Damage model for the effects of laser weapons on composite structures

With the progress in laser technology, compact and efficient laser systems have become available for some years, which combine high output powers of several 10 kilowatts with a very good beam quality and can therefore be focused even over long distances. This will enable future military applications. Various nations are currently investigating the potential of such laser systems as an effector for a laser weapon. Experts assume that the further development of the technology will enable an output power in the 100-kilowatt class

Research capabilities for laser effects at Fraunhofer EMI

Against the background of these developments, the question arises for the Bundeswehr to what extent future systems can also be designed to be robust against the effects of high-energy laser radiation. To investigate such issues, Fraunhofer EMI operates special laboratories in which the conditions for a high-energy laser beam can be adjusted over a very wide range of parameters. The occurring effects can be observed very precisely under laboratory conditions at short distance with industrial lasers that are already available today. In addition to the physical effects for classic materials such as metals, the effects on new materials such as composite materials are also considered.

An important representative of the latter material class is carbon fiber reinforced plastic (CFRP). This is manufactured using layers of pre-impregnated carbon fibers, so-called prepregs, which are laminated into a plastic matrix (Figure 1). The carbon fibers give the material high mechanical stability combined with low weight. Because of these properties, CFRP is increasingly being used for lightweight constructions in the aerospace industry.

Analysis of damage mechanisms of CFRP under laser irradiation

In joint studies, the Bundeswehr Research Institute for Materials, Fuels and Lubricants (WIWeB) and Fraunhofer EMI are investigating the fundamental effects of high-energy laser radiation on CFRP. The first series of tests were initially carried out with laser powers of up to 10 kilowatts and varying beam diameters (1-3 centimeters) on planar CFRP samples with material thicknesses of up to 6 millimeters. During the investigations, special high-speed measurement technology was used to analyze the effective processes occurring during laser exposure. Images taken with a high-speed camera clearly show how the laser first ablates the plastic matrix, while damage to the carbon fiber occurs at a later stage due to its much higher melting and vaporization temperature (Figure 2).

Derivation of damage models

For further analysis, the samples were examined in a high-resolution computer tomograph (µ-CT) after the laser irradiation. This enables a non-destructive visualization of the damage inside the sample and, in particular, a quantification of the damaged volume (Figure 3). The evaluation of the test results shows that, regardless of the laser parameters used and the duration of the irradiation, there is a linear relationship between the laser energy applied and the volume removed in the investigated samples. This relationship can be formulated mathematically as a simple damage model, which allows predictions to be made about the extent of the damage that has occurred for the irradiation parameter range under consideration. Thus, statements about perforation times or the mechanical weakening of the material can be derived. The results of the joint investigations were published in the Journal of Composite Materials [1]. The publication also presents further results such as perforation times, electron microscopic analyzes of the morphology of the damaged material, and experimental studies of the decrease in mechanical stability of the exposed CFRP samples.

Outlook for future studies

Previous studies have shown that complex mechanisms of interaction can occur during lasers irradiation of composite materials, which are related to the inhomogeneous structure of this type of material. Due to the orientation of the carbon fibers in CFRP, there are preferred directions that lead to anisotropic heat conduction and damage to the material even outside the exposed area. These effects are currently being investigated in further series of tests. In addition, current studies also address the question which damage mechanisms occur at very high laser powers when the laser power will be increased further in the next development cycles and could reach more than 100 kilowatts. Will the observations made so far at lower beam powers continue to be valid, or will qualitative changes in the material behavior occur at very high power? In order to be able to classify such fundamental phenomena, it is intended to scientifically investigate the thermomechanical load behavior of materials in the high power regime.

[1] J. Wolfrum, S. Eibl, E. Oeltjen, J. Osterholz, and M. Wickert, High-energy laser effects on carbon fiber reinforced polymer with a focus on perforation time, Journal of Composite Materials (2021), https://doi.org/10.1177/0021998320988885