MEASURING STRESS OF A WIND TURBINE BLADE AND CONTROLLING THE WIND TURBINE

It is described a method of determining a value (12) of a stress related quantity of a rotor blade (3) of a wind turbine (1), the method comprising: emitting a primary radar signal (9) towards a portion (21, 22) of the blade (3); receiving a secondary radar signal (10) emanating from the blade (3) due to interaction with the primary radar signal (9); analysing at least the received secondary radar signal (10); and deriving, based on the analysis, the value (12) of the stress related quantity as related to or indicating a blade acceleration and/or a first temporal derivative of the blade acceleration and/or a higher temporal derivative of the blade acceleration.