THE ADVANTAGES OF LASER ULTRASONIC FOR THE INSPECTION OF COMPLEX COMPOSITE PARTS

Laser Ultrasonic Composite Testing, MISTRAS NDT TRAINING INSTITUTE, NDT TRAINING INSTITUTE IN TRICHY, NDT INSTITUTE IN TRICHY, NDT TRAINING IN TRICHY IN CHENNAI, TRICHY NDT INSTITUTE IN CHENNAI, NDT IN TRICHY, NDT TRICHY, VISUAL TESTING NDT, NDT,  NON DESTRUCTIVE TESTING IN CHENNAI, NDT ULTRASONIC TESTING IN CHENNAI, TEST NDT IN CHENNAI, TRAINING CHENNAI, PLC TRAINING IN CHENNAI, SAFETY TRAINING IN CHENNAI

INTRODUCTION

Laser ultrasonics is rapidly becoming a well-known industrial technique for inspecting composites that provides large benefits over conventional systems in the inspection of complex contoured carbon-fiber reinforced polymer (CFRP) components (Osterkamp and Kaiser, 2008). The advantages of laser ultrasound over conventional systems for the inspection of complex shaped CFRP are multi-fold. Systems are typically able to rapidly scan the laser beam across the surface of the part with no knowledge of the part curvature, no special tooling or fixtures, and no contour following.

Teaching procedures are quick and simple and there is minimal recurring setup time. typical laser ultrasonic systems use a robot manipulator to move a scan head around a complex shape from a standoff distance of approximately 1.8 m (6 ft). A fast, two-axis galvanometer scanner is used to scan the laser beams across the surface, thereby covering large areas of complex shaped parts in a rapid fashion. These types of system characteristics make laser ultrasound  extremely versatile for rapidly testing a large variety of part types. In addition, the variety of system configurations available provides the flexibility needed for all types of factory environments and requirements.

Laser generation of ultrasound in CRFP materials occurs due to the absorption of the laser wavelength in the top 10 to 100 μm of the surface layer. This surface layer is generally an organic matrix such as an epoxy resin, bismaleimide or thermoplastic, although other surfaces such as paint topcoats can also provide excellent generation. The size of the laser spot hitting the target is approximately 5 mm (0.2 in.) in diameter. This provides an absorption volume that efficiently directs the bulk of the ultrasonic energy in a longitudinal mode normal to the material surface regardless of the laser angle of incidence. A short pulse laser, often CO2 (10.6 μm) or Nd:YAG (1.064 μm), is used as the generation laser. Laser energies are kept below the damage threshold, and the generation mechanism is therefore a thermo-elastic process in which the material returns to its prior state and the laser leaves no lasting effect. The rapid expansion of the material due to heating causes a stress wave in the material, which is the ultrasound.

In general, ultrasound generated by a laser is similar to that from conventional ultrasonic devices with the exception that laser-generated ultrasound typically contains a much broader frequency spectrum. To some degree, both frequency and mode content are dependent on the generation laser spot size. Although the laser generates several types of spatial modes, as the spot size increases, a higher percentage of the energy is shifted to the longitudinal mode (a more piston-like source) and less is available for other modes such as shear or lamb modes. In addition, a higher percentage of the total energy tends to shift to the lower frequency domain as the spot size is increased. This is analogous to transducer diameters increasing as their center frequency decreases. For detection, a second laser is directed to the same point on the material under test. The detection pulse (typically an Nd:YAG at 1.064 μm) is a long pulse laser of 50 to 100 μs in length. This pulse must illuminate the surface of the material long enough for the ultrasound to reverberate several times in the material thickness. The ultrasonic vibrations on the surface modulate the phase and frequency of the light scattered from the surface and collected by the system optics.

This light is sent to a discriminating device where these phase and frequency modulations are converted to amplitude modulations and in turn are converted to electrical signals. During the scanning process all data acquisition, system timing and laser light intensity are automatically controlled by the system. There are no user parameters to input or adjust. Optional user defined filter parameters and techniques are available in the post-processing and data analysis phase to optimize the detection of anomalies. For production components, material integrity is of the utmost importance. Nondestructive testing (NDT) is by definition nondestructive. To that end, the laser pulse energies and spot sizes used in laser ultrasonics are designed to keep the energy density below the damage threshold of the CFRP matrix. Different materials may have different damage thresholds and can react differently to the laser wavelengths. Material testing to determine the characteristic damage threshold is needed prior to production use. For a number of years, laser ultrasonics has been used on the factory floor at a military aircraft manufacturing facility in Fort Worth, Texas and more recently at a military and commercial aircraft manufacturing facility in Winnipeg, Canada to inspect production composites parts for military aircraft (Drake et al., 1998; Yawn et al., 1999). The technology has been approved and implemented on an array of programs such as the F-16, F-22, F-2 and F-35. To date, the systems installed in the Fort Worth facility have tested in excess of 40 000 flyaway production parts.

