Damage detection - the phased array approach

TWI Bulletin, November - December 2006

Under the spotlight - non destructive testing of non-metal aerospace materials

Channa Nageswaran is an Engineering Doctorate student based in the NDT department of TWI Ltd. His primary interest is in phased array ultrasonic testing. The present piece is generated from a European funded project for the evaluation of various NDT techniques for use on composite materials. He is associated with the Metallurgy and Materials department of the University of Birmingham.

Colin Bird joined TWI in 2000 after twenty years in the nuclear power generation industry developing remote inspection methods including high temperature TOFD techniques; turbine disc inspection systems as well as providing training in NDT methods for engineers and technicians to nuclear industry standards. His work at TWI has concentrated on ultrasonic phased array techniques for critical defect sizing and the development of ultrasonic methods for defect detection in friction stir and other advanced welding processes. He has continued with nuclear inspection work with stainless steel vessels.

Carbon Fibre Reinforced Plastic materials are increasingly being used in modern civilian aircraft for structural applications. There is an urgent need to detect and classify delamination defects and evaluate the threat to the integrity of the component in-service. Channa Nageswaran and Colin Bird evaluate the possible use of ultrasonic phased array (PA) technology.

Linear array ultrasonic probes are used to scan CFRP panels that contain both Teflon simulated delaminations and barely visible impact damage (BVID) caused by drop weight impacting. A comparison is made between the use of phased array scanning as opposed to conventional single-crystal transducer scanning. The advantages and disadvantages of using PA technology for thin composite material components are discussed.

History

This paper follows on from work carried out on the CRAFT Framework V project NANOSCAN (G4ST-CT-2002-5028). The aim of the project was to study the effectiveness of several current NDT techniques for the inspection of aerospace composites. The techniques evaluated include air-coupled ultrasonics, thermography, shearography and resonance testing. The materials of primary concern were CFRP laminates and honeycomb structures. A range of defect types can degrade the quality of both these composite materials and it was the aim of each technique to both detect and classify these defects.

Carbon fibre reinforced plastics and the critical defects

The manufacturing process (hand lay-up) can lead to several flaws:

• Foreign body inclusions (backing film, dust)
• Voids (entrapped air, moisture ingress)
• Incorrect lay-up order (fibre/ply misalignment)
• Incorrectly applied curing cycle
• Bonding failure between plies during cure

In service damage primarily leads to delaminations that could arise from voids and inclusions introduced during fabrication, but missed by QA procedures, which then act as initiator sites once the component becomes a load bearing structure. A major form of damage is induced by relatively low velocity impacts on the component such as dropping tools on the wing skin and bird strikes.

These events lead to extensive subsurface delaminations without any obvious visual clues on the impacted surface; this is often referred to in industry parlance as barely visible impact damage (BVID).

Phased array equipment

From its initial NDT applications in the nuclear industry, phased array techniques are being considered for use in different fields and the NANOSCAN project evaluated the use of PA technology for composites in the aerospace sector.

The R/D Tech FOCUS 32/128 PA system was used in this project. It is able to build an active aperture using a maximum of 32 elements and address 128 channels. Data collection software was used for data representation and analysis.

Experiments

Specimens

All the specimens in this project are based on the aerospace grade prepregs ('pre-impregnated' continuous fibres in resin) AS4/8552 and HTS/6376 manufactured to aerospace specifications (HS-CP-5000). The AS4 (12K fibre count/tow) was infused in a part cured epoxy matrix to derive the 8552 system. It is tough in resisting impacts and is highly damage tolerant. It is amine cured to increase the toughness of the epoxy resin and to operate in temperatures of up to 121°C.

Impact damage was introduced to the Q-ID specimen using the Rosand Instrumented Falling Weight Impact Tester (IFWIT). Energy of impact was controlled by changing the height of the drop weight (potential to kinetic energy). The specimen was rigidly supported on a rectangular window of 100mm x 110mm; the impactor head was 20mm in diameter and weighs 4.72kg. After several trials 3mm thick neoprene padding was placed over the impact point to prevent surface damage; the neoprene would have absorbed some of the impacting energy and hence the imparted energy was less than that calculated. Since the aim was to study the characteristics of the impact damage rather than the conditions and mechanisms by which it took place, this discrepancy is not expected to be an issue.

Artificial delamination damage was introduced using Teflon inserts before final cure. Porosity was introduced into several specimens during manufacture by not adhering to the recommended curing cycle. Table 1 summarises the specimen details.

Table 1 Specimen details and lay-up

A number of specimens were destructively evaluated to measure and confirm the presence of impact induced and porosity defects. The length of delaminations and their through thickness positions, as measured using micrographs, were compared to ultrasonic evaluations (both PA and conventional).

