direction of the test specimen, as observed on the fracture surfaces of Al alloy plate 1 and steel plate 2, and was measured using an optical camera.

[0042] (4) Correlation The relationship between the amplitude reduction rate of the Lamb waves obtained from each test piece and the crack length in the adhesive layer is summarized in Figure 3. As is clear from Figure 3, the two were approximately proportional. In other words, the larger the amplitude reduction rate (in other words, the more the amplitude of the Lamb waves decreased due to the passage through the adhesive layer), the larger the crack length in the adhesive layer.

[0043] In addition, taking into account the mechanism of crack initiation and propagation in the adhesive layer, if the amplitude reduction rate is below a predetermined value (for example, below 4%), it is also possible to evaluate that cracks have not occurred or have barely propagated.

[0044] From the above, it has been shown that according to the present invention, the presence or absence and progression of cracks that may occur at the joint of a thin plate portion can be easily and non-destructively inspected, monitored, or quantitatively evaluated based on the amplitude or index value of the Lamb wave that propagates through the thin plate portion via the adhesive layer. [Explanation of symbols]

[0045] 1 Al alloy plate (first thin plate part) 2 Steel plate (2nd thin plate part) 3 Adhesive layer D Evaluation equipment M conjugate (joint structure) W1, W2 Ultrasound L1, L2 Lamb wave

Claims

1. A non-destructive evaluation method for a bonded structure in which a first thin plate portion and a second thin plate portion are bonded together with an adhesive layer between opposing bonded surfaces, comprising: a transmitting step of transmitting ultrasonic waves obliquely to the first thin plate portion; a receiving step of receiving Lamb waves propagated by the ultrasonic waves through the adhesive layer from the second thin plate portion in an oblique direction; and evaluating a crack that has appeared in the adhesive layer based on the amplitude of the Lamb wave, The evaluation step evaluates the crack as being larger as the amplitude decreases relative to a reference amplitude.

2. The nondestructive evaluation method for a joint structure according to claim 1 , wherein the transmitting step and the receiving step are performed in a non-contact state with the first thin plate portion and the second thin plate portion.

3. 3. The nondestructive evaluation method for a bonded structure according to claim 1, wherein the ultrasonic waves are transmitted and the Lamb waves are received from one side of the bonded structure.

4. The nondestructive evaluation method for a bonded structure according to claim 3 , wherein an angle at which the ultrasonic waves are transmitted to the first thin plate portion is substantially equal to an angle at which the Lamb waves are received from the second thin plate portion.

5. The method for nondestructive evaluation of a joint structure according to claim 1 , wherein the first thin plate portion and / or the second thin plate portion is made of a metal, a resin, or a composite material.

6. the first thin plate portion and the second thin plate portion have a plate thickness of 0.5 to 10 mm, 6. The method for non-destructive evaluation of a bonded structure according to claim 1, wherein the frequency of the ultrasonic waves is 0.1 to 5 MHz.

7. The nondestructive evaluation method for a joint structure according to claim 1 , wherein the adhesive layer is a resin layer.

8. The non-destructive evaluation method for a joint structure according to claim 1 , wherein the crack is generated or propagates due to a load applied to the joint structure.

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