Evaluation of Ultrasonic Bonding Strength with Optoacoustic ... - MDPI

applied sciences Article

Evaluation of Ultrasonic Bonding Strength with Optoacoustic Methods Takumi Kamimura 1 , Sanichiro Yoshida 2 and Tomohiro Sasaki 1, * 1 2

*

Graduate School of Niigata University, 8050 Ikarashi-ninocho, Nishi-ku, Niigata-shi, Niigata 950-2181, Japan; [email protected] Department of Chemistry and Physics, Southeastern Louisiana University, Hammond, LA 70402, USA; [email protected] Correspondence: [email protected]; Tel.: +81-25-262-6710

Received: 31 May 2018; Accepted: 19 June 2018; Published: 23 June 2018

Abstract: This study reports the application of an optoacoustic method for evaluating the bonding strength of ultrasonically bonded joints in a non-destructive and non-contact fashion. It is proposed that the bonding strength is correlated with the resonant frequency of bonded joints. The bonding strength measured with a destructive tensile test roughly increased with the vibration time, however, it varied, causing the transitional and dispersed formation of micro-bonds at the bonding interface. Scanning Electron Microscopic observation of the fractured surface suggested that the bonding strength depends on the total bonded area of micro-bonds. Frequency response of the bonded joint was examined with a non-destructive method using a piezo-electric vibrator. The experiment revealed that the resonant frequency exponentially increased with the bonding strength. In addition, this vibration behavior was dynamically visualized with electronic speckle pattern interferometry (ESPI). The correlation between the bonded area and the resonant frequency is discussed based on finite element analysis. The results indicate the possibility for in-situ evaluation of the ultrasonic bonding strength. Keywords: ultrasonic bonding; bonding strength; nondestructive testing; resonant frequency; electronic speckle pattern interferometry (ESPI)

1. Introduction Ultrasonic bonding is one of the solid-state bonding techniques that uses high frequency vibration to bond metal sheets. This technique is utilized in the automotive industry for joining parts such as wire bonding on a semiconductor substrate, wiring components with electrode terminals, and thin film electrodes of lithium ion batteries. The importance of the ultrasonic bonding is increasing with electrification of automobiles and the expansion of demand of electric vehicles. In addition, as the ultrasonic bonding has advantages such as short-time and low energy compared with other solid-state bonding techniques [1], the ultrasonic bonding is expected to be a promising tool in dissimilar bonding for automotive body panels. Since the industrial importance and the demand of the bonding quality increase, inspection methods for assuring the reliability of the bonding are required. Destructive methods including a tensile testing, tensile shear testing, etc. are utilized to evaluate the bonding strength. The bonding reliability is evaluated statistically by a sampling inspection. On the other hand, nondestructive methods including ultrasonic testing or radiographic testing are conducted for evaluation of the state of the bonding. Some researchers evaluated the deterioration inside materials with ultrasonic waves [2–4], electrical methods [5], thermography [6,7], and radiography [8]. The ultrasonic testing detects the reflected waves coming from the discontinuous interfaces due to cracks and un-bonded regions. In addition, some researches [9–11] tried to detect the residual stress around the bonded area by nonlinearity of the sound wave. These methods used the acoustoelastic Appl. Sci. 2018, 8, 1026; doi:10.3390/app8071026

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effect, in which the sound velocity depends on the elasticity of materials. The bonding strength is often estimated with the bonded area. The bonded area was visualized by ultrasonic C-scan testing [12–15] and radiography [16,17]. In these studies, it was indicated that the relation between the bonded area and the bonding strength are linearly correlated. However, C-scan testing and radiography are not suitable for in-situ measurement of the bonding strength, because it needs measurement equipment outside of the production line and is time-consuming. There have been other ways to evaluate the bonding area with ultrasonic techniques including the A-scan method [18,19], resonant frequency measurement [20], cantilever test [21], electrical method [22], and thermographic test [23]. Application of an optical technique is an effective methodology for non-contact measurement. However, there are few studies that relate the optical measurement to evaluation of the bonding strength. The bonded area detected by the optical coherence tomography [24] had a correlation with the bonding strength. Furthermore, nondestructive evaluations are of technical interest, as well as fundamental interest, in the measurement of interfacial strength between dissimilar materials such as metal/polymer dissimilar joints [25] and fiber reinforced composites [26–28]. The interfacial strength is extremely important in controlling the mechanical property of joints. In our study, we propose a new method to evaluate the bonding strength that is non-destructive and quick by analyzing the elastic behavior of the bonded materials. We expect that the elastic behavior will change depending on the bonding strength. By analyzing the elastic behavior of bonded materials, it is expected that the bonding strength can be evaluated. We focus on the optoacoustic methods to analyze the elastic behavior in a non-destructive way. The ultrasonic bonding uses acoustic vibration, so we considered that it will be possible to measure the elastic behavior in situ. In addition, the vibration behavior can be measured without contact using the optics with the acoustics. 2. Theory First, we describe formation and enlargement of the bonded area in an ultrasonic bonding process. The ultrasonic bonding process consists of two main steps: a clamping step and a vibrating step. At the clamping step, a normal force is applied to the bonding part by the ultrasonic horn and the faying surfaces get into an intimate contact under the exertion of the normal force. During the vibrating step, the ultrasonic horn vibrates parallel to the contact area with the bonding tip at the ultrasonic frequency, causing relative motion between the plates to be bonded. At the initial phase in the bonding mechanism, oxide films on the faying surface break, and new surfaces get into contact. By achieving intermetallic bonding, micro-bonds are created. These micro-bonds create a convex contact part, so that micro-bonds are formed dispersedly at the macroscopic bonding interface. With increase in the vibration time, each micro-bond gradually enlarges, leading to an increase of the bonding strength. In our study, we consider the bonded specimen that consists of two pieces of metal sheet as shown in Figure 1. From the bonding mechanism described above, we hypothesize an elastic modulus of the micro-bond is the same as the elastic modulus of other areas. The apparent spring constant of the total bonded area K (N/mm) is expressed by the spring constant per unit area k (N/mm · mm2 ) as K = k · A, where A (mm2 ) is the total area of micro-bonds. Since the total area of micro-bonds is small at the initial phase during bonding, the number of springs is also small. Thus, it is considered that the spring constant, K, is small if the bond is weak. The micro-bonds develop with the vibration in the latter bonding phase, resulting in an increase of the total bonded area, A, and the total spring constant, K. Considering the specimen as a spring-mass system in which two plates are connected via micro-bonds, the resonant frequency f is given as Equation (1). 1 f= 2π

r

K M

(1)

where K is the apparent spring constant at the bonded part. M is the mass of the bonded specimen. As it is considered that the mass is constant during the bonding, the resonant frequency of the bonded

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specimen is solely affected by the apparent spring constant K. Namely, the bonding strength could be specimen specimen is is solely solely affected affected by by the the apparent apparent spring spring constant constant K. K. Namely, Namely, the the bonding bonding strength strength could could evaluated by measuring the resonant frequency. be be evaluated evaluated by by measuring measuring the the resonant resonant frequency. frequency.

Figure Figure 1. 1. Change Change of of apparent apparent spring spring constant constant by by enlargement enlargement of of bonded bonded area. area. Figure 1. Change of apparent spring constant by enlargement of bonded area.

