Abstract

 

Keywords: Fatigue crack; Crack depth; Microwave; Nondestructive testing; Coaxial line sensor

 

Introduction

 

Recently, we have developed a microwave imaging system utilizing an open-ended coaxial line sensor to inspect the delamination in IC packages [13]. Coaxial line can support transverse electromagnetic (TEM) waves without cutoff frequency for the fundamental TEM mode. Therefore, the operating frequency band can be broad, and it is also possible to decrease the size of the aperture for increasing the spatial resolution. Moreover, since the electric field at the sensor aperture is center of symmetry, the detection capability is independent of the scanning direction.

 

Principle

 

(1)

 

, can be written as

r=e-2ad

(2)

 

r=r0 e-2a d

(3)

 

.

By transforming Eq. (3) into a decibel form, the amplitude of the reflection coefficient in decibel, A, can be written as

A=20 log r0  - 40(log e)ad

(4)

 

Hence, an amplitude difference of the reflection coefficient in decibel, DA , between the case that no crack exists and the case that a crack is under the sensor aperture, can be written as

DA = 40(log e)ad

(5)

 

The constant a is related to the closure stress of the crack, and is a function of the operating frequency. Once a is determined, crack depth d could be eval‎uated by using the measurement result of DA.

 

Experimental Procedure

 

Fig 1: Configuration of the microwave inspection system.

Fig 2: Photograph of the open-ended coaxial line sensor.

 

Fig 3: Distribution of the electric field at the sensor aperture.

Fig 4: Descript-xive geometry of the specimen.

 

 , was determined with the average value of the crack depths measured on both sides of the specimen. The stress ratio, R, defined as a ratio of the minimum to the maximum value of the stress intensity factor, was maintained during the crack growth. After the desired depth of the crack was reached, the plate was machined and polished to remove the initial notch, leaving a true fatigue crack in the remaining material. The conditions used for introducing fatigue cracks in the specimens are listed in Table 1. Since fatigue cracks introduced in the same material under the same conditions as the present experiment had been eval‎uated to be closed by ultrasonic technique [6], the fatigue cracks introduced in S1 to S3 are considered to be closed. After the microwave inspection was carried out, the specimens were fractured for the measurement of the depth of the cracks. The value of the crack depth, d, indicated in Table 1 is an average of the measurements performed at the five locations in the direction of the specimen width on the fractured surface.

22 6 0.1 4.51 Table 1: Conditions used for introducing fatigue cracks.Specimen

Maximum stress intensity factor KImax (MPa·m1/2 )

Frequency (Hz)

Stress ratio R

Crack depth d (mm) S1 22 6 0.1 1.70 S2 22 6 0.1 2.44 S3

 

 

Results and Discussion

 

 and P₂

 shown in Fig. 6, was used to calculate the amplitude difference DA.

(6)

 

 and P₂

 express the magnitude of the two separated peaks of the W-shaped signal, respectively.

Fig 5: A 3D view of the amplitude of the reflection coefficient obtained by scanning the crack in S3 in the x- and y-directions.

Fig 6: The amplitude of the effective reflection coefficient obtained by scanning the crack in S1 to S3 in the x-direction.

 

It should be noted that the intensity of the microwave transmitted into the crack is significantly small, therefore, the characteristic signal is difficult to be measured if the spatial resolution and S/N are not high enough.

Fig 7: The Relation of the microwave response with the sensor position for the detected crack.

Fig 8: Relationship between the magnitude of the characteristic signal and the crack depth.

 

Figure 8 shows the relationship between the magnitude of the characteristic signal,DA, and the crack depth d. For each specimen, the crack was scanned in the x-direction, at seventeen locations with a pitch of 2 mm in the y-direction. As shown in Fig. 8, the magnitude DA is observed to be in a linear relation with the crack depth d, which is consistent with Eq. (5). By using this relationship, once the characteristic signal is measured, the depth of the crack could be eval‎uated quantitatively.

