Laser-Ultrasonics - LUS

Laser-Ultrasonics - LUS

Principle

Laser-Ultrasonics is a method for non-destructive testing of materials, similar to conventional ultrasound systems, but with significant advantages for specific applications. Both the generation and detection of ultrasound are conducted laser-based and thus completely contactless, which leads to following advantages:

No couplant needed
Automation possible
Distance to the object possible
Harsh industrial environment feasible
Difficult measuring positions possible
High ultrasound bandwidth

Usually, various kinds of ultrasonic waves are produced directly at the sample surface by short laser pulses in the ns-range. Those waves then spread out into the volume. Waves reflected and scattered by structures and defects can in turn be detected at different positions on the sample surface using a laser vibrometer.

By scanning the laser positions and applying reconstruction algorithms, images of the object can be created and elastic properties can be determined. The broadband nature of the excited ultrasound allows very high axial resolutions (exact values depending on the material). For investigations inside the object the so-called "bulk waves" are evaluated, while for surface analysis the simultaneously generated surface acoustic waves (SAW) are used.

Recommended as further reading about Laser-Ultrasonics: Scruby, C. & Drain, L., 1990 Laser Ultrasonics. Bristol: Adam Hilger and our publications.

Typical applications

Laser-Ultrasonics is basically recommended as possible modality, where conventional ultrasonic methods are not suitable, as the before-mentioned benefits are required.

Detection of internal defects in hot/cold/dangerous/sensitive objects
Microstructure analysis of metals during thermo-mechanical cycles
Determination of hardness penetration depth
Detection of delamination in fiber reinforced polymers (e.g. CFRP, GRP) and hybrid materials of those with metals
Determination of elastic/physical properties

Laboratory equipment

    • Detection systems / Vibrometers
      2x Tecnar: TWM / PDL for laboratory and industrial use
      BossaNova: Tempo
      In-house developments based on TWM, Confocal Fabry Perot, Sagnac
      Xarion: Eta250 Ultra (broadband microphone)
    • Ultrasonic excitation lasers (q-switched ns- and ps-lasers)
      Quantel: Q-smart 850, Brilliant B, Ultra
      InnoLas: SpitLight Hybrid
      CryLas: ps-Laser
      Bright Solutions: Wedge
    • DAQ-Systeme
      Oscilloscopes: Rohde & Schwarz, LeCroy
      Data acquisition cards: GaGe 14- and 16-bit
    • Laboratory / Infrastructure
      Laboratory space: 64m2
      7 optical tables for experimental setups
      Linseis quenching and deformation dilatometer with LUS-system

Analysis of a CFRP plate by means of Laser-Ultrasound

LUS4Metals Workshop in Linz

From 25th to 26th of September 2019 RECENDT organizes the 4th International Workshop on Laser-Ultrasonics for Metals (LUS4Metals) in Linz. Topics of the workshop are metallurgy and materials characterization, modelling and simulation, novel instrumentation and advanced sensing as well as defect detection and NDE for industrial applications. Read more...

International LUS-Symposium

In July 2016, RECENDT hosted the 5th International Symposium "Laser-Ultrasonics and Advanced Sensing" (short LU2016). At the 5-day conference, new developments in the field of laser-ultrasonics were presented.
Read more...

Examples of Applications

Metal processing

Since Laser Ultrasonics can be operated from a safe distance, this technology can be used in very early stages of metal processing at high temperatures. For example, the thickness of plates and the wall thickness of pipes can be measured in the still glowing state (even with only one-sided access). Also changes of the microstructure (e.g. grain growth), that are interesting in the rolling process, can be characterized with LUS.

Different materials (steel, aluminum, copper, semiconductors) - a wide range of issues (sheet thickness, defects, elastic properties, grain growth, anisotropy): the LUS technology combined with innovative data evaluation is highly versatile.

Casting process

For the casting of metals it may be interesting to monitor the depth of melting, but also to find hot cracks (aka solidification crack) in the freshly cast and still hot bars. Both is possible with Laser-Ultrasonics and was already investigated at RECENDT. In the figure a reconstruction and the corresponding photo of the cross section through a circular ingot of aluminum can be seen, which has an obvious center crack.

Hardness penetration depth in steel

The determination of the hardness penetration depth in thermally hardened components is essential for quality control. State-of-the-art is to cut samples and do etching and hardness measurements. Laser-Ultrasonics opens a non-destructive alternative. Thereby, zones of different microstructures at arbitrary positions can be imaged up to a tomographic representation of the hardness penetration depth.

Microstructure of metals in-situ

By offering the possibility to investigate glowing samples, Laser Ultrasound also enables the determination of in-situ metallurgical information during thermo-mechanical cycles of new steel grades. Therefor, samples can be inductively heated and deformed in our modified quenching and deformation dilatometer (by Linseis Messgeraete). From the measured parameters, like speed of sound and acoustic damping, it is possible to conclude on grain growth, phase transitions and texture changes with appropriate calibration.

Carbon fiber reinforced composites - CFRP

In the mainly manual production process of CFRP components delamination, inclusions and inner cracks represent critical problems for the mechanical strength. These errors are not visible from the outside because of the black coloring of the material. Using Laser-Ultrasound components can be scanned non-destructively and imperfections can be detected.

Fiber directions in composite materials

Do you want to gain a deeper insight and understanding of interior properties of your fiber-reinforced plastics? Are you interested in the anisotropies of your components or do you need to determine the orientation of the fibers inside a CFRP-injection moulded component?
We can help with our Terahertz-Technology (THz), OCT technology, and Laser Ultrasound Technology.

Sealing or bonding processes

Do you operate sealing processes e.g. for food packaging? With our Laser Ultrasonics or (depending on the material) OCT technology we are able to measure the sealing seams inline in the process. We provide similar testing methods for bonding processes, e.g., hot melt adhesives for packaging.

Bondings and Solder Joints

The Laser Ultrasonics technology is also applicable for the testing of adhesion and bonding layers, e.g., solder joints. As the high-frequency ultrasonic waves are influenced at the interfaces, an image of this critical inner zone can be generated.

3D printing / generative manufacturing / additive manufacturing

In recent years the rapid development and quality improvements in the field of 3D printing for plastic and metal components has enabled many applications, including the serial production of critical components. Using the OCT inspection technique (for plastics, ceramics) and Laser-Ultrasonics (for metals, hard plastics) allow the detection of certain defects offline or even inline and thus enable to monitor the process closely in order to set corrective actions if needed.

Inline quality assurance in welding

Non-destructive testing of welds is possible with Laser-Ultrasound. Thanks to the good automation capacity also inline measurements at high speed can be carried out, depending on the required resolution. With a repetition rate of 10 Hz, 100 Hz or even higher, flaws, inclusions, hot cracks and pore concentrations can be found. 

Spot welding

By means of Laser-Ultrasound also spot welds can be scanned in a short time and zones of actual material connection (weld nugget) can be imaged and evaluated.

Analytics for paper & the paper industry

Paper is an extremely diverse material and its characterization requirements are relevant for production, processing, and application. We can support you in many ways with our testing methods (IR & Raman Spectroscopy, THz Spectroscopy, OCT, and Laser-Ultrasound), for example, in chemical analytics (directly in the production process) and the characterization of surfaces and fibers for both absorbency and anisotropy.