3. 3. Experimental Experimental Procedure Procedure 3. Experimental Procedure 3.1. Specimens Bonded by the Ultrasonic Bonding 3.1. Specimens Specimens Bonded Bonded by by the the Ultrasonic Ultrasonic Bonding Bonding 3.1. Figure illustrates the bonding tip geometry. Figure222 illustrates illustrates the the schematics schematics of of the the ultrasonic ultrasonic bonding bonding machine machine and and Figure schematics of the ultrasonic bonding machine and bonding bonding tip tip geometry. geometry. An ultrasonic bonder with an output power of 2.4 kW and aa driving frequency of 14.71 kHz was An ultrasonic bonder with an output power of 2.4 kW and driving frequency of 14.71 kHz was used used An ultrasonic bonder with an output power of 2.4 kW and a driving frequency of 14.71 kHz was for the bonding. The amplitude of the ultrasonic horn under no load was 53 µµm (peak-peak). An for the bonding. The amplitude of the ultrasonic horn under no load was 53 m (peak-peak). An used for the bonding. The amplitude of the ultrasonic horn under no load was 53 µm (peak-peak). industrial aluminum alloy, AA6061-T6 sheet, was used for the bonded specimen. The sheet was cut industrial aluminum alloy, AA6061-T6 sheet, was used for the bonded specimen. The sheet was cut An industrial aluminum alloy, AA6061-T6 sheet, was used for the bonded specimen. The sheet was cut into two metal sheets as shown in Figure The upper sheet had size 11 ×× 0.5 intotwo twometal metalsheets sheetsas asshown shownin inFigure Figure3.3. 3.The Theupper uppersheet sheethad hada aasize sizeofof of5050 50 11× 0.5 mm. mm. The The lower lower into ×××11 0.5 mm. The lower sheet had aa size of 50 ×× 11.5 ×× 0.8 mm. The lower sheet was placed on an anvil and clamped by a sheet had size of 50 11.5 0.8 mm. The lower sheet was placed on an anvil and clamped sheet had a size of 50 × 11.5 × 0.8 mm. The lower sheet was placed on an anvil and clampedbybya fixture to suppress slippage. The upper sheet was lapped on the lower sheet in such a way that the upper sheet was lapped on on thethe lower sheet in such a way thatthat the afixture fixturetotosuppress suppressslippage. slippage.The The upper sheet was lapped lower sheet in such a way total length of the bonded specimen was 85 mm. Vibration was applied by the bonding tip of the total length of the bonded specimen was 85 mm. Vibration was applied by the bonding tip of the total length of the bonded specimen was 85 mm. Vibration was applied by the bonding tipthe of ultrasonic horn under aa normal force of 588 N. Bonding strength was controlled by changing the ultrasonic horn under normal force ofof 588 changing the the the ultrasonic horn under a normal force 588N. N.Bonding Bondingstrength strengthwas was controlled controlled by by changing vibration time within 1000 ms. the ultrasonic horn and the anvil had knurled vibration time time within within 1000 1000 ms. ms. The The bonding bonding tip tip of of the the ultrasonic ultrasonic horn horn and and the the anvil anvil had had aaa knurled knurled vibration The bonding tip of surface shown in Figure 2b. There were 13 ×× 13 protrusions with aa 0.8 mm pitch, 0.3 mm height, and surface shown in Figure 2b. There were 13 13 protrusions with 0.8 mm pitch, 0.3 mm height, and surface shown in Figure 2b. There were 13 × 13 protrusions with a 0.8 mm pitch, 0.3 mm height, and 90° angle of groove. The total tip area was 10 × 10 mm. The bonding was carried out by aligning the 90° angle of groove. The total tip area was 10 × 10 mm. The bonding was carried out by aligning the 90◦ angle of groove. The total tip area was 10 × 10 mm. The bonding was carried out by aligning the protrusions of the horn tip and the anvil tip with each other. protrusionsof ofthe thehorn horntip tipand andthe theanvil anviltip tipwith witheach eachother. other. protrusions

(a) (a)

(b) (b)

Figure geometry. Figure 2. 2. (a) (a) Ultrasonic Ultrasonic bonder; bonder; (b) (b) Bonding Bonding tip tip geometry. geometry. Figure 2. (a) Ultrasonic bonder; (b) Bonding tip

The The bonding bonding strength strength was was measured measured by by aa tensile tensile shear shear test test (Figure (Figure 4). 4). The The test test was was conducted conducted at at aa cross-head cross-head speed speed of of 0.1 0.1 mm/s. mm/s. The The maximum maximum load load during during the the tensile tensile shear shear test test was was defined defined as as the the bonding bonding strength strength in in this this study. study. The The fractured fractured surface surface was was observed observed by by aa scanning scanning electron electron microscope microscope (SEM) (SEM) and and results results were were output output to to aa digital digital image image with with aa constant constant brightness brightness and and contrast. contrast. The The image image was was binarized binarized with with aa constant constant intensity intensity threshold, threshold, then, then, the the bonded bonded area area was was estimated estimated by by measuring measuring

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The bonding strength was measured by a tensile shear test (Figure 4). The test was conducted at a cross-head speed of 0.1 mm/s. The maximum load during the tensile shear test was defined as the bonding strength this PEER study. The fractured surface was observed by a scanning electron microscope Sci. 2018, 8,in x PEER FOR REVIEW 4 of 17 Appl.Appl. Sci. 2018, 8, x FOR REVIEW 4 of 17 (SEM) and results were output to a digital image with a constant brightness and contrast. The image the total ofawhite the white region. details of bonded thethe bonded estimation are described in Section the total areaarea of the region. The The details of the areaarea estimation are described in Section was binarized with constant intensity threshold, then, bonded area was estimated by measuring 4.1. area of the white region. The details of the bonded area estimation are described in Section 4.1. 4.1.total the

Figure 3. Bonded specimen. Figure 3. Bonded Bonded specimen. Figure 3. specimen.

Figure 4. Tensile shear test. Figure 4. Tensile Tensile shearshear test. test.