 

Conclusions

 

The detection of closed fatigue cracks by using microwaves under a condition of no contact and without any coupling medium has been demonstrated. A coaxial line sensor with high spatial resolution has been proved to be a powerful tool for inspection of small fatigue cracks. The magnitude of the measured characteristic signal was observed to be in a linear relation with the depth of the crack. Thereby, a promising new technique could be built for reliable quantitative eval‎uation of closed fatigue cracks without any coupling medium.

Acknowledgements

 

We thank Mr. M. Mikami for help with preparing the specimens. This work was partly supported by The Ministry of Education, Culture, Sports, Science, and Technology under Grant-in-Aid for COE Research 11CE2003, and by Japan Society for the Promotion of Science under Grant-in-Aid for Scientific Research (B)(2) 13555023 and 13555192, and Encouragement of Young Scientists (A) 12750065.

References

 

 

1. Buck O, Skillings BJ. Effects of closure on the detection probability of fatigue cracks. Review of Progress in Quantitative Nondestructive Eval‎uation, New York, 1982;1:349-53.

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3. So KK, Sinclair AN. Size measurement of tightly closed surface cracks by surface acoustic waves. Nondestructive Testing Communication 1987;3(3):67-74.

4. Elber W. Fatigue crack closure under cyclic tension. Engineering Fracture Mechanics. 1970;2(1):37-45.

5. Saka M, Schneider E, Hoeller P. A new approach to detect and size closed cracks by ultrasonics. Research in Nondestructive Eval‎uation 1989;1(2):65-75.

6. Ahmed SR, Saka M. Quantitative nondestructive testing of small, tight cracks using ultrasonic angle beam technique. Materials Eval‎uation 2000;58(4):564-74.

7. Pozar DM. Microwave Engineering, Wiley, New York, 1998.

8. Ju Y, Saka M, ABé H. Microwave nondestructive detection of delamination in IC packages utilizing open-ended coaxial line sensor. NDT & E International 1999;32(5):259-64.

9. Hruby RJ, Feinstein L. A novel nondestructive noncontacting method for measuring the depth of thin slits and cracks in metals. The Review of Scientific Instruments 1970;41(5):679-83.

10. Auld BA. Ferromagnetic resonance flaw detection. Physics in Technology 1981;12(7):149-54.

11. Yeh CY, Zoughi R. Sizing technique for slots and surface cracks in metals. Materials Eval‎uation 1995;53(4):496-501.

12. Huber C, Abiri H, Ganchev S, Zoughi R. Analysis of the crack characteristic signal using a generalized scattering matrix representation. IEEE Transactions on Microwave Theory and Techniques 1997;45(4):477-84.

13. Ju Y, Saka M, ABé H. NDI of delamination in IC packages using millimeter-wave. Proc. the 17th IEEE Instrumentation and Measurement Technology Conference, IEEE, Baltimore, 2000;3:1597-602.

14.

 

 

 

 

 

Applications of Microwaves in Non-Destructive Testing H S Ku PhD Graduate, IRIS, Swinburne University of Technology and Faculty, University of Southern Queensland (USQ), Australia F Siu PhD Candidate, IRIS, Swinburne University of Technology; Lecturer, Kwun Tong Campus, Institute of Vocational Education, Hong Kong, China E Siores Professor and Executive Director, Industrial Research Institute, Swinburne (IRIS), Swinburne University of Technology (SUT), Henry Street, Hawthorn, VIC 3122, Australia. J A R Ball Prof & Head, Department of Electrical, Electronic & Computer Engineering, University of Southern Queensland (USQ), West Street, Toowoomba, 4350 Australia. Corresponding Author: Harry Siu-lung Ku Affiliation: Faculty of Engineering and Surveying University of Southern Queensland Tel. No.: (07) 46 31-2919 Email : ku@usq.edu.au Fax. No.: (07) 4631-2526 Address: Faculty of Engineering and Surveying University of Southern Queensland West Street, Toowoomba, 4350 Australia.

