Diode Lasers Enable Rapid, Strong Repairs for Combat Aircraft
Dr. Jonathan Lawrence
One of the most intractable problems confronting modern air force ground crews is the effective repair of combat aircraft skins. Typically, a crew must fix the holes caused by bullets and shrapnel in the outer skin of various parts of a number of types of aircraft. This problem is compounded when it must perform maintenance in the field.
The options available for carrying out such field repairs are limited. None are rapid, and such methods are usually centered on the use of bolted or riveted metallic patches. Although these patches often are reasonably effective, the counterproductive generation of stress concentrators by the fastener holes makes them inefficient.
In contrast, composite patches, bonded by a high-power diode laser, distribute loads more evenly, and the repair does not generate stress concentrators. For this reason, along with their high specific strength, stiffness, and resistance to fatigue and corrosion, composite patches are used extensively for fixing aluminum aerospace structures.
Figure 1. Tests of the high-power diode laser bonding technique were performed in a vacuum box. In application, the necessary work environment could be obtained with the use of a material that adheres to the part being processed and that is transparent to the laser radiation.
Researchers at Nanyang Technological University in Singapore thus are investigating the use of a high-power diode laser to bond composite patches to aerospace materials, with the goal of developing a method that will render combat aircraft airworthy again in the shortest possible time.
In tests of the approach, samples were placed in a vacuum box (Figure 1). Prior to laser treatment, the pressure in the box was reduced to 2 t, causing a movable glass window to press an APC-2 repair patch onto the Alclad substrate. This ensured that the cured thermoplastic adhesive would be a narrow (<0.2 mm) and even cross section and would be free of trapped air pockets. (In the field, the proper work conditions could be provided by a material, transparent at the output wavelength of the diode laser, that adheres to the contoured aircraft skin to provide a sealed edge and therefore to allow vacuuming.)
A high-power diode laser from Rofin-Sinar GmbH of Hamburg, Germany, supplied 940-nm radiation in output powers ranging from 1.5 to 2.3 kW. The defocused, multimode beam illuminated the surface of the samples through the glass window of the vacuum box. The 6 × 4-mm beam was scanned across the 25-mm width of the APC-2 repair patch five times, overlapping itself by 1 mm on each pass. No melting of the surface of the patch was observed on any of the samples, either during or at the end of bonding.
Measurements of single-lap shear strength revealed that the laser bond was stronger than one produced by resistive implant welding and significantly stronger than one produced by induction welding (Figure 2). A possible explanation for the latter phenomenon is that induction welding heats the Alclad substrate but not the APC-2 patch. As a result, fusion bonding on the underside of the patch is dependent exclusively on the conduction of heat from the substrate to the thermoplastic adhesive interlayer, then to the patch. Consequently, the bond that is formed will not be flawless.
A visual inspection of separated pieces of an induction-welded joint confirmed this; small particles of the repair patch were observed on the pieces of the thermoplastic adhesive that remained on the Alclad, indicative of cohesive failure within the aluminum oxide layer. The small amounts of APC-2 found also indicated cohesive failure within the adhesive.
Figure 2. Results of single-lap shear tests for the bonds produced with the laser, induction welding and resistive implant welding demonstrate that the high-power diode laser (HPDL) produces stronger bonds than resistive implant welding and much stronger ones than induction welding.
Such problems did not accompany the laser bond because heating and melting of the thermoplastic adhesive in that case was the result of direct conduction through the APC-2 repair patch. A small amount of heat was conducted to the Alclad, allowing the adhesive to bond more fully and evenly with both materials.
In the case of resistive implant welding, the bond line similarly heated up directly and created a bond superior to that achieved with induction welding. Failure occurred primarily as a result of tensile failure in the implant and of delamination of the implant from the APC-2 patch, making it inherently weaker than the laser-generated bond.
The results of Boeing wedge tests for the induction-welded and laser bonds were plotted and normalized so that they showed only the length of the cracks that propagated following precracking of the bonds with the wedge. The laser samples had a one-hour crack growth rate of 1.9 ±0.5 mm/h, which is deemed to be very good. The induction-welded samples displayed a one-hour crack growth rate that was rated as good, at 2.7 ±1.2 mm/h, but these results again indicated that the high-power diode laser bonding process is superior, albeit marginally in this case.
The enhanced performance of the laser bond also is the result of the more consistent melting of the thermoplastic adhesive across its section, which in turn promotes fuller bonding both to the patch and to the substrate.
One of the most crucial factors in the repair of battle-damaged aircraft is speed. To this end, the thermal history of the bonds was recorded in terms of the heating rate. Thermocouples were inserted between the Alclad and the thermoplastic adhesive. In the case of resistive implant welding, a thermocouple also was positioned between the implant and the APC-2 patch.
The results demonstrate that resistive implant welding takes much longer than either laser bonding or induction welding to reach the 250 °C melting temperature of the thermoplastic adhesive. Whereas the bond line reaches 250 °C in approximately one minute using laser bonding or induction welding, resistive implant welding takes approximately six minutes to reach this mark.
Although these times are similar for laser bonding and induction welding, the actual times to complete a bond are very different. The processing time achieved using the high-power diode laser was 2.75 minutes, compared with 11.75 minutes using the latter. This is attributable to the fact that the heating is too localized in the induction welding technique, a problem that is avoided when using the laser by scanning its wide beam over the surface of the repair patch.
Moreover, the temperature keeps rising for the induction and resistive implant welding techniques. Indeed, the recorded temperature exceeded 300 °C with induction welding, resulting in damage to the Alclad substrate. This was not the case for the bonds produced with the laser, again because of the scanning of the beam.
The preliminary results suggest that the high-power diode laser-based technique is a feasible solution to the potentially debilitating problem of grounded combat aircraft. The approach produces strong repairs and can render units airworthy again in a very short time. What is more, the compact form factor and modest power requirements of the diode laser naturally make the method suitable for battlefield deployment.
The high-power diode laser is a bonding tool superior to its contemporary counterparts. It also is possible that it will produce even stronger bonds with the use of different adhesives.
MORE FROM PHOTONICS MEDIA