Perhaps you should try some spelling variations:
20 instability » 2 instability, 26 instability, 27 instability
10 instability » 1 instability, 17 instability, _ instability
2c instabilit » sc instability, 2.2 instability, 2.4 instability
5 instability » _ instability, 7 instability
im lehrbuch » ein lehrbuch, zum lehrbuch, ihk lehrbuch

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %22str%c3%b6mungsmechanik %((((((2c instabilit) OR (20 instability))) OR (5 instability))) OR (10 instability))%c3%a4t %im lehrbuch%22.
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((((2c instabilit) or (20 instability))) or (5 instability))) or (10 instability))%c3%a4t %im lehrbuch%22*.