Abstract 

 

This paper describes a new area of application of microwave energy in non-destructive testing (NDT), in which the quality of adhesively bonded products can be detected by the intrinsic spectrum signals generated by a variable frequency microwave (VFM) source. When microwave energy is launched into a metallic cavity, which is partially or fully loaded with a material, the electromagnetic energy reflects backward and forward between the cavity walls and travels through the material many times until a final standing wave condition is established. Materials property variables like internal or surface defects, dielectric properties, physical geometry and, physical and chemical properties can contribute to its unique signal output characteristics. The reflected and input signals form a ratio, percentage of reflectance, which can be monitored and plotted as a function of the frequency. The percent of reflectance against frequency curve is called the microwave reflective spectrum. The spectrum generated can be used as a signature curve for assessing bond quality during processing. By this way, the same material under the same processing parameters provides a common characteristic curve, which can be used as a tool to provide a rapid, on-line, non-intrusive, non-destructive and volumetric monitoring of adhesively bonded polymer materials.

Keywords: variable frequency microwaves (vfm), reflective spectrum, microwave-assisted non-destructive eval‎uation and adhesives.

 

Introduction

 

Microwave irradiation is a unique energy source, which offers an alternative means to provide fast processing times for an array of advanced materials using either single or multi mode fixed frequency, or multimode variable frequency. The unique feature about microwaves is their deep penetration into materials with substantial reduction in process time, often by as much as 10 to 1 (Siu et al., 1999a). The material properties of greatest importance to microwave processing of a dielectric are the complex relative permittivity e =e¢ - je²and the loss tangent, tan d= e²/ e¢(NRC, 1994). The real part of the permittivity, e¢, sometimes called the dielectric constant, mostly determines how much of the incident energy is reflected at the air-sample interface, and how much is absorbed. The imaginary part of the permittivity e² called the dielectric loss. The most important property in microwave processing is the loss tangent, tan d, which predicts the ability of the material to convert the absorbed energy into heat. For optimum microwave energy coupling, a moderate value of e¢ to enable adequate penetration, should be combined with high values of e² and tan d, to convert microwave energy into thermal energy. Microwave heating is basically volumetric and provides an even temperature distribution throughout the material. It is based on the interaction of an electromagnetic field with both the adherend (polycarbonate) and the adhesive. The amount of power absorbed (P) by the adhesive greatly influences its curing time and resulting bond strength, and is dependent on both the permittivity and the loss tangent of the adhesive as follows:

(1)

 

 is the dielectric permittivity in free space;

E is the electric field strength.

Note that the adherend will also absorb the microwave energy in the same behaviour as the adhesive. But in this case the adherend is polycarbonate which is a low loss material and hence its microwave absorption capability is neglected.

In this experimental task uses the lap joint of two samples in a microwave field for non-destructive eval‎uation (NDE). The variable frequency microwaves (VFM) are generated by high power helix travelling wave tubes (TWTs) (Everleigh et al., 1994). Helix TWTs provide all possible bandwidth requirements for applications requiring modest power levels. The VFM facilities available at the Industrial Research Institute, Swinburne, Swinburne University of Technology are Microcure 2100 model 250 and Microcure VW 1500. They are shown in Figures 1and 2 respectively. Microcure 2100 model 250, which has a maximum power, output of 250 W and operates within a frequency range of 6.5-18 GHz. Microcure VW 1500 operates within a frequency range of 2-8GHz and has a maximum power output of 125 W. The cavity dimension of Microcure VW1500 are 250 mm x 250 mm x 300 mm and the Microcure 2100 model 250 has a cavity size of 300 mm x 275 mm x 375 mm. A block diagram of the system of Microcure 2100 is shown in Figure 3.

Fig 1: A VFM Facility, Microcure 2100 model 250.

Fig 2: Another VFM Facility, Microcure VW 1500.

 

Fig 3: A Block Diagram of the System of Microcure 2100.

 

 

Principles of the Resonant Mode MA-NDE

 

The resonant mode MA-NDE technique is based on the variable frequency microwave (VFM) concept and interactions between microwaves and materials at high frequencies. When a microwave signal of a given frequency is launched into a cavity which is fully or partially filled with material, the microwaves will reflect back and forth between cavity walls and travel through the material many times before establishing a final standing wave condition. There are three possible outcomes for the interactions of these waves:

 

15. partially confined and partially reflected,

16. totally reflected back to the launcher, and

17. totally confined within the cavity. The final condition depends on the cavity dimensions, frequency launched and material properties. The ratio between the reflected signal and the input signal can be monitored and plotted as a function of the frequency. This signal ratio versus frequency curve is called the microwave reflective spectrum.

