Original Date: 03/08/1999
Revision Date: 01/18/2007
Best Practice : Portable Phase-Stepping Digital Shearography
The inability to detect subsurface flaws in composite materials under field conditions (e.g., onboard a carrier) often requires aircraft to be taken out of service and sent to a depot facility whenever damage is suspected. This procedure is expensive and detrimental to Fleet readiness. In response to this problem, the Applied Research Laboratory at the Pennsylvania State University (ARL Penns State) developed the Portable Phase-Stepping Digital Shearography system which can perform full-field, non-contact inspection of suspected damage to aircraft structure in situ.
Shearography is an optical interferometric technique which detects small, out-of-plane displacements. In detecting subsurface flaws, shearography systems rely on static or dynamic excitation (e.g., heat, internal pressure) to flush out flaws. The result is a fringe pattern whose anomalies display approximate flaw size and position. However, commercial shearographic systems produce qualitative results with low signal-to-noise ratio which results in low detection sensitivity. The ARL Penn State’s Portable Phase-Stepping Digital Shearography system utilizes optical phase-shifting, advanced digital signal processing and image processing techniques. As a result, this system provides detection sensitivity improvements of almost two orders of magnitude greater than commercial systems. The output is displayed as a real-time image of high resolution (1000 x 1000 pixels), capable of large area airframe nondestructive inspection.
The Portable Phase-Stepping Digital Shearography system is completely self-contained and can be operated by a single individual. Figure 2-3 depicts the system’s schematic. The system also features a hand-held detection head which contains the optics, display, and containment for the vacuum system used to provide surface deflection. The operator holds the detection head (weighing less than 20 pounds) to the surface to be inspected, turns on the vacuum, and obtains the shearogram in real time. The laser source, vacuum source, and controller are connected to the head via an umbilical, allowing relatively unlimited access to the aircraft structure.
The Portable Phase-Stepping Digital Shearography system, as configured, is capable of detecting surface deformations with a sensitivity of 1/100th of an optical wavelength (five nanometers), which greatly increases the flaw detection capability. Inspection of a one-square foot area takes several seconds, an order of magnitude faster than traditional ultrasound techniques. The portability of the system allows the device to be used in nearly any environment including shipboard. The ARL Penn State estimates the system will reduce annual maintenance costs by $240,000 per depot facility.
Figure 2-3. System Schematic
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