Hi @Norbert 

Am sure you can use the flowcharts you created in Lucidchart in your engineering thesis while ensuring that you meet all necessary requirements and give proper credit to the source. 
If you have any clarifications or questions, it's always a good idea to consult with your academic advisor or thesis committee

Lucidchart typically allows users to use and distribute content created in their platform for personal or academic purposes,
 but it's essential to verify this for your specific use case

Provide a clear citation for the flowcharts you've created using Lucidchart. 
This citation should include the title of the flowchart, the author (you), the date it was created, and the fact that it was created using Lucidchart

Hope this helps - Happy to help further!!
Thank you very much and have a great one!

Warm regards