 

 

The study focused on the non-destructive testing method for adhesive bonding application of polycarbonate by microwave irradiation. Characterisation studies through utilisation of an appropriate frequency band for microwave processing of polycarbonate were performed. In a VFM environment, the microwave oscillator generated a signal, which was launched into the multimode microwave cavity. A directional coupler identified the strength of the forward and reflected microwave signals. A controller compensated for the gain of the amplifier to ensure a constant forward power level across a range of frequencies, while sweeping occurred. The generated data was transmitted to an off-line PC which recorded the information including, frequency forward and reflected powers and also calculated a percentage of reflected energy to form the microwave reflective spectrum. This signal ratio output at a given frequency was used as an indication of all the interactions with materials inside the cavity.

 

Experiments and Results

 

It was found that the best frequency to process the adhesive was in the frequency range of 9.5-12.5 GHz (Siu et al., 1999a; 1999b), which could also be conventionally cured at 65^o

C isothermally for 40 minutes. Microcure 2100 model 250 VFM facility was therefore employed for the experiment. The central frequency selected was 11 GHz with sweeping bandwidth of 1.1 GHz and sweeping rate of 0.1 second. The power output was 200 W. Polycarbonate samples of 101 mm x 25 mm x 1 mm were joined and the lapped area was 6.5 cm²

. After applying a calculated amount of the adhesive to the lapped area, the samples were then placed in the center of the cavity on top of a Teflon block.

Microwave reflective spectra were then taken at the following moments during the cure status of the adhesive:

 

18. before cure with sample at 45^o

C for 6 minutes,

19. isothermally cured sample at 45^o

C for 10 minutes,

20. isothermally cured sample at 65^o

C for 6 minutes,

21. after sample isothermally cured for 20 minutes, and

 

 

Fig 4: Microwave Reflective Spectra during Isothermal Adhesive Curing at 65o C.

 

In general, the spectra shift to the left during curing and the changes in peak location and shape are directly related to the change of the dielectric properties. Comparing spectrum A to B, the peaks not only shift to the left, but also change in shape and become flattened. Comparing spectrum B to C, the peaks become more flattened. Spectrum change between C and D is not much. Spectrum change between D and E is even less. In other words, the adhesive almost reached the ultimate extent of cure after isothermal curing at 65^o

C for 6 minutes by VFM irradiation and essentially no further reaction occurred after 20 minutes.

 

Conclusion

 

Microwave assisted nondestructive (MA-NDT) eval‎uation techniques using a resonant microwave mode was presented. Such an eval‎uation system provides an on-line, volumetric, non-contact, non-intrusive and non-destructive monitoring feature. By comparing a microwave reflective spectrum during the production processes to the standard spectra, a computerized monitored system can be used to regulate the process-input parameters for proper adjustment and compensation. Such methodology can be used for on-line assessing and real-time eval‎uating product quality. Successful application of this technique depends largely on the database gathered, which can greatly reduce or eliminate products with defects generated during the manufacturing process.

References

 

 

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26. Everleigh, C A, Johnson, A C, Espinosa, R J and Garard, R S (1994), Use of High Power Travelling Wave Tubes as a microwave Heating Source, Material Research Society Symposium Proceeding, Vol. 347, pp 79-89.

27. National Research Council (NRC) (1994), Microwave Processing of Materials, National Materials Advisory Board, Commission on Engineering and Technical Systems, National Academy Press, USA, pp1-7, 11-2, 100, 105.

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31. Siu, F, Siores, E, and Taube, A (1999b), Variable Frequency Microwaves (VFM) for Non-Destructive Testing and Eval‎uation of Adhesively Bonded Polymers, ASME, Vol. 234, pp77-85.

32. Steinberg, B D and Subbaram H M (1991), Microwave Imaging Techniques, p 3, John Wiley & Sons, Inc.

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