A diamond saw was used to section specimens and subsequently polished to a 0.25mm finish and mounted in CitoFix. A digital optical microscope is used for imaging (x1000 maximum magnification).

Defects

Three types of defect were investigated ultrasonically in this project. During hand lay-up of the specimen Teflon inserts were deliberately inserted between plies. These are known to simulate inter-ply delamination artificially and are also representative of manufacturing inclusion type defects. The acoustic impedance mismatch between the bulk CFRP and the Teflon should lead to their detection. The N-series panels are manufactured with entrapped air; this leads to the introduction of gross porosity where there is impedance mismatch between the material and air.

Impact induced damage is known to lead to severe delamination below the impact surface. The panels are scanned after the introduction of impacts ranging from 30 to 70 Joules. We noted that on an 8mm (64 plies) thick quasi-isotropic panel 45 Joules represented the threshold for initiation and perforation took place around 60 Joules ( Fig.1). However, the threshold values are dependent on many factors and researchers have found a very large variation.

Fig.1. Damage size related to energy of impact under present set-up scenario

Probes

CIVA and Quicksonic modelling software are used to check theoretically the integrity of the incident PA sound field and remove unexpected grating lobes. An example of using CIVA is given in Fig.2.

Scanning

All the data presented in this paper were collected in pulse echo mode, because in-service inspections of aircraft components usually only allow one-sided access. Noting that delaminations occur between plies and are always parallel to the surface, 0° incidence testing offers the best inspection strategy (in terms of defect detection and coupling efficiency).

An important aspect of the present work is the need to immerse the specimens. When several elements of an array are phased time delays are applied to focus the resultant sound beam within a desired zone, in our case at 0° incidence on to the specimen. The number of elements, hence the active aperture, determines the distance required in the medium for effective beam forming to take place ( Fig.2). A practical value for the minimum focal range distance is:

Where B is the distance in medium, A is the active aperture length and l is the wavelength in the medium.

Fig.2. 16-element linear aperture with 0.6mm pitch; calculated field for point focusing without significant grating lobes

Results and discussion

Material characteristics

For the current study, an accurate value for the longitudinal velocity of the sound in the material was evaluated to be 2850 m/s with a standard deviation of 1.3%. Similarly the attenuation on the sound travel (combined absorption and scattering) was evaluated as a function of sample thickness and frequency; this then allows us to choose an appropriate frequency for the study.

Artificial delaminations

For aircraft in-service, the maximum allowable defect size, especially delamination, before it is considered to be beyond airworthiness depends on several factors:

• Design philosophy (fail-safe/safe life)
• Material type
• Impact risk zones (eg dropped tools, maintenance traffic, runway debris).

Similarly the criteria relating to monolithic components is different to sandwich components due to differing damage morphology. A typical critical delamination size in CFRP wing skin exposed to direct airflow is 10mm x 10mm for a modern fighter aircraft. The -6dB drop method is used for sizing; here a locus line tracing a 6dB drop in the amplitude from the maximum value over the defect represents the defect size.

Figure 3 shows a scan of the N1-D specimen. It shows the presence of the artificial delamination and the presence of through thickness gross porosity. The PA system is able to impose the -6dB sizing criterion for both the delamination and the porosity regions automatically. The through thickness positions can also be evaluated accurately.

Fig.3. 32-element linear array scan of N1-D showing artificial delamination and gross porosity (B-scan to the left, C-scan on the right).

Manufacture induced porosity

A study of the morphology of porosity in three different lay-ups shows that the weave of the material plays an important role in the degree of damage. Figure 4 shows C-scans of N2-D and N3-D that are of cross-ply and quasi-isotropic lay-up, respectively. All the panels have flawed cure cycle induced porosity. Three equal volumes of ultrasonic data are extracted for the three lay-up specimens of 1.4 x 180 x 100mm ³ .

Fig.4. Entrapped porosity in three lay-up schemes

The maximum echo amplitude within this volume above 20% of the front wall echo was plotted on a two-dimensional map; hence the area of the total cross-section covered by porosity is evaluated by image thresholding ( Fig.5). The total volume of porous region is greatest in the cross-ply panel then the quasi-isotropic and least in the unidirectional laminate; furthermore the fibres tend to guide the growth direction. Porosity is created due to entrapped air between the fibres and can be used to deduce the lay-up of a laminate.

Fig.5. A two-dimensional map of the porosity damage for unidirectional (top), cross-ply and quasi-isotropic laminates

The porosity levels in the unidirectional, cross-ply and quasi-isotropic laminates are 6.94%, 19.83% and 10.33%, respectively.

