Diagnosis of hysteretic dampers used for seismic protection of structures by means ultrasonic measurements

•A UT damage index for inspection of hysteretic seismic dampers has been validated.•A strong linear relationship UT index – ID mechanical index has been demonstrated.•The traditional mechanical index ID can be determined indirectly from UT index.•UT damage index behavior is scarcely affected by the loading pattern.•UT damage index works well above 20 dB of signal noise ratio.