What is Laser Ultrasonic Testing?

Laser ultrasonic testing (LUT) combines the sensitivity of ultrasonic inspection with the flexibility of optical systems to deal with complex inspection challenges. It has advantages for testing metals, composite materials, ceramics, and liquids. It can remotely and rapidly inspect curved surfaces on fixed or moving parts. It can measure parts in hostile environments and at temperatures well above those that can be tolerated by other instruments. Its accuracy and flexibility have made it an attractive new option in the non-destructive testing market.

LUT is a remote implementation of conventional ultrasonic testing (UT), which otherwise applies contact transducers, squirter transducers, or immersion systems. In the laser ultrasonic testing system illustrated schematically here, the small, fiber-coupled measurement head is placed near the part to be evaluated, and the remote base station contains all the support equipment.

Schematic layout of laser ultrasonic inspection system.

Schematic layout of laser ultrasonic inspection system.

LUT systems first generate ultrasonic waves from a pulsed laser. The laser pulse strikes the component, generating ultrasonic waves through a thermoelastic process or by ablation. As shown, laser pulses can generate the full complement of waves (longitudinal, shear, surface, and plate). These waves interrogate a feature of interest in the interior or surface of the component, and then propagate to the surface position of the detection laser beam. The resulting surface displacement is measured with the laser ultrasonic receiver. The measured signal is then processed to yield and display the required information. As with conventional UT, measurements can be performed in pulse-echo, through-transmission, and pitch-catch configurations.

With laser ultrasonics, manufacturers can perform critical measurements early in the manufacturing process, enabling them to post-process parts for quality control. It also enables overhaul and repair shops to test manufactured components of equipment such as aircraft for structural defects and metallurgical properties while they are still in service. These types of measurements will enable manufacturers to improve their level of product quality and to control their cost of parts. They will reduce the cost of stopping production lines to correct part defects that can be identified by real-time laser ultrasonic monitoring.

Broad Uses of Laser Ultrasonic Testing

Laser UT is fast and effective on rough surfaces. It can function effectively in a factory environment. It is ideally suited for many applications that are beyond the capabilities of conventional ultrasonic testing. The applications span three broad areas:

  • Process monitoring: measurements early in an industrial process on parts that are hot and/or moving at high speed
  • Post-process evaluation: high resolution inspection of small parts; fast areal scans of large components or structures
  • In-service inspection: inspection of complex structures (turbine blades); inspection under hazardous conditions (in nuclear power plants); fast scanning of safety-critical oil and gas pipelines.

Sample Applications

In-line thickness gauging

  • Alloy strip (1.04mm thickness) moving at 2 m/s
    Resonant frequency gives thickness to high accuracy

Thickness measurement in 1/2” steel

  • Spacing of echoes gives thickness to high accuracy

Disbond detection in composite honeycomb structure

  • Excellent imaging of simulated disbond using IOS LaserScan™ Software