Del Mar Photonics

17th World Conference on Nondestructive Testing, 25-28 Oct 2008, Shanghai, China
 

Aerospace NDT with Advanced Laser Shearography
John W. NEWMAN
Laser Technology Inc.

Abstract:
Shearography nondestructive testing has evolved considerably since first used on a
production aircraft program in the USA in 1986. Shearography laser interferometric imaging
methods measure test structure deformation due to an applied engineered change in stress. The
resulting changes in Z-Axis strain component reveal images of subsurface defects such as
disbonds, delaminations, core defects and impact damage in aerospace structures. Shearography
NDT provides high thru-put, cost-effective productivity enhancements, improved manufacturing
processes and quality. Development of digital CCD cameras, the PC and small, high power
solid-state lasers have led to dramatic performance improvements in shearography instruments
and systems. Shearography is currently in use on a wide variety of aircraft including F-22, F-35
JSF, Airbus, Cessna Citation X, Raytheon Premier I and the NASA Space Shuttle. This
presentation will provide a brief background on shearography NDT technology as well as recent
developments in production and portable on-aircraft shearography inspection technologies and
applications.
Key Words: Aerospace NDT, shearography NDT, honeycomb structure, NDT, disbond,
damage, delamination
1.0 Background
In today’s highly competitive aerospace environment, a capable high-speed inspection
technology is critical. Shearography nondestructive testing is providing a better and faster means
to nondestructively inspecting new aircraft both during manufacturing and in the field.
In the quest to maximize fuel economy and performance, engineers have turned from riveted
and bonded aluminum structures to solid composite laminates, composite sandwich panels with
honeycomb or foam cores and tape wound composite structures such as fuselages. The
traditional methods for nondestructive testing, such as ultrasonic (UT) C-Scan, may not provide
the best defect detection capability for these new materials and geometries and are slow with a
typical through-put of just 10 sq. ft./hour. Further, the process of manufacturing complex
composite structures requires a means for fast inspection to provide a process control feedback
and to ensure quality and reliability at the lowest possible cost. In many aerospace programs
today, laser shearography is providing a large part of the solution.

2
2.0 History of Shearography NDT
The electronic laser shearography imaging interferometer was pioneered in the early 1980’s
by three researchers, Dr. John Butters at Loughborough University in the UK, Dr. S. Nakadate in
Japan and Dr. Mike Hung at Oakland University in the USA. The author’s team at Laser
Technology Inc. led the development of the shearography camera as a tool for nondestructive
testing, delivering the world’s first production shearography NDT system to Northrop Grumman
in 1987 for the manufacturing of the USAF B2 Stealth Bomber.
In the last twenty years more than 1,200 shearography systems have been integrated into the
manufacturing process for aircraft composites, tires and high-reliability electronics. As with all
NDT methods and technologies, shearography’s strengths and weakness must be completely
understood, and applications qualified through Probability of detection (PoD) verification with
written procedures and rigorous training for operators and engineers alike. Once qualified,
however, shearography systems can operate with extraordinary efficiency reaching through-puts
from 25 to 1200 sq. ft per hour, 2.5 to 120 times the typical 10 sq. ft./hour inspection rate for
ultrasonic C-Scan.
Photo 2
This executive jet flap is
manufactured with GRP skins and
multiple foam cores with internal
bond lines making this part
extremely difficult to inspect using
traditional NDT technologies.
Photo 1
The USAF B-2 stealth
bomber was the first
aircraft to incorporate
Shearography NDT
technology in the
manufacturing of
complex composite
structures.

3. Shearography NDT
Unlike UT C-Scan, which uses a single transducer that requires a raster scan over the part to
build up an image, Shearography is a whole field, real-time imaging technique that reveals outof-
plane deformation derivatives in response to and applied stress. Using a slight pressure
reduction in a shearography test chamber, critical defects are imaged and measured in seconds.
The shearography camera detects surface bumps as small as 3 nanometers caused by local strain
changes around subsurface defects as the pressure is reduced on the part. Vacuum shearography
is highly effective for image disbonds, delaminations, core damage and core splice-joint
separations. Other Shearography NDT techniques that are frequently used include thermal pulse
shearography for non-visible impact damage, pressure shearography for damage to composite
wrapped pressure vessels. Vibration shearography has been highly developed in the last several
years by the author to inspect the foam on the external tank of NASA’s Space Shuttle.
o
Photo 3
Shearography images above show disbonds in the flap, between the foam core and
GRP skins, on the left as the white/black indications in the unwrapped image or, on
the right, as areas with fringe concentrations in the wrapped phase map. Time to
perform a shearography test of this 1.5 sq. ft. section of the flap is 4 seconds, 135
times faster than testing with UT C-Scan, if the test could be accomplished at all
given the complex shape.
Photo 4
A typical digital shearography camera,
the LTI-5100, includes a built in laser, the
shearing image interferometer and image
processing computer. The aircraft radome
may be tested with thermal stressed by
applying a slight temperature change on the
order of 3-10°C.

(pdf)

DPSS lasers for Aerospace NDT with Advanced Laser Shearography

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