Announcement of the Laser Ultrasonic System

With the support of the Defense University Research Initiative grant, we have purchased the Lasson Technologies' Laser Ultrasonic Receiver TWM-500 The system which is now fully installed is intended for the optical detection of ultrasound at the surface of a workpiece. The system is now fully installed at the university's ATMC (Advanced Technology and Manufacturing Center) Ultrasonic NDE Laboratory. A typical setup is as follows.

Laser ultrasonic testing combines the sensitivity of ultrasonic inspection with the flexibility of optical systems in dealing with complex inspection problems. It works well in the testing of metals, composite materials, ceramics, and liquids. Its remote nature allows the rapid inspection of curved surfaces on fixed or moving parts. It can measure parts in hostile environments or at temperatures well above those that can be tolerated using existing techniques. Its accuracy and flexibility have made it an attractive new technique in the non-destructive testing market.

Laser-based ultrasound ("LBU") is a remote implementation of conventional ultrasonic inspection systems that normally use contact transducers, squirter transducers, or immersion systems. LBU systems operate by first generating ultrasound in a sample using a pulsed laser. When the laser pulse strikes the sample, ultrasonic waves are generated through a thermoelastic process or by ablation. When this ultrasonic wave reaches the surface of the sample, the resulting surface displacement is measured with the laser ultrasonic receiver based on an adaptive interferometer.

Laser ultrasonics allows manufacturers to test materials and components for structural integrity, metallurgical purity, temperature, and other properties. It also allows overhaul and repair shops to test manufactured components of equipment such as aircraft for structural defects and metallurgical properties while still in service. This type of testing is crucial to manufacturing industries both to improve the level of quality in products as well as control the cost of parts. It permits industry to minimize the cost of stopping production lines to correct part defects that can be identified during real-time non-destructive testing using laser ultrasonic technology. Advantages of the LBU system can be summarized as follows.

1. Broad frequency band (generation and detection): From DC up to 1 GHz. With our system, the detection has a flat response from about 100Hz to 125MHz.
2. High spatial resolution. The generation and detection beam can be focused to very small size (µm size). In our system, the detection is focused down to 50 µm. The generation spot will depend on the focusing lens that will be used.
3. Remote generation and detection, without physical contact. With laser ultrasonics, we don't need coupling liquid. Laser-based ultrasound system serves industry's need for rapid and non-destructive inspection of parts during manufacture and in-service maintenance. Laser-based ultrasonic systems solve a variety of complex real-world industrial measurement and testing problems. Consult the list of applications below to see how laser-based ultrasonic systems can solve these problems.

Thickness Measurement

An important application for laser ultrasonics is the measurement of thickness in metals, ceramics, glass, and semiconductors. Laser ultrasonics provides a very precise (few micrometer accuracy) method of measuring thickness during manufacture of the components. It also has many maintenance applications as it can measure the thinning or erosion of walls in hot process piping and vessels without the need to shut down and cool an entire system or plant for the inspection.

Here we can use the time-of-flight approach for ultrasonic thickness measurement. We determine the thickness using the measured time-of-flight and the known sound velocity of the material. A peak detection algorithm allows the detection of the arrival time of each pulse and thus the round-trip time-of-flight (TOF). Knowing the sound velocity v in the material, we can determine the thickness d of the sample using d=(v x TOF)/2.

Defect Detection

The above are raw signal outputs of the laser ultrasonic system for a sample with several hole-shape defects at different depth beneath the sample surface. Analyzing the signal features by suitable advanced signal & image processing algorithms we can visualize the defects by 2-D or 3-D image representation.