Carbon-fiber Reinforced Polymer Inspection Difficulties

As the use of composites in the aerospace industry continues to expand, more complex and difficult to inspect components are becoming common. Part diversity can range from small brackets and clips, floor panels, stringer sections and complex inlet ducts up to large wing and fuselage skins. In commercial aircraft, very large barrel structures for the fuselage and large flat floor panels are commonplace. The amount of square footage of CFRP that must be tested daily to meet production rates can be enormous. Throughput of this magnitude and of such a wide variety of components often requires companies to purchase and maintain a number of different machine configurations for ultrasonic inspection alone. Often, these systems are dedicated to inspecting
only one type of part configuration such as floor panels and still may require extensive setup time when parts are changed in the system. Laser ultrasound has the potential to alleviate many of those difficulties. Often, inspections of large complex contoured parts with tight radii, ply drops and drilled holes can be accomplished on the same machine as a large batch of small clips and brackets. Setup time and machine configuration changes between parts are simple or virtually nonexistent. The next section will describe two common production testing problems and their laser ultrasonic solutions.

Two Inspection Problems and Their Solutions

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Two specific examples of difficult inspection scenarios will now be detailed. In the first example, consider a set of small brackets approximately 152.4 mm (6 in.) in length, each of a slightly different configuration and size. These brackets typically have two or three sides oriented at 90° to each other with the corners having approximately 6.4 mm (0.25 in.) radii. Production rates may require on the order of 2000 of these various brackets to be tested each month. Inspection of large numbers of small, complex shaped brackets such as these create two main difficulties: part transfer and setup for each bracket would be extremely time consuming, and the subsequent  inspection of that number of brackets in a month would swamp any system. On the other hand, the inspection of individual brackets with a laser ultrasonic system is easily accomplished with little setup, and the inspection of a three-sided bracket can be completed from a single inspection view of the part. In addition, a laser ultrasonic system could easily be designed with automated ingress and egress to the inspection cell of a rack loaded with many brackets at one time, all tested from a single inspection view.

Figure 1 shows an amplitude C-scan of a two-sided bracket scanned from a single view. The angle of incidence of the laser beams to the bracket faces is approximately 45°.

Figure 2 shows a B-scan cut across the (four) drilled holes in the two-sided bracket. The image clearly shows the four holes by the absence of a back wall signal. (The front wall signal inside the holes is the result of the generation laser interacting with the floor behind the bracket.) The front surface of the part together with the back wall is noted in the figure.

The three-sided bracket of this material (shown in Figure 3) can also be tested from a single view with an angle of incidence of over 65°. In principle, this would allow a single, simply designed rack to hold many different styles of brackets at one angle convenient for the system to scan with minimal adjustment to the scan head angle or position. The rack could be designed to move in and out of the inspection cell in an automated fashion. The only manual task at that point would be placing the brackets on the rack and removing them after inspection.

The second example is of a hat stringer section. In commercial aircraft production these types of stringer section can be upwards of 10 m (32.8 ft) in length and of varying widths and curvatures.
These types of stringers can present many difficulties for conventional systems. Part loading and unloading into fixtures, system setup and calibration can all be time consuming and difficult to get correct, producing results that are frequently dependent upon the skill of the system operator.
Figure 4 shows an example of a commercial aircraft stringer section with challenging tight radii. Figure 4a shows a photo of the part taken by the system camera located on the scan head. This is a view from the actual scanning position. From this view the system is able to scan the flange, inside radius, web, outer radius and top of the hat without repositioning. The amplitude C-scan image from such a scan appears in Figure 4b. Only one additional view from a similar orientation on the other side would be required to test the entire part. In laser ultrasound, software controls the entire ultrasound generation and detection process. By removing adjustable parameters, the data quality is consistent from operator to operator.

CONCLUSION

In over 15 years of daily production inspection of complex aerospace composite structures at the Fort Worth and Winnipeg facilities, laser ultrasound has shown itself to be an extremely versatile, adaptable and cost-effective NDT technique. This article has shown two examples of component types that, in a high production rate environment, are very difficult to inspect quickly and easily with conventional NDT. Herein has been demonstrated how laser ultrasound can be used as a simpler, faster and more efficient solution to the inspection of these challenging components
while providing results that are less dependent on the skill of the operator running the system. Laser ultrasound, while still a new technology to many in the aerospace community, is rapidly proving itself to be a capable and versatile NDT tool.

REFERENCES
Drake, T.E., Jr., K.R. Yawn, S.Y. Chuang, M.A. Osterkamp, P. Acres, M. Thomas, D. Kaiser, C. Marquardt, B. Filkins, P. Lorraine, K. Martin and J. Miller, “Affordable NDE of Aerospace Composites with Laser Ultrasonics,” Review of Progress in Quantitative Nondestructive Evaluation, Vol. 17, 1998, p. 587.
Osterkamp, M. and D. Kaiser, “Application of Laser Ultrasonics for the Non-destructive Inspection of Complex Composite Aerospace Structures,” 1st International Symposium on Laser Ultrasonics: Science, Technology and Applications, Montreal, Canada, 16–18 July 2008.
Yawn, K.R., T.E. Drake, Jr., M.A. Osterkamp, S.Y. Chuang, P. Acres, M. Thomas, D. Kaiser, C. Marquardt, B. Filkins, P. Lorraine, K. Martin and J.Miller, “Large-scale Laser Ultrasonic Facility for Aerospace Applications,” Review of Progress in Quantitative Nondestructive Evaluation, Vol. 18, 1999, p. 387.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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