Ultrasonic Time-Frequency Response Characteristics for Damage Evolution of Back-Filling Concrete under Staged Loading Conditions

Figure 10

.Two-dimensional spectrum of received ultrasonic signals at different stress levels. (a) 5.96 MPa. (b) σ = 16.78 MPa. (c) σ = 27.48 MPa. (d) σ = 35.52 MPa.

(a)

(b)

(c)

(d)