**4. Conclusions**

Three methods for interlaminar strength evaluation were analysed. Data evaluated using the AITM standard give significantly different results from the data based on the ASTM procedures. This implies that results from different methods cannot be directly compared and used for numerical analyses.

On the basis of statistical evaluation of three different composite materials, it can be stated that the choice of thermoplastic type can have a significant effect on the values of interlaminar strength. The values measured on the PAEK samples were statistically significantly higher than those measured on the PPS and PEEK samples (valid for both test temperatures).

The metallographic analysis performed on the samples showed similar homogeneity of all investigated materials, and the void size and quantity were not sufficient for a negative effect on interlaminar strength.

Cold conditions at −55 ◦C increased interlaminar strength in all investigated cases compared with RT conditions. The PAEK material gives significantly higher interlaminar shear strength in comparison with the PPS and PEEK materials. Additionally, the PAEK material has the lowest scatter in measured data. The PAEK thermoplastic material is a new composite material planned for application in future aircrafts.

Further tests on the coupon with TPC matrix are planned for the future in order to expand the material database. Based on these tests, a material will be selected that could replace the thermosetting materials used in aircraft structures for interior panels.

**Author Contributions:** Conceptualization, R.H., M.K. and R.R.; Methodology, R.H., M.K. and R.R.; Validation, R.H., M.K. and R.R.; Formal analysis, R.H., M.K. and R.R., Investigation, R.H., M.K. and R.R.; Resources R.H., M.K. and R.R.; Data curation, R.H., M.K. and R.R.; Writing—original draft preparation, R.H., M.K. and R.R.; Writing—reviewing and editing R.H., M.K. and R.R.; Visualization, R.H., M.K. and R.R.; Supervision, R.H., M.K. and R.R.; Funding acquisition, R.H., M.K. and R.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Ministry of Industry and Trade of the Czech Republic in the frame of "TRIO" call. Project No. FV 30033—Design and technological development of primary aircraft parts of advanced shapes made of composites with a thermoplastic matrix.

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