3.2. Measurement of Frequency Response 3.2. Measurement Frequency Response 3.2. of Frequency Response experimental arrangement to detect the resonant frequency of bonded the bonded specimen experimental the bonded specimen The The experimental arrangement to detect the resonant frequency of the specimen is is shown in Figure 5. The 20 mm portion of bonded the bonded specimen was fixed on a piezoelectric actuator. shown in Figure 5. The 20 mm portion of the specimen was fixed on a piezoelectric actuator. was fixed a piezoelectric actuator. A sinusoidal-voltage wave generated a function generator input topiezoelectric the piezoelectric actuator A sinusoidal-voltage sinusoidal-voltage wave generated by aaby function generator A wave generated by function generator was was input to the actuator through an amplifier. amplitude of input the input voltage to piezoelectric the piezoelectric actuator 100 amplifier. of the voltage to the actuator was 100 through an amplifier. The The amplitude piezoelectric actuator waswas 100 mV, mV,mV, which corresponded to the displacement amplitude of about 2.1 μm. A capacitance displacement whichcorresponded correspondedtotothethe displacement amplitude of about 2.1 A μm. A capacitance displacement which displacement amplitude of about 2.1 µm. capacitance displacement gauge gauge placed at theofother end of specimen theinspecimen in order to detect theofamplitude of vibrating the vibrating gauge was was placed at the other endspecimen of the tothe detect the amplitude of the was placed at the other end the orderin toorder detect amplitude the vibrating bonded bonded specimen. The output voltage of the capacitance gauge recorded into a (PC), personal bonded specimen. The output voltage of the capacitance gauge was into a personal specimen. The output voltage of the capacitance gauge was recorded intowas arecorded personal computer computer (PC), changing the driving frequency of piezoelectric the The piezoelectric actuator. The driving frequency computer (PC), changing the driving frequency ofactuator. the actuator. Thewas driving frequency changing the driving frequency of the piezoelectric driving frequency increased from 0.1 increased from toof1000 Hzand at increment the increment of Hz, 0.5 and from to 10,000 Hz in the was increased 10 to101000 HzHz, at the of10,000 0.5 and from 10001000 to increment 10,000 Hzofin 10 towas 1000 Hz at from the increment 0.5 from 1000 to HzHz, in the logarithmic 10the 0.1 0.1 logarithmic increment of 10 Hz. We computed the frequency responses at each driving frequency logarithmic increment of 10 Hz. We computed the frequency responses at each driving frequency Hz. We computed the frequency responses at each driving through Fourier transformation, through Fourier transformation, the peak frequency the spectrum found through Fourier transformation, and and thespectrum peak frequency in the spectrum was was found to beto be and the peak frequency in the amplitude was found toinbeamplitude ±5amplitude Hz around the driving frequency. ±5 around Hz around the driving The maximum amplitude inthe thedriving Fourier spectrum plotted ±5 Hz the driving frequency. The maximum in the Fourier spectrum wastowas plotted The maximum amplitude in thefrequency. Fourier spectrum wasamplitude plotted against frequency obtain against the driving frequency to obtain the frequency response. From the frequency response, against the driving frequency obtain the frequency response. From the frequency response, the frequency response. From thetofrequency response, resonant frequencies of the bonded specimen resonant frequencies offrequency the bonded specimen identified. Sampling frequency of output resonant frequencies of the bonded specimen were identified. Sampling frequency of output and were identified. Sampling of output andwere input voltage was 100,000 Hz, the number of and input voltage was 100,000 Hz, the number of samples was 10,000. input voltage was 100,000 Hz, the number of samples was 10,000. samples was 10,000.

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Multifunction I/O Device (National Instrument Co. USB-6361)


Amplifier

PC

5 of 17

Displacement gauge

Multifunction I/O Device (National Instrument Co. USB-6361)

PC

Detector

Jig Amplifier

Displacement gauge Welded specimen

Vibration

Detector

Piezoelectric actuator Jig

Welded specimen

Vibration

Stage

Piezoelectric actuator

Figure 5. Experimental setup formeasuring measuring the the resonant byby a piezoelectric actuator and aand a Figure 5. Experimental setup for resonantfrequency frequency a piezoelectric actuator capacitance displacement gauge. I/O = input/output; PC = personal computer. capacitance displacement gauge. I/O = input/output; PC = personal computer. Stage

3.3. Electronic Speckle Pattern Interferometry (ESPI)

3.3. Electronic Speckle Pattern Interferometry (ESPI) Figure 5. Experimental setup for measuring the resonant frequency by a piezoelectric actuator and a

Electronic speckle pattern interferometry (ESPI) was used to visualize bending motion in the capacitance displacement gauge. I/O = input/output; PC = personal computer. Electronic speckle pattern interferometry was used to visualize bending in the above vibration test. In addition, this technique(ESPI) can measure the resonant frequency andmotion vibration above vibration test. In addition, this technique measure theofresonant and vibration mode simultaneously [29–34]. Figure 6 illustrates the schematic ESPI to frequency measure out-of-plane 3.3. Electronic Speckle Pattern Interferometry (ESPI) can displacement. A semiconductor laser with wavelength 660 nm was light source. The mode simultaneously [29–34]. Figure 6 illustrates theofschematic ofused ESPIfortothe measure out-of-plane Electronic speckle pattern interferometry (ESPI) was used to visualize bending motion in the light was expanded by a lens and splitwith into wavelength two paths (arm 1660 andnm armwas 2) with a beam splitter, and displacement. A semiconductor laser of used for the light above vibration test. In addition, this technique can measure the resonant frequency and vibration source. used to irradiate the specimen a split reference An A6061-T6 sheet, which is the same material The light was expanded by[29–34]. a lensand and into plane. twothe paths (arm 1ofand arm 2) with aout-of-plane beam splitter, and mode simultaneously Figure 6 illustrates schematic ESPI to measure as the bonded specimen, was utilized for the reference plane. The roll direction of the reference plane semiconductor with wavelength of 660 nm was used forwhich the light The used todisplacement. irradiate theAspecimen andlaser a reference plane. An A6061-T6 sheet, is source. the same material was aligned with the specimen. When the laser is irradiated on a rough surface, the lights of light wasspecimen, expanded by a lens and split paths (arm 1 and with a beam splitter, and theplane as the bonded was utilized for into the two reference plane. Thearm roll2)direction of the reference random overlap and interfere with each other to thesheet, coherence thesame laser,material thereby a used tophase irradiate the specimen and a reference plane. Andue A6061-T6 which of is the was aligned with the specimen. When the laser is irradiated on a rough surface, the lights of the speckle formed.was Theutilized superposed pattern of the and the specimen as thepattern bonded in specimen, for thespeckle reference plane. The rollreference direction plane of the reference plane random phase overlap and interfere with each other duecamera. to theonBy coherence of the laser, therebyimage a speckle wasphotographed aligned with the When the laser(CCD) is irradiated a subtracting rough surface, the lights of the were by aspecimen. charge coupled devise a photographed pattern in vibration formed. speckle pattern plane and thethereby specimen random phase The overlap and interfere with other of duethe to reference the of the a ofwere before fromsuperposed another image after each deformation, and thencoherence calculating thelaser, absolute value photographed by a charge coupled devise camera. By aplane photographed image before speckle pattern in formed. superposed speckle pattern of subtracting the reference and the specimen them, a subtract image wasThe created to (CCD) recognize the deformation of the bonded specimen. The were photographed by a charge coupled devise (CCD) camera. abeam photographed imageoftothem, vibration from another image after deformation, and then By calculating absolute subtracted speckle intensity changes with the phase difference ofsubtracting the objectthe (arm 2)value relative vibration from another image after deformation, andby then calculating theof absolute valuesubtracted of thebefore reference beam (arm 1). The phase difference is caused displacement the specimen. a subtract image was created to recognize the deformation ofthe the bonded specimen. The them, a subtract image was created to recognize the deformation of the bonded specimen. The

speckle intensity changes with the phase difference of the object beam (arm 2) relative to the reference subtracted speckle intensity changes Multifunction I/O with Device the phase difference of the object beam (arm 2) relative to beam (arm 1). Thebeam phase difference isCo. caused by the displacement of the specimen. (National USB-6361) the reference (arm 1).Instrument The phase difference is caused by the displacement of the specimen. PC

Multifunction I/O Device Amplifier (National Instrument Co. USB-6361)

Laser

PC Amplifier

Screen

Laser

Screen Beamsplitter Beamsplitter

Jig VibrationJig Piezoelectric actuator Vibration

Piezoelectric actuator

Charge coupled devise (CCD) Charge coupled devise (CCD)

Bonded specimen

y

Bonded specimen

Stage Stage

x

y

z

x

z

Figure 6. Experimental setup of electronic speckle pattern interferometry (ESPI).

