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NDI on Bonded Sandwich Structures with Foam Cores and Stiff Skins - Shearography the Answer?By Joakim Andersson and Barend van den Bos
CSM Materialteknik AB
P.O Box 1340 Sweden
|Fig 1: Typical structure with artificial defects|
|Fig 2: Disbond type describtion.|
|Panel||Core density and height||Skin||Doubler||Defect size||Remarks|
|A1||51 kg/m3 - 30 mm||1 mm||-||50x50 mm||Defect type III + impact damages|
|A5||110 kg/m3 - 45 mm||2 mm||-||30x30 & 50x50 mm||Defect type III + impact damages|
|A6||51 kg/m3 - 50 mm||1,25 mm||-||50x50 mm||Defect type III + impact damages|
|A7||51 kg/m3 - 50 mm||0,6 mm||-||30x30 & 50x50 mm||Defect type III + impact damages|
|NDT1||51 kg/m3 - 50 mm||1 mm||-||Ø 50 mm||Defect type I and II|
|NDT2||-||1 mm||1 mm||Ø 50 mm||Defect type I and II|
|NDT3||51 kg/m3 - 50 mm||1 mm||1 mm||Ø 50 mm||Defect type I and II|
|Table 1: Panel design, ref. Figure 1. All panels have stainless steel sheets with Rohacell foam core (except NDT2) and Scotchweld 9323 B/A adhesive.|
The investigation showed that within the round robin test, shearography was the only technique that managed to detect all defects. It was also possible to detect defects in areas with very stiff skins (up to 3 + 2 mm) with shearography if heat was used as excitation method. The depth position could be estimated from characterisation of the indications. More detailed results are given separately for the round robin test and the application trials.
5.1) Round robin test
Techniques to be used were chosen from previous knowledge and techniques described in (1). With regards to the critical criteria, see Background, the following inspection methods were chosen to be evaluated in the Round-Robin test:
The round robin tests were performed on test samples NDT1 through NDT3 only.
The inspections with the techniques 1) through 3) have all been performed with the Sonic Bond Master from Staveley. Specific information on that equipment can be found in Staveley operations handbook. In addition to the methods above radiographic inspection was used to verify some of the results. On test sample NDT1 through NDT3 were voids detected with radiography for all defects except the defect type II on sample NDT3.
5.1.1) Ultrasonic, resonance
For the resonance inspection three different probes and frequencies were used, 65 kHz, 110 kHz and 250 kHz.
On sample NDT2 (metal-to-metal bonding) both defect types (I and II) could be detected with the 65 kHz and 110 kHz probes. Sample NDT1 and NDT3 (skin-to-core bonding) was inspected but no reliable indications could be obtained from any of the two defect types.
5.1.2) Mechanical Impedance analyser
The probes S-MP-3 and S-MP-4 from Staveley were used.
During the inspection the defects were indicated in sample NDT2 using the amplitude mode. A frequency of approximately 7 kHz gave the best signal to noise ratio for the amplitude mode. No reliable results could be obtained in the phase mode at any frequency.
No reliable results could be obtained from the defects in NDT1 and NDT3.
5.1.3) Pitch and catch technique
During the trials Staveley probe SPO-5699-8 was used.
No good results were obtained on samples NDT1 and NDT3.
The defect type I was detected on test sample NDT2 with the frequency range 37 to 43 kHz. Defect type II in NDT2 gave only vague indications that were not different to the general variation obtained in defect free areas.
5.1.4) Bondimeter technique
The Bondimeter technique uses vacuum to stress the part. A micrometer measuring skin surface deformation is fixed into the centre of a vacuum cup. In areas of disbond, the negative pressure will cause a surface deformation that can be picked up by the micrometer.
On sample NDT1 different surface deformation could be obtained at the defects. On sample NDT3, no difference in deformation was indicated for the defects.
Trials have also been made on the panels A1 through A7. Some positive results were achieved on the 50x50 mm defects, see table 2. As the defect type III is at the edges of the part the micrometer could not get closer than 45 mm to the edge due to the size of the vacuum cup (90 mm diameter). The 30x30 mm defects will therefore never affect the micrometer. Even for the 50x50 mm defects only the outer edges of the defect affected the micrometer. A disbond further away from the edges is expected to give stronger indications.
Uniform indications were obtained in areas without defects. At the 50x50 mm defects a noticeable increase in deformation was indicated.
|Panel and defect size||Initial value||Value over defect||Difference|
|A1 - 50x50 mm||30||36||6|
|A5 - 30x30 mm||28||28||0|
|A5 - 50x50 mm||27||32||5|
|A6 - 50x50 mm||32||35||3|
|A7 - 30x30 mm||32||33||1|
|A7 - 50x50 mm||31||44||13|
|Table 2: Bondimeter readings in defect areas. (Readings in 10-2 mm)|
A Dr. Ettemeyer GmbH shearography system with a 50mW Diode laser (wavelength 780 nm) has been used at the investigations. The optics in the video camera was equipped with a daylight filter.
