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A researcher uses laser bond inspection (LBI) on a large structure at the Boeing Laser Bond Inspection Laboratory in Seattle, Washington (Image source: USAF).

Breaking bonds

AFRL, LSP Technologies develop NDT method for bonded composite structures

Bonded composite airframe structures offer a potential to achieve more affordable manufacturing and more efficient structures, ultimately meeting United States Air Force (USAF) goals to increase range and reduce fuel consumption. The Lockheed Martin Skunk Works X-55 Advanced Composite Cargo Aircraft is perfect case study of the benefits a bonded composite airframe with 90% part count reduction.

However, composite structures, when joined using adhesive bonding rather than traditional rivets or fasteners, require a new inspection process to verify the strength and safety of the composite bonds. And testing the integrity of bonded composite structures is often difficult to perform without compromising the structure.

For the past decade, Air Force Research Laboratory (AFRL) Materials and Manufacturing Directorate at Wright-Patterson Airforce Base—in conjunction with Boeing and LSP Technologies—has been developing a process called laser bond inspection (LBI), a nondestructive testing (NDT) methodology for inspecting composite structure bonds. This process, combined with bond process control—a Bayesian process control method to eliminate bad composite bonds—is an enabling technology to transition lightweight primary bonded composite structures.

LBI is used to ensure that adhesively fastened structures are joined correctly. The method is based in stress wave dynamics and uses a bond’s response to a high-energy, laser-generated stress wave (pulsed at 100-300ns) to detect structurally substandard bonds.

When performing an LBI inspection, operators protect the surface of the bond with tape and an electromagnetic acoustic transducer (EMAT). As the stress wave spreads through the composite material, it bounces off the back of the surface as a tensile wave. The tensile wave then passes through the bond line and stresses the bond at a pre-determined strength value (approximately 80% of the threshold value required for separation of a proper bond). The EMAT sensor registers the vibrations which are compared against a pre-inspection baseline.

The pulse will break a weak, improperly prepared bond, but a standard bond will remain unchanged after pulse excitation even after multiple inspections. The substandard adhesive bonds that LBI breaks can result from errors created by poor adhesive mixing, improper surface preparation, or contamination.

LBI is the only reliable and mature method capable of assessing the integrity of adhesive bonds in composite structures and detecting “kissing bonds.” Kissing bonds are a bond line defect where two surfaces are in intimate contact but not properly bonded and can go undetected by typical ultrasound inspection.

The LBI process can be calibrated to test bonds between composite structures of variable geometries. And while the window of structural thickness that can be tested depends on the type of composite material and bond, LSP Technologies has recorded successful testing on fiber reinforced polymer composites up to an inch in thickness.

“This effort is a significant progression of more than a decade of collaboration and development work by the U.S. Air Force, The Boeing Company, LSP Technologies, and additional major aerospace original equipment manufacturers,” said Andrea Helbach, structures team lead at the AFRL Materials and Manufacturing Directorate.

“However, it is far from finished. On-going efforts will prove the technology on multiple representative aircraft structures and prototype a fiber optic delivery system to increase device reach.”

Future work in this area will include analyzing data for bond integrity in the field. While LSP Technologies states that LBI can be performed successfully on all types of metal, polymer, and ceramic composites, tests to define a wider gamut of materials, thicknesses, and joint designs are needed to expand the technology to other platforms.

The process has already been repurposed to inspect welded joint integrity for unfused welds and subsurface cracking.

Notwithstanding continuing development efforts, LSP Technologies recently sold a cart-mounted LBI system to Northrop Grumman at the end of this past January.

Finding new ways to save time in manufacturing and increasing aircraft range are uphill challenges. Bonded composite structures enabled by LBI are a step in the right direction.

This technology, along with a Defense Advanced Research Projects Agency (DARPA) program to establish bond process control, are significant steps in enabling bonded aircraft structures for use in the field.

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