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Figure 6. of speckle interferometry The specimen was vibrated insetup the same manner as pattern the measurement of frequency response. Figure 6. Experimental Experimental setup of electronic electronic speckle pattern interferometry (ESPI). (ESPI). The input voltage to the piezoelectric actuator was 5 mV, which is lower than that used in the The was in same as the of response. The The specimen specimen was vibrated vibrated in the the same manner manner the measurement measurement of frequency frequency response. The the measurement of frequency response described in theasSection 3.2, because the ESPI cannot detect input voltage to the piezoelectric actuator was 5 mV, which is lower than that used in the input voltage to the piezoelectric actuator was 5 mV, which is lower than that used in the per largemeasurement displacement owing to response high sensitivity. The frame rate3.2, ofbecause the CCD camera was detect 30 frames of frequency described in the Section the ESPI cannot the measurement of frequency response described in the Section 3.2, because the ESPI cannot detect the second (fps). The brightness of high the subtract images was multiplied by 10camera to make the large displacement owing to sensitivity. The frame rate of the CCD was 30images frames brighter. per

large displacement owing to high sensitivity. The frame rate of the CCD camera was 30 frames per second (fps). The secondand (fps).Discussions The brightness brightness of of the the subtract subtract images images was was multiplied multiplied by by 10 10 to to make make the the images images brighter. brighter. 4. Results 4. Discussions 4. Results Results and and Discussions 4.1. Formation of Bonds under the Ultrasonic Bonding

4.1. Formation of under Ultrasonic Bonding 4.1. Formation of Bonds Bonds under the thebetween Ultrasonicthe Bonding Figure 7 shows the relation vibration time and the bonding strength. The vertical Figure 7 shows the relation between the time the The vertical axis is the vibration time. The horizontal axis is the bonding strength. Withstrength. vibration times shorter Figure 7 shows the relation between the vibration vibration time and and the bonding bonding strength. The vertical axis is the vibration time. The horizontal axis is the bonding strength. With vibration times shorter thanaxis 200 isms, joint strong to measure bond strength was With not obtained. period theavibration time.enough The horizontal axis the is the bonding strength. vibration This timestime shorter than 200 strong enough to the bond strength not This can be considered as an incubation to form micro-bonds the removal of period oxide film. than 200 ms, ms, aa joint joint strong enoughtime to measure measure thethe bond strength was wasthough not obtained. obtained. This time time period time to the removal of The can bonding strength as roughly increased with the vibration time ofthough 200 ms,the while it had largefilm. variance can be be considered considered as an an incubation incubation time to form form the micro-bonds micro-bonds though the removal ofaoxide oxide film. The bonding strength roughly increased with the vibration time of 200 ms, while it had a large The bonding strength roughly time. increased with thethe vibration of changed 200 ms, while it had a largetime. especially in the shorter vibration In addition, fracturetime mode with the vibration variance in shorter vibration time. In the fracture mode changed with variance especially especially in the thethe shorter vibration time.time In addition, addition, changed with the the two The specimen bonded with shorter vibration broke atthe thefracture bondedmode interface between the vibration time. The specimen bonded with the shorter vibration time broke at the bonded interface vibration time. The specimen bonded with the shorter vibration time broke at the bonded interface sheets as showntwo in Figure as 8a. Wheninthe vibration time increased to over 400 ms, the fracture occurred between between the the two sheets sheets as shown shown in Figure Figure 8a. 8a. When When the the vibration vibration time time increased increased to to over over 400 400 ms, ms, the the around the bonded part as shown in Figure 8b. We call this fracture mode the base material fracture. fracture fracture occurred occurred around around the the bonded bonded part part as as shown shown in in Figure Figure 8b. 8b. We We call call this this fracture fracture mode mode the the base base During the vibration,During the uppervibration, bonding sheet in contact with thecontact ultrasonic horn is compressed by material material fracture. fracture. During the the vibration, the the upper upper bonding bonding sheet sheet in in contact with with the the ultrasonic ultrasonic horn horn the normal force. This causes thinning of the bonding part. the The base metal facture is due tofacture a decrease is is compressed compressed by by the the normal normal force. force. This This causes causes thinning thinning of of the bonding bonding part. part. The The base base metal metal facture is strength of of the fracture strengthof ofthe thefracture specimen by the thinning. is due due to to aa decrease decrease of the fracture strength of the the specimen specimen by by the the thinning. thinning.

Figure timevs. vs.bonding bonding strength. Figure7.7.Vibration Vibration time strength. Figure 7. Vibration time vs. bonding strength.

Figure 8. Fracture pattern after the tensile shear test. (a) Interface fracture, the vibration time 300 ms; (b) Base material fracture, the vibration time 400 ms.

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Figure 8. Fracture pattern after the tensile shear test. (a) Interface fracture, the vibration time 300 ms; (b) Base material fracture, the vibration time 400 ms.

The changes in bonding strength are associated with the development of the bonded area. In order to estimate the bonded area, the fracture surface that exhibited the interface fracture was observed. The changes in bonding strength are associated with the development of the bonded area. In Figure 9 shows an example of an SEM of thesurface interface fractured surface. This specimen showed order to estimate the bonded area, image the fracture that exhibited the interface fracture was a bonding strength of 532 N at the vibration time of 200 ms. In the fracture surface, two characteristic observed. Figure 9 shows an example of an SEM image of the interface fractured surface. This patterns are observed white on the fractured surface: the scratch in Figure 9b,c, and specimen showed ain bonding strength of 532 N at the vibration time ofpattern 200 ms.shown In the fracture surface, the dimple pattern inpatterns Figure are 9d,e. In the initial stage offractured ultrasonic bonding, the ultrasonic vibration two characteristic observed in white on the surface: the scratch pattern shown in Figure and the dimple pattern in Figure 9d,e. Inpattern. the initial stage of scratch ultrasonic bonding, the in causes friction9b,c, between metal sheets forming the scratch Thus, the pattern observed ultrasonic vibration causes friction between metal sheets forming the scratch pattern. Thus, the the SEM image is not considered to be the “un-bonded region”. The intermetallic bond is achieved scratch pattern in and the SEM imageadhesion is not considered to be thesurface, “un-bonded region”. The thorugh removal of observed oxide films interfacial on the scratched resulting in formation intermetallic bond is achieved thorugh removal of oxide films and interfacial adhesion on of micro-bonds. In addition, these phenomena mainly occur at the stress concentration part the caused scratched surface, resulting in formation of micro-bonds. In addition, these phenomena mainly occur by the horn and anvil tips, or uneven contact part due to rolling. Thus, the formation of micro-bonds at the stress concentration part caused by the horn and anvil tips, or uneven contact part due to rolling. occurs dispersedly at the bonding interface. Since the micro-bond is torn off in the tensile shear test, Thus, the formation of micro-bonds occurs dispersedly at the bonding interface. Since the micro-bond the resultant patternshear is observed in the fractured variation in fractured the bonding strength is torn offdimple in the tensile test, the resultant dimple surface. pattern isThe observed in the surface. observed in Figure 7 isbonding considered to be due to in theFigure transient formationtoofbemicro-bonds. Figure 10 The variation in the strength observed 7 is considered due to the transient shows the relation between the bonding strength and the total area of the white region in the SEM formation of micro-bonds. Figure 10 shows the relation between the bonding strength and the total image computed with the image The total area both the scratch pattern area of the white region in the binarization. SEM image computed with theincludes image binarization. The total area and includespattern. both theThe scratch pattern and the increases dimple pattern. Thecalculated bonding strength with the the the dimple bonding strength with the bondedincreases area, indicating calculated of bonded area, indicating the development of a bonded region. However, it was difficultarea to by development a bonded region. However, it was difficult to identify the accurate bonded identify the accurate bonded area by the image analysis, because the size of the micro-bonds, about the image analysis, because the size of the micro-bonds, about 10 to 100 µm, was so small that it was 10 to 100 μm, was so small that it was difficult to distinguish between the scratch pattern and the difficult to distinguish between the scratch pattern and the dimple pattern. Actual area of the bonded dimple pattern. Actual area of the bonded region is surmised to be smaller than the values shown in region is surmised to be smaller than the values shown in Figure 10. Figure 10.

