Multichannel fiber laser Doppler vibrometer studies of low momentum and hypervelocity impacts

Significance Statement

In the lifetime of physical structures such as bridges, tunnels and aircraft etc. impacts due to shrapnel and other airborne particles can be expected and can cause damage to the structure. It is therefore necessary to determine the effects of impact damage during design, prototyping and the early manufacturing phase for industries including, for example, aircraft manufacturers and potential operators. A number of studies have been undertaken to simulate several types of impact. Air-gun systems, where a small projectile is propelled at hypervelocity, are used to simulate impacts experienced during aircraft service. Drop-weight tests are used to simulate low-velocity impacts.

In most studies with light gas guns polyvinylidene fluoride probes (PVDF) are attached to the target edges in order to keep track of the tests and development of damage induced during impacts. Ideally the measurements should be made as close to the center of the target as possible. Optical fiber probes offer the possibility of making impact measurements. Initial studies with Fiber Bragg Gratings (FBG) mounted on a carbon composite plate target inside a light gas gun have demonstrated comparability with the PVDF transducers. This is because the FBG can be bonded closer to the impact-point of the projectile on the selected target, providing more accurate data on the impact dynamics.

However, there are still some issues concerning the FBG. There are problems with the displacement resolution and the need for the grating to be mounted with a known strain. The target mounted probes could be destroyed by the impact even when they are not hit directly by the particles. In addition, the contact sensor must be calibrated when set up on the target.

Julio Posada-Roman and Jose Garcia-Souto at Universidad Carlos III de Madrid, in collaboration with David Jackson and Mike Cole at the University of Kent (UOK), adapted the multichannel fiber laser Doppler vibrometer developed at UOK, which does not suffer from the problems associated with the FBG approach. It has miniature variable focus non-contact collimator probes mounted inside the light gas gun low vacuum chamber at a distance of ~80cm from the target A displacement resolution of ~ 1nm√Hz was obtained, with the carrier frequency optimally selected for digital processing. These are some of the advantages of the method. Their work is published in Optics and Lasers in Engineering.

The authors demonstrated the multichannel optical fiber laser Doppler vibrometer with the capacity to make non-contact measurements of three different impact locations, simultaneously. They undertook two sets of measurements, first with small ball bearings falling under the influence of gravity and the second with small projectiles fired from a high-speed light gas gun with 1-8km/s speed range.

The implementation of data processing enabled the authors to determine frequency, phase, and vibration amplitudes at high resolution. The authors speculate that it is feasible to design a small portable instrument with conventional optics except for the optical fiber transceiver links. The unit combined with flexible fiber links and miniature collimators can be applied on mobile platforms, including Mach 2 jets and in the next generation of high speed passenger planes. This would be particularly valuable where simultaneous measurements of impacts on wings could be achieved in flight, and would apply equally well to other large objects, such as racing cars.

From the study, it was possible to increase the number of channels to 8 or even 16, therefore increasing the number of locations where the effects of impact and vibration could be measured simultaneously with 1mW of optical power channel.

Reference

Julio E. Posada-Roman, David A. Jackson, Mike J. Cole, Jose A. Garcia-Souto. Multichannel fiber laser Doppler vibrometer studies of low momentum and hypervelocity impacts. Optics and Lasers in Engineering.

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