Positive results were obtained on the samples NDT 1 through NDT 3. All disbonds were indicated. At the inspections heat was used as the excitation force. In general the defect type I was easier to detect in all test samples. The defect type II needed more excitation to be detected. The heating has to be adapted to the defect that is hardest to detect.
Even on the sample NDT3, with skin and doubler, both 1 mm of steel, bonded to the core, the defects were indicated, see figure 3. These defects were not detected by any other inspection method.
|Fig 3: Shearography results on NDT3.||Fig 4: Shearography results on NDT5.|
5.2) Application trials
During the application trials different full scale related items were investigated that were not covered at the Round-Robin test. In addition to this the best way to apply shearography for this project was also studied.
Following items were investigated:
The application trials showed that heat was easier to implement directly to the structure than vacuum. Larger panels were actually easier to inspect than small test samples due to less full body movement and more rapid cooling in the large panels. Defects down to 50 mm could be detected in a bondline between core and doubler (2 mm) underneath a 3 mm skin, see figure 4.
5.2.1) Excitation method
Even though vacuum excitation is easier to control and repeat, there were a number of reasons to chose heat in favour of vacuum as excitation method:
18.104.22.168) Temperature change for different structures
For metal-to-metal bonds (NDT2) a temperature difference of 1/2 degree Celsius was enough to get clear defect indications. Higher temperatures resulted in larger deformations and slightly larger affected area (making the defects look bigger). Maximum temperature difference used was 10o C.
It was very difficult to get good results for skin to core disbonds on NDT1 and NDT3. Before any clear defect indications were obtained the whole part would bend which caused a disturbing deformation.
Inspections on panels A1 through A7 and R1 gave better results. Clear defect indications were easily obtained on these panels. A temperature difference of approximately 5o C was enough to get defect indications from 1 mm skin + 0,8 mm doubler to core disbonds. The conclusion was made that larger panels gave faster cooling and less full body bending which affected the inspection quality in a positive direction.
22.214.171.124) Initial temperature. How does it affect the inspection?
The temperature of the part compared to the ambient air was important. If the temperature difference between the part and the ambient air exceeded 10o C the measurements were disturbed. The laser fringes got very turbulent, probably due to air turbulence.
Within the 10o C temperature range the increase and decrease of temperature was not affected by the initial temperature of the part. The reference image can be taken before or after heating. The trials showed that a higher sensitivity was achieved with heating prior to taking the reference image
126.96.36.199) Heating sources
Air guns and Halogen lamps were used as heating sources in the investigation. Halogen lamps have the advantage of being fast and easy to turn on and of without creating to much air turbulence. If high temperatures are to be achieved air guns are faster. As the investigation showed that temperature increases exceeding 10o C actually not are practically useful the halogen lamp was enough. A 500 W lamp at approximately 300 mm distance created a 5o C increase of the surface temperature in 20 seconds. More lamps and/or higher effects will reduce the heating time. The heat from the lamp itself created air turbulence in front of the lamp even after switching the light off. This made it necessary to screen the heating source after heating the part. Also it was not possible to make measurements while heating since the halogen lamp included wavelengths that were not filtered out by the optics which caused the video camera to saturate.
Optical filters and lamps with light outside the wavelength area of the laser may allow heating during measurements without saturating the video camera.
5.2.2) Surface geometry and inspectable areas
The affect of surface geometry on the shearography results was investigated on sample R1. Sample R1 have approximately a 0,5 m radius. No change in sensitivity was indicated in the curved area. The affect of impact damages was studied on panels A1 through A7. Even in areas with heavily deformed skins due to high impact energies, shearography could indicate the damages. Geometry changes did not affect the results significantly. Defects close to the edges and in some cases open to the edges were also indicated as good as in the middle of the panels.
Trials were made on thicker and thicker test sample to try to find the skin thickness limitation for shearography. For thicker skins larger temperature differences were necessary to detect defects. Indication from the Æ50 defect was vaguely distinguished from the background noise on NDT5 with 3 mm skin + 2 mm doubler bonded to the foam core, see figure 4. The Æ25 defect was detected in bondline between the 3 mm skin and 2 mm doubler, see figure 4.
Each different combination of skin thickness and core density may give different results.
5.2.3) Defect characterisation
When a defect has been indicated it is essential to know as much as possible about the defect. In the trials to find the skin thickness limitation it was evident that defects appeared at different excitation levels depending on the depth position. The part was heated and a reference image was taken as soon as possible heating. As the part cooled down measurements were made. Longer time between reference image and measurements resulted in higher excitation level (temperature difference). If measurements are made at different time intervals, different excitation levels are achieved. A typical inspection cycle consisted of:
The size of a defect was measured directly in the software. If a very high excitation level was used the size of the indication got slightly larger than the defect. On the other hand the defect was not indicated or indicated as much smaller if a very low excitation level was used. The heating will therefore influence the size of the defect indication.
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