Impact induced delaminations

It was confirmed that impact damage exists at different levels through thickness.

The delamination near the back wall is larger than the ones near the front wall see Fig.6. When impacted there is rapid local bending at the impact point ( Fig.7).

Fig.6. TOF showing multiple level delaminations (55J impact energy)

Fig.7. Local deformation scenario on impact

The in-plane shear stresses created between successive plies in different orientations leads to a break down in the bonding, creating delamination; the mismatch in bending stiffness has been directly related to the severity. It was also possible to deduce the major orientation directions (0°, +/- 45°, 90°).

Figure 8 shows a PA amplitude scan. RF data is collected at a digitising frequency of 100 MHz. The probe is a 16 element 7MHz linear probe. Gates can be used to select delaminations at different levels.

Fig.8. Corrected C-scan showing delamination at three levels

A study was carried out to confirm the -6dB ultrasonically determined sizes of the delaminations in the sample by sectioning. A through thickness datum plane was identified accurately and corrected depth and size measurements were made of the delaminations. The sample was then sectioned and micrographs were taken at magnifications from x16 to x1000 (eg Fig.9.)

Fig.9. Delamination damage between two plies (45° (top) and 90°) showing typical measured wall gap in Q-ID-2 (Magnification x1000)

Table 2 shows that PA testing is able to locate and size multiple level delaminations within an acceptable margin of error (satisfying the minimum size criteria).

Table 2 PA testing is able to locate and size multiple level delaminations within an acceptable margin of error (satisfying the minimum size criteria).

It was not possible to transmit significant energy beyond the first incident delamination. For sound propagation in a medium (both planar and spherical) the particle vibration amplitude is related to the sound pressure using the following relation.

A further quantity of interest in understanding plane and spherical sound propagation is the intensity of the sound in W/m ² .

Where Z (= ρc) is the acoustic impedance. The above is then rearranged for particle displacement, as follows:

This feature of sound propagation in solids is used to detect delaminations in CFRP. The gap between plies (where disbond leads to delaminations) is reported to be around 0.7µm, whereas we measured an average of 3.24µm on sectioned specimens. Assuming bulk measured longitudinal velocity of 2850m/s and a frequency of 5MHz, the wavelength is 0.57mm. Hence the particle displacement is around 0.00000114 mm (2x10 ^-6 of wavelength). This implies that at the face of the delamination the interface will see a free boundary and almost all the energy will be reflected back to the transducer as an echo.

Delamlination gap is sufficient for the propagating wave to interact with the incident wall as a free boundary

Due to this effect it is not possible to penetrate past the first incident delamination to those in its shadow. All the energy is then confined between the first incident delamination and the front wall to create repeat echoes. Multiple level delaminations are detected by virtue of the conical damage feature, those near the back wall being larger in area.

Components made out of CFRP are often thin (1 to 15mm). Hence short broad band pulses are needed to resolve features along the axial direction. Conventional probes have a -20dB pulse length between 200 and 800ns whereas the PA probe used in this study had a measured value of 1300ns. Since the delamination defects and porosity are parallel to the front wall and the components are thin, the beam steering feature of the PA is redundant. Similarly focusing within the thickness has been shown to do little to illuminate defects better near the back wall (for an 8mm thick specimen).

Practical implementation

The specimens are immersed to allow sufficient coupling distance for effective beam forming. In-service inspection of aircraft components would require dismantling and provision for large immersion tanks that will increase the overall cost. When collecting RF, the data volume transfer is a limiting factor on the size of the array that can be used effectively.

A major known advantage of using array transducers, not necessarily working in phased array configuration, is the increase in scanning speeds that can be achieved. A scan of a 100mm x 100mm panel with an 8MHz conventional and 7MHz PA probes, motor speed of 5mm/s and resolution of 0.6mm x 1mm was set up. The total scanning time for conventional was 33 minutes while it was one minute 46 seconds for PA. Noting that these results are highly probe (length of array) specific, it is a good illustration of the performance increases that can be achieved with an array.

Conclusions

Advantages

• Both delamination and porosity damage can be effectively detected
• Total time to complete scans can be reduced very significantly when an array is used, not necessarily in phased mode
• It is possible to create live images of component sections to study damage growth characteristics and mechanisms (fatigue loading)
• Active aperture size of the probe can be altered electronically to improve sizing (beam spot size) depending on the precision required

Acknowledgements

• The EC for part funding the NANOSCAN project.
• All partners of the consortium for the successful completion of the project.
• EPSRC for funding the studentship at TWI/University of Birmingham.