Figure 9. Fracture surface observation at bonded area by SEM. (a) Bonded interface; (b) Scratch part;

Figure Fracturescratch surface observation bonded by SEM. (a) Bonded (c) 9. Magnified part; (d) Dimpleat pattern; (e)area Magnified dimple pattern. interface; (b) Scratch part; (c) Magnified scratch part; (d) Dimple pattern; (e) Magnified dimple pattern.

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Figure 10. Relation between bonded area and bonding strength.

Figure 10. Relation between bonded area and bonding strength. 4.2. Evaluation of Bonding Strength with Frequency Response Figure 10. Relation between bonded area and bonding strength.

4.2. Evaluation of Bonding Strength with Frequency Response

Figure 11 illustrates the frequency response of the bonded specimen. The horizontal axis is the

4.2.driving Evaluation of Bonding with Frequency Response The vertical axis response is an amplitude that means The the displacement of is the Figure 11 frequency. illustrates theStrength frequency of the spectrum bonded specimen. horizontal axis The fivevertical peaks are seen 606, 1918,spectrum 6033 Hz in thethe frequency response.ofWe drivingspecimen. frequency. The is at an163, amplitude thatspecimen. means displacement Figure 11 illustrates theaxis frequency response of3487, the and bonded The horizontal axisspecimen. is the interpret that these peaks are the resonant frequencies. Figure 12 shows the bonding strength plotted driving frequency. vertical axis is3487, an amplitude means response. the displacement of The five peaks are seen The at 163, 606, 1918, and 6033 spectrum Hz in thethat frequency We interpret against the resonant frequency. The red circle plots show the interface fractures. Blue triangle plots specimen. The five peaks are seen at 163, 606, 1918, 3487, and 6033 Hz in the frequency response. We that these peaks are the resonant frequencies. Figurefracture, 12 shows bonding strength plotted against show the base material fractures. In the interface thethe bonding strength exponentially interpretfrequency. that these peaks are the resonant frequencies. Figure 12 shows theBlue bonding strength the resonant The red circle plots show the interface fractures. triangle plotsplotted show the increased with the resonant frequency. This tendency appeared at each of the resonant frequencies against the resonant frequency. The red circle plots show the interface fractures. Blue triangle shown in Figure 11. It is noted that the bonding strength of the specimen exhibits a correlation with base material fractures. In the interface fracture, the bonding strength exponentially increased plots with the show the basefrequency material better fractures. In the interface fracture, the bonding strength exponentially thefrequency. resonant than the vibration time. this study is shown similar to resonant This tendency appeared at each ofThe theapproach resonantinfrequencies inthe Figure 11. increased theinresonant frequency. This(AFM), tendency appeared at forced each of the resonant frequencies “tappingwith mode” atomic force utilizes the vibration a cantilever. It is noted that the bonding strengthmicroscopy of the specimenwhich exhibits a correlation with theofresonant frequency shown in Figure 11. It isenergy noted that theobject bonding of the specimen exhibits a correlation In AFM, the potential of the altersstrength the resonance behavior of the cantilever. In thewith betterthe than thecase, vibration time. The approach in thistime. study is similar to changes the “tapping in resonant frequency better than the vibration this study is mode” similar to atomic the present the resonant frequency of the vibrating jointThe (theapproach cantilever)in depending on the force “tapping microscopy (AFM), which utilizes the forced vibration of a cantilever. In AFM, the potential interfacial strength between themicroscopy two sheets.(AFM), The results indicate this method in the mode” in atomic force which utilizesusefulness the forcedofvibration of a cantilever. energy of the object alters the resonance behavior the cantilever. In the present case, theOn resonant the bonding strength in aobject non-destructive manner in the case of interface fractures. In evaluation AFM, the ofpotential energy of the altersofthe resonance behavior of the cantilever. In the the of other in thejoint case of the base material fracture, the bonding is deviated from the frequency thehand, vibrating (the cantilever) changes depending onstrength thechanges interfacial strength between present case, the resonant frequency of the vibrating joint (the cantilever) depending on the relation inbetween theindicate interfacial fracture. Figure 13 results shows bonding of specimens. interfacial strength theusefulness two sheets. indicate usefulness ofofthis method the the two sheets.observed The results ofThe this method in theparts evaluation theIndentations bondinginstrength on the bonding part became bigger due to penetration of the knurled edges on the ultrasonic horn On evaluation of the bonding strength in a non-destructive manner in the case of interface fractures. in a non-destructive manner in the case of interface fractures. On the other hand, in the case of the base Withhand, the development ofofthe micro-bonds at thefracture, bondingthe interface, a relative motion betweenfrom the the thetip. other the casestrength the base material bonding strength deviated material fracture, thein bonding is deviated from the relation observed in isthe interfacial bonding sheets during the ultrasonic bonding is hindered. This may cause the relative motionfracture. relation observed in the interfacial fracture. Figure 13 shows bonding parts of part specimens. Indentations Figure 13 shows bonding of specimens. Indentations on the bonding became bigger between the horn tipparts and the upper sheet, causing the enhancement of edge penetration and the due to on the bonding part became bigger due to penetration of the knurled edges on the ultrasonic horn penetration of the knurled on resonant the ultrasonic horn tip. With thestudy development ofthe thesecond micro-bonds thinning of the bondingedges part. The frequency measured in this is affected by tip. With the development of the micro-bonds at the bonding interface, a relative motion between the moment of the area of the bonded part, because the the specimen vibrates withduring a bending Thus,bonding at the bonding interface, a relative motion between bonding sheets themotion. ultrasonic bonding sheets ofduring thefrequency ultrasonicinbonding is hindered. Thisis may cause to theberelative the deviation resonant the basebetween material fracture considered duesheet, to motion the is hindered. This may cause the relative motion the horn tip and the upper between the tip and the upper sheet, causing the enhancement of edge penetration andcausing the thinning of horn the bonded part. the enhancement edge penetration and the thinning of the bonding part. The resonant frequency thinning of theofbonding part. The resonant frequency measured in this study is affected by the second measured in of this affected thebecause secondthe moment ofvibrates the area of athe bonded part,Thus, because moment thestudy area ofisthe bondedby part, specimen with bending motion. the specimen vibrates with a bending Thus, the deviation frequency the deviation of resonant frequencymotion. in the base material fracture of is resonant considered to be dueintothe thebase thinning of the part.to be due to the thinning of the bonded part. material fracture is bonded considered

(a)

(a)

(b)

(b)

Figure 11. Frequency response of bonded specimen. (a) Driving frequency from 10 to 1000 Hz; (b) Driving frequency from 1000 to 10,000 Hz.


