# The Key Role of Laser Ultrasonics in the Context of Sustainable Production in an I 4.0 Value Chain
## 4 The Key Role of LUS in a Smart Manufacturing Production Site
### Operating Principle  
The laser ultrasonic method is based on the contactless introduction and detection of ultrasonic signals in the material. The core of the system is an excitation laser and a detection laser--most commonly Nd:YAG lasers are used for both--and integrated interferometry, such as a Fabry-Perot or a two-wave mixing interferometer. The pulsed excitation laser transmits up to several mJ of energy to the material surface, where either thermoelastic excitation or ablative excitation occurs. Both variants generate a more or less intense stress field, resulting in a broadband ultrasonic signal (usually in the range of 500-50 MHz). In the case of thermoelastic excitation, mainly surface and plane waves are generated, while ablative excitation tends to produce more pronounced longitudinal waves. These waves pass through the material and are reflected at the back side, creating an echo. The detection laser can be mounted either in transmission or reflection mode, where the smallest deflections of the surface, due to reflection lead to a frequency shift, which is processed by an interferometer into signals that can be evaluated [107]. Figure 1 shows a schematic visualization of a LUS system.  
Apart from other types of waves generated by the excitation laser, such as surface waves or Lamb waves--an interaction of bulk waves occurring to (a-) symmetric vibrations in thin samples -, the essential ones here are the bulk waves. The bulk waves in a solid body can be subdivided depending on the mode of propagation. If the displacement of the involved particles is in the direction of propagation and results in compression, these waves are called longitudinal or compression waves; if the atomic motion is perpendicular to the direction of propagation, these waves are called transverse or shear waves. Bulk waves have the same frequency propagation, which means that they travel at the same speed, or in other words are non-dispersive. Since the compression waves propagate about twice as fast as the shear waves, these two types of waves can be clearly distinguished [107]. By analyzing the different types of waves in terms of propagation mode, direction and velocity, frequency and/or temperature dependence, or penetration depth into the material, different properties of the material can be determined.  
The most important analysis parameters of the detected signals are the Time of Flight (ToF), i.e., the propagation velocity, and the amplitude or its decrease after passing through the material, i.e., the attenuation. These two parameters can be used to generate a wide range of information about the microstructural or geometric configuration of the component, by conducting suitable evaluation operations. The evaluation of the velocity of sound provides specific information about  
* Geometry
* Phase transformation
* Phase composition
* Recrystallization
* Texture  
while the evaluation of the attenuation covers the aspects of
* Grain size and grain growth
* Phase constitution
* Dislocations  
In the following subsections the underlying principles of gaining insight into microstructural changes are briefly described.