Perhaps you should try some spelling variations:
2.2 instability » 7.2 instability, 6.2 instability, 2.4 instability
20 instability » 10 instability, 2.4 instability, 2.6 instability
27 instability » 7 instability, 17 instability, 37 instability
26 instability » 2.6 instability, 16 instability, 2.4 instability
2c instabilit » sc instability, 2.4 instability, 2.6 instability
6 instability » 7 instability, 5 instability, 4 instability
2c lehrbuch » 2 lehrbuch, 1 lehrbuch, im 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 (2.2 instability))) OR (27 instability))) OR (((26 instability) OR (6 instability))))))%c3%a4t %2c lehrbuch%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((2c instabilit) or (((((((20 instability) OR (2.2 instability))) or (27 instability))) or (((26 instability) or (6 instability))))))%c3%a4t %2c lehrbuch%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (((((((20 instability) OR (2.2 instability))) or (27 instability))) or (((26 instability) or (6 instability))))))%c3%a4t %2c lehrbuch%22*.