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Figure 11. Frequency response of bonded specimen. (a) Driving frequency from 10 to 1000 Hz; (b) Appl. Sci. 2018, 8, 1026 Driving frequency from 1000 to 10,000 Hz.

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Figure 12. 12. Relation frequency each of vibration and bonding Figure Relationbetween between resonant resonant frequency at at each of vibration modemode and bonding strength.strength. The The resonances were observed at the driving frequency of (a) 173 Hz; (b) 626 Hz; (c)(d)1485.9 Hz; resonances were observed the driving frequency of (a) 173 Hz; (b) 626 Hz; (c) 1485.9 Hz; 1717.9 Hz, Hz, and Hz.Hz. (d) 1717.9 and(e) (e)2013.7 2013.7


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Figure 13. 13. Indentation atatcontacted withthe thehorn horn bonding (a) Vibration is 200 Figure Indentation contacted area area with bonding tip. tip. (a) Vibration time istime 200 ms; (b) ms; (b) Vibration ms. Vibrationtime time is is 1000 1000 ms.

4.3. Evaluation of Bonding Strength with ESPI The ESPI arrangement in this study visualizes the out-of-plane displacement distribution of the specimen surface (the out-of-plane component of the bending displacement). Since the frame rate of the CCD camera (30 fps) is much lower than the driving frequency of the vibration test, the ESPI image shows the averaged displacement during a given frame. Figure 14 shows interference fringes observed at four driving frequencies. Here, the left and right ends of the image are the clamped and

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Figure 13. Indentation at contacted area with the horn bonding tip. (a) Vibration time is 200 ms; (b) Vibration time is 1000 ms. 4.3. Evaluation of Bonding Strength with ESPI

TheEvaluation ESPI arrangement in this study visualizes the out-of-plane displacement distribution of the 4.3. of Bonding Strength with ESPI specimen surface (the out-of-plane component of the bending displacement). Since the frame rate of The ESPI arrangement in this study visualizes the out-of-plane displacement distribution of the the CCD camera (30 fps) is much lower than the driving frequency of the vibration test, the ESPI image specimen surface (the out-of-plane component of the bending displacement). Since the frame rate of shows averaged during giventhe frame. Figure 14 shows interference fringes thethe CCD camera displacement (30 fps) is much lowera than driving frequency of the vibration test, the observed ESPI at four driving frequencies. Here, the left and right ends of the image are the clamped and oscillating image shows the averaged displacement during a given frame. Figure 14 shows interference fringes (driven by theattransducer) of the specimen, respectively. In the fringe patterns (a)–(c), itand is seen observed four driving ends frequencies. Here, the left and right ends of the image are the clamped that oscillating in going from the by clamped to the oscillating end, the fringe interval decreases. This indicates (driven the transducer) ends of the specimen, respectively. In the fringe patterns (a)– that (c), it is seen that in going from the clamped to the oscillating the fringe interval decreases. the out-of-plane displacement increases nonlinearly from theend, clamped to the oscillating end.This In other indicates that the out-of-plane displacement increases nonlinearly from the clamped to the oscillating words, the specimen underwent a bending motion. As the driving frequency increases from 160 Hz to other words, underwent a bending motion. As the driving frequency increases 170.5end. Hz In (Figure 14a–c),the thespecimen fringe interval becomes smaller, indicating that in this frequency range from 160 Hz to 170.5 Hz (Figure 14a–c), the fringe interval becomes smaller, indicating that in this the vibration amplitude increases with the driving frequency. At the driving frequency of 173 Hz frequency range the vibration amplitude increases with the driving frequency. At the driving (Figure 14d), the type of fringe pattern observed in Figure 14a–c that represents a monotonic increase frequency of 173 Hz (Figure 14d), the type of fringe pattern observed in Figure 14a–c that represents in the out-of-plane displacement from the clamped to oscillating end disappears. Instead, a bright a monotonic increase in the out-of-plane displacement from the clamped to oscillating end disappears. region appears in the middle of the specimen. This is This considered indicate an mode Instead, a bright region appears in the middle of the pattern specimen. pattern istoconsidered to eigen indicate (resonance) oscillation, and is called the characteristic pattern hereafter. an eigen mode (resonance) oscillation, and is called the characteristic pattern hereafter.

Figure Increasein innumber number ofoffringes as resonance is reached.(a) - (c) show patterns the driving Figure 14. 14. Increase fringes as resonance is reached. (a–c)fringe show fringeatpatterns at the frequency near the resonance, and (d) 173 shows a fringe pattern at thepattern resonance. driving frequency near the resonance, andHz(d) 173 Hz shows a fringe at the resonance.

In a characteristic pattern, the bright region is interpreted as representing a node of the oscillation. Since dark fringes generally represent null displacement and at a node of eigen mode oscillation the oscillation is null, this interpretation may sound counterintuitive. However, we believe this is a correct interpretation. The ground for this argument is as follows. When the specimen is oscillated at resonance, the vibration is close to null in the vicinity of nodal points and much larger in the other areas. In the present experimental arrangement, it is expected that the oscillation in non-nodal areas is so large that the speckle pattern loses correlation between the subtracted (after deformation) frame and the subtracted-from (before deformation) frame. (See Section 3.3. Electronic Speckle Pattern Interferometry (ESPI) for the image subtraction.) Consequently, the gray scale level of the captured image tends to be saturated in both the after-deformation and before-deformation frames, converging to the maximum level of 255 (in the 8-bit format). Therefore, when the image subtraction is made, the gray-scale values of the before and after deformation images are close to each other in the vicinity of 255. This makes these areas appear to be dark. On the other hand, the gray-scale value in the area near a nodal point

in the other areas. In the present experimental arrangement, it is expected that the oscillation in nonnodal areas is so large that the speckle pattern loses correlation between the subtracted (after deformation) frame and the subtracted-from (before deformation) frame. (See Section 3.3. Electronic Speckle Pattern Interferometry (ESPI) for the image subtraction.) Consequently, the gray scale level of the captured image tends to be saturated in both the after-deformation and before-deformation Appl. Sci. 2018, 8, 1026 11 of 16 frames, converging to the maximum level of 255 (in the 8-bit format). Therefore, when the image subtraction is made, the gray-scale values of the before and after deformation images are close to each the vicinity of 255. This makes areas appear to be dark.Consequently, On the other hand, the graytendsother to bein a low value that fluctuates fromthese subtraction to subtraction. in the subtracted scale value in the area near a nodal point tends to be a low value that fluctuates from subtraction to image the nodal area appears to be whiter. subtraction. Consequently, in the subtracted image the nodal area appears to be whiter. Figure 15 shows the frequency response of the bonded specimen detected by ESPI. The total Figure 15 shows the frequency response of the bonded specimen detected by ESPI. The total intensity of the specimen was plotted against the driving frequency. Original data was smoothed with intensity of the specimen was plotted against the driving frequency. Original data was smoothed the 10-point The averaged total intensity showsshows a rapid decrease at certain with themoving 10-pointaverage moving method. average method. The averaged total intensity a rapid decrease at frequencies, which appears as appears sharp minima Figurein 15.Figure Based the above we interpret certain frequencies, which as sharpinminima 15.on Based on the argument, above argument, we that these minima represent resonant frequencies. interpret that these minima represent resonant frequencies.

