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Using ‘ASSESS’ to detect subtle flaws in structures and materials

A new technology could make rocket launches more successful

When it comes to materials, engineers are always looking for something better. There is a constant need for stronger, lighter and easier-to-produce materials that can withstand harsh environments.

This is particularly true for aerospace applications. The smallest defect in a material used in rockets can have catastrophic, costly consequences. Safely launching and landing spacecraft, especially reusable spacecraft, requires thorough inspections to identify potential manufacturing flaws.

There are, of course, nondestructive evaluation techniques that aim to find defects in materials before launch, but current techniques are limited in what they can detect. For example, thermography, which uses an infrared camera to reveal material imperfections, is limited by relatively small scanning areas. X-ray radiography and computed tomography, other common NDE techniques, while detailed can be prohibitively slow.

To address these issues, researchers at Los Alamos National Laboratory created a new technology called Acoustic Steady-State Excitation Spatial Spectroscopy, or ASSESS, that can rapidly and non-intrusively detect a wide variety of defects in materials used in reusable rockets.

Originally developed to support inspection needs in the oil and gas sector, ASSESS can also meet the high-resolution and stringent time requirements of the aerospace industry. ASSESS uses high-speed, high-resolution ultrasonic wavefield measurements that can highlight subsurface defects such as corrosion, thickness reduction, cracking and delamination in metals, composites and hard plastics. It can scan and analyze structures in three dimensions, providing a wealth of information previously unavailable from two-dimensional scans.

ASSESS rapidly scans structures by continuously exciting them with ultrasonic tones and measuring the mechanical surface response of the structure with a scanning laser Doppler vibrometer. An LDV is a scientific instrument that can measure very small (nanometer scale) mechanical vibrations by shining a laser on the surface and comparing the phase of the reflected laser light with that of the laser at the source.

The ASSESS system scans the LDV measurement point over the region of interest to collect a full-field surface velocity measurement that contains valuable information about how the ultrasonic waves interact with the structure and detects potential defects or damage. Depending on the structure being scanned and the defect type, the velocity measurement and other supplemental information is fed into different defect-detection algorithms to produce 3D defect and damage assessments.

A key component of ASSESS is its damage detection algorithm based on local wavenumber analysis, which can reveal hidden corrosion and delamination in thin structures. The research team is also developing techniques to detect flaws in coatings and additive manufactured parts.

As reused rockets become more common in the aerospace industry, the need to detect structural flaws becomes more important; ASSESS can provide tools to help ensure successful launches, saving money and, potentially, lives.”

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