Figure 15. Frequency response of bonded specimen detected by ESPI.

Figure 15. Frequency response of bonded specimen detected by ESPI.

Characteristic patterns were observed at other frequencies as shown in Figure 16. Figure 16a, b, e–g

Characteristic patterns were observed at other as shown in Figure 16. Figure 16a,b,e–g show the similar bending motion to Figure 14d. frequencies The nodes appeared bright. The number of nodes with the increase in driving frequency, as expected. In Figure bright. 16c, d, h,The the number fringes appear showincreased the similar bending motion to Figure 14d. The nodes appeared of nodes in thewith transverse direction. These patterns canasbeexpected. interpreted representing vibration increased the increase in driving frequency, In as Figure 16c,d,h, torsional the fringes appear in modes. Appl. Sci. 2018, 8, x FOR PEER REVIEW of 17 the transverse direction. These patterns can be interpreted as representing torsional vibration12modes.

Figure 16. Visualized vibration modes by ESPI. (a) First mode; (b) Second mode; (c) Third mode; (d)

Figure 16. Visualized vibration modes by ESPI. (a) First mode; (b) Second mode; (c) Third mode; Fourth mode; (e) Fifth mode; (f) Sixth mode; (g) Seventh mode; (h) Eighth mode. (d) Fourth mode; (e) Fifth mode; (f) Sixth mode; (g) Seventh mode; (h) Eighth mode.

In this fashion, the ESPI method allowed us to find several resonant patterns and frequencies. Accordingly, we compared the bonding strength with the resonant frequency. Figure 17 plots the bonding strength as a function of the resonant frequency determined by the above-mentioned method. Yellow rhombus plots show the interface fracture detected by EPSI. Red circle plots show the interface fracture detected by the capacitance displacement gauge (Figure 12). Similarly, blue

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In this fashion, the ESPI method allowed us to find several resonant patterns and frequencies. Accordingly, we compared the bonding strength with the resonant frequency. Figure 17 plots the bonding strength as a function of the resonant frequency determined by the above-mentioned method. Yellow rhombus plots show the interface fracture detected by EPSI. Red circle plots show the interface fracture detected by the capacitance displacement gauge (Figure 12). Similarly, blue triangle plots show the base material fracture detected by the capacitance displacement gauge. Notice that the results from the capacitance displacement gauge and those from the ESPI measurement show the same general dependence on the resonant frequency. The two data sets clearly indicate that the bonding strength exponentially increases with the resonant frequency. This result demonstrates that the optoacoustic method can evaluate the bonding strength in a non-destructive and non-contact fashion. It also indicates that the resonant frequency can be detected by monitoring the total intensity of speckles in the observing Appl. Sci. 2018,surface. 8, x FOR PEER REVIEW 13 of 17

Figure 17. Relation between resonant frequency ESPI and andbonding bonding strength. (a) 1st Figure 17. Relation between resonant frequencydetected detected by by ESPI strength. (a) 1st resonant frequency (173Hz), and frequency(626Hz). (626Hz). resonant frequency (173Hz), and(b) (b)2nd 2nd resonant resonant frequency

5. Finite Element Analysis 5. Finite Element Analysis Model 5.1. 5.1. Model We carried out eigenvalue analysis using the commercial finite element analysis (FEA) solver We carried out eigenvalue analysis using the commercial finite element analysis (FEA) solver LS-DYNA (R 7.1, Livermore Software Technology Corp., Livermore, CA, USA). Figure 18 shows the LS-DYNA (R 7.1, Livermore Software Technology Corp., Livermore, CA, USA). Figure 18 shows the specimen model in FEA. The size of the upper and lower plates was commonly 50.0 mm in length, specimen model in FEA. sizeinofthickness. the upper andsheets lowerwere plates wasin commonly length, 11.5 mm in width, and The 0.8 mm Two lapped such a way50.0 thatmm the in total 11.5 length mm inof width, and 0.8 mm in thickness. Two sheets were lapped in such a way that the total length the bonded specimen was 85 mm. The 8-node hexahedron element was employed. The of the bonded specimen was 85 mm. The 8-node hexahedron element was employed. The element element size was 0.125 × 0.125 × 0.1 mm. The numbers of nodes and elements were 671,142 to 670,313 size was depending 0.125 × 0.125 × 0.1 mm. area The and numbers of respectively. nodes and elements were was 671,142 to 670,313 on the bonded 588,800, The specimen modeled as an depending elastic material. Physical of A6061-T6 are shown Table 1. The mm edgeas of an the elastic lower plate on the bonded area properties and 588,800, respectively. The in specimen was10modeled material. was fixed. Because the specimen was fixed by the screw of a jig at a point 10 mm away from the edge Physical properties of A6061-T6 are shown in Table 1. The 10 mm edge of the lower plate was fixed. in thethe experiment, was considered thatscrew the clamp force applied at this area. from The other side ofin the Because specimenit was fixed by the of a jig atwas a point 10 mm away the edge the specimen was a free edge. Assuming that the contact area with the bonding tip was 10 × 10 mmof the experiment, it was considered that the clamp force was applied at this area. The other side and the micro-bond was preferentially formed in the bonding interface under the knurled edges on specimen was a free edge. Assuming that the contact area with the bonding tip was 10 × 10 mm and the bonding tip, 13 × 13 bonded areas were arranged on the bonded interface. As shown in Figure 19, the micro-bond was preferentially formed in the bonding interface under the knurled edges on the the bonded area was expressed by merging nodes on the contact surfaces of two plates. In addition, bonding tip, 13 of × the 13 bonded areas were arranged the bonded interface. As shown Figure 19, enlargement bonded area was modeled by anon increase in number of the merged nodesinshown the bonded area was expressed by merging nodes on the contact surfaces of two plates. In addition, 2 2, as black points in Figure 19b–d. The total bonded area was set to five conditions: 2.06 mm , 2.64 mm 2 2 2 enlargement of the bonded area was modeled by an increase in number of the merged nodes shown as 5.90 mm , 6.89 mm , and 12.51 mm .

black points in Figure 19b–d. The total bonded area was set to five conditions: 2.06 mm2 , 2.64 mm2 , 5.90 mm2 , 6.89 mm2 , and 12.51 mm2 .

Figure 18. Bonded specimen model by finite element analysis (FEA). Table 1. Physical properties of A6061-T6.

and the micro-bond was preferentially formed in the bonding interface under the knurled edges on the bonding tip, 13 × 13 bonded areas were arranged on the bonded interface. As shown in Figure 19, the bonded area was expressed by merging nodes on the contact surfaces of two plates. In addition, enlargement of the bonded area was modeled by an increase in number of the merged nodes shown 2 2 Appl. as Sci.black 2018, points 8, 1026 in Figure 19b–d. The total bonded area was set to five conditions: 2.06 mm , 2.64 mm13, of 16 2 2 2 5.90 mm , 6.89 mm , and 12.51 mm .

Figure Bondedspecimen specimen model model by analysis (FEA). Figure 18.18. Bonded byfinite finiteelement element analysis (FEA). Table 1. Physical properties of A6061-T6. Table 1. Physical properties of A6061-T6. Material

Density

Young’s modulus

Poison’s ration

Material

3 Density (Kg/mm 3 )

Young’s Modulus (GPa)

Poison’s Ration

A6061-T6

2.7 × 10−6

68.9

0.33

(Kg/mm )


A6061 – T6

2.7

×10-6

(GPa) 68.9

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0.33

Figure 19. Enlargement of bonded area.

Figure 19. Enlargement of bonded area.

5.2. Results

5.2. Results

Figure 20 illustrates the relation between the bonded area and the resonant frequency at each

vibration mode obtained by FEA. The resultsthe show that thearea resonant goesfrequency up as the bonded Figure 20 illustrates the relation between bonded and frequency the resonant at each area increases. The relation between them is exponential. The bonded area increased as the resonant vibration mode obtained by FEA. The results show that the resonant frequency goes up as the increased, similar to the experimental indicated in Figures 12 and This factas bondedfrequency area increases. The relation between them results is exponential. The bonded area17. increased supports the claim that the resonant frequency of the bonded part increases with the bonded area. the resonant frequency increased, similar to the experimental results indicated in Figures 12 and 17. The vibration modes and the resonant frequency analyzed by FEA are illustrated in Figure 21. The This fact supports the claim that the resonant frequency of the bonded part increases with the colors of the pattern presented in Figure 21 represent the relative magnitude of the displacement bondedvector. area. The Thered vibration and the resonant frequency FEA displacement. are illustrated means modes the larger displacement, and the blue analyzed means theby smaller in Figure 21. 21a, Theb,colors of the pattern presented inmodes. FigureThe 21 nodes represent the relative magnitude Figure e, f, h showed the bending vibration of vibration were seen as a blueof the displacement red means the larger displacement, and the blue means smaller area. Figure vector. 21c, d, g The illustrated the torsional vibration modes. These vibration modes are the consistent displacement. Figure 21a,b,e,f,h showed the bending vibration modes. The nodes of vibration were with the experimental result observed in the ESPI measurement, except for the eighth vibration mode. seventh shown in Figure 21g coincides with the vibration eighth mode observed in ESPI (Figure modes 16h). seen asThe a blue area.mode Figure 21c,d,g illustrated the torsional modes. These vibration The resonant frequency obtainedresult by FEA was plotted the experimental result ESPI in are consistent with the experimental observed in theagainst ESPI measurement, except forinthe eighth Figure 22. The result indicates that the FEA result is correlated with the ESPI result, and the vibration vibration mode. The seventh mode shown in Figure 21g coincides with the eighth mode observed can16h). be visualized using ESPI. in ESPImode (Figure The resonant frequency obtained by FEA was plotted against the experimental result in ESPI in Figure 22. The result indicates that the FEA result is correlated with the ESPI result, and the vibration mode can be visualized using ESPI.

Figure 20. Relation between first resonant frequency and bonded area.

Figure 21a, b, e, f, h showed the bending vibration modes. The nodes of vibration were seen as a blue area. Figure 21c, d, g illustrated the torsional vibration modes. These vibration modes are consistent with the experimental result observed in the ESPI measurement, except for the eighth vibration mode. The seventh mode shown in Figure 21g coincides with the eighth mode observed in ESPI (Figure 16h). The resonant frequency obtained by FEA was plotted against the experimental result in ESPI in 22. Appl. Figure Sci. 2018, 8, The 1026 result indicates that the FEA result is correlated with the ESPI result, and the vibration 14 of 16 mode can be visualized using ESPI.

Figure 20. Relation between first resonant frequency and bonded area.

Figure 20.REVIEW Relation between first resonant frequency and bonded area. Appl. 2018, x FOR PEER Appl. Sci.Sci. 2018, 8, x8,FOR PEER REVIEW

15 17 of 17 15 of

Figure Composite displacements in x, zzy,directions. z directions. (a) First mode: 118.03 Figure 21. Composite displacements in x, (a) First mode: 118.03 Hz, (b)Second Second Figure 21.21. Composite displacements in the thethe x, y, y, directions. (a) First mode: 118.03 Hz,Hz, (b)(b) Second mode: 701.60 Hz, (c) Third mode: 1426.74 Hz, (d) Fourth mode: 1659.86 Hz, (e) Fifth mode: 1994.54 mode: 701.60 Hz, (c) Third mode: 1426.74 Hz, (d) Fourth mode: 1659.86 Hz, (e) Fifth mode: 1994.54 mode: 701.60 Hz, (c) Third mode: 1426.74 Hz, (d) Fourth mode: 1659.86 Hz, (e) Fifth mode: 1994.54 Hz, Sixth mode: 4002.46 Hz, Seventh mode: 4019.39 Eighth mode: 6747.37 (f) Sixth mode: 4002.46 Hz, (g) Hz, Seventh mode: 4019.39 Hz, (h) Eighth mode: 6747.37 Hz. Hz,Hz, (f) (f) Sixth mode: 4002.46 (g)(g) Seventh mode: 4019.39 Hz,Hz, (h)(h) Eighth mode: 6747.37 Hz.Hz.

Figure 22. Resonance detected ESPI vs. resonance by FEA. Figure Resonance detected by ESPI vs. resonance byby FEA. Figure 22.22.Resonance detected byby ESPI vs. resonance FEA.

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6. Conclusions We applied the optoacoustic method to evaluate the bonding strength in a non-destructive and non-contact fashion. The experiment showed that the bonding strength of an ultrasonically bonded joint increased with the enlargement of the bonded area. The increase in the bonded area led to the higher resonant frequency of the joint in the vibration test: the resonant frequency increased exponentially with the bonding strength. It was indicated that the joint strength is correlated with the resonant frequency of the joint better than with the vibration time. In addition, the resonant frequency and the vibration modes were visualized by ESPI. The exponential correlation between the bonded area and the resonant frequency was confirmed by FEA. The vibration modes obtained by FEA were consistent with the experimental results in ESPI. This study indicated that it is possible to evaluate the ultrasonic bonding strength by obtaining the correlation curve beforehand between the bonding strength measured by the tensile shear test and the resonant frequency with a non-destructive method. Author Contributions: Conceptualization: S.Y., Funding acquisition: T.S., Investigation: T.K. and T.S., Project administration: T.S., Writing, original draft: T.K., Writing, review and editing: S